How a Computer Simulation Works
by Chris Sweeney

October 28, 1997

A computer simulation of an aircraft is based upon the mathematics and physics of aerodynamics and flight, converted into computer code. The accuracy and realism of the simulation is only as good as the mathematical equations which describe the aircraft. The better the model description, the better the pilot will like the simulation.

A computer simulation works by calculating all of the equations of the aircraft as fast as it can. One pass through all of the equations is called one cycle. The faster the computer can cycle through the equations, the better the simulator will "fly." The computers we use now can cycle through the equations once every 0.01 seconds or 10 milliseconds. This is a 100 Hertz frequency or 100 cycles per second.

At the beginning of the cycle, the computer reads the pilot's inputs from the control stick, the rudder pedals, the throttles, and any other device the pilot may be touching. These commands are sent to the flight control system equations to calculate how these pilot commands should deflect the control surfaces of the airplane like the elevators, the ailerons, the rudder, the flaps, and the engine. These surface positions are used by the aerodynamic equations to calculate the aerodynamic forces exerted on the aircraft due to the deflections of the control surfaces and the speed of the plane. The throttle position is used by the engine model equations to determine how much thrust is being generated by the engines.

The aircraft model uses equations of motion to move the motion system and to move the aircraft over the computer-generated ground. From physics, we know F=M*A (force equals mass times acceleration). We divide the mass of the aircraft by the forces (aerodynamic, engine, and gear) acting on the airplane to calculate an acceleration. We mathematically integrate the acceleration to get the aircraft velocity and integrate the velocity to get the aircraft position. We send an acceleration command to the motion system which moves the cab to generate an acceleration the pilot can feel. We send the position of the aircraft to the visual system to update the visual display so the pilot sees the plane moving down the runway or moving through the air.

The pilots respond to the feel of the motion and what they see out the window by moving the controls, and the whole process begins again.