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from failures or unusual attitudes (ref. 4). This training is
considered too hazardous to perform in the actual aircraft.
Although the previous discussion relates to fixed-wing
transport training, a similar use for flight simulation is
under way for helicopter training. For helicopters,
however, less is known about what level of fidelity is
needed for these simulators. The FAA has released an
Advisory Circular suggesting fidelity requirements for
helicopter simulators (ref. 5), but little data exist to
support the requirements. Its development started with the
fixed-wing Advisory Circular (ref. 3), and the specifications
in most areas were made more stringent owing to
the greater dependency a pilot places on external cues in
helicopter flight than in fixed-wing flight.
The other principal use of flight simulation is for research
and development. When an aircraft system, or component,
reaches a mature level of development, it is often evaluated
by a pilot in simulation. These simulations may be
used to evaluate a new vehicle’s handling qualities or the
functionality of a new system component in a more
realistic and safer environment prior to flight testing.
Flight simulation results may also yield a final product,
such as data for a handling-qualities specification. Finally,
flight simulation may be used to determine the causal
factors in an accident. An accident scenario can be
duplicated in order to hypothesize crew action in response
to events.
In the above instances, flight simulation attempts to
imitate flight. Figure 1 illustrates the key components of
simulation and flight. In flight, a pilot receives cues that
indicate vehicle motion in three main ways. First, motion
is perceived from visual cues with the eyes. Second, the
pilot perceives motion from the vehicle’s acceleration.
Third, the pilot can infer, or predict motion, via the
kinesthetic force and position cues that the vehicle’s forcefeel
system provides. The latter is an often neglected, but
important, cueing source (refs. 6, 7).
In contrast, the pilot seldom receives any of these cues
accurately in simulation. The aircraft is now represented
by a mathematical aircraft model, which is likely to
contain inaccuracies. The visual system, which is
typically computer generated, does not provide the cueing
richness of the real world. The simulator visual field of
view is usually less than that of the vehicle, and the
visual acuity provided today is incapable of rendering
20/20 vision. The vehicle’s force-feel system is usually
the easiest to replicate, although matching the nonlinear
effects (friction, free-play, and hysteresis) and the inertia
characteristics can be challenging. This challenge results
both from a surprising lack of flight data and from
simulator force-feel system limitations. And because the
simulator displacements are constrained, the motion
system can typically provide only a subset of the in-flight
accelerations. It is the motion system that is the focus of
this report.
Of the above cueing sources, only the motion platform
has practical hard technological limits in its capability to
reproduce the in-flight cues. Thus, in light of those hard
limits and the associated costs of providing them, establishing
reliable motion fidelity requirements is warranted.
This is especially true for helicopters, since the pilot often
stabilizes the pilot-vehicle system, and this stabilization
is only possible via feedback from the simulator’s cueing
systems.
The Role of Platform Motion in Flight Simulation
The role of platform motion has been the subject of great
debate. Some researchers and users believe in the extreme
that no platform motion is necessary. Some believe in the
exact opposite. As pointed out by Boldovici (ref. 8),
“Debates about whether to buy motion bases often include
anecdotes, misinterpretation of research results, and
incomplete knowledge of the research issues that underlie
the research results.” Toward understanding the role of
motion in flight simulation, the arguments for the
support of each of these views are given below.
4
Flight
Task
demands
Stick
force
cues
Stick displacement cues
Real-world motion cues
Real-world visual cues
Force-feel
Pilot dynamics Aircraft
Pilot
Simulation
Task
demands
Simulator
stick
force
cues
Simulator stick
displacement cues
Simulator motion cues
Simulator visual cues
Simulated
force-feel
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Helicopter Flight Simulation Motion Platform Requirements(4)
