Frequency spread

A wide range of input frequencies (up to around 2 Hz) without ‘holes’ in the spectrum is essential. In other words, it is important neither to omit a particular sub-range of frequencies nor to dwell on them for too long. One of the most difficult aspects of a frequency sweep is the maintenance of a progressive increase in frequency. Moderate frequency inputs are easy to achieve and impatient pilots tend to progress to the higher frequencies too quickly. The result of this is the omission of many low-to – middle frequencies. This problem is best overcome by using a cadence count technique as well as coaching the pilot to prevent him dwelling on a particular frequency.

5.6.2.2 Trim condition

The maintenance of the desired trim condition (airspeed and/or attitude) is important. This is best achieved by conducting a symmetrical frequency sweep which starts and stops at an accurate trim condition. Depending on the trim condition it may be necessary to bias the central control position in order to maintain the correct trim attitude or airspeed. To prevent corruption of the power spectrum it is important that such bias movements take place at a low frequency (over several input cycles). Typically the aircraft is kept within 10 knots of the trim airspeed as it passes through the level pitch attitude. Stick trim is generally beneficial as positive centring provides useful cueing to the trim position, however, large breakout forces may introduce discontinuities in the inputs.

5.6.2.3 Off-axis response

It is inevitable that off-axis responses will take place and it is important to allow them to occur for parameter identification purposes and for cross-coupling specification compliance testing. If the responses become so large that the pilot must intervene then he should make corrective inputs which are not correlated with the frequency of the primary control inputs. Thus if a high frequency pitch sweep is being made and suppression of a roll response is required this should be made at low frequency and vice-versa.

5.6.2.4 Resonance

It is important to avoid any dominant rotor or structural modes which could cause adverse structural or aerodynamic resonance that may damage the aircraft. Such frequencies should be identified by engineering analysis prior to a sweep. A well – designed helicopter should have structural, rotor and control system natural frequencies well above the range of frequencies a pilot would be likely to generate under normal circumstances. However, the deliberate use of high frequencies during testing may erode the design safety margin in this respect.

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