Va and turbulent air penetration speed.

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

On Fri, 09 Jan 2004 02:03:13 GMT, Robert Moore
wrote:

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.

Since the maximum vertical gust that the plane is designed to handle
is 30 knots (3000 feet per second) based upon a 30 kt vector and a 120
kt vector by my calculations the maximum change in the AOA before
reaching the design limit of the plane should be 14 degrees. An upward
gust would increase the AOA but that would also cause a rapid increase
in altitude which would be naturally corrected by a decrease in AOA
(even if on auto pilot). A downward gust would similarly be corrected
by trying to regain altitude. At cruise altitude in a small plane
neither of these should be particularly exciting as long as you don't
bounce off the ceiling too hard, something that is less likely at Va
than at a higher 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.

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