String in the middle does not protect you from a spin

Balanced flight (string in the middle) does not protect you from stalling or
a spin entry, but it helps. This is true both in straight flight and
turning flight.

Stalling is about angle of attack, above the critical angle you are stalled,
below it you are not. Whether you are in balanced or unbalanced flight
makes no difference to this.

Spinning and spin entry are stalled manoeuvres, unstalled and they cannot
happen, stalled and they may.

If you are just below the stalled angle of attack, then unbalanced flight
(slip or skid, string to one side) may stall part of the wing, so the glider
will stall.

If you are stalled and in unbalanced flight, then it is more likely that the
stall will become a spin entry.

Some types of glider will enter a spin even if stalled wings level in
balanced flight, whether this happens may be affected by C. of G. position;
different examples of the same type of glider may vary. Stalling in
balanced flight when turning is more likely to result in a spin entry than
when the wings are level.

Any turbulence, roughness, gustiness or gradient in the air will increase
the likelihood of stalling when close to the stall, and will increase the
likelihood of a spin entry when stalled.

Unbalanced flight makes it more difficult to sense and control the angle of
attack, especially when close to the stall. Unbalanced flight can conceal
the symptoms of the approaching stall.

Derek Piggott wrote (5th February 1994):
"I think lots of people still think that pro-spin controls means having a
lot of rudder or aileron on and don't realise that the important thing is
the stick position. If the stick is well back, spinable machines spin:
without the stick being back they don't spin."

In steady turning flight the stick will be further back for a particular
angle of attack compared with straight flight. The angle of attack with
the stick on the back stop in a steady turn will be lower than with the
stick on the back stop in straight flight. These statements are not true
when making a rapid entry into a turn.

The change in stick position in steady turning flight is due to circular
flow, as explained in "Understanding Gliding" by Derek Piggott, see chapter
2, use the index; the section is titled "Stalling in turns". However, note
the section titled "The function of the elevator" earlier in the same
chapter, note in particular where he says:
"In a tight turn, the nose may even be a little lower than the normal
cruising position but if the stick is being held well back near the end of
its range, the angle of attack _is_ large and the aircraft _is_ close to the
stall."

When instructors are being trained in the U.K., a standard demonstration
which every instructor must show is to get the glider to stall off a steep
turn. The K13 shows this very well, it departs abruptly into a steep spin
entry. Usually there is either only very subtle symptoms of what is about
to happen, or no symptoms at all; other of course from the position and
movement of the controls.

The best book I know for basic "theory of flight", an understanding of angle
of attack and the stall, and the importance of stick position and movement
is still "Stick and Rudder" by Wolfgang Langewiesche published 1944.

Fly safe, careful when you pull the stick!

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"Jim" wrote in message
news
I think the recent threads on spinning have been wonderful. I find
them a great help to focusing on such an important issue.

Somewhere in my dark student-pilot experiences I was shown that
coordinated flight, with the yaw string kept carefully in the middle,
does not guarantee against spin entry. As I recall, it was pointed
out to me that even with the string in the middle, in a steep turn the
inside wing is flying at a higher angle of attack than is the outer
wing. If a stall is induced in such a steep turn the inside wing is
likely to stall before the outside wing and thus will have higher drag
than the outer wing and the glider will likely fall off over the
inside wing. If confusion or inadequate skill or distraction get in
the way of an immediate recovery the higher drag of the inside,
falling wing may initiate an autorotation and possible spin entry.

Does this seem like a real possibility?

In any case, it instilled in me the knowledge that the yaw string is
not an indicator of the relative angles of attack on the two wings.

W.J. (Bill) Dean (U.K.). wrote:
Balanced flight (string in the middle) does not protect you from stalling or
a spin entry, but it helps. This is true both in straight flight and
turning flight.

Interesting problem. Reminds me of flying multi-engine noncenterline
with one engine out, where to keep the string in the middle you have to
fly with the ball to the side.

So (my swag analysis) if you are in a right hand turn, the forces on the
wings are as below, with the inner wing having more drag, because it is
slower and must fly at a higher AOA (gained through aileron deflection).
To counteract the forces now trying to rotate the aircraft, one must
toss in some rudder. To then keep from side slipping, one must fly in
what would feel like a skidded turn to balance all of the forces.


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Does this make sense?


In any case, it instilled in me the knowledge that the yaw string is
not an indicator of the relative angles of attack on the two wings.

true dat.

nafod40 wrote:
...
So (my swag analysis) if you are in a right hand turn, the forces on the
wings are as below, with the inner wing having more drag, because it is
slower and must fly at a higher AOA (gained through aileron deflection).
To counteract the forces now trying to rotate the aircraft, one must
toss in some rudder. To then keep from side slipping, one must fly in
what would feel like a skidded turn to balance all of the forces.
...

The inner wing wing has a higher drag *coefficient*, but a lower speed,
so it is not obvious if the drag is higher or lower. Near the speed of
best L/D, drag an lift coefficients increase in the same proportion with
increasing AOA, so if the difference in lift coefficient compensates
exactly for the difference in speed, the same should be true for
drag. At lower speeds the (relative) drag coefficient increase is higher
than the lift coefficient increase and this has to be conterd by some
inside rudder. This is confirmed by experience on most gliders, where
the need for inside rudder increases when you come closer to the stall
speed/angle. But this is an oversimplified view. There are at least to
other source of rolling moment beside the overbanking moment due to
the different speed of both wings. One of them is the difference in
AOA due to the fact that both wings have the same sink speed (vertical
component of velocity) but a different horizontal speed. Another one
is due to inertial forces, the difference between the centrifugal forces
on both wings. These both effects results in an underbanking rolling
moment, so the aileron input has to counter less than the overbanking
moment due to the speed difference. And aileron deflection is not just
a change in AOA, but a change in airfoil shape, and it usually affects
only the outer part of the wing, the inner part may be in the opposite
condition (lower lift on inside wing) so that the global effect is what
is wanted.