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

For a complete discussion *why* Va changes with weight (and is the only
V speed that does, at least in our small pistons) go find Kershner's
book - any of Kershner's books. The equation is there, also, so you
can create a table of your favorite weights.

"Blanche" wrote in message
...
For a complete discussion *why* Va changes with weight (and is the only
V speed that does, at least in our small pistons) go find Kershner's
book - any of Kershner's books. The equation is there, also, so you
can create a table of your favorite weights.

Kerschner did give an excellent treatise on why and how Va (maneuvering
speed) changes weight, and Langeweische may have done so as well. But, Va
is most cetainly not the only V-speed that changes with wieght, and a
partial list should include:
Vx (best angle of climb speed)
Vy (best rate of climb speed)
Vso (stall speed--cleam)
Vc (design max cruising speed)
Vr (rotation speed) althought more commonly associated with heavy
aircraft operation and arguably related to Vsse and Vmc in multi-engined
aircraft.

There are more, but some of the more technically interesting examples, such
as Vh (maximum level flight speed at maximum power), might be considered
more appropriate for rec.aviation.homebuilt

I agree with you about Kerschner's calculation, and also suggest that it
could also be expecially usefull with regard to Vso; although the practice
is probably not approved for light aircraft and would be questioned it an
incident were to occur. Be aware, however, that weight based tables for all
of the common V-speeds are routinely used on transport aircraft.

Peter

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

On Fri, 11 Sep 2009 01:50:08 -0700 (PDT), "Flaps_50!"
wrote:

On Sep 7, 4:10*am, a wrote:
On Sep 5, 10:20*pm, "Flaps_50!" wrote:



Ever watch that video of the Air Tanker C-130 snap it's wings after
dropping retardant on a fire? Imagine pulling out of a dive (while
banking) in a valley and then within a second losing 10,000 lbs of
cargo... when the weight is reduced, load factor goes up for a given
value of lift. In the case of the C-130 there may have been other
factors such as metal fatigue but the increased load was the primary
cause.

=======
I watched the video -- seems to me what happened is aoa was
appropriate for the load, when the load was lost the sudden excess
lift over weight acted pretty much the same *as if the pilot suddenly
yanked the yoke fully aft, but will let others more skilled make their
points.


this diagnosis didnt make sense to me. how could an aircraft that has
just shed it's load fail? with the shedding of the load the airframe
gets relatively stronger.
the locheed reports indicate that the aircraft had an undetected crack
in the root of the mainspar that had grown to such an extent that the
structure was compromised. the events that you see on the video are
coincidental and not the cause of the crash.
the crack grew to the point that it broke up in flight. that was the
cause.
Stealth Pilot

"Stealth Pilot" wrote

this diagnosis didnt make sense to me. how could an aircraft that has
just shed it's load fail? with the shedding of the load the airframe
gets relatively stronger.
the locheed reports indicate that the aircraft had an undetected crack
in the root of the mainspar that had grown to such an extent that the
structure was compromised. the events that you see on the video are
coincidental and not the cause of the crash.
the crack grew to the point that it broke up in flight. that was the
cause.

It shed its load of water, by dropping it on a fire. If you keep your
control surfaces in the same position, you will suddenly pull more G's when
the plane is much, much lighter. Those G's were more than a plane with an
already compromised wing could stand, so it broke up.

Does that make more sense?
--
Jim in NC