Aileron control with negative flaps or spoilers on initial roll
David Bingham
provide better aileron control on initial roll (low air speeds). It is
not intuitively obvious why to me. Please point me to past
publications or articles that cover this subject. Thanks.
Dave
miriano
If you can accept a layman description... Regarding the negative
flaps, it seems that when you're on the ground the wing's angle of
attack is very high, so that at low air speed it's actually stalled.
By using negative flaps you "reduce" the angle of attack, thus somehow
anticipating the moment where the wing starts flying.
About the spoilers the point is different, and probably easier to
figure out. The spoilers are actually "destroying" the lift from an
area of the wing, conseguentely transferring it to the remaining of
the span. So the ailerons are more charged e more effective. You can
note this transfer of lift if you observe a glider on final,
preferably one with a large span or a flexible wings (like an ASW20):
everytime the spoilers are extended the wings bend up, and if
retracted they flat down, and this withoud any change on the g's.
Of course now I'm looking forward for a really technical
description...
Miriano
John Giddy
"miriano" <mir...@ravazzolo.org> wrote in message
news:4f91360e.02063...@posting.google.com...
|
| If you can accept a layman description... Regarding the
negative
| flaps, it seems that when you're on the ground the wing's
angle of
| attack is very high, so that at low air speed it's
actually stalled.
| By using negative flaps you "reduce" the angle of attack,
thus somehow
| anticipating the moment where the wing starts flying.
I agree with most of your explanation except the discussion
above of angle of attack.
Stalling is a function of true angle of attack, not
airspeed. My approach to the use of negative flap reduces to
the same argument you used regarding spoilers:
i.e. that the lift of the wing is reduced, thus any changes
at the aileron produce a bigger *proportional* effect.
To get back to the stall argument, there is a belief that
there is a "stall speed" below which the wing will be
stalled. This can be demonstrated.
However this speed is dependent on the wing loading (All-up
weight of the glider/pilot combination). What actually
happens is that as you reduce the speed, to keep a
reasonable glide angle, you are increasing the angle of
attack until at some speed the required angle of attack
reaches the limit and the stall occurs.
Remember that it is the true angle of attack we are talking
about, not the angle made between the wing chord and the
apparent (to the pilot) path of the glider. In other words,
as the glider gets slower, the sink rate starts to increase,
so the actual air flow direction is pointed further upwards,
giving a larger angle between the direction of airflow and
the wing chord than you might expect.
Discussion anyone ?
Cheers, John G.
Centurion
> To get back to the stall argument, there is a belief that
> there is a "stall speed" below which the wing will be
> stalled. This can be demonstrated.
> However this speed is dependent on the wing loading (All-up
> weight of the glider/pilot combination). What actually
> happens is that as you reduce the speed,
> Discussion anyone ?
How about this... at zero G you cannot stall any wing. Think about it. 0G
and 0kts = wing not stalled. 0G and Mach3 = wing not stalled.
You are absolutely correct in your statement about a 'stall speed'. This
speed is only relevant in a given flap/spoiler configuration at a fixed wing
loading (read g-load). Depart from any of those parameters and the wing
will stall at a different speed. I've stalled an aircraft at Va to prove a
point to a student.....+6G was not fun for either of us!
$0.02
--James
Pat Russell
Miriano -
Your layman description is quite excellent. Don't change a word.
-Pat
Larry Goddard
John,
Taking the example or situation originally described, ie, aileron
control at takeoff, the glider (as far as wing loading is concerned) is
at 0 g's... there is no wing loading initially. It only gradually
builds as the glider picks up speed.
Larry Goddard
"01" USA
Bert Willing
--
---------------------
Bert Willing
Calif A21S
Come fly at La Motte du Caire in Southern France
http://perso.wanadoo.fr/cunimb/index.htm
"Centurion" <spam...@nowhere.com> a écrit dans le message de news:
o35pfa....@10.0.0.1...
> "John Giddy" wrote...
Robert Ehrlich
I disagree with both explanations.
For the case of flaps: although the wing's angle of attack is high, it is below
stall angle of attack. Stall is not a question of speed but of angle of attack.
Precisely the stall angle of attack is the angle of angle of attack where the
lift coefficient reaches its maximum value, so that a further increase in angle
of attack results in a decrease of the lift coefficient. The notion of stall speed
is only related to this as far as the plane is airborne, since any decrease of
speed must be compensated by an increase in lift coefficent in order that the
lift balances the weight of the aircraft, and when the angle of maximum lift coefficient
is reached, no further reduction of speed is possible while maintaining the
balance of forces. No sailplane when sitting on its landing gear on the ground
has an angle of attack equal or very close to the stall angle. However, as the
angle is nevertheless higher than in normal flight, the increase of lift coefficient
as a function of aileron deflection may be not so steep as at lower angles of
attack, so reducing it may help. This is especially true on sailplanes where
a coupling between flaps and ailerons let the ailerons drop with the flaps, although
with less deflection, or on sailplanes with flaperons. But even with ailerons
independant from flaps, negative flaps decrease drag and so allow a better acceleration
and the speed where ailerons work is reached earlier. Another reason for negative
flaps during initial roll with ailerons coupled to flaps is that in this lowered
position increased by the attitude on the ground, moving them, beside generating
changes of lift, also genarate huge changes of drag and so much adverse yaw.
I experienced personnally on a LAK-12 during landing that with flaps in landing
position, this effect is so important that the rudder is unable to counter it.
For the case of spoilers: again this explanation is only valid when the ship is
airborne, so that the lift must balance the weight. It then happens because the
lack of lift produce an increased sink, i.e. an increase of angle of attack until
the balance of forces is reached again. On the ground, if you cancel some part
of the lift, you simply loose is, nothing compensate it. The explanation is probably
rather in the fact that the airstream is forced to accelarate around the spoilers
and some of this accelarated airstream flows along the ailerons. However this may
not be true for all ships. I read in the flight manual of the ASW24 that extending
the airbrakes helps the efficiency of the ailerons, but nothing similar is said for
the LS4 and my own feeling is that the effect is opposite, during the ground roll
after landing I find it is easier to keep its wings level with airbrakes retracted.
This is consistent with the fact that on this glider there is some overlap between
airbrakes and ailerons, so with airbrakes extended, some part of the ailerons is
in turbulent flow behind the airbrakes.
My conclusion is that the reasons may be various or inexistant and only the
flight manual of the glider gives the good answer and maybe the explanation.
E.g. the flight manual of the LS6 says keep flaps at +5 from the beginning of
the ground roll when taking off.
Marty Pautz
I own an LS3. This is the ship with full span flaperons. For some time,
I used the negative flap method and moved the flaps from -7 position to
zero, then to +10 degree. The +10 degree is the recommendation in the
manual.
I really couldn't tell you why I did this, other than other people telling
me that this was the thing to do. Then, I had a life changing
experience. As I transitioned from -7 degrees, one wing dropped and hit
the ground. Before I could realize it, I was sliding backwards down the
runway. I came up with a couple of presumptions as to why, but the "why"
part doesn't matter. I now follow the instruction manual and do not have
a problem with a lack of control.
Marty Pautz
Stephen Cook
----- Original Message -----
From: "Robert Ehrlich" <Robert....@inria.fr>
Newsgroups: rec.aviation.soaring
Sent: Monday, July 01, 2002 7:18 PM
Subject: Re: Aileron control with negative flaps or spoilers on initial roll
> miriano wrote:
> >
> > d.bi...@verizon.net (David Bingham) wrote in message
news:<7e709dac.02062...@posting.google.com>...
> > > I would like to understand why the use of negative flaps or spoilers
> > > provide better aileron control on initial roll (low air speeds). It is
> > > not intuitively obvious why to me. Please point me to past
> > > publications or articles that cover this subject. Thanks.
> > > Dave
> >
> > If you can accept a layman description... Regarding the negative
SNIP
> > Of course now I'm looking forward for a really technical
> > description...
> >
> > Miriano
>
> I disagree with both explanations.
>
> For the case of flaps: although the wing's angle of attack is high, it is
below
> stall angle of attack. Stall is not a question of speed but of angle of
attack.
SNIP
>
> My conclusion is that the reasons may be various or inexistant and only
the
> flight manual of the glider gives the good answer and maybe the
explanation.
> E.g. the flight manual of the LS6 says keep flaps at +5 from the beginning
of
> the ground roll when taking off.
I aggree (I disagree with the original explanation). I asked this question
when I first started gliding and the answer I got ties in with an
explanation in Derek Piggott's book Understanding Gliding. (p183).
Flaps (and ailerons when going down) increase lift for the first 8-10 deg of
deflection. Thereafter the increase in lift is minimal but the increase in
drag is significant. Many gliders have coupled flaps and ailerons where
selecting flap causes the ailerons to go down as well. Some gliders such as
the LS6 have flaperons where the entire surface works as both flap and
aileron. If you've got the flaps down, further deflection of the surface
for down aileron isn't going to increase the lift on that wing significantly
because the deflection will be approaching or beyond 8-10 deg. Hence the
reduced aileron effectiveness with flap selected.
And something else from the LS6 manual which I guess is related. If you
discover you have taken off with one flaperon disconnected you must
select -5deg flap in order to maintain control.
Stephen
Finbar
I can touch down on the tailwheel first with full landing flaps,
without stalling. When sitting on the runway for takeoff, the wing is
at a lower angle of attack with both wheels on the ground than it
would be in a shallow descent with the tailwheel lower than the main.
At flying speed, therefore, with both wheels on the ground, the wing
is certainly not stalled, even with landing flaps down.
I can certainly confirm that negative flaps produce a huge improvement
in aileron effectiveness at low speeds (at least on my DG-202), so
there's something to the technique. Negative flaps, of course, reduce
the tendency for pressure to rise on the upper aft portion of the
wing, and therefore reduce the likelihood of flow separation on the
aft portion of the wing - a stall. Raised spoilers probably do
something different - create a sizeable vortex near each end, pulling
high-speed air down onto the upper surface and, again, helping to keep
flow attached at least in some places on the outboard wing, where the
ailerons are. So, in both cases, the techniques sure look like
they're addressing flow separation on the upper surface of the
ailerons. It looks like the wing is stalled.
We have an apparent contradiction: the wing cannot be stalled, yet it
appears to act that way.
I'm wondering whether the contradiction is resolved by the very
different Reynolds numbers early in the takeoff run vs at flying
speed. I seem to remember the stalling angle of attack is a function
of Reynolds number - it's lower at lower Reynolds numbers. The
Reynolds number (RN) would be lower at low speeds - is the stalling
angle of attack that much lower at low speeds? I've not yet found any
actual details on how much lower it might be. If the effect were
large, it would account for the flow separating in front of the
ailerons at low speeds (stalled) but not at flight speeds (same angle
of attack, but not stalled).
Does anyone know how RN affects stalling angle of attack?
John Giddy
"Larry Goddard" <la...@siriusimages.com> wrote in message
news:3D204C22...@siriusimages.com...
| John,
|
| Taking the example or situation originally described, ie,
aileron
| control at takeoff, the glider (as far as wing loading is
concerned) is
| at 0 g's... there is no wing loading initially. It only
gradually
| builds as the glider picks up speed.
|
| Larry Goddard
| "01" USA
I agree Larry,
However I don't think it changes the argument, as the angle
of attack is set by the dimensions of the aircraft, and the
airflow direction is exactly parallel to the ground. If the
designer set the wing incidence at a large angle, the wing
would be at stall at all speeds and would never take off.
My point is that as the speed builds up, negative flap or
extended spoilers change the lift distribution so that
relatively more lift occurs at the outer part of the wing
where the ailerons are placed, thus giving them a greater
proportional effect than with a clean wing.
Cheers, John G.
Greg Arnold
I used to have an LS-3 with the full span flaperons. In negative flap, the
'3 stick movement from side to side is limited. So you get the benefit of
negative flaps, but you lose the benefit of full aileron deflection. If I
remember correctly, my manual said to start the takeoff run with zero flaps,
and then move to positive.
Steffen Engel
At first, there is better aileron control due to negative flapsetting.
There are two reasons for it:
1. you lower the angle of attack. Due to this, the wing gets unstalled
earlier (as with flaps positive). Stall is a function of angle of attack and
with lower speed, the maximum angle of attack is much lower than with higher
speed (energy of flow is less, so the air can't follow surface, low
reynoldsnumber with separation of flow). Additional there is a histeresis in
maximum angle off attack: the glider can fly with this angle of attack
unstalled, but only when coming from low angle of attack.
2. the aileron is in higher energy flow (increase of velocity on the upper
surface)
These points are matter of fact.
The second is deployment of brakes. The flow around brakes consists of two
parts: Flow over and through the brakes and a a side flow around the brakes.
The side flow to the outer wing side improves flow to a part of the aileron
leading to better efficiency of the aileron.
This effect is dependant to the interlap of aileron and flap: the higher the
interlap, the larger part of the aileron is covered with turbulence and less
efficiency, lowering the effect of increasead flow to the outboard aileron.
hope my english is not to bad to explain :-)
Bye, Steffen
Robert Ehrlich
> ...
> Second Efffect:
>
> This effect is seldom discussed, yet I think it may be at
> least as important as the first one above. Further - it has
> the same basic explanation for both flaps and airbrakes - a
> reduction in the lift the wing produces, either by reducing
> AOA (flaps) or by spoiling lift (brakes).
>
> Imagine that the aileron lift differential is unchanged
> between brakes out and brakes in (or flaps neg/pos). We can
> think of trying to raise one wing, and producing a maximum
> Torque about the fuselage due to Aileron deflection. We'll
> call that torque TA.
>
> Now imagine that there is a differential airflow across the
> wings. Perhaps a gust hits one wing, but not the other, or
> the towplane is off to one side and the propblast hits only
> the wing on that side. By placing the flaps negative or by
> extending the airbrakes, we reduce the extra lift generated
> by the wing exposed to the increased airflow and we reduce
> the torque that the asymmetric lift produces about the
> fuselage. If we reduce that torque below TA, the ailerons
> can overcome it.
>
> For example, if a gust that hits the right wing causes that
> wing to produce 10% more lift (brakes in or flaps positive),
> and if we can reduce the AOA or spoil lift on the right by
> 50%, we would get only a 5% increase in lift on that side
> and the asymmetric airflow would have a reduced effect.
>
Yes, I completely agree with this explanation. For airbrakes, the
differential in drag is also increased, improving yaw stability.
I already heard it as a reason for which you should keep airbrakes
open when inadvertently caught in a cloud: beside limiting your
speed, yaw and roll stability are increased, because of the loss
of external references you can only rely on this stability to
avoid abnormal attitude.
Robert Ehrlich
Even if low RN reduce the stalling angle of attack, I doubt that any glider
reaches it when sitting on the ground. Your post shows a confusion that is
frequently encountered between separation and stall. Of course both things
are related. But separation is a progressive thing which starts in a narrow
region near the trailing edge and this region grows when the angle of attack
grows beyond some value well below stall angle. As an effect of this, the lift
coefficient, which is increasing quite linearly with angle of attack at low
angles, increase more slowly when separation grows and finally reaches a
maximum beyond which it can no more grow and this is the stall. So there
is no contradiction, you can have flow separation but this is not a stall.
I remember just having had a look in the appendix of the book of Martin
Simmons "Model Airplane Aerodynamics" on the graphs of Cl vs angle of
attack for various airfoils at various RN. As far as I remember the curve
for high RN is quite a straight line up to an angle just below the
stall angle where it ends with a short hook, while the curve at low RN, which
is below the former one (lower Cl at same angle) is rather a curve with
up convexity, i.e. its slope decreases progressively when approaching the
maximum, which may be reached at a higher angle. So even if the stall may
be reached at a higher angle, the separation seems to begin at a lower
angle.
Mark Navarre
ASW-20 when in the first notch of negative flap. Coincidentally this is the
setting recommended in the manual for the beginning of the takeoff roll with a
transition to positive flap at approx 30 kts. Other procedures result in wing
dropping more often on my glider. Funny thing is that almost always it's the
left wing that drops. And it seems to circle to the left easier as well.
The original question in the thread was why negative flap works
better......don't know......it just does.
Mark Navarre
-
Roger Druce
the flapped fixed undercarriage Janus B & C models and the later model
flapped Janus CE with its retractable undercarriage. I have around 1000+
hours on the B&C fixed undercarriage Janus and now around 300 hours on the
later retractable U/C Janus CE.
The wings are all the same except for the extra span with the C models (20m)
as compared to the B models (18.2m).
The fixed U/C B&C models definitely have more (= better response) aileron
authority during the take-off ground roll in zero degree flap setting than
the later retractable U/C Janus CE.
I make this comparison and all this commentary based on tail skid on the
ground (= nose up) initial ground attitude on take off, and not a nose wheel
on the ground attitude due to heavy pilots (which I ain't!).
With the fixed gear B&C models it is entirely satisfactory to use zero flap
for take-off in all circumstances except in an extreme crosswind, that is to
say the aileron control authority is satisfactory. This simpliifes things
by avoiding having to fiddle with the flaps on take-off.
However the retractable U/C Janus CE has weaker aileron control authority (=
weaker reponse) during initial ground roll in zero flap setting.
Accordingly I use negative flap on take-off more frequently when flying the
CE, that is to say when there is a modest or greater crosswind.
The key to this is that with the CE, the undercarriage geometry with the
undercarriage down extends the mainwheel further out than with the fixed
gear Janus, so that the fuselage and wing sit at a higher angle. The nose
sits markedly higher on the horizon with the CE when seated in the cockpit
on the ground than with the fixed U/C Janus B & C.
The higher the ground attitude, the more the inner flapped section of the
wing generates lift. If the lift is different one side to the other due to
crosswind & wing dihedral effect or tug aircraft slipstream offset in the
crosswind, then the more the differential lift to be overcome with aileron.
A solution, use more negative flap during initial ground roll to
reduce/remove the flapped inner sections of the wing from contributing to
differential lift one side to the other.
Roger Druce
"David Bingham" <d.bi...@verizon.net> wrote in message
news:7e709dac.02062...@posting.google.com...
Bob Gibbons
<Steffe...@epost.de> wrote:
>Hi David,
>
>> I would like to understand why the use of negative flaps or spoilers
>> provide better aileron control on initial roll (low air speeds). It is
>> not intuitively obvious why to me. Please point me to past
>> publications or articles that cover this subject. Thanks.
>
... portions of response deleted for brevity
>
>These points are matter of fact.
>
>The second is deployment of brakes. The flow around brakes consists of two
>parts: Flow over and through the brakes and a a side flow around the brakes.
>The side flow to the outer wing side improves flow to a part of the aileron
>leading to better efficiency of the aileron.
>
Since this subject comes up frequently, I have gone to Google and
copied the response I posted to the group 2 years ago to a similar
question, after talking with Dick Johnson.
I guess we really need a FAQ for this type of repeated question.
Bob
============================================================
From: Bob Gibbons (r...@tartarus.rsc.raytheon.com)
Subject: Re: Dropping a wing on takeoff
Newsgroups: rec.aviation.soaring
View: Complete Thread (63 articles) | Original Format
Date: 2000-11-02 21:18:53 PST
This topic, that is, why deploying spoilers seems to help roll control
at low speeds, has come up often in RAS during the
years. Unfortunately, we seldom seem to get responses from
knowledgeable aeronautical engineers as to the real cause for what is
generally acknowledged as a real effect.
Earlier today I asked Dick Johnson the cause of this effect. I will
try to summarize his reply, but I may miss some of the finer details,
so any aerodynamicists, feel free to correct me. BTW, for those who
are not familiar with Dick credentials, in addition to his long
history of contest successes, Dick is a professional aerodyamicist
with 50+ years of experience.
Dick's response was that the effect of increased roll effectivness
with spoiler deployment is real, and is caused by a 3-dimensional flow
effect initiated by the spoiler deployment disturbing the normal low
pressure area over the top of the wing in the area of the
spoilers. The breakup of the normal low pressure in the spoiler area
results in a lateral spanwise flow in the airfoil forward of the
ailerons. This lateral flow modifies the normal pressure distribution
on the airfoil forward of the ailerons and allows the airfoil in the
aileron section to operate at a higher angle of attack (without flow
separation, i.e., stalling) than would otherwise be possible, thus
allowing greater aileron effectivness during the takeoff roll.
The effect is more pronounced in standard class gliders since without
the benefit of flaps to modify the airfoil camber, standard class
airfoils are often set on the fuselage at a higher angle of attack
than the corresponding airfoil on a 15m flapped ship. Furthermore, 15m
ships, with interconnected ailerons, can decamber their airfoil
(select negative flap) and generate increased resistance to flow
seperation at the ailerons during the early ground roll.
Hope this helps, and, as I said, this is an restatement by a
non-professional, corrections by practicing aerodynamicists welcome.
Bob
=========================================================