The boundary layer and its control

One of the most important advances in the study of aerodynamics was the discovery of the influence of the ‘boundary layer’. This is a very thin layer of air adjacent to the surface of the aircraft. Despite its thinness, it holds the key to understanding how air flows behave, and in particular how lift is generated.

In this chapter we shall show how a knowledge of the behaviour of the boundary layer can enable us to improve the lift, drag and general handling characteristics of aircraft.

A major breakthrough

At the time of the Wright brothers’ first flight, a significant part of the basis of modern aerodynamic theory already existed in the form of the so-called clas­sical theory of fluid mechanics. In the early stages of its development, this theory took no account of the effects of viscosity (the stickiness of the air), with the unfortunate result that lift and drag forces were not predicted. For some time, therefore, it was little more than a toy for mathematicians. Equations which correctly took account of viscosity had been derived, but these ‘Navier-Stokes’ equations are extremely complex, and in their complete form they were of little practical use, until invention of the digital computer.

A big breakthrough occurred a few years after the Wright Brothers’ historic flight, when Prandtl found that the effects of viscosity were only important and apparent in a very thin layer adjacent to the surface. He called this the boundary layer. For an aircraft wing in cruising flight, it is, at most, only a few centimetres thick.

Although an exact analysis of the flow in the boundary layer was not possible, approximate methods based on experimental observation were developed. Outside the boundary layer, the effects of viscosity are negligible, and develop­ments of the classical theory can be used. By combining boundary layer theory

with the classical theory, it eventually became possible to produce results that were of real practical use.