Factors affecting lift and drag

2.1 ACTION AND REACTION FROM AIR

The air forces which act upon a model, both those which support it and those which resist its movement, arise from die properties of the air, which has mass. To generate supporting force a mass of air must be accelerated or deflected to yield an upward reaction which, for equilibrium, equals the weight. To work on the air the wing or wings of the model must move through it, disturbing it In addition to the wing, all other components of a model, such as fuselage, tailplane, undercarriage, etc., also disturb the air and add to the total of energy needed, without, in general, adding any lift. The greater the expenditure of energy required to generate a given lift force, the less the efficiency of the model.

The mass of air available for a model to work on depends on three factors: 1) the amount of air in a given space, i. e. the mass density of air where the model operates; 2) the size of the model; and 3) the speed or velocity of its flight (Fig. 2.1 a, b and c).