Scale effect and the boundary layer

3.1 THE BOUNDARY LAYER

The most important differences between model and full-sized aircraft aerodynamics can be attributed to the boundary layer, the thin layer of air close to the surface of a wing or any solid body over which the air flows. Two properties of air, its mass and its viscosity, determine the behaviour of the boundary layer. Viscosity may be roughly described as the stickiness of any fluid. Treacle and glycerine are highly viscous at normal temperatures. Cream and water are less viscous, air and other gases are less viscous still. The viscosity, like the density of air, is beyond control for practical purposes in model aerodynamics. Like air density, it does vary with temperature and air pressure, as Lnenicka’s chart in Appendix 1 shows. Inertia opposes change of direction or velocity. Viscosity resists shearing flows and tends to keep the fluid in contact with surfaces. In situations where fluid in the boundary layer over a surface is accelerating or decelerating, forces arising from mass and from viscosity interact, sometimes reinforcing one another, sometimes in mutual opposition. Where velocities of flow are high and the curvature of surfaces relatively large in radius, as with full-sized wings at high speeds, mass inertia is dominant, the effects of viscosity, though not negligible, are smaller. With model wings, at low speeds, viscous forces become relatively much more important A very small wing, such as that of an insect, operates in a fluid which seems relatively much more viscous than the air does to the wing of an airliner. Model aircraft, and full-sized sailplanes, man-powered aircraft, hang-gliders, etc., come somewhere between. It cannot be expected that a model wing, even one made to exact scale from a full-sized prototype, will behave in exactly the same way as its larger counterpart Unfortunately such scale effects almost invariably work to the disadvantage of the smaller aircraft.