hi,

Im needing to move the firewall back in order to fit my engine in the Sig 1/5th Cub.
As i am only new to kit building , im not sure what to watch out for when doing so. Doing this (shortening the nose) must change thrust angles together with changing the angles of the nose itself for the cowl to fit on?
Im really not sure what to do...can i just put in a second firewall behind the original one where i need it to be and cut an opennng in the original to suit ( that would keep all the nose parts the same without alteration)

Kind regards
Steve</p>

You could do that...

Or, you can cut away the doublers that the firewall will be up against equadistancely (is that a word??)

Meaning, if you want a 1/2 inch move back....then take a 1/2 inch off the doublers. Make the cuts before you glue and it will be leaps and bounds easier. This will also preserve your thrust angles.

This is all assuming there's enough room up front to allow for this. I'm not sure what's laid our for your fuel tank support, landing gear support, etc. You'll need to think about all this first.

Thanks Articatrider,

I really am struggling with this in my head!!!! its starting to hurt.

If i move the firewall back and keep the trust angle the same as you suggest. Now bear with me here!

Wont the cowl openning for the thrust washer on the engine position now be different? My thinking is it probably will.
Is it not worth worrying about?

Steve

The first thing I would do is make a tracing off the plans of the front end, move the tracing back 1/2" and compare the engine and cowl parts to the new arrangement. See how that would affect the doublers (they might taper a bit and cause some problem with the cowl). Also take a look at the consequences for the fuel tank area, landing gear, etc. when things are shoved back. Paper "construction" is always simpler to alter and adjust than the real thing! Good luck, Dzl

As you are going to put in a new firewall, you can just move the motor mounts so the thrust washer is centerred in a straight cowl.

I'm curious though why you need to move the firewall. If you are using a recomended engine, that shouldn't be necessary.

What engine are you using?

Don

Thanks for the advice.
There would be no problem with landing gear or tank or any other foreseeable issues.
For some reason i just couldnt get my head around what the consequenes would be by doing this.
Lots of people seem to move it to suit the engine.
I will trace it out and see , it think i may need to make a new one slightly larger if i move back as the nose is tapered slightly to the front.
I will just need to shorten the fuse ply sides to suit.

I think i just worked it out in my little head, tell me if im wrong.
With the firewall back say 1/2" from normal then to keep the same thrust LINES i would need to mount the motor slightly HIGHER and slightly more LEFT than if it were in the plans position?

Steve

Don,


Im using a saito 65, The specs suggest up to a 65 . But a 65 requires modification.
As i will need to modify it , i was thinking my old 90TS flat twin would look nice and is about the same power

Steve

The actual angles that are set by the doublers are what you have to worry about. Most of the time the fuse is built to provide a little bit of downthrust, and a little bit of right thrust to account for torque. So....if you look closely at the front of the fuse, from the top down....you'll notice the right doubler will seem a little shorter than the left one. This is to provide the right thrust. Just make sure when you do your cuts the firewall "sits" the same in the front of the fuse as it did before you made the cuts. This can be accomplished by "shaving" the same amount of each side.

The cowling I wouldn't worry about too much if all you're talking is a 1/2 inch. If it's a straight back shot then the thrust wsher should come out in the same place...or close enough. The cowl on a cub you should be able to fudge a little bit

Did you build this plane already? Are you building it? Did you cut glue the doublers to the fuse sides yet?

Arcticcat,

Thanks again,

Ive just received the kit and will start assembly soon. The kit looks fantastic - laser cut and good quality woods.
Cant wait to start!!

Steve

Thats it. It won't take much.

One of our guys built an old school kit, th plans showed a tube type receiver, and the plans called for 7 degrees right thrust. That's what he built in, but didn't move the motor mount centers. It kind of looked like a prize fighter that had his nose broke a few times to many. It flew well though.

Four strokes do sometimes present a bit ofa problem due to the carb being behind the engine. The Dave Brown Long mounts are good ones for cramped spaces, but you still have the extra "nose" haning out there. Some of the kit motor mounts, the two piece ones are probably the best for cramped spaces. You can usually swing the carb so it clears the lug and tuck it back between the mounts to give you some extra room.

Don

What I would do is leave the firewall where it is and cut off and extend the cheek pieces 1/2" out. Just a suggestion.

Sorry, didn't read before writing this, that you have a cowl. Is it possible to move the cheek pieces and cowl 1/2" out and leave the firewall where it is?

No .i cant extend out from the firewall. that would not be an option as it would lengthen the plane.</p>

I had the same issue with a 1/6 cub. I built the fuselage as per plans then when I mounted the engine, OS 40FS, I noted that the cowl was 3/4" farther forward than it was indicated on the plans. I measured 20mm back from each side of the current firewall and cut using a razor saw - in this way the relative thrust offsets were transferred. Then I drew around the cut section on 1/4" birch ply (not lite ply!) and made a new firewall to fit snugly into the opening of the fuselage. The cowl is such that in the was enough purchase on the cut back fuselage to properly mount the cowl and keep the propellor in the "scale position".

The important thing is to keep the propellor in the correct position and work back from there.

Andrew.

Thanks for your input.
So the parts in the kit actually built a longer fuse than the plan indicated?

No I built the original fuselage as per plans but with OS 40FS the Carb is at the back which pushes the engine forward. The cowl overlap would originally have been about 40mm now it is 20mm - which is fine for attaching the to the fuselage.

When you get to this stage of the build just mount your engine on the engine mounts and see where the prop will sit then adjust the firewall back if necessary.

Thanks to all who helped me. I really appreciated your help and its quite clear now .
Its great to know there is so much knowlege out there in the ether.

Regards
Steve