Hi, I'm putting together a rather old ARF called a "Slick Baby". The instructions are somewhat lacking, but the design apparently calls for a beam mounted engine, with the cylinder head upright - see the illustration on the cover of the instructions. However, with my chosen powerplant (an OS .45 FSR), this would mean that I would have to cut away much of the top of the cowl and that there would be a rather unsightly protrusion on top of the fuselage.

Instead, I was thinking of mounting the engine inverted along the same rails - see photo. In so doing, I would be be able to maintain the thrust angles, but would in effect be mounting the engine approximately 5 mm above the aircraft datum (the thickness of the engine moutning lugs). The advantage of this is that the engine would be entirely enclosed within the cowl, except for the muffler. The drawback, as I see it would be that would induce a resultant down thrust (because the elevation creates a moment of force about the datum?)

I was wondering how adversely this would affect the flight characteristics of the plane?

Guess I could probably try it inverted, but without the cowl first. At worst, I would always cut off the mounting beams and install a glass filled nylon engine mount. But if anyone could offer some guidance, I'd be mighty grateful....

You'll likely notice very little in the way of handling changes. The small amount of change in the position of the engine may, and only may, give you a very slight amount of effective downthrust. Likely, you won't notice anything. If there's a way to move the mount down slightly, then your problem is moot, of course.

Thanks Bill, that's very reassuring. Unfortunately, there does not appear to be any easy way to move the mount itself.

However, I am hoping, as you have suggested, that any problems would remain merely hypothetical and purely in the realm of the theoratical. Unlike yourself, I just don't have the experience nor the technical knowledge to make a confident decision.

Thanks again.

If you raise the engine thrust line the cowl will most likely not fit correctly.

About the only problem you'll have is getting the prop shaft thru the hole in the cowl.

If you raise the engine, the spinner and cowl will not line up.

The model's picture shows a spinner. The cowl and spinner make a decent looking nose together. They make an ugly, out-of-line nose when they don't line up.

If the firewall is not pre-drilled for the motor mount, you'll have no problem locating the mount so that the prop shaft of the engine is centered for a straight, in-alignment cowling. If the firewall is pre-drilled, I would suggest that you study the problem of filling the holes and drilling new ones.

You know, I just looked closely at the attached pictures. The motor mount is actually constructed of wood isn't it? The beam mount is actually a couple of sticks of hardwood projecting through the firewall isn't it?

Since it's wood, if it's a very hard wood, things are a little different.

Test fit the cowling to see if it is going to look skewed with the spinner up about that 5mm. A smaller than recommended spinner might look ok to you or not. Misalignment looks worse at the top, where the spinner would stick above the cowl. A smaller diameter spinner would line up with the top of the cowling.

You can also trim hardwood motor mounts down about the thickness of the engine mounting flanges and have no problem. Don't do that unless the wood is very hard wood however.

Thanks everyone.

Darock - you're right, its a hard wood beam mount. However, the cowling provided is pretty generous lengthwise and isn't a really tight fit. I think I can shift it around a couple of degrees with no problems, then fix and trim (if required).

Another approach would be to make another cowl. Have been meaning to try my hand at moudling a fibreglass one.

Will go fiddle around and see how much a problem I have. Thanks very much for the heads up.

Length isn't the problem.
You've shifting the motor UP. The present cowling, and actually the entire airplane, is shaped so that the cowling lines up with the spinner. The spinner is going to be displaced 5mm UP from where it'd be if the motor's prop shaft was with the engine upright.

5mm actually isn't much.

The cowling might be able to be shifted up front and rear and not look out of place. Every ARF cowling I've fitted in the last year or so has been larger than the forward part of the fuselage. The usual practice seems to be to align the top of the cowling tightly against the top of the fuselage and have the excess at the bottom serve as heat exhaust.

I just finished fitting the cowling of an ARF Skybolt. I wound up gluing 3/32" plywood pads to the fuselage sides underneath the cowling where the hold-down screws go. Standing off the sides of the cowling gives better airflow out the cowling. And the bottom still had more room. I could have stood that cowling off along the top if I'd wanted. But the Skybolt is a scale airplane and that wouldn't have looked scale. Your model isn't scale, so it wouldn't matter.

Don't sweat making a new cowl. A new one can't clear up the misalignment problems. If you need to stand the top off by 5mm, just do it with pads. If the present cowl isn't big around enough, then split the bottom so it will. A strip of epoxy/glass to bridge the split and you're good to go.

I'd just use a smaller spinner so it lines up along the top edge. Yeah it won't match the rest of the front of the cowl but as long as the upper sight line matches I'll bet only one in 20 observers will notice that the spinner isn't fully matched all around.

Heck, you can even turn it to your advantage. Cut away a little of the lower cowl where it's flat for the spinner mating surface so that some air can come in through the gap to help cool the crankcase and provide fresh air to the carb.

It shouldn’t be a big problem to simply saw the engine bearers off flush with the firewall and put a firewall mount in the proper place. The firewall might need to be strengthened a little but that shouldn’t be such a big deal. A 1/8 thick plywood doubler on the firewall with a little epoxy and two flat head screws in the ends of the sawed off bearers should be plenty. blind nuts can be threaded to the back of the original firewall to hold the new mount.

Thanks everyone.

Darock - ok, I think I got it. But you're right - the cowl is slightly big and should allow me to stand it off the top if need be.

Bruce - will keep you suggestion in mind if I really can't get the cowl to line up.

Lou - well, yours is still the option that was in reserve should all else fail.

Right now, I'm just keen to get everything setup so that I can go fly the plane this weekeend!

Epoxy a 1/4 inch piece of hardwood the same width of the beam to he bottom of the beam full length. Inset the engine lugs on the top of the original beams till the bottom of the lugs are flush with the top of the original beams. Mount the engine with the top of the lugs down. be sure the bottom of your insets are exactly parallel with each other, and you can't rock the engine. Don't do this if the lug tops are not straight and flat.

Thanks Scratchonly, but I've already mounted the engine.

Just wanted to thank everyone who helped out. Everything was exactly as you guys said - the plane handles real sweet, but the spinner is offset a little. Bruce - you were right, of the 20 odd people who've admired the plane, only one picked up that the spinenr didn't quite line up (and he thought that it was a design feature!). Had to put in a ST .45 just to get the plane to balance, and that puts out more than enough power.

I've put in over 20 flights on the plane already and its both fun and easy to fly. Would like to post a picture but I realise that I've been spending so much time flying the plane that I haven't even done the customary photo shoot!

Thank you all once again!