TANDEMS AND CANARDS

If the total lift load required to support the weight of the aircraft is shared between two – main lifting surfaces disposed fore and aft, as in a true tandem layout, then as with the biplane there will be four instead of two tips and four tip vortices. As mentioned before (2.5) an aircraft which has a load-carrying tail, or forewing in the case of a canard, is in a strict sense a tandem and there will be some excess vortex drag. Only if the stabiliser is

rigged to carry zero load in normal flight does the difference in pressure above and below the surface disappear. With it goes the tip vortices and the associated drag. Neither the tandem nor the canard permits this arrangement, which is achieved by appropriate positioning of the centre of gravity. (See further explanation in Chapter 12.)