I suspect there are many reasons why negative flap
and spoilers help roll response. In all cases when
there is no airflow over the wing unless the wings
are supported or they are very well balanced one will
drop. Airflow is required for the ailerons to have
any effect. As regards spoilers I suspect that their
use and resultant spoiling of lift results in the extremities
of the wing producing more lift than the inner parts
and of course the outer extrememities are where the
ailerons are resulting in the forces being greater
where there is more leverage. Even a very small airflow
will produce some lift and it is surprising just how
much even a gentle breeze can generate. Try weighing
your glider in a 5 knot breeze and then in the hangar
in still air, you may be amazed at the difference.

Negative flap will have the same sort of effect but
the premise that this is only the case where flaperons
as opposed to seperate flaps and aileron is in my experience
correct. The Kestrel 19, which has a separate land
flap was quite happy starting the roll if flying flap
which was also connected to the ailerons was left in
neutral and I found no problem with low speed roll
control. The ASW 17 is totally different and I find
requires full negative flap at the start of the take
off roll. An additional benefit is that the glider
will not leave the ground with negative flap connected
which means that the tailwheel can be firmly pegged
to the ground with full up elevator with no danger
of getting airborne helping to keep straight in a crosswind.

On a winch launch with the Kestrel one stage land flap
could be selected before takeoff, and the same is true
of the Janus A which has seperate flaps and ailerons,
not something I would care to do with the 17.

On landing the Kestrel full landing flap could be retained
for the landing roll with the flying flap in neutral,
with the 17 full negative is applied carefully after
touchdown. Sudden application of full negative can
cause both wingtips to strike the ground. I have never
experience a problem with wing drop on the landing
roll except in a significant crosswind. I suppose I
am really not sure why it works, I am only very glad
that it does.


At 17:00 30 July 2005, Eric Greenwell wrote:
David Smith wrote:

I dont buy Chris's point about angle of attack changing
as a wing drops =
on the ground, if the tail is on the ground there
is no pitch change, =
atlthough it would be true banking into a turn at
flying speed. =
Personaly I think the propwash and/or crosswind effect
is much more =
important, given that the prop wash on most tugs
affects the right =
wing, if that is tending to drop, it will lift in
the propwash ( we use =
that affect in a downwing takeoff). Conversely a crosswind
from the =
right and the propwash causing the left wing to drop
can quickly get out =
of hand.=20

The effects we're discussing also appear during landing
in calm air, so
while crosswinds and tug wash affect the situation,
they aren't
necessary for the wing dropping. A possile reason some
people might
notice it as often on landing is they almost always
have the spoilers
out (unlike takeoff).


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Eric Greenwell
Washington State
USA