Unlike step response testing, the pulse input is specifically mentioned in the Ministry of Defence Standard 00-970 [5.1]. It is quoted as a suitable test technique for evaluating the transient response characteristics of rotorcraft whose handling qualities are affected by the incorporation of sophisticated stability and control augmentation systems. Although the assessment of augmented rotorcraft is the subject of a later chapter in this book it is pertinent to include pulse inputs here. Since the pulse input has the advantage of returning the control inceptor to its trimmed position any ensuing response will be dictated by both the underlying stability characteristics of the helicopter and the action of any automatic control system. In practice the input will be similar to the release-to-trim technique described earlier although since the magnitude of the peak response is a specification compliance parameter a control fixture is often used and the test approached in the same incremental manner as step response testing.
The desired response to a pulse input is specified using a set of quantitative data equivalent to the control response parameters introduced above. These include the peak response, the time required for the flight parameter to make a first pass through the datum (T01), the time for a second pass in the sense of the original disturbance (T02) and the time to return to datum (TF). The rate of return to datum, the size of any overshoots and the tolerance of datum re-capture are all specified using fractions of the peak response. Thus (T30) and (T11) are the times taken for disturbance to reduce to 30% and 10% of the peak response. For responses that are oscillatory, unlike the dead-beat response portrayed, the magnitudes of the first and second overshoots (x1 and x2) are typically set at 15% and 10% respectively for Level 1 handling qualities and the tolerance for datum capture (xF) at 10%. The initial part of the pulse, or ‘boxcar’, input is also a means of assessing the control response of the helicopter. Therefore the rate of onset of the pilot-induced ‘disturbance’ is also specified. This is achieved by requiring that the response (y1) exceed a certain percentage of the peak response within a given time of the input being made (T1). Figure 5.13 shows an example time history of a pulse input and subsequent dead-beat response.