Hi. I'm building a P39 Airacobra. Unfortunately, when I built the firewall, I built it backwards. Not a huge problem. I simply flipped the engine around. It won't look as good, but I'm wondering if I can expect any problems due the fact that the muffler is on top and the needle-valve is on the bottom (see pic).

I don't see a problem. Which kit is your P-39

You might have difficulty setting you needle valve adjustment when the engine is running, that might not be very safe with a spinning prop.
nedyob

This could cause a lot of problems. First, if there is an offset angle (right thrust) built into the firewall you will have reversed that and have the angle to the left. This could cause the plane to fly weird because the offset will now be set to the left. Second, with the carb so low you will more than likely have a lot of problems with fuel siphoning through it. Third, looking at the position of the engine at the spinner plate it looks like the engine is not centered in the cowl.

If the firewall is flat and doesn't have a thrust angle built into it then you have an easy fix. Simply epoxy dowel rod into your existing mounting holes in the firewall and drill new mount holes in the correct position. Then you can mount the engine correctly. If there is a thrust angel built into the firewall you can still redrill new mount holes, but you will have to shim the engine mount in order to get the thrust angle correct.

Ken

A few more problems you may encounter are:

1) With the muffler like that, you will get a lot of fuel residue on the plane that will make for a messy clean-up

2) With the needle valve facing down, if you have a crash, even a minor one, you might easily break it off and have to replace the whole carb or at least the whole needle valve housing.

If it were me, I'd fix it now before it's covered. It's a big fix, but not really too bad.

It's a Global Quality Kit. I have a build thread available ( [link=http://www.rcuniverse.com/forum/m_6334567/tm.htm]here[/link] ).

I just placed the engine on the mount to take a picture. It's not screwed down to the mount, but once it is screwed in place, it is centered.

The instructions didn't mention anything about the offset angle and I didn't see one once I installed the firewall. The engine is basically centered with no offset. The problem with reversing the firewall installation is that it caused the engine to be about 1/4 inch off center. This picture shows it a little better.




The engine should be on the left side of the mounts when looking from the front. I had to flip the engine to the other side to center it.

Yeah, that's not going to be fun. I think I may purchase one of those hoses to extend the muffler and route the residue down.

Unfortunately, there is no turning back now. I weighed all the factors and decided it simply wasn't worth tearing everything apart.

I'm mainly worried that having the muffler on top will cause the residue to leak back into the cylinder. Or that the fact that the needle valve in on the bottom, fuel would have a problem reaching the carb. Is this a possibility?

Just get really good at flying inverted! Seriously, I'd just bite the bullet & chop/cut/rebuild. Wouldn't take long at all.

If you ditch that wooden mount and get a good glass filled nylon mount, it will make for an even easier fix, and a much better mount.

MinnFlyer and Deadeye had the best suggestion just redo the front now that you are still in the building process. by the look of your build so far you seem to have the experience the fix would be a pain but not that bad just time consuming. you could just cut the wooden mount off flush I would add 1/8 hard ply to the back of the bulk head and some triangle stock and mount a composite engine mount center the thrust line and add blind nuts to secure it in place, and by the looks of what you have there with a dremmel tool you could make a neat exit hole for your engine.

i have one of these kits & it is very nice quality. P-39's fly very well. Id ditch that 3 blade prop however.

Could just flip the carb around, would be tight on the muffler but at least the needle valve wouldn't be pointed straight down. Too bad you couldn't just flip the cylinder around 180 degrees too.

I have to side with the rebuild the whole front end right now crew. You'll be kicking yourself a year down the road when that plane is a big PITA to work with at the field....

Andy

2 problems with that solution.
1. the needle would practically hit the propeller
2. Throttle lever on wrong side and operate in reverse requiring yet more changes.

Another thing I see in the pictures is the plywood bearer. They flush into the firewall with small triangular stock holding them. First time you go over on the nose, there goes the mount - the required support is just not there. Wood bearers are usually maple or a similar hard wood and run from the engine back through the firewall to the next former back. They also generally are glued to the fuselage sides along with the bulkheads they pass through. There is a lot of stress on this part of the airframe and they learned long ago how to build them so they don't break.

Hogflyer

FBM,
The right trust (some time a little down trust as well) is built in the plans and normally is some where between 3 to 5 degrees which can not be easily seen or noticed with bare eyes. If you get a straight edge and a protractor you can check it out in you plans.
Flipping the firewall will turn every thing around 180 deg causing the plane to fly like a squirrel
The best solution IMHO is the chop, cut, rebuild approach. Good luck 2 U.

Look at your plans in the picture..the pic showing the firewall from the back, the second pic you posted..see the slot for the motor mount isn't centered when compaired the the nose wheel bearing mount? thats your thrust angle....the pic showing the engine from the front clearly shows the the thrust angle is backwards

Well, I was able to get everything nice and centered. I think it's pretty close to the original set-up, except that the muffler is on top and the needle valve is on bottom. I'll have to see what happens when I fire her up.

Thanks for all the suggestions.

i would just go with it. the angal of the needle valve should not be to hard to adjust. c-crunch

When you change the angle of the needle you also change the angle of the throttle arm on the opposite side.

Why would I need to change the angle of the needle valve? It should still start and run the way it is currently set-up, correct?

I would not change the angle of the needle valve. It looks fine the way it is.

If you leave everything as is, and have trouble getting the engine adjusted:
Buy, or build, a cradle that will hold the plane inverted. That will put everything in a more 'natural' position and make the needle easier to reach. Once the engine is running, flip it upright when you pick it up to carry it to the taxiway.

I've seen several people with inverted engines who flip the model to make starting easier.

Fooseball, if you don't flip it over to start, residual oil will be trapped in the crankcase rather than being drained from a side/down muffler. You will get some floods. You can well get floods when you flip it over as you will have fuel running into your muffler while starting. It's gonna be messy to say the best.

Now as I see that picture of an engine mount simply extending into a notch in the firewall, well good buddy, you better get some extra fingers or a hand because one day that engine, will come right out of the front of the machine when you are starting it. Like the firewalls in a number of ARFs, that ain't worth a SHOT! I have seen too many front ends come off models and I heve seen a guy lose most of his right hand when the firewall came off during a restart after a rather hard landing.

If you don't cut that machine apart, and rebuild the front-end, all I can say is "Best of Luck and you were warned."

The plans call for the firewall to be angled, so I was unable to use a standard motor mount. And I was unable to find an angled mount. I was also worried about the strength of simply glueing the motor mount into the firewall, but I asked my local hobby shop pro and he said it shouldn't be a problem with a .15 sized motor. I used 30 minute epoxy and lots of it.



My question is: Why would a kit manufacturer design a plane that could potentially be dangerous?

As someone said. The oil will drain back into the engine.
It wouldn't be a problem if you get it running, but you're going to be fighting it everytime to get
engine started everytime becuase your glow plug will be wet with oil and it will be harder to flush
and you're be buying glow plugs up the waa zoo from the hobbie shop, becuase eventaully the oil
will leave a coating of burnt oil and you'll have problems with the coil not glowing enough. So you'll
crank up the juice more and blow the plug. It's cheaper to just buy $5.00 of wood now.

You can always wack it off. it's only wood. just add light ply and add spaces to the back of the
engine mount.

You could have easily make an engine mount from wood with a slighty off set
angle to compensate, to get the proper thrust angle. If you wish not to install
a plastic engine mount. The eagle II use plywood engine mount.

You can always start it on a craddle with the modle upside down...but it's going to cost
you a lot more than $5.00 to make a craddle if you don't have one and you always gatta be hauling a cradle around
instead of a Y steak you can steak to the ground.

I don't like handing the modle more than i have too with the prop spining.lol