Any theoretical attempt to evaluate the forces and couples experienced by an aircraft when in flight by a mathematical analysis of the pressures exerted by the air when in motion about the various parts, leads to what have, so far, proved insuperable difficulties. It involves the integration of the equations of motion of a real fluid, and, except in a few very special cases, these have been insoluble. The actual motion of a fluid is affected by a number of its properties, and, in general, accounts would have to be taken of its density, viscosity, and at high speeds its compressibility. In certain circumstances the effect of these last two can be neglected and the classical theory of hydrodynamics dealing with the motion of a non-viscous, incompressible or perfect fluid can be applied. A further simplification consists in supposing that the motion is irrotational, that is, any small portion of the fluid at a point has no angular velocity about its centre of gravity.