# 2 stroke alternative ?

Discussion in '2-Stroke Aircaft Engines' started by MadProfessor8138, Jun 6, 2019.

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1. Jul 1, 2019

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You know....that's a good point
Now I'm curious if the converted sled and motorcycle engines (4 stroke ) leave the rev limiter in the loop or if they delete the rev limiter and depend on prop load to set max rpm.
Just depending on prop load during a decent could potentially get nasty....

Kevin

2. Jul 2, 2019

### mm4440

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Outboard powerheads are worth looking at. I have seen a 4 stroke Honda conversion. They are available in a useful range of outputs and are tuned to turn a prop with a similar duty cycle to aircraft. They are expensive new, are vertical mounted and have water cooled exhaust manifolds which are not desirable in an aircraft, otherwise should be great. The supply of snowmobile engines will dry up due to Al Gore caused Global Warming.

3. Jul 2, 2019

### Vigilant1

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But it does work just fine on purpose-built acft engines (and they have redlines even closer to their normal operating speeds than you'd be considering, I think). These are airplanes after all, and there's some presumption that the operator is at least vaguely cognizant of what the machine is doing. In a descent that allowed a normally-loaded cruise prop to exceed redline RPM by a significant amount for a significant time, Vne might be a bigger worry than the RPM being run.

4. Jul 2, 2019

### BJC

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What aircraft was that Honda outboard used on?

The early Scorpion helicopters were designed around converted outboards, but they proved to be inadequate.

Outboards operate with relatively large quantities of cooling water. My understanding is that their cooling systems aren’t easily converted to a closed system.

Supposedly, a Honda outboard was being developed as the engine of choice for the Fretaga J 6, but I can’t find any data about it.

BJC

5. Jul 2, 2019

### Vigilant1

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That would make a lot of sense. If the water jacket/contact area/pump/available channels are built to use an unlimited amount of
75F liquid, it might not work well for recirculated coolant that is considerably warmer.

6. Jul 2, 2019

### pictsidhe

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Outboards have interested me, too. They are somewhat weight conscious as they often need to be manhandled, the duty cycle is similar to aircraft. Water cooled engines rarely have trouble getting the heat into the water, so I suspect that a non-leaky pump may be all the engine side mod needed.
They don't have a sturdy crank output. It's designed to run a quill, not gears or belts. That looks like the toughest nut to me. Oh yeah, they tend to be pricey. Might be worth looking for a model that had chronic ignition woes that boaters have given up on. It's easier to retrofit a different system when you aren't trying to cram it inside a tight cowling.

7. Jul 2, 2019

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I think the main issues with using an engine designed for marine use are :

1. BOAT: Break Out Another $Thousand$
2. Crankshaft orientation
3. Carb design and carb jetting
4. Availability of parts..not much aftermarket
5. Heat dissipation....most outboards arent set up with a "standard" waterpump per-say...they use an impeller on the shaft and they move huge amounts of water.
I would think that an electric waterpump would have to be utilized and a very efficient closed loop cooling system be designed....this could prove very frustrating,costly and may be a problem that has no real resolution.

Yes,the Scorpion helicopters did use outboard engines...I knew a guy that built and flew one.
He fought constantly with cooling issues and so did everyone else that used the outboards.
Also,look at how the drive system is set up on a Scorpion.....the factory output shaft on an outboard is acceptable without modifications needing to be done to its location.

While the marine engine technology is truly outstanding....there are to many obsticals to overcome with the design in general.
But that's just my opinion...

Kevin

8. Jul 2, 2019

### mm4440

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Most outboards are now four strokes. I did not say it would be easy. If you can get rid of the water cooled exhaust the cooling requirements should be similar to a snowmobile or motorcycle engine of similar power. I assume they have thermostats. Anyone know for sure? The Honda project is on Facebook.

9. Jul 2, 2019

### pictsidhe

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Now I remember the bad about outboards. The exhaust is routed through water cooled passages out the side of the block. That's a lot of extra cooling load. There's about as much heat in the exhaust of a regular engine as in dumped into its cooling system, so an outboard is likely to need nearly double the cooling. Best solution is mod the head so the exhaust port is conventionally short.

10. Jul 2, 2019

### BBerson

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A 20hp Honda outboard is \$4000 new. It might be rough to find a used engine with micro-processor and whatever else is needed to make it run.

11. Jul 3, 2019

### mm4440

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Used jet skis are another source of engines. Tend to be high power but at least they are mounted horizontally.

12. Jul 3, 2019

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Pictsidhe......you are correct on the cooling set-up of most outboards.
The work that is involved with redesigning the cooling system passages(head & exhaust),water pump location/function and designing/building a cooling system radiator with enough area to dissipate the heat.....well,I would rather p*ss broken glass than to get involved in that endeavor.
I watched a man struggle for years with tons of money,tons of parts and tons of advice .....all to no avail.
Sometimes you just can't make something into something it's not.

There's still the issue of output shaft orientation.....it's going the wrong way for aircraft use.

That's one conversion that I wouldn't even attempt.......been there to watch others fail and smart enough not to go down that road myself.

Kevin

Last edited: Jul 3, 2019
13. Jul 3, 2019

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mm4440......you are correct about the jetski engines.
They are called Rotax's & Bombardier's.....they have been flying in planes for about 30 years.
Rotax 377,447,503,582,583,670......etc.

Rotax didnt start out to make aircraft engines.....they built snowmobile and jetski engines which were adapted to aircraft use by individuals.
Once Rotax figured out that "if you can't beat them,then you might as well join them" they started producing parts for aircraft
The engines were going to be converted whether they liked it or not......so they figured they might as well make some money off of the situation.

Kevin

14. Jul 3, 2019

### mm4440

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A 6 cylinder Rotax 900 series would be very interesting. 150 to 200+ hp, smooth and light.

15. Jul 3, 2019

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I'm not familiar with that engine.....didnt think Rotax ever made anything with more than 4 cylinders.

Kevin

16. Jul 3, 2019

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From what I just read.....its a 3 cylinder 4 stroke.
2 options....standard and HO

Kevin

17. Jul 3, 2019

### mm4440

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Some years ago Rotax had a certified V-6 aircraft engine project, 285 hp as I recall. Was halted. I am speculating on a flat 6 version of the current 912- 914 4 cylinder engines.

18. Jul 3, 2019

### pictsidhe

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I think Rotax started with motorcycles, but they've been making engines for nearly 99 years, so they probably have got the hang of most small engines by now. They mist be one of the biggest powersport engine manufacturers.

Hmmm, they make motorcycle engines, powersport engines and even aircraft engines.
Their geared 91x aircraft engines specced at 5800rpm for takeoff, piston speed 2320fpm. I can assure you that their motorcycle engines are quite competitive with the Japanese ones, piston speed is a lot higher on those...

19. Jul 3, 2019

### pictsidhe

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a '456' would be great. The bad is it would probably be 3/4 the price of a 912...

20. Jul 3, 2019

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