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The bob-up task was to be performed as fast as possible. Five bob-ups were completed for each motion-system configuration. Pilots repositioned the helicopter downward to the starting position between each bob-up. This repositioning was performed at the pilot’s discretion without performance standards. Thus, the bob-down was not part of the evaluation.
Simulated Vehicle Math Model
The vertical-axis dynamics were selected by averaging the hovering characteristics (at sea level) of five helicopters: the OH-6A, BO-105, AH-1G, UH-1H, and the CH-53D. Heffley et al. (ref. 58) was the source for these dynamics. The resulting vertical-acceleration-to-collective-position dynamics were as follows:
(11)
freedom was used. A Singer Link DIG-I image generator provided the visual scene, which was representative of the state of the art in the late 1970s. This visual image generator was different from that reported in section 3, a result of scheduling constraints at the VMS facility. The image generator that was used did not have a texturing capability. The visual delay from the math model to the visual image was 83 msec (ref. 52), which is a typical delay for flight simulator image systems. Visual lead compensation was not used to reduce the visual delay, the purpose being to more closely match the visual and motion delays. This matching is imperfect, since the vertical-axis frequency response can be approximated by an equivalent time delay of 140 msec (eq. (4)). Thus, in this experiment, the visual response effectively leads the motion response.
The visual field of view was presented on three windows that spanned ±78° horizontally and +12° and –17° verti-cally as shown in figure 36. The center window had the principal objects in the field of view for the task, and the information presented in the left and right windows was limited to polygonal color variations on the ground. The image that the pilot viewed for the task is shown in figure 37.
A conventional left-hand collective lever was used. It had a travel of ±5 in, had no force gradient, and the friction was adjustable by the pilot. All flight instruments were disabled so that all cues were from the visual scene and the motion system. Rotor and transmission noises were present to partially mask the motion-system noise. The head-up display shown in figure 37 was stowed for the task. Three NASA Ames test pilots participated; they are hereinafter referred to as pilots A, B, and C. All three fly rotorcraft and had extensive simulation experience.
Motion System Configurations
A second-order high-pass filter, typical of that used in most flight simulators, was placed between the math model vertical acceleration and the simulator-commanded acceleration. It had the form of
˙˙ 2
hcom Ks
()s = (12)
˙˙2 2
hs + 2ζωs +ω
where ˙˙ is the commanded vertical acceleration of the
hcomsimulator cab, ˙˙h is the math model’s vertical acceleration at the pilot’s station, K is the motion gain, ζ is the damping ratio, and ω is the filter’s natural frequency.
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