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Va and turbulent air penetration speed.



 
 
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  #1  
Old January 8th 04, 10:22 PM
Doug
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Default Va and turbulent air penetration speed.

Kershner's "The Advanced Pilot's Flight Manual" has the following
definition for Va.

Va - The maneuvering speed. This is the maxiumu speed at a particular
weight at which the controls may be fully deflected without
overstressing the airplane.

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.

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.
  #3  
Old January 8th 04, 11:28 PM
Robert Moore
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(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






  #4  
Old January 9th 04, 01:47 AM
Maule Driver
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"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...


  #5  
Old January 9th 04, 02:00 AM
Maule Driver
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"Doug"
Va - The maneuvering speed. This is the maxiumu speed at a particular
weight at which the controls may be fully deflected without
overstressing the airplane.

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.

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.


There should be no reason to go slower than Va for turbulence penetration.
The case is made later in this thread for using the faster Vb for such
conditions (I disagree but can be convinced).

However, one point that is often overlooked is that the published Va is
usally/always stated for max gross weight. At lesser weights, Va is lower
and can be significantly lower. Va isn't marked on the airspeed indicator
but it is an indicated airspeed.


  #6  
Old January 9th 04, 02:01 AM
ArtP
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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.
  #7  
Old January 9th 04, 02:22 AM
Robert Moore
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"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
  #8  
Old January 9th 04, 02:29 AM
Robert Moore
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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

  #9  
Old January 9th 04, 02:37 AM
Teacherjh
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Even if Vwhatever is "close to stall" (inasmuch as a gust could stall it), when
the gust is over, so is the stall. However, if that gust ripped your wings
off, when the gust is over, you don't recover your wings.

Jose

--
(for Email, make the obvious changes in my address)
  #10  
Old January 9th 04, 02:49 AM
ArtP
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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.
 




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