Date of this Version
Published in Transactions of the Nebraska Academy of Sciences, Volume 1 (1972).
The indoor model is a free-flying rubber-powered model aircraft designed to achieve maximum flight duration through light construction and proper aerodynamic configuration. So fragile it can be flown only indoors, a world competition model weighs approximately one gram, is powered by a motor of equivalent weight, and under ideal conditions in sufficiently large halls is capable of single-flight durations exceeding forty-five minutes. World competition regulations established by the Federation Aeronautique Internationale restrict only wing span, thus design problems arise from a different direction from those of man-carrying aircraft. With the wing planform largely determined by the given span, the designer is faced with the problem of coordinating the horizontal tail surface area and center of gravity location of the ship with respect to the wing to minimize power consumption and thus to achieve maximum duration.
Tailplanes of man-carrying aircraft are used exclusively for stabilizing purposes but on a model it is possible to obtain lift from them as well. This implies a center of gravity aft of the center of lift of the wing. Under certain conditions a model will perform significantly better with the center of gravity 15 to 50% of the wing chord behind the center of lift of the wing.
The builder, in the "design" of his model, is then confronted with decisions on three important related variables: How large shall he make the horizontal tail surface? How far aft is the center of gravity to be placed? What angle of wing incidence is best? These variables, unfortunately, are not independent. A large stabilizer, for example, admits of a farther aft center of gravity than a small one and may, in fact, require it for equivalent performance. Aerodynamic design has hitherto been largely empirical in nature, values for the above variables being chosen by the builder on the basis of prior experience (or upon values embodied in the latest record-breaking model). No mathematical solution, valid for indoor models, to establish optimum model dimensions and to determine force vector locations and surface settings, is known. There exists a stability equation for man-carrying aircraft which incorporates the above variables. This equation is linear and plots as a straight line whose slope denotes the degree of stability or instability. This equation is, however, linearized by aerodynamicists using two important simplifications which render it invalid for indoor model aircraft design, namely, that the wing provide all of the lift, and that the wing lie on the action line of the propellor thrust vector.