may suppose that the air skirting the upper surface of the aerofoil has a component motion imparted towards the axis of flight,
Fig. 84. and that skirting the under surface in the opposite direction, so that when the aerofoil has passed there exists a Helmholtz surface of gyration. This surface of gyration will, owing to viscosity, break up into a number of vortex filaments or vortices after the manner shown.
§ 127. Peripteral Motion in a Real Fluid (continued).—The cyclic flow of the vortices to the right and left hand of the aerofoil finds itself superposed on the main cyclic system of the aerofoil,
Fig. 85. so that the axes of these vortices will not be parallel to the axis of flight as might be supposed, but will take up a resultant direction and may be conceived to spread out as shown in Fig. 85. The compounding of two cyclic systems into a resultant system is illustrated diagrammatically in Fig. 84, in which the circle represents the main cyclic system, that whose supporting reaction is concerned in sustaining the load; represents the cyclic system of one of the vortex filaments, and the resultant.
