Paul Hanson wrote:
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,
stuff snipped...

"Not quite."

Writing as a non-practicing aerospace engineer (that's what they called
aeronautical engineering in the 1960's in the U.S.), a dormant teacher
gene compels me to comment. I'd have written "Agreed," IF the words
"capable of" were inserted between "is" and "generating."

As has been pointed out, in steady state flight, pure speed has
esentially zero to do with the amount of lift a wing generates. It
generates an amount essentially equal to the glider's weight *if in
steady state flight*! Why no excess? That pesky elevator, which allows
the whole flying system to reduce the main wing's angle of attack (AOA),
in conjunction with an increasingly descending flight path. Not unless
a gust, or elevator use changes AOA will momentarily excess lift appear
(or, disappear).
- - - - - -

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.

"It" (i.e. lift) does not directly increase as a function of speed.
(Just the *capability* of momentarily creating it does.) Considering
steady state high speed finishes of long wing birds (for the sake of
discussion...note that these principles hold true for any wing, with or
without flaps or spoilers), given the likelihood (either aerodynamic or
geometrical) washout does exist, the lift distribution DOES change for a
fixed trailing edge configuration with reduced/changing AOA (imagine
reducing it to the point of inverted flight). Decambering the trailing
edge with negative flap will of course further affect lift distribution.
The presence of wing bending may be due merely to the normal
(washout-affected) lift distribution of high speed flight, or it could
be increased by spoiler use or the presence of aft stick, but it is
incorrect to conclude it is entirely due to "excess lift due to speed."
So long as Joe Pilot does not create (or encounter a gust that
creates) excess lift, in what used to be steady state flight, note that:
Speed alone will NOT overstress the wing in bending.
- - - - - -

more snips
I stand by my statement. The wing can only take so
much stress from EXCESS LIFT generated at higher speeds,
Agreed, as stated. But see below...

and that usually determines a glider's VNE.

Um...unless I was the designer, I'd be loath to be so dogmatic.
Especially when enthusiastic elevator use above maneuvering speed
definitionally implies capability to generate lift generating G
exceeding design factors (which may or may not be the spar, incidentally).
- - - - - -

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.
Um...With respect to the last sentence, I could have sworn AOA enters
the picture somewhere. An equation for a symmetrical airfoil comes to
mind...

CL = Lift/(0.5*air density*free stream velocity[squared]* wing area

For a wing SECTION (beloved of mathematical types), replace wing area
with wing chord, and (as way too many college teachers told me) "It can
be shown that" the section lift coefficient of a thin, low-speed,
symmetrical airfoil solves to 2*pi*AOA. Camber (which gliders obviously
have), changes the *location* of a lift curve when plotted vs. AOA
graph, but not the linear relationship with AOA.

So, "Agreed," speed has a BIG impact on (potential) lift for any given
airframe/glider. That pesky velocity squared term.

But it isn't speed that directly affects lift, rather it is AOA. This
(obviously!) isn't obvious, but further research and thought should
clarify things for you. The way I think of it is speed *depends* on
AOA, as does the potential for "excess lift." But (always assuming
steady state flight for ease of our thought experiment) speed by itself
is NOT the driver of things, it's merely along for the AOA ride.
_ _ _ _ _ _


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,
Agreed.
and can happen at any airspeed, any gross
weight.
Agreed, despite what Tom Knauff semantically preaches (for
understandable if arguable reasons).
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.

Discussion of the 3rd sentence of this paragraph is probably better left
for a real conversation. I'd need to hear more to decide if I agreed or
not. I've never heard, or thought, of the degree of turbulence being a
factor in where definitional stall separation occurs. (Let's ignore
laminar airfoils during our thought experiments.)

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

Len Niemi (whom I never met) was one of my heroes.

Regards,
Bob - pedantically apologetic - W.