Much valuable information can be derived from wind-tunnel model tests. Sometimes the behavior of a specific airplane is so complex that the accuracy of simplified theoretical analysis becomes doubtful; then model testing is almost indispensable in arriving at a sound design. Model testing is often used to determine the optimum location of engines or fuel tanks, and other design parameters.
Because of the requirements on geometrical, kinematical, and dynamic similarity, wind-tunnel flutter models are often quite expensive and difficult to construct. Elaborate techniques of model construction and test instrumentation have been evolved in the past decades.5-fi6~6-72 In recent years attention is called to the method of support of the model in the wind tunnel. For example, the rigid-body degrees of freedom (translation and rotation of the airplane as a whole), may have important effects on the flutter of swept wings and tails. Yet it is impractical in model tests to allow all the degrees of freedom corresponding to free flight conditions. Some simplification is achieved by separating the constituent oscillations of the airplane into symmetric and antisymmetric types and examining them separately.
The critical condition can be found by observing either the free oscillation of the structure following an initial disturbance, or the response of the structure to an external periodic excitation. In the former method, the airspeed is increased until there results a maintained oscillation of a specific amplitude in a chosen degree of freedom. In the second method, one or several exciters (e. g., eccentric rotating masses, air pulse exciter, etc.) are used to excite the oscillation. At each airspeed, the amplitude response is recorded for varying exciter frequencies. The critical flutter condition is specified as the extrapolated airspeed at which the amplification becomes very large.