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Ha Bart,
Volgens mij is het niet nodig een grotere prop te hanteren bij een dikke radiaal-motorkap dan bij een vergelijkbare gestroomlijnde kap. De motor bepaald de propellorkeuze niet de vorm van de kap, samen met het doel van het vliegtuig zoals Henri ook al aangeeft; kleine diameter en grote spoed is voor snelheid, grote diameter en kleine spoed is voor trekkracht.
Ik vond deze onderbouwingen op rcg (in dezelfde strekking als Henri`s opmerking):
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Propellers don't 'screw' their way through the air. They fly. Propellers are rotating wings. So long as there's clear air in front of it, the propeller will develop lift. What happens behind the propeller is drag. Lift, and forward thrust, still exist, but there is a component of drag, frontal area impedance if nothing else.
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Propellers don't work by "blowing air backwards" as is commonly thought. Like any wing, they work by creating a pressure differential; the airplane is literally SUCKED forward by the low pressure area the propeller creates ahead of the plane.
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However, considering that the most thrust from a prop comes from the outer 2/3 of the prop disk (WAG), if the prop is even a little bigger than the fuselage (9 inch prop, 6 inch fuselage) that should be sufficient to get it into the air, considering most RC models are way overpowered.
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It is surprising how well a plane flies with just the propellor tips extending beyond the cowl. Tests have shown, that in AT-6 racing, it was beneficial to have tips with mor pitch than the root. The reasoning behind it was, that air speed in front of the cowl was lower. The inner flow design of the cowl is very important to drag. If the cowl is designed to let only cooling air for the engine pass (well baffled engine), drag is lowest, and most air will flow around the cowl instead of through it.
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The extreme example I've seen of your perceived problem is a Gee Bee, the R1 or R2 I believe. Most models of this fat, stubby-winged bird have propellers that barely stick out beyond the edge of the cowl, yet they fly fine. That's because the propeller is still doing its job, creating a low pressure area ahead, and a high-pressure area behind. All that really matters is the area immediately ahead of, and immediately behind the propeller. Is it more efficient to have the propeller clear of the cowl.
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Area is proportional to radius squared, so the last 20% of the prop sweeps 36% of the area. Lift is proportional to speed squared, and speed is proportional to radius, so we might (without doing the calculus it really needs) be up to radius^4 - that's gives 59%
Then there are tip losses, which will reduce this figure...
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Er zijn zoals hierboven ook al aangegeven flink wat voorbeelden van 1:1 vliegtuigen te geven waarbij de propellor minder dan de helft (of zelfs nog veel minder) voorbij de grote ronde motorkap uitsteekt, en toch vliegen ze prima! Zelfs racers als de GeeBee:
of jagers als de peashooter:
Bij deze I-16 Rata steekt er zelfs maar een heel klein deel van de prop buiten de cowl:
Groeten, Ramses
