ORIGIN OF AERODYNAMIC DATA OF DESIGN - AEROPLANE or AIRPLANE.
The designer bases his calculations on data derived from four main sources : ( 1) Theoretical investigations, such as the Lan chester-Prandtl theory of the action of wings. (2) Experiments on models in wind tunnels. (3) Experiments on full scale aero planes in flight. (4) Analysis of the measured performance of aeroplanes. References to the first of these divisions will be found in the article on AERODYNAMICS, where also the principles under lying experiments on models are dealt with.
The wind tunnel has played a great part in the development of aeronautics. Many manufacturing firms possess their own tunnels, used mainly for measurements of the resistance of aeroplane bodies, etc., with a view to improving per formance. National wind tunnels exist in many countries, gener ally reserved in principle for research on problems of wide interest and application.
The main advantage of model experiments is the ease with which the experimenter can control the conditions.
Most of the leading aircraft-pro ducing countries carry out full scale experiments. In Great Britain they are made at the Royal Aircraft establishment under the control of the director of scientific research at the Air Ministry. In the United States they are carried out by the National Advisory Committee on Aeronautics, by The Material Division of The Army Air Corps and by certain manufacturers.
The measurements of speed and climb which are made during the acceptance tests of aero planes are a valuable source of information for the designer. Their analysis enables an overall comparison between various types to be made on a rational basis, but necessarily lacks some thing in precision. For each type differs from the others in many features, so that, for example, an observed difference in speed cannot safely be ascribed to any one peculiarity. But with suitable precautions reliable information can be obtained in a convenient form. It is found that the factors which have the greatest in fluence on the speed and climb of an aeroplane are the horse-power per unit of total weight and the total weight carried per unit of wing surface. For further details of the method, and its limita tions, reports of the National Advisory Committee on Aeronautics (U.S.) or of the Aeronautical Research Committee (British) should be consulted.
