Flight Simulation Software

3 Aircraft Equations of Motion

If anybody ever tells you anything about an aeroplane which is so bloody complicated you can’t understand it, take it from me: it’s all balls.

R. J. Mitchell

3.1 Atmospheric Model

3.1.1 The Atmosphere

Most flight simulation covers flight in the troposphere, which is the lowest layer of the atmosphere from the surface of the earth up to an altitude of approximately 18,000 m. In the troposphere, Boyle’s law applies in terms of the relationships between pressure, temperature and density. As pressure varies with altitude, an altimeter, which is effectively a barometer, measures altitude. The temperature of the air entering an engine affects engine performance for both piston-engine aircraft and jet-engine aircraft. The density of air is used in the computation of lift, drag and other aerodynamic terms, affecting aircraft performance. Consequently, accurate modelling of the atmosphere is a critical component of flight simulation. The equations covered in this chapter are mostly based on the US Standard Atmosphere (Anon, 1976).

The International Standard Atmosphere (ISA), which corresponds to a temperature of 15°C at sea level, a pressure of 101,325 Pa and an air density of 1.225 kgm⁻³, provides a set of reference values. These variables, which define weather conditions, vary over the surface of the earth and with altitude and time. In practice, temperature and pressure are normally set to default (ISA) or specific values in simulation, to define ...