"Ian" wrote in message
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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.).
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