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maximum distance front engine bearing to prop centerline?

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maximum distance front engine bearing to prop centerline?

Old 12-20-2003, 02:05 PM
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mikenlapaz
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Default maximum distance front engine bearing to prop centerline?

I'm putting together an air boat with a Homelite 25 cc gasser. I like the idea of using the the factory engine shourd and 5" of the handle extension. The handle extension (two parts) is held together with with 2 self tapping screws and attached the the main shourd with 4 self tapping screws. The 1" alum wand tube is held by compression and the end would have a hose clamp.

The idea is to make a 5" plus prop drive shaft to screw into the existing flywheel retaining nut (5/16-24) and give it albeit nominal support on the output end with a 1"- 5/16" bearing and a seal.

Cowled planes have long shafts, I just need some input as to maximum lengths and whether the end bearing is practical.

Thanks
Old 12-24-2003, 03:17 AM
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Capgains
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Default RE: maximum distance front engine bearing to prop centerline?

Boy I was lost on this one till I read it 5 times. I would guess from a hack engineering stand point (My dad and a good friend of mine are engineers) I would be more interested in what the shaft will plug into. The flex shaft was meant to spin a plastic line head and a little piece of string that really provides little resistance (ei even if it hits something, it will break the line with little interuption of power to the head) Mounting the prop hub right on to the flywheel is the way most people convert these. I can see how your installation would be clean, and easy, but I would bet that the propeller's resistance is going to waste either the flex cable, or the points where the cable attaches to the flywheel, or at the prop hub. The flex in the cable alone will cause problems, when you goose the throttle, the cable will load up, and then transfer the torque to the prop, and when you decelerate, the prop's momentum and weight will load the cable the other way. The bearing on the end may serve to center the cable coming out of the tube, but that cable will be flexing in front of and behind it, and possible cause a flutter on the bearing.

My guess,

Good luck.

Better yet, why don't you just get rid of the flex shaft, and attach a long aluminum or titanium bar to the flywheel, maybe a u joint at the prop, so you can get your angle, and Git R Done?
Old 12-25-2003, 01:33 AM
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mikenlapaz
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Default RE: maximum distance front engine bearing to prop centerline?

Capgains
We are in agreement that trying to use the flex cable is a bad idea!
The idea is to make a 5" plus prop drive shaft to screw into the existing flywheel retaining nut (5/16-24) and give it albeit nominal support on the output end with a 1"- 5/16" bearing and a seal.
I have drilled and tapped out the square recess in the flywheel retainer nut to 5/16-24 for the attachment point for the referenced 'extension'. Found an 8mmx28mm ( .31496" x 1.1" ) bearing and a grade 5 1/2 bolt which will be machined and tap both ends to 5/16-24 and leave the an 8mm bearing surface area about 4.5" out from flywheel.

My primary concern was/is the advisability of putting the bear in a housing that is attached to engine via plastic case and 4 self tapping screws. Without advise of others experience, I will experiment and hope for best.

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