IN most aerodynamical problems it is convenient to consider the aircraft to be stationary and the air to be flowing with a speed equal to the flight speed in a direction opposite to the direction of the flight. This practice simplifies the investigation of the motion of the aircraft and is perfectly legitimate from the mathematical point of view. Thus, if the flight speed is v, the kinetic energy or the dynamic head, q, of unit mass of air is ½ρv2. It the air is allowed to flow through a restriction, for example, a radiator or an air intake duct, the velocity of flow is reduced, and the dynamic energy thus lost is converted into the pressure energy. In most problems it is sufficiently accurate to assume that air is an incompressible fluid so that the density remains constant, and the flow changes can be investigated by applying the well‐known Bernoulli's Equation, viz., p+½ρv2=constant. This assumption is not strictly true, as the flow changes are usually brought about very quickly and there is little chance for the heat generated to be dissipated. However, the error made by this simplified assumption is not very large if the flight speed is fairly low. But in the case of high‐speed aircraft, e.g. a fighter aircraft, the above assumption involves a considerable error. To make a due allowance for the suddenness of the change, it would be necessary to discard the notion of the incompressibility of air and to use the adiabatic law between the pressure and the density of air. Thus, a better and a truer picture of the actual state of affairs would be obtained by assuming the air to be compressible and to investigate its effect on the pressure, density, and the temperature of the air. It is proposed to make a theoretical investigation of this problem on these lines and to present the results in the form of tables, graphs, and nomograms which could be easily applied in the solution of any practical problem on the flow changes.