DENSITY

Air is a mixture of gases, mostly nitrogen and oxygen. At the fundamental level, gases are regarded as consisting of enormous numbers of separate particles, called molecules, which are in violently agitated motion. The temperature of a gas is the measure of this molecular motion; low temperatures are states of less molecular motion than high temperatures. It is the impact of the moving particles which creates gas pressure on objects immersed in gas. Density is the measure of the number of molecules in a given space.

In low speed aerodynamics it is not necessary to consider the molecular structure of die air. The medium in which models fly is a fluid. This is not to say that air is a liquid. Liquids are fluids which are almost totally incompressible, gases are compressible fluids. Model aircraft do not (as yet) fly at such speeds that the compressibility of the air needs to be allowed for. This is true also for full-sized sailplanes and hang gliders, ultra light, light * and commercial aircraft up to medium sized piston-engined transport aeroplanes.

Compressibility problems do arise for jet-driven aeroplanes and for the tips of «propellers and helicopter rotors. For modelling purposes, fortunately, the air may always be regarded as an incompressible fluid. Even so, significant variations of density occur. *■ They are related to altitude and weather. In Appendix 1, charts (prepared by Jaroslav V, Lnenicka) indicate the magnitude of these effects. At high altitudes and in hot weather, i the air is less dense than near sea level when cold. Modellers operating on the high. plateaux of East Africa or the Americas find air density does make a difference since to

achieve the same air mass reactions to gain lift, their models have to fly faster. Engines and propellers are also adversely affected.

Density is usually expressed in kilogrammes per cubic metre (i. e. mass per unit volume), or in Imperial measure, slugs per cubic foot In aerodynamics a standard value for density of 1.225 kg/m3 (.002378 slugs/ft3) is assumed, corresponding to a sea level value at normal temperature and pressure. For most purposes in design this figure is • adequate. In formulae, the Greek letter p (rho) is used to stand for density (Fig. 2.1a).