In the chapter on transonic aircraft (Chapter 9) we dealt with the buffeting that occurs at transonic speeds. This unsteadiness is primarily the result of unsteadiness in the position of the shock wave at the end of any patch of supersonic flow. Buffeting of the aircraft, and shaking of the controls can also happen at low speeds, when flow separation occurs at the onset of the stall. This can be regarded partially as a benefit, since it gives the pilot a warning of the approaching stall.
A mild form of buffeting is often felt when the flaps are lowered, but this is rarely a problem since it is neither severe nor sustained for long periods. A more serious form of buffeting may occur when the wake from the wing interacts with the tail surfaces.
A major problem in aircraft structural design is that of resonances, which occur when a source of vibration has a frequency that coincides with one of the natural frequencies of the structure. Many of the sources of vibration are purely mechanical in origin. Engine vibration is one obvious example. However the forcing frequency can also come from aerodynamic sources such as propeller wash.
Flow separations can generate turbulence that is sufficiently regular and periodic to set up resonances. In particular, bluff (non-streamlined) shapes such as fuselage-mounted dive brakes can generate a periodic shedding of vortices known as a Karman vortex street. Vortices are shed alternately from either side of a component such as a dive brake, and this produces an alternating force on the brake, and anything in its wake. The ‘singing’ of telephone wires is caused by this effect.
The preferred cure for resonances is to increase the stiffness until the natural frequency of vibration is well above the forcing frequency. Alternatively, the mass distribution can sometimes be changed, so that the natural frequency is much lower than the forcing frequency. Moving the engines outboard on a wing will reduce the natural frequency of bending oscillations. Care must then be taken to ensure that the forcing frequency does not coincide with one of the harmonics of the structure’s natural frequency.
Noise from engines and propellers, whether airborne as pressure waves, or directly transmitted, can result in structural fatigue due to the fluctuating loads that it produces. Noise-induced fatigue is particularly likely to occur in helicopters and with unducted fan propulsion.