Flutter

Flutter involves:

•aerodynamic forces.

•inertia forces.

•the elastic properties of a surface.

The distribution of mass and stiffness in a structure determine certain natural frequencies and modes of vibration. If the structure is subject to a ‘forcing’ frequency near these natural frequencies, a resonant condition can result giving an unstable oscillation which can rapidly lead to destruction.

An aircraft is subject to many aerodynamic excitations (gusts, control inputs, etc.) and the aerodynamic forces at various speeds have characteristic properties for rate of change of force and moment. The aerodynamic forces may interact with the structure and may excite (or negatively damp) the natural modes of the structure and allow flutter. Flutter must not occur within the normal flight operating envelope and the natural modes must be damped if possible or designed to occur beyond VD / MD. A typical flutter mode is illustrated in Figure 14.12.

Since the problem is one of high speed flight, it is generally desirable to have very high natural frequencies and flutter speeds well above the normal operating speeds. Any change of stiffness or mass distribution will alter the modes and frequencies and thus allow a change in the flutter speeds. If the aircraft is not properly maintained and excessive play and flexibility (backlash) exist, flutter could occur at flight speeds well below the operational limit speed (VMO / MMO).

Wing flutter can be delayed to a higher speed, for a given structural stiffness (weight), by mounting the engines on pylons beneath the wing forward of the leading edge, Figure 14.13. The engines act as ‘mass balance’ for the wing by moving the flexural axis forward, closer to the AC.

Figure 14.13 Wing mass balanced by podded engines

Limitations 14

Limitations 14

Aileron Reversal

Figure 14.15 Low speed aileron reversal

Low Speed

It was described on page 147 that if an aileron is lowered when flying at high angles of attack, that wing could possibly stall, Figure 14.15. In that case the wing will drop instead of rising as intended. Hence the term low speed aileron reversal.

Figure 14.16

High Speed

Aileron reversal can also occur at high speed when the wing twists as a result of the loads caused by operating the ailerons. In Figure 14.16 the aileron has been deflected downwards to increase lift and raise the wing. Aerodynamic forces act upwards on the aileron, and as this is behind the flexural axis of the wing, it will cause a nose-down twisting moment on the wing structure. This will reduce the angle of attack of the wing which will reduce its lift. If the twisting is sufficient, the loss of lift due to decreased angle of attack will exceed the gain of lift due to increased camber, and the wing will drop instead of lifting.

SPOILER SURFACES

OUTBOARD AILERONS (LOW SPEED ONLY)

INBOARD AILERONS

(HIGH SPEED AND LOW SPEED)

Figure 14.17 Inboard & outboard ailerons & roll spoilers

High speed aileron reversal can be delayed to a speed higher than VD / MD by having inboard and outboard ailerons and/or roll control spoilers. The inboard ailerons, Figure 14.17, are mounted where the wing structure is naturally stiffer and work at all speeds. The outboard ailerons work only at low speed, being deactivated when the flaps are retracted.

On most high speed jet transport aircraft roll control spoilers assist the ailerons. Because they are mounted further forward and on a stiffer part of the wing, roll control spoilers do not distort the wing structure to the same degree as ailerons.

Limitations 14