Page 595 - Aircraft Stuctures for Engineering Student

576 Elementary aeroelasticity

are recorded and analysed to determine the decay rate. The procedure is repeated at
increasing speeds with smaller increments being used at higher speeds. The measured
decay rates are plotted against speed, producing a curve such as that shown in Fig.
13.24. This curve is then extrapolated to the zero decay point which corresponds to
Vf. Clearly this approach requires as accurate as possible a preliminary estimation
of flutter speed since induced oscillations above the flutter speed diverge leading to
possibly catastrophic results.
Other experimental work involves wind tunnel tests on flutter models, the results
being used to check theoretical calculations”

13.4.5 Control surface flutter

If a control surface oscillates about its hinge, oscillating forces are induced on the
main surface. For example, if a wing oscillates in bending at the same time as the
aileron oscillates about its hinge, flutter can occur provided there is a phase difference
between the two motions. In similar ways elevator and rudder flutter can occur as the
fuselage oscillates in bending. Other forms of control surface flutter involve more
than two different types of motion. Included in this category are wing bending/aileron
rotation/tab rotation and elevator rotation/fuselage bending/rigid body pitching and
translation of the complete aircraft.
It can be shown4 that control surface flutter can be prevented by eliminating the
inertial coupling between the control rotation and the motion of the main surface.
This may be achieved by mass balancing the control surface whereby weights are
attached to the control surface forward of the hinge line.
All newly designed aircraft are subjected early in the life of a prototype to a ground
resonance test to determine actual normal modes and frequencies. The primary
objects of such tests are to check the accuracy of the calculated normal modes on
which the flutter predictions are based and to show up any unanticipated peculiarities
in the vibrational behaviour of the aircraft. Usually the aircraft rests on some
low frequency support system or even on its deflated tyres. Electrodynamic exciters
are mounted in pairs on the wings and tail with accelerometers as the measuring
devices. The test procedure is generally first to discover the resonant frequencies
by recording amplitude and phase of a selected number of accelerometers over a
given frequency range. Having obtained the resonant frequencies the aircraft is
then excited at each of these frequencies in turn and all accelerometer records
taken simultaneously.

Babister, A. W., Aircraft Stability and Control, Pergamon Press, London, 1961.
Duncan, W. J., The Principles of the Control and Stability of Aircraft, Cambridge University
Press, Cambridge, 1959.
Bisplinghoff, R. L., Ashley, H., and Halfman, R. L., Aeroelasticity, Addison-Wesley
Publishing Co. Inc., Cambridge, Mass., 1955.
Dowell, E. H. et al., A Modern Course in Aeroelasticity, Sijthoff and Noordhoff, Alphen am
den Rijn, Netherlands, 1978.