Negative flaps for better low speed aileron control?

I dont buy Chris's point about angle of attack changing as a wing drops =
on the ground, if the tail is on the ground there is no pitch change, =
atlthough it would be true banking into a turn at flying speed. =
Personaly I think the propwash and/or crosswind effect is much more =
important, given that the prop wash on most tugs affects the right =
wing, if that is tending to drop, it will lift in the propwash ( we use =
that affect in a downwing takeoff). Conversely a crosswind from the =
right and the propwash causing the left wing to drop can quickly get out =
of hand.=20


David Smith

5Z wrote:
Chris Nicholas wrote:

Now, why does opening the airbrake (usually called spoilers in the USA
for some unknown reason) help on some gliders? And did it ever help on
gliders that really did have spoilers (UK-speak for things that spoil
lift but don't add much drag, unlike airbrakes [in UK-speak], which do
both in spades)?


The best argument FOR this technique was presented in Soaring magazine
quite a few years ago.

At low speed the wing is producing some lift over the entire span.
This provides a damping effect to any rolling tendency. Imagine a
whole bunch of weak springs attached to the spar every few inches and
attached to the ceiling. The plane will resist a wing drop. Now,
remove the springs in the spoiler region and the glider will tip more
easily, as there are fewer springs working to keep it level.

So with the spoilers extended, the wing is easier to pick up as the
aileron has a little bit less force to fight.

The "spring" analogy doesn't sound right to me: it implies the wing is
attempting to hold the glider level, which is desirable. I suspect it is
the opposite: when a wing moves down a bit, it moves into slower moving
air nearer the ground, and loses some lift (and vice versa for the wing
going up a bit). This loss of lift tends to make that wing go down even
faster. Or if the wing is already down, it's definitely in slower moving
air compared to the up wing. The ailerons have to overcome this
disparity in lift and CG shift from the glider being tilted to the side.

So, with the spoilers out, the lift disparity is reduced (both wings
lose some lift), and it's easier for the ailerons to lift the wing off
the ground.

The above argument probably makes more sense when there is a wind. If
it's calm and the air motion is just the glider rolling on the ground
through the air, it might still make sense if the air near the ground is
more disturbed (somehow) the air several feet higher.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

David Smith wrote:

I dont buy Chris's point about angle of attack changing as a wing drops =
on the ground, if the tail is on the ground there is no pitch change, =
atlthough it would be true banking into a turn at flying speed. =
Personaly I think the propwash and/or crosswind effect is much more =
important, given that the prop wash on most tugs affects the right =
wing, if that is tending to drop, it will lift in the propwash ( we use =
that affect in a downwing takeoff). Conversely a crosswind from the =
right and the propwash causing the left wing to drop can quickly get out =
of hand.=20

The effects we're discussing also appear during landing in calm air, so
while crosswinds and tug wash affect the situation, they aren't
necessary for the wing dropping. A possile reason some people might
notice it as often on landing is they almost always have the spoilers
out (unlike takeoff).


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

I suspect there are many reasons why negative flap
and spoilers help roll response. In all cases when
there is no airflow over the wing unless the wings
are supported or they are very well balanced one will
drop. Airflow is required for the ailerons to have
any effect. As regards spoilers I suspect that their
use and resultant spoiling of lift results in the extremities
of the wing producing more lift than the inner parts
and of course the outer extrememities are where the
ailerons are resulting in the forces being greater
where there is more leverage. Even a very small airflow
will produce some lift and it is surprising just how
much even a gentle breeze can generate. Try weighing
your glider in a 5 knot breeze and then in the hangar
in still air, you may be amazed at the difference.

Negative flap will have the same sort of effect but
the premise that this is only the case where flaperons
as opposed to seperate flaps and aileron is in my experience
correct. The Kestrel 19, which has a separate land
flap was quite happy starting the roll if flying flap
which was also connected to the ailerons was left in
neutral and I found no problem with low speed roll
control. The ASW 17 is totally different and I find
requires full negative flap at the start of the take
off roll. An additional benefit is that the glider
will not leave the ground with negative flap connected
which means that the tailwheel can be firmly pegged
to the ground with full up elevator with no danger
of getting airborne helping to keep straight in a crosswind.

On a winch launch with the Kestrel one stage land flap
could be selected before takeoff, and the same is true
of the Janus A which has seperate flaps and ailerons,
not something I would care to do with the 17.

On landing the Kestrel full landing flap could be retained
for the landing roll with the flying flap in neutral,
with the 17 full negative is applied carefully after
touchdown. Sudden application of full negative can
cause both wingtips to strike the ground. I have never
experience a problem with wing drop on the landing
roll except in a significant crosswind. I suppose I
am really not sure why it works, I am only very glad
that it does.


At 17:00 30 July 2005, Eric Greenwell wrote:
David Smith wrote:

I dont buy Chris's point about angle of attack changing
as a wing drops =
on the ground, if the tail is on the ground there
is no pitch change, =
atlthough it would be true banking into a turn at
flying speed. =
Personaly I think the propwash and/or crosswind effect
is much more =
important, given that the prop wash on most tugs
affects the right =
wing, if that is tending to drop, it will lift in
the propwash ( we use =
that affect in a downwing takeoff). Conversely a crosswind
from the =
right and the propwash causing the left wing to drop
can quickly get out =
of hand.=20

The effects we're discussing also appear during landing
in calm air, so
while crosswinds and tug wash affect the situation,
they aren't
necessary for the wing dropping. A possile reason some
people might
notice it as often on landing is they almost always
have the spoilers
out (unlike takeoff).


--
Change 'netto' to 'net' to email me directly

Eric Greenwell
Washington State
USA

writes:

Will someone please explain why negative flaps supposedly provides
better aileron control. I know conventional wisdom says that it does
but WHY? It is not intuitively obvious at least to me. Yes I have
tried negative flaps at low speeds both on the roll and braking but
its effect as far as I could judge was marginal and my thoughts were
that it reminded me of a placebo. So please direct me to the
authorative articles on the subject or if there is a simple
explanation please educate me. Thanks.

With -ve flap, the AoA of the wing centre is less, so less lift unless
you increase the pitch angle. Net result is increased lift from the
outer wing sections, where the ailerons are, and reduced lift in the
centre. Hence the lift distribution is wider and more effective at
damping out roll.


--
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+61 (08) 9257-1001 Kalamunda.
West Australia 6076
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Raw, Cooked or Well-done, it's all half baked.
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wrote:
writes:


Will someone please explain why negative flaps supposedly provides
better aileron control.
snip

With -ve flap, the AoA of the wing centre is less, so less lift unless
you increase the pitch angle. Net result is increased lift from the
outer wing sections, where the ailerons are,

I don't think the lift in the outer sections would increase; instead, it
ought to stay the same (for gliders that don't move the ailerons with
the flaps) or decrease (in most gliders the ailerons also rise when
negative flap is selected).

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Aileron Authority & Flaps at Take-off and Landing.

Summary.

There are two types of launch.

Slow, such as aerotow, car and reverse pulley, when it is necessary to
control the glider at low speed, perhaps with a large crosswind component,
before the glider gains flying speed. It is necessary to start with the
controls set to give control at low speed, and perhaps to change the setting
as flying speed is gained.

Fast, such as winch and bungee (catapult), when the glider gains speed so
fast that it is not possible to change the control setting during the ground
run, and the glider should start with the control setting needed when first
airborne.

The problem.

We discovered the problem, the solution and the explanation at Lasham in the
early 70's when we started flying the Slingsby Kestrel 19.

At take-off we were in the two-point tail down pitch attitude.

In light winds on aerotow take-off, in neutral or thermal flap setting, and
especially when also cross-wind; we found that we had no lateral control at
the start of take-off. If a wing went down it stayed down. When the
airspeed was above about 30 knots we did have control, even if we were still
tail down.

The explanation.

Ailerons and lateral stability.

When we move an aileron down, we increase the Angle of Attack (AoA) at that
wingtip. This increases the lift at that wingtip PROVIDED the new, higher,
AoA is below the stalling AoA; the wing has lateral stability.

If the new, higher, AoA is above the stalling AoA the lift at the wingtip
will be reduced. The effect of moving the aileron down will be that the
wingtip goes down, the exact opposite of what the pilot intended. The wing
has lateral instability.

If the wingtip is at or above the stalling AoA with the aileron neutral, the
effect of moving the aileron down is immediate and marked, the wing goes
straight down.

Flying flap setting at takeoff.

The effect of moving the flaps (so far as affects flying the machine at
take-off) is to change the camber, i.e. as if we changed the angle at which
the wing is set on the fuselage. Since at take-off the glider is tail down
in the two-point attitude, this changes the AoA.

If the ailerons move with the flaps, then with the flaps down the neutral
aileron AoA will be higher than when the flaps are up, so we are more likely
to have lateral instability.

With the flap lever fully forward and the flaps fully up, we have the best
chance of lateral stability, the ailerons will work.

Change of stalling AofA with speed.

When we learn to fly, we are taught that the stalling AoA is the same at all
speeds, so that if we achieve the stall AoA at any speed, we will stall even
if the speed is high.

However, this is not true at very low speeds, due to Reynolds number
effects. At 30 knots the stalling AoA will be at the normal flying figure,
say 18 degrees. But at 5 or 10 knots the stalling AoA will be about 10
degrees. This explains why we found in our Kestrel 19s (in neutral flap)
we had no control at 10 knots and full control at 30.

This change of stalling AoA with speed explains why we need full negative
flap to have aileron control on take-off at low speeds, but can still have
full control with thermal flap setting at 30 knots.

The solution.

Aerotow.

Start the take-off run with the flap lever fully forward, flaps fully up
(fully negative). If you have a separate landing flap control (e.g.
Kestrel) this flap should also be up.

If using a C of G hook, it may be wise to start by holding the wheel brake
on to ensure that there is no overrun, this may mean taking up slack with
the air-brakes out; warn the launch point crew first!

If not holding the wheel brake, or as soon as you have let it off and locked
the air-brakes, the left hand should be touching but not holding the
release.

When you are sure you have full control and will not have to release, move
your left hand to the flap lever.

As the speed builds, move the flap lever back to the position you intend to
use when flying. If you start to lose aileron control, move the flap lever
forward again at once, because you moved it back too soon.

Start with the stick fully forward. Obviously, if you get the tail up, the
angle of attack is lower, and also the effect of gusts is reduced. Lower
the tail to the normal take-off attitude when the speed is high enough for
good aileron control.

Cable launching.

For car or reverse pulley launching, use the same method as for aerotow.

For winching, start with the setting you need once airborne. If the winch
and its driver behave as they should, the glider will not have time to drop
a wing, and you will not have time to move the flap lever. Use the same
method for bungee launching.

For winching with a Kestrel, use the half landing flap position (if fitted)
for launching, and neutral flying flap position; this setting will be
correct for an immediate landing after a low launch failure.

If it goes wrong.

If the wing goes down release at once. Do not hang on to see if you can
get the wing up.

If the glider does start to groundloop, it will happen so quickly that the
glider will be broken before you can release. If there is any appreciable
speed or wind, the groundloop will turn into a cartwheel, which will hurt
the pilot as well as the glider.

Remember, all the time the launch continues, energy is going into the
glider. If you lose control, this energy has to go somewhere.

Unflapped gliders.

Some unflapped gliders are very close to tip stalling (lateral instability)
at the start of the ground run. There are two strategies to try.

Stick forward.

Start with the stick fully forward. Obviously, if you get the tail up, the
angle of attack is lower, and also the effect of gusts is reduced. Lower
the tail to the normal take-off attitude when the speed is high enough for
good aileron control.

Airbrakes.

Start with the airbrakes open. This gives better lateral control; I don't
know why, but it seems to. If you want to start with the wheel brake on,
and it is worked by the air brake lever, you are going to have to do this
anyway. Clearly the tug pilot must be warned, and anyone at the launch
point who may give a stop signal must be told.

Use of rudder.

A sharp application of rudder makes the glider roll as well as yaw. This
can be used as a last resort if the wings are not responding to aileron;
this will put you out of line with the tug, but this can be sorted out when
you have aileron control. Obviously, this cannot be used if it runs you or
the tug off the runway or otherwise into trouble.

Landing.

Putting the flaps up after landing achieves two things. It dumps lift,
making it less likely that bumpy ground or a gust will put you in the air
again when you thought you had landed. It improves aileron control when
you are moving slowly, but this is less important than when taking off
because you are losing energy and speed not gaining it, and you can use the
wheel brake. You have to let go of the air brake to move the flaps, if you
have near flying speed they may close and cause you to take off again before
you get the flaps up; consider raising the tail to reduce angle of attack
until the flaps are up.

In a Kestrel it is the flying flaps which should go up, to increase aileron
authority.

Beware of using the wheel brake unless you are dead straight, if you are
turning or drifting it may provoke a ground loop.

Flight manuals.

In general one should always read and obey the flight manual. However the
Kestrel manual was written before we knew much of the above, and does not
reflect current knowledge and practice. There may be other types to which
this applies. Use full negative (fully up) flap for starting aerotow
take-offs!

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


wrote in message
oups.com...

Hi Group
Will someone please explain why negative flaps supposedly
provides better aileron control. I know conventional wisdom
says that it does but WHY? It is not intuitively obvious at
least to me. Yes I have tried negative flaps at low speeds
both on the roll and braking but its effect as far as I could
judge was marginal and my thoughts were that it reminded me
of a placebo. So please direct me to the authorative articles
on the subject or if there is a simple explanation please
educate me. Thanks.
Dave

PS Also posted on the Stemme Owners Group where there is a
thread running on the use of negative flaps for better control.

Hi Bill,

Your comments are spot on. I'm a PIK 20B jockey and always use full
-ve flap for take-off, whether I have a wing runner or it's a
wing-down launch. Landings are treated similarly, moving to full -ve
flap as soon as the mainwheel is on the ground to ensure maximum
aileron authority.

Regards,

Geoff Vincent
Grampians Soaring Club
Australia


On Mon, 1 Aug 2005 23:45:04 +0100, "W.J. \(Bill\) Dean \(U.K.\)."
wrote:

Aileron Authority & Flaps at Take-off and Landing.

Summary.

There are two types of launch.

Slow, such as aerotow, car and reverse pulley, when it is necessary to
control the glider at low speed, perhaps with a large crosswind component,
before the glider gains flying speed. It is necessary to start with the
controls set to give control at low speed, and perhaps to change the setting
as flying speed is gained.

Fast, such as winch and bungee (catapult), when the glider gains speed so
fast that it is not possible to change the control setting during the ground
run, and the glider should start with the control setting needed when first
airborne.

The problem.

We discovered the problem, the solution and the explanation at Lasham in the
early 70's when we started flying the Slingsby Kestrel 19.

At take-off we were in the two-point tail down pitch attitude.

In light winds on aerotow take-off, in neutral or thermal flap setting, and
especially when also cross-wind; we found that we had no lateral control at
the start of take-off. If a wing went down it stayed down. When the
airspeed was above about 30 knots we did have control, even if we were still
tail down.

The explanation.

Ailerons and lateral stability.

When we move an aileron down, we increase the Angle of Attack (AoA) at that
wingtip. This increases the lift at that wingtip PROVIDED the new, higher,
AoA is below the stalling AoA; the wing has lateral stability.

If the new, higher, AoA is above the stalling AoA the lift at the wingtip
will be reduced. The effect of moving the aileron down will be that the
wingtip goes down, the exact opposite of what the pilot intended. The wing
has lateral instability.

If the wingtip is at or above the stalling AoA with the aileron neutral, the
effect of moving the aileron down is immediate and marked, the wing goes
straight down.

Flying flap setting at takeoff.

The effect of moving the flaps (so far as affects flying the machine at
take-off) is to change the camber, i.e. as if we changed the angle at which
the wing is set on the fuselage. Since at take-off the glider is tail down
in the two-point attitude, this changes the AoA.

If the ailerons move with the flaps, then with the flaps down the neutral
aileron AoA will be higher than when the flaps are up, so we are more likely
to have lateral instability.

With the flap lever fully forward and the flaps fully up, we have the best
chance of lateral stability, the ailerons will work.

Change of stalling AofA with speed.

When we learn to fly, we are taught that the stalling AoA is the same at all
speeds, so that if we achieve the stall AoA at any speed, we will stall even
if the speed is high.

However, this is not true at very low speeds, due to Reynolds number
effects. At 30 knots the stalling AoA will be at the normal flying figure,
say 18 degrees. But at 5 or 10 knots the stalling AoA will be about 10
degrees. This explains why we found in our Kestrel 19s (in neutral flap)
we had no control at 10 knots and full control at 30.

This change of stalling AoA with speed explains why we need full negative
flap to have aileron control on take-off at low speeds, but can still have
full control with thermal flap setting at 30 knots.

The solution.

Aerotow.

Start the take-off run with the flap lever fully forward, flaps fully up
(fully negative). If you have a separate landing flap control (e.g.
Kestrel) this flap should also be up.

If using a C of G hook, it may be wise to start by holding the wheel brake
on to ensure that there is no overrun, this may mean taking up slack with
the air-brakes out; warn the launch point crew first!

If not holding the wheel brake, or as soon as you have let it off and locked
the air-brakes, the left hand should be touching but not holding the
release.

When you are sure you have full control and will not have to release, move
your left hand to the flap lever.

As the speed builds, move the flap lever back to the position you intend to
use when flying. If you start to lose aileron control, move the flap lever
forward again at once, because you moved it back too soon.

Start with the stick fully forward. Obviously, if you get the tail up, the
angle of attack is lower, and also the effect of gusts is reduced. Lower
the tail to the normal take-off attitude when the speed is high enough for
good aileron control.

Cable launching.

For car or reverse pulley launching, use the same method as for aerotow.

For winching, start with the setting you need once airborne. If the winch
and its driver behave as they should, the glider will not have time to drop
a wing, and you will not have time to move the flap lever. Use the same
method for bungee launching.

For winching with a Kestrel, use the half landing flap position (if fitted)
for launching, and neutral flying flap position; this setting will be
correct for an immediate landing after a low launch failure.

If it goes wrong.

If the wing goes down release at once. Do not hang on to see if you can
get the wing up.

If the glider does start to groundloop, it will happen so quickly that the
glider will be broken before you can release. If there is any appreciable
speed or wind, the groundloop will turn into a cartwheel, which will hurt
the pilot as well as the glider.

Remember, all the time the launch continues, energy is going into the
glider. If you lose control, this energy has to go somewhere.

Unflapped gliders.

Some unflapped gliders are very close to tip stalling (lateral instability)
at the start of the ground run. There are two strategies to try.

Stick forward.

Start with the stick fully forward. Obviously, if you get the tail up, the
angle of attack is lower, and also the effect of gusts is reduced. Lower
the tail to the normal take-off attitude when the speed is high enough for
good aileron control.

Airbrakes.

Start with the airbrakes open. This gives better lateral control; I don't
know why, but it seems to. If you want to start with the wheel brake on,
and it is worked by the air brake lever, you are going to have to do this
anyway. Clearly the tug pilot must be warned, and anyone at the launch
point who may give a stop signal must be told.

Use of rudder.

A sharp application of rudder makes the glider roll as well as yaw. This
can be used as a last resort if the wings are not responding to aileron;
this will put you out of line with the tug, but this can be sorted out when
you have aileron control. Obviously, this cannot be used if it runs you or
the tug off the runway or otherwise into trouble.

Landing.

Putting the flaps up after landing achieves two things. It dumps lift,
making it less likely that bumpy ground or a gust will put you in the air
again when you thought you had landed. It improves aileron control when
you are moving slowly, but this is less important than when taking off
because you are losing energy and speed not gaining it, and you can use the
wheel brake. You have to let go of the air brake to move the flaps, if you
have near flying speed they may close and cause you to take off again before
you get the flaps up; consider raising the tail to reduce angle of attack
until the flaps are up.

In a Kestrel it is the flying flaps which should go up, to increase aileron
authority.

Beware of using the wheel brake unless you are dead straight, if you are
turning or drifting it may provoke a ground loop.

Flight manuals.

In general one should always read and obey the flight manual. However the
Kestrel manual was written before we knew much of the above, and does not
reflect current knowledge and practice. There may be other types to which
this applies. Use full negative (fully up) flap for starting aerotow
take-offs!

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


wrote in message
oups.com...

Hi Group
Will someone please explain why negative flaps supposedly
provides better aileron control. I know conventional wisdom
says that it does but WHY? It is not intuitively obvious at
least to me. Yes I have tried negative flaps at low speeds
both on the roll and braking but its effect as far as I could
judge was marginal and my thoughts were that it reminded me
of a placebo. So please direct me to the authorative articles
on the subject or if there is a simple explanation please
educate me. Thanks.
Dave

PS Also posted on the Stemme Owners Group where there is a
thread running on the use of negative flaps for better control.

On Fri, 29 Jul 2005 17:31:08 UTC, wrote:

: Will someone please explain why negative flaps supposedly
: provides better aileron control.

Here's a guess.

At low speeds on the ground, the wings are very near their stalling
AOA. If on wing does start going down, for whatever reason, the
additional vertical movement may cause it to increase its AOA beyond
stalling. Hence much reduced lift on that side, lift still being
produced on the other side, rolling moment, wing goes down faster. An
unstable situation.

By using negative flap you are effectively reducing the AOA of both
wings. Now a downward movement is less likely to lead to stall, and
more likely to lead to an increase (but sub-stall) AOA at the same
speed, so more lift and back up comes the wing. A stable situation.

This is all guesswork, since I don't fly flapped gliders. I'd expect
my explanation to work best on gliders with high wing loadings and
relatively high angles of attack sitting on the ground. Does that fit?

Ian
--

On Sat, 30 Jul 2005 00:05:03 UTC, "
wrote:

: at low speeds
: and "high angle of attack", the airflow around the wing is probably
: highly turbulent

I doubt that. Turbulence is associated with high Reynolds number, and
that depends directly on speed. Low speed - low Re - less
turbulence.

Ian
--