The lift produced by a wing is dependent on the flow speed and the circulation, which is related to the strength of the vortex system. In level flight, the lift is equal to the weight. Thus, at constant altitude and aircraft weight, the required vortex strength is reduced as the speed increases. Since the trailing vortex drag also depends on the strength of the vortex system, the trailing vortex drag also reduces with increasing speed. In fact, the drag coefficient for trailing vortex drag is proportional to CL2, and it may be remembered that for level flight the CL value required reduces with increasing speed.
In contrast, the boundary layer normal pressure and surface friction drag rise roughly as the square of the speed. From Fig. 4.21 we see that as a result, there is a minimum value for the overall drag, and this minimum occurs when the trailing vortex drag is equal to the boundary layer drag. There is, therefore, a disadvantage in trying to fly any aircraft too slowly. The implications of this, in terms of performance and stability, are discussed in later chapters.
It is important to note that trailing vortex drag is not the only drag contribution that is lift-dependent. If a symmetrical wing section is set at zero angle of attack to a stream of air, the boundary layers on both upper and lower surfaces will be identical, but once the angle of attack is increased, and lift is generated, the boundary layers will alter, together with the amount of drag produced. Thus, it will be seen that some of the boundary layer (profile) drag is also lift-dependent.
For further information on drag, the reader is referred to Hoerner (1965), who gives an excellent detailed treatise on the subject.
Recommended further reading
Lachmann, G. V., (editor), Boundary layer and flow control, Vols I & II, Pergamon Press, 1961.
Hoerner, S. F., Fluid dynamic drag, Hoerner, New Jersey, 1965.