Vne, Va and lift?

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?

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?

Cheers

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.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

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

In article
,
"Flaps_50!" wrote:

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

Clearly my understanding of this subject was inadequate. Thank you for
this link.

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.

Any vertical gust which approaches 45 degrees AoA is likely to destroy
your aircraft outright no matter what speed you're flying. We're talking
something near a 60kt vertical gust if you're flying at 60kts. That kind
of gust is beyond extreme. You're very unlikely to ever encounter such a
beast unless you're in a thunderhead or something of that nature.

Note that Va is set for a certain maximum vertical gust speed. For gusts
beyond that speed, no guarantees are made. My plane's manual explicitly
calls this out, saying:

"Note: According to the Regulations the term "severe turbulence" means
air movements which might be encountered in wave rotors, storm clouds,
visible whirlwinds and when overflying mountain ranges and ridges. As we
observed in Chapter II.1 this level of turbulence is reached when the
variometer indicates about 7m/s (+1378ft/min) momentary peak indication.
The experienced flyer knows that he can expect even more severe
turbulence in storms and in high mountain ranges."

So, yes, if you expect turbulence in excess of the numbers used to set
your Va, you should fly even slower yet. However I think this still
won't save you if it's as extreme as you describe, but fortunately such
extremes are very rare indeed.

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon

Clearly my understanding of this subject was inadequate. Thank you for
this link.

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.

One other point that may add to your understanding, on Hershey Bar wings.
It is my understanding that HB wings have little to no need for washout, as
their stall characteristics are very controllable with zero washout. It has
been a while since I read on that subject, but I recall the distribution of
lift tends to make the stalls occur at the root and progress out to the tips
as the stall deepens, just like a "nice" wing should do.

But then, I could be wrong! g
--
Jim in NC

On Sep 12, 11:45*am, "Morgans" wrote:
Clearly my understanding of this subject was inadequate. Thank you for
this link.

*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.

One other point that may add to your understanding, on Hershey Bar wings.
It is my understanding that HB wings have little to no need for washout, as
their stall characteristics are very controllable with zero washout. *It has
been a while since I read on that subject, but I recall the distribution of
lift tends to make the stalls occur at the root and progress out to the tips
as the stall deepens, just like a "nice" wing should do.

But then, I could be wrong! g
--

Yes, quite right. I was using the term to imply constant chord which
makes it easier to think about Cl and lift variation along a wing with
washout. Some actual Hershey bar wings have very little washout
because they start their stall in the center and it may not rapidly
propagate, especially if fences or similar devices are added I
believe. That said, the constant chord wing with no washout introduces
other disadvantages...

Cheers