Frequency sweeps at low speed should only be attempted in zero wind or very light wind conditions. Similarly frequency sweeps at high airspeed should not be made if significant turbulence is present. Usually two pilots are used with one pilot performing the sweep while the other controls the remaining three axes. A ‘quarter count’ cadence count technique is often used for the low to medium frequencies. Using a longitudinal cyclic sweep as an example and starting with a 20-second period, the pilot would count from one to five as he moves the cyclic aft, then again as he returns the cyclic to the trim position. The process is then repeated for the forward deflection. The period is then reduced by counting to four for each quarter, and so on. Ideally the sweep is performed three times using a minimum of two pilots as each pilot will employ a slightly different technique. A 10-second ‘trim-shot’ is typically recorded before and after each sweep.
188.8.131.52 Incremental approach
There is no need to cover the entire frequency spectrum in a single sweep. In view of the independence of the results from input amplitude, it is perfectly feasible to combine the data from several test runs. From the point of view of aircraft damage, it is desirable to minimize the exposure of the airframe to this type of testing. Thus it is important that the test objectives are clearly defined as it may not be necessary to go up to potentially damaging high frequencies if adequate results can be obtained without doing so.
For ADS-33E work it is only necessary to cover the frequency range which will permit the bandwidth and phase delay calculations to be completed. It is sensible to conduct an initial frequency sweep with a low limiting frequency and analyze the results before continuing. Having determined the frequency range(s) of interest from the first ‘overview’ test, the optimum range(s) of inputs for subsequent, more detailed, tests is determined. Risk mitigation is achieved by minimizing the amount of testing and avoiding high frequencies wherever possible so that the potential for airframe damage is reduced. The conduct of limited range and, therefore, duration frequency sweeps will also facilitate the maintenance of an accurate trim condition throughout the test input.