POWER DURATION: VARIABLE TRIM

The spiral climb is effective in that it allows motors to be operated at maximum power. High rates of climb are achieved, but even better rates of climb would result if the model did not have to spiral. The speed of flight up the climb path would be greater if the excess lift force could be prevented from appearing. This can be done by reducing the angle of attack and camber of the wing, but this unfortunately spoils the gliding trim (Fig. 4.9). The best aerodynamic solution to the problem is variable trim and/or variable wing camber. By trimming the model under power to climb with a low Cl, and therefore no excess lift, energy wastage is reduced and there is no need to spiral. The wing camber should be reduced to that which gives least drag at the low lift coefficient, and the tailplane trim adjusted accordingly. When the motor run ends, both camber and trim should change mechanically to give the best possible glide. During the climb, torque and slipstream effects tending to make the model turn should be trimmed out as far as possible, to keep the flight straight

Since the climb is at low Cl and high velocity, as Figure 2.11 indicates, vortex – induced drag will be low, much lower than with the high Cl spiral climb. The parasite drag will be high, but if the correct camber is chosen, profile drag can be reduced as discussed in Chapter 7. A considerable improvement in climb results.

The rubber powered model also tends to loop under the surge of power from the motor just after release, and there is a good case for variable trim in this situation too. The

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tailplane setting should change progressively from that for low Cl when the motor is at full power to high Cl as the power fades. (Practical mechanisms were published in the ; Aeromodeller Annual for 1972, page 78, and in A. M. Annual 1974-5, pp. 122- 127).