Before leaving the rubber motor there is one otber subject worthy of discussion. In terms of getting the most out of the motor the conventional folding prop, design shows up badly in theoretical analysis, mainly because the trim required produces a most inefficient set-up towards the end of the power run. To overcome the change in trim as the motor stops, the propeller folds and the model goes into its glide, the conventional folder-design is generally losing height quite appreciably over the latter part of the power run—Fig. 6. The model becomes progressively more and more under-elevated as the end of the power run is approached so that it is, often appreciably, diving under power, A well trimmed model with a non-folding prop., on the other hand, will at least maintain height during this period, or even continue to climb slowly. Cutting off the power run early to prevent this is a partial solution, but the non-folder still has the advantage in this respect. Even when the power output level has fallen below that necessary to maintain level flight, the model is descending more slowly than il would on the glide. Hence it has a longer effective power run.
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