stunthanger.com
Engine basics => Engine set up tips => Topic started by: Brad Smith on January 13, 2013, 11:34:09 AM
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Im ordering a shaft extention for a fox 35 what is the size of the shaft 1/4-28or5/16-24 thanks fo the help im at work and dont have a fox 35 to look at.
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It is 1/4-28. Almost all engines of that size use that thread.
F.C.
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Thanks
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Is it hard on the fox 35 to add weight to the engine shaft to add front weight? Or should you add it elsewhere?
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Is it hard on the fox 35 to add weight to the engine shaft to add front weight? Or should you add it elsewhere?
You could add a heavy hub or a Prather spinner weight for a few flights to test, but then add the equivalent to the nose structure some where. It doesn't mess with the way the engine runs, but the vibration from running makes the shaft bounce around in the bushing. Over time, the crankshaft can break right in front of the counter weight disk. That just happened to a club member hear back in August. I was holding the model, (a Barnstormer with a L&J Fox .35 ) and just before he gave me the release signal, the engine suddenly went quiet!! The heavy hub, prop, and cranks shaft went flying about 10 feet in front of the airplane! We warned him about that!! You will have no warning that it is going to happen. It just will let go.
Type at you later,
Dan McEntee
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Melt lead into the back plate. Might have to put a strap across it to make sure it stays in place. D>K
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How did the OP's desire to add a shaft extension get misinterpreted to adding weight to the nose?
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How did the OP's desire to add a shaft extension get misinterpreted to adding weight to the nose?
The original poster asked about a shaft extension, but Onelife asked about adding balance weight to the cranks shaft:
" Is it hard on the fox 35 to add weight to the engine shaft to add front weight? Or should you add it elsewhere?"
This is what prompted my answer.
Type at you later,
Dan McEntee
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The original poster asked about a shaft extension, but Onelife asked about adding balance weight to the cranks shaft:
" Is it hard on the fox 35 to add weight to the engine shaft to add front weight? Or should you add it elsewhere?"
This is what prompted my answer.
Type at you later,
Dan McEntee
Ah! I see now! What I have heard referred to as a "turn signal response". Ya 'know: a 90-degree turn from the direction we're going! A response that has little-to-nothing to do with the original conversation. Yes!!! (As is this one! :-) )
;D ;D ;D
PS: Mine was a "rhetorical question", Dan. ;) ;)
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Ah! I see now! What I have heard referred to as a "turn signal response". Ya 'know: a 90-degree turn from the direction we're going! A response that has little-to-nothing to do with the original conversation. Yes!!! (As is this one! :-) )
;D ;D ;D
PS: Mine was a "rhetorical question", Dan. ;) ;)
I know Bill!! No worries! ;D Just pointing it out as a public service!
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Well I have an excuse for my response, its called old age and dementia. S?P
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And I'm glad Dan added the "fatigue from weight" thing. I use a heavy hub maybe too often, when I know it makes sense to glue on some lead.
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Fox .35 cranks break. You don't really need to add a "heavy hub" to make it happen. A heavy nylon propeller will work just fine for that. LL~ Steve
PS: I wouldn't put a shaft extension on a Fox .35 for any reason.
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Fox .35 cranks break. You don't really need to add a "heavy hub" to make it happen. A heavy nylon propeller will work just fine for that. LL~ Steve
PS: I wouldn't put a shaft extension on a Fox .35 for any reason.
A nylon prop is heavier than wood? I used to use a nylon 8-8 on a Fox Competition Special and didn't think it was heavy but never actually weighed it either.
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The plastic props are a lot heavier than an equivalent wood prop. The issue with the Fox .35 crank is that the torsional load of the engine firing is reacted into the rotating mass of the prop, causing fatigue failure at the discontinuity of the intake port. So keeping the propeller weight down is likely more important than keeping the spinner/nut/hub weight down.
If it was fatigue due to rotating bending loads, you would expect to see failures right where the shaft journal reduces down to the size needed for the drive hub and threaded portion--which I can't recall seeing.
The other failure that is pretty common is to see the crank disk fail between the crankpin and the shaft journal. And typically it seems the line of failure goes tangent to the shaft. Again, an area of stress concentration and maximum bending moment coming from the cyclical rod forces.
The design margins on the Fox Stunt .35 crank are pretty low all around, so you will see multiple types of failures which have different causes. I know people have had success running different props, different hubs, shaft extensions, etc. The question is how long will the engine stay in one piece? And, there are an awful lot of used engines we have collected with no real idea of how many hours are already on them. They are low cost, so if it breaks, just grab another one. Unless it fails at a bad spot and you lose an airplane that you really liked.
Here is a sampling of measured propeller weights so you can see just how large the differences are.
Dave