# Yamaha 701 PWC engines -- why not?

Discussion in '2-Stroke Aircaft Engines' started by Andy Garrett, Jan 5, 2020.

1. Jan 6, 2020

### pictsidhe

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A universal one just isn't possible. They need tuning to avoid destructive resonance, or have a short life. It would be possible to design them for categories of engines. I am on and off working on one for an industrial V twin. It would work on several different engines with minor tweaks for mounting etc. RPM range, flywheel and prop MOI must be within a certain range.

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2. Jan 7, 2020

### Hot Wings

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Kind of off topic:
VW did just that in the beginning. We had a VW dealer here in the middle of nowhere back in the late 50's. They wanted twice the money for a VW bus than GMC wanted for a V8 Suburban. My father was kind of cheap - guess what I learned to drive in?
Got to drive my first VW, a '57, in 1974.

Fortunately for a lot of EAB builders VW changed their marketing strategy.

3. Jan 7, 2020

### Andy Garrett

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Lively conversation. Engine options at realistic prices have always been a challenge in HBA.

I agree that a PRSU shouldn't be too much of a challenge even if it must be fabricated.

As much as anything, I just want to add something meaningful to the world of completed homebuilt aircraft once I finish my plane. Another 'built to the plans' machine, while sensible and arguably safe, is just not that interesting. I like to embrace the spirit of 'experimental' and I enjoy studying the creative and ingenious ways people tackle challenges and make a plane their own. It's the aircraft that deviate from plans in intelligent and innovative ways that get my attention and often my admiration. I don't think I'm alone in that opinion. After all, if everything was just built to plans, then if you've seen one, you've truly seen them all. We would only be comparing paint jobs.

Last edited: Jan 7, 2020
4. Jan 7, 2020

### cheapracer

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To cover all bases, I'm guessing weight is your enemy there.

There has been plenty of failures, and a few deaths, from no other reason than harmonics.

If you have thoughts, please share, no one is going to laugh at you, it's something that a few of us would like to resolve and any ideas, even if a bit obtuse, sometimes leads to other ideas.

5. Jan 11, 2020 at 2:21 PM

### Andy Garrett

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Turd, My knowledge of snowmobiles is limited. Are there closed-looped cooled engines with the kind of power and weight as the Gen II Yamaha 701 that I should be considering?

6. Jan 11, 2020 at 3:23 PM

### cheapracer

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7. Jan 11, 2020 at 4:59 PM

### Andy Garrett

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At Turd's suggestion, I am looking at liquid cooled snowmobile engines. They tend to be very high horse-power and finding listed weights has so far, been unsuccessful. The Yamaha VMax 600 engine is intriguing, but at 92hp would have to be de-rated. Perhaps this would increase reliability--don't know. Prices are a bit higher than PWC options and finding rebuilt engines ready for a core trade has also been fruitless thus far. It's just a tougher space to navigate. I'm not sure why.

I think I need to ask around that FB page cheapracer.

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8. Jan 11, 2020 at 8:07 PM

### pictsidhe

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It's difficult to derate two strokes much. That involves smaller ports...

9. Jan 11, 2020 at 8:24 PM

### Rik-

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You know there’s a good idea when a company starts modifying things.

Edge performance, who specializes in Rotax power upgrades is going after the Yamaha Apex snowmobile engines and basically stripping off everything and getting a tune to work, I.e. Steve Henry’s 300 hp Yamaha powered Just aircraft Highlander.

in its natural, natural aspirated state, the Apex makes like 100-150 hp and people are purchasing a sled for less than 10k used and they basically strip it for the engine. There’s a Facebook page “Yamaha Aircraft Conversions” that has a lot of information on the harness, engine mount and exhaust.

The engines spin a lot of rpm to make big hp, a PSRU is required and some are using the Rotax unit and others are using a custom built one by “Sky Trax” (around $2500) but you can cruise the engines at like 5-6k and make 120-150 hp in he turbo configuration and not be so bad on fuel. Point is, for less than 15k a Kitfox, or Highlander can have an engine that is more powerful than a Rotax that cost$55k.

10. Jan 11, 2020 at 8:51 PM

### Turd Ferguson

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There's some limited flexibility with the powerband without re-porting. By the time you get a re-drive and prop installed the hp will be limited.

The Yamaha R1 variants are interesting. The Apex seems to have much greater reliability. Someone sells an adapter so a C type Rotax gearbox can be mated up and I've seen someone else modify a C gearbox to fit if you don't mind a one-way alteration.

11. Jan 11, 2020 at 11:08 PM

### Rik-

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Sky Trax is making the adapter as well as his own gear box

12. Jan 11, 2020 at 11:46 PM

### Andy Garrett

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Rik-, I am familiar with Steve Henry. He hangars his planes at KMAN where I work part time. Fantastic engines! But I'm looking for Affordaplane power, where 75hp is MASSIVE. The big R1s are an order of magnitude or three beyond where I need to be in power and price.

During my day of learning the range of liquid cooled snowmobile engines that might work, and thinking on the omni-present and not inconsiderable PSRU issues, it occurred to me why motorcycle engines with the built in transmissions might be appealing. The transmissions (which are made to handle heavy torque loads) offer multiple reduction ratios. Of course, I googled the hell out it, and found some examples. A guy with a Hawk powered by a Honda GL500 for example--shifting gears in flight--not to speed up the prop, but to reduce the RPM for fuel savings.

So, down I went... another rabbit hole. Turns out, that 250cc, 2-stroke, liquid cooled and 450cc, 4-stroke, liquid cooled, dirt bike engines come in at 60-65 pounds at 49-53hp. That's with the transmission, so no need to add weight for the re-drive. First and second gears on the Yamaha YZ250 and Honda CRF450R (the two I studied most, and which represent the numbers I just gave) are appropriate for reduction to prop speed at the engines' max HP RPM. These engines can be had in and around \$1K give or take depending on vintage.

It seems the legendary Yamaha Virago V-twin engines are also an option. I don't know what they weigh though and can't seem to find that info on the web.

Alas, rather than narrowing my options, I seem to be expanding them. It's an enjoyable mental exercise at least. I appreciate all the comments that keep me thinking.

13. Jan 12, 2020 at 1:56 AM

### pictsidhe

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Dana modified a KX100 engine for paramotor use. I believe he swapped the clutch for a compliant element to deal with TV. TV is likely to be your biggest issue if adapting an engine to swing a prop.

14. Jan 12, 2020 at 6:43 AM

### Rik-

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‘here’s an inexpensive choice that has power upgrades out there and can be sourced at harbor freight.. They actually have quite a following.

15. Jan 12, 2020 at 7:37 AM

### cheapracer

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Longer header pipe is the easiest, an inch per 500 rpm drop required is a rule of thumb that works, and if you want to go further, mill the bottom of the barrel and add spacer up top to lower the port heights.

Besides being lighter, the YZ250 will lose power if you start to have trouble, but will still kind of still run, if the 450 goes bang, that's it, it stops - and is very expensive to repair.

16. Jan 12, 2020 at 1:31 PM

### wsimpso1

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The motorcycle engines look attractive from the perspective of having high power and a gearbox. Motorycle vibration characteristics are completely different from airplanes, and their vibe isolation schemes tend to be incompatible. Replacing the clutch with an elastic element, properly sized, and perhaps adding some inertia on the engine side of the elastic element could be very good ideas indeed. You will be innovating here with both innovator's benefits and risks. The other thing you will have to do with any of the PWC/snowmachine/motorcycle engines is add a suitable shaft and support for the prop. Look up how to calculate gyroscopic moment and p-factor and inertia of the prop.

17. Jan 12, 2020 at 1:46 PM

### Andy Garrett

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Forgive my ignorance... 'TV'?

My main concern with the idea of these two particular motorcycle engines is, as you indicate, the vibration associated with a single jug. Though more modern engines seem to go a long way to address this. It's hard to anticipate. Probably best to talk to someone who has made a similar conversion, though this path is not currently preferred.

18. Jan 12, 2020 at 2:39 PM

### wsimpso1

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Torsional vibration. You get firing pulses and other pulses that can not be allowed to run close to the set of natural vibration frequencies in the system.

Big vibe inputs are firing and first order piston accels, while lowest natural frequency of the system is usually engine MMOI rotating opposite prop MMOI with the torsionally soft shafts and gears between them.

See https://www.homebuiltairplanes.com/...-and-resonance-basic-theory-and-issues.14215/ for the basics and a discussion that followed. Naysayers exist, but I have seen negative results - destroyed machinery and test rigs. It is very real, and when handled wrong, spectacular. When handled right, folks ask "what's the big deal?".

Billski

Last edited: Jan 12, 2020 at 4:59 PM
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19. Jan 12, 2020 at 2:48 PM

### Andy Garrett

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Thank you. I will add this to the many things I plan to study.

20. Jan 12, 2020 at 4:39 PM

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