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band decreases with increasing altitude.
51
Figure 63. Visible polygons for the three level-of-detail visual databases.
Another metric, mean visual form ratio, was also
examined. Mean visual form ratio is the average angular
width divided by angular length. It is simply a function of
altitude and range to an object. Figure 64 shows that the
mean form ratio was invariant for the high condition,
decreased slightly with altitude for the medium condition,
and dramatically decreased with altitude for the low
condition.
Motion System Configurations
Two motion configurations were presented: full motion
and no motion.
Procedure
Five NASA Ames test pilots flew the two tasks. All of
the pilots had extensive rotorcraft and simulation experience.
Pilots signaled the initiation and conclusion of the
task by pulling a trigger on the centerstick. The pilots
Figure 64. Mean visual form ratio for the three level-ofdetail
visual databases.
participated in one or two sessions per day, with each
session lasting 1 to 2 hr. Pilots received training during
their first session and a short refamiliarization on subsequent
sessions. These training sessions used several high
level-of-detail databases constructed especially for training
purposes (they were not used in the data collection). Pilots
evaluated the configurations in a randomized order.
Results: Objective Performance Data
All of the results for the altitude repositioning task were
analyzed using a repeated measures analysis of variance
(ref. 54). For this task, the pilot’s altitude repositioning
error was determined by
% repositioning error =
actual altitude change
desired altitude change
desired altitude change
-
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ù
û úú
(20)
Figure 65 shows the mean percent repositioning error for
three of the five experimental factors: vertical motion
presence, reposition direction, and initial altitude. Across
all visual databases, accuracy in altitude repositioning
improved when motion was present (F(1,4) = 39.347,
p = 0.003), with an overall tendency to overshoot the
desired altitude change. This result was very surprising,
for conventional wisdom would suggest that estimating
the required altitude change would be a purely visual task.
It would be expected, based on the results of Vertical
Experiments I and II, that the trajectory quality between
the two final altitude points would be improved with
motion (motion allowing the generation of lead, thus
improving the open-loop phase margin). However, it was
not expected that the presence of motion would affect the
52
Figure 65. Altitude repositioning error.
altitude endpoint. Apparently the pilot was combining the
visual and motion cues in a way that improved his
estimate of the vehicle’s state.
In comparing climbs versus descents, pilots were better at
halving the altitude than they were at doubling it (F(1,4) =
23.339, p = 0.008); there was a tendency to ascend too
high when doubling altitude and to descend too little when
halving altitude. Finally, figure 65 shows that better
accuracy resulted when starting at higher initial altitudes
(F(1,4) = 14.064, p = 0.002); there was an overall
tendency to overshoot the desired altitude. Interestingly,
there were no statistically significant main effects found
when varying the level-of-detail quality among the
databases, nor for the minimum resolution size.
No main effects were found for visual scene level-of-detail
manipulations, but there was a statistically significant
interaction between level-of-detail and initial altitude
(F(4,16) = 3.451, p = 0.032). Figure 66 illustrates this
interaction. For the ascents, the final altitude more closely
matches the desired doubled altitude as the level-of-detail
becomes more constant. Only for the 21-ft initial-altitude
descents did the high constancy level-of-detail database not
result in the best repositioning performance. So, it
appears useful to attempt to have the visual system mimic
the level-of-detail changes, as one would experience in the
real world.
Altitude-Rate Control Task. The performance
measurement for the altitude-rate control task was mean
absolute vertical rate during the climb or descent. Only
data between vehicle altitudes of 10 and 40 ft were used,
thus eliminating the initiation of either the climb or the
descent.
Figure 66. Altitude reposition versus level-of-detail and
initial altitude.
Figure 67. Vertical speed dependence on motion and
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Helicopter Flight Simulation Motion Platform Requirements(35)
