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