Firewall Strength
 

Im building a Top FLite Bearcat and i had to do some modifications to make the engine mount inverted. the firewall is laminated ply and balsa, the two small support formers are ply, along with the two small stringers attahced to the firewall, and the rest of the tall formers are balsa. it will have stringers and be sheeted later. do you think this will stand up to a saito 1.20? thanks, todd

Firewall Strength
 

From that description, it is hard to make any judgements. I would say if it is as strong as the one shown on the plans and the engine is withing the recommended range, you will be fine.

A picture would be helpful.

Tom

Firewall Strength
 

i forgot to attach the photo. doh! :eek:

Firewall Strength
 

sorry its blurry :-/

Firewall Strength
 

DogEater:

If you have effectively inverted the designed mount, and the 1.20 is within the recommended range, you should not have any worry.

That being said, it looks like the engine mounts on the wooden beams. If yes, I'd cut them off flush with the firewall and use a soft mount for the engine, bolted to the firewall.

Bill.

Firewall Strength
 

just looking for opinions here not an argument :) but why do you prefer engine mounts v.s. a beam mount? i've never used beams but i dont see anything particulary wrong with them. maybe you see something i dont? :) thanks

Firewall Strength
 

Dingo:

The beam mounts work fine, a soft mount works better.

If your planes last no more than a yeaar or two, go with the beams.

But.

You are getting set to put a shaky engine on the plane, the soft mount just makes it last longer by isolating the vibration.

Even a two stroke engine will vibrate, and over a period of time the beam mounts will be crushed and become fuel contaminated.

Using just an ordinary reinforced plastic mount will prevent the crushing and soaking, so even if you don't spend the money for a soft mount you can have great benefits by not using the wooden beams.

But if your planes only last a year or two...

Bill.

Firewall Strength
 

Bill, you have some good points there with the vibration issue. i would like her to last! I still have some time till engine mounting, so i may just take your advice and just that system. the way it is now though the frewall is just as strong as using beams right? do you think structurally the beams are stonger? it'd seem they would be, but the firewall is attached to the beams anyways so it should be just as stong. see this mess of ideas in my head!! haha, thanks again, you have some strong points

Firewall Strength
 

Dingo:

Initially the strength comparison is a wash. But unless the beams are carefully fitted into the formers they will shake, and cause the glue joints to fail. But if you put ply gussets between the beams and the back of the firewall, the composite mount wins.

Age? My oldest flyable airplane was built in 1960, oldest r/c plane is more than 20 years old, my oldest (and first) r/c twin is about ten years old. It's in my avatar. And none of these planes have the engines on wooden beam mounts.

Bill.

Firewall Strength
 

another reason im worried about firewall strength is because the firewall is a 1/16" piece of ply laminated to 2 pieces of 1/8'' balsa, thats not going to and wasnt designed to support the wieght of an engine mounted to it. it was meant to support part of the load of the beams....i may have to go with beams.

Firewall Strength
 

Dingo:

Sorry, didn't think about that. Replace the firewall with 1/4" ply using a soft mount, or 3/8" with an ordinary composite mount.

An alternate that you might use is a modification of the beam mount system. Doesn't do anything about the vibration, though.

Open the beams wide enough to bolt a plywood plate on, then mount the engine to the plywood. Then, when the ply mount is soaked/crushed just rip it out and make new plates.

My old Eaglet is built this way, it has survived for many years.

Bill.

Firewall Strength
 

1/4" ply is WAY more than adequate with a 1.20 4-stroke no matter what type of mount you use. I've built many TOC size aircraft with 4.2's in the nose and none had anything more than 1/4" AC ply. Some even had lightening holes in the firewall. Not a single one ever had a longevity problem and in fact several are still flying nearly 10 years later.

John

Firewall Strength
 

You can back up balsa formers by laminating them with VERY thin plywood using epoxy and this results in great strength and light weight.