Va and turbulent air penetration speed.

(Doug) wrote

Now, Va is commonly taught as turbulent air penetration speed. But
nowhere in the definition does it say that Va will protect the
airframe from damage due to turbulence.

And commonly incorrectly taught at that!

Does slowing down even slower than Va protect the airframe from even
more severe turbulence? Or is Va the best speed for turbulence
penetration? Or is Va just used as a turbulence air penetration speed
becauase of tradition or some other non-technically correct reason.

Probably taught because of igorance.


Quoted from FAR Part 1:

VA means design maneuvering speed.

VB means design speed for maximum gust intensity.

VC means design cruising speed.


Quoted from Aerodynamics for Naval Aviators:

"The loads imposed on an aircraft in flight are the result of maneuvers and
gusts."
"As a general requirement, all airplanes must be capable of withstanding an
approximate effective +/- 30 foot per second gust when at maximum level
flight speed for normal rated power. Such a gust intensity has relatively
low frequency of occurrence in ordinary flying operations. The highest
reasonable gust velocity that may be anticipated is an actual veritical
velocity, U, of 50 feet per second."

Quoted from FAR 23:

Section 23.333: Flight envelope
(c) Gust envelope. (1) The airplane is assumed to be subjected to symmetrical
vertical gusts in level flight. The resulting limit load factors must
correspond to the conditions determined as follows:

(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC must be
considered..
(ii) Positive and negative gusts of 25 f.p.s. at VD must be
considered...........

End Quoting......

As can be seen, Vb, the speed for the maximum gust has nothing to do with Va.
How does one find Vb for most of the GA fleet?.....simple...just look
for the top of the green arc on the airspeed indicator.
By flying at Va during turbulence, one is flying un-necessarily close to
stall and a possible upset.
The aircraft can be loaded to a much higher value with the controls than
by a 50 fps gust.
I would suggest a look at the flight envelope picture in FAR Section 23.333.

Bob Moore
ATP CFI

"Robert Moore"
As can be seen, Vb, the speed for the maximum gust has nothing to do with
Va.
How does one find Vb for most of the GA fleet?.....simple...just look
for the top of the green arc on the airspeed indicator.
By flying at Va during turbulence, one is flying un-necessarily close to
stall and a possible upset.

At Va, it would take a full, instaneous deflection of the controls to stall
the aircraft. How is that un-necessarily close to stall?

The aircraft can be loaded to a much higher value with the controls than
by a 50 fps gust.

Whicih is why it is recommended to fly at Va in turbulent conditions. That
is, 1) because a full deflection of the controls, presumably commanded to
recover from a turbulence induced upset, will result in a stall instead of a
breakup, and 2) incidently because it is conservatively lower than Vb and
therefore well within any expected maximum gust.

I sense this is going to be a long one...

"Maule Driver" wrote

At Va, it would take a full, instaneous deflection of the controls to
stall the aircraft. How is that un-necessarily close to stall?

A gust of sufficient value will also stall the a/c at Va.

Whicih is why it is recommended to fly at Va in turbulent conditions.

With the FAA saying that Vb is the speed for maximum gust intensity,
who do find suggesting otherwise.

2) incidently because it is conservatively lower than Vb and therefore
well within any expected maximum gust.

This is true, but also closer to a stall

I sense this is going to be a long one...

Just post the documentation. :-)

Bob Moore

"Robert Moore" wrote in message
. 6...
"Maule Driver" wrote

At Va, it would take a full, instaneous deflection of the controls to
stall the aircraft. How is that un-necessarily close to stall?

A gust of sufficient value will also stall the a/c at Va.

Whicih is why it is recommended to fly at Va in turbulent conditions.

With the FAA saying that Vb is the speed for maximum gust intensity,
who do find suggesting otherwise.

2) incidently because it is conservatively lower than Vb and therefore
well within any expected maximum gust.

This is true, but also closer to a stall

I sense this is going to be a long one...

Just post the documentation. :-)

Bob Moore

How about you post an example of an airplane with a published Vb that is the
same as Vmo (or Vno)?

Mike
MU-2

On Thu, 08 Jan 2004 22:28:48 GMT, Robert Moore
wrote:

By flying at Va during turbulence, one is flying un-necessarily close to
stall and a possible upset.

Which in my plane (SR20) is also 60% power at 50 degrees LOP, my
normal cruise (120 IAS) and I am nowhere near stall which is 65 IAS.

ArtP wrote

Which in my plane (SR20) is also 60% power at 50 degrees LOP, my
normal cruise (120 IAS) and I am nowhere near stall which is 65 IAS

And I thought that every private pilot was taught that an airplane
can be stalled at any airspeed and any attitude. I assure you that I,
or a gust of sufficient value can stall your SR20 at 120 kts.

Bob Moore

On Fri, 09 Jan 2004 01:29:05 GMT, Robert Moore
wrote:


And I thought that every private pilot was taught that an airplane
can be stalled at any airspeed and any attitude. I assure you that I,
or a gust of sufficient value can stall your SR20 at 120 kts.

By my calculations, if I am flying in cruise it would take a tail wind
gust of 56 knots to stall me. I suspect that would fall in the
category of sever turbulence and I don't think there is any airspeed
that would be safe under those conditions in a single engine normal
category aircraft. In any case a stall at cruise altitude should not
be a problem but parts (like the engine or the wings) falling off the
aircraft would be.

ArtP wrote

By my calculations, if I am flying in cruise it would take a tail wind
gust of 56 knots to stall me.

That's not the type of gust being discussed, try "vertical" gust.

Bob Moore

ArtP wrote:
By my calculations, if I am flying in cruise it would take a tail wind
gust of 56 knots to stall me.

Arrrgghhhh! No, that's not what's going on at all.

You stall when you exceed the critial angle of attack. One way to do
that is to slowly reduce airspeed under 1g conditions (i.e. straight and
level flight in non-turbulent air) until you reach Vs. At Vs and 1g,
the wing has to be operating at the critical AOA to generate lift equal
to the plane's weight, so you stall. This is your standard private
pilot power-off stall demo.

But, there are many other ways to reach critical AOA. One way is to
instantaneously change the direction the relative wind is coming from.
Let's say you're in straight and level flight at 120 KTAS. That's just
about 200 ft/s (feet per second). I'm going to pull two plausable
numbers out of my butt for the sake of argument:

1) At 120 KTAS, 1g, and whatever you weigh at the moment, the wing is
operating at an AOA of 5 degrees.

2) The critial AOA for your wing is 18 degrees.

The relative wind is coming from straight ahead and the wing is tilted
up at an angle of 5 degrees from the horizontal. This gives you your 5
degree AOA. NOTE: this doesn't mean your nose is pitched up 5 degrees,
because the wing has a built-in angle of incidence, i.e. the chord line
of the wing is not parallel with the longitudinal axis of the plane.

Now, let's say you hit a vertical gust of 50 ft/s. That means the
relative wind is now 200 ft/sec straight back plus 50 ft/sec up. If I
did my vector sums right, that means it's now 206 ft/sec coming from an
angle of 14 degrees below the horizontal. Since your wing is tilted up
at 5 degrees, the AOA is suddenly 5 + 14 = 19 degrees, which is greater
than the critical AOA. Presto, you are suddenly stalled!

If the gust were just a little bit weaker, you would only reach an AOA
of, say, 17 degrees. This is below the critcial AOA so you're not
stalled. But, the AOA has suddenly gone up by a factor of 3.4. Since
you're still going at the same airspeed (actually, a little bit more,
206 instead of 200 ft/s), you're suddenly generating 3.4 times as much
lift, and you will find yourself being accelerated upward at 3.4 g's,
which is enough to make your socks droop down around your ankles. Well
endowed men or women will be wishing they had worn more supportive
underwear about now. But, since your wings were probably designed to
withstand 3.8 g's (plus 150% safety factor), they should stay on and not
even get bent or wrinkled.

Here's the mantra; repeat after me:

"Stall is when you exceed the critial AOA. You can do this in any
flight attitude, at any airspeed"

ArtP wrote:
By my calculations, if I am flying in cruise it would take a tail wind
gust of 56 knots to stall me.

Wings don't stall because of a lack of airspeed.

Hilton