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0,2,4,0
V7
2,4,0,0
V5 V2
5,1,0,0
V1
3,1,0,2
Figure 61. Pilot motion fidelity ratings for vertical task.
49
6. Vertical Experiment III: Altitude
and Altitude-Rate Estimation
Background
Relative to flight, simulation has perennially produced
less effective control of altitude and altitude rate. Fixedwing
landings in simulation usually have higher runway
position dispersions and higher mean touchdown velocities
than in flight. Bray showed that with a 16-in blackand-
white television monitor, simulator vertical
displacements of at least ±20 ft are required in order to
achieve the desired fidelity in the landing task (ref. 60).
In helicopter simulations, pilots often comment that the
vertical damping (as perceived from the visual and motion
cues) is less than it is in flight (refs. 59, 63–65). Since
the math models have often been developed and validated
from in-flight measurements, attempts to determine the
cause have focused on the simulator visual and motion
cues.
Often, the assumption is made that pilots obtain vehicle
acceleration information from the motion system, and rate
or position information from the visual system, as shown
in figure 62. Thus, decreasing motion gain or increasing
washout frequency is assumed not to affect the outer rate
and position loops. In this work, an experiment was
designed to test this assumption by exploring the effects
of visual scene properties and vertical motion on the
estimation of altitude and altitude rate.
Experimental Setup
Five factors were incorporated in Vertical Experiment III:
(1) reposition direction, (2) initial altitude, (3) presence of
vertical motion, (4) visual scene level-of-detail control,
and (5) visual scene mean object size.
Tasks
The experiment involved two tasks.
1. In the first task, the altitude repositioning task, pilots
were instructed either to double or halve their initial
altitude. The initial altitudes were 15.6, 18.3, and
21.0 ft. Altitude was the only degree of freedom under
the pilot’s control. No time requirement was placed
on the task, and no requirements were imposed on
overshooting or undershooting what the pilots felt to
be their doubled (or halved) altitude.
2. In the second task, the altitude-rate control task,
pilots were instructed to climb or descend at a
constant rate of 3 ft/sec. The climbs started at an
altitude of 7.5 ft and ended at 42.5 ft. The descents
started at an altitude of 42.5 ft and ended at 7.5 ft.
These unusually specific altitudes were selected to
allow one-to-one motion and visual cues in the
Vertical Motion Simulator.
For both tasks, all instruments were disabled so that
pilots would have to estimate their altitude and altitude
rate from the visual scene, motion system, or cockpit
collective movement. Although pilots would ordinarily
accomplish these tasks in flight by referring to an
instrument that measures either altitude or altitude rate,
these instruments were intentionally disabled so that the
possible value of the motion and visual cueing variations
could be determined.
Pilot
Acceleration
cue
Rate-of-climb and
altitude cues
Aircraft
model
Motion
system
Visual
system
Figure 62. Typical assumptions in the apportioning of
simulation cues.
Simulated Vehicle Math Model
The math model represented the AH-64 Apache helicopter:
˙˙
˙
.
. .
˙ .
.
h
z
h
z
c
1 1
0 122 118
0 0 12 9
14 6
1 00
é
ë
êê
ù
û
úú
=
- -
-
é
ë ê
ù
û ú
é
ë
êê
ù
û
úú
+ é
ë ê
ù
û ú
[d ] (18)
The state z1 was added to approximate the effects of
dynamic inflow into the rotor. All other displacements and
orientations were held fixed at zero.
Simulator and Cockpit
The simulator and cockpit were the same as for the yaw
experiment described in section 3. The Evans and
Sutherland CT5A visual system was used, but both the
hardware and software were modified to achieve the visual
variations described below.
50
Design of Visual Scenes
Simulated scenes are typically less dense (fewer objects
per degree of field of view) at lower altitudes than at
　
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