Precision Hover Task

In order to investigate the influence of multi-axis tasks upon the use of PAC, the Preci- sion Hover task was selected. The manoeuvre is a multi-axis reposition and stabilisation task to assess low speed performance. It provides a method to appraise both the ability of the aircraft to transition from translating flight to hover and the ability to maintain precise position. Task performance is driven by a series of visual elements, positioned within the environment, shown in Fig. 4.23. The investigation was conducted with four pilots (A,B,C,D), and conducted using HELIFLIGHT-R.

Figure 4.23: External view of the Precision Hover course.

Figure 4.24 displays a schematic of the course layout. The manoeuvre begins from a point behind and to the left of the intended hover point (large cross in Fig. 4.23). To reach the hover point, the pilot is required to hold 6-10 knots ground speed whilst transitioning at 45◦. The transition is performed at 45◦ to observe both lateral and longitudinal HQs, alongside deficiencies caused by cross-couplings. As the vehicle ap- proaches the hover point, s/he must initiate a deceleration, to bring the helicopter to hover at the reference point. When in the hover, the primary height and lateral cueing is given by the ‘hover board’. The pilot must also maintain lateral and longitudinal position, by using a series of cones located around the hover reference point. During completion of the manoeuvre, the pilot, from his sight, must keep the view of the top of the reference pole within the hover board (Spherical marker shown in Fig. 4.24). The distance between the reference pole and the hover board defines the required lateral and height tolerances. The reference cones define the necessary longitudinal position of the helicopter, and assist the pilot in judging whether he is at the correct hover point.

The manoeuvre is initiated with the aircraft travelling at a ground speed of between 6 and 10 knots, at an altitude of less than 20 feet. The target hover point is oriented at 45◦ relative to the heading of the rotorcraft. The ground track should be such that the rotorcraft will arrive over the target hover point, which should be captured in one smooth manoeuvre following the initiation of deceleration. This is to stop the pilot decelerating, and ‘creeping’ to the final hover point.

Unlike the Pitch Tracking investigation, PIO incipience was engineered through the addition of time delays only. However, these time delays were applied to both the Pitch and Roll control channels. The manoeuvre requires the pilot to maintain performance standards in all controlled axis (pitch, roll, yaw, heave). Furthermore, if the vehicle

Figure 4.24: Schematic of Precision Hover course layout.

exhibits high levels of cross-coupling, maintenance of performance becomes a complex task, with un-predictable vehicle behaviour.

Triggers (i.e. time delays) were applied in both the longitudinal and lateral control channels to observe whether the task was suitable for exposing PIOs in both axes. A matrix of combinations was completed, with configurations informed through the use of the Bandwidth Phase Delay results, shown in Chapter 3. As displayed in the preceding section, a delay of 200ms in the longitudinal axis was determined to cause a PIO prone system. For the lateral axis, a value of 200ms was not enough to cause a PIO prone vehicle. However, a delay above 300ms was found to cause a PIO prone configuration.

Through observation of predictions, time delays were applied to both the lateral and longitudinal axes; in the lateral axis, delays of 200ms and 400ms and in the longitudinal axis, delays of 100ms and 200ms. The cases were tested, alongside addition cases with no delays that were added for the completion of the Precision Hover manoeuvre. During the investigation, HQRs and PIORs were collected, awarded by all pilots during an evaluation run of each configuration. All results are contained within Appendix C.

Figure 4.25 displays HQRs awarded during completion of the manoeuvre. As shown, pilots found the manoeuvre most challenging with delays in both axes (as expected). With a lateral time delay = 200ms, pilots consistently awarded HQRs within Level 2. One observation was the lack of difference observed between cases with 0ms and 100ms longitudinal time delay. However, small noticeable differences were observed between cases with 100ms and 200ms longitudinal delay. This suggests that longitudinal oscillations were not encountered until reaching a delay of 200ms. Furthermore, a

noticeable difference between HQRs awarded with a lateral time delay = 400ms was observed. However, with one exception, all ratings awarded for this case were within HQL 2 (HQR 4-6). 0 2 4 6 8 10

Handling Qualities Rating

Pilot A Pilot B Pilot C Pilot D [400;200] [400;100] [400;0] [200;100] [200;200] [200;0] [0;200] [0;100] [0;0]

Figure 4.25: Precision Hover HQRs. [Lateral Delay; Longitudinal Delay]

Figure 4.26 displays the PIO Ratings awarded during the investigation. As shown, a large amount of scatter was found for results. This is believed to be due to defi- ciencies within the PIO Rating Scale, and will be discussed in detail in a Chapter 6. Scatter made it difficult to analyse results obtained. Traditionally, PIOR>= 4 is used to denote a PIO that has been observed during the test campaign. This is due to the descriptive wording used in the scale decision tree. Following this rule, pilots encoun- tered oscillations for lateral delays greater or equal than 200ms, and for a longitudinal delay of 200ms. This reflects predictions made prior to the investigation. However, it is noted that for the majority of vehicle configurations, the observation of oscillations was rare, with only one observation. Only with a lateral delay = 400ms, did multiple pilots believe that they encountered unintentional oscillations.

0 1 2 3 4 5 6

Pilot Induced Oscillation Rating

Pilot A Pilot B Pilot C Pilot D [400;200] [400;100] [400;0] [200;100] [200;200] [200;0] [0;200] [0;100] [0;0]