poor lateral control on a slow tow?
Doug
off, people keep asking me hard questions. One that has come up recently is
why a heavy glider on tow feels horrible, but thermalling in the same glider
at lower speeds is fine? (see also Mike Fox's article on aerotowing in the
October issue of S&G).
I did some calculations, and I reckon it's probably due to the tug wing wake
(tip vortices generating a downwash inboard, upwash outboard) changing the
lift distribution on the glider wing - with an increased angle of attack out
at the tips reducing aileron effectiveness. There's possibly an interesting
academic research project here, but it's always best to get a reality check
first ...
Is poor handling at low speed on tow a common experience? I'd appreciate
any thoughts/comments/war stories ... particularly bad tug/glider/speed
combinations, incidents of wing drop during a tow etc etc?
Doug Greenwell
wladkummer76
Sure, even more with flaps equipped ships! But yaw control seems to be
more affected than roll. Can we explain that?
Just a little faster please is what doctor ordered here...usually 5
mph more is enough to a more pleasant tow.
Andy
> Is poor handling at low speed on tow a common experience?
Well not for me, but only because I don't often get towed slow. For
my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd
like to be. On the few tows that the speed has dropped to 60kts it's
barely possible to stay in control in any sort of turbulence. Both
pitch and roll control are greatly reduced, I never noticed a problem
with yaw control.
Slow tows only seem to be a problem for me when I go to an site that
uses tow pilots that have no high performance glider experience but
they still know better than the glider pilot what speeds are needed.
Fortunately there are not too many of those.
The worse possible case is the tow pilot that refuses to follow the
simple instruction to stay in ground effect until a safe tow speed is
reached.
Some glider pilots don't realize how hopelessly inaccurate the tug
airspeed indicator may be on tow and they ask for the speed they'd
like to see in the glider. Bad idea. Far better to ask for a speed x
kts or mph higher than they use for the gliders they tow everyday.
Andy
Doug
"wladkummer76" <wladimi...@gmail.com> wrote in message
news:78c15469-76d8-4c3a...@m11g2000vbs.googlegroups.com...
Good question ... the only thing I can think of at the moment is that the
tug tip vortices would also generate an inwards 'sidewash' flow above the
wake, which would be stronger the heavier and slower the tug was. If you
are then for some reason off to one side of the tug the sidewash at the fin
from the nearer vortex would tend to yaw you 'nose-out', in the wrong
direction.
Doug
John Cochrane
>
> Well not for me, but only because I don't often get towed slow. For
> my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd
> like to be. On the few tows that the speed has dropped to 60kts it's
> barely possible to stay in control in any sort of turbulence. Both
> pitch and roll control are greatly reduced, I never noticed a problem
> with yaw control.
>
This is precisely the aerodynamic puzzle. The same standard class
glider stalls in the low 40s and is perfectly happy thermaling at
45-50. So why does 60 feel so awful on tow?
This does seem a lot worse in standard class than 15 meter, another
hint for the puzzle.
If it were downwash, one would think that just flying higher would
solve it. But that's the best theory I've heard so far.
I have seen quite a few contests with tow pilots who had little
experience with fully ballasted gliders. It's really important to get
the word out to them 1) yes, we really want to tow that fast. 2) get
up to speed in ground effect, then start climbing.
I distinctly remember that helpless feeling sitting in a tanked up
discus, crossing the end of the runway, as the towplane departed what
looked like straight up at about 50 knots.
On another occasion, every single tow by one pilot was accompanied by
a chorus of demands for more speed on the radio. Eventually he piped
up "what do y'all want to fly so fast for anyway?'' I guess the 2-33
never wanted to go 70 on tow.
John Cochrane
jcarlyle
Once the tugs (I've seen this behind an L-19 and a Pawnee) reached
cruising altitude, it became extremely difficult to stay behind them.
I asked them to slow down (went from 85 to 65 kt), got into low tow,
slipped and even pulled on some spoiler, but my LS8 (unballasted) was
a handful in roll and pitch. Once on a turbulent blue day I
deliberately released 25 miles before I had final glide, preferring
the excitement of finding a means of staying up rather than continuing
on tow.
-John
Doug Greenwell
>> > Is poor handling at low speed on tow a common experience?
>>
>> Well not for me, but only because I don't often get towed slow.
>> my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd
it's
>> barely possible to stay in control in any sort of turbulence. =A0Both
>> pitch and roll control are greatly reduced, I never noticed a problem
>> with yaw control.
>>
>
>This is precisely the aerodynamic puzzle. The same standard class
>glider stalls in the low 40s and is perfectly happy thermaling at
>45-50. So why does 60 feel so awful on tow?
>
>This does seem a lot worse in standard class than 15 meter, another
>hint for the puzzle.
>
>If it were downwash, one would think that just flying higher would
>solve it. But that's the best theory I've heard so far.
>
>I have seen quite a few contests with tow pilots who had little
>experience with fully ballasted gliders. It's really important to get
>the word out to them 1) yes, we really want to tow that fast. 2) get
>up to speed in ground effect, then start climbing.
>
>I distinctly remember that helpless feeling sitting in a tanked up
>discus, crossing the end of the runway, as the towplane departed what
>looked like straight up at about 50 knots.
>
>On another occasion, every single tow by one pilot was accompanied by
>a chorus of demands for more speed on the radio. Eventually he piped
>up "what do y'all want to fly so fast for anyway?'' I guess the 2-33
>never wanted to go 70 on tow.
>
>John Cochrane
>
In the normal tow position I don't think you can get high enough to get
away from the wing wake without tipping the tug on its nose. On the other
hand, my simple modelling suggests that a typical low tow position should
be a lot better.
15m vs standard class ... Forgive my ignorance (I've never flown a
flapped glider), but presumably a 15m glider is towed with the flaps
deflected, which would tend to unload the tips and hence maintain aileron
authority at high incidence. Would the ailerons typically be drooped at
this flap setting?
Doug
Andy
wrote:
>The same standard class glider stalls in the low 40s and is perfectly happy
>thermaling at 45-50.
Not mine, at least not when ballasted. Most of my flying is done
where the thermals are small cored and often rowdy. 50kts would only
work in a very smooth thermal that was big enough to use a low bank
angle.
However, the D2 which does seem to climb well at low speeds seems to
be really sensitive to being towed slow, at least that I understand
from one of the local pilots. Maybe D2 owners could comment on that?
Andy
sisu1a
> Sure, even more with flaps equipped ships! But yaw control seems to be
> more affected than roll. Can we explain that?
> Just a little faster please is what doctor ordered here...usually 5
> mph more is enough to a more pleasant tow.
I'm going to assume your flying with a nosehook, in which case the big
tether is resisting motion in the yaw axis... whereas for roll the
towrope runs directly down the longitudinal axis, leaving airspeed/AoA
as the dominant external forces affecting roll.
The rope also holds the glider at unnatural attitudes for given
airspeeds, which I think contributes a lot to the lack of perceived
(less than usual for that particular airspeed) aileron authority,
contributing greatly to making the glider feel horrible on tow,
despite being well within it's stall speed.
Not as an endorsement for or against this arrangement but from an
aerodynamic prospective rather, a CG hook leaves the yaw axis pretty
free to swivel accordingly and likely reduces the rope induced AoA
affecting aileron authority as well *once in a steady climb. (*not to
be confused with the up pitching tendency these hooks are famous for
during the initial acceleration of a launch...)
-Paul
Andy
Since the possible influence of hook position has been introduced,
I'll have to add that all my tows in the 28 have been on the CG hook
as were all my tows in the 19 I had before that.
One factor that has not been mentioned yet is the fact that a
ballasted glider has much higher roll inertia that a dry one. This
will require larger aileron inputs to get the same roll response and I
suppose could lead to an aileron stall.
I don't remember ever having loss of control authority on an
unballasted tow and maybe there is more to that than just the
difference in stall speeds.
Andy
Doug Greenwell
>
>> Sure, even more with flaps equipped ships! But yaw control seems to be
>> more affected than roll. Can we explain that?
>> Just a little faster please is what doctor ordered here...usually 5
>> mph more is enough to a more pleasant tow.
>
>I'm going to assume your flying with a nosehook, in which case the big
>tether is resisting motion in the yaw axis... whereas for roll the
>towrope runs directly down the longitudinal axis, leaving airspeed/AoA
>as the dominant external forces affecting roll.
>
good point - a nose hook would tend to improve yaw and pitch stability on
tow, but have little effect on roll, so it's difficult to split out the
effects of aerodynamics and tow cable dynamics. I did some work years ago
on towed bodies (aerial targets and sonar fish) and the coupling effects
between cable and body motion get really complicated.
>The rope also holds the glider at unnatural attitudes for given
>airspeeds, which I think contributes a lot to the lack of perceived
>(less than usual for that particular airspeed) aileron authority,
>contributing greatly to making the glider feel horrible on tow,
>despite being well within it's stall speed.
yes - I'm sure there must be a pyschological element as well
>
>Not as an endorsement for or against this arrangement but from an
>aerodynamic prospective rather, a CG hook leaves the yaw axis pretty
>free to swivel accordingly and likely reduces the rope induced AoA
>affecting aileron authority as well *once in a steady climb. (*not to
>be confused with the up pitching tendency these hooks are famous for
>during the initial acceleration of a launch...)
>
>-Paul
>
I think the rope effects on stability with a nose hook are generally
favourable - its the aerodynamic effects of the tug wake that cause the
problems
bildan
I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons. Overuse of
ailerons is very common and it makes aero tow 'wobbly'. If you
consciously use rudder to aim the nose at the tug's tail and just keep
the same bank angle as the tug with ailerons, it might work better.
Wake effects are generally favorable if you stay at the right height
relative to the tug. Using a slightly higher tow position can
sometimes help a lot.
The tip vortices rotate inward above the propwash which, if allowed to
do so, will drift the glider to the center position and help keep it
there. I haven't noticed any tendency for them to yaw a glider
towards a tugs wing tip.
Doug Greenwell
>On Dec 31, 4:40=A0am, "Doug" wrote:
>> As an aerodynamicist/flight dynamicist recently re-soloed after 25
years
>recent=
>ly is
>> why a heavy glider on tow feels horrible, but thermalling in the same
>der
>> at lower speeds is fine? (see also Mike Fox's article on aerotowing
in
>e
>> October issue of S&G).
>>
>> I did some calculations, and I reckon it's probably due to the tug
wing
>ake
>> (tip vortices generating a downwash inboard, upwash outboard) changing
>e
>> lift distribution on the glider wing - with an increased angle of
attack
>out
>> at the tips reducing aileron effectiveness. =A0There's possibly an
>intere=
>sting
>> academic research project here, but it's always best to get a reality
>ck
>> first ...
>>
>> Is poor handling at low speed on tow a common experience? =A0I'd
>apprecia=
>te
>> any thoughts/comments/war stories ... particularly bad
tug/glider/speed
>> combinations, incidents of wing drop during a tow etc etc?
>>
>> Doug Greenwell
>
>I suspect, but can't know unless I flew with you, that you are
>unconsciously trying to "steer" the glider with ailerons. Overuse of
>ailerons is very common and it makes aero tow 'wobbly'. If you
>consciously use rudder to aim the nose at the tug's tail and just keep
>the same bank angle as the tug with ailerons, it might work better.
>
>Wake effects are generally favorable if you stay at the right height
>relative to the tug. Using a slightly higher tow position can
>sometimes help a lot.
>
>The tip vortices rotate inward above the propwash which, if allowed to
>do so, will drift the glider to the center position and help keep it
>there. I haven't noticed any tendency for them to yaw a glider
>towards a tugs wing tip.
>
Certainly my early aerotows were a bit wobbly, although they've settled
down now - I (re-)soloed on an aerotow, which I think is relatively
unusual in the UK, but most of my launches since then have been on the
winch.
The problem is a bit more than wobbliness though - there does seem to be a
common theme of experienced pilots having real control difficulties when
heavy and a bit slow on tow (not a situation I've been in myself ...
yet).
Part of the difficulty in analysing this is splitting out the effects of
the aerodynamic wake/wing/fin interaction(s) and the mechanical cable
dynamics.
It just occured to me that it might be interesting to do this by flying a
motor glider in formation with a tug - no cable, so only the aerodynamic
effects to contend with. There has been a lot of work done on
aerodynamics of flrmation flight and towing, but everything published has
been on aircraft with either the same span or with the lead aircraft much
larger than the trailing aircraft.
John Cochrane
> However, the D2 which does seem to climb well at low speeds seems to
> be really sensitive to being towed slow, at least that I understand
> from one of the local pilots. Maybe D2 owners could comment on that?
>
> Andy
The D2 also has a very low angle of incidence and thus a high nose
attitude on tow. Could this be part of the issue -- it's just darn
uncomfortable to fly with the nose pointed above the towplane? That
would also account for why 15 meter seems easier. We fly with flaps;
they are interconnected to the ailerons so we're not getting great
roll rates on tow. But it does give a nose down attitude so we can see
the towplane.
John Cochrane
Todd
Looking at things from and aerodynamics standpoint (and I am about as
far from and aerodynamicist as you can get) it should seem that part
of the empirical data would suggest an experiment where you fly a
glider equipped with and Angel of Attack meter at your typical tow
speeds and record the AoA at various speeds. Then fly that glider on
tow at those same speeds and record the results.
I would suspect that we will find that the AoA is higher (e.g. closer
to the critical stall AoA ) and that this would explain the handling
characteristics. So the million dollar question is why? My guess is
that in soaring flight we are essentially coasting down hill at say
60Kt. In towing flight at 60Kt, we are being "dragged" up the hill
(plus, maybe, some effect from tow rope attachment point leverage) so
something, aerodynamically. has to be different and that has (???) to
be the AoA. As we all know from out stall training, as we approach
the critical angle of attack, we loose out roll (aileron)
effectiveness.
...or should I go back to learning to flip burgers at McD's?
Derek C
>
> - Show quoted text -
There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being asked
to climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.
Derek C
bildan
> I too agree with the real or perceived tow handling characteristics.
>
> Looking at things from and aerodynamics standpoint (and I am about as
> far from and aerodynamicist as you can get) it should seem that part
> of the empirical data would suggest an experiment where you fly a
> glider equipped with and Angel of Attack meter at your typical tow
> speeds and record the AoA at various speeds. Then fly that glider on
> tow at those same speeds and record the results.
Done that - and as nearly as I can see, there's no difference in AoA.
I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on tow.
If I'm careful not to over-control the ailerons, there's no problem at
all.
Heavily ballasted gliders respond sluggishly in roll just due to the
extra roll inertia. A pilot trying to hold a precise position behind
a tug needs and expects crisp aileron response. When he doesn't get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. If he's less than equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.
Martin Gregorie
If the tug's downwash field extends back far enough to include the
glider, its AOA will be relative to the downwash streamlines. Add the
downwash angle to the climb angle of the tug-glider combination will make
the glider look quite nose-high to its pilot.
I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
chords behind the wing, i.e. about where the tailplane is, but not what
its angle might be at the end of a tow rope.
--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |
Andy
wrote:
I'd be surprised if the flow field from the towplane wake is
significant for gliders in normal high tow position. I do wonder if
the "sluggish controls" effect is to some extent psychological because
flying formation requires much more precision than normal slow flight
off tow. I'm most uncomfortable when I find myself slow and below the
towplane and need to climb up.
Unless the glider is accelerating vertically, I'm pretty sure that
steady climb requires the same amount of lift as steady glide. Steady
climb is not the same as accelerating climb. (F=MxA so if the lifting
force exceeds the glider's weight by definition it accelerates
vertically).
The towplane provides thrust to overcome the frictional and lift-
related drag losses, but unless you are well below the towplane the
force on the rope is, for all practical purposes, horizontal. If you
have a cg hook you will get a modest nose-up pitching moment from the
rope, but this is a trim issue more than an AOA issue I believe. The
tension on the rope could also provide some counter-force to rudder
and elevator inputs, but I don't think you'd feel much for small
angular displacements.
9B
Eric Greenwell
> As an aerodynamicist/flight dynamicist recently re-soloed after 25 years
> off, people keep asking me hard questions. One that has come up recently is
> why a heavy glider on tow feels horrible, but thermalling in the same glider
> at lower speeds is fine? (see also Mike Fox's article on aerotowing in the
> October issue of S&G).
>
> I did some calculations, and I reckon it's probably due to the tug wing wake
> (tip vortices generating a downwash inboard, upwash outboard) changing the
> lift distribution on the glider wing - with an increased angle of attack out
> at the tips reducing aileron effectiveness. There's possibly an interesting
> academic research project here, but it's always best to get a reality check
> first ...
The wake behind a climbing towplane should be well below a glider in the
high tow position. How can it lift it's weight and the glider, if the
wake isn't descending? Recall one of the exercises a student does while
learning to tow is to start in the high tow position, then move straight
down until the wake turbulence is felt. With the usual 150' to 200'
rope, it's way below the high tow position. So, I don't think it's wake
turbulence, IF the glider is in the high tow position.
I think a big part of the answer is the pilot's perception of
"attitude": the glider has an additional attitude (relative to the
horizon) over it's normal angle of attack because the glider and tug are
ascending at about a 6 degree angle. T His is perceived by the pilot as
a very nose high attitude, and makes him feel uncomfortable; that, along
with reduced control response, makes him feel it's worse than it really
is. Usually, this happens close to the ground, making the perception
even worse.
The unusually nose-high attitude can keep the glider pilot from using
enough up elevator, with the consequence that he does sink down into the
wake. That will make the situation actually worse, not just perceptually
worse. But, it's because he is far from the high tow position, not just
because the speed is slower than normal.
This analysis obviously assumes a high tow as the normal situation, and
may not apply to the situation where low tow is the norm.
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
Bruce Hoult
wrote:
> The D2 also has a very low angle of incidence and thus a high nose
> attitude on tow. Could this be part of the issue -- it's just darn
> uncomfortable to fly with the nose pointed above the towplane? That
> would also account for why 15 meter seems easier. We fly with flaps;
> they are interconnected to the ailerons so we're not getting great
> roll rates on tow. But it does give a nose down attitude so we can see
> the towplane.
I had some really horrible feeling tows in a PW5. The thing felt
mushy, nose high, couldn't see the towplane, needed a lot of back
stick, afraid it was going to stall etc.
Then I realized that when you're going up at over 1000 fpm in still
air, keeping ANY part of the tug on the horizon (even wheels) is far
too high a position. I dropped down until I could start to feel the
wash and then came up a little. It felt much better but the tug seemed
WAY UP THERE.
Work it out ... at 65 knots and going up at 10 knots, the other end of
a 50m rope will be 7.7m above you if you're following the same path.
Even guessing 3m to get out of the wake, the tug should still be
nearly 5m above you.
And maybe it's 11 or 12 knots climb (I can't tell because the vario is
pegged), in which case that's another 1 or 1.5 m.
Since then I tow with the Pawnee horizontal stabilizer in the same
position against the forward parts of the tug no matter what glider
I'm in and just ignore the horizon. Even in the DG1000 two-up and
climbing at 700 fpm this still results in the tug's wheels being a
fraction above the horizon (and I've been criticized for this on
biannuals) but I'm still comfortably above the turbulence of the tug's
wake.
Tony V
> ....Since then I tow with the Pawnee horizontal stabilizer in the same
> position against the forward parts of the tug no matter what glider
> I'm in and just ignore the horizon.
Yes, use the tug as a reference. Using the horizon doesn't work on real
hazy days and it doesn't work in the mountains.
Tony V.
Anne
John Cochrane has the answer right, at least for standard class ships
like the Discus 2. I can verify that you run out of elevator control
at tow speeds significantly lower than the free-flight stall speed.
The reason is that the tow rope applies a downward thrust at the nose
- I have wing tip-camera video that confirms the tow rope has a
significant downward pull on the nose. I always try to stay away from
tow plane wash, so don't think that's a major component. I've never
experienced as marked a behavior in flapped ships, so I put it down to
AOA.
Mike
John Vella Grech
comfortable with this method gliding in Aus. At least the tow rope has an
upward componenet.
John
Doug Greenwell
>On Dec 31, 1:47=A0pm, Martin Gregorie
>> On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
>> >> On Dec 31, 4:40=A0am, "Doug" wrote:
>>
>> >> > As an aerodynamicist/flight dynamicist recently re-soloed after
25
>com=
>e
>> >> > up recently is why a heavy glider on tow feels horrible, but
>> >> > thermalling in the same glider at lower speeds is fine? (see also
>> >> > Mike Fox's article on aerotowing in the October issue of S&G).
>>
>> >> > I did some calculations, and I reckon it's probably due to the
tug
>> >> > wing wake (tip vortices generating a downwash inboard, upwash
>> >> > outboard) changing the lift distribution on the glider wing -
with
>n
>> >> > increased angle of attack out at the tips reducing aileron
research
>> >> > project here, but it's always best to get a reality check first
..
>>
>> >> > appreciate any thoughts/comments/war stories ... particularly bad
>> >> > tug/glider/speed combinations, incidents of wing drop during a
tow
>> >> > etc etc?
>>
>> >> > Doug Greenwell
>>
>> >> I suspect, but can't know unless I flew with you, that you are
>> >> unconsciously trying to "steer" the glider with ailerons.
>o=
>f
>> >> ailerons is very common and it makes aero tow 'wobbly'. =A0If you
>> >> consciously use rudder to aim the nose at the tug's tail and just
>keep
>> >> the same bank angle as the tug with ailerons, it might work better.
>>
>> >> Wake effects are generally favorable if you stay at the right
height
>> >> sometimes help a lot.
>>
>> >> The tip vortices rotate inward above the propwash which, if allowed
>to
>> >> do so, will drift the glider to the center position and help keep
it
>towa=
>rds
>> >> a tugs wing tip.- Hide quoted text -
>>
>> >> - Show quoted text -
>>
>> > There was a debate on our club forum about why gliders feel
>> > uncomfortable on slow tows that are still well above their normal
>> > stalling speed. We think the answer is that the glider is being
asked
>o
>> > climb with the tug providing the thrust via the rope. The glider is
>> > still effectively in free flight and therefore has to fly at a
greater
>> > angle of attack for a given airspeed to produce the extra lift for
>> > climbing. Hence its stalling speed is somewhat increased.
>>
>> If the tug's downwash field extends back far enough to include the
>> glider, its AOA will be relative to the downwash streamlines. Add the
>> downwash angle to the climb angle of the tug-glider combination will
>make
>>
>> I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
>> chords behind the wing, i.e. about where the tailplane is, but not
what
>> its angle might be at the end of a tow rope.
>>
>> --
>> gregorie. | Essex, UK
>> org =A0 =A0 =A0 |
>
>I'd be surprised if the flow field from the towplane wake is
>significant for gliders in normal high tow position. I do wonder if
>the "sluggish controls" effect is to some extent psychological because
>flying formation requires much more precision than normal slow flight
>off tow. I'm most uncomfortable when I find myself slow and below the
>towplane and need to climb up.
>
>Unless the glider is accelerating vertically, I'm pretty sure that
>steady climb requires the same amount of lift as steady glide. Steady
>force exceeds the glider's weight by definition it accelerates
>vertically).
>
>The towplane provides thrust to overcome the frictional and lift-
>related drag losses, but unless you are well below the towplane the
>force on the rope is, for all practical purposes, horizontal. If you
>have a cg hook you will get a modest nose-up pitching moment from the
>rope, but this is a trim issue more than an AOA issue I believe. The
>tension on the rope could also provide some counter-force to rudder
>and elevator inputs, but I don't think you'd feel much for small
>angular displacements.
>
>9B
>
It is surprising, but part of the problem is the word 'wake' ... in
order to generate lift a wing has to move a fair amount of air around
(although let's not start the bernoulli argument now!), so its influence
on the surrounding atmosphere extends a surprising distance away from it.
Tip vortices are also a very stable flow structure, so don't begin to
break up or decay for a very very long way downstream.
The climb angles are too small to make a significant difference to the
lift required from the glider wing (assuming the tow rope is straight),
since the effect on lift goes with the cosine of the angle
On the other hand, if the tow rope is not straight then there could be a
significant lift component from the tension force (going with the sine of
the tow rope angle) ... but you would have to be quite a long way above
the tug to make a big difference.
Doug Greenwell
>On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
>
>> On Dec 31, 6:19 pm, bildan wrote:
>>> On Dec 31, 4:40 am, "Doug" wrote:
>>>
>>>
>>>
>>>
>>>
>>> > As an aerodynamicist/flight dynamicist recently re-soloed after 25
>>> > years off, people keep asking me hard questions. One that has
come
>>> > up recently is why a heavy glider on tow feels horrible, but
>>> > thermalling in the same glider at lower speeds is fine? (see also
>>> > Mike Fox's article on aerotowing in the October issue of S&G).
>>>
>>> > I did some calculations, and I reckon it's probably due to the tug
>>> > wing wake (tip vortices generating a downwash inboard, upwash
>>> > outboard) changing the lift distribution on the glider wing - with
an
>>> > increased angle of attack out at the tips reducing aileron
>>> > effectiveness. There's possibly an interesting academic research
>>> > project here, but it's always best to get a reality check first
..
>>>
The downwash angle doesn't change much past the tail, and a half to a
third of the tug AoA is a good first guess.
My modeling suggest that there does seem to be an overall reduction in the
glider wing lift (downwash over the centre wing having more of an effect
than upwash over the tips), so the glider requires another degree or two
in AoA - so feeling even more nose-up to the pilot!
Doug Greenwell
>
>> On Dec 31, 6:19 pm, bildan wrote:
>>> On Dec 31, 4:40 am, "Doug" wrote:
>>>
>>>
>>>
>>>
>>>
>>> > As an aerodynamicist/flight dynamicist recently re-soloed after 25
>>> > years off, people keep asking me hard questions. One that has
come
>>> > up recently is why a heavy glider on tow feels horrible, but
>>> > thermalling in the same glider at lower speeds is fine? (see also
>>> > Mike Fox's article on aerotowing in the October issue of S&G).
>>>
>>> > I did some calculations, and I reckon it's probably due to the tug
>>> > wing wake (tip vortices generating a downwash inboard, upwash
>>> > outboard) changing the lift distribution on the glider wing - with
an
>>> > increased angle of attack out at the tips reducing aileron
>>> > effectiveness. There's possibly an interesting academic research
>>> > project here, but it's always best to get a reality check first
..
>>>
The downwash angle doesn't change much past the tail, and a half to a
Doug Greenwell
Possibly two (or more) different handling problems on tow then ...
1) Running out of nose-up elevator authority when in a 'high' high tow
position due to a combination of increased AoA required due to tug
downwash and downward force component from the rope + a nose down pitching
from the rope
2) degradation of lateral control due to changes in spanwise lift
distribution
I've certainly sparked some interest here - considering it's New Year
:-)
Doug Greenwell
>On Dec 31, 1:06=A0pm, Todd wrote:
>> I too agree with the real or perceived tow handling characteristics.
>>
as
>> far from and aerodynamicist as you can get) it should seem that part
>> of the empirical data would suggest an experiment where you fly a
>> glider equipped with and Angel of Attack meter at your typical tow
on
>> tow at those same speeds and record the results.
>
>Done that - and as nearly as I can see, there's no difference in AoA.
>
>I've flown some pretty heavy high performance gliders behind some
>pretty bad tow pilots - one of them stalled the tug with me on tow.
>If I'm careful not to over-control the ailerons, there's no problem at
>all.
>
>Heavily ballasted gliders respond sluggishly in roll just due to the
>extra roll inertia. A pilot trying to hold a precise position behind
>a tug needs and expects crisp aileron response. When he doesn't get
>it, he increases the amount and frequency of aileron with a
>corresponding increase in adverse yaw. If he's less than equally
>crisp with rudder to oppose the adverse yaw, it gets wobbly.
>
Where did you mount the AoA meter?
It's not the angle of attack that's the problem, but the change in local
incidence along the wing. The overall lift may not change by very much
when near to the tug wake, but its distribution along the wing does, with
increased lift at the tips and reduced lift at the root - putting the
aileron region close to the stall and hence reducing control
effectiveness.
I agree that increased roll inertia due to ballast is a factor, but since
the same factor applies to maintaining bank angle in a thermalling turn I
don't see how it can account for a significant difference in handling
between tow and thermalling?
Doug Greenwell
The wake does descend, although this is not necessary for a wing to
generate lift (otherwise wind tunnels would not work!) ... actually, the
downwash is a consequence of a reduction in lift and increase in (induced)
drag for a three-dimensional wing.
However, the turbulent prop wash also descends with it, so setting a tow
position on the basis of a reasonable distance above the prop wash would
automatically position you close to the tip vortices.
PS I've only ever come one other Greenwell outside the North East of
England ... any relation?
Doug Greenwell
There do seem to be many advantages to low tow - I'm not sure why it's
not used much in the UK. On the Junior the rope apparently fouls the nose
in low tow, so perhaps its a problem with some hook positions?
Big Wings
soarable lift - I don't hang on until an arbitrary height like 2,000' is
reached. (I have been known to release at 700' - but my club now charges
for a minimum of 1,000' even if one releases earlier so I tend to go a
bit higher now - I might have become a bit wiser as well!)
Since I must release from the high-tow position to ensure adequate
clearance from the metal rings on the rope immediately after I release -
if I'm in low tow I must go up to high-tow first - by which time I'm
well past the lift and will probably fail to find it. Hence my preference
for high-tow during a launch into a soarable sky. During a retrieve I will
often go low-tow.
Andy
> I've flown some pretty heavy high performance gliders behind some
> pretty bad tow pilots - one of them stalled the tug with me on tow.
> If I'm careful not to over-control the ailerons, there's no problem at
> all.
>
> Heavily ballasted gliders respond sluggishly in roll just due to the
> extra roll inertia. A pilot trying to hold a precise position behind
> a tug needs and expects crisp aileron response. When he doesn't get
> it, he increases the amount and frequency of aileron with a
> corresponding increase in adverse yaw. If he's less than equally
> crisp with rudder to oppose the adverse yaw, it gets wobbly.
Bill, I use whatever aileron is required to establish and maintain the
bank angle I need. I also use exactly the right amount of rudder to
maintain coordinated flight. My low speed ballasted tows are not
"wobbly". I am momentarily out of control since the ailerons are on
the stops and I'm still not getting the roll response I need.
Please don't assume that the problem is caused by pilots not
understanding how to use the controls. The problem is caused by being
towed at a speed lower than that at which the glider is controllable
in rough air.
The solution is simple and entirely in the hands of the tow pilot.
The aerodynamic explanation is of little interest to me.
Your insistence on using the term "wobbly" to describe the problem
convinces me that you have never experienced it.
Andy
Derek C
>
> - Show quoted text -
What started the debate at Lasham was using a Rotax engined Falke as a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,
Derek C
Doug Greenwell
>On Jan 1, 11:15=A0am, Doug Greenwell wrote:
>> At 20:23 31 December 2010, bildan wrote:
>>
>>
>>
>>
>>
>> >> I too agree with the real or perceived tow handling
characteristics.
>>
>abou=
>t
>> as
>> >> far from and aerodynamicist as you can get) it should seem that
part
>> >> of the empirical data would suggest an experiment where you fly a
>> >> glider equipped with and Angel of Attack meter at your typical tow
>glider
>> on
>> >> tow at those same speeds and record the results.
>>
>> >Done that - and as nearly as I can see, there's no difference in
AoA.
>>
>> >I've flown some pretty heavy high performance gliders behind some
>> >pretty bad tow pilots - one of them stalled the tug with me on tow.
>> >If I'm careful not to over-control the ailerons, there's no problem
at
>> >all.
>>
>> >Heavily ballasted gliders respond sluggishly in roll just due to the
behind
>> >a tug needs and expects crisp aileron response. =A0When he doesn't
get
>> >it, he increases the amount and frequency of aileron with a
>> >crisp with rudder to oppose the adverse yaw, it gets wobbly.
>>
>> Where did you mount the AoA meter?
>>
>> It's not the angle of attack that's the problem, but the change in
local
In a steady climb in any light aircraft the climb angles are so low (<
10deg) that the lift remains pretty well equal to weight. For example a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift = Weight/cos(10deg) = 1.02 x
Weight. You don't need to increase lift to climb - you increase thrust
to overcome the aft component of the weight, and the stick comes back to
maintain speed ... at constant speed the increased power input comes out
as increasing potential energy = increasing height.
I think a lot of people confuse the actions needed to initiate a climb
with what is actually happening in a steady climb.
On your second point, if you are on tow anywhere sensible behind a tug you
are in its wake and are being affected by the wing downwash. Wake is not
really a good word, since it seems to get confused with the much more
localised (and turbulent) propwash.
A (very) crude way of visualising the affected wake area is to imagine a
cylinder with a diameter equal to the tug wing span extending back from
the tug - that's the downwash region, and then in addition there's an
upwash region extending perhaps another half-span out either side.
Doug Greenwell
>10deg) that the lift remains pretty well equal to weight. For example a
>10deg climb angle at 60 kts corresponds to an impressive climb rate of
>10.5kts - but that would only give Lift = Weight/cos(10deg) = 1.02 x
>Weight. You don't need to increase lift to climb - you increase thrust
>to overcome the aft component of the weight, and the stick comes back to
>maintain speed ... at constant speed the increased power input comes out
>as increasing potential energy = increasing height.
>
whoops - I should have said Lift = Weight*cos(10deg) = 0.985 x
Weight, since in a climb the thrust (or tow cable) is supporting part of
the the weight .... long night, early morning :-)
Derek C
>
> - Show quoted text -
So why did a K13 feel on the verge of a stall at 50 knots on tow? All
the classic symptoms of a stall were there, including mushy controls,
wallowing around and buffeting. If you got even slightly low it seemed
quite difficult to get back up to the normal position. Lack of
elevator effectiveness is yet another sympton of the stall!
Fortunately we have given up aerotowing with the Falke. It just seemed
like a good idea at the time because its flying speeds are more
closely matched to a glider; in theory anyway.
Derek C
Eric Greenwell
>
> PS I've only ever come one other Greenwell outside the North East of
> England ... any relation?
My great-grandfather, Henry Nicholas Greenwell, left Greenwell Ford
around 1850 for Australia, finally settling in Hawaii. Anything sound
familiar?
Doug Greenwell
>On Jan 1, 3:34=A0pm, Doug Greenwell wrote:
>> At 15:09 01 January 2011, Derek C wrote:
>>
>>
>>
>>
>>
>> >> At 20:23 31 December 2010, bildan wrote:
>>
>> >> >> I too agree with the real or perceived tow handling
>> characteristics.
>>
>am
>> >abou=3D
>> >t
>> >> as
>> >> >> far from and aerodynamicist as you can get) it should seem that
>> part
>> >> >> of the empirical data would suggest an experiment where you fly
a
>> >> >> glider equipped with and Angel of Attack meter at your typical
tow
that
>> >glider
>> >> on
>> >> >> tow at those same speeds and record the results.
>>
>> >> >Done that - and as nearly as I can see, there's no difference in
>> AoA.
>>
>> >> >I've flown some pretty heavy high performance gliders behind some
>> >> >pretty bad tow pilots - one of them stalled the tug with me on
tow.
>> >> >If I'm careful not to over-control the ailerons, there's no
problem
>> at
>> >> >all.
>>
>> >> >Heavily ballasted gliders respond sluggishly in roll just due to
the
>> behind
>> >> >a tug needs and expects crisp aileron response. =3DA0When he
doesn't
>> get
>> >> >it, he increases the amount and frequency of aileron with a
>equally
>> >> >crisp with rudder to oppose the adverse yaw, it gets wobbly.
>>
>> >> Where did you mount the AoA meter?
>>
>> >> It's not the angle of attack that's the problem, but the change
in
>> local
=
>a
>> 10deg climb angle at 60 kts corresponds to an impressive climb rate of
x
>> Weight. =A0You don't need to increase lift to climb - you increase
>thrust
>> to overcome the aft component of the weight, and the stick comes back
to
>> maintain speed ... at constant speed the increased power input comes
out
>>
>> I think a lot of people confuse the actions needed to initiate a climb
>>
>> On your second point, if you are on tow anywhere sensible behind a tug
>u
>> are in its wake and are being affected by the wing downwash. =A0Wake
is
>n=
>ot
>> really a good word, since it seems to get confused with the much more
>> localised (and turbulent) propwash.
>>
>> A (very) crude way of visualising the affected wake area is to imagine
a
>> cylinder with a diameter equal to the tug wing span extending back
from
>> the tug - that's the downwash region, and then in addition there's
an
>> upwash region extending perhaps another half-span out either side.-
Hide
>quoted text -
>>
>> - Show quoted text -
>
>So why did a K13 feel on the verge of a stall at 50 knots on tow? All
>the classic symptoms of a stall were there, including mushy controls,
>wallowing around and buffeting. If you got even slightly low it seemed
>quite difficult to get back up to the normal position. Lack of
>elevator effectiveness is yet another sympton of the stall!
>
>Fortunately we have given up aerotowing with the Falke. It just seemed
>like a good idea at the time because its flying speeds are more
>closely matched to a glider; in theory anyway.
>
>Derek C
>
good question - which suggests that something more complicated was going
on?
Lack of elevator effectiveness is not really a symptom of stall as such
.. it's a symptom of low airspeed. So for buffeting and mushy,
ineffective elevator to be happening at an indicated airspeed of 50-55
knots I'm wondering whether the tailplane was stalling rather than the
wing?
In this case you'd a tug with a wing span of a similar size to the glider
(14.5m to 16m), which would put the tug and glider tip vortices very close
together. Two adjacent vortices of the same sign tend to wind up round
each other and merge quite quickly - if this happened with the two sets of
tip vortices it would generate an increased downwash near the tail and push
the local (negative) incidence past the stall angle.
I'd be the first to admit this is getting rather speculative - but these
possible interaction effects would be amenable to some fairly
straightforward wind tunnel testing ... a good student project for next
year!
Doug Greenwell
I had to look it up - I was born 15miles away in Sunderland and had never
heard of Greenwell Ford. Doesn't ring a bell, but I'll have a look in
the family history files my wife has put together. It's certainly in the
right part of the NE, not far from Easington and the Durham coalfields
Doug
Gary Osoba
Span differential, the nature of wake roll-ups, and effects in the
larger free stream.
An airfoil moving through a viscous media makes quite a disturbance.
Among other things, it results in relative upwash upstream in the flow
field, downwash aft in the flow field, and effects which are
vertically displaced in the flow field as well.
However, lateral influence in the flow field- outside the wake rollup
at the tips- is of special interest here. Wake rollups with vorticity
do not spread the energy spent in achieving pressure equilibrium very
efficiently. That is why displacing the event over a larger area such
as laterally (as in more span) or vertically (as in the case of
winglets) makes the wing more efficient. Since the wing doesn’t do a
very good job of inducing lift beyond the tip on the other side of the
wake rollup, the downwash immediately aft of the wing is significantly
greater than the downwash aft of the wing and a meter or two outboard
of the tips.
This lateral downwash differential is preserved in the aft flow field,
albeit to lesser degrees with increasing distance until the free
stream reaches unity. However, when being towed slow and heavy it
doesn't take much to create a noticeable effect. In the case of a tow
plane with 10-11 meter span towing a glider of 15 meter span, the
downwash aft of the towplane and inboard on the glider span is greater
than the free stream field meeting the tips and the ailerons. The
effect is that a glider under tow can behave more like a design with
wings geometrically twisted in the wrong direction- with ailerons
operating near the stall. The effect increases with increasing
downwash required of the towplane.
One way to check this effect would be tow behind a motorglider of
greater span than your glider. This should provide for a better match
of downwash angles across your span.Get all the climb you can for a
given airspeed. Time your roll rates. Then tow behind a conventional
towplane at the same speed and same climb rate as the first case. Time
and compare roll rates.
You can also check numerically by calculating the rolling moments and
taking into account the assymetrical lift distributions using the
methods of Multhopp and Redeker. However, arriving at the effective
angles of attack across the span, in a modified and vertically
displaced flow field 200 feet aft of the tow plane might be rather
difficult. Several angle of attack probes positioned in front of your
wing and distributed along the span would likely be the better
approach.
If a towplane could push rather than pull a glider, the effect would
be reversed and the aileron authority would increase.
Best Regards,
Gary Osoba
Free Flight 107
> At 06:24 01 January 2011, Anne wrote:
>
>
> I've certainly sparked some interest here - considering it's New Year
>
And I mignt add this is a very fast moving discussion too! While I was
loging in 2 messages were posted..
Concerning the Tow Plane position while on tow, two of my CFIs have
said to position yourglider as if you were going to Machine Gun the
pilot of the Tow Plane. this is equivelent of aligning the horizontal
of the TP with a portion of his foweward fuslage, like the wheels on a
Pawnee.
Works great in all conditions I've come accross in 15 years flying 8
different types from 2-33 to Duo Discuss. Never been criticized for it
either in BFRs.
Wayne
Doug Greenwell
>
>
>Span differential, the nature of wake roll-ups, and effects in the
>larger free stream.
>
>An airfoil moving through a viscous media makes quite a disturbance.
>Among other things, it results in relative upwash upstream in the flow
>field, downwash aft in the flow field, and effects which are
>vertically displaced in the flow field as well.
>
>However, lateral influence in the flow field- outside the wake rollup
>at the tips- is of special interest here. Wake rollups with vorticity
>do not spread the energy spent in achieving pressure equilibrium very
>efficiently. That is why displacing the event over a larger area such
>as laterally (as in more span) or vertically (as in the case of
Absolutely - I used a simple vortex lattice method (AVL) to come to the
same conclusion. For the relatively short spacing between glider and
towplane the lack of a wake roll-up model in this code probbaly doesn't
affect the adverse change in spanwise lift distribution on the glider wing
- however, modelling any more complex interactions between tug and glider
vortices would need a proper CFD study. The other interesting element is
the effect of bank - with 15m+ wing spans it doesn't take much of a roll
angle to put one tip right into the tug wake, leading to yet more
asymmetric effects.
Most experimental wake vortex interaction studies (eg recent Airbus A380
studies) have used models in big ship tow tanks to get long vortices, but
because glider and tug are so close, we could probably use a large wind
tunnel. Another possibility would be to fly a motor glider behind a tug
aircraft, in order to take out any effect of rope angle.
It would probably be difficult to get someone to fund it though, since the
solution is simple - tow faster :-)
basic conclusions towthis doesm
Peter Smith
Winter has certainly arrived in the northern hemisphere.Shall we next
discuss how many angels can dance on the head of a pin?
twocool...@juno.com
>
> - Show quoted text -
"aft component of weight??"
Not that this adds anything to the discussion, but.....weight acts in
a "downward" direction toward the center of the earth.
In a climb, on tow, the "aft" forces are drag (mostly) and a small bit
of lift.
Anyway, interesting topic.......has been beat to death at our local
field...EVERY pilot seems to have had it happen, in all different
kinds of gliders......many explainations....not one all-encompassing
explaination yet.
Cookie
twocool...@juno.com
>
> - Show quoted text -
Just looking at the vectors..........lift + drag + weight + thrust(tow
rope)... must = zero
Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) then lift has to be GREATER than what you might at first
think. A lot more than if the thrust(tow rope) was pulling along in
the direction of flight. So...the angle of attack has to be higher at
a given speed on tow than it would be in free flight at the same
speed.....
plus all that other stuff already mentioned..........
Cookie
Tom Claffey
compared to free glide so requires more speed on tow. Heavy standard class
probably the worst needing 70-75kts on tow but thermalling happily at
60kts.
Re low tow, we use it in Australia, it feels more stable [to me] and we
release in low tow with no problems.
Doug Greenwell
>On Jan 1, 10:34=A0am, Doug Greenwell wrote:
>> At 15:09 01 January 2011, Derek C wrote:
>>
>>
>>
>>
>>
>> >> At 20:23 31 December 2010, bildan wrote:
>>
>> >> >> I too agree with the real or perceived tow handling
>> characteristics.
>>
>am
>> >abou=3D
>> >t
>> >> as
>> >> >> far from and aerodynamicist as you can get) it should seem that
>> part
>> >> >> of the empirical data would suggest an experiment where you fly
a
>> >> >> glider equipped with and Angel of Attack meter at your typical
tow
that
>> >glider
>> >> on
>> >> >> tow at those same speeds and record the results.
>>
>> >> >Done that - and as nearly as I can see, there's no difference in
>> AoA.
>>
>> >> >I've flown some pretty heavy high performance gliders behind some
>> >> >pretty bad tow pilots - one of them stalled the tug with me on
tow.
>> >> >If I'm careful not to over-control the ailerons, there's no
problem
>> at
>> >> >all.
>>
>> >> >Heavily ballasted gliders respond sluggishly in roll just due to
the
>> behind
>> >> >a tug needs and expects crisp aileron response. =3DA0When he
doesn't
>> get
>> >> >it, he increases the amount and frequency of aileron with a
>equally
>> >> >crisp with rudder to oppose the adverse yaw, it gets wobbly.
>>
>> >> Where did you mount the AoA meter?
>>
>> >> It's not the angle of attack that's the problem, but the change
in
>> local
=
>a
>> 10deg climb angle at 60 kts corresponds to an impressive climb rate of
x
>thrust
>> to overcome the aft component of the weight, and the stick comes back
to
>> maintain speed ... at constant speed the increased power input comes
out
>>
>> I think a lot of people confuse the actions needed to initiate a climb
>>
>> On your second point, if you are on tow anywhere sensible behind a tug
>u
>> are in its wake and are being affected by the wing downwash. =A0Wake
is
>n=
>ot
>> really a good word, since it seems to get confused with the much more
>> localised (and turbulent) propwash.
>>
>> A (very) crude way of visualising the affected wake area is to imagine
a
>> cylinder with a diameter equal to the tug wing span extending back
from
>> the tug - that's the downwash region, and then in addition there's
an
>> upwash region extending perhaps another half-span out either side.-
Hide
>quoted text -
>>
>> - Show quoted text -
>
>"aft component of weight??"
>
>Not that this adds anything to the discussion, but.....weight acts in
>a "downward" direction toward the center of the earth.
>
>In a climb, on tow, the "aft" forces are drag (mostly) and a small bit
>of lift.
>
>Anyway, interesting topic.......has been beat to death at our local
>field...EVERY pilot seems to have had it happen, in all different
>kinds of gliders......many explainations....not one all-encompassing
>explaination yet.
>
>Cookie
>
>
the direction of motion (relative to the air), which is inclined upwards -
so if you take 'aft' as relative to the glider flight path rather than
the earth, then there is an aft component of weight.
Doug Greenwell
>On Jan 1, 10:11=A0pm, "twocoolglid...@juno.com"
> wrote:
>> On Jan 1, 10:34=A0am, Doug Greenwell wrote:
>>
>>
>>
>>
>>
>> > At 15:09 01 January 2011, Derek C wrote:
>>
>> > >> At 20:23 31 December 2010, bildan wrote:
>>
>> > >> >> I too agree with the real or perceived tow handling
>> > characteristics.
>>
I
>=
>am
>> > >abou=3D
>> > >t
>> > >> as
>> > >> >> far from and aerodynamicist as you can get) it should seem
that
>> > part
>> > >> >> of the empirical data would suggest an experiment where you
fly
>a
>> > >> >> glider equipped with and Angel of Attack meter at your typical
>w
>> > >> >> speeds and record the AoA at various speeds. =3D3DA0Then fly
>that
>> > >glider
>> > >> on
>> > >> >> tow at those same speeds and record the results.
>>
>> > >> >Done that - and as nearly as I can see, there's no difference
in
>> > AoA.
>>
>> > >> >I've flown some pretty heavy high performance gliders behind
some
>> > >> >pretty bad tow pilots - one of them stalled the tug with me on
>tow.
>> > >> >If I'm careful not to over-control the ailerons, there's no
>problem
>> > at
>> > >> >all.
>>
>> > >> >Heavily ballasted gliders respond sluggishly in roll just due to
>e
>> > >> >extra roll inertia. =3DA0A pilot trying to hold a precise
position
>> > behind
>> > >> >a tug needs and expects crisp aileron response. =3DA0When he
>doesn'=
>t
>> > get
>> > >> >it, he increases the amount and frequency of aileron with a
>ly
>> > >> >crisp with rudder to oppose the adverse yaw, it gets wobbly.
>>
>> > >> Where did you mount the AoA meter?
>>
>> > >> It's not the angle of attack that's the problem, but the change
in
>> > local
>ve=
>ry
>> > >much
>> > >> when near to the tug wake, but its distribution along the wing
>does,
>> > >with
>> > >> increased lift at the tips and reduced lift at the root - putting
>e
>> > >> aileron region close to the stall and hence reducing control
>> > >> effectiveness.
>>
>> > >> I agree that increased roll inertia due to ballast is a factor,
but
>> > >since
>> > >> the same factor applies to maintaining bank angle in a
thermalling
>> > turn
>> > >I
>> > >> don't see how it can account for a significant difference in
>g
>> > >> between tow and thermalling?- Hide quoted text -
>>
>> > >> - Show quoted text -
>>
>> > >What started the debate at Lasham was using a Rotax engined Falke
as
>a
>> > >glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
>> > >with a normal tug), but K13s with a stalling speed of 36 knots felt
>> > >very unhappy behind it, especially two up. In a conventional
powered
>> > >aircraft you pull the nose up (to increase the angle of attack and
>> > >produce more lift) and increase power to climb, the extra power
being
>> > >used to prevent the aircraft from slowing down. I don't see why
>> > >gliders should behave any differently, except that the power is
>coming
>> > >from an external source. As you try not to tow in the wake and
>> > >downwash from the tug, I can't see that this is particularly
>> > >significant,
>>
>> > >Derek C
>>
>> > In a steady climb in any light aircraft the climb angles are so low
(
exampl=
>e a
>> > 10deg climb angle at 60 kts corresponds to an impressive climb rate
of
1.02
>=
>x
>> > Weight. =A0You don't need to increase lift to climb - you increase
>thru=
>st
>> > to overcome the aft component of the weight, and the stick comes
back
>o
>> > maintain speed ... at constant speed the increased power input comes
>t
>> > as increasing potential energy =3D increasing height.
>>
>> > I think a lot of people confuse the actions needed to initiate a
climb
>>
>> > On your second point, if you are on tow anywhere sensible behind a
tug
>you
>> > are in its wake and are being affected by the wing downwash. =A0Wake
>is=
> not
>> > really a good word, since it seems to get confused with the much
more
>> > localised (and turbulent) propwash.
>>
>> > A (very) crude way of visualising the affected wake area is to
imagine
>a
>> > cylinder with a diameter equal to the tug wing span extending back
>from
>> > the tug - that's the downwash region, and then in addition there's
an
>> > upwash region extending perhaps another half-span out either side.-
>e quoted text -
>>
>> > - Show quoted text -
>>
>> "aft component of weight??"
>>
>> Not that this adds anything to the discussion, but.....weight acts in
>> a "downward" direction toward the center of the earth.
>>
>> In a climb, on tow, the "aft" forces are drag (mostly) and a small
bit
>> of lift.
>>
>> Anyway, interesting topic.......has been beat to death at our local
>> field...EVERY pilot seems to have had it happen, in all different
>> kinds of gliders......many explainations....not one all-encompassing
>> explaination yet.
>>
>> Cookie- Hide quoted text -
>>
>> - Show quoted text -
>
>Just looking at the vectors..........lift + drag + weight + thrust(tow
>Then.....if the tow rope provides a forward and Downward pull........
>(which was pretty much proven in an earlier discussion, by virtue of
>the 'sag" in the rope, the angle at which the rope meets the
>glider) then lift has to be GREATER than what you might at first
>think. A lot more than if the thrust(tow rope) was pulling along in
>the direction of flight. So...the angle of attack has to be higher at
>a given speed on tow than it would be in free flight at the same
>speed.....
>
>plus all that other stuff already mentioned..........
>
>
>Cookie
>
>
There does seem to be a consensus that the effects on lateral control get
worse as you get closer (lower) to the wake/propwash, so I think there's
got to be more to it than just geometry.
Derek C
Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose. The elevator should still
be effective at 50 knots, so it's more likely that the wing is close
to the stall. The stall is only strictly related to the angle of
attack. During a aerotow climb the wing has to support an additional
weight component as well as drag, so the effective wing loading may
well be increased, requiring a greater angle of attack for a given
airspeed. Going 10 knots faster seems to cure the problem.
Derek C
twocool...@juno.com
>
> - Show quoted text -
Yes, this is true......but to me it is better to keep the vectors
simple. If you apply a component along the line of the fuselage (aft
vector) then you have to add in the other component too. What
direction? Remember aft is parallel to the glider, not the flight
path of the glider.
We could in fact break any vector up into any number of
components......but eventually you have to combine them again.
To me, using the Earth (as horizontal and vertical reference) is
best. Then we can easily see the climb angle of the glider, the
direction of flight if you will, and the speed. We can also easily
see the angle of attack. With this reference we need to apply only 4
vectors (forces) lift, drag, weight, thrust. IF we use the glider
itself, longitudinal axis as reference, we right away have 8 vectors
to contend with. On tow, if we know any three forces, we can
calculate the forth. In gliding flight its only three forces (thrust
= 0) so its even easier.
Ultimately, if in "steady flight" there is in fact no force acting on
the glider......because the sum of all of the components = 0.
I like your explaination of climbing aircraft above. Another way to
look at it: (speed kept constant) IF thrust is greater than drag, the
aircraft will climb, IF thrust = drag, the aircraft will fly level
with the Earth. IF thrust is less than drag, the aircraft will
descend. If thrust = 0 the aircraft will descent at its L/D angle.
(assuming thrust is applied along the direction of flight)
Oh yeah...yet another factor from the earlier version of this
discussion. The force of the tow rope(thrust) does not necessarily
act through the glider's center of gravity. Neither does the drag
vector. This can cause a pitching moment, which will require elevator
input to counteract. Another factor that can give a different "feel"
on tow.
Cookie
twocool...@juno.com
>
> - Show quoted text -
'Actually the only totally reliable sysmptom of being stalled is that
> the elevator will no longer raise the nose.'
HUH? Many cases possible where we could have full elevator and not
be stalled. (I demonstrate this is 2-33 and grob 103 and ask-21.
All you need is heavy pilot (forward CG) and gentle stick back to the
stop. Glider will mush, but not stall. Elevator will not raise the
nose........wing does not have angle to stall.
On tow the only additional "weight component" would be a downward
component to the tow rope (thrust). Since the tension on the tow rope
is fairly low........it should not have a big effect, but there is
some effect.
But yeah, that extra 10 knots makes all the difference in the world.
(I remember occasionally getting a "slow tow" when flying a 2-32 with
three aboard..........what a handful!!!
Cookie
twocool...@juno.com
>
> - Show quoted text -
Lift is perpendicular.......drag is parallel........
n7ly
> 'Actually the only totally reliable sysmptom of being stalled is that
> > the elevator will no longer raise the nose.'
>
> HUH? Many cases possible where we could have full elevator and not
> be stalled. (I demonstrate this is 2-33 and grob 103 and ask-21.
> All you need is heavy pilot (forward CG) and gentle stick back to the
> stop. Glider will mush, but not stall. Elevator will not raise the
> nose........wing does not have angle to stall.
>
whoa - depends on who's defining "stall". The FAA definition is indeed
that when the aircraft does not respond in the direction of the
control input that it's done. When you can no longer move the elevator
up, you're done. Nose doesn't respond in direction of aft stick
deflection, you're stalled. I don't remember exactly the way they
word it, but the result is that touch the elevator limit, that's it.
Slow entry rates result in higher stall speeds. Forward cg's give
higher stall speeds. Trim settings (on some configs) affect stall
speeds. Weight, etc., etc. The scene that seems the most insidious
is the slow entry rate. They sneak up on you, kind of like a slow tow.
Andy
wrote:
> Then.....if the tow rope provides a forward and Downward pull........
> (which was pretty much proven in an earlier discussion, by virtue of
> the 'sag" in the rope, the angle at which the rope meets the
> glider) then lift has to be GREATER than what you might at first
> think.
I was not part of that earlier discussion and I certainly don't accept
that conclusion.
All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.
When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. Maybe it all depends on what
you call high tow. I've seen may pilots tow tens of feet higher than
I regard as normal high tow.
Andy
Mike the Strike
Martin Gregorie
> All I have read here is that the D2, because of its very low angle of
> incidence, may have a downward pull on the nose (and even here downward
> would mean below the glider longitudinal axis, not necessarily below the
> horizon). I'm quite sure that my ASW 28 being towed on the CG hook has
> no downward force on the nose.
>
Hmmm, My Libelle glides at around 55 kts with the trim full forward so
should need its nose held down a bit when being towed at 60-65 kts on the
nose hook. Its possible that I am holding the nose down - all I can say
is that I'm not aware of doing so once I'm off the ground, stabilised
behind the tug and waiting for it to unstick, gain speed and start to
climb.
There is a noticeable catenary in the tow rope and, since that is a thin,
flexible rope the pull on the nose hook will be at the same angle as the
rope leaves the nose and not on the direct line between my nose-hook and
the rope attachment point on the tug. This probably puts the force line
above the glider CG and so is contributing a nose down moment.
FWIW I estimate that climbing at 600 fpm at 60 kts is a 5.67 degree climb
and that the tow rope tension is 37.62 kg for my glider (10 kg is drag
due to the glider and the rest is due to the glider hanging from the
rope).
However, I don't know rope weight or exact length or how to calculate the
sag in the rope and hence can't estimate the distance of the force line
above or below the CG.
--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |
kirk.stant
Without a gunsight, how do you do that? ;^)
I don't understand why the high tow position is taught by reference to
the towplane or horizon, when what should be taught is how to find the
correct tow position (just above or below the wake, which is actually
the propwash). Simple - once safely airborne (usually before the
towplane), just ease down until you feel the towplanes turbulence,
then ease up a bit. THEN look at the towplane and pick whatever
convenient references you need to maintain this vertical alignment.
Any significant change in towplane speed will require a readjustment
of the tow position (normally only a factor if on an aerotow
retrieve).
Obviously, if you only tow behind the same towplane on every flight,
you will quickly learn where to position your glider. But if you have
a variety of towplanes, or are towing behind something different
(Agcat, Wilga, AN-2, whatever) for the first time, you can use this
process to find the correct position quickly.
Many US instructors seem to only teach HOW to do something without
going into WHY it is done. As a result, there are a lot of "shortcuts"
being taught, and a lot of poorly trained pilots, IMHO. A result of
not having a standardized curriculum, a la BGA, perhaps?
Kirk
66
Doug Greenwell
>
>Yes, this is true......but to me it is better to keep the vectors
>simple. If you apply a component along the line of the fuselage (aft
>vector) then you have to add in the other component too. What
>direction? Remember aft is parallel to the glider, not the flight
>path of the glider.
>
>We could in fact break any vector up into any number of
>components......but eventually you have to combine them again.
>
>To me, using the Earth (as horizontal and vertical reference) is
>best. Then we can easily see the climb angle of the glider, the
>direction of flight if you will, and the speed. We can also easily
>see the angle of attack. With this reference we need to apply only 4
>vectors (forces) lift, drag, weight, thrust. IF we use the glider
>itself, longitudinal axis as reference, we right away have 8 vectors
>to contend with. On tow, if we know any three forces, we can
>calculate the forth. In gliding flight its only three forces (thrust
>
>Ultimately, if in "steady flight" there is in fact no force acting on
>
>I like your explaination of climbing aircraft above. Another way to
>look at it: (speed kept constant) IF thrust is greater than drag, the
>with the Earth. IF thrust is less than drag, the aircraft will
>(assuming thrust is applied along the direction of flight)
>
>Oh yeah...yet another factor from the earlier version of this
>discussion. The force of the tow rope(thrust) does not necessarily
>act through the glider's center of gravity. Neither does the drag
>vector. This can cause a pitching moment, which will require elevator
>input to counteract. Another factor that can give a different "feel"
>on tow.
>
>
>Cookie
>
There are a multiplicity of possible axes systems that are used in flight
dynamics - which one you use (and what you call it) depends on where you
are (US, UK, rest of world), and which one makes the sums simpler!
The equations of motion for climb and descent in free-flight are exactly
the same - just some terms disappear, or change sign. On tow is another
matter, since the tow rope angle introduces an additional force, and a
constraint on the motion.
DLR, the German aero reseach instititute, did do some work in 1999 on the
longitudinal dynamics of aerotowing, looking at the effect of downwash,
rope forces and hook position on pitch stability ... at least I think they
did - I have the report, but need to get it translated! Doesn't look to
contain anything concrete on lateral stability though.
Doug Greenwell
>On Jan 2, 6:01=A0am, Derek C wrote:
>> On Jan 1, 5:27=A0pm, Doug Greenwell wrote:
>>
>>
>>
>>
>>
>> > At 16:43 01 January 2011, Derek C wrote:
>>
>> > >> At 15:09 01 January 2011, Derek C wrote:
>>
>> > >> >> At 20:23 31 December 2010, bildan wrote:
>>
>> > >> >> >> I too agree with the real or perceived tow handling
>> > >> characteristics.
>>
>(a=
>nd I
>> > >am
>> > >> >abou=3D3D
>> > >> >t
>> > >> >> as
>> > >> >> >> far from and aerodynamicist as you can get) it should seem
>t
>> > >> part
>> > >> >> >> of the empirical data would suggest an experiment where you
>y
>> > a
>> > >> >> >> glider equipped with and Angel of Attack meter at your
>typical
>> > tow
>fly
>> > that
>> > >> >glider
>> > >> >> on
>> > >> >> >> tow at those same speeds and record the results.
>>
>> > >> >> >Done that - and as nearly as I can see, there's no
difference
>in
>> > >> AoA.
>>
>> > >> >> >I've flown some pretty heavy high performance gliders behind
>e
>> > >> >> >pretty bad tow pilots - one of them stalled the tug with me
on
>> > tow.
>> > >> >> >If I'm careful not to over-control the ailerons, there's no
>> > problem
>> > >> at
>> > >> >> >all.
>>
>> > >> >> >Heavily ballasted gliders respond sluggishly in roll just due
>to
>> > the
>posi=
>tion
>> > >> behind
>> > >> >> >a tug needs and expects crisp aileron response. =3D3DA0When
he
>> > doesn't
>> > >> get
>> > >> >> >it, he increases the amount and frequency of aileron with a
than
>> > >equally
>> > >> >> >crisp with rudder to oppose the adverse yaw, it gets wobbly.
>>
>> > >> >> Where did you mount the AoA meter?
>>
>> > >> >> It's not the angle of attack that's the problem, but the
change
>> > in
>> > >> local
change
>=
>by
>> > >very
>> > >> >much
>> > >> >> when near to the tug wake, but its distribution along the wing
>> > does,
>> > >> >with
>> > >> >> increased lift at the tips and reduced lift at the root -
>putting
>> > the
>> > >> >> aileron region close to the stall and hence reducing control
>> > >> >> effectiveness.
>>
>> > >> >> I agree that increased roll inertia due to ballast is a
factor,
>ut
>> > >> >since
>> > >> >> the same factor applies to maintaining bank angle in a
>g
>> > >> turn
>> > >> >I
>> > >> >> don't see how it can account for a significant difference in
>> > handling
>> > >> >> between tow and thermalling?- Hide quoted text -
>>
>> > >> >> - Show quoted text -
>>
>> > >> >What started the debate at Lasham was using a Rotax engined
Falke
>s
>> > a
>> > >> >glider tug. This towed best at about 50 to 55 knots (c.f. 60+
>knots
>> > >> >with a normal tug), but K13s with a stalling speed of 36 knots
>felt
>> > >> >very unhappy behind it, especially two up. In a conventional
>d
>> > >> >aircraft you pull the nose up (to increase the angle of attack
and
>> > >> >produce more lift) and increase power to climb, the extra power
>ng
>> > >> >used to prevent the aircraft from slowing down. I don't see why
>> > >> >gliders should behave any differently, except that the power is
>> > coming
>> > >> >from an external source. As you try not to tow in the wake and
>> > >> >downwash from the tug, I can't see that this is particularly
>> > >> >significant,
>>
>> > >> >Derek C
>>
>> > >> In a steady climb in any light aircraft the climb angles are so
low
>(
>> > >10deg) that the lift remains pretty well equal to weight. =3DA0For
>exa=
>mple
>> > =3D
>> > >a
>> > >> 10deg climb angle at 60 kts corresponds to an impressive climb
rate
>of
>> > >> 10.5kts - but that would only give Lift =3D3D Weight/cos(10deg)
>=3D3=
>D 1.02
>> > x
>> > >> Weight. =3DA0You don't need to increase lift to climb - you
>increase
>> > >thrust
>> > >> to overcome the aft component of the weight, and the stick comes
>k
>> > to
>> > >> maintain speed ... at constant speed the increased power input
>comes
>> > out
>>
>> > >> I think a lot of people confuse the actions needed to initiate a
>mb
>> > >> with what is actually happening in a steady climb. =3DA0
>>
>> > >> On your second point, if you are on tow anywhere sensible behind
a
>ug
>> > >yo=3D
>> > >u
>> > >> are in its wake and are being affected by the wing downwash.
>ke
>> > is
>> > >n=3D
>> > >ot
>> > >> really a good word, since it seems to get confused with the much
>e
>> > >> localised (and turbulent) propwash.
>>
>> > >> A (very) crude way of visualising the affected wake area is to
>ne
>> > a
>> > >> cylinder with a diameter equal to the tug wing span extending
back
>> > from
>> > >> the tug - that's the downwash region, and then in addition
there's
>> > an
>> > >> upwash region extending perhaps another half-span out either
side.-
>> > Hide
>> > >quoted text -
>>
>> > >> - Show quoted text -
>>
>> > >So why did a K13 feel on the verge of a stall at 50 knots on tow?
All
>> > >the classic symptoms of a stall were there, including mushy
controls,
>> > >wallowing around and buffeting. If you got even slightly low it
>seemed
>> > >quite difficult to get back up to the normal position. Lack of
>> > >elevator effectiveness is yet another sympton of the stall!
>>
>> > >Fortunately we have given up aerotowing with the Falke. It just
>seemed
>> > >like a good idea at the time because its flying speeds are more
>> > >closely matched to a glider; in theory anyway.
>>
>> > >Derek C
>>
>> > good question - which suggests that something more complicated was
>g
>> > on? =A0
>>
>> > Lack of elevator effectiveness is not really a symptom of stall as
>such
>> > ineffective elevator to be happening at an indicated airspeed of
50-55
>> > knots I'm wondering whether the tailplane was stalling rather than
the
>> > wing?
>>
>> > In this case you'd a tug with a wing span of a similar size to the
>er
>> > (14.5m to 16m), which would put the tug and glider tip vortices very
>ose
>> > together. =A0Two adjacent vortices of the same sign tend to wind up
>rou=
>nd
>> > each other and merge quite quickly - if this happened with the two
> of
>> > tip vortices it would generate an increased downwash near the tail
and
>push
>> > the local (negative) incidence past the stall angle.
>>
>> > I'd be the first to admit this is getting rather speculative - but
>e
>> > possible interaction effects would be amenable to some fairly
for
>n=
Depends on your definition of 'stall' - whether the nose drops or not
depends so much on the aircraft configuration, the aerofoil section and
the stall entry technique.
NASA did a 'deep-stall' test program in the early 80s with a modified
1-36 which was able to carry on pitching up to 70deg AoA ... look at the
incidence on this photo!
http://www.dfrc.nasa.gov/gallery/photo/Schweizer-1-36/Medium/ECN-26845.jpg
Doug Greenwell
>On Jan 2, 2:49=A0am, Doug Greenwell wrote:
>> At 03:11 02 January 2011, twocoolglid...@juno.com wrote:
>>
>>
>>
>> >> At 15:09 01 January 2011, Derek C wrote:
>>
>> >> >> At 20:23 31 December 2010, bildan wrote:
>>
>> >> >> >> I too agree with the real or perceived tow handling
>> >> characteristics.
>>
>(and=
> I
>> >am
>> >> >abou=3D3D
>> >> >t
>> >> >> as
>> >> >> >> far from and aerodynamicist as you can get) it should seem
that
>> >> part
>> >> >> >> of the empirical data would suggest an experiment where you
fly
>> a
>> >> >> >> glider equipped with and Angel of Attack meter at your
typical
>> tow
fly
>> that
>> >> >glider
>> >> >> on
>> >> >> >> tow at those same speeds and record the results.
>>
>> >> >> >Done that - and as nearly as I can see, there's no difference
in
>> >> AoA.
>>
>> >> >> >I've flown some pretty heavy high performance gliders behind
some
>> >> >> >pretty bad tow pilots - one of them stalled the tug with me on
>> tow.
>> >> >> >If I'm careful not to over-control the ailerons, there's no
>> problem
>> >> at
>> >> >> >all.
>>
>> >> >> >Heavily ballasted gliders respond sluggishly in roll just due
to
>> the
>positi=
>on
>> >> behind
>> >> >> >a tug needs and expects crisp aileron response. =3D3DA0When he
>> doesn't
>> >> get
>> >> >> >it, he increases the amount and frequency of aileron with a
than
>> >equally
>> >> >> >crisp with rudder to oppose the adverse yaw, it gets wobbly.
>>
>> >> >> Where did you mount the AoA meter?
>>
>> >> >> It's not the angle of attack that's the problem, but the
change
>> in
>> >> local
>examp=
>le
>> =3D
>> >a
>> >> 10deg climb angle at 60 kts corresponds to an impressive climb rate
>of
=3D3D
>=
>1.02
>> x
>> >> Weight. =3DA0You don't need to increase lift to climb - you
increase
>> >thrust
>> >> to overcome the aft component of the weight, and the stick comes
back
>> to
>> >> maintain speed ... at constant speed the increased power input
comes
>> out
>>
>> >> I think a lot of people confuse the actions needed to initiate a
>climb
>>
>> >> On your second point, if you are on tow anywhere sensible behind a
>tug
>> >u
>> >> are in its wake and are being affected by the wing downwash.
>> is
>> >n=3D
>> >ot
>> >> really a good word, since it seems to get confused with the much
more
>> >> localised (and turbulent) propwash.
>>
>> >> A (very) crude way of visualising the affected wake area is to
>imagine
>> a
>> >> cylinder with a diameter equal to the tug wing span extending back
>> from
>> >> the tug - that's the downwash region, and then in addition
there's
>> an
>> >> upwash region extending perhaps another half-span out either side.-
>> Hide
>> >quoted text -
>>
>> >> - Show quoted text -
>>
>> >"aft component of weight??"
>>
>> >Not that this adds anything to the discussion, but.....weight acts in
>> >a "downward" direction toward the center of the earth.
>>
>> >In a climb, on tow, the "aft" forces are drag (mostly) and a small
bit
>> >of lift.
>>
>> >Anyway, interesting topic.......has been beat to death at our local
>> >field...EVERY pilot seems to have had it happen, in all different
>> >kinds of gliders......many explainations....not one all-encompassing
>> >explaination yet.
>>
>> >Cookie
>>
>> it depends on your reference frame - lift and drag are perpendicular
to
>> the direction of motion (relative to the air), which is inclined
upwards
>-
>> so if you take 'aft' as relative to the glider flight path rather
than
>> the earth, then there is an aft component of weight.- Hide quoted text
-
>>
>> - Show quoted text -
>
>Lift is perpendicular.......drag is parallel........
>
.. oh alright - I left a few words out :-)
Derek C
wrote:
>
> 'Actually the only totally reliable sysmptom of being stalled is that
>
> > the elevator will no longer raise the nose.'
>
> HUH? Many cases possible where we could have full elevator and not
> be stalled. (I demonstrate this is 2-33 and grob 103 and ask-21.
> All you need is heavy pilot (forward CG) and gentle stick back to the
> stop. Glider will mush, but not stall. Elevator will not raise the
> nose........wing does not have angle to stall.
>
> On tow the only additional "weight component" would be a downward
> component to the tow rope (thrust). Since the tension on the tow rope
> is fairly low........it should not have a big effect, but there is
> some effect.
>
> But yeah, that extra 10 knots makes all the difference in the world.
> (I remember occasionally getting a "slow tow" when flying a 2-32 with
> three aboard..........what a handful!!!
> Cookie
I know that you have some 'persons of size' out in the States, but I
have never flown a glider that could not stalled in straight flight
with a legal weight (less than 110kg, 242lbs) person in the front
seat. This includes the K21 and the G103. Sometimes you run out of
trim when circling in a thermal with such folk. I weigh about 190lbs
by the way.
Derek C
Doug Greenwell
>..
>> =A0'Actually the only totally reliable sysmptom of being stalled is
that
>> > the elevator will no longer raise the nose.'
>>
>> be stalled. =A0(I demonstrate this is 2-33 and grob 103 and ask-21.
>> All you need is heavy pilot (forward CG) and gentle stick back to the
the
>> nose........wing does not have angle to stall.
>>
>whoa - depends on who's defining "stall". The FAA definition is
indeed
>that when the aircraft does not respond in the direction of the
>control input that it's done. When you can no longer move the elevator
>up, you're done. Nose doesn't respond in direction of aft stick
>deflection, you're stalled. I don't remember exactly the way they
>word it, but the result is that touch the elevator limit, that's it.
>Slow entry rates result in higher stall speeds. Forward cg's give
>higher stall speeds. Trim settings (on some configs) affect stall
>speeds. Weight, etc., etc. The scene that seems the most insidious
>is the slow entry rate. They sneak up on you, kind of like a slow tow.
>
Not necessarily - have the CG too far forward, and you'll run out of
elevator before you stall.
Admittedly that is still a stall according to FAR23/25 definitions
"a stall is produced, as shown by either:
(1) An uncontrollable downward
pitching motion of the airplane;
(2) A downward pitching motion of
the airplane that results from the activation
of a stall avoidance device (for
example, stick pusher); or
(3) The control reaching the stop."
Doug Greenwell
>..
that
>> > the elevator will no longer raise the nose.'
>>
>> be stalled. =A0(I demonstrate this is 2-33 and grob 103 and ask-21.
>> All you need is heavy pilot (forward CG) and gentle stick back to the
the
>> nose........wing does not have angle to stall.
>>
>whoa - depends on who's defining "stall". The FAA definition is
indeed
>that when the aircraft does not respond in the direction of the
>control input that it's done. When you can no longer move the elevator
>up, you're done. Nose doesn't respond in direction of aft stick
>deflection, you're stalled. I don't remember exactly the way they
>word it, but the result is that touch the elevator limit, that's it.
>Slow entry rates result in higher stall speeds. Forward cg's give
>higher stall speeds. Trim settings (on some configs) affect stall
>speeds. Weight, etc., etc. The scene that seems the most insidious
>is the slow entry rate. They sneak up on you, kind of like a slow tow.
>
Not necessarily - have the CG too far forward, and you'll run out of
Doug Greenwell
>On Jan 1, 12:44=A0pm, Free Flight 107 wrote:
>> On Jan 1, 3:21=A0am, Doug Greenwell wrote:> At 0=
This is what I was taught - once I'd done a few tows it seemed pretty
straightforward even for an inexperienced pilot like me.
sisu1a
> have never flown a glider that could not stalled in straight flight
Not that this adds to the main discussion, but...
160lb (normal sized) pilot here... my SZD-59 couldn't be stalled
straight ahead without 'tricking' it (ease nose up till *near stall
and yank the stick for the last bit of travel). No mods were done to
plane to cause this, and a reliable W&B with me in it determined C/G
to be at 37%, well within the limits (it's JAR22 cert'd...). This is
actually common with this ship as well as the Jantar Std3 from which
it's derived. I added 5.5lbs to the tail via a brass/lead tailwheel
and now fly it at 50%, where it flies much nicer, with an honest (and
predictable) stall and a more usable trim range. At some point I'm
going to make an externally visible, removable weight set (replacement
rudder hinge access panels milled out of brass or tungsten) to add up
to 3 more lbs, to get it to 65% for mid-late season flying but plan on
leaving it at 50% for early season.
-p
Bruce Hoult
> Actually the only totally reliable sysmptom of being stalled is that
> the elevator will no longer raise the nose.
But that is neither necessary nor sufficient!
If you put enough weight in the front cockpit then there are plenty of
gliders where you reach the back stop while they are still flying just
fine.
Conversely, there are also plenty of gliders with sufficiently
powerful elevators that the wing can be stalled and you're mushing at
500+ fpm but you still have perfect control over the attitude of the
nose and can raise or lower it at will. Not to mention other aircraft
such as the F/A-18 which can be flown in perfect control with the wing
stalled at huge angles of attack.
I see in another post the definition:
> Admittedly that is still a stall according to FAR23/25 definitions "a stall is produced, as shown by either:
> (1) An uncontrollable downward pitching motion of the airplane;
> (2) A downward pitching motion of the airplane that results from the activation of a stall avoidance device (for example, stick pusher); or
> (3) The control reaching the stop."
Without having that document in front of me I will hazard a guess that
this is not a definition of a stall, but rather a definition of the
standards for what a pilot should do in order to pass a practical
flight examination. They're not going to fail him when the aircraft
fails to actually stall because the elevator reaches its stop first,
so they explicitly allow that as a signal that the pilot is allowed to
terminate the "stall" attempt and commence the stall recovery
procedure.
The only true definition of a stall is when the wing is at an angle of
attack such that a further increase of AoA produces a decrease of
lift.
*Usually* this will be accompanied by a large increase in drag such
that the combination of lift and drag is easily capable of supporting
the aircraft against gravity at a low speed and steep nose up descent
angle, but that may not necessarily always be the case and some
aircraft might speed up while stalled (perhaps at high altitude?).
John Chapman
interesting.
However, has anyone considered descent on tow?
A SGS 2-33 glider descending on tow behind a Cessna 182 has some
interesting lateral control issues.
* Descending behind a 182 with 10 degrees of flap at 65 mph in high
tow with full spoilers is reasonably stable.
* At 40 degrees of flap the 2-33 is almost uncontrollable. You need
full stick and some patience to recover from the frequent level flight
excursions.
The 182 high lift/high drag wake is the obvious difference.
Would anyone care to venture an analysis or opinion?
Cheers, John Chapman, 1DG
twocool...@juno.com
>
> - Show quoted text -
This is one of my pet peeves.........I don't like to use the term
"high tow" because beginners seem to assume it means that the glider
is flying higher than the tow plane. I use the terms "normal tow"
and low tow. (because around here we noramlly tow above the wake.)
But I do take the time to explain to my students that "high" tow means
"high" compared to the wake(not campared to the tow plane), and you
need only to be above the wake a little, and low tow is "low" compared
to the wake, and you want ot be just barely below the wake. Also that
flying "too high" on tow is a "mortal sin", and extremely dangerous.
In rough air gliders seem more prone to "bounce up" than to "bounce
down" so if your already running toward the high side on tow, one or
two good bounces will make you too high. If you fly just above the
wake, you have room to "bounce up" once or twice, and have time to re-
position, before getting "too high" giving the tow pilot a hard time.
As far as visual reference....there are several....some pilots and
some instructors seem to prefer some or one more than others. I like
to use the tow plane compared to the horizion. Another is the
position of the tow plane in the glider's canopy...if the towplane
appears low in the canopy, like under the inst panel, the glider is
too high......if the tow plane appears way high above the inst panel,
the glider is too low. My least favorite visual is lining up stuff on
the tow plane itself, like top of fin to pilot's head, or stab to
strut or whatever.....every make and modlel of tow plane is different,
and tow pilot may change pitch attitude........The problem with the
horizon method is if in hilly or mountainous terrain, the horizon is
not always visible, students sometimes use the wrong reference
then...The best is to use a sort of combination of all the visual
references at once......not hard after the student has a few flight
under the belt.
On X-C retreive, I find that once the towplane levels out, that the
wake goes way high, and to fly above the wake makes the glider too
high.......I recommend, and usually go to low tow then...and since the
wake is way up there, the low tow is not very low at all. In fact
I've done low tow retreives that were higher than some high tows
behind a strong tow plane!
Cookie
twocool...@juno.com
>
> - Show quoted text -
I didn't say you "can't" stall these gliders. What I did say was that
you "can" get the stick back to the rear stop, without stalling, if
you are gentle, and if the GC is forward.
But then again.....a couple of new (to me) definitions of "stall" have
come up. I just attended a FAA seminar on stalls, and nobody defined
stall as "stick all the way back and nose won't come up".
I still go by the definition of stall as "when the angle of attack of
the wing reaches or exceeds the critical angle. "
Cookie
Chris Nicholas
positions, are to some extent using different definitions of high and
low tow.
When I started my UK glider training in 1970, a "high" tow position in
the glider was level with, or even higher than, the tug. It was way
above the tug wake and prop wash. It was the normal position for
towing at my gliding club, and as I understood it at the time, the
same for most UK gliding club training.
A "low" tow position meant below the tug wake and propwash. It was
normally only used for long cross-country tows, and was allegedly
easier for the glider pilot, particularly in thermic conditions. I saw
it and experienced it also when dual tows were practised. The glider
on the short rope went to the high tow position, and the glider on the
long rope into low tow.
After a series of tug upset accidents, UK practice was changed. The
normal tow position now became a lower "high" tow, not far above the
tug wake and propwash. With a tug that was climbing well, this placed
the glider below the tug. That is now the norm, in the UK, as far as I
know. Consequently, with anything other than a very low powered tug, a
glider on tow often has its longitudinal axis horizontal, or even
inclined above the horizontal.
Chris N
twocool...@juno.com
Yes...the upset accidents.......I watched from the ground as a tow
pilot got killed that way.....
Once you are above the wake, there is no reason you need to go
higher....and lots of reasons not to. But how did that "above the
towplane" stuff get started in the old days? I think that was a
misconception from the start!
I know a couple of glider ports around here were their answer is to
use low tow exclusively. While this does address the upset problem, I
feel that there are more disadvantages and dangers to low tow (for
routine tows) than for "normal" tow.
If a pilot can't keep a fairly steady tow position, above the wake and
below the tow plane, even if rough air, he needs more training.
Flying in low tow is not a substitute for pilot skill.
Cookie
twocool...@juno.com
The context of those stall definitions have to to with aircraft
certification. I think our towing discussion should stick with the
"aerodynamic" (AoA) definition.
As far a stalls on the practical test......the practical test
standards leaves a lot up to the examiner. I train my students for
what I call "baby stalls"....where you just creep up on it, and the
glider barely stalls, and the recovery is almost immediate. I also
train them for "monster stalls" where you really pull back hard, and
fast, resulting in a major nose up, followed by major nose down
attitude and a more "active" recovery needed.
The former is perhaps more realistic....in that in the real world
flying, the pilot might be more likely to stall this way. It also is
a good way to teach stall recognition and stall avoidence.
The later however shows more "plane handling", and more of the flight
envelope, almost more like an aerobatic maneuver.. Also shows no
"fear" of stalling. But unlikely to happen this way in the real
world.
I have sent students to examiners who like the stalls demonstrated in
the "baby" way, and others who like the "monster" way. I tell my
students to ask the examiner what he/she wants and perform the stall
and recovery accordingly.
During flight reviews, I find many pilots who, when I ask them to
demonstrate a stall and recovery, simply lift the nose up a bit, the
glider slows, and they push the nose back down.....never an actual
stall. To me this shows either fear, lack of understanding, or lack
of the "feel" of the glider. Thjen I demonstrate a "real stall" and
have then practice a bit.
Cookie
twocool...@juno.com
Which part don't you accept? The part about rope pulling downward, or
the part about the required lift being greater if/when it does?
In the previous discussion we all seemed to agree that the tow rope
has a consicerable sag during tow, and that the pulling force of the
rope acts in the direction of the rope meeting the tow hook, which is
not along the long. axis of the glider, and not parallel to the
direction of flight of the glider.
Now, how significant? I dunno!
With a mid-mounted wing glider and a nose hook, the forces of the tow
rope and the drag all run pretty close to the CG.....so probably
little to no pitching effect.......On a 2-33 for instance, where the
tow hook is mounted low, and the wing is high, I believe there is a
nose up pitching moment created, and in fact the 2-33 needs full
forward trim and considerable forward stick pressure on tow. Where a
mid wing nose hook glider flys nicely with about neutral trim and
little stick force if any.
But if we were to agree that the tow rope does not pull in the
dircetion of flight of the glider, and in fact pulls somewhat
"downward" compared to the direction of flight, we need to balance
this force......the only way to balance this force is for lift to
become greater, since weight, and drag remain the same. More lift
comes from more AoA.
I am not saying this is the only factor in this mushy tow deal, but I
think it contributes along with the other factors mentioned.
Cookie
Eric Greenwell
>> I know that you have some 'persons of size' out in the States, but I
>> have never flown a glider that could not stalled in straight flight
>
> Not that this adds to the main discussion, but...
>
> 160lb (normal sized) pilot here... my SZD-59 couldn't be stalled
> straight ahead without 'tricking' it (ease nose up till *near stall
> and yank the stick for the last bit of travel). No mods were done to
> plane to cause this, and a reliable W&B with me in it determined C/G
> to be at 37%, well within the limits (it's JAR22 cert'd...). This is
> actually common with this ship as well as the Jantar Std3 from which
> it's derived. I added 5.5lbs to the tail via a brass/lead tailwheel
> and now fly it at 50%, where it flies much nicer, with an honest (and
> predictable) stall and a more usable trim range.
Wow, what's the minimum cockpit load for the glider? Must be less than
140 pounds!
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
sisu1a
> Wow, what's the minimum cockpit load for the glider? Must be less than
> 140 pounds!
close...
Min -143lb
Max -256lb
-Paul
Frank Whiteley
wrote:
Where was that?
Answer me off group, okay?
Frank Whiteley
Derek C
>
> - Show quoted text -
The correct position for 'high tow' is just a few feet above the tug
propwash. If you are not sure, ease the glider down until you feel the
turbulence and then ease back up a few feet. The appearance of the tug
and its position relative to the horizon will depend on its angle of
climb, and for that matter the nature of the horizon because a
mountain range is somewhat higher than a plain or a seascape. When the
tug levels out on a cross-country retrieve the correct high tow
position looks a lot higher than the normal climb position.
Derek C (UK Instructor)
Andy
Maybe the disagreement is only what is meant by downwards. I disagree
that for a glider towing just above the wake, using a CG hook, and
with the tug in a full power climb at normal tow speed, that the rope
applies any force to the glider in a direction below the local
horizontal plane. All the qualifiers above describe a normal tow for
me.
Andy
Craig
> At 21:47 31 December 2010, Martin Gregorie wrote:
>
>
>
>
>
> >On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
>
> >> On Dec 31, 6:19 pm, bildan wrote:
> >>> On Dec 31, 4:40 am, "Doug" wrote:
>
> >>> > As an aerodynamicist/flight dynamicist recently re-soloed after 25
> >>> > years off, people keep asking me hard questions. One that has
> come
> >>> > up recently is why a heavy glider on tow feels horrible, but
> >>> > thermalling in the same glider at lower speeds is fine? (see also
> >>> > Mike Fox's article on aerotowing in the October issue of S&G).
>
> >>> > I did some calculations, and I reckon it's probably due to the tug
> >>> > wing wake (tip vortices generating a downwash inboard, upwash
> >>> > outboard) changing the lift distribution on the glider wing - with
> an
> >>> > increased angle of attack out at the tips reducing aileron
> >>> > effectiveness. There's possibly an interesting academic research
> >>> > project here, but it's always best to get a reality check first
> ..
>
> >>> > Is poor handling at low speed on tow a common experience? I'd
> >>> > appreciate any thoughts/comments/war stories ... particularly bad
> >>> > tug/glider/speed combinations, incidents of wing drop during a tow
> >>> > etc etc?
>
> >>> > Doug Greenwell
>
> >>> I suspect, but can't know unless I flew with you, that you are
> >>> unconsciously trying to "steer" the glider with ailerons. Overuse
> of
> >>> ailerons is very common and it makes aero tow 'wobbly'. If you
> >>> consciously use rudder to aim the nose at the tug's tail and just
> keep
> >>> the same bank angle as the tug with ailerons, it might work better.
>
> >>> Wake effects are generally favorable if you stay at the right height
> >>> relative to the tug. Using a slightly higher tow position can
> >>> sometimes help a lot.
>
> >>> The tip vortices rotate inward above the propwash which, if allowed
> to
> >>> do so, will drift the glider to the center position and help keep it
> >>> there. I haven't noticed any tendency for them to yaw a glider
> >towards
> >>> a tugs wing tip.- Hide quoted text -
>
> >>> - Show quoted text -
>
> >> uncomfortable on slow tows that are still well above their normal
> >> stalling speed. We think the answer is that the glider is being asked
> to
> >> climb with the tug providing the thrust via the rope. The glider is
> >> still effectively in free flight and therefore has to fly at a greater
> >> angle of attack for a given airspeed to produce the extra lift for
> >> climbing. Hence its stalling speed is somewhat increased.
>
> >If the tug's downwash field extends back far enough to include the
> >glider, its AOA will be relative to the downwash streamlines. Add the
> >downwash angle to the climb angle of the tug-glider combination will make
> >the glider look quite nose-high to its pilot.
>
> >I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
> >chords behind the wing, i.e. about where the tailplane is, but not what
> >its angle might be at the end of a tow rope.
>
> >--
> >martin@ | Martin Gregorie
> >gregorie. | Essex, UK
> >org |
>
> third of the tug AoA is a good first guess.
>
> My modeling suggest that there does seem to be an overall reduction in the
> glider wing lift (downwash over the centre wing having more of an effect
> than upwash over the tips), so the glider requires another degree or two
> in AoA - so feeling even more nose-up to the pilot!
Many thanks to the aerodynamics folks for cogent replies. From a
structures and vectors standpoint, the greatest amount of downward
catenary force possible from the rope is the rope's own weight (in
other words, damn little). If the towplane and glider are at exactly
the same elevation the vertical component of the catenary force equals
half the rope weight. Any other vertical forces imparted to the
sailplane result from the vector generated by the relative positions
of the towplane and glider. Kudos to Doug for the stimulating
discussion.
Thanks,
Craig
Doug Greenwell
>On Jan 3, 12:01=A0am, Derek C wrote:
>> Actually the only totally reliable sysmptom of being stalled is that
>> the elevator will no longer raise the nose.
>
>But that is neither necessary nor sufficient!
>
>If you put enough weight in the front cockpit then there are plenty of
>gliders where you reach the back stop while they are still flying just
>fine.
>
>Conversely, there are also plenty of gliders with sufficiently
>powerful elevators that the wing can be stalled and you're mushing at
>500+ fpm but you still have perfect control over the attitude of the
>nose and can raise or lower it at will. Not to mention other aircraft
>such as the F/A-18 which can be flown in perfect control with the wing
>stalled at huge angles of attack.
>
>I see in another post the definition:
>
>> Admittedly that is still a stall according to FAR23/25 definitions "a
>ll is produced, as shown by either:
>> (1) An uncontrollable downward pitching motion of the airplane;
>> (2) A downward pitching motion of the airplane that results from the
>vation of a stall avoidance device (for example, stick pusher); or
>> (3) The control reaching the stop."
>
>Without having that document in front of me I will hazard a guess that
>this is not a definition of a stall, but rather a definition of the
>standards for what a pilot should do in order to pass a practical
>flight examination. They're not going to fail him when the aircraft
>fails to actually stall because the elevator reaches its stop first,
>so they explicitly allow that as a signal that the pilot is allowed to
>terminate the "stall" attempt and commence the stall recovery
>procedure.
>
>The only true definition of a stall is when the wing is at an angle of
>attack such that a further increase of AoA produces a decrease of
>lift.
>
>*Usually* this will be accompanied by a large increase in drag such
>that the combination of lift and drag is easily capable of supporting
>the aircraft against gravity at a low speed and steep nose up descent
>angle, but that may not necessarily always be the case and some
>aircraft might speed up while stalled (perhaps at high altitude?).
>
Exactly - FAA 'legal' definitions of stall are aimed at defining speeds
for certification purposes. Most recently, more use has been made of the
'1g stall speed', which is the slowest speed you can fly and still
maintain level flight (obviously a bit tricky for a glider!) - which
corresponds to your definition of teh AoA at which a further increase
produces a decrease of lift (= maximum lift). The problem for the
regulators is that some (many?) aircraft become uncontrollable one way or
another before you get to this point - which is why stall such a bl*^dy
difficult thing to define precisely
Doug Greenwell
At what speed are you using 40deg of flap?
In a descent I would think you are likely to be closer to the tug wake
than in a climb
Doug Greenwell
>On Jan 1, 3:06=A0am, Doug Greenwell wrote:
>> At 21:47 31 December 2010, Martin Gregorie wrote:
>>
>>
>>
>>
>>
>> >On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
>>
>> >>> On Dec 31, 4:40=A0am, "Doug" =A0wrote:
>>
>> >>> > As an aerodynamicist/flight dynamicist recently re-soloed after
25
>> come
>> >>> > up recently is why a heavy glider on tow feels horrible, but
>> >>> > thermalling in the same glider at lower speeds is fine? (see
also
>> >>> > Mike Fox's article on aerotowing in the October issue of S&G).
>>
>> >>> > I did some calculations, and I reckon it's probably due to the
tug
>> >>> > wing wake (tip vortices generating a downwash inboard, upwash
>> >>> > outboard) changing the lift distribution on the glider wing -
with
>> an
>> >>> > increased angle of attack out at the tips reducing aileron
>research
>> >>> > project here, but it's always best to get a reality check first
>> ..
>>
>> >>> > Is poor handling at low speed on tow a common experience?
>> >>> > appreciate any thoughts/comments/war stories ... particularly
bad
>> >>> > tug/glider/speed combinations, incidents of wing drop during a
tow
>> >>> > etc etc?
>>
>> >>> > Doug Greenwell
>>
>> >>> I suspect, but can't know unless I flew with you, that you are
>> >>> unconsciously trying to "steer" the glider with ailerons.
>> of
>> >>> ailerons is very common and it makes aero tow 'wobbly'. =A0If
you
>> >>> consciously use rudder to aim the nose at the tug's tail and just
>> keep
>> >>> the same bank angle as the tug with ailerons, it might work
better.
>>
>> >>> Wake effects are generally favorable if you stay at the right
height
>> >>> sometimes help a lot.
>>
>> >>> The tip vortices rotate inward above the propwash which, if
allowed
>> to
>> >>> do so, will drift the glider to the center position and help keep
it
>> >towards
>> >>> a tugs wing tip.- Hide quoted text -
>>
>> >>> - Show quoted text -
>>
>> >> There was a debate on our club forum about why gliders feel
>> >> uncomfortable on slow tows that are still well above their normal
>> >> stalling speed. We think the answer is that the glider is being
asked
>> to
>> >> climb with the tug providing the thrust via the rope. The glider is
>> >> still effectively in free flight and therefore has to fly at a
>greater
>> >> angle of attack for a given airspeed to produce the extra lift for
>> >> climbing. Hence its stalling speed is somewhat increased.
>>
>> >If the tug's downwash field extends back far enough to include the
>> >glider, its AOA will be relative to the downwash streamlines. Add the
>> >downwash angle to the climb angle of the tug-glider combination will
>e
>> >the glider look quite nose-high to its pilot. =A0
>>
>> >I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
>> >chords behind the wing, i.e. about where the tailplane is, but not
what
>> >its angle might be at the end of a tow rope.
>>
>> >--
>> >gregorie. | Essex, UK
>> >org =A0 =A0 =A0 |
>>
>> The downwash angle doesn't change much past the tail, and a half to a
>> third of the tug AoA is a good first guess.
>>
>> My modeling suggest that there does seem to be an overall reduction in
>e
>> glider wing lift (downwash over the centre wing having more of an
effect
>> than upwash over the tips), so the glider requires another degree or
two
>> in AoA - so feeling even more nose-up to the pilot!
>
>Many thanks to the aerodynamics folks for cogent replies. From a
>structures and vectors standpoint, the greatest amount of downward
>catenary force possible from the rope is the rope's own weight (in
>other words, damn little). If the towplane and glider are at exactly
>the same elevation the vertical component of the catenary force equals
>half the rope weight. Any other vertical forces imparted to the
>sailplane result from the vector generated by the relative positions
>of the towplane and glider. Kudos to Doug for the stimulating
>discussion.
>
>Thanks,
>Craig
>
It's been very interesting - and sparked off a few potentially very
interesting research topics (typical academic - always an eye to the next
journal paper!)
Good point on the rope forces - I hadn't looked at it that way, but as
you say any bow in the tow rope won't actually have a significant effect
on the static forces/moments on the glider .. just as well, because it's
quite difficult to calculate the shape once you take drag forces into
account!
Doug
ProfChrisReed
cause.
Imagine you are flying free @55kt. You have a sink rate of, say,
1.5kt. Now you are on tow, again @55kt, but this time the combination
is climbing @5kt. Your wings are generating 6.5kt more lift than in
free flight, and must therefore be at a substantially higher AoA.
Additionally, the faster you are climbing (in still air) the greater
the AoA must be for you to keep station with the tug.
I fly an Open Cirrus, towing from the C of G hook without ballast, and
never experienced this at my previous club which had a Citabria tug.
My current club has a Pawnee, and I have from time to time felt the
tow was too slow because the controls felt mushy and the glider
wallowed about, feeling as if it was close to the stall. The Pawnee
climbs much faster than the Citabria.
If in addition the tug's slipstream imparts a downward flow to the
airmass, even more lift and higher AoA is required.
twocool...@juno.com
Actaully, comparing climbing steeply say, 10:1 on tow, to gliding at
40:1, the lift vector is (a tiny bit) SMALLER during the tow!
During the 10:1 tow, lift would be 99.5% of the glider's weight, while
during a 40:1 glide, lift would be 99.97% of the glider's weight!
(the missing 0.5% on tow is made up by the thrust vector...the missing
0.03% in glide is made up by the drag vector.
Cookie
Cookie
twocool...@juno.com
>
> - Show quoted text -
Actually, 5 or 10 pounds of down force at the glider's nose would be
significant. Every loosen your shoulder belts and lean
forward?.....this little weight shift will change pitch and speed.
Now with a cg hook ...probably not significant.
Cookie
twocool...@juno.com
>
> - Show quoted text -
Just some real fast and dirty assumptions.........say your climb angle
is 5 or 6 degrees.......200' rope. Rope could easily sag 10' in the
middle........I eyeball this to be 10 degrees "off horizontal" at the
ends.......this would net 10 degrees downward using the level earth as
a reference......and 15 degrees compared to the flight path of the
glider.
But I gotta agree that the numbers and angles are kinda small.....so
significant? Maybe, maybe not......Very little vertical force at the
nose can make a big difference......with a cg hook.....probably not
anything noticable...
Cookie
twocool...@juno.com
I also disagree with you statement that the AoA must be greater if
you climb more rapidly......not so....
Assuming a constant airspeed....
The rate of climb is strictly a factor of the power available. More
powerful towplane = faster rate of climb......lift on the glider's
wing, and the towlane's wing stays practically constant, therefore
the angle of attack is just about constant.
It is the climb angle (direction of flight) which changes with power,
not the AoA.
Cookie
Darryl Ramm
wrote:
Ugh?
The glider is flying, the towplane is not dragging the glider up an
incline. If the combination is going up faster (=steeper climb rate/
angle) then both aircraft wings are generating more lift and they get
this this from some combination of increased AoA and airspeed. The
more powerful towplane may allow both aircraft to fly at an increased
AoA and overcome the associated drag. The increased climb angle comes
from the increased lift. Assuming a constant airspeed means all the
increase is coming from an increase in AoA and the more powerful
towplane thrust is offsetting the increased drag. I'd be interested to
see an explanation of any other way of generating an increase in climb
angle without increasing the lift of the glider and/pr towplane.
Darryl
Eric Greenwell
> On Jan 3, 5:23 pm, "twocoolglid...@juno.com"<twocoolglid...@juno.com>
>> The rate of climb is strictly a factor of the power available. More
>> powerful towplane = faster rate of climb......lift on the glider's
>> wing, and the towlane's wing stays practically constant, therefore
>> the angle of attack is just about constant.
>>
>> It is the climb angle (direction of flight) which changes with power,
>> not the AoA.
>>
>> Cookie
>
> Ugh?
>
> The glider is flying, the towplane is not dragging the glider up an
> incline. If the combination is going up faster (=steeper climb rate/
> angle) then both aircraft wings are generating more lift and they get
> this this from some combination of increased AoA and airspeed. The
> more powerful towplane may allow both aircraft to fly at an increased
> AoA and overcome the associated drag. The increased climb angle comes
> from the increased lift. Assuming a constant airspeed means all the
> increase is coming from an increase in AoA and the more powerful
> towplane thrust is offsetting the increased drag. I'd be interested to
> see an explanation of any other way of generating an increase in climb
> angle without increasing the lift of the glider and/pr towplane.
Actually, I do think the towplane is pulling the glider up an incline!
The flight path is inclined, and the towplane is the only one that can
provide the force. In fact, I think the lift required *decreases* with
increased climb rate during tow! How could that be? The tow rope
provides some of the force needed to hold the glider in the air.
Imagine an extreme tow, a 50 knot airspeed, but climbing at 35 knots (45
degree angle). The tow rope is providing 70% of the force holding the
glider in the air, so the wing needs to supply only 30% of the force.
Or imagine a really extreme, vertical tow: all the force required to
keep the glider moving steadily through the air is provided by the
towrope/towplane, and none by the wing.
Let the games begin!
Derek C
>
> - Show quoted text -
If you had a really powerful tug that was capable of climbing
vertically, then the glider would just be dangling on the end of the
rope and would not have to produce any lift. The tension in the rope
would be equal to the weight of the glider plus any drag components.
While this is not a very likely scenario, I do think that the thrust
vector must be greater in a 10% climb than you are claiming.
Derek C
Derek C
I had a high tow behind a 180 hp Piper Super Cub (a fairly slow tug)
to practice aerobatics in a K21 on Sunday. The nosehook on the K21 is
situated under the nose, just in front of the nosewheel, and I was
flying it solo. For the last thousand feet of the tow the airspeed
dropped to about 56 knots and I got the same symptoms as described
above. The normal free flight stalling speed for a K21 is only about
39 knots, and I had no problems in the early part of the tow when the
airspeed was 60+ knots. If the glider feels as though it is close to
the stall, then it probably IS close to the stall!
Derek C
Darryl Ramm
I think you are trying to push this argument up an incline with a
rope. :-) But I'll take your points into consideration next time I'm
vertically towing behind a helicopter.
---
I think Chris Reed well nailed the (somewhat bleeding obvious when you
think about it) issue here with AoA and handling on slow tow.
Darryl
Bruce Hoult
wrote:
> Just some real fast and dirty assumptions.........say your climb angle
> is 5 or 6 degrees.......200' rope. Rope could easily sag 10' in the
> middle
10 *feet* ???!!!!
When I watch a glider taking off, the tow rope is entirely clear of
the ground as soon as the tow plane has taken up slack and started to
accelerate, and this continues to be true when the glider is just off
the ground (in case you argue there is more tension when the glider is
rolling on the ground).
The sag might or might not be a bit more than 10 inches, but it's
certainly nowhere near ten feet!
BruceGreeff
As I understand things - you are confusing climbing with vertical
acceleration.
While flying without vertical acceleration within the performance
available to gliders (I.e. constant vertical speed, modest climb angles,
tiny descent angles) the wing has to support pretty much one glider
weight (1g*mass). This is true whether you are gliding at 1:nn, towing
behind an anaemic cub at 9000" density altitude, or screaming skywards
behind a turbo Cmellak (Zlin 37). All that changes is the angle your
flight path makes relative to the ground. While the rate of climb may
seem significant it has nothing to do with AoA.
So - the flight path angle to any given frame of reference is largely
irrelevant to the AoA required. The AoA required is dependant on many
things - as I understand it these include -
- the Cl of the wing,
- relative velocity of the air over the airfoil,
- relative density of the air,
- the surface area of the wing
- the force it must support.
The force it must support will vary slightly depending on the vector of
force applied by the propulsive device. This could be an engine, a tow
plane or gravity. But it is only significant for one propulsive tool I
am aware of - winching does involve a short period of significant
vertical accelleration. In the transition from the initial climb to the
steep climb part of a winch launch the accelleration changes from ~1g to
about 2g. So the wing has to generate enough lift to generate the force
to change the flight vector - for a short while the AoA is high, close
to the ground. If it goes wrong here the prospects for a stall are good.
In the steep climb the angle described by the flight path relative to
the ground can easily reach 45 to 50 degrees, but the AoA on the wing
remains constant. It will be supporting a constant about 1g after the
transition.
The bending moment on the wing root is higher for reasons related to
where the winch vector is applied, and to the direction and magnitude of
that force, but this is not the load the wing must support. As the
glider has no significant vertical acceleration, the wing is
aerodynamically supporting a constant ~1g. (It must be a little higher
because of the added component of the winch force vector normal to the wing)
Of course - the angle that the flight path can make relative to the
ground is proportional to the excess power available - hence the low
rate of climb behind the cub, versus the extreme angle on a winch.
Aerodynamics guys - Am I confused?
Bruce
--
Bruce Greeff
T59D #1771 & Std Cirrus #57
Martin Gregorie
> Of course - the angle that the flight path can make relative to the
> ground is proportional to the excess power available - hence the low
> rate of climb behind the cub, versus the extreme angle on a winch.
>
> Aerodynamics guys - Am I confused?
>
Sounds fair to me except that you omitted two fairly significant forces:
- the weight of the cable
- the tension in the cable.
Both will add to the load carried by the wing. The tension should add a
fairly constant load to the wing once the glider has rotated into full
climb since the throttle setting remains fairly constant[*] from rotation
until the glider is near the top, but the effective cable weight will
increase as more of it is lifted off the ground and then as the whole
cable gets closer to vertical.
[*] this is true on a calm day but is obviously incorrect in the presense
of turbulence or a significant wind gradient.
--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |
twocool...@juno.com
>
> - Show quoted text -
Do some simple vector diagrams....lift, weight drag and thrust.....do
these for various climb, (and glide) angles)... you will soon see that
the greatest lift occurs at "level" flight......the steeper the climb,
or the steeper the descent, the less the lift.
You will see that to achieve steep climb 9with constant airspeed) , we
need only to increase the thrust vector.......more power required.
In level flight, power is required......lift = weight lift tapers
off to zero as we approach straight up flight, and / or straight down
flight. In descending flight (gliding) less and less power is
required as we steepen the descent, until we reach best L/D, when no
power at all is required.
What you are thinking about is trying to increase climb angle with a
fixed amount of power.......to do this we have to increase the angle
of attack, and of course fly slower........Vx....Vy.......come in to
play now.......best rate, best angle ....at fixed power, we vary speed
and AoA to achieve Vx or Vy or whaterver rate or angle we want.....
But if we have a very powerful towplane, we can climb fairly steeply,
at a normal towing speed, and the AoA will be fairly low, certainly
not near stall....
But remember the premise of this discussion.......towing at a speed at
which the glider performs nicely in gliding flight, yet had control
issues in towing flight.....but speed constant. A glider which will
"glide" nicely at say 50 MPH, may not tow nicely at 50
MPH.............vector diagrams will show that the lift is nearly the
same in both cases, therefore the AoA is about the same........so some
or all of the other factor discussed in this thread must come into
play.
But rapid climb rate = high AoA is a fallacy.
High climb rate = high power is correct.
Cookie
Doug Greenwell
In a steady climb (or descent) lift is very close to being equal to
weight, even taking account of tow rope inclination. You don't need
extra lift to climb, you need extra thrust to increase potential energy.
(A pull-up or zoom climb is different, because in this case you are
trading speed for height).
The Pawnee is a significantly heavier aircraft than the Citabria, so would
generate stronger tip vortices at a given tow speed, and hence have more
effect on a glider behind it.
Doug Greenwell
>On Jan 3, 3:34=A0pm, Doug Greenwell wrote:
>> At 19:12 03 January 2011, Craig wrote:
>>
>>
>>
>>
>>
>> >> At 21:47 31 December 2010, Martin Gregorie wrote:
>>
>> >> >On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
>>
>> >> >>> On Dec 31, 4:40=3DA0am, "Doug" =3DA0wrote:
>>
>> >> >>> > As an aerodynamicist/flight dynamicist recently re-soloed
after
>> 25
that
>h=
>as
>> >> come
>> >> >>> > up recently is why a heavy glider on tow feels horrible, but
>> >> >>> > thermalling in the same glider at lower speeds is fine? (see
>> also
>> >> >>> > Mike Fox's article on aerotowing in the October issue of
S&G).
>>
>> >> >>> > I did some calculations, and I reckon it's probably due to
the
>> tug
>> >> >>> > wing wake (tip vortices generating a downwash inboard, upwash
>> >> >>> > outboard) changing the lift distribution on the glider wing -
>> with
>> >> an
>> >> >>> > increased angle of attack out at the tips reducing aileron
>> >research
>> >> >>> > project here, but it's always best to get a reality check
first
>> >> ..
>>
>> >> >>> > Is poor handling at low speed on tow a common experience?
>> >> >>> > appreciate any thoughts/comments/war stories ... particularly
>> bad
>> >> >>> > tug/glider/speed combinations, incidents of wing drop during
a
>> tow
>> >> >>> > etc etc?
>>
>> >> >>> > Doug Greenwell
>>
>> >> >>> I suspect, but can't know unless I flew with you, that you are
>> >> >>> unconsciously trying to "steer" the glider with ailerons.
>> >> of
>> >> >>> ailerons is very common and it makes aero tow 'wobbly'.
>> you
>> >> >>> consciously use rudder to aim the nose at the tug's tail and
just
>> >> keep
>> >> >>> the same bank angle as the tug with ailerons, it might work
>> better.
>>
>> >> >>> Wake effects are generally favorable if you stay at the right
>> height
>can
>> >> >>> sometimes help a lot.
>>
>> >> >>> The tip vortices rotate inward above the propwash which, if
>> allowed
>> >> to
>> >> >>> do so, will drift the glider to the center position and help
keep
>> it
>glide=
>r
>> >> >towards
>> >> >>> a tugs wing tip.- Hide quoted text -
>>
>> >> >>> - Show quoted text -
>>
>> >> >> There was a debate on our club forum about why gliders feel
>> >> >> uncomfortable on slow tows that are still well above their
normal
>> >> >> stalling speed. We think the answer is that the glider is being
>> asked
>> >> to
>> >> >> climb with the tug providing the thrust via the rope. The glider
>is
>> >> >> still effectively in free flight and therefore has to fly at a
>> >greater
>> >> >> angle of attack for a given airspeed to produce the extra lift
for
>> >> >> climbing. Hence its stalling speed is somewhat increased.
>>
>> >> >If the tug's downwash field extends back far enough to include
the
>> >> >glider, its AOA will be relative to the downwash streamlines. Add
>the
>> >> >downwash angle to the climb angle of the tug-glider combination
will
>> >e
>> >> >the glider look quite nose-high to its pilot. =3DA0
>>
>> >> >I know that the downwash angle is roughly 1/3 of the wing AOA at
4-5
>> >> >chords behind the wing, i.e. about where the tailplane is, but not
>> what
>> >> >its angle might be at the end of a tow rope.
>>
>> >> >--
>> >> >gregorie. | Essex, UK
>> >> >org =3DA0 =3DA0 =3DA0 |
>>
>> >> The downwash angle doesn't change much past the tail, and a half to
a
>> >> third of the tug AoA is a good first guess.
>>
>> >> My modeling suggest that there does seem to be an overall reduction
>in
>> >e
>> >> glider wing lift (downwash over the centre wing having more of an
>> effect
>> >> than upwash over the tips), so the glider requires another degree
or
>> two
>> >> in AoA - so feeling even more nose-up to the pilot!
>>
>> >structures and vectors standpoint, the greatest amount of downward
>> >catenary force possible from the rope is the rope's own weight (in
>exactly
>> >the same elevation the vertical component of the catenary force
equals
>> >sailplane result from the vector generated by the relative positions
>> >of the towplane and glider. Kudos to Doug for the stimulating
>> >discussion.
>>
>> >Thanks,
>> >Craig
>>
>> It's been very interesting - and sparked off a few potentially very
>> interesting research topics (typical academic - always an eye to the
>next
>> journal paper!)
>>
>> Good point on the rope forces - I hadn't looked at it that way, but
as
>> you say any bow in the tow rope won't actually have a significant
effect
>> on the static forces/moments on the glider .. just as well, because
it's
>> quite difficult to calculate the shape once you take drag forces into
>> account!
>>
>> Doug- Hide quoted text -
>>
>> - Show quoted text -
>
>Actually, 5 or 10 pounds of down force at the glider's nose would be
>significant. Every loosen your shoulder belts and lean
>forward?.....this little weight shift will change pitch and speed.
>
>Now with a cg hook ...probably not significant.
>
>
>Cookie
>
>
true - but it would take a very small elevator deflection to trim it out
twocool...@juno.com
>
> - Show quoted text -
Yeah.........Hey, I am not saying that this is the answer to the
question........I have yet to see any answer which fully explains the
phenomon........I am just bringing up this issue of the tow rope
because it was brought up in the earlier discussion. It is just one
of the "suspects" in the investigation.
We all agree that at a given speed, faily slow, that a glider handles
fine in gliding flight, and has troubles on tow at the same speed.
So there are obvious differences during tow.........the tow rope
hooked to the nose is one, along with all the others we have
discussed, like down wash, vortex, etc.......
Cookie