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a slight decrease in performance, and the compensation
increased over that of the V1 case. Yet all pilots rated the
configuration high fidelity, since they felt is was “close to
visual flight.” Here is where the fidelity definitions could
be made more precise, because a pilot is faced with a
potential dilemma with a configuration that has noticeable
but not objectionable differences (medium fidelity definition),
yet is close to visual flight (high fidelity definition).
Pilots apparently felt that when it was necessary to
group the ratings into three categories, the V2 configuration
was in the most favorable category. Yet they all
perceived a difference. It appears that the pilots may have
been sensitive to the differences in the immediate acceleration
because of collective input. This initial acceleration
is proportional to the high-frequency motion gain,
and it is reasonable that the 10% change between these
configurations could be noticed. It is also reasonable that
the variation was not objectionable.
For the high-gain and moderate phase-distortion case,
configuration V3, all pilots noticed the necessity to
change their control input technique. Two pilots noticed
the unpredictability, or the “slipperiness,” of this configuration.
Only one downgraded the fidelity rating to
medium. The objective performance did not change
appreciably in the bob-up, but difficulty was perceived in
repositioning downward. Although the ratings indicated
the configuration was high fidelity, the comments and the
performance did not agree. Thus, the V3 configuration
should not be placed in the high-fidelity region.
Increasing the phase distortion further to the V4
configuration resulted in a perceived degradation in
39
performance, and this is evident from the phase-plane data.
The aggressiveness level was reduced, and the required
compensation increased. Two of the three pilots noticed a
phasing difference between the visual and the motion cues,
yet with all of the above, they rated this configuration no
worse than medium. However, one pilot thought that it
was close to being objectionable. Considering the
comments and the performance, it appears that the fidelity
was closer to low than the rated medium, and thus no
change is suggested.
Comparing these near-unity motion gain results with the
only other set of high quality experimental data for helicopters
(ref. 24) shows similar trends. That experiment
showed that for tracking a randomly moving target, while
the open-loop crossover frequency remained nearly
invariant with increases in motion-system phase
distortion, the phase margin increased by 30° when going
from a motion phase distortion of 108° to a phase distortion
of 16°. For a phase distortion of 43°, which is similar
to configuration V3, a phase margin degradation of 15°
was measured. The comments and objective performance
data from the present experiment, for these cases, indicated
a similar trend in stability degradation, yet the subjective
fidelity ratings for this task still rated the V4
configuration as medium fidelity.
In another experiment (ref. 60), for aircraft with poor
longitudinal flying qualities in a landing task, it was
observed that phase distortions higher than 90° caused
“essentially uncontrolled touchdowns.” Thus, considering
previous data and the lack of consistency in this experiment’s
objective and subjective results for these configurations,
it seems appropriate to retain the hypothesized
fidelity breakpoints suggested by Sinacori (ref. 44) for the
near-unity gain configurations.
Returning to the low phase-distortion configuration with
moderate gain, V5, pilot comments and objective performance
generally indicated no degradation from the fullmotion
case. Only one pilot rated the fidelity as medium
(owing to a slight vertical oscillation). From the consistency
of these results, it appears that when the phase
distortion is less than 30°, lowering of the gain required
for high motion fidelity from 0.8 to 0.6 is appropriate.
Keeping essentially the same motion gain, but increasing
the phase distortion to configuration V6 resulted in
comments indicating a performance loss because of a
reduction in precision and aggressiveness. Fidelity ratings
of medium are consistent with Sinacori’s hypothesized
boundaries (ref. 44).
A reduction of gain to 0.3, but with a low phase
distortion in configuration V7 resulted in adequate
performance, fears of overshooting, and comments that the
motion was almost unusable. These comments were
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Helicopter Flight Simulation Motion Platform Requirements(27)
