07 How does the flight model influence fidelity?
Description
This article is from the Flight Simulators FAQ, by with numerous contributions by Bob Wittick rwittick@msu.edu others.
07 How does the flight model influence fidelity?
True fidelity in a flight simulator comes from the flight equations
used in the flight model. In general, the more complex the flight
model, the better performance you are going to get, though there are
instances where even a *good* flight model can lead to poor flight
simulation (more on that in a minute).
In general, most of the low-end, low-cost simulators on the market use
what is known as a "3 Degree of Freedom", or 3 DOF, flight model.
This means that the equations of motion only determine x, y, and z
displacements of the aircraft in space, and then use this information
to determine the flight attitude. The actual characteristics are
based on the so-called "performance" equations, which themselves are
usually only defined for steady-state situations. Various other
characteristics, such as roll rate, must be fudged by the author.
Some simulations don't even pay any attention to angle of attack,
using stick input or airplane pitch as the final determining
characteristic.
Most high-end simulators use a 6 DOF model, described below, and a
lot of PC-based simulators tend to ignore these kinds of models
completely, and rely on a "point-space" performance model instead.
The equations of motion do not make the flight model, however; they
merely set the limit on what is and is not possible. In order to
support these equations, you must also have good models for finding
the lift-curve slope, drag coefficients, stability derivatives, and
other parameters.
In addition, you have to decide how you want to calculate these
parameters. Should you calculate your lift on each wing
independently, or just the lift on the whole wing surface area? The
latter method would be faster and easier, but the former would allow
you to model such flight dynamics as the "Dutch roll" modes,
stall-spin conditions, and other common effects. How about downwash
effects, which alter the effective angle of attack of the tail?
There are several issues that need to be considered.
Finally, after you have your flight model together, you need to find
the parameters that fit your aircraft, so that your *plane* flies
just as realistically as your flight model does. For example, you
could have a high-end, 6 DOF flight model, but if your Cessna 172 has
the wrong wing area modeled, it won't *fly* like one.
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