MODEL SIZE

A large model, flying through air of standard density, must create more disturbance and hence generates more air reaction, both lift and drag, than a small model, at similar speed. The wing span in relation to the model weight, or span loading, is of some importance. A large span wing at a given speed sweeps through a larger mass of air than a short wing. To gain the same reactive forces, with a larger total mass to work on, smaller accelerations are needed. Span loading is expressed as a ratio, weight-per-unit-length (Newtons per metre, or pounds per foot). The capital letter W stands for weight, and the small letter b Sir span (breadth). Span loading = W/b.

Model size is more conveniently expressed in terms of wing area. Units such as square metres or square feet are employed, though these are rather large for modelling purposes and the F. A.I. Sporting Code quotes areas in square decimetres. (One square decimetre equals l/100th. sq. metre.) In this book areas will be given in square metres to conform to standard aerodynamic conventions. The capital letter S is used to stand for square measure, i. e., area.

2.2 VELOCITY

With a model of given span and area, a larger mass of air will be disturbed if speed is high than if low. Velocity, V, is expressed in metres or feet per second, rather than kilometres or miles per hour, in standard formulae (Fig. 2.1c).