voyager_663rd

I need to add 1/2" ply extention to the already-epoxied existing firewall (this is an arf so the firewall was already fuel-proofed) so that I can extend the motor mount out by another 1/2".

The plans call for 5 7/8" from backplate to fw but I changed engine mounts. As a result I can only get 5 5/8" distance. The extra 1/4" I'm wanting on the extention is because when the engine is on the mount, it has to be set up at the VERY END of the arms to get the clearance. The extra 1/4" on the extention will allow me set the engine up 1/4" back from the very end of the arms.

Two questions:

1) I'm planning on using two 1/4" pieces of regular ply (not a/c ply) glued together to form the 1/2" extention I need.

a) Is it ok to use regular ply for this?
b) Just regular yellow glue ok for the initial bonding of ply to ply?

2) Seeing as the existing firewall is already epoxied, do I need to "scrape/sand" it off from the existing firewall where the extention is going to be glued (or should it be epoxied) so that the ply extention is glued to bare wood rather than epoxied wood ?

Don Presten

I had to do the same thing on an old Ultra Stick because there were already 8 holes drilled into the stock firewall and I was putting a different mount on which would have required 4 more holes.

Heres what I did:

- built a new firewall using 1/8" firewall plywood that fit perfectly over the existing firewall.
- drilled the large center hole to allow for the fuel lines.
- rough sanded both the existing firewall and one side of the new one.
- painted the other side and the edges of the new firewall with a white primer sealer.
- drilled four pilot holes in the four corners of the new firewall that would take four small servo mounting screws.
- applied 10 minute epoxy to the sanded surface of the existing firewall and the non-painted side of the new firewall.
- applied new firewall to old firewall surface.
- cinched it tight by driving the four servo mounting screws to the corners.

It was well strong enough and has lasted many flights with no issues.

BillyGoat

I take it your motor mount will be bolted through the extensions and existing firewall?

Regular ply will get the job done but personally I would use maple, sand the firewall to wood and use epoxy

Ed Smith

I would suggest not even gluing it, then it can't come unglued. Use longer screws through the 1/2" spacer and into the existing mount holes in the original firewall.

Ed S

voyager_663rd

What I was planning.

I didn't want to use new blind nuts (have to gut the whole front end to get at them. Still in place and i'll bolt the mount through the ext thru the existing fw. I'll still sand it to bare wood and epoxy it.

I know that maple, etc would be best but I don't know that I can find a small enough piece. Thanks for the tips in any case.

CafeenMan

I think your plan is fine. I laminate plywood and leave it loose between the mount and the firewall whenever I need to extend the engine. It works fine. I soak the extension in solvent based clear polyurethane paint for an hour and let it drip dry. That provides plenty of fuel-proofing. Just clean off the dust after your done sanding it and drop it right in the can.

CafeenMan

One thing to note is that the plywood will compress. What you should do is check the bolts between flights for the few few weeks until everything takes a "set".

voyager_663rd

Thanks for the 'proofing tip. And confirmation that the ply I will use is ok to use.