1/2 vw prop hub fiasco?

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Hummelbird Hunter

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I am looking at improving the prop hub assembly on my Hummelbird. There are so many spacers all of which can create vibration and other problems on their own. The bolts involved tightening down the prop are 4 1/2 inches long. Starting at the prop flange extension of the crankshaft there is a wooden disk 3/4 inch thick which clears the big hub bolt. Then against that is the prop. On top of the prop is the spinner mounting disk and on top of that is another wooden disk 3/8" thick. Finally an aluminum disk 1/4" thick sits on that which is all bolted together with the six 4 1/2" by 5/16" bolts. All these disks are joined with drivers [aluminium tubing about 1/2 inch dia.and about 1 inch long- 12 of them] Would it be better to remove the two wooden spacers and mount the prop against the metal flange and the aluminum disk on that. Do you need drivers? Friction and the bolts on their own should keep the prop in place. How are other Hummelbirds set up?
 

Rockiedog2

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Neither my 37 or 45 Casler(Hummel) built motors required those wooden discs. I'm wondering what their purpose is...something to do with the spinner/cowling fitup?
I run the prop against the flange and a 1/4 AL disc on the front. The 5/16" prop bolts aren't mounted in any bushings just straight thru the 5/16" holes in the prop center. There are no dowels in the flange to take the shear loads so the bolts are in shear as well as tension. The bolts carry fiber lok nuts behind the flange and I torque the nuts to spec with a crowfoot I think they call it. No problems with any of that as far as i know. (my motors were/are on a Legal Eagle so may not apply to your cowling setup.Just the motor)
It seems that all that potentially compressible wood in the mountup would possibly allow the torque to vary more than normally expected. If it was miine I would want it outa there unless a compelling reason not to. If I understand correctly the spacer at the hub prevents the prop from engaging the big centering dowel. That further loads the bolts and puts them relying on crushable plywood to brace against shear. I couldn't live with that even with the 1/2" bushings.
 

Dana

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Why do you have all those spacers? If you really need one for prop to cowl clearance or such, use something more rigid. I've used lexan
(polycarbonate), aluminum is of course ok too.

Dana
 

TFF

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Was it done to clear the cowl or fine tune the CG? I would shorten the cowl if not for CG.
 

Pops

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Was it done to clear the cowl or fine tune the CG? I would shorten the cowl if not for CG.


Also for extra clearance between the left exhaust stack and the prop for hand propping.
 

Hummelbird Hunter

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Removing all those timber spacers required me to cut a 50 mm hole in the spinner plate and into the prop hub to clear the central hub bolt. The previous builder just used a spacer to clear it and added another one on top of the prop for good luck I guess.
 
Last edited:

Magisterol

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Jul 22, 2021
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Anyone with the 1/2 VW 45hp engine on a Hummelbird? What prop (DxP) and static rpm are you getting out of it? I have a 47x37 DxP prop and get less than 3100rpm static but less than 3300rpm in the air doing 85 KIAS. Thx.
 
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