Methods of excitation

Having trimmed the aircraft as accurately as possible in the desired flight condition, the long-term response is excited using one of the methods detailed below. The magnitude of any artificial excitation should be chosen to give a response which is representative of that occurring naturally. Excessive excitation may lead to an unrepresentative response because the pilot may often have to intervene before the motion has developed. As a general rule, the smallest excitation which produces a significant oscillatory response or a divergent aperiodic response is used.

(1) Natural turbulence. The effect of imperfect trim conditions or natural turbulence may be sufficient to excite an aperiodic or lightly damped oscillatory response. Such initiation of the long-term response is desirable in that there can be no doubt that the excitation method is representative. However, these responses are usually contaminated by subsequent atmospheric disturbance before the motion is complete and it may be impossible to extract meaningful quantitative data. Nevertheless, natural turbulence does provide the opportunity to qualitatively evaluate the difficulty of suppressing the long-term response under representative conditions.

(2) Release to trim. If there is a benign response to natural turbulence then verification of such a desirable aircraft characteristic is usually made using artificial excitation. Equally a pilot generated disturbance may be required if the level of turbulence is insufficient to generate a meaningful response from the aircraft. Often the best method is to accelerate or decelerate from the trim airspeed and then to smoothly release the controls back to their trim positions. The controls may then be left free (but monitored) until recovery action, or the suppression of an off-axis response, becomes necessary. A speed increment of 5-15 knots is used for this method.

(3) Pulses. Longitudinal cyclic or collective pulses may also be employed to verify any lightly damped or aperiodic long-term response obtained from natural turbulence. Unless of extreme magnitude and direction (that is virtually a release to trim), pulses typically provide insufficient excitation to initiate a long-term response which is well damped, especially in forward flight.

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