I've never really known for sure why some aircraft have their engines mounted inverted. At first, I thought it was for a scale appearance as the cylinder head protruding from the underside of the aircraft wasn't as much a detraction for a scale or semi-scale aircraft as if it protruded from the top.

However some aircraft have a completely enclosed cowl and a round fuselage, such as many warbirds. And the engine is still mounted inverted. So maybe it's to lower the CofG.

I intend to build a Corsair with a .30 Saito. The kit instructions say to mount the engine inverted. Is there any reason one couldn't mount the engine "right side up"? I am concerned about the crankshaft housing drain and of course the slight problems of starting an inverted engine.

Thank-you.

I think the CofG thing is part of it, but in most scale type planes, the prop shaft is closer to the top of the cowling than the bottom. So the choice is a small amount of engine sticking out the bottom, or a Lot of it sticking out of the top.

I think the corsair is one of the few that has the shaft right in the center. I think you would be fine to mount your engine upright. Im pretty sure Ive seen some corsairs with the engine mounted on its side.

It also depends where the fuel tank is in relation to the carburetor.

An inverted engine allows you to cut a larger air exit hole where it can't be seen and cools the engine better; also check your tank location, if it is quite low, it may be a better fit for an inverted engine. Good luck.

Four strokes don't seem to suffer any running problems being inverted. The scale cooling air exit is generally in the bottom of the cowl, this allows better cooling air ventilation to the cylinder when the engine is inverted. However, with baffles in the engine compartment the engine can be adequately cooled in any position. From a personal point of view I will opt for side mounting, head to the right if at all possible for both 2 & 4 strokes. Others will invert anything and tell you it works fine. I've been doing this since 1968 and my experience just tells me my way positively works every time, so I stick with it.

Engines without float type carbs such as our model engines will run in any position. That's why we can fly inverted all day long. The issue is always tank to carb placement. If you can get the tank located correctly, an inverted engine will run fine, assuming you don't flood it during start. Otherwise you will need a pump for consitent runs.

The only ones I've seen or had to mount inverted were simply for a better fit inside the cowl.

The issue is also the head of a two stroke engine's deepest point (when inverted) is the glow plug's location. Becasue of this, this is a natural well for fuel to migrate too. On ocasions, this will cool the glow plug to the point of killing the engine. Because the head shape and glow plug location is not necessarilly the deepest point on an inverted four stroke, in most 4 strokes this is not as big an issue.

Well....lots of good advice and good information. I forgot about some aircraft having the prop center-line closer to the top of the cowl. Odd about the fuel tank location. I think my manual says something specific about the height of the tank in relation to the carb. I'll have to check it out. What's odd, is I wouldn't have thought that was critical with a pressurized fuel tank, especially when the fuel level changes in flight anyway (due to consumption as well as aircraft attitude). On the Saito 30, the rear-mounted carb is quite close to the crank center-line. I'll have to check this out. As far as flying the engine inverted, it would be convenient to mount the engine upright and vent the engine breather nipple with a short piece of fuel line. This line would exit behind the cowl in the existing small muffler "in-dent" in the fuselage. The cowl itself is not pre-cut for a muffler, so I can do that anyway I like. I will have to check out what provisions are there for cowl venting one the aircraft kit arrives. I think my biggest concern for writing the original post, was that if I mounted the engine upright on a low-wing aircraft, it woudl raise the Cof G significantly in the "z" direction, making it a little less stable in "roll". Presumeably the "pitch" CofG would be unaffected. Anyway, thanks everyone for taking your time to respond.

If you think about it upright, inverted or side mounted engines will not affect the roll provided the designed thrust line is not changed.

Ed S

Higher centers of gravity, certainly do effect roll stability. It won't effect the roll moment of inertia, enough to cause concerns. However, the designer takes these effects into consideration, when he designs the airplane. So, Argess, unless you are designing an airplane for yourself, as a beginner (not recommended) you don't really need to concern yourself about whether the engine is upright, side mounted, or inverted from this standpoint. However, from an ease of operation standpoint, side mount or up right. Leave inverted until you've got some experience.

I am assuming you are a beginner, if not I've meant no disrespect

I guess the question of wether or not roll stability will be affected significantly by altering the engine mounting, is really particular to a certain plane/engine configuration. So I'll start with it inverted. If I don't get too lazy, I'll try changing it once I have a "feeling" for the plane's response and see if it changes noticeably. Thanks All.

PS: No, Mode One, I am not a beginner in this hobby. Started in 1968 with a Cox PT-19 Control Line airplane. I became more interested in RC boats during the 70's and early 80's and with a long gap in between, finally started with on-road and off-road RC vehicles a few years back. I kind of have an inquiring mind, so I like to understand what's going on before I change things. It's odd that I should now find myself asking about inverted engines after all these years, plus working in the aerospace industry since 1984, but I am the first to admit, I can learn something from anyone, so I come here to ask questions from people more knowledgable than myself. Forums can sure be an informative place to find answers. Thanks again.

i hate mounting engines inverted, seems like i always had a hard starting or poor running issue, i prefer mounting the engine on its side,vertical,or angled a little like on the hangar 9 pts mustang.

I've mounted some, even on ugly 'ol bare faced Stiks, with the muffler low and centered. That usually puts the engine head at a 8:00 position or so. Centralizes the muffler weight and drag and gives a less detractive look. And, most importantly, spits the exhaust oil down and away from the airframe.