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From: (Mary Shafer)
Subject: Re: su27 cobra manuver
Date: Wed, 24 Jan 1996 17:07:57 GMT

On 23 Jan 1996 19:30:51 GMT, (Robin Kim) said:

R> No.  That's why there's an AOA limiter that prevents the F-16 from
R> doing anything remotely resembling a cobra.  I believe it becomes
R> unstable in yaw at higher AOAs, unlike the Su-27.  I think the
R> F/A-18 either does not have an AOA limiter or it can be disabled.

The F/A-18 has no AOA limiter; if you can get there, you can fly
there, as far as I know.  This is part of the reason I think the F-18
is a better airframe than the F-16 (notice that I said _airframe_).

The F-16 has a deep stall problem for about ten degrees up in the
medium-high alphas.  You can only get out of this by rocking out of
it.  That is, you have to pull up to a higher alpha and then push down
though the deep stall region very quickly (this is sort of the reverse
of the cobra, if you see what I mean--it's an overshoot that carries
you through the positive augmented C_m_q region).  The airplane has
always had this problem; the first fix they tried was a bigger tail,
which I think helped some, but they ended up with putting a software
limiter in, too.  However, it is possible to get into the deep stall
less directly, tricking the FCS.  Falling out of the sky in a deep
stall is not a happy event and to my mind, it's a severe deficiency in
the airframe.

Mary Shafer               NASA Dryden Flight Research Center, Edwards, CA
SR-71 Flying Qualities Lead Engineer     Of course I don't speak for NASA                               DoD #362 KotFR   

Newsgroups: rec.aviation.military
From: (Mary Shafer)
Subject: Re: su27 cobra manuver
Date: Thu, 25 Jan 1996 20:23:35 GMT

On 25 Jan 1996 16:38:58 GMT, (Robin Kim) said:

R> Thanks much for the correction and explanation.  I'd ask you to
R> explain "positive augmented C_m_q region," but since I'm sure I
R> would understand it, don't bother.  :^)

Let's just call it the pitching moment coefficient--if it's negative,
the plane wants to pitch the nose down, which gets you moving back
towards level flight, which is good.  If it's at or near zero, the
plane wants to leave the nose where you pulled it up to, which is not
so good, and if it's positive, the plane wants to keep pitching up
after you stop pulling, which is very bad.  Fortunately for the F-16,
it actually gets only a tiny bit positive, which is why it deep-stalls
instead of pitching up, stalling, departing, and spinning.

Let's think about balance here--if you want to balance a wooden pencil
on your finger, you have to put your finger, which is the center of
rotation, under the longitudinal center of gravity, which is closer to
the eraser than to the tip.  This is static stability.  If you put the
center of gravity behind your finger, the tip will pitch up and the
pencil will fall off.  If you put the center of gravity in front of
your finger, the tip will pitch down and the pencil will fall off.

Planes work the same way, except that they've got lift coming from the
tail to help balance things out.  Unlike our balanced pencil, the
airplane's center of rotation isn't right at the center of gravity,
but the lift makes it all work.  You can make it work with the pencil
if you use your other hand to hold the eraser end up or down, too.
For an unaugmented airplane, we put the center of rotation in front of
the center of gravity and then use the lift on the tail to pull the
back end up and keep the plane level.  For a plane like the F-16, that
is not statically stable (so it will be more maneuverable), we move
the elevator to give us the force to push the tail up, and that's the
augmentation I mentioned above.

However, as you move through different angles of attack, the center of
rotation moves because the amount of lift you get from various parts
of the airplane changes.  What happens in the F-16 is this change
makes it so the elevators can't push the nose over enough to get the
plane to pitch down, back to level flight.  Rather, the airplane just
stays at the same angle of attack, not pitching up, not pitching down,
but just falling out of the sky.  However, if you can get up to a
higher angle of attack, when the center of rotation changes again,
you'll be able to push the nose down and the momentum you have will
carry you through that area where it doesn't actually work, just like
the cobra works by using pitching momentum to flip the plane back
beyond the vertical.  In both cases, you're going to use the momentum
to make you overshoot a region that you can't really fly in.  So you
use the elevons to push the tail down until you get to the higher
alpha and then you push the stick forward very, very quickly, and down
you go to below the deep stall region, and the augmented stability
pulls the tail up and you get level.

(Yes, I know I've glossed over lots of stuff here and it's possible
that not everything is exactly 100% correct, but if you know enough to
know that, we'll start from the equations and FCS block diagrams and
go from there.  This is about as good an explanation as I can make for
folks outside the field in a reasonable number of lines, even if it
does have some over-simplifications.)

Mary Shafer               NASA Dryden Flight Research Center, Edwards, CA
SR-71 Flying Qualities Lead Engineer     Of course I don't speak for NASA                               DoD #362 KotFR   

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