BALANCE WITH ZERO TAIL LOAD

As mentioned above, if the mainplane is of symmetrical section, the tailplane will carry no load. (A stabiliser will still be needed.) This situation can be attained at one flight speed, if the centre of gravity position is aft of the mainplane aerodynamic centre, as suggested in Figure 12.5. Here, there is a nose up moment produced by the lift and weight couple. This can be adjusted, by careful c. g. location, so that it exactly equals and so balances the

With the canard, balance is achieved with an upload on the forewing.

[L + Lt = W]. The forewing creates some vortex drag but relieves the mainplane of some load. Stability problems would arise with this arrangement. The mainplane creates upwash in the neighbourhood of the foreplane. This has an effect on rigging angles.

camber-induced nose-down moment. The tail then has no balancing role in this flight attitude and is only a stabiliser.

In the days when designers thought in terms of a moving centre of pressure, it was often recommended that the centre of gravity should be located at about 33% of the mean wing chord, to produce this kind of balance. This was because the models were intended always to fly at low speeds and the wind tunnel charts (see Figure 10.4) available in those times showed that at high lift coefficients the centre of pressure on many well cambered wing sections just below the stalling angle was at about 33%. In modem terms, the effect is still the same. It happens that with well cambered profiles, tat high cj the pitching moment coefficient at the aerodynamic centre is roughly equivalent to a rearward movement of the centre of pressure of about 8%. Hence the old balance remains about right for the one, slow, flight speed. It is clear, all the same, that at any other trimmable speed, faster or slower than the one favoured for the old free-flight models, the tail load can be brought to zero only if the centre of gravity is in some other location. Putting this again in centre of pressure terms, at high speeds, as the old chart shows, the c. p. moves back, so the tail load can be made zero only by moving the centre of gravity back, to increase the strength of the lift-weight couple.

The advantage of trimming for zero tail load is that when the tailplane is not lifting, it creates no tip vortices and hence no vortex drag. With a thin, symmetrical profile its parasitic drag is then at an absolute minimum. Fortunately, at low speeds the parasitic drag contribution to the total drag of a model is quite small (see Figure 4.10) so on the free flight models such a balance was of very slight advantage. Saving a fraction of the tail drag saves only a fraction of a fraction of the parasitic drag of a model. At low speed, this itself is a small fraction of the whole. There are dangers in this trim as far as stability is concerned, but for the moment consider only the case of a dive. In a vertical dive, there is no wing lift. The weight of the model acts straight down. There is now NO pitching moment or couple of the weight with the lift to produce a nose up moment. Yet the wing camber remains and the nose down pitching force from this cause is very powerful, because the airspeed is high. The tailplane, even though originally rigged for zero load, must provide a powerful balancing down force to prevent the model bunting. It is clear again that although the zero load trim operated at slow speed, at high speeds the tail must carry a down load if balance is to be achieved.

On fast models with wings of slight camber, the zero tail load condition can be’ achieved by locating the centre of gravity aft, as before. This can result in quite substantial drag savings because parasitic drag becomes very important at high flight speeds. It still follows that the tail must produce a download at steep diving speeds, and an upload at low speeds, to maintain balance. In general, then, any model with a cambered wing profile

may be trimmed for zero tail load at one flight speed, but at faster speeds than this it will carry a download and at slower speeds, an upload.

With canards all the same points apply, but the load’s direction is the other way, and stability problems arise, more severely as the centre of gravity is moved aft.