Control in transonic flight

When an aircraft is flying close to the speed of sound, the operation of a con­trol surface may cause the flow to change locally from subsonic to supersonic type, or vice versa. This means that the handling characteristics can change significantly, and in extreme cases, the controls may even reverse, making the aircraft almost unflyable. For example, application of right rudder will cause the left wing to travel a little faster than the right. If the aircraft is flying at the point where compressibility effects are causing a loss in lift and a rise in drag, then the faster moving wing may drop, so that the aircraft rolls away from the turn, instead of into it. This is a potentially dangerous characteristic, and great care has to be exercised when using the rudder, or indeed when making any control movements in the region of flight close to Mach 1.

Control reversal was sometimes encountered by the faster aircraft of the Second World War straying too far into the transonic region, but in most cases, this reversal was due to insufficient structural stiffness, leading to aeroelastic effects, as described in Chapter 14.