TAIL UNIT DRAG

As part of the total drag of a model, the contribution of the tail unit is small, but the same rules apply as for wings. The vortex drag of tailplanes depends on the trim of the aircraft,’ which is discussed in Chapter 12. Otherwise, it becomes important at high speeds for aircraft with strongly cambered wings. The whole unit should be as small as possible commensurate with its necessary function of stabilising and steering. Some reduction of total drag is possible in theory if the three surfaces of fin, port and starboard tailplane, are

reduced to two by arranging the unit as a V tail, the angle between the two surfaces being then approximately 110 degrees. However, the required total area for such an arrangement is no less, and may be slightly more in practice, to achieve the same stability. In some flight positions it is possible for one side of a V tail to be stalled or to be blanketed by the other and this had been known to cause control difficulties. Since the elevator and rudder control effects are obtained by coupling only two hinged surfaces, there are some situations where full control is not available. If, for example, full elevator is applied, and full rudder is required at the same time, the angle to which one of the control surfaces is required to move is very large and it may stall. (Recovery from spins, requiring full rudder against the rotation with elevator down to unstall the wing has been found difficult or even impossible in some full sized ‘V’ tail aircraft)