WASH IN

‘Wash in’, the twisting of a wing to a higher angle of incidence at the tip, promotes tip stalling, increases the strength of the tip vortex and should be avoided. If, in order to trim a model for turning flight or to balance out torque reactions from the propeller, it is necessary to rig one wing at a different angle from the other, this should normally be done by warping one tip to a smaller angle, ‘wash out’, rather than ‘washing in’ the opposite tip.

‘Wash in’ has the same effects as drooping an aileron. Differential ailerons (see Chapter 13) are geared so as to mitigate the ill effects of this by raising (‘washing out’) the aileron on one side much more than drooping the other. Adverse aileron drag, causing an undesired yaw, is caused by the difference in vortex drag between the downgoing and the upgoing sides.

6.4 THE ELLIPTICAL WING

Mathematical analysis and experiment show that the only type of wing that will produce, at all speeds, constant down wash and a load distribution exactly matching the area is one with elliptical planform distribution (Figure 6.Id). This is not quite the same thing as saying the wing should be a perfect ellipse. It may be so, but any other form which gives a chord at each point the same as the pure ellipse will have the same effect In Figure 6.2 some of the possible variations are illustrated. The effective angle of attack everywhere is the same and the Cl max. is reached simultaneously along the entire span. This follows from the equal distribution of load, area for area, of the wing. In practice, such a simultaneous stall is rarely achieved, since the wing is usually slightly yawed or ‘one wing low’ prior to the stall, and the elliptical planform will then appear to cause tip stalling of a mild kind. Tip stalling is also encouraged by the lower Re of the outer wing. To prevent this an increase of outer chord above that of the pure ellipse is needed. The perfect load distribution is also upset to some extent by the presence of the fuselage which disturbs the flow and, with some wing positions, may reduce the load-carrying capacity of the centre section of the wing to nothing. For these reasons the ellipse is best regarded as an ideal to be approached as closely as possible, rather than the best practical wing planform. For small models the bad effects of low Re at the tip may dictate a rectangular plan, as mentioned above.