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when the aircraft was 10 ft below the target, the target
remained in the field of view of the visual scene.
The desired performance standard for this task required that
the pilot make only two or fewer reversals outside the red
region before stabilizing in the red. Adequate performance
required two or fewer reversals outside the top and bottom
horizontal boundaries of the objects, which are 2.25 ft
apart.
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:
˙˙
( )
.
h
s
s
dc s
=
+
9
0 3
(11)
A delay is usually added to a model such as this to
approximate the lag caused by the rotor dynamics, but this
delay was instead subsumed by the simulator motion and
visual systems. This technique was successfully used in a
previous model validation experiment (ref. 59). In
addition, no torque or rpm limits were imposed on the
pilot. Also, since the task was limited to a single axis,
there was no coupling into the directional axis. Atmospheric
conditions were calm air, no turbulence, and
unrestricted visibility. The math model cycle time was
25 msec.
Simulator and Cockpit
The NASA Ames VMS, described in section 2, was again
employed. For this experiment, only the vertical degree of
freedom was used. A Singer Link DIG-I image generator
provided the visual scene, which was representative of the
32
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° vertically
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.
Figure 36. Cockpit field of view for vertical tracking.
Figure 37. Pilot’s visual scene for vertical tracking.
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
˙˙
˙˙ ( )
h
h
s
Ks
s s
com =
+ +
2
2 2zw w2
(12)
where ˙h˙com is the commanded vertical acceleration of the
simulator cab, ˙h˙ is the math model’s vertical acceleration
at the pilot’s station, K is the motion gain, z is the
damping ratio, and w is the filter’s natural frequency.
The damping ratio of the above filter was held fixed at
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Helicopter Flight Simulation Motion Platform Requirements(23)
