FAI Tandem (oz15566)

About this Plan

FAI Tandem. Indoor duration model.

Quote: "Until the FAI adopted a one gram minimum weight rule for the 1972 Indoor World Championships, the design of indoor microfilm stick models had reached a plateau. A competitive model was invariably a conventional tractor with pylon mounted wing, a tail boom of about the same length as the motor stick, and a horizontal tail with area between 30% and 40% of the wing area.

This basic configuration had evolved over many years in which indoor model weight was not restricted by the rules. In the absence of weight restrictions, the conventional configuration represents an optimum compromise between aerodynamic efficiency and structural weight. Successful designs could and did differ from each other in detail, but for a design to be competitive, its basic configuration had to fall within certain limits. Most experienced indoor modelers realize this fact intuitively and Max Hacklinger theoretically demonstrated it in the early Sixties.

The new one gram weight rule, however, poses a different set of requirements than those which produced the conventional layout. It could well be that the optimum model for this new set of rules is quite different from the models which evolved in the absence of weight rules.

The search for a new optimum configuration has already produced some unconventional models, such as the extremely low aspect ratio design flown by Jim Richmond in the 1971 team selection finals, and the biplanes by Clarence Mather and Tom Vallee. Designs along these same lines have also appeared in the Pennyplane class, as a result of the fact that the Pennyplane rules impose some of the same design constraints as the new FAI rules.

These unconventional approaches illustrate the design dilemma posed by rules which impose a maximum wingspan and a minimum weight. In theory, indoor model duration can always be increased by reducing the wing loading. In the past this was often done by reducing the weight of the model. With a minimum weight rule, the only way you can reduce the wing loading is to increase the wing area. But, because the span is limited, increasing the wing area means reducing the aspect ratio, thus increasing the drag. So, the rules force a trade-off between increased wing area and reduced flight efficiency.

I order to see how various design factors can affect this trade-off, we spent some time developing a theoretical method for predicting indoor model performance. The method involves making theoretical estimates of a model's lift coefficient, overall drag coefficient, and propeller efficiency..."

Direct submission to Outerzone.

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