Va and turbulent air penetration speed.

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

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.

Art..

With all due respect.. you really need to go and review the section on
angle of attack and accelerated stalls.. I've been able to intentionally
stall a light (ASEL) aircraft in smooth air at Va.. its called a steep
turn with extra back pressure. It has nothing to do with "tailwind
gusts".. it has EVERYTHING to do with angle of attack.

Dave

ArtP wrote:
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.

"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.

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.

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.

The desired effect is to have the airfoil stall before breaking but at the
same time that the pilot not lose control. Gusts may increase indicated
airspeeds and consequently produce more stress on the airplane, so from this
standpoint slower is always better. On the issue of controlablity, faster
is better. Vb is the speed that is supposed to provide the best compromise.
There was an article in Business and Commercial Aviation a few years ago
that had a comprehensive explanation but I no longer have the issue. I seem
to recall that for swept wing jets Vb is greater than Va and may even be
greater than normal cruise. Of course jets are also concerned with mach
exceedances and upsets so the issue is more complicated for them

Mike
MU-2

"Doug" wrote in message
om...
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.

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.

(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.
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
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

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

Ok... all you closet aeronautical engineers... I'm asking for someone to
help do my work for me.. with regards to Va..

I have an Excel Spreadsheet application that does W&B and plots it on a
graph... The form also lists certain speeds that are "static": Vx/Vy,
Vne, etc.. I would like to modify this form to list Va dependent on
the given calculated gross weight, and perhaps even doctor it up to do
density altitude computations..

If anyone HAS or KNOWS (or has the formulas)how to do this in Excel,
please feel free to pass it on.. I'm sure I will figure out or find what
I need sooner or later, but I'm not wanting to reinvent the wheel if I
dont have to.. this is for myself and some flying club members (and for
anyone else who happens to see it on here).. not a school project or
work assignment of any kind.

Dave PP-ASEL

Doug wrote:

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.