I-Love-Jets

The prop choice for maximum speed is mainly determined by two variables:
[ul][*] the aerodynamic quality of the airframe[*] the available power (HP and torque peak rpm)
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So let’s take the WESTON MAGNUM R airframe and engine combo as an example for prop selection:

According to Weston the Eurotec V1 engine - a modified Webra .50 - should be combined with the APC 8x8 or 8x9 sport prop for maximum top speed results. In fact these prop sizes do match the Magnum aerodynamic layout quite well. After all true 175 mph top speed is possible in stock configuration – not more!





But why should the MAGNUM owner not take the APC 7x10 prop for instance to gain more top speed?

Well, theoretically increasing prop pitch means increasing prop slip-stream velocity at same rpm. Theoretically the 8“ pitch of the 8x8 APC produce at least 175 mph thrust at estimated 23000 rpm (unloaded). So in this case the calculated slip-stream velocity corresponds perfectly with the real maximum flight speed of the Magnum. This "calculated versus real speed compliance" does apply for many square sized props such like 5x5, 6x6, 7x7 etc if the plane’s airframe is not too draggy.

But in case of the oversquared 7x10 prop size difficulties will occur such like
[ul][*] too much prop stall especially during take-off and during hard turns (the prop doesn’t “bite“)[*] insufficient static thrust[*] Magnum airframe drag kills prop efficiency[*] possible lack of engine torque to maintain the optimum engine and prop operating rpm
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So this MAGNUM example shows how important the reduction of airframe drag is to effectively make use of highly oversquared prop pitches. If the MAGNUM airframe would feature
[ul][*] a sleeker and longer fuselage[*] FIRE or RIRE engine configuration[*] full engine and pipe cowling[*] a wing featuring much more aspect ratio and less sweep[*] a wing featuring a different airfoil[*] a non-flat sectioned elevator and tail fin[*] no canopy[/ul]
then the 7x10 prop size (e.g. the APC 7x10 as a continuous strand carbon fiber copy - a thicker prop hub diameter is a must!) could be used potentially leading to much higher top speeds than 175 mph with a .45 sized ducted fan engine (OPS .45 or OS .46 VX-DF) for instance.




Last not least the prop design itself is most important. There are factors like
[ul][*] prop pitch distribution per blade[*] airfoil type selection and distribution per blade[*] prop tip speeds that should be kept below transonic speeds[*] etc
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For the Speed-Cup participants therefore a sophisticated propeller design is of major interest. It is obvious that each purpose built airframe design does require an individually matching prop design and size...