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Outboard used on a amphibian aircraft

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Paul Minnicks

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" The plans for my home built

" The plans for my home built (NOT FAA CERTIFIED) Coot-A amphibian I am
> building call for the engine to be installed on top of a pylon. I want
> to move the center of gravity down and therefore make it more stable in
> flight and on the water. I am now in the process of investigating ways
> of accomplishing this. One idea I have is to install the engine in the
> haul and drive the propeller with an up side down boat out board drive.
> I know just about everything there is to know about aircraft and very
> little about water crafts. That is why I am contacting you. The engine I
> am going to use is a water cooled, 230 hp, fuel injected, Subaru car
> engine. It will have to incorporate a gear reduction drive get the
> aircraft propeller tip speed under supersonic. Boats with high rpm V/8
> car engines, must also have reduction drives incorporated in their out
> drives . But, gear lubrication when upside down and the shear weight of
> an outboard installed in an aircraft could be problems. To lighten it
> up, I would be able to cut away some of the drive case housing and still
> have enough strength to push a light aircraft. May be what I need are
> blueprints of existing out boards and drives. or a very knowledgeable
> mechanic. I just don't know at this point.
>
> Anyway, if you would help me or turn me in the right direction to get
> information on this subject I would really appreciate it.
>
> Paul Minnicks, A&P, IA, Commercial Pilot (and novice seaman) "
 
" Interesting proposition. I a

" Interesting proposition. I am not properly qualified to address what you want to do but let me throw out a few thoughts.

In an outboard the oil reservoir for the gearcas is located above. By flipping it you will have to provide for this.

Next consider the prop in an outboard pushes against the gearcase. I don't know if you invision a pusher or puller for your aircraft but your prop would need to push against the gearcase also.

Then there is the gears themselves. It would be a real bummer to be flying along enjoying the cool afternoon air and have some type of problem in the gearcase causing the prop to want to go into reverse.

Another element is the upper seal of the gearcase and bearing is cooled by water from the waterpump in a boat. In an aircraft ???

Also keep in mind the intended diameter of the prop versus the clearence available from the lower unit of the outboard. The deepest lower unit I know of in a boat is 25".

Last, the boat propshaft will be horizontal, lower unit driveshaft will be vertical and I assume that you will have the Subaru horizontal. That is a 180 degree change in force????

Not sure that I would want to fly with you on this project. "
 
" Interesting proposition! Hav

" Interesting proposition! Have you considered resonant frequencies of the drive system and propeller? Your outboard drive unit is not nearly as rigid as any of the various reduction units available. (my outboard allows nearly half a turn of the prop before engaging the engine!)Resonant torque pulses can do bad things to propellers, right? You must find a way to keep the resonant frequency of your system out of the operating RPM range, or else you are in for one bad day. Even with elastomeric dampers, sprags and such, it would still be difficult to keep your drive system alive never mind the prop. Outboard drives are not designed with an aircraft propeller's large radius and momentum in mind. But it might be kind of cool to taxi up somewhere and apply "beta range" (reverse). Oooh, just think of the torque reversal required for that with a big prop on it! Ouch! (you'd want eliminate the reverse gear and shift mechanism altogether) Seriously, I believe this would end up heavier than going ahead with the pylon mount. More stable CG, sure. Adequate reliability I doubt seriously. The Subaru is a good choice for many homebuilts. (I am a big advocate of using the superior technology and lower cost of auto engines in homebuilts; I am planning to build my own airplane someday. I will almost certainly use an auto engine) And thinking outside the normal envelope is the only way to create innovation. But I'm a little skeptical of this particular concept. The torque moment of the outboard trying to twist forward is partially offset by the drag of the water. You will see proportionally much less resistance from the air, even at normal cruise speed. Water drag is just a lot more however you slice it. I suspect that you'd have to stiffen/strengthen the housing quite a bit to keep it from developing cracks in the trailing edge. I'm not as concerned about this, however, as I am about keeping the prop from going resonant and coming apart. Mount the engine vertically and develope a sufficiently stong and stiff drive train and you could really have something. But this will be a HUGE undertaking to R&D if you want to see it through to actually be practical. By the time you got done with it there would no longer be anything left from the outboard. That is my 2 cents worth. "
 
" Thanks for the in put. If I

" Thanks for the in put. If I do this, I probably will build my own 4130 steel tubing pylon. You guys have convinced me, reworking an outboard just to use the gear train would not be worth the time and money.

Thanks again. "
 
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