Propeller clearance to tailboom

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DaveK

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I know the minimum clearance per the FAR is 1 inch between propeller tip and structure, but looking for experience with what is desirable. Minimizing the clearance is good for my design to allow a larger diameter boom, but I’m concerned with vibration at that location. Any better guidance on clearance to avoid excessive vibration and fatigue at that location, beyond the minimum due to engine shifting?

I’ve been searching here and elsewhere without much luck.
 

DaveK

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To be clear, engine mounted above tailboom, pusher configuration like a Drifter or Woody’s Pusher. Question is how close is reasonable allowable for propeller tip to come to top of boom after accounting for engine shifting and mount sag. Boom is semi-monocoque aluminum.
 

TFF

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I would want around 5” clearance. To clear, what if mount problems, and just in case something got kicked up into the prop. It would not be 1”.
 

Victor Bravo

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Some of the Kolb guys would go down below one inch, which scared the crap out of many other Kolb guys... but nobody has ever had a tailboom strike apparently.

IMHO, within a couple of inches would seem to me like close enough for the inflow and outflow near the prop to get affected, which in turn would cause all sorts of weird stresses, oscillations, loads, etc.
 
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BBerson

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To be clear, engine mounted above tailboom, pusher configuration like a Drifter or Woody’s Pusher. Question is how close is reasonable allowable for propeller tip to come to top of boom after accounting for engine shifting and mount sag. Boom is semi-monocoque aluminum.
Might make some noise. The wing down wash and cabin vortex shedding is probably much worse.
 

nestofdragons

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The Mignet HM1000 had a prop passing close to structure. The disadvantage is very known: NOISE! If you are already using a V-8 engine with open exhaust, you don't need to worry. 🙉Going for ultralight, i would take the complaining neighbors in count.
 

Lucky Dog

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Most Rans and Kolb aircraft with tail booms have three inches of clearance in stock configuration. If you are designing, I'd stick with that number so you have room to add some prop diameter in case you discover you need it later.
 

AeroER

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The closer the propeller plane to the root of the boom, the smaller the tolerable clearance due to deflections.

A large overhang without other changes should probably add clearance.
 

DaveK

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With a 72” prop it would be 3 inches of clearance as drawn. With a 100hp Rotax it might end up with a slightly smaller diameter prop, so I’m thinking it’s good as planned.
Thanks for the input.
 

raytol

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Dec 17, 2021
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Put the engine in place without the rubber engine mounting blocks. Deflect everything you can and then add 1 inch.
 
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