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.