On Sep 6, 4:49*pm, Mike Ash wrote:
In article
,

*"Flaps_50!" wrote:
I wonder, does anyone routinely recalculate limiting V speeds on the
basis of TOW? I assume that limiting Va speeds go with the square root
of the fraction MTOW loading so for 75% MTOW *Va would drop to 86% Va..
But the question is, if Vne is limited by arodynamic issues such as
flutter or windshield how would that change with load? *Put another
way, is Vne ever load dependent and/or does anyone use a rule like
that for Va?

I must admit that I never knew that Va went down, rather than up, with
weight. It makes sense now that I've read about the phenomenon, but this
is the first time I've heard of it.

As for Vne, my understanding of the causes of it (flutter, aerodynamic
loads, etc.) would indicate that it's not dependent on weight at all,
except for how weight might help you get to that speed, but I could very
well be wrong.

A second sort of connected question is: is there any wing that can
produce more lift at 45 degrees AOA than at the stall point (I know
that most airfoils produce about the same lift at 45 AOA as at the
stalling point)? What I'm thinking about is wings with washout or drag
reducing devices that will reduce maximum lift at the stall point but
not the 'flat plate at 45 AOA' lift. Put another way, how much loss of
lift do we get from typical washout?

The coefficient of lift is at its maximum at the stall angle of attack,
by definition. It's not possible for 45 degrees to give the same lift as
stall unless the wing actually stalls at 45 degrees AOA, and that would
be really unusual. Don't know the answer to the washout question, but I
think it would be better framed as how much unnecessary drag is
produced, rather than "loss of lift", which is confusing because a wing
always produces the same amount of lift for a given weight in steady
level flight no matter what the speed, AOA, or wing configuration.


The reason why I riase this is beacuse the variation of Cl with lift
is rarely shown at high AOA but I found some test diagrams and they
show that for typical foils, Cl at 45 is almost the same as the the
stall point. see:

http://www.aerospaceweb.org/question...s/q0150b.shtml

Now, as I understand/see it, with washout, the overall max lift in a
wing must be less than that given by just max Cl at stall and
planaform. This would not be the case with the 'flat plate lift'. for
a hershey bar wing with say 3 degrees washout, I'd say that flat plate
at 45 could generate at least as much lift as at the stalling point.
This may be a bit esoteric but I think it's interesting and might
indicate an even lower Va if vertical winds are anticipated.

Cheers