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movement direction.
Statistically, there were significant effects for the presence
of vertical platform motion (F(1,4) = 78.846, p = 0.001)
and for movement direction (F(1,4) = 14.806, p = 0.018).
There was also a significant interaction between these two
factors (F(1,4) = 12.379, p = 0.024). Figure 67 shows
these effects. Note that the vertical rates were slower with
motion than without motion, and that the vertical rates
were slower when descending than when climbing. The
presence or absence of platform motion had a stronger
effect on performance than the movement direction.
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Figure 68 shows the mean vertical speeds versus altitude
for all of the pilots for the four combinations of movement
direction and motion presence. These profiles
illustrate that for climbs, vertical speed increased with
increasing altitude, and that the increase was more
pronounced when motion was absent. What seems to be
occurring is that the presence of platform motion reduces
the influence of optical flow rate changes on a pilot’s
control of vertical speed. Optical flow rate when moving
vertically (the angular rate at which objects move visually
in elevation) is proportional to vertical speed divided by
altitude (ref. 66). So, at constant vertical speed, optical
flow rate continuously decreases during a climb. If pilots
try to maintain a nearly constant optical flow rate, they
will increase speed with increasing altitude.
The theory that pilots try to maintain a constant optical
flow rate is supported by Johnson and Awe (ref. 67).
They found that during a fixed-base simulation in which
pilots were asked to maintain speed that the pilots often
slowed as their altitude decreased. Since Figure 68 shows
this same tendency, but less so with motion, it is believed
that the acceleration cue mitigates the attempt to maintain
a constant optical flow rate. Manipulations in level-ofdetail
did not affect the pilots’ ability to control vertical
rate.
Summarizing the experiment discussed in this section,
platform motion improved pilots’ accuracy in the altitude
repositioning task, which was surprising. It is hypothesized
that an integration of the visual and the motion cues
is occurring, and that the integration affects a pilot’s
estimate of altitude and altitude rate. The specifics of this
integration are still unknown. That is, future work is
needed to determine how many of the altitude and altituderate
cues are derived from the visual and how many are
derived from the motion system. Still, this study showed
that one cannot ascribe any of the vertical states to a
single cue. The overall conclusion is that for the control
of altitude in simulation to be more like that of flight,
vertical motion should be provided.
Figure 68. Mean vertical speed versus platform motion and movement direction.
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7. Roll-Lateral Experiment
Background
In flight simulation, the roll and lateral translational
degrees of freedom are treated together for several reasons.
First, owing to the motion platform geometry, a certain
amount of lateral translational motion must accompany a
rolling motion to locate the center of rotation properly.
Second, for coordinated maneuvers, in which the body-axis
lateral aero-propulsive forces are zero, lateral translational
platform motion provides an acceleration to counteract the
“leans” that would result from only rolling the cockpit.
Figure 69 shows a coordinated and an extremely
uncoordinated case (roll only) with a pendulum hanging
from the top of a rolling flight simulator cockpit. In both
cases, altitude remains constant. In the coordinated case,
the platform moves laterally with an acceleration of
g*tanfplat; however, the body-axis lateral component of
the aero-propulsive force is zero which keeps ay = 0.
Sustaining the platform acceleration to maintain this
coordination consumes available lateral displacement
quickly.
Coordinated
Lift
Cockpit
Yplat = gtanfplat
Uncoordinated
ay = 0
fplat
Yplat = 0
Lift
g g
ay = –gsinfplat
fplat
Figure 69. Coordinated vs. uncoordinated flight.
Most motion-base flight simulators are hexapods with
similar displacement capabilities. In these devices,
sufficient lateral translational platform travel is not
available to simulate exactly the motion cues that a pilot
would receive in coordinated flight. To reduce the amount
of lateral displacement used, platform drive commands use
several methods. In one method, the roll angle of the
　
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