POWER DURATION MODELS: CLIMBING

Many models, including modem electric-powered aircraft, have much greater power available than is necessary merely for sustained flight or a slow climb. This applies to rubber driven models immediately after launching and to all successful engine driven duration models. With these, the best trim for the glide is incompatible with that for the fastest climb at maximum power.

Consider a model with a fixed trim flying straight and level under power. The tailplane holds the wing at a constant angle of attack and so at constant Cl – If the power is increased, slightly, the first result is a forward acceleration. The Cl remains the same so this increase in V causes an increase of the lift force, and this accelerates the model upwards. It begins to climb at some angle. When it settles down again to equilibrium, as was shown in Figure 1.4, the lift force is reduced because some of the weight is supported by a component of propeller thrust To get a reduced lift with fixed Cl velocity along the inclined flight path must be reduced. This is essential to balance the lift equation.

Suppose that after a flight in this condition, a little more power is added. The result will be, again after a short period of non-equilibrium, a climb at a steeper angle, but again, velocity must be reduced. The wing is still held firmly by the tailplane at its original angle of attack to the airflow, and as more and more power is applied, the wing lift force required is progressively reduced. For each power setting of the motor, there is one angle of climb, and only one, at which equilibrium can be established, and the steeper the angle, the slower the flight speed. Going to the extreme position, represented in Figure 1 Ad, it is possible to increase power until the angle of ‘climb’ is 90 degrees. The wing then must yield no lift. Since the tail is still holding the wing at its constant angle of attack, the only way the wing can give no lift is if its forward velocity is zero. For equilibrium in such an attitude, a fixed trim power model must hover with no rate of climb at all. Any forward speed would generate lift on the wing and the model would begin to loop the loop. To achieve the vertical attitude and hold it, the model requires more power than it did at some less-steep attitude. (It actually climbed quite well under reduced power, whereas now at a higher power it gains no height at all.)