Now that we have established what produces a stall, we are in a much better
position to comprehend the factors that produce a spin, and what
we can do to lessen the chances of entering one. For the moment,
we'll go back to the important factor of angle of attack. We know
that the more AOA you have, the more lift you have, up to a point where
you reach the Critical Angle of Attack, where the airflow stalls and lift
is no longer produced. If we made a graph to represent this, it would
look something like Diagram 5. We
also know that the more airspeed we have, the more lift we produce.
Diagram
5
In EAW,a spin is usually encountered when
you are at low speeds, and turning hard. A number of factors contribute
to the spin, and I will walk you through a typical spin. You are
on the tail of a bandit, and he is turning hard up in front of you.
As you have been chasing him for some time, your airspeed has dropped and
you are in a situation where less lift is generated by both wings.
You are ususally turning hard with a high AOA, which gives you a good turn
rate, but puts your wings close to the critical AOA.- When you execute
a banked turn, the inside wing is has a slower airspeed than the outer
wing, and therefore produces less lift, so the aircraft rolls slightly
more into the turn.
Experienced pilots know how to co-ordinate their turns with the rudder,
so that the aircraft doesn't "skid" through a turn, and this helps avoid
spins, but in combat this usually goes straight out the window, and we
tend to use whatever gives us the quickest rate of turn. Most pilots
find that the aircraft will turn quicker if you use a a bit of rudder in
the directioin of the turn, and this is where you get even closer to a
spin. By using the rudder in the direction we are turning (e.g. left rudder
while banking and turning left), in effect we are "skidding" the
aircraft through the turn. Even if you don't use rudder into the
turn, at slower speeds gravity will make the nose want to drop, giving
the same effect as using the rudder. This does a number of things
that contribute to a spin.
Firstly, by yawing the aircraft to one side, one wing is thrust forward
and the other wing is pulled backwards. In effect this means that one wing
is travelling even slower again than the other one, creating a lift differential
- the slower wing produces less lift, the faster wing produces more, and
you roll a little bit more towards the slower wing.
Secondly, because the aircraft is skidding through the air, the airflow
to the slower wing is disturbed by the nose of your aircraft, and in effect
part of the wing is "shaded" from good airflow.
(Diagram 6) Once again, this means
that wing with the disturbed airflow produces less lift, and so you roll
a little more towards the slower wing.
Diagram
6
Diagram 7
The third thing that puts you right over the edge is that you change the
AOA of the wings. Remember that we were already near the critical
AOA, with both wings close to stalling. The extra yawing of the aircraft
changes the AOA of each wing separately. The wing moving more slowly
is actually at a higher AOA to the airflow, since it's horizontal component
is smaller, and the wing that moves forward decreases it's AOA. If
we take a look at the lift we are generating on
Diagram
7 , we can see that one wing is creating a
helluvalot more lift than the other. One wing has exceeded the Critical
AOA, has stalled and produces no lift, but a lot of drag. In contrast,
the other wing is below the Critical AOA, and is producing maximum lift.
The result? A Spin. The aircraft appears to roll towards the
direction of the stalled wing sharply and very fast, but you actually rotate
around the stalled wing. This motion is called "AUTOROTATION" and
is used in many aerobatic maneuvers. The "snap roll" is one such
move where you do a 360 degree autorotation and come out of the autorotation
with level wings. Getting out of a spin is quite easily done if you
know what to do, and do it quickly. You will notice that if you are
slow in executing your recovery moves, then it will take some time to get
out of the spin.
OK, so that's the physics of a spin, and now that you are armed with that
knowledge it should make it the recovery moves a little clearer and easier
to understand, but before that, I'll briefly outline a few points about
recognizing an imminent stall.
Click here to go to Recognizing an imminent
stall
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