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Prop shaving down cowl of J3

Old 08-17-2008, 06:35 PM
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PetesCub
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Default Prop shaving down cowl of J3

My prop (15X8) is attached to an AXI 4120/20 engine via a collet prop adapter. My aircraft is a 1/5 scale SIG Cub J3 which I built electric. My question is whether I can put a washer on the engine prop shaft to try and have the rear prop plate stick a little further out so the prop won't shave the cowling prop opening. I this something I might want to do, or might it create other problems?

Any advice appreciated,

Thanks!
Old 08-17-2008, 06:42 PM
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TedMo
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Default RE: Prop shaving down cowl of J3

Probably not a good idea, prop may slip without the drive washer in contact with back of prop. Perhaps you could add a pc. of 1/8 ply behind the motor mount to bring motor out beyond cowl. Otherwise will have to re locate position of cowl further back. Should be able to do that with very little problem.
Old 08-17-2008, 07:53 PM
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RCKen
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Default RE: Prop shaving down cowl of J3

While you "might" get away with putting a washer behind the prop, I'm with Ted and don't recommend it. The proplem with a plain washer is that they can slip if you don't tighten the prop down with brute force. The better way is to do what TedMo suggested. Cut a spacer to put between the engine mount and the firewall to move the entire engine out the required amount. This is actually a quite common practice when assembling planes with cowls. Many planes require the drive washer of the engine to sit a specific distance from the firewall in order to get proper fit of the cowl. Sometimes variances in engines used will make it impossible to get that distance unless you build a spacer for the engine mount. I had to do this very thing on the Top Flite B-25 review that I just finished.

Look at the section on mounting the engines in this review and you will see exactly what Ted and I are talking about.

[link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=1011]RCU Review of the Top Flite B-25[/link]

Ken
Old 08-17-2008, 07:59 PM
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Kemosobie
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Default RE: Prop shaving down cowl of J3

I fully agree w/posts 2 & 3. Spacer between firewall and motor is best way to go.
Old 08-17-2008, 08:21 PM
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ndb8fxe
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Default RE: Prop shaving down cowl of J3

I'm a little bit confused about some of the responses to this problem. Isn't the plane designed to have the prop plate a specific distance from the firewall? This is the place to start to determine the propper placement of the motor and subsequently the prop. I only recommend moving(spacing) the motor if the prop plate is too close to the firewall. If the prop plate is the propper distance from the firewall and you are getting contact between the prop and the cowl then the cowl must move.
Old 08-17-2008, 09:24 PM
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Default RE: Prop shaving down cowl of J3

Either way, we're probably only talking about 1/16" or less. If the cowl is already "indexed" to sit tight where it is at, then shimming the mount makes more sense. Final trimming of the model might involve correcting the engine thrust angle, anyway.
Old 08-18-2008, 11:36 AM
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PetesCub
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Default RE: Prop shaving down cowl of J3

Thanks for all the advice. A few of you were questioning the "design" of the SIG plans, but again, I went off trail and went and installed an electric, so I'm sure to be off spec. I'll try the shim method. I think your right, it will only be a 1/16 to 1/8 of an inch tops. Could I simply just continue running the prop and have it shave the cowling down to the length it feels comfortable with??

Thanks all again!
P
Old 08-19-2008, 04:49 AM
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gunfighterII
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Default RE: Prop shaving down cowl of J3

ndb8fxe,
It's much easier to move the mount than to reposition the cowl. Especially when the cowl is already mounted.

I had a similar problem with my U-Can-Do, mounted the engine to specs and the paint lines on the cowl did'nt match, had to move the engine out 1/2"
Much better result than a mismatched paint scheme.
Old 08-22-2008, 03:31 PM
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ndb8fxe
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Default RE: Prop shaving down cowl of J3

Ah, I see. Good points.

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