BALANCE WITH NO TAIL

Figure 12.7 shows that a wing which has no pitching moment at any normal angle of attack may be balanced without a tail or forewing. Such a wing may be of symmetrical profile, since such aerofoils do not produce any pitching forces except when stalled, or a profile with a reflex or‘cubic’ camber line may be employed (see Figure 7.3). Figure 12.8 shows a common tailless aircraft layout which helps to overcome stability problems.

12.3 BALANCE WITH FORWARD CENTRE OF GRAVITY

Figure 12.9 shows that balance is possible with the centre of gravity ahead of the mainplane’s aerodynamic centre, on a model of orthodox layout The tailplane and wing will generate additional vortex drag, so there is no advantage in such an arrangement except that it tends to be very stable, as will appear. Some scale model aircraft with stability problems have been flown successfully with this kind of trim.

12.4 TAILPLANE LOADS AND THE ELEVATOR POSITION

Modellers frequently are puzzled at the fact that tail loads are normally downwards in a dive or fast trim, when the elevator has to be held down to hold the attitude.

This is related closely to stability. In Figure 12.5 the tailplane is shown carrying zero load and its exact angular setting to achieve this depends on the downwash from the main wing. Suppose the pilot wishes to re-trim for a faster flight speed. The elevator (or all – moving tailplane) must first be moved down and this momentarily produces a lifting force on the tail which causes the nose to go down. But the change of wing angle of attack reduces the Cl and this in turn reduces the downwash at the tail. There is, therefore, an increase of angle of attack at the tail tending to produce a lifting force there. If the nose – down pitch brought about by the initial elevator movement is/ not quickly checked, the combined effect of down elevator and reduced downwash. will cause the nose-down motion to continue and become exaggerated. Hence as soon as the new flight attitude is reached or earlier, the elevator must be re-set for a new set of conditions. If the model is stable, it will finally be trimmed to give a download where previously it was at zero, but this condition will be reached with a small amount of down elevator at the control end.

An unstable model will react differently. The initial down elevator movement will, as before, produce a nose-down pitch, increase of flight speed, and reduced downwash, but to prevent the motion going too far, the elevator will have to be checked more quickly and the eventual trimmed position will be elevator up. That is, an unstable aircraft will dive with elevator up, and fly slowly with elevator down, even thought the initial control movements needed to bring these attitudes into being will still be in the usual sense.

A neutrally stable model will respond normally to control movements, but every flight attitude will trim out with the elevators neutral.

To achieve zero tail loads at a number of flight speeds, for the sake of reducing parasitic drag, it is possible to use an adjustable centre of gravity. This is done in some full-sized sailplanes, by means of mercury reservoirs in nose and tail. At low flight speeds when zero tail load is required with high Cl and strong downwash, the mercury is all pumped into the forward tank to bring the c. g. as far forward as possible. When high speed trim is required, and tail drag becomes more significant, an aft c. g. is required and the mercury is pumped to the tail.

Unfortunately, this reduces stability and as any radio controlled model flier knows, as the stability is reduced the elevator control becomes increasingly sensitive and even ‘twitchy’. To fly a sailplane, or any other aeroplane, at very high speeds with aft centre of gravity is very dangerous since a small twitch on the controls can precipitate a severe pitching nose up or down, and this can break the wing or tail. It is probably wiser to put up with some loss of high speed performance for the sake of safety. At low speed the gain is very slight in any case.