Negative flaps for better low speed aileron control?

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.