LOADS IN FLIGHT

A constant pitching coefficient in the steady speed flow of a wind tunnel does not imply a constant pitching/b/re when a cambered wing is in real flight The standard equations of Chapter 2 point to the powerful influence of flight velocity: all aerodynamic forces increase in strength with the square of the airspeed. Thus, a constant pitching coefficient means a nose down force which increases enormously as the airspeed rises. This force tends to distort the wing, raising the trailing edge and, since the tips are less rigid than the root, the wing acquires a ‘washout’ that was not intended by the designer. If the wing is suitably stiff in torsion, the twisting will be slight, although there is always some. If the model is comparatively flimsy, with wings covered with plastic film, the distortion may be very severe and has highly undesirable effects. In extremis, the wing itself may twist so far that the tips are ‘lifting’ downwards and they may break off in the downward direction. At best, the carefully designed elliptical lift distribution will be lost at high speed. The twist may also initiate flutter or jam aileron control rods.

The pitching force, nose down, must be balanced in some way, or the model as a whole will be incapable of flight in equilibrium. The tailplane, in an orthodox model, provides the balancing force. At high speed with a cambered wing the direction of this tail force is invariably downwards – the pitching moment tries to pitch the model nose down, the tailplane must restrain this. The more cambered the wing, the larger this load on the tail will be, at a given speed. Some radio controlled model sailplanes, designed primarily for thermal soaring and based on ‘free flight’ model principles, have been known to break up in the air when ‘penetrating’. The tailplanes may break, or the wings, or both. For high speed flight, wings must be stiff in torsion and tails strong in downward bending. A sailplane may ‘tuck under’ into a dive beyond the vertical, if the tailplane is incapable of resisting the pitching force of the cambered wing at speed. (See also 12.22) Another reason for reducing camber on all fast flying models, including pylon racers and multi-task sailplanes is to reduce the download on the tailplane.

7.12 AILERON REVERSAL

The effect of ailerons is not only to change the section ci of the parts of the wings where they operate, but also the pitching moment. A down-going aileron tends to twist the wing to smaller angle of attack and an upgoing aileron vice-versa. As before, such twisting forces increase when the model is at high speed. If the wing is flexible, the effect of the camber change may be equalled and cancelled out completely by the effect of the wing distortion on the angle of attack. A model which suffered from this, as some do, might be deemed to have suffered a radio failure or the servos might be thought overloaded. While such faults as these do sometimes develop, torsionally stiff wings are essential for aileron control at high speeds, particularly on high aspect ratio sailplanes which tend to be flexible and which also require large ailerons.