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I’m still deciding the way forward power-wise to get into this PPG thing – engine choice been a big part of whats holding me up – which engine should I go for?

Should I build my own from scratch (been there done that - built a 11 cylinder 23hp radial engine in my workshop at home a few years ago), should I collate data on Yamaha and Suzuki 2 stroke air-cooled blocks and see if I can find a suitable option from amongst the many 1970's and 1980's options built for MX racing? Okay 2 -stroke has crap torque (compared to 4 stroke), and you need torque for thrust, but they have something 4 strokes can't offer - less parts to break (and they are lighter).

Hell, there are soooooooo ......many engine options for PPG flying - what about rotary.

In theory rotary ticks all the boxes, and it suprizes me it's taken so long for anyone to get the rotary ball rolling in PPG. Parajet now have as most folk will be aware. Problem with rotary,is not reliability or specific fuel consumption (those issues are largely deal't with now that advanced ceramic coating technology has matured and electronics have got EFIS units down to negligable weight and accuracy) - the problem remains that, even with CAD/CAM software as powerful is it gets, it still takes a fair amount of R&D capital to engineer rotary engines, and the PPG market is just not large enough to justify that capital investment.

It would have to be an adaptation of an existing design for some other application.

So off I went looking for rotary options......

Parajet has to be the starting point with their new Rotron 294cc motor …..mmmm, reasonable price (???), but don’t forget it’s water cooled, so that’s extra hardware and weight. Wouldn’t it be nice if there was an air-cooled rotary? – and guess what, there is – its made by UAV engines, their 731 and 741 engines, both weigh in at a touch over 10kg’s, producing a very useful 38hp. That makes everything else out there on the PPG market currently look over-weight and under-powered. 38hp @ 10kg's is fantastic - it's brilliant!

But how to speak to UAV - that’s not easy, they hardly ever answer their website published phone number – I had to go do some company research. UAV is owned by Silver Arrow, Silver Arrow is owned by Elbit … and get through to Elbit (who do answer their phone - albeit you have to phone Isreal!!) and you’ll get a tel number for UAV that someone answers. Don't know why UAV is so shy - they do get a lot of "steam" from UK anti war groups because their engines are used in Isreali UAV's.

Then it all fell apart – wait till UAV tell you how much they sell their engines for and you won’t bother anymore with that option. You can purchase a complete top shelf PPG setup, wing, harness, risers, motor - the whole lot and fuel for a month of Sundays, for less than what UAV sell just the 731 and 741 engines for!

Oh well - back to the drawing board ………..

A great rotary engine is made for karting by a small company called Woelfle Engineering GmbH (German ………of course, UK gov policies threw UK engineering to the dogs years ago). It’s called the Aixro XR50 – close on 50hp and with a torque figure that’s plenty plenty sufficient at rpm/gearing that is easily tailored for PPG work.

Just one problem with this block – it weighs in at 15-17kg’s (versus the UAV +/-10kg’s), and its water-cooled (well so is Parajet’s Rotron 294cc rotary engine) – that’s the same as CRE’s popular PPG engine, the MZ34/35 (as used in some Black Hawk paraglider models).

The MZ for the record produces just over 27hp @ 6250rpm, versus XR50’s close on 50hp @ 8500rpm. Much more than needed, so run the XR50 at the MZ’s rated power output rpm of 6250, and you'll get just under 32hp, but with substantially higher torque of 40Nm. No problem for the heaviest of PPG drivers.

In terms of fuel consumption, weight and practicality, rotary blocks really come into their own for PPG as air-cooled blocks.

This is starting to look good ...........

A study of the Aixro XR50 indicates that the power output could be de-rated – bring it down to around 32-35hp (give or take around 10%), and there is every indication from a thermal perspective, you could then bin the water-cooled case and engineer an air-cooled finned outer-case - don’t ask me the fin size/length/spacing – haven’t got there yet – but it wouldn’t be much, if any, larger than the UAV 7 – after all they are getting 38hp from an air-cooled 208cc chamber (versus Parajet’s and Aixro’s 294cc chamber).

This along with a few other mods, should enable the Aixro XR50 engine to be brought down to around 10kg's as well. That’s good – things start to look real attractive now for this engine as a PPG power source.


Good question. I’m pretty sure that the 741’s short TBO of around 150hrs (the 731 version TBO is even less - around 50hrs!) has a lot to do with it’s air-cooled nature, versus been configured as a water-cooled engine, like the Aixro XR50.

Still, I’m motivated, more online research – open up a folder, start collecting all info I can on rotary engines, sizes, displacements, manufacturers ect ect …….

As part of all this hunting around trying to get to grip with possible options, I took a look at Parajet’s USA webpages, where they have a series of pictures outlining the Everest Parajet Deveopment Story (http://www.parajet.com/index.php?id=118). Giles provides interesting info on the engineering issues that had to be tackled in the design of the PPG that was used to get to the top of Everest, but note how little, if anything he said at all about the actual rotary engine its-self.

Interesting ..... and while scrolling through the pictures on the Parajet USA website, I came across a picture titled “XR50 Rotor, Apex & Side Seals” … and it all fell into place.

Nothing original about my idea at all – Parajet’s been there and done that – the Parajet Rotron is a repackaged Woelfle Engineering AIXRO XR50 karting engine! I was wondering how the hell Parajet had managed to fund the R&D for a rotary engine – they didn’t, they use the Woelfle Engineering GmbH engine.

In saying that I don’t wish to belittle Parajet’s efforts here, considerable work has had to be undertaken by Parajet to repackage the XR50, no question about it, but for those of you who are engineering minded, this does get one thinking.

Okay, its going to take some work to re-configure a used AIXRO XR50 for PPG use (just ask Parajet!!), in either its original water cooled format, or as an air-cooled lump. You’ll need to de-rate the power, its got plenty so no prob’s there, you best off taking off everything not needed, and play with fueling to get an ideal hp/torque relationship, especially if you are going to get an air-cooled case machined up, but its by no means not possible – and it appears to be a damn good power source option for heavier or power hungry PPG drivers – and at an unbeatable power to price ratio.

Rotary engines have a lot to offer the PPG community:

- they are a lot more comfortable (none of the vibration associated with reciprocating IC engines)

- they offer better centralised mass distribution

- comparable fuel consumption (when setup carefully)

- significantly greater power and torque outputs for the same mass

- greater gearing options (and therefore greater scope for prop choice)

- less moving parts to break (improved reliability)

I don't think its going to be long before a lot of other manufacturers start out on the rotary engine road.

…….. and the cost of a used AIXRO XR50 on the karting scene? Not much more than a 1000 - 1200 Quid. New they cost just under 3k. I suspect rotary is on its way big time into the PPG scene. Parajet took the plunge, others are going to follow suit quickly I suspect.

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OK, short answer to a very long and good post.

Ref, not enough funding. (you may be supprised to hear that Parajet secred MOD funding over a year ago to produce a light rotary motor.)

They started off using the AX50 unit some time ago but had problems, and as you can see, they moved over to another unit which I am told is designed to run at much higer revs than the AX50 lump, (which means that is will not be reving as much as it's proven existing use.

I just wanted to answer the things I knew about :D


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Nope - that Simon, I was not aware of - so the engine rotor/internals/seals ect ... in the serial production engines are not sourced from Woelfe Engineering?

Well, well ........ a very useful piece of info - thanx. Will have to keep that in mind re: gearing ratio if I go down that route.

KISS - not much around for PPG use that coud be more simple than the mechanics on a rotary.

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Being fascinated by the rotary for its benefits I watch with great interest, I am also in the bracket of interested weight ranges being tall and elegantly well built. :lol:

Any thoughts on the rotor tip wear problem Gulfstream? I understand that is a potential weak spot. Time between overhaul (TBO) is an issue in reality but not in any form of certification of course.

Compared to a Go-Kart the loading on the motor is smooth and even, this must up the life of the motor and components by a huge margin.

Cooling, flow direction might help, there may be an opportunity there with CF ducting - 'know someone (SW) not to far from a keyboard who know loads about CF. You might even think of forcing the flow (elect/mechanical) on a cue from CHT? Just a thought.

http://fsae.com/eve/forums/a/tpc/f/1256 ... 55451/p/12

How great it is to listen to someone who has skills like yours - 'would love to see a picture of the radial - who doesn't LOVE radials.

When Gilo came back from Everest I had a chat with him (was buying a motor at the time) about rotary motors and he was very open about the source of his power-plant - Germany and an adaptation of an existing unit. The tweaking by the Germans made the installation make sense I understand, as you suggest.


Note the TBO - I guess they don't expect them to survive too many encounters with guns or missiles. A lot depends on how they run the thing, probably at 100% continuously to keep the speed up for the encounter.

Also, military contractors thrive on reorders and replacements, I wonder what that does for the stated TBO times. Perhaps the 'real' life of the unit is a little higher - but if the cost is off the clock who cares anyway. :roll:

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Norman - to take up on your comments: cooling is the big issue.

Rotor tip wear has always been one of the issues in rotary engine design – both Apex tip seals and side seals. Big progress has been made over the last few years regards tip seals, primarily through the use of using ceramics as the seal material, and having them run against chamber surfaces that also have ceramic coatings.

The problem is that ceramic coatings are not good conductors of heat, and one of the ways rotors dump excess heat energy is through the seal interface with the chamber. If the efficiency of that “interface” is comprised by the ceramic coating used to reduce wear, you have thermal problems that have to be engineered for – and the fall back to deal with that is either by way of reducing power output to whatever the ambient air temp/flow can absord and draw off, or choosing to keep the power output and using water-cooling – and that means greater mass with the added mechanical complications.

Depending on the end-use of the engine, air-cooling may be preferable in one case, but water-cooling may be preferable in another case.

It’s a “ balance” and as far as the UAV 731 rotary engine goes, it would appear they scarificed wear and weight to get performance i.e. against the background that many target drones do get blown out the sky before 150 hrs (or 50hrs) are up, they have done away with the ceramic coatings and water cooling so that thermal transfer is more efficient and hp can be maximised – and that is reflected through the price difference between the 741 and 731 (which is more-or-less in line with what it would cost to ceramic coat the parts on a 731 and bring it up to 741 spec – along with a few minor other changes).

The other evidence that supports my thinking along these lines is that a couple years back I happened to walk into Apicote in Gloucester to pick up a pair of Ducati cylinders and pistons one Saturday morning I had sent them in for coating, and guess what were on the bench lined up waiting for some or other type of ceramic coating (just before the guy pulled a long sheet over them to cover them up – I pointed towards them – he knew I was about to ask some or other question about them)? – about a dozen UAV 741 rotary chambers!

Personly I’d prefer an air-cooled block – so the chamber of the XR50 will have to be changed. Cooling fin design and position will the issue: so long as that can be incorporated without exceeding water-cooled weight, it makes sense. I don’t know what coatings are used in the XR50 engine, but the TBO strongly suggests they are pretty good coatings. And while on the subject of coatings there are ceramic heat barrier coatings which can be applied to the rotor which will drop the rotor temp between 30degree C – 45degree C – now that may well be an answer. Would be nice if they could be applied to the inside of the chamber, but unfortuneatly they can’t be engineered smooth enough for that purpose.

Why doesn’t Woelfe use the heat barrier coating I am talking about? Because it would push the cost of the engine over and above what end-users I think would be willing to pay. It would increase the engine around Euro’s 300 – 500’s. Still, worth it as a one off I would think.

Not withstanding Parjet’s experience to date with the XR50 engine, I’m confident it can be done. Even if the power has to dropped back to around 38hp – 40hp, if the weight can be brought done to around 10 – 12kg’s, this along with the other benefits that rotaries offer, will make it a viable power source project.

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Fantastic Gulfstream,

I for one hope that a reasonably priced rotary can be developed. What a step forward that would be. Get the cooling issues sorted out and the technical challenges resolved will leave quite a product on the bench.

I am tempted to think that making that product retrofitable to almost any model of paramotor could make quite a return. :idea:

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Building a one off “good for purpose”, is one thing – commercialising the product for successful/profitable retail to the PPG community is something else.

To be successful - commercialy, it will have to be offered with a warranty, it will require agreements with distributors/retailers, it will require the logistics of parts & service support, it will require marketing, it will require packaging & presentation – and it will have to compete with established brand-names. Speak to anyone with an engineering production background, who also retails, and they’ll all say the same thing: its one matter designing something "good for purpose" – it’s another matter turning it into a viable business. I have no business skill/retail experience.

I think my mind is made up: I’ll chase options for another week or so, if nothing else turns up, think I’ll get an AIXRO XR50 – break it down, study it, then attempt re-engineering & re-building it as an air-cooled PPG motor – should make an interesting project and good read (or good laugh) as a serialised thread on the forum.

Attached below are a few pic's of the AIXRO XR50 rotary motor - and some figures from karting folk who have "played" with one on a bench.

Tech Spec's

Specification: Single rotor, Wankel design

Power: 46hp @ 7800 rpm

Weight: ~ 17 kg

Torque: 47 Nm at 4500 rpm

Chamber volume: 294cc

Max. engine speed: 10500 rpm

Clutch: 2-disc centrifugal

Drive: 428 pitch

Ignition: Magnetic ignition with adjustable timing

Housing: Water cooled, partially Nicasil coated

@ 8750 RPM

50hp - no air filter, with a 160 jet and good exhaust flow - water temp 50C

@ 8750 RPM with restrictor in periphal inlet

40.3hp - no air filter, biland exhaust - more torque in the mid range

@ 8750 RPM with K&N filter

43.7hp - with the good exhaust and a new k&n air filter, bit more torque than the 50hp run

@ 8750 RPM as above with 150 jet

47hp - no air filter, good exhaust - way better response & torque in the mid range!

All above done with a restrictor to get a more torque in mid range - because the flapper valve opens the periphal port to early (so the karting guys say).

Conclusion: this engine has lotsssssss .. of tuning potential (up and down depending on thrust/torque required and where you want it in the rpm range) Remember Parajet ran this thing to 96hp @ 8600rpm with a supercharger and normal 95 Octane fuel!

With open and lengthend exhaust and good carb/tuning/variable ignition 60hp is reachable..

Want more(?) - use ceramic needle bearing and ceramic apex seals (thought it came standard with ceramic Apec seals - it doesn't, so they will certainly reduce friction) A reliable 65hp would be possible.

If anyone has any comment/idea - good/bad, for/against, positive/negative …. whatever, I’m all ears.






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Fantastic idea and I take your point, I don't have a commercial head so haven't a clue what it would entail putting something like this into production. 'Just excited by the idea that we might all be flying around with rotarys one day.

Gulfstream, if you decide to do it you would have an avid readership here. There are a bunch of techies and bikers (or ex bikers) who I am sure would love to read about your project.' A few might even join in... :roll:

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Nope - that Simon, I was not aware of - so the engine rotor/internals/seals ect ... in the serial production engines are not sourced from Woelfe Engineering?

Well, well ........ a very useful piece of info - thanx. Will have to keep that in mind re: gearing ratio if I go down that route.

KISS - not much around for PPG use that coud be more simple than the mechanics on a rotary.

so you would be able to fix it out on the field if it went wrong?

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Nope, not likely - rotaries are inherently and in orders of magnitude more reliable and less likely to breakdown in the first place compared to conventional IC engines.

That is precisely what makes them so attractive .............. and yes, of course that is all about setting them up properly in the first place - but, whats the point with any engine (let alone one you fly with) if it's not setup "good for purpose" to start with(?).

So to answer your question: nope - field repair will be a less a concern than it is with other PPG motors

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Is that a Bing carb on that Norman?


..... hope Norman doesn't mind - yes, that is a Bing carb: the XR 50 can be purchased with any one of a number of carbs. The motor is easy to setup by end-users with any one of a number of branded or OEM and after market carb options.

Fuel injection is also possible - and turbo-charging - and supercharging ................. this motor has loads of scope of mod's and tuning, many of the options not even tried out yet by th karting community (the main users of this engine to date).

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  • 6 months later...

Some more info on this...

The reason why Parajet chose to use liquid cooling is because by the time you add all

the metal you need to cool the engine it is the same weight as the 800ml of

water that is required to cool is and the 750grams of radiator and hose that

you also need in the system.

The aixro engine is heavy, to get the weight down on our own 294cc engine we

manufactured a hollow main shaft, machined every last bit out of the rotor

and made new side plate castings with an integrated coolant pump to reduce

weight still further.


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