The equations of motion have reduced the solution of aircraft motion
from determining the time derivatives of the state variables to
determining the resultant forces and moments on the aircraft. The
external forces and moments on the aircraft come from four general
sources: aerodynamic force, propulsive force, force on the landing
gear, and body forces (mostly gravity). The simulator calculates
these separately and then adds them together. For example, the force
in the x-direction is given by:
X = X_{A} + X_{P} + X_{G} + X_{B} | (39) |
Of the different types of force, calculation of body forces is the
most straightforward. The most important body force in flight
simulation is gravity. Gravity acts parallel to the local z-axis.
Thus, the components of gravity force along body axes is the component
in the local z-direction are:
X_{B} | = | (40) | |
Y_{B} | = | (41) | |
Z_{B} | = | (42) |
Another body force on the airplane is the Coriolis force due to the Earth's rotation. This force is important for high-speed, high-altitude flight, but is not that important for low-altitude, low-speed flight.
Calculation of the other types of force is difficult. The following two sections describe in detail the calculation of these forces.