If the manufacturer angles the fire wall 3degrees, I am suppose to offset the engine so that the crank shaft comes out center of the cowl, right? The current plane I am working on now, the manufactuer drew the center lines on the firewall with the vertical line offset about a 1/4 inch, but when the engine is mounted there, it does not come out of the cowl centered. is it critical I leave the engine on there marks, or center it in the cowl?

Thanks !

Flyin High,
That right thrust is built in for a reason, and you shouldn't try to correct it. Rigth thrust is built into a plane to counter the tendency a plane will have to pull to the left due to the rotation of the engine. Depending on the type of cowl you have on the plane you can build/assemble the cowl so that it matches the angle of the engine so it's less noticable.

Hope this helps

Ken

You mis-read the question Ken.

You do want that offset, but you also want the prop to come out on center.

You have three choices:

1 mount the engine further to the left-hand side,

2 cock the cowl to the right

3 add washers to the right-hand side of the mount to remove some of the offset (3 degrees sounds like a lot - 2 is a more usual amount)

Check the distance of the prop hub of the engine from the firewall. On planes with an angled firewall and cowl, the instuctions should tell you how far forward to mount the engine. If you move the engine forwards or backwards from that point, the end effect will be for the prop hub to move left or right of center.

The only real thing I see with that (the original question) is that the prop and spinner may not be exactly square with the cowl. I don't think that's gonna happen no matter what you do if you maintain that thrust angle, or do I have a misunderstanding of the problem?

I have two cowled planes, and both of them have the cowl that is not exactly square with the cowl and that's because the cowl is square with the plane but because of the thrust angle of the engine, it cannot be square with the cowl.

Please correct me if I am wrong because I am putting together a Seagull Super Star and it has a rather large cowl and I want this to not only perform well, but to look good too.

DS.

I thought the O.P. was talking about the crankshaft/spinner not being centered, ie, being shifted left or right.

The problem you describe is seperate. I've seen some planes where the cowl front was at a small angle to compenstate, but for the most part, you're stuck. However, there is 1 thing you can do if you want to put in the effort. Set up the plane, get it all mounted, and flight test to make sure you get the thrust line you want for your engine/prop and flying style. Then add a balsa or ply shim to the front of the cowl and sand to exactly match the spinner, leaving a little space. Seal and paint to match the cowl, and you can get a perfect fit. If you really want it to be perfect, apply a bit of glass cloth over top, and do all that fiberglass finishing stuff. Labor intensive, but it is cool when you see a plane with the spinner so well fitted you almost can't see the gap between spinner and cowl.

That's a great idea, and thanks for that. But, I just leave it alone. It seems that I am probably the only one that notices it at the field (no one comes up to me and tells me that my cowl is not straight or something like that). Perhaps they are being polite???

But, that's good poop. I may just do that with the Super Star. I want it to look good, and it's coming out that way. Not that the others do not look good, but they are 'work planes' where the Super Star will be a 'fun plane'.

DS.