Negative flaps for better low speed aileron control?

wrote:
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.

I posted a description if this earlier in the thread - go back and take
a look.

Bottom line is that if the full span of the wing is producing some
lift,the roll is damped. If the spoilers are opened the lift of that
portion of the wing is reduced and so the roll is less damped and the
feeble aileron inputs have more effect on keeping the wings where you
want them.

-Tom

On Sat, 06 Aug 2005 15:10:05 +0100, W.J. (Bill) Dean (U.K.). wrote:

In all normal flight the maximum lift, i.e. stalling, AoA is constant.
However, at the very low speeds met with in light winds at the start of
take-off and the end of landings, the stalling AoA is very much less.

Whereas in all normal flight conditions, from 1G "stalling speed" to Vne,
the stalling AoA remains constant at a figure probably between 16 and 19
degrees, at 5 or 10 mph it is around 10 degrees.

This is interesting, it explains a lot. I would like to read up some
more. Do you know of any references (perhaps on the net, or Reichman) that
describe this effect?

Also is there any "hysteresis" in this effect. In other words, if an
airfoil is "flying" at say 12 degrees angle of attack, and you reduce the
airspeed, to say 20 km/h, until it "stalls", not in the conventional
sense - in that it is not producing lift to support the aircraft mass at
1G, but because the airflow separates and the co-efficient of lift
deteriorates.

Will the same airfoil, at the same angle of attack "unstall" if the speed
is increased above 20 km/h, or will it require a higher speed, say 25
km/h before the airflow normalises and the expected co-efficient of lift
returns?

In my experience, in a glider with marginal aileron (or rudder) control,
in a hot & high cross wind take off, it is better to keep the controls
neutral until the glider has some airspeed before correcting for a wing
drop (or yaw). If you immediately apply full control deflection then wait
for the speed to build up, it seems to take longer before the controls
"unstall" and become effective enough to correct the situation. (Either
way the left hand is never far from the release...)

Thanks

Ian

"Ian" wrote in message
news
On Sat, 06 Aug 2005 15:10:05 +0100, W.J. (Bill) Dean (U.K.). wrote:


In all normal flight the maximum lift, i.e. stalling, AoA is constant.
However, at the very low speeds met with in light winds at the start of
take-off and the end of landings, the stalling AoA is very much less.

Whereas in all normal flight conditions, from 1G "stalling speed" to Vne,
the stalling AoA remains constant at a figure probably between 16 and 19
degrees, at 5 or 10 mph it is around 10 degrees.


This is interesting, it explains a lot. I would like to read up some
more. Do you know of any references (perhaps on the net, or Reichmann)
that describe this effect?

Also is there any "hysteresis" in this effect. In other words, if an
airfoil is "flying" at say 12 degrees angle of attack, and you reduce the
airspeed, to say 20 km/h, until it "stalls", not in the conventional
sense - in that it is not producing lift to support the aircraft mass at
1G, but because the airflow separates and the co-efficient of lift
deteriorates.

Will the same airfoil, at the same angle of attack "unstall" if the speed
is increased above 20 km/h, or will it require a higher speed, say 25
km/h before the airflow normalises and the expected co-efficient of lift
returns?

In my experience, in a glider with marginal aileron (or rudder) control,
in a hot & high cross wind take off, it is better to keep the controls
neutral until the glider has some airspeed before correcting for a wing
drop (or yaw). If you immediately apply full control deflection then
wait for the speed to build up, it seems to take longer before the
controls "unstall" and become effective enough to correct the situation.
(Either way the left hand is never far from the release...)

Thanks

Ian


I do not know of any references.

We discovered this effect when we first flew the Slingsby Kestrel 19 at
Lasham in the 1970s. We discovered that we had a low speed control problem
on take-off; there were many theories as to why, and as to what to do.
Then someone discovered that using negative flaps at the start of the ground
run "worked", so we all did it without understanding why it worked.

Later we were told it was a Reynolds number effect, which resulted in a
lower stall AoA at very low speeds; my memory is that it was Derek Piggott
(our Chief Flying Instructor) who told us, whether he worked it out himself
or whether he consulted others such as Frank Irving I have no idea.

I do not understand about Reynolds numbers, but I understand that they
change with density as well as with speed, so that the behaviour of a glider
at say 20,000 ft. at a 1 G stall may well be different to that at sea level
at a 1 G stall. Also, I understand that model builders find that an exact
model of a glider may fly differently from the real thing, because both the
wing chord and the speed are different and it flies at a very different
Reynolds number; I think that models cannot get as good an L over D as the
real thing.

I should be very surprised if there is any hysteresis effect as the speed
changes. If you are using aileron when this results in tip stalling and
lateral instability, you will have more to do when the aileron starts flying
properly; just as if you use aileron when stalling in flight it can trigger
a wing drop against the aileron moving down.

I don't know what you mean by " "stalls", not in the conventional sense - in
that it is not producing lift to support the aircraft mass at 1G,". The
wing is either stalled or it is not. If it is stalled then an increase in
AoA will produce a decrease in lift, and whether or not the lift at the
stalling AoA is more or less than the weight of the glider is irrelevant.

Those who cable launch must be trained and practiced at recovering from
launch failures. A recovery from a break when in a steep climb may involve
a push over to the recovery attitude during which the speed drops well below
the 1G stall speed as you go over the top; provided the AoA is kept well
below the stall AoA the glider is not stalled even though the wing is not
producing enough lift to support the glider.

I have no experience of "hot and high", the highest glider site in the UK is
the Long Mynd at about 1,400 ft. It may well be that there is a change in
behaviour at high density altitudes, either in quality or in relation to
indicated speeds; but I would expect the same principals to apply.

I think most glider handbooks give an indication of the maximum density
altitudes flown when test flying (by implication the height at which the
glider was tested to Vne).

I suspect that glider flying in the hot and high parts of the USA may reveal
things about a glider which the designers and test pilots did not know.

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

"W.J. (Bill) Dean (U.K.)." wrote in message
...
"Ian" wrote in message
news
On Sat, 06 Aug 2005 15:10:05 +0100, W.J. (Bill) Dean (U.K.). wrote:


Snip------
Those who cable launch must be trained and practiced at recovering from
launch failures. A recovery from a break when in a steep climb may
involve
a push over to the recovery attitude during which the speed drops well
below
the 1G stall speed as you go over the top; provided the AoA is kept well
below the stall AoA the glider is not stalled even though the wing is not
producing enough lift to support the glider.

Very true.


I have no experience of "hot and high", the highest glider site in the UK
is
the Long Mynd at about 1,400 ft. It may well be that there is a change
in
behaviour at high density altitudes, either in quality or in relation to
indicated speeds; but I would expect the same principals to apply.

I think most glider handbooks give an indication of the maximum density
altitudes flown when test flying (by implication the height at which the
glider was tested to Vne).

I suspect that glider flying in the hot and high parts of the USA may
reveal
things about a glider which the designers and test pilots did not know.

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

I have spent my whole flying career flying gliders and airplanes at very
high and hot western US conditions and have never experienced or heard of
any differences in handling or stall/spin characteristics due to high
density altitudes. The noticeable differences are the faster ground speed
at which the glider takes off and lands, the amount of runway required, the
much lower rate of climb on air tow and the higher cross country speeds
achievable in the high thin air. Of course, you need oxygen on almost every
flight.

Bill Daniels

On Sat, 06 Aug 2005 23:36:39 +0100, W.J. (Bill) Dean (U.K.). wrote:

I have no experience of "hot and high", the highest glider site in the
UK is the Long Mynd at about 1,400 ft. It may well be that there is a
change in behaviour at high density altitudes, either in quality or in
relation to indicated speeds; but I would expect the same principals to
apply.

It's very simple, but very real. In a "hot & high" (low altitude
density) take off, you need a significantly faster ground speed in before
you gain control authority. However your average wing runner does not
run any faster. Net result there is more time for things to go wrong
between the time the wing runner lets go and the time the pilot has
control.

I used to fly off a narrow tar strip at 5200' altitude and 30 deg plus
temperatures common in summer. The prevailing wind was 60 degrees off the
runway, but fortunately not normally stronger than 10 knots. But when it
did blow, we discovered many gliders have marginal control characteristics
taking off in these conditions.

Lots of time was spend briefing wing runners. They had to run the
downwind wing (there is a natural tendency to hold the wing back a
little which helps keep the glider straight). A skilled wing runner would
start running with the wing held a little behind his back, then when he
reached full speed he would allow the wing to overtake but still kept it
under control until it was at full stretch in front, effectively add some
speed to their run. Some guys would hold the wing by the leading edge then
when they let go it would fly out from under their palm, allowing them to
prevent it lifting up (which the downwind wing tends to do) for a short
extra distance of the run.

We lined up the gliders pointing to the downwind side of the centerline,
so the weathercock effect would turn them straight. Tail wheels were
better than skids. If you had one, you held the stick back to keep it on
the ground (except if you had a V tail). C of G hooks were always more of
a problem than nose hooks. The standard class guys often used airbrakes.
Then off course the flap ships always started in negative flaps.

But most important of all was to be ready to release when things went
wrong. Sometimes it took two or three attempts to get a sensitive glider
airborne in marginal conditions. So long as the run off area was kept
clear and the pilot aborted at the first signs of things going wrong,
there was no danger of damage.

Regards

Ian

On Tue, 2 Aug 2005 20:01:36 UTC, wrote:

: 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!

I believe that one of the main reasons for wing drops at launch is
wings being held level by tip runners. If the runner has to exert a
significant force - wind, non-central ailerons, combination - then as
soon as s/he lets go the glider will roll. If I'm holding the tip
before launch and, say, having to apply a significant up force, I let
it go down about half way. Normally the pilot's reflexes kick in with
some opposite aileron, the load comes off and I bring the wing up to
level again. If that doesn't work I tell the pilot what the problem
is.

Ian


--

Ian Johnston wrote:

If I'm holding the tip
before launch and, say, having to apply a significant up force, I let
it go down about half way. Normally the pilot's reflexes kick in with
some opposite aileron, the load comes off and I bring the wing up to
level again. If that doesn't work I tell the pilot what the problem
is.

I instruct my wing runners to balance the wing, not hold it level. I've
found some wing runners don't really know what level looks like when
they are 50 to 60 feet from the other tip, and the wing is drooping
because of the water weight. Wind and slight control deflections play a
role, too, but if the wing is balanced, it won't drop. Being level is
generally not important.

If balanced is way off level, the pilot should notice something is
wrong, and he should determine the cause. It might be the ballast is not
equal, for example. I would rather not have the wing runner attempting
to "correct" the situation.


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

Eric Greenwell
Washington State
USA

Hi all,

1.I have a guess at the question of airbrakes improving low speed
response-they simply deflect extra air around their sides, increasing
the airflow over ailerons and wing root.

2.One point I must take issue with was in regards to Reynolds numbers.


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


At low RE the boundary layer is much more easily transitioned to
turbulent flow(In fact at RE=60,000 flow is considered critical and
boundary layer attachmnent will fail), therefore turbulence is much
more likely at low RE NOT high RE.

Interesting that no one else noticed that eh?

But as a modeller as well as fullsize pilot I have studied the effects
of low RE a lot more I guess.....

3.I fly a Hornet often, an aircraft with I believe the same fuselage as
the Mosquito, and it also shows the wing drop on ground run tendency,
due to high AOA in the two point position.

4. Small chord tips at low speed may well be sub RE60,000, not creating
ANY lift until RE60K is reached!(RE is proportional to speed, chord and
air density).

Regards & Safe flying to all

Dave Lawley


--
dav
------------------------------------------------------------------------
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dav wrote:

1.I have a guess at the question of airbrakes improving low speed
response-they simply deflect extra air around their sides, increasing
the airflow over ailerons and wing root.

Looking at top views of gliders, it appears the spoiler is always well
inboard of the aileron, so I don't see how significant air can be
deflected to them. With my ASW 20, the improvement occurred whether the
airbrakes were open 10 mm or 50 mm, so I think the improvement is caused
by spoiling lift rather than any deflected flow.

Tufting the wing between the spoilers and ailerons would allow a direct
check of the change in airflow. This could be done in a 20 mph wind, so
the glider would not even have to be moving.

--
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Eric Greenwell
Washington State
USA

Once it starts moving, even slowly, the wing is producing lift. The
more lift is produced, the more difficult it is to cause a wing to
drop, as this lift distributed over the whole wingspan damps a tendency
to roll.

Eliminate this damping by opening the spoilers and now the feeble
aileron is able to cause a roll.

Try this experiment: On a day with a light wind, sit in the glider
while it's pointed into the wind. Starting from a wings level
position, start rocking the wings. With the spoilers open, you will
find that you are able to go steeper and make quicker reversals. If
the wind is very light, it may all you can do just to keep the wings
level.

-Tom