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Prop to Fuselage clearance


Chris Walby
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I am having some fun with a 38” electric twin I am about to build and while selecting a suitable prop for the motors I already have may have a potential problem or perhaps not!

It is a warbird, but only stand away scale and really more practice balsa bashing for me.

So how close can/should I run a prop to the fuselage as upping the diameter yields suitable gains in thrust (well within the motor/esc ratings) that I am looking for?

Is 8mm (5/16) a bit close on a 8 inch prop? I could just make the wings slightly longer or move the nacelles out a bit… but how far?

No clue about prop tip vortex and its interaction, but I assume it won’t end with a prize!

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Hi Chris,

Except if you're planning to build a Cessna Skymaster, I don't see the need for a genuine 'pusher' prop.

I suppose you simply want the tractor propellers to contra-rotate, so why not using CW and CCW props from any suitable size (from multirotors)?

As you're using 8", plenty of choices for 7"-3 bladed screws, giving you an additional 1/2" clearance to your fuselage.

Cheers

Chris

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Chris

Compared to all the other losses the prop and its wash suffer a bit of fuselage interaction will not be noticeable and the benefits of using the biggest possible diameter are very real.

I doubt a smaller 3 blade will ever match the performance of a bigger 2 blade even taking into account any fuselage interaction

About the only thing you may notice from a close running prop is an increase in noise as the prop tip vortex hits the fuselage but even so it will be nothing like the racket you get from a pusher running close to the wing trailing edge! wink 2.

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Take a look at a full size DC3 - clearance isn't exactly massive (ISTR mention of the propeller tips being a foot from the pilot's heads) and various full size references quote a minimum distance from a prop tip to airframe as 1 inch radially and a foot longitudinally so interaction shouldn't be a major consideration. Ducted fans certainly manage with minimal clearances!

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