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Old Aug 26, 2012, 02:17 PM
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Hanger 9 Piper J-3 Cub 40


I have the above mentioned plane with a Saito FG-14B 4 stroke gas engine. In the Hanger 9 manual, it states that when mounting the engine, the drive washer must be 4.5" (112.5mm) ahead of the firewall. On the saito, the closest that I can get, due to the location of the carb at the back of the engine, is 124.5mm or 4.98". Will having the motor mounted 0.48" (11.5mm) further AWAY from the firewall then indicated in the manual make any difference that I should be concered about (other then CoG and difficulity fitting the cowl)?

Jim
Last edited by Newfie Pilot; Aug 27, 2012 at 08:42 AM.
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Old Aug 26, 2012, 06:18 PM
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Quote:
Originally Posted by Newfie Pilot View Post
I have the above mentioned plane with a Saito FG-14B 4 stroke gas engine. In the Hanger 9 manual, it states that when mounting the engine, the drive washer must be 4.5" (112.5mm) ahead of the firewall. On the saito, the closest that I can get, due to the location of the carb at the back of the engine, is 124.5mm or 4.98". Will having the motor mounter 0.48" (11.5mm) further AWAY from the firewall then indicated in the manual make any difference that I should be concered about (other then CoG and difficulity fitting the cowl)?

Jim
Hi Jim,

Just install another former behind the existing one and make openings in the present one to clear the carburetor.

You will then keep the same overall dimensions and have no problem witht the cowl installation.

Zor
Old Aug 26, 2012, 06:57 PM
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Could you please clarify further? Do you mean place a second firewall behind the original and the cut a hole in the forward one to allow me to move the engine in the mount the 1/2" back, thus giving me the 4.5" from outer firewall to the drive washer? Why not just cut a hole in the existing one without placing a second one behind the first? Is the 1/2" really worth the effort i.e. is it going to cause problems? Don't get me wrong, if it will greatly affect the flight characteristics of the plane, then I will modify it...
Last edited by Newfie Pilot; Aug 26, 2012 at 07:06 PM.
Old Aug 27, 2012, 08:12 AM
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Outside of looking a little weird, I doubt that the .48" will make much difference in performance.
Old Aug 28, 2012, 02:00 AM
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Newfie Pilot,

I fully agree with Tom that there will be no noticeable difference in the way the model flies.

What I undestand however is that your cowl is too short to intall the engine further ahead. By cutting the firewall to accommodate the carb the firewall is weakened. A second firewall behind the existing one brings back the strength of the fuselage front and allows your cowl to be installed normally.

Also a firewall should be sealed so that if any exhaust leakage takes place in the engine compartment it does not get into the fuselage. The second firewall will allows that seallng.

Also there would be much less effect on the CG location.

I am not telling you what to do. I am just explaining my understanding of your situation and mentioned what I would do.

Zor
Last edited by Zor; Aug 28, 2012 at 02:13 AM. Reason: Just checking
Old Aug 28, 2012, 04:47 AM
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OK, got it. Thanks for your help. I will more then likely end up following your suggestion.

Jim.
Old Aug 29, 2012, 02:56 PM
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Official reply from Horizon Hobby:

Quote:
Hello Jim,

Thank you for contacting Horizon Hobby's Product Support Department...

...It will effect CG but the motor should be in a safe range of distance not to add much strain to the mount.
Old Aug 29, 2012, 03:23 PM
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Quote:
Originally Posted by Newfie Pilot View Post
Official reply from Horizon Hobby:
Hi Newfie Pilot,

Quote:
Official reply from Horizon Hobby:

Quote:
Hello Jim,
Thank you for contacting Horizon Hobby's Product Support Department...
...It will effect CG but the motor should be in a safe range of distance not to add much strain to the mount
Of course you already knew that moving the engine forward would affect the CG (useless mention by the fellow at Horizon Hobby).

The rest of the text (the motor should be in a safe range of distance not to add much strain to the mount) does not make much good sense to me.

I interpret "should be in a safe range" as meaning "further away" which was your necessity to clear the caburetor. When the range (distance) increases there is more moment arm for the motor mass to create objectionale forces to the firewall.

I found that the replies we get from such places mainly involve selecting pre-written text that often has no relation to our queries or simply do not make any sense as selected by the person replying.

I think you had better advice from this thread.
Your turn to select what makes better sense to your judgment.

Zor
Old Aug 29, 2012, 03:28 PM
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The Hangar 9 Cub, and your Saito are both sold by Horizon. I would think that they would be fully aware of any problems if you use this application.

You could always add triangle stock to the rear of the firewall, and pin the firewall too the fuselage sides using bamboo skewers, if you are still worried about it.
Old Sep 11, 2012, 09:59 PM
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I just installed a FA 56 in mine. Fit great. 4.5 inches with no problems. Could always sell engine and put 56 in.
Old Dec 02, 2015, 06:18 AM
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Sorry for dragging this out of the past but can anyone tell me about the original fire wall engine mounting holes. I've picked up a sad old Cub that had the remains of a bodged fire wall which was all rotten.
Is there any offset on the engine mounting position to allow for any right thrust angle ? Or is the engine just mounted in the centre with no right hand thrust angle ?
I've stuck a new fire wall in as near as I can to the original position but it was a bit of a guessing game.
Old Dec 02, 2015, 05:04 PM
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Hanger 9 ARF's usually have right & down thrust built into the firewall, never owned the 40 Cub though.
See how she flies and shim the motor mount to suit your liking.
Old Dec 03, 2015, 02:09 AM
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I thought about the shimming but would like to get it near so I don't end up with a big prop shaft hole in my new cowl.
Old Dec 03, 2015, 08:40 PM
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Find a straw or other straight tubing and slip it on the crankshaft eye ball it up, you want around two degrees down and right. You could fine tune it without the cowl, and when satisfied, finish the cowl. I say less down / right is better than too much.
Old Dec 09, 2015, 12:31 PM
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