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Control reversal
When a control surface such as an aileron is deflected downwards, the centre of lift moves aft. If the centre of lift under normal conditions is near the flexural centre, then the rearward shift will produce a nose-down twisting moment. The effect of this twisting is to reduce the wing angle of attack, as shown in Fig. 14.3. If the wing is too flexible, the reduction in angle of attack can have a greater effect than the increase in camber, so that the lift decreases instead of
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Fig. 14.3 Control reversal Deflection of the control surface produces a large amount of camber at the rear of the section. This causes the resultant lift force to move rearwards, thus tending to twist the section nose-down. If the structure is insufficiently stiff in torsion, the resulting decrease in angle of attack can cause a loss of lift; the reverse of the effect intended |
increasing as expected. The consequence is that the control action is reversed, and the aircraft can become virtually impossible to fly.
Such control reversal was sometimes encountered in power-dives, by fast piston-engined aircraft during the Second World War. As we have seen, the centre of lift moves rearwards as the aircraft approaches the speed of sound, so that the possibility of control reversal is increased. The fact that control reversal often occurred as an aircraft approached the speed of sound, led to a belief among pilots that control reversal was an inherent feature of supersonic flight. This is not true. It was simply that the deficiencies in torsional stiffness became critical as the aerodynamic loads and moments increased.
The movement of the centre of lift in high speed flight produces a major design problem, since it is clearly impossible to arrange for the resultant lift force to pass near the flexural centre, in both high and low speed flight cases. The solution is to make the wing sufficiently stiff in torsion (twist), although this may be difficult to achieve without incurring a high weight penalty.
Control reversal occurs mainly with outboard ailerons, and one solution is to use a secondary set in inboard ailerons for high speed flight. The use of spoilers instead of ailerons can provide an alternative solution. The use of slab or all-moving surfaces for control may also help to reduce the problem.
Other static problems
In addition to the rather dramatic cases described above, the flexibility of the aircraft can produce other, sometimes fatal conditions, such as the jamming to control cables, the fracture of hydraulic pipes, and the rupturing of fuel tanks.
