Motor Inboard or Outboard on Profile Plane

Hi fellas. I have read various things on motors for Profile planes being on the inboard and outbord side. I have personally always put it on the outboard side simply because I felt that the outboard side could use the extra weight. Since joining this forum I have seen both, what are the pros and cons for either [photo not recovered: sipcof.gif]

For one thing mounting it inboard will demand that the engine overcome centrifigal(sp) force in drawing fuel. Not a good thing at best.
Jon Ramsden
<@_@>

>For one thing mounting it inboard will demand that the engine
>overcome centrifigal(sp) force in drawing fuel. Not a good
>thing at best.
>Jon Ramsden
><@_@>

Maybe not. The tank mounted in the normal outboard position might present draw problems simply due to a longer height differential between the spraybar and the fuel pickup. So what are your thoughts if the tank is also mounted on the inboard side? Does it become a pressurized system?

>Maybe not. The tank mounted in the normal outboard position
>might present draw problems simply due to a longer height
>differential between the spraybar and the fuel pickup. So what
>are your thoughts if the tank is also mounted on the inboard
>side? Does it become a pressurized system?
>

This is what is usually done--mount the tank inboard (square clunk tank) with the engine.

As stated, however, mounting the engine and tank outboard saves on tip weight. Sometimes this is sufficient in itself. Mounting it inboard means adding sufficient tip weight to off set this weight plus what is normally needed for tip weight. You also bring the thrust line further inboard which can add to yaw problems.

On the plus side, the fuel is thrown away from the plug and the engine acts like it is upright at all times. On the other hand, although I have seen a problem on an inverted (or upright) engine when the plane is turned over, I have never really seen a problem with the engine mounted sideways outboard. Every stroke is the same so I haven't seen the detrimental affects here. Some claim that it runs better mounted inboard, but if it is running fine outboard I don't see how it will improve the other way.

Personally, I would (and do) mount the engine outboard.

Well, this one's not a stunter, but the weight of the three Baby Bees mounted inboard (looked better in flight) required excessive tip weight. I kept adding weight until the poor thing quit flying with the outboard wing panel high --- at which time the model's total weight was approximately that of a Mack truck.

Mike Keville

[photo not recovered: 22138.jpg]

Attachment #1, (jpg file)

Biggest problem with the inboard engine is the exhaust is thrown up instead of down. Getting someone to launch with his face getting sprayed with oil can be a minor problem. This is where a K&B 40, old Royal 40, and other engines where the exhaust can be moved to the opposite side is useful.

Fuel draw is no worse, possibly better than when mounted outboard. The height the engine has to draw fuel against centrifugal force is never more than from the engine bearers to the spray bar. On an outboard mounted engine the distance could be greater especially if using something like a GRW chicken hopper tank. There is less concern of the tank flying off if something gives because of reduced stress on the mounting system.

Uncle Mickey; Could your leadouts have been too high causing the outboard tip to be high?

>Uncle Mickey; Could your leadouts have been too high causing
>the outboard tip to be high?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Doubtful. See the bent wire leadout guide in that photo (barely visible there). The leadouts extended straight out from the bellcrank that was located slightly below center in the profile fuselage, passing through the guide seen (faintly) on the underside of the inboard panel.

Can't complain about that one too much; it made beautiful touch-and-goes, and in 1990-91 it won a 1st in 1/2A Multi-Engine Profile Scale plus a Pilots' Choice "Outstanding Craftsmanship" award. (Though that was probably due more to the fact that Floyd Carter, PTG and Ron Burn weren't there!)

Mike Keville

Glen,

If the tank is uniflow vented, or muffler pressure assisted, it should make little if any difference... ...other than the exhaust port (for most engines) being in the face of the launcher.

The problem of 'swamping' the glow plug should appear in outboard mounted sidewinders, and affect inboard mounted engines less.

The "height" to draw the fuel to the spraybar jet is also worse in normal outboard tank and engine sidewinders. (The reference for the uniflow tank is where the air vents into the inside of the tank shell. This is generally accepted by now, I guess.) On a 2" wide tank, that vent ends about 1 7/8" "off the mounts" outboard, so about 1 1/4" to 1 3/8" outboard of the spraybar jet.

On an inboard/inboard setup, the uniflow interior vent opening is about 1/8" inboard of the mounts, i.e., closer to the spraybar jet height. ...unless you inset the tank in the fuselage...

Muffler pressure is usually more of an influence than venting.

Swamping the plug might be more likely in rich 4-cycling, than in the recently stylish low-2 cycle mode that works well with basically RC designed, recent engines. Such engines have a wide RPM spread possible in 2-cycle, and where they're running 4-cycle (quite often) doesn't reach into their design power band. (Modified engines can be exceptions to that.) So, if plug swamping is to be a problem, we'd really see it when 4-cycling in "normal" profile setups.

Frank Williams, I think it was, designed a profile F8F Bearcat for an inboard mounted ST60 many years ago. Frank wouldn't do a layout that had inherent problems. (FYI, if you didn't know, Frank wrote the Powertrain column for Stunt News for several years, too.)

The major negative to inboard/inboard for me is the need for added tip weight it requires, to trim a stunter. Mikey's tri-motor didn't have that as a "critical design element."

Thank you all for your replies....really good stuff, and lots to think about.

Can we please have a definition of inboard and outboard? I think I've got them wrong.

I always thought that the inboard side faced the inside (center) of the circle.

You are correct.