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The spin
We have previously mentioned the control problems which may be caused by the stall occurring at the wing tip before the root. If the aircraft is not flying perfectly symmetrically when such a stall occurs, one tip will stall before the other resulting in a rolling moment because of the reduction of lift on the stalled tip. This will also be accompanied by a yawing moment because of the locally increased drag (Fig. 12.14). The result of this is that the aircraft will enter a spiral path and is then said to be spinning. Figure 12.15 shows how the aircraft can get ‘locked’ into the spin. Although the rising wing is at a lower angle of attack its lift coefficient may be the same as the stalled falling wing which is operating ‘over the hump’ of the lift curve. Thus there will be no overall rolling moment and the rolling rate becomes constant.
The overall motion is a mixture of roll, sideslip and yaw (Fig. 12.16). If the spin is steep, roll is more important than yaw. If it is flat the reverse is true as can also be seen from Fig. 12.17.
The presence of the rolling component causes the incidence of the stalled wing to increase and the unstalled wing to decrease, thus strengthening the basic aysmmetry of the flow which ‘locks in’ as described above. Similarly the yawing motion will cause the fin to supply a yawing moment which balances that due to the stalled wing. The yaw then settles at a steady rate.
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Fig. 12.14 Asymmetric stall Stall on one wing results in roll and yaw |
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Fig. 12.17 Steep and flat spins In steep spin rotation is primarily in roll, in flat spin primarily in yaw |
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Fig. 12.18 Effect of mass distribution on spin Masses at nose and tail tend to move outwards under rotation thus flattening the spin |
Once established, the spin will thus persist and in some cases correction can be difficult. For example recovery is not possible from an inverted spin on many swept-wing fighters.
The inertial properties of the aircraft have an important bearing on its spinning characteristics. Figure 12.18 shows that the spin will tend to be flattened by the presence of mass concentrations towards the nose and tail.
Spin recovery, like recovery from a simple stall, requires the separated flow over the stalled wing to be reattached. In the spin this is done by first removing the yaw by applying rudder in the opposite sense to the direction of rotation, and when the aircraft is established in a steady dive, pulling out by means of the elevators. For difficult cases other techniques, such as a tail parachute, may be employed.
Recommended further reading
Abzug, M. J. E. and Larrabee, E., Airplane stability and control: a history of the technologies that made aviation, Cambridge University Press, Cambridge, 1997, ISBN 0521809924.
Cook, M. V., Flight dynamics principles, Arnold, London, 1997, ISBN 0340632003. A good standard undergraduate text.
Nelson, R. C., Flight stability and automatic control, 2nd edn, McGraw Hill, Boston, Mass., 1998, ISBN 0070462739. An integrated treatment of aircraft stability, flight control, and autopilot design, presented at an accessible mathematical level, using standard terminology and nomenclature.
Nickel, M. W., Tailless aircraft in theory and practice, (Eric M. Brown translator), Edward Arnold, London, 1994, ISBN 1563470942. A well known standard work which originally appeared in German.
