Hi

I had an accident yesterday. Ran out of fuel and dead sticked...realised it to late (couldn't hear my engine, friend of mine was starting his next to me - not his fault though) and was on a long low pass and way too slow when I realised the engine is not running anymore. I couldn't turn the plane around for none of the runways...it stalled and just before it hit I got the nose up. It hit on a slope, which probably safed it from most damage. The landing gear bent - easy to fix. The cowl got a hit here and there - already fixed. The firewall also broke out, this was also a place which was repaired previously and it broke on the joints which was glued - so a kind of a clean break it was. Now I want to know, would epoxy hold the firewall efficiently? The firewall takes a lot of strain when he engine is running WOT right?

What I did so far was to clean up where it broke, I put A LOT of epoxy on it and glued it back into place. I did however see that it's not a 100% fit (possible gaps between the firewall and sidewalls here and there), so I also put some more epoxy inbetween any joints and held the plane nose pointing up so that epoxy could flow backwards into any gaps which might still be there.

I then pulled on the firewall where the engine will be putting strain on it...I used quite a bit of force and it held. Did this a couple of times and nothing moved. I was thinking today however about gluing four pieces of plywood (long strips) on each joint where the sidewall and firewall comes together for extra support. I'm not too worried about weight. Before the repair I had +- 80grams of lead infront which could be taken out if neccessary.

Would Epoxy do the trick or is there actually something else I should have used?

For wood, a good epoxy is about the strongest bond their is. I have built boats that will run in excess of 100mph with a big block chevy in the back of them out of nothing but wood, fiberglass and a good epoxy. And depending on the size of your plane and engine, you can put more force on the firewall with your hand than the engine ever could. More than likely youve got it all fixed up already.

TonyG

Busted up the firewall of one plane last week. Once you've cleaned it up and epoxied it back together like you've done, and if you're not too concerned with any additional weight, you can take some ply (I used 1/4") and cut a piece to fit behind the original firewall between the engine mounts. Epoxy that to the repaired firewall and it ain't going anywhere. Drill up some new holes to run fuel line or bolt the engine back up and I should think it would be good to go. Just need to be as close as you can when you glued it back up to begin with that you didn't change the thrust angle too bad.

Just my thoughts based on what I've done in the past................

Pdm, I think I'll put some ply in just for extra support as you said. Will work on the plane tonight again. Yes, I have kept the thrust angle in my mind, but wasn't sure where it originally was. I used the "old" glue marks as references. The thrust angle seems to be fine, but only flying it will really tell. The firewall is not the straightest thing in the world anymore - the ideal would have been to cut a new one. But since it has been a bit crooked after the last crash, it doesn't bother me to much since I know it holds up. I will however thoroughly check it before taking it up for the real test.

An easy way to add strength to a firewall in a situation such as yours is to pin the firewall. Drill holes through the side of your fuselage into the firewall. Then use a small amount of epoxy and drive the dowel rod into the drilled hole. Cut the dowel off flush with the fuselage side.

Ken

That's a good idea, but what size dowel? The firewall I have is only about 8mm thick (have to go check, but around there - don't think it's much thicker) - if I drill a hole into the side the ply will split won't it?

The size of the dowel is going to depend on the size of the wood in the firewall. You want a dowel that's about 1/4 the size of the firewall. It doesn't take a whole lot to do the job here.

Ken

Thank you Ken. I think that is my way out for piece of mind.

For added support, RCKen is right on with dowel pins. There are any number of places that dowel pins can be used in one of these things but you had said that the previous repair joints are where the firewall broke the second time and pinning the repaired part, although it makes the existing firewall stronger where it attaches, the dowles won't do much if it decides to break at the joints again. This is why I suggested a secondary firewall. You can also use micro balloons mixed with the epoxy, or some fiberglass cloth to help out. It's all good. Good luck!

I've also used steel wire to pin a firewall instead of wooden dowels. I just countersunk them slightly below the surface of the fuselagelage sides. The only down side is that it is heavier than wood dowels.

A piece of triangle stock on each side reinforces a firewall very well too. And it's easier than pinning.

Put some triangle stock on the back of the fire wall ( bottom, sides and maybe top ) This will make sure it won't go anywhere. You can also mix some thirty minute epoxy with micro balloons or cotton flock to make a great gap filler.

Round Toothpicks work great for pinning the firewall !!

Quick question on thrust angle - don't know why I suddenly don't seem to remember.

Standing behind your plane (cockpit view) the engine should be slightly pointing towards the right or left? How many degrees is the thrust angle usually, probably differs - but what is a good starting point?

Korps, standing behind the plane, the thrust angle should be right. My guess is everyone will tell you something different when it comes to what angle it should be but I would start at about 2 or 3 degrees and see how it flys. If it isn't to your liking, you can adjust it with washers where the mount is. What's right is basically what ever makes it fly the way you want it to.

Okay so here is what I have so far. This week has been hectic so haven't really gotten to work a lot on the plane. I can't get the correct size dowel so that idea is out for me.

The picture shows what I have done so far.

I epoxied the 4 sides of the firewall (NOT shown on drawing) - fixing the firewall back into it's position.
Then also epoxied the gaps/joints (as shown on drawing)
I'm going to glue the triangle stock as shown aswell.
The piece of plywood was installed after the previous crash which took out all the supports in the nose - the tank rests on this.

The drawing may not be 100% correct but it's an idea of whats going on inside.

Should this be sufficient?