POWER DURATION: THE SPIRAL CLIMB

For the most efficient results in terms of rate of ascent, ways of using excess power without producing perpetual looping must be found. A model with fixed trim for minimum sink on the glide allows only one solution. Since equilibrium is impossible at full power, there must be acceleration. Looping flight is a form of acceleration, the inertia being directed outwards while the excess lift, generated by excess speed and power, is used to oppose the inertia. Instead of looping the loop, such a model must be made to turn in a spiral as it climbs. An inertia force will appear, directed outwards against the turn. The excess lift generated by the high speed will be opposed to the turning inertia by banking the wing and directing some lift force sideways. If the rate of turn is not rapid enough, excess lift will raise the model to a steeper climb angle, and rate of ascent will slow down. If the turn rate is too fast, too much lift will be directed laterally and again, the climb rate will suffer (fig. 4.8).

Although very effective, the spiral climb is wasteful of power, some of that thrust being used only to generate the sideways component of wing lift. This creates high vortex drag. Stability problems also arise.