Va and turbulent air penetration speed.

On 9 Jan 2004 14:18:14 -0800, (Dan Thomas)
wrote:

(Doug) wrote in message . com...
Kershner's "The Advanced Pilot's Flight Manual" has the following
definition for Va.

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

Note that this definition DOES NOT say that the airplane will stall
before it breaks due to turbulence.

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.

Doesn't have to. Pulling full up elevator loads the wings the
same as a strong vertical gust; both increase AOA and the airplane and
occupants both feel increased G loading. At or below Va the airplane
will stall and thereby unload the structure somewhat if the AOA
reaches stall angle, and the load factor won't exceed the designed
structural limits.

IF you are cruising at Va and encounter a vertical gust that causes a
stall right at the design limit you survive. What happens when you
hit a vertical gust of twice the velocity of the first? According to
the ABS and Airsafety Foundation, you are going to break your
airplane.

Va is lower at lower weights because the airplane is going to
tend to change direction more easily in a gust or sharp pull-up, and
the directional change keeps AOA below stall angle and maintains the
high wing loading. Lowering Va will allow it to stall sooner.
Everybody fears wing failure, but many airplanes will suffer tail
failure first. Bonanzas and 210s are famous for such accidents, with a


Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

VFR pilot entering IMC and losing control. He pops out of the overcast
at 400 feet in a screaming spiral dive, and promptly pulls up hard.
The stabilizer fails downward, then the airplane pitches forward onto
its back and the wings fail downward (negative Gs).

Dana

Roger Halstead wrote

IF you are cruising at Va and encounter a vertical gust that causes a
stall right at the design limit you survive. What happens when you
hit a vertical gust of twice the velocity of the first? According to
the ABS and Airsafety Foundation, you are going to break your
airplane.

Again referencing a couple of previous posts:
-------------------------------------------------------
Quoted from Aerodynamics for Naval Aviators:

"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."
-------------------------------------------------------
And 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...........
--------------------------------------------------------

Now since 50 fps is the highest reasonable gust that may be anticipated,
and all aircraft are designed to withstand this gust all the way up to
Vc, what causes the wings to come off? As Dana has posted:

VFR pilot entering IMC and losing control. He pops out of the overcast
at 400 feet in a screaming spiral dive, and promptly pulls up hard.
The stabilizer fails downward, then the airplane pitches forward onto
its back and the wings fail downward (negative Gs).

Yes, I understand that older aircraft may have been certificated to only
a 30 fps gust value, but as pointed out in AFNA above, that will be
encountered very infrequently and in my opinion, never outside of a
thunderstorm. In fact, the aiframe must withstand the 25 fps gust (not
far from 30 fps) all the way to the maximum demonstrated dive speed.

The aircraft is already designed for the maximum anticipated gust. There
is no gust "twice the velocity" for which the aircraft is designed.


Is there some reason that you don't copy/paste or at least provide a url
for the material that you have referenced? Who is (are) the ABS and what
engineering credentials are possesed by members of the AOPA's Airsafety
Foundation?

Bob Moore