How to get maximum height on a winch launch?
Dan G
winch launch, without compromising safety in any way.
This question has two parts really - how should the pilot fly the
launch (pretty much how should he operate the elevator during the
launch), how fast should the glider fly, and how should the winch
driver feed the power in? For the sake of argument, let's assume the
glider is a K13.
For reference, I fly from an airfield with a 3,300' long cable, and the
maximum height I achieve repeatedly is 1,700'. Headwind typically goes
from around 10kts at the ground to something like (I estimate) 20kts at
height.
bob...@aci.on.ca
2) Do nothing different, you are already acheiving beyond average
heights, changing your current habits is maybe looking for a problem.
If I could get 1700 feet on 3300 feet of cable I would change
absolutely nothing. Look no further.
Bob
Bill Daniels
there are a series of truely excellent papers in the folder "Winch Dynamics
and Performance" by George Moore. (soargsm)
Essentially what George determened is that the major sensitivities are:
1.Field length
2. Headwind
3. Cable tension as a percentage of glider flying weight.
4. Weight & drag of the cable
There is very little extra height to be gained by agressive technique beyond
just hitting your target airspeed and holding it there with smooth control
inputs. This gives an angle of attack just slightly greater than that for
best L/D. If you want help with this, put some AOA strings on the canopy
sides and calibrate them for L/D max and Vmin sink. Of course, it goes
without saying that you should never put yourself in danger by rotating into
the climb so quickly that a recovery from a rope break is impossible.
Smooth and accurate flying are what you want.
So, if you want to get higher, get a longer field with more wind, replace
steel with high strength Dyneema and get a winch that can accurately hold
tension at 80 - 90% of the weak link.
Bill Daniels
"Dan G" <dan...@gmail.com> wrote in message
news:1165953472....@l12g2000cwl.googlegroups.com...
Dan G
There is one thing in particular I am unsure about however - how much
up elevator I should be putting in. With the K8, I fly the launch at
50-55kts with the stick fully back for perhaps the last 30% of the
launch after smoothly transitioning from full forwards on the ground
run through neutral once in the full climb, until I feel the glider
being pulled level, when I relax the aft pressure in anticipation of
putting the nose down to reuce cable tension prior to releasing. (This
later movement often results in quite an increase in speed, which I
convert to height after releasing. Is this an indication I'm releasing
back pressure too early, or the winch driver not backing off the
throttle enough?)
With a K21, which winch drivers here (which includes me) are told to
give "full power all the way up" from our Oldsmobile V8. Climb speed is
at lesst 65kts and I'm reluctant to pull back too hard. Why? A K21
launches on a black weak link (1,000kg). I imagine full up elevator in
a K21 would produce enough lift to break that at well below max winch
speed, 81kts.
Is there a way to calculate how much lifting force full up elevator
should produce for any given airspeed? Or should I not be putting full
up in? If not, just how far should I bring the stick back, and when?
Bill Daniels
launch. All modern trainers like ASK 21's or Grob 103's will fly an almost
perfect launch without the pilot touching the controls if trim and winch
power are right. Demonstrating hands off launches is a good start to winch
training. (The take home lession is the launch will be nearly perfect if the
pilot doesn't screw it up.)
You should be using just enough elevator to keep the airspeed where you want
it. (Typically 55 - 60 knots, depending on the glider) The basic principle
is that the winch driver controlls power and the glider pilot controlls
airspeed. If it requires large amounts of up elevator to control airspeed,
signal for less power. As you point out, some gliders with high CG
locations like a K8 will need nearly full down elevator from the start to
control the natural pitch up. It shouldn't need much up elevator at all
later.
The winch driver should be slowly backing off the power as the glider
approaches the top of the launch. Take a good look at George Moores charts
that show cable tension - tension is a direct function of winch power.
In any event, there's very little additional height that can be achieved
late in the launch. Pulling hard near the top just loads up the glider and
may break the weak link. (You are using the correct Tost weak links,
right?)
Suggestion: Get a copy of a good glider flight simulator like Condor and
practice lauches on a computer. There's a lot to learn with a good sim. I
use Condor as an "animated white board" when I teach winch ground school.
Bill Daniels
"Dan G" <dan...@gmail.com> wrote in message
news:1165997669.7...@l12g2000cwl.googlegroups.com...
Dan G
> Dan, your questions concern me. Full up elevator is dangerous on a winch
> launch. All modern trainers like ASK 21's or Grob 103's will fly an almost
> perfect launch without the pilot touching the controls if trim and winch
> power are right.
>
> You should be using just enough elevator to keep the airspeed where you want
> it. (Typically 55 - 60 knots, depending on the glider) The basic principle
> is that the winch driver controlls power and the glider pilot controlls
> airspeed. If it requires large amounts of up elevator to control airspeed,
> signal for less power.
>
> As you point out, some gliders with high CG
> locations like a K8 will need nearly full down elevator from the start to
> control the natural pitch up. It shouldn't need much up elevator at all
> later.
>
> The winch driver should be slowly backing off the power as the glider
> approaches the top of the launch. Take a good look at George Moores charts
> that show cable tension - tension is a direct function of winch power.
>
> In any event, there's very little additional height that can be achieved
> late in the launch. Pulling hard near the top just loads up the glider and
> may break the weak link. (You are using the correct Tost weak links,
> right?)
We set the trim well forwards to give approach speed in the event of a
launch failure (I am not going to stop doing that, but hypothetically
where would you put the trim for best launch height?), and for K21s
winch drivers are briefed to give full throttle the whole way up (winch
is c.300bhp).
I've also heard several pilots be admonished by instructors recently
for "not pulling back" after somewhat low launches. Surely pulling back
increases the lift the aircraft is producing, and therefore gives more
height? If the airspeed is well below placard max winch speed, I would
have thought it would not result in enough force to break the link.
BTW we do, without fail, use the correct weak links (and not doubled-up
or anything stupid either). That said, we do break weak links with some
regularity.
>
> Suggestion: Get a copy of a good glider flight simulator like Condor and
> practice lauches on a computer. There's a lot to learn with a good sim. I
> use Condor as an "animated white board" when I teach winch ground school.
Heh, I have Condor but find it much harder to fly than "real life",
probably as my comp is rather weedy so it's a bit jerky. Thanks for the
suggestion though!
Dan
Bill Daniels
"Dan G" <dan...@gmail.com> wrote in message
news:1166030195.0...@j72g2000cwa.googlegroups.com...
> We set the trim well forwards to give approach speed in the event of a
> launch failure (I am not going to stop doing that, but hypothetically
> where would you put the trim for best launch height?), and for K21s
> winch drivers are briefed to give full throttle the whole way up (winch
> is c.300bhp).
Same as always. Set the trim for zero stick force. It has no effect
whatever on release height.
>
> I've also heard several pilots be admonished by instructors recently
> for "not pulling back" after somewhat low launches. Surely pulling back
> increases the lift the aircraft is producing, and therefore gives more
> height? If the airspeed is well below placard max winch speed, I would
> have thought it would not result in enough force to break the link.
I sounds like your winch may reach redline RPM just as the glider rotates
into the climb. If the pilot doesn't rotate quickly enough, the winch
driver will have to throttle down to save the engine.
Pilots trained on aero tow will often resist pitching up into the climb and
need to be admonished that this is not an aero tow and they need to climb.
As they climb the engine RPM will diminish. I'm not saying pull hard, just
pitch up smoothly to control airspeed.
The very best height is achieved by flying the glider at just over the best
L/D angle of attack. Pulling up harder and flying at a greater AOA will
actually reduce height achieved. To repeat, fly the selected airspeed. You
may improve things slightly by using an AOA indicator. Strings work fine.
>
> BTW we do, without fail, use the correct weak links (and not doubled-up
> or anything stupid either). That said, we do break weak links with some
> regularity.
Whew! Thanks for that.
>
>>
>> Suggestion: Get a copy of a good glider flight simulator like Condor and
>> practice lauches on a computer. There's a lot to learn with a good sim.
>> I
>> use Condor as an "animated white board" when I teach winch ground school.
>
> Heh, I have Condor but find it much harder to fly than "real life",
> probably as my comp is rather weedy so it's a bit jerky. Thanks for the
> suggestion though!
>
It is harder than real life - but it's good training.
Bill Daniels
Derek Copeland
of thinking, that the rotation into the full climb
should be gentle and controlled, and should not put
you in an unrecoverable position if the launch fails
for any reason.
The exact position to hold the stick during a winch
launch is very type dependent. Gliders with way back
c of g hooks, such as the K21, K8 and Pirat will require
the stick to be held neutral or even forward of neutral
during the full climb. Gliders with slightly further
forward hooks such as the K13 will need the stick held
fairly well back, although probably not on the back
stop. Gliders with single 'compromise' hooks such as
the Slingsby Skylark and the Bocian will need the stick
to be held on the back stop if they are to climb at
all steeply.
The best advice I can give to Dan G is to look at the
wingtips during the launch and find what stick position
will hold them at about 45 degrees to the horizon AND
give a safe speed. If you have a low powered winch
you may have to adopt a lesser angle.
Derek Copeland
At 15:18 13 December 2006, Bill Daniels wrote:
>Dan, your questions concern me. Full up elevator is
>dangerous on a winch
>launch. All modern
At 15:18 13 December 2006, Bill Daniels wrote:
>Dan, your questions concern me. Full up elevator is
>dangerous on a winch
>launch. All modern trainers like ASK 21's or Grob
>103's will fly an almost
>perfect launch without the pilot touching the controls
>if trim and winch
>power are right. Demonstrating hands off launches
>is a good start to winch
>training. (The take home lession is the launch will
>be nearly perfect if the
>pilot doesn't screw it up.)
>
>Suggestion: Get a copy of a good glider flight simulator
>like Condor and
>practice lauches on a computer. There's a lot to learn
>with a good sim. I
>use Condor as an 'animated white board' when I teach
>winch ground school.
>
>Bill Daniels
>
>'Dan G' wrote in message
>>> 'Dan G' wrote in message
>>> news:1165953472....@l12g2000cwl.googlegroups.com...
Martin Gregorie
> We set the trim well forwards to give approach speed in the event of a
> launch failure (I am not going to stop doing that, but hypothetically
> where would you put the trim for best launch height?),
>
> and for K21s
> winch drivers are briefed to give full throttle the whole way up (winch
> is c.300bhp).
>
That sounds excessive to me. The only glider that gets full power all
the way is an ASH-25.
We launch K.21s on a 240 hp diesel winch (Deutz air cooled V8) and never
reach full power in normal circumstances. If an ASK-21 or G.103 is being
launched the driver goes smoothly "through the stop" on "all out" and
smoothly back to the stop when he sees the glider rotate into full
climb. "Through the stop" means just that - not "on the end of throttle
travel". After that it depends on conditions. In calm that setting can
be kept most of the way up, but if there's a wind gradient the driver
will be backing off as the glider gets into progressively stronger wind.
The "stop" I mentioned is a spring loaded bar somewhere around (at a
guess) 75% power.
We use 3500 ft of cable and expect 1200 - 1500 ft from an ASK-21 flown
two-up in normal conditions - 6 to 12 kts headwind.
I'd endorse Bill's comment about hands-off trim. Our CFI regularly
demonstrates this in an ASK-21. He claps slowly from just after lift-off
to the top of the launch to show the student that he is flying hands off.
--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |
Ian
Martin Gregorie wrote:
> Dan G wrote:
> > and for K21s
> > winch drivers are briefed to give full throttle the whole way up (winch
> > is c.300bhp).
> >
> That sounds excessive to me. The only glider that gets full power all
> the way is an ASH-25.
It shouldn't, since an ASH-25 is a brown weak link, not a black one.
Many clubs, pilots and winch drivers ignore this, but at their peril.
When I am driving I will refuse to launch an ASH-25 unless someone I
trust will confirm to me that it has the right link in place. 60:1 does
not bring immortality.
Ian
Ian
Derek Copeland wrote:
> The exact position to hold the stick during a winch
> launch is very type dependent. Gliders with way back
> c of g hooks, such as the K21, K8 and Pirat will require
> the stick to be held neutral or even forward of neutral
> during the full climb.
My Pirat will fly a very nice winch launch which is almost completely
independent of anything the pilot chooses to do with the stick! It's
not so much the fore-and-aft position of the hook, I think, as the
vertical one: since it is at the bottom of a deep fuselage there is a
considerable nose up pitching moment during the ground run. As soon as
it takes off, therefore, it does a lovely rotation into full climb all
by itself: full forward stick has no effect at all.
> Gliders with single 'compromise' hooks such as
> the Slingsby Skylark and the Bocian will need the stick
> to be held on the back stop if they are to climb at
> all steeply.
All the Bocians I have ever flown had C of G and nose hooks. The Pirat
manual specifically permits winching on the nose hook: using full back
stick and forward trim from the "All Out", and warning that it will
only give 60% of normal release height. I have never tried this, for
several reasons, one being that I rather like having the back release
there ...
Ian
Ian
Dan G wrote:
> With a K21, which winch drivers here (which includes me) are told to
> give "full power all the way up" from our Oldsmobile V8.
Then with all due respect, I think your club needs to think about its
training policy for winch drivers. "Give full power all the way up" is
a recipe for disaster - it should be "Give enough power all the way up,
and don't assume that means full throttle". A good winch driver knows
how much welly to give it by the bow in the cable, the speed of the
drums and the appearance of the glider. Whacking in full throttle
without a care is as bad as giving full back stick from the start of
the launch.
If your club is training winch drivers to think about throttle position
without considering effects and training pilots to think about stick
position without considering effects, it may have to do some
fundamental thinking about flying and training. In both cases the
principle should be "Know what effect you want and use the right amount
of control to achieve it, monitoring the situation and varying inputs
as required." I would be very cautious about an organisation which
taught crude open-loop control in two separate safety-critical areas.
Please don't take this personally, because it's not so intended.
Ian
Bert Willing
winch launching my Calif over 8 years on a 280 hp winch was exactly "full
power until release", and that was the only way to go.
So, in some cases this type of instructions actually does work.
Bert
"Ian" <ian.g...@btinternet.com> wrote in message
news:1166192664....@73g2000cwn.googlegroups.com...
Bill Daniels
"Bert Willing" <willing_no_...@ir-microsystems.com> wrote in message
news:1166193...@sicinfo3.epfl.ch...
> Ian,
>
> winch launching my Calif over 8 years on a 280 hp winch was exactly "full
> power until release", and that was the only way to go.
> So, in some cases this type of instructions actually does work.
>
> Bert
Full power for the whole launch works when the winch is severely
underpowered. As more powerful winches are introduced, you will find there
are points in the launch where the driver must reduce power to maintain
constant tension.
Bill Daniels
Dan G
course we back off at the top. But there was some suprise at the launch
point recently when a K21 waved off... the common thought was "K21s
should be able to handle it". Maybe it could if it was no wind, but
with a good headwind they just can't.
So far the sole answer to my original question has been "look at the
wingtips during the launch and find what stick position
will hold them at about 45 degrees to the horizon AND
give a safe speed. If you have a low powered winch
you may have to adopt a lesser angle" from Derek.
Does everyone agree with this?
Dan
Frank Whiteley
Dan G wrote:
> Should say that it's "full power all the way up" the main climb, of
> course we back off at the top. But there was some suprise at the launch
> point recently when a K21 waved off... the common thought was "K21s
> should be able to handle it". Maybe it could if it was no wind, but
> with a good headwind they just can't.
>
the twins easily if the wind gets above about 15knots. If indicated
airspeed is above 60knots, you're getting diminishing returns from the
launch.
Frank Whiteley
Martin Gregorie
launched, but it certainly seemed to need a lot of grunt to get it
moving and rotated into full climb.
Ian
Martin Gregorie wrote:
> Ian wrote:
> > It shouldn't, since an ASH-25 is a brown weak link, not a black one.
> > Many clubs, pilots and winch drivers ignore this, but at their peril.
> > When I am driving I will refuse to launch an ASH-25 unless someone I
> > trust will confirm to me that it has the right link in place. 60:1 does
> > not bring immortality.
> >
> Fair comment - I've never been in the winch or at that end when one was
> launched, but it certainly seemed to need a lot of grunt to get it
> moving and rotated into full climb.
Thanks. Yes, they are heavy (about 1600lbs max AUW, iirc) but the
designers have specified a brown link and I respect that. It's not as
if the damn things need to get high anyway!
Ian
Ian
Bill Daniels wrote:
> Full power for the whole launch works when the winch is severely
> underpowered. As more powerful winches are introduced, you will find there
> are points in the launch where the driver must reduce power to maintain
> constant tension.
It's the inflexibility of "full power all the way" which worries me.
I've lobbed K21's off at Sutton Bank on windless days which needed full
throttle, windy days on which a whisker over idle was quite enough and
days when things changed from calm to windy half way up the launch. A
driver who thinks "K21 - right, full power" is ... I was going to right
"get himself into trouble oneday" but that isn't quite right. He's
going to get two other people into trouble one day.
Ian
Ian
Dan G wrote:
> Should say that it's "full power all the way up" the main climb, of
> course we back off at the top. But there was some suprise at the launch
> point recently when a K21 waved off... the common thought was "K21s
> should be able to handle it". Maybe it could if it was no wind, but
> with a good headwind they just can't.
And that was a surprise? Find yourself another club, man, because if
headwinds on a winch launch are unexpected your current lot are going
to damage someone one day.
> So far the sole answer to my original question has been "look at the
> wingtips during the launch and find what stick position
> will hold them at about 45 degrees to the horizon AND
> give a safe speed. If you have a low powered winch
> you may have to adopt a lesser angle" from Derek.
>
> Does everyone agree with this?
No. I think it's bad advice. Yes, you should aim to have a proper climb
angle and air speed, but the stick position is irrelevant. Just use
enough to do what's needed.
How many inches of rudder movement would you use to balance a 20 degree
per second roll into a turn? Or would you just aim to us enough to keep
the string straight?
Ian
Dan G
>how should the winch driver feed the power in?
I received a very detailed reply in the Yahoo winch design group. With
permission, I reproduce it below:
---------- Forwarded message ----------
From: andyh418
Date: Dec 14, 2006 12:01 PM
Subject: [winchdesign] Re: Throttle during rotation
To: winch...@yahoogroups.com
Hi Dan
There are lots of ways to do this. After training lots of drivers,
and consulting with other experienced drivers of LPG V8 winches, this
is the method I teach. People operating diesel winches may have to
use a different technique, although I know at least some drivers of
diesel winches operate like this.
1. Before launch, pick a throttle position which you estimate is
correct for an initial climb setting for the glider and wind
conditions. (In a winch with throttle guides this is easy; without
them, use your experience. Marks next to the throttle lever help you
learn)
2. At "ALL OUT", accelerate smoothly to that position in 2-3 seconds.
Keep accelerating smoothly past your chosen position if the glider
does not rotate into the climb.
3. Hold throttle steady as the glider rotates into the climb and
assess airspeed from comparison with other launches and how
quickly/hesitantly the glider rotates (and of course any signals. Too
slow, lower nose, smoothly feed in power. Too fast, yaw the glider,
smoothly back off power). Most people use how quickly the glider
moves up the windscreen of the winch to assess speed. Listening to
the engine also helps. Make small corrections if necessary.
4. If the correct initial position is chosen, you will find you can
hold the throttle absolutely still for the first 1/2-2/3 of the
launch, then you will be gradually backing off the power towards the
top, otherwise the glider will accelerate.
5. Give a nice brisk cut to idle once you have assessed the glider is
at the top of the launch, to give a positive signal to the pilot.
After release, wind in the cable.
HOW THIS CAME ABOUT?
It was found that the existing method of giving a bit of extra power
initially, then backing off as the glider rotated to avoid overspeed,
then feeding the power back in once in the full climb was an
overcomplicated process with minimal gain, but often interpreted
incorrectly and not very standard between drivers. It was also found
that these power changes were centred about a common position - the
throttle position for the first half of the launch. The recommended
technique is much simpler, and easier to standardise drivers.
Standardising drivers has obvious benefits for pilot safety and
consistency of launch.
Also, during rotation, there are many things changing. The glider is
accelerating, cablespeed increasing. As the pilot rotates, the
geometry of the launch is changing the relationship of cablespeed to
glider airspeed and increasing the load on the winch. The glider may
be climbing into the wind gradient. Introducing another changing
factor - who is driving and what they do with the throttle - is a bad
idea. Constant throttle works.
BUT WE NEED TO SLOW THE CABLE DOWN AS THE GLIDER ROTATES, DUE TO
GEOMETRY!!
Yes, but that doesn't necessitate moving the throttle. This is why
I'm such a big fan of LPG/Gas/Petrol V8's as winch engines. If the
climb throttle position is set correctly for glider type and wind,
the engine will accelerate the glider to a safe speed for rotation,
but once the glider starts to rotate into the climb, the cable load
increases. This has the effect of reducing the cablespeed for a given
throttle position, in proportion to the increase in airspeed caused
by the changing geometry of the launch. Only minor correction are
required, but a novice who just holds the throttle in the suggested
position (throttle stops or marks next to the lever) and backs off
the power towards the top will achieve a good launch.
The video of the Tost record has the engine accelerate to a high rpm
and stabilise there briefly, before the rpm reduces and the load
increases (from the engine note) as the glider rotates into the climb
and loads up the winch. After that you only hear minor corrections,
with a trend of the power and rpm gradually reducing in the top 1/2
of the launch. We can't see the throttle lever or linkage. We can
make no assumptions about how the driver is handling the power. If he
was using the technique above, it would sound like that. If he was
adding abit extra then taking it off then feeding it back in, it
would also sound like that. These corrections are very minor which is
why we have learned they are unnecessary.
Are there any days when you need to modify this technique? Only one
scenario that I am aware of, and that is when operating in strong
headwinds. In these conditions, the climb power is relatively low,
but of course we need to get the glider through the wind gradient and
into the full climb before this is the correct power to set. The
solution is easy. Simply accelerate past your target climb throttle
setting, to a position about halfway between your target climb
setting and a no headwind setting for the glider you're launching.
Then gradually reduce back to your target setting as the glider
climbs through the wind gradient.
As for how much to back off in the top half of the launch? On a still
day you wont need to back off much. On a windy day this will be a
gradual continuous process as the glider moves into stronger wind
layers. Watch the glider and listen to the engine load. Pilot
feedback via radio etc is always useful, but particularly on windy
days. Marks next to the throttle or throttle stops helps make a
quantitative adjustment after feedback. They are an invaluable aid.
Andy
Ian
Dan G wrote:
> Dan G wrote:
>
> >how should the winch driver feed the power in?
>
> I received a very detailed reply in the Yahoo winch design group. With
> permission, I reproduce it below:
Interesting ... but once more, beware of rules. Lots of factors can
make differences to technique: whether the drive is direct, fluid
flywheel or torque convertor, size of drums (and consequently inertia)
and turbocharged vs normally aspirated.
Ian
Dan G
> Dan G wrote:
> > Should say that it's "full power all the way up" the main climb, of
> > course we back off at the top. But there was some suprise at the launch
> > point recently when a K21 waved off... the common thought was "K21s
> > should be able to handle it". Maybe it could if it was no wind, but
> > with a good headwind they just can't.
>
> And that was a surprise? Find yourself another club, man, because if
> headwinds on a winch launch are unexpected your current lot are going
> to damage someone one day.
Easy now, my club has a long safety record any would be proud of. AFAIK
there's never been a winch launch accident. In that particular example
there was a very strong wind gradient. I was using as an illustration
of the belief that K21s can take everything the winch gives.
> > So far the sole answer to my original question has been "look at the
> > wingtips during the launch and find what stick position
> > will hold them at about 45 degrees to the horizon AND
> > give a safe speed. If you have a low powered winch
> > you may have to adopt a lesser angle" from Derek.
> >
> > Does everyone agree with this?
>
> No. I think it's bad advice. Yes, you should aim to have a proper climb
> angle and air speed, but the stick position is irrelevant. Just use
> enough to do what's needed.
Well, "find what stick position will hold them at about 45 degrees to
the horizon AND give a safe speed" and "you should aim to have a proper
climb angle and air speed" seems to be the same thing...
Dan
Pete Brown
still do not understand what the theoretical factors are
that determine max launch height.
Neglecting wind speed and wind gradient for the moment, as
well as safety concerns relating to rope breaks down low,
what is the technique to max out height given the changing
climb and tow line angles and geometry?
Derek Piggot writes in his book "Ground Launches , 1996, pg
34.):
“The optimum speed on the launch depends to some extent on
the wind strength with a slightly higher speed giving the
best launch height in calm conditions. Best L/D speed is
close to the optimum, with the minimum cruising speed or
speed for minimum sink being the lowest safe speed.”
Derek Piggott, Ground Launches , 1996, pg 34.
If the speed for best L/D is close to optimum, what is optimum?
The American Soaring Handbook, Vol 3, Ground Launch, 1980,
pg 32 says the following about auto tow
“The factor which determines the towing speed of the
automobile is the indicated air speed (IAS) of the
sailplane. It has been shown that the launching forces on
the sailplane are at a minimum just below the IAS for
maximum L/D of the sailplane”
Just how far below the best L/D IAS is the point of minimum
stress on the airframe?
Once we have derived the factors to max out launch height,
how do you then balance efficiency versus climb rate after
that point in the launch when you have enough altitude for
a safe recovery from a low rope break?
Pete
--
Peter D. Brown
http://home.gci.net/~pdb/
http://groups.yahoo.com/group/akmtnsoaring/
Marc Ramsey
> Derek Piggot writes in his book "Ground Launches , 1996, pg 34.):
>
> “The optimum speed on the launch depends to some extent on the wind
> strength with a slightly higher speed giving the best launch height in
> calm conditions. Best L/D speed is close to the optimum, with the
> minimum cruising speed or speed for minimum sink being the lowest safe
> speed.” Derek Piggott, Ground Launches , 1996, pg 34.
>
> If the speed for best L/D is close to optimum, what is optimum?
The optimum is the best glide speed at the wing effective load factor
based on the takeoff weight of the glider and the cable tension. For
example, for an ASK-21 with a best L/D speed of 49 knots at a weight of
1300 lbs, if the cable tension is 1900 lbs, the effective load factor is
1950/1300 or 1.5, the optimum launch speed is 49*SQRT(1.5) or 60 knots IAS.
> The American Soaring Handbook, Vol 3, Ground Launch, 1980, pg 32 says
> the following about auto tow
>
> “The factor which determines the towing speed of the automobile is the
> indicated air speed (IAS) of the sailplane. It has been shown that the
> launching forces on the sailplane are at a minimum just below the IAS
> for maximum L/D of the sailplane”
>
> Just how far below the best L/D IAS is the point of minimum stress on
> the airframe?
What they appear to be discussing here is a rule of thumb for automobile
speed based on the non-adjusted best glide IAS of the glider. Given a
nominal climb angle of 45 degrees, if the automobile is driving at 50
mph (44 knots), the glider will be flying at 62 knots TAS. So, in this
case, once the glider has rotated to climb angle you'd want to drive 6
or 7 MPH slower than the gliders best L/D speed at sea level density
altitude. If the density altitude is higher, you'd want to drive faster.
> Once we have derived the factors to max out launch height, how do you
> then balance efficiency versus climb rate after that point in the launch
> when you have enough altitude for a safe recovery from a low rope break?
Assuming the winch (or automobile) is providing a constant cable
tension, the max launch height will be pretty close to optimal if the
pilot maintains the optimum launch speed IAS as calculated above. This
should result in a climb angle for most of the launch of about 45 degrees.
Marc
Ian
Dan G wrote:
> Ian wrote:
> > And that was a surprise? Find yourself another club, man, because if
> > headwinds on a winch launch are unexpected your current lot are going
> > to damage someone one day.
>
> Easy now, my club has a long safety record any would be proud of. AFAIK
> there's never been a winch launch accident. In that particular example
> there was a very strong wind gradient. I was using as an illustration
> of the belief that K21s can take everything the winch gives.
I am glad to hear about the safety record. I only hope they can keep
it.
> > No. I think it's bad advice. Yes, you should aim to have a proper climb
> > angle and air speed, but the stick position is irrelevant. Just use
> > enough to do what's needed.
>
> Well, "find what stick position will hold them at about 45 degrees to
> the horizon AND give a safe speed" and "you should aim to have a proper
> climb angle and air speed" seems to be the same thing...
No, they are completely different. Stick position is irrelevant. All
that matters is whether you need to move it.
Let me give a simple example. The exact position of the temperature
lever on a shower doesn't matter, does it? If it's too hot you move it
"cold" a bit and if it's too cold you move it "hot" a bit. You don't
say "I'll have a position 5 shower, no matter how hot that is".
Same with the stick.
Ian
Derek Copeland
make! Stick position is type dependent. Where you hold
the stick and how much you have to pull back, or push
forward, may be different to what you are used to,
if you fly a different type of glider. It depends on
the hook position, the exact c of g and centre of pressure,
and the trim position. The advice I gave was to hold
the stick in a position that gives the optimum angle
of climb for your type of winch. Full stop, period!
Real life winch launching takes place using different
types of gliders in differing wind strengths. No one
throttle setting can cover all cases. If you launched
a slow glider (e.g. a K8) into a 45 knot headwind,
you could stop the winch altogether once it has got
airborne and allow it to launch like a kite. The wind
would be providing the energy while the winch would
just act as an anchor point on the ground. Conversely
launching a heavy two seater on a still day with a
moderately powerful winch might require absolutely
full throttle.
Derek Copeland
walkera...@gmail.com
thank you ray
AS
>
> looking for a used winch or plans for making one.
> thank you ray
Where in the world are you located? There are a good number of winches for sale on the German server:
http://www.segelflug.de/osclass/index.php?page=search&sCategory=107
Uli
Winchmeister of the Carolina Soaring Association
Don Johnstone
>I thought that was exactly the point I was trying to
>make! Stick position is type dependent. Where you hold
>the stick and how much you have to pull back, or push
>forward, may be different to what you are used to,
>if you fly a different type of glider. It depends on
>the hook position, the exact c of g and centre of pressure,
>and the trim position. The advice I gave was to hold
>the stick in a position that gives the optimum angle
>of climb for your type of winch. Full stop, period!
>
>Real life winch launching takes place using different
>types of gliders in differing wind strengths. No one
>throttle setting can cover all cases. If you launched
>a slow glider (e.g. a K8) into a 45 knot headwind,
>you could stop the winch altogether once it has got
>airborne and allow it to launch like a kite. The wind
>would be providing the energy while the winch would
>just act as an anchor point on the ground. Conversely
>launching a heavy two seater on a still day with a
>moderately powerful winch might require absolutely
>full throttle.
>
>Derek Copeland
If you take a winch launch with a really skilled winch driver you will
experience a constant speed all the way up the launch. How is that
done? There are a combination of things that can give clues to the
winch driver, the bow in the cable, if it is wire, does not work so
well with dynema/sky rope. Marked throttle settings as on a
Skylaunch winch can help but it really is a matter of feel and
experience. For most glass gliders 55-65kts seems to work best.
I always reduce power at the top, back to idle once the glider gets
near to the max angle, instigating a back release, this avoids the
possibility of a loop if the cable is released under tension. It is
absolutely essential to ensure that the power is cut, before the
glider is vertically over the winch in nil/very low headwind
conditions, it is very unpleasant to have 1000ftl of cable drop on top
of the winch, it it can take a bit of time to get it off as well.
Bill.D...@coloradosoaring.org
The winch does not and, in fact, cannot control glider airspeed. The pilot is the only one who can do that. The winch merely controls power (think thrust). It works exactly as with airplanes, pitch controls airspeed and power (thrust) controls rate of climb. The pilot merely adjusts pitch attitude as necessary to maintain the desired airspeed.
Reduce winch power and the rate of climb (angle) decreases but the airspeed is unchanged. Reduce it all the way to zero and the glider is left in a glide at the desired airspeed. Increase power and the rate of climb (angle) increases but the airspeed stays the same. Increase it too much and the weak link breaks - at exactly the desired airspeed - or the glider runs out of up elevator and the pilot must then ask for less power.
It's really very simple.
Don Johnstone
wrote:
>> well with dynema/sky rope. Marked throttle settings as on a=20
best.
>> I always reduce power at the top, back to idle once the glider
>> glider is vertically over the winch in nil/very low headwind=20
>
>If you know how to fly you can have exactly the airspeed you want
all the
>ay up the launch. =20
>The winch does not and, in fact, cannot control glider airspeed.
The
power (think
and
>
>Reduce winch power and the rate of climb (angle) decreases but
the
in a
and the
runs out
>
>It's really very simple.
>
Andreas Maurer
Bill.D...@coloradosoaring.org wrote:
>If you know how to fly you can have exactly the airspeed you want all the way up the launch.
>
>The winch does not and, in fact, cannot control glider airspeed. The pilot is the only one who can do that. The winch merely controls power (think thrust). It works exactly as with airplanes, pitch controls airspeed and power (thrust) controls rate of climb. The pilot merely adjusts pitch attitude as necessary to maintain the desired airspeed.
>
>Reduce winch power and the rate of climb (angle) decreases but the airspeed is unchanged. Reduce it all the way to zero and the glider is left in a glide at the desired airspeed. Increase power and the rate of climb (angle) increases but the airspeed stays the same. Increase it too much and the weak link breaks - at exactly the desired airspeed - or the glider runs out of up elevator and the pilot must then ask for less power.
>
>It's really very simple.
unfortunately it's not that simple. Your description does not even
remotely match the experiences I made I made in the last couple of
thousand winch launches.
Best regards
Andreas
Peter F
Unfortunately the gliding movement certainly in the UK, but also in Europe,
seems to have taken the "Easy" option of Skylaunch winches.
400hp and auto Transmission mean that the pilot has little control of the
airspeed during the launch.
Too fast? ease back, go even faster!
PF
Steve Leonard
> Actually if we had better designed winches Bill would be correct.
>
> Unfortunately the gliding movement certainly in the UK, but also in Europe,
> seems to have taken the "Easy" option of Skylaunch winches.
> 400hp and auto Transmission mean that the pilot has little control of the
> airspeed during the launch.
>
> Too fast? ease back, go even faster!
>
> PF
Steve Leonard
Bill.D...@coloradosoaring.org
Bill.D...@coloradosoaring.org
> Actually if we had better designed winches Bill would be correct.
>
> Unfortunately the gliding movement certainly in the UK, but also in Europe,
> seems to have taken the "Easy" option of Skylaunch winches.
> 400hp and auto Transmission mean that the pilot has little control of the
> airspeed during the launch.
>
> Too fast? ease back, go even faster!
>
> PF
>
>
Don Johnstone
wrote:
Andreas Maurer wrote:
>> On Tue, 20 Dec 2016 16:59:34 -0800 (PST),
>> Bill.D...@coloradosoaring.org wrote:
>>=20
want all
>e way up the launch. =20
>> >The winch does not and, in fact, cannot control glider
airspeed. The
power
airspeed
pitch
>> >
>> >Reduce winch power and the rate of climb (angle) decreases
but the
left in
climb
much and
runs
>> >
>> >It's really very simple.
>> Hi Bill,
>>=20
even
>> remotely match the experiences I made I made in the last
couple of
>> thousand winch launches.
>> Andreas
>
>It is just that simple and it PRECISELY matches my experiences in
my last
>ouple of thousand launches.
Well Bill, we will have to differ because it certainly does not match
my experience in the last 10,000 launches. Both the MVG and
Skylaunch winch will exceed the max winch launch speed easily
whatever you do in the cockpit, except of course yawing to tell the
winch driver he is applying too much grunt.
If you think winch launching is simple it is going to bite you one
day.
Andreas Maurer
Bill.D...@coloradosoaring.org wrote:
>On Wednesday, December 21, 2016 at 6:01:50 AM UTC-7, Andreas Maurer wrote:
>> On Tue, 20 Dec 2016 16:59:34 -0800 (PST),
>> Bill.D...@coloradosoaring.org wrote:
>The winch does not and, in fact, cannot control glider airspeed. The pilot is the only one who can do that.
reaction to lower the nose and keep up airspeed.
>Reduce winch power and the rate of climb (angle) decreases but the
airspeed is unchanged. Reduce it all the way to zero and the glider
is left in a glide at the desired airspeed.
reaction to lower the nose and keep airspeed up. Without reaction the
glider pilot is very, verfy close to stall and crash.
Seen a couple of bad crashes due to winch failure.
> Increase power and the rate of climb (angle) increases but the airspeed stays the same.
heavy open class double seater.
> Increase it too much and the weak link breaks - at exactly the desired airspeed - or the glider runs out of up elevator and the pilot must then ask for less power.
ever ran out of elevator authority.
Maybe us Europeans are doing something fundamentally different. ;)