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.