Negative flaps for better low speed aileron control?

Geoff:
I also have a PIK-20B and have a question for you (or anyone in the group).

Given the PIK's high AOA when both wheels are on the ground, and given the
fact that going to full negative flap after touchdown will cause the tail
wheel to drop to the ground, do you attempt to keep the tail wheel up as
long as possible by applying forward stick when rolling up the flaps? As you
know, in a moderate headwind this is not likely an issue but in very light
wind or no wind, the abrupt loss of aileron control during the late stage of
rollout is irritating, especially if there is a crosswind component where a
mild groundloop is virtually guaranteed.

Paul
ZZ
"Geoff Vincent" wrote in message
...
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.

Ok, I'll buy that - makes sense. The explanation that the dropping
wing has a much higher momentary angle of attack also makes sense -
especially if your response is to bang on full aileron!

One hard-core solution is to fill up to the gills with water and let
inertia keep the wings level until aerodynamic control is reached -
just don't move that stick off the forward & center position until you
have enough speed to keep the wings level!

Only partially joking - heavy wings do seem less susceptible to wing
drops, in my limited experience.

Kirk
66

For what its worth, based in my 18 years and 2000+ hours in a PIK20B,
I usually landed with 45-60 deg of flap and held the ship off slightly
for a normal 2-point landing. I would then leave the flaps down for
the initial high-speed rollout (on both main and tail wheel) to aid in
decelleration. As the speed decreased, I would roll the flaps to full
up, which would firmly plant the tail wheel. Since the ship was
already rolling on both wheels from initial touchdown till flap
transition, there was no occasion to have the tail wheel "drop to the
ground" as you mention. In any significant crosswind, when I rolled
the flaps to full negative, I would also apply back elevator to help
keep the tail planted and assure directional stability.

Hope this helps.

Bob

On Tue, 02 Aug 2005 18:59:30 GMT, "P. Corbett"
wrote:

Geoff:
I also have a PIK-20B and have a question for you (or anyone in the group).

Given the PIK's high AOA when both wheels are on the ground, and given the
fact that going to full negative flap after touchdown will cause the tail
wheel to drop to the ground, do you attempt to keep the tail wheel up as
long as possible by applying forward stick when rolling up the flaps? As you
know, in a moderate headwind this is not likely an issue but in very light
wind or no wind, the abrupt loss of aileron control during the late stage of
rollout is irritating, especially if there is a crosswind component where a
mild groundloop is virtually guaranteed.

Paul
ZZ
"Geoff Vincent" wrote in message
.. .
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.

Paul,

Personally I found it a bit difficult to consistently 2-point the PIK
20 due to its inherent tendency to lift off again at the slightest
bump. Now I fly it onto the main wheel and move flaps smoothly to
full -ve using a little forward stick to ensure a soft tailwheel
touch-down. Once both wheels are on the ground full back stick
ensures that they stay there and minimises the risk of cross-wind
drift and ground loops. I agree that in nil-wind conditions it is
common for a wing drop in the final couple of metres but this is never
a problem.

On Tue, 02 Aug 2005 18:59:30 GMT, "P. Corbett"
wrote:

Geoff:
I also have a PIK-20B and have a question for you (or anyone in the group).

Given the PIK's high AOA when both wheels are on the ground, and given the
fact that going to full negative flap after touchdown will cause the tail
wheel to drop to the ground, do you attempt to keep the tail wheel up as
long as possible by applying forward stick when rolling up the flaps? As you
know, in a moderate headwind this is not likely an issue but in very light
wind or no wind, the abrupt loss of aileron control during the late stage of
rollout is irritating, especially if there is a crosswind component where a
mild groundloop is virtually guaranteed.

Paul
ZZ
"Geoff Vincent" wrote in message
.. .
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 the subject of roll authority early on ground roll how about some comments about standard class ships starting ground roll with air brakes deployed? My ASW 28 definitely benefits from this procedure. I have heard various rational for the practice but am curious about other comments.

Thanks,

Bob

Read back in this thread for the effect of spoilers on aileron
authority at low speed.

-Tom

Sounds like this might be a lot like the Glasflugel Mosquito. If you do a 2
point landing any bump will have you airborne. If land tail first though,
that doesn't happen.

on topic.... full -ve at the beginning of the roll & move quickly to the
first position, then neutral for lift off. No idea why that works best. It
does, so I do it.

"Geoff Vincent" wrote in message
...
Paul,

Personally I found it a bit difficult to consistently 2-point the PIK
20 due to its inherent tendency to lift off again at the slightest
bump. Now I fly it onto the main wheel and move flaps smoothly to
full -ve using a little forward stick to ensure a soft tailwheel
touch-down. Once both wheels are on the ground full back stick
ensures that they stay there and minimises the risk of cross-wind
drift and ground loops. I agree that in nil-wind conditions it is
common for a wing drop in the final couple of metres but this is never
a problem.

On Tue, 02 Aug 2005 18:59:30 GMT, "P. Corbett"
wrote:

Geoff:
I also have a PIK-20B and have a question for you (or anyone in the
group).

Given the PIK's high AOA when both wheels are on the ground, and given
the
fact that going to full negative flap after touchdown will cause the tail
wheel to drop to the ground, do you attempt to keep the tail wheel up as
long as possible by applying forward stick when rolling up the flaps? As
you
know, in a moderate headwind this is not likely an issue but in very
light
wind or no wind, the abrupt loss of aileron control during the late stage
of
rollout is irritating, especially if there is a crosswind component where
a
mild groundloop is virtually guaranteed.

Paul
ZZ
"Geoff Vincent" wrote in message
.. .
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.

Hi Gang
I have received an enormous number of responses to my original posting
the use of negative flaps to improve effective aileron control while on
the ground. I think I now have a reasonable understanding of the issues
involved and would like to present a summary of my understandings.
Probably the worst situation occurs with flaperons whereby the flaps
are combined with the ailerons from the wing tips to the root of the
wings. I own 2 gliders - a Stemme S10 VT and a DG800B that have these
flaperons and these 2 ships are difficult to handle in gusty conditions
both on the initial role and breaking on landing. Definitely not entry
level gliders!
So lets make sure we first agree on some of the fundermentals:

1) The angle of attack AOA of the wing is defined by the angle between
the cord of the wing and the effective direction of the flow of the air
that the wing is moving in.

2) The stall of a wing (or a plank of wood) occurs when the AOA reaches
about 17 degrees IRRESPECTIVE, to the first order, of the speed that
the wing is moving through the air whether it be 5mph or 150mph. For
almost all wings ever invented the stall AOA is between 16 and 19
degrees. This is a remarkable fact - not intuitively obvious. Onset of
stall is loosely defined as a dramatic drop of lift.

3) As the AOA of a wing is increased from zero lift also increases
somewhat linearly to an AOA of about 8 degrees. Then the rate of
increase of lift decreases for a further increase of AOA and lift
reaches a maximum at around 12 degrees (minimum sink). At AOAs greater
than 12 degrees lift then diminishes at an ever increasing rate so that
around 17 degrees lift is a small fraction of what is was at 12
degrees. Note that, AND THIS IS EXTREMELY IMPORTANT, except at minimum
sink there are 2 AOA values that will give the IDENTICAL value of lift.
I will show this to be the Archille's Heel for many of our low speed
control problems.

OK now a typical situation with a flaperon ship such as my Stemme on
initial role say with 5 degrees of flaps. The tail wheel is on the
ground and the Stemme, because of its high undercarriage, is pointing
its nose upward. The AOA of the wings are around 12 degrees near
minimum sink. A gust hits me from the side and a wing drops. I react by
full aileron usage and the wing I am trying to lift now has an
effective AOA of 16 or 17 degrees whereas the opposite wing has an AOA
of 7 or 8 degrees. Which wing has the highest lift? The stalled wing
or the one with the AOA of 8 degrees? THE UNSTALLED WING has the
highest lift! In other words the the aileron control has reversed
itself and I am aggravating the problem rather than solving the
problem. If I am unlucky the wing that the gust has hit will itself hit
the runway through my over reaction with the ailerons. What should I
have done? 2 things - the first started off with negative flaps and
secondly have been gentler on the aileron control. In so doing the AOAs
of both wings would have been less than 12 degrees (minimum sink) and
aileron control would be normal not reversed.

Lets recap for a moment. What I am saying is that, if the AOA is
around 12 degrees (minimum sink), and you use use full aileron
deflection, you have a good chance of reversing the operation of the
ailerons. On the ground that means loss of control and in the air the
very real possibility of a spin. All this because there are 2 values of
AOA that give the same value of lift. The only exception is exactly at
minimum sink.

OK What to do? Clearly if you have flaperons use negative flaps for
the initial roll until the tail comes up and then go to whatever the
book says (normally 5 or 8 degrees positive). On landing do what the
book says and then on braking go to full negative flaps. Avoid large
movements of the ailerons. Don't over react!

I would recommend for all gliders that you be aware of the effective
AOA of the wings with the tail wheel on the ground. Some gliders such
as the Mosquito, I am told, are in a stalled condition until the tail
is raised. Again small movements of the ailerons are called for and get
that tail wheel up as soon as possible. Know your glider.
Dave

PS It is claimed that spoilers improve low speed aileron control. Well
maybe. How about someone figuring out in a rational manner if this is
so. I would be real interested in the science if it were shown to be
true.

It seems to me that the various comments on this subject
are confused. If there is any kind of twist in the
wing - aerodynamic or otherwise, then each part of
the wing has its own angle of attack. Moving an aileron
changes the angle of attack only of that part of the
wing (plus whatever disturbance it creates for a short
distance inboard).

The original question was about what effect flaps had
on aileron efficiency. I presumed this to mean a wing
with no interconnection between flaps and ailerons
and definitely not a wing with flaperons. With such
a wing in mind, it appears to me that any effect the
flap setting would have on the aileron would be the
disturbance the flap causes at its outboard end and
across the inboard end of the aileron. I'd like to
read something about that. (I'm no aerodynamicist,
and have no mathematical skill.)



At 13:54 05 August 2005, T O D D P A T T I S T wrote:
' wrote:

Probably the worst situation occurs with flaperons


I'm not sure what you mean by 'worst situation.' My
Ventus
C has flaperons. It does have reduced aileron effectiveness
at low speed when the flaps are positive or zero, but
not so
much that it's uncontrollable. I flew it that way
for my
first few flights. Putting the flaps in negative position
improves aileron effectiveness significantly.

So lets make sure we first agree on some of the fundermentals:

2) ... Onset of stall is loosely defined as a dramatic
drop of lift.

Not really. Stall is at maximum lift, and lift drops
off
moderately after that. The big difference is that
at AOA
above stall, the lift decreases with AOA. When flying,
this
means that beyond stall, the nose drops, the wing descends,
which increases the AOA even more, which reduces lift
more,
which decreases lift more, etc. This runaway decrease
in
lift is why so many think that stalling means that
lift
drops to zero or near zero at stall. It's actually
at the
maximum there and just beyond stall..

On the ground, this effect is different, as the weight
of
the aircraft is not supported by the air, so it can't
drop
and thereby increase the AOA in the same way it drops
in
the air.

3) As the AOA of a wing is increased from zero lift
also increases
somewhat linearly to an AOA of about 8 degrees.

Yes.

Then the rate of increase of lift decreases for a further
increase of AOA

I agree - the 'rate of increase' decreases. This is
the
nonlinearity of the CL curve I discussed.

and lift reaches a maximum at around 12 degrees (minimum
sink).

No. Lift is max at around 17 degrees - at the critical
AOA
(stall angle).

At AOAs greater
than 12 degrees lift then diminishes at an ever increasing
rate so that
around 17 degrees lift is a small fraction of what
is was at 12
degrees.

This is wrong. Lift increases smoothly to its maximum
up
to 17 degrees. The rate of that increase varies, but
it's a
positive rate up to the critical angle and then the
'rate
of increase' is zero and it's about to turn negative.

Note that, AND THIS IS EXTREMELY IMPORTANT, except
at minimum
sink there are 2 AOA values that will give the IDENTICAL
value of lift.

No. Lift is a function of airspeed and AOA (and air
density,
which we can ignore) There are an infinite number
of AOA
values that give the same lift. You tell me the AOA
and
lift you want, and I'll calculate the airspeed.

I will show this to be the Archille's Heel for many
of our low speed
control problems.

OK now a typical situation with a flaperon ship such
as my Stemme on
initial role say with 5 degrees of flaps. The tail
wheel is on the
ground and the Stemme, because of its high undercarriage,
is pointing
its nose upward. The AOA of the wings are around 12
degrees near
minimum sink. A gust hits me from the side and a wing
drops. I react by
full aileron usage and the wing I am trying to lift
now has an
effective AOA of 16 or 17 degrees whereas the opposite
wing has an AOA
of 7 or 8 degrees. Which wing has the highest lift?
The stalled wing
or the one with the AOA of 8 degrees? THE UNSTALLED
WING has the
highest lift!

No, the wing at 17 degrees has the highest lift. In
fact,
it will have the higher lift, even if it's stalled
at 18
degrees. ( I should mention that you can't just assume
that
the aileron changes the AOA of the wing. Lowering
the
aileron changes the camber of the wing, which produces
a
different airfoil having a different CL curve.

In other words the the aileron control has reversed
itself and I am aggravating the problem rather than
solving the
problem.

No. Control reversal does not occur on the ground.


If I am unlucky the wing that the gust has hit will
itself hit
the runway through my over reaction with the ailerons.
What should I
have done? 2 things - the first started off with negative
flaps

Yes.

and secondly have been gentler on the aileron control.
In so doing the AOAs
of both wings would have been less than 12 degrees
(minimum sink) and
aileron control would be normal not reversed.

No, although the earliest possible response is best,
using
the least aileron required to do the job.

Lets recap for a moment. What I am saying is that,
if the AOA is
around 12 degrees (minimum sink), and you use use
full aileron
deflection, you have a good chance of reversing the
operation of the
ailerons.

No.

On the ground that means loss of control

No.

and in the air the very real possibility of a spin.


'In the air' is a much different condition.

All this because there are 2 values of
AOA that give the same value of lift. The only exception
is exactly at
minimum sink.

No.

OK What to do? Clearly if you have flaperons use negative
flaps for
the initial roll until the tail comes up and then go
to whatever the
book says (normally 5 or 8 degrees positive). On landing
do what the
book says and then on braking go to full negative flaps.

Agreed.

Avoid large
movements of the ailerons. Don't over react!

You shouldn't need full aileron, and I agree overreaction
is
bad, but if you need full, then use it. It may not
be
enough, but don't expect more control from less aileron.
You won't get it.



T o d d P a t t i s t - 'WH' Ventus C
(Remove DONTSPAMME from address to email reply.)

The original question was about what effect flaps had
on aileron efficiency. I presumed this to mean a wing
with no interconnection between flaps and ailerons
and definitely not a wing with flaperons.

I've flown a 15-meter glider with no *rolling* connection between flaps and
ailerons since 1981. By 'no rolling connection' I mean only ailerons impart
roll, regardless of flap position...the flaps lack capability for
differential movement. On this particular ship, assuming neutral
roll-stick, at zero and negative flap settings the entire trailing edge is
in-line (i.e. ailerons and flaps camber-track identically). During the
camber-changing portion of positive flap deflection, the flaps droop twice
as much as the ailerons. Once the ailerons have reached 'max droop' they
remain there while the flaps continue down to ~75-degrees for glide path
control.

From the pilot's perspective, when the ship is on the ground, at
low/early-in-takeoff-roll/post-landing-rollout speeds, the ailerons are
distinctly most effective with flaps negative, less so with flaps neutral
and worst with flaps positive. (Experimenting - ahem, 'roll playing!' -
with a stationary glider, whether flapped and/or spoilered, etc., in a
steady headwind is recommended for the curious. If 'it' happens - i.e.
altered aileron effectiveness - it must be possible.)

For the purposes of the following discussion, I'll take the paraphrased
question about 'what effect flaps had on aileron efficiency' to mean 'what
effect flap position has on perceived aileron effectiveness at
low-relative-wind speeds.'

Though there may be an exception or two out there (I know of none), in
general, negative flaps early in the roll make ailerons 'more effective.'
One way to view why this is so is inertially. In a given state (i.e.
un/partially/fully ballasted), any glider has its minimum roll inertia at
zero airspeed. Any lift created by air flowing past the wing effectively
increases roll inertia by 'stiffening' the glider in roll, meaning aileron
effect will be diminished from what it would be if (say) the air flowed ONLY
over the wings over the span of the ailerons. In other words, anything that
can be done to minimize lift produced from the aileron-less portion of the
wings is to the relative good of maximizing the ailerons' roll
effectiveness. I suspect this relative reduction in roll inertia is the
largest contributor to the 'improved aileron effect.' (Consider fully
ballasted roll inertial effects for example...assuming an equal and
nominally good wing run, which is more likely to drop a wing, a ballasted or
an unballasted otherwise identical glider? For skeptics who claim I'm
confusing aerodynamic effects with mass/inertial effects, F=ma.)

With such
a wing in mind, it appears to me that any effect the
flap setting would have on the aileron would be the
disturbance the flap causes at its outboard end and
across the inboard end of the aileron. I'd like to
read something about that. (I'm no aerodynamicist,
and have no mathematical skill.)

Any break in the trailing edge between flap and aileron will indeed have
some 2nd-order (e.g. transverse airflow) aerodynamic effects beyond the
1st-order (i.e. 2-dimensional-flow-based) ones...exactly zero of which will
be distinguishable to the pilot in the cockpit. Additionally, Todd
Pattist's conception of what's happening when viewing things from the
perspective of the lift-curve vs. angle-of-attack is on-target...but again,
the extent a pilot in the cockpit actually *uses* these sorts of thoughts in
the heat of the moment is debatable. Prioritizing things, it seems to me
what matters to a (non ab-initio) PIC's perspective is: 1) s/he conceptually
grasps 'what's likely to happen when,' maximizing chances of remaining ahead
of the plane and doing the right things in a timely manner, prior to 2)
comprehending 'technically why'. If one's grasp of 'technically why' is
missing, incomplete or downright inaccurate at mathematical or scientific
levels, it matters not so long as it doesn't interfere with 'conceptual
reality,' in which case "No harm, no foul," applies.

For me, having some grasp of 'technically why' helps me remember 'conceptual
reality.' Ideally that grasp will also be accurate, of course!

Regards,
Bob W.