I have a question on the prop type for a hovercraft. I know this is a plane newsgroup but hope they are similar enough to answer the question.
I have an OS40 engine with a 9 x 6 prop. I can not use a bigger prop due to the size of the housing but want to get more power. Would I get more power by using a 3 or 4 blade prop or is this too much for the engine?
Can anyone suggest a better prop set up or is the 9 x 6 prop the best?
Many thanks Steve
A higher pitch prop may not be a solution since it tends to "slip" substantially at low forward velocity (relative to the centerline of the crankshaft). A three or four bladed prop used on a conventional RC model is less efficient than a two bladed prop (too much space needed to go into air turbulence and the trailing blade moving through disturbed air from the leading blade and stuff). That said, because this is a hovercraft and because I infer they prop is in a duct type housing, you might want to look at using an impeller for a ducted fan jet model. I confess I don't know how large a diameter fan you can get or if it would function at the relatively low speed of your .40 (say 15,000 rpm vs. 25,000 or more rpm for a purpose built ducted fan engine).
It should be an interesting project nonetheless so keep us posted.
With a plane you can sometimes switch to less pitch and gain more pull, but have less speed. Do you want more power or more speed?
Try a 9x5, 9x7, and 9x8.
An OS40 normally uses a 10x6, so a 9x7, or 9x8 should not over load the engine.
Finding the best prop is always trial & error. Buy a bunch and see which one works best.. A different brand prop may also work better.
More blades are less efficient, more might help maybe/maybe not.
Bolly makes 1/2" increment props would a 9.5 x 6 fit in the housing. APC has recently started make 1/2" increment props also.
it is not likely that you will get any more power by going to a different prop, especially if you go to more blades.....the 2 blade will be much more efficient. You may be able to drop the pitch a bit like use a 9x5 but I would first step up to 30% nitro fuel and see what you think if you are not already doing so.
Brad L.
We all have been assuming that you are using the .40 for forward thrust, not to fill the skirt of the HC. I built a balsa and tissue hovercraft back in the 70s that used a Cox .049 for lift. It sat in a venturi that resembled a fan duct. The old Cox spun a three-blade "air tractor" or something prop. It did much worse with a two blade.
That said, I run a MasterAirscrew three-blade on my airboat. The thing is so F'ing loud on the water that all the neighbors complain. The three-blade loads the engine more at lower RPMs and since the boat cant attain the same speeds as my .40 sized planes, the less efficient prop works very well. Pulls well from a stop and idles great. There is a real trade-off here, but I am willing to loose speed for more low-speed pull, less noise and a small dia. prop that still idles like a champ. Hope this helps.
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