Negative flaps for better low speed aileron control?

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.

Dave,

Sorry I don't know the aerodynamics (was trained in fluid dynamics of
bioreactors, not aerodynamics) only that it made a world of difference
on both my Nimbus 2 and Ventus B. The wings will drop on both without
it, where with it I can usually come to a full stop before the wing
will drop. It is definitely not a placebo. Try it in an 8-10 knot
breeze. Point the plane into the wind and try both positions.

Tim

It's mainly useful on gliders that sit at a high angle of attack on the
ground. At low speeds, the wing is stalled, reducing aileron
effectiveness as they are in the detached airflow; selecting negative
flaps decreases the effective angle of attack of the wing and can
provide some aileron effectiveness at low speeds by unstalling the wing
earlier.

Same thing while slowing down - negative flaps will delay the wing
stalling and losing aileron effectiveness.

It also helps to get the tail up as soon as possible, although by the
time you can raise the tail the ailerons will probably be
working...what some gliders really need is an extended (and
retractable) tailwheel - start the roll with the tail level, lower the
tail as speed increases.

Just joking (kinda).

Kirk
66

Going to negative on the ground run as you slow reduces
the angle of attack and stops the wing from stalling
on gliders that are prone to wing dropping.

Also reduces lift (AoA) and helps glider stick to the
ground, ie, no desire to keep flying as the wing can't
generate enough lift.

John

PS Some glider have ailerons that go negative (or do
not go as positive) as flaps are increased towards
landing, eg, Mosquito, ASH 25.

At 18:18 29 July 2005, wrote:

It also helps to get the tail up as soon as possible,
although by the
time you can raise the tail the ailerons will probably
be
working...what some gliders really need is an extended
(and
retractable) tailwheel - start the roll with the tail
level, lower the
tail as speed increases.

Just joking (kinda).

Ironically, the Slingsby Vega is the only glider I
know of with a retractable tailwheel and it's also
the only flapped glider I've ever seen with a severe
tendency to drop a wing on takeoff even in full negative
flap (caused by the huge main wheel).

All of the following applies only to gliders, that have a Flap-Aileron
connection, i.e. changing the flap position changes the aileron
position as well.
I can confirm that the ground-runs with negative flaps make a
difference with my Libelle H301. The AoA is the angle between the chord
line and the relative wind. Try this: look at the wing from the tip
towards the fuselage with the stick centered and the flaps full
positive. Draw a mental line from the leading edge to the trailing edge
of the aileron. The airflow (relative wind) will be parallel to the
ground during the ground roll. Now put the flaps up full negative and
repeat the above. The line is now at a shallower angle since the
leading edge is still where it was before but the trailing edge is up.
The thing that really helps to gain roll control on take-off with the
Libelle is to get the tail up and 'flying' asap. That way, the wing is
at a small AoA and the ailerons are effective right away.

Uli Neumann

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?

The flap/aileron interconnect has a lot to do with it.
With the help of another person;
Put the flaps in full negative then measure the max deflection of the
aileron (full up to full down).
Then put the flaps in full positive and again measure the max aileron
deflection.

With many sailplanes you will find that with negative flaps the interconnect
allows a greater range of aileron deflection.

Duane

T o d d P a t t i s t wrote: Once you are on the ground, with your front
and back wheels

rolling (or skid/wheel, etc.) the angle of attack remains

unchanged. Stalling is a function of the AOA, not your

airspeed. Stall is only indirectly related to airspeed when

your aircraft weight is supported by lift. On the ground

your weight's supported by your wheel(s), so you can go very

slowly and not be stalled. (Unquote).

I'm open to correction, but I think Todd is only right in the
circumstances that the glider stays level laterally.

As soon as a wing starts to drop, and the other to rise, the dropping
wing has a higher AoA and the rising wing a lower one. On a nil wind
day, with the glider having reached say 10 knots during the take-off run
(faster than the wingtip runner can run), a tip dropping at even 1 knot
has an extra AoA of 6 degrees - may well be enough to stall it with
neutral aileron. Putting that aileron down (to try to lift the tip) just
increases the AoA even more into its stall - hence, ineffective.
Negative flaps, if ailerons move in sympathy with them, can be enough to
unstall that downgoing tip.

By the way, what Todd says about non-linear CL curve makes entire sense
to me, and Duane's point about geometry, and others' comments. I just
think all effects are in play, at least on some gliders.

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)?

Chris N.

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What Todd says about angle of attack on the ground makes sense and is
absoulutely correct - the problem is that we are working with wings
well below their stall speeds, with weight supported by the landing
gear, so "angle of attack" takes on a different meaning: at low speeds
and "high angle of attack", the airflow around the wing is probably
highly turbulent, and control surfaces will be inefficient. Reduce
that "angle of attack" and the control surface is working in
undisturbed (or less disturbed) air, and should be more effective.
Raise the tail, and turbulence should be at a minimum.

I would love to see some wind tunnel testing of, say, a Pik-20 wing at
low (below stalling) speeds. I would bet a high tow that at the normal
ground attitude, on the ground (i.e. a ground plane below the wing,
weight supported by the gear), at low speeds (zero to 20 knots?), the
airflow over the wing is probably really turbulent, and that setting
negative flaps/ailerons moves those control surfaces closer to
"undistrubed" flow, giving them a little more effectiveness.

Remeber, this is a short, transitory phase of the development of
airflow and lift over the wing - I wonder if much study of this
"pre-flight" regime has been done.

Then again, I may be full of sh*t!

I do know it works on a Pik-20b, and that my LS6 doesn't really care
where my flaps are during takeoff (but I use negative anyway, and go
full negative on rollout - gets the flaperons out of the rocks!).

Kirk
66

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.

Another argument is that on some wings, the turbulence caused by the
spoiler might help attach some flow to the aileron.

In any case, then next time you're sitting in the glider waiting for a
tow and there's a bit of a breeze on the nose, try some experiments.
Have someone hold your wing level, then do some slight rolls with and
without spoilers. I have found that there is a slight but noticeable
increase in response with the spoilers open.

-Tom