At 19:06 30 June 2007, Martin Gregorie wrote:
Paul Hanson wrote:
VNE in free air is determined by the amount of lift
a wing can generate vs. it's load strength; ie the
wings can only generate as much lift as the spar/structure
can safely handle.

With respect, this is entirely wrong. In straight free
flight the wings
generate exactly enough life to counter the weight
of the airframe and
its contents. If the wings generate more lift than
that the aircraft
will loop: if they generate less its called a 'stall'.

I suspect that Vne is more often determined by the
torsional resistance
of the wing. That's certainly the case for an ASW-20.


You suspect incorrectly. The faster you fly, the more
lift the wing is generating, until it can no longer
safely bear the bending (compression) loads, or in
some cases I suppose can no longer be countered by
the other flight control surfaces, but certainly not
as a function of parasite drag (especially on a 20).
Perhaps some gliders wings twist (torsional load) a
bit more than others, but most bend upwards more than
they twist. I'm sure this varies with washout as well,
but just watch a high speed finish or any high speed
flying, particularly on a long wing. You can see them
bending upwards (not backwards or twisting) due to
the excessive (excessive in this case meaning more
than is needed to simply offset the glider against
gravity) lift being generated at higher speeds, and
it most certainly increases as a function of speed.

By the very same phenomenon, the outboard wing generates
more lift in turning flight, since the outboard wing
is moving faster through the air--hence the over banking
tendency. This is why when once established in a bank
, it usually requires somewhere between neutral stick
and top aileron to maintain the same bank angle without
increasing (not on all gliders though). Of course while
turning other forces are at play too, like increased
drag creating adverse yaw, diving tendency etc, but
that is a different subject.
I stand by my statement. The wing can only take so
much stress from EXCESS LIFT generated at higher speeds,
and that usually determines a glider's VNE. Other
factors (besides the center of lift usually closely
coinciding with the center of gravity) keeping the
glider form 'looping' at higher speeds are being applied
by other flight control surfaces, like the elevator
for instance. There may be some specific cases where
VNE is determined by the speed at which the other flight
controls are no longer effective enough to counter
the lift the wings generate, but no examples I can
site off hand. The generation of lift is in direct
mathematical relation to the speed of the relative
wind, period.
BTW, a stall only in the simplest sense is from the
wing generating 'not enough lift'. It is from exceeding
the critical angle of attack for any given loading
condition, and can happen at any airspeed, any gross
weight. It happens when the airflow over the wing becomes
too turbulent to provide the needed aerodynamic reaction
to offset it's current load requirement, any angle,
any speed.

Paul Hanson
"Do the usual, unusually well"--Len Niemi