The Moni was originally designed to be powered by the 2-cylinder horizontally-opposed KFM 107 2-stroke engine.. claimed to put out 25 hp at 6,000 RPM with a direct-drive 33" diameter propeller with 18" pitch .. see picture above at left. The small, high-revving prop is inefficient, but actually kind of a clever setup for a motorglider where you're (supposedly) powering up primarily to catch thermal or ridge lift ... you get light weight and a small, low-drag prop for better engine-off soaring performance... but since the Moni design turned out to need a heavy nose ballast weight for proper CG location anyway, the lightweight aspect of the motor was a bit of a joke. And, to be honest, touring motorgliders are flying around like powered airplanes most of the time anyway.. why not enjoy flying even when there's no hope of finding much lift? But above all, personally, I HATE 2-STROKE ENGINES!! They are smelly, polluting, gas-guzzling, noisy, and unreliable ... but they're simple, lightweight, and available in a wide variety of models. I removed the KFM engine from Moni #053 and sold it, so now I'm committed to installing a small 4-stroke engine with direct-drive prop into what will become the Mosi. The engine choices are few and far between, but I'm hoping to emulate what Jim Hardy started doing a few years ago.. see picture on right above.. more details on his website:
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For good reasons, airplane engines like to be horizontally-opposed (AKA boxer) cylinder arrangements, usually 4 or 6 cylinders. So just go out and buy a 70 pound 25hp flat-4 4-stroke engine, right?... I WISH!!.. ain't no such animal to be found. There are a few flat-2s in that power range, like the 1/2 VW, or 1/3 Corvair engines... but they require extensive machining, and they are a bit too heavy, and they tend to shake far more than flat-4s do... the shaking is even worse with in-line 2s. Flat-4s are very well balanced, which is one reason why they are so common in airplanes. The U.S. government surplus market has a few interesting choices.. the 4A032 engine is the one I'm focusing on for now... it's the world's smallest flat-4, only 8 cubic inches displacement per cylinder, 32 cid total. The main problem: it may be a bit TOO small. It was rated at 6hp by Uncle Sam, but that is with a small inlet restriction plate, high altitude, hot, low octane operating conditions. The 4A032 was intended for powering generators, air compressors, welders, etc... designed to run continuously at 3,600 RPM.. and they reportedly last for tens of thousands of hours without major overhaul. A few rather ingenious folks out there are using them to power lawn tractors, log splitters, and who knows what else. Some people claim that it's easy to get 16 hp out of them by just drilling out the restriction plate, and putting on better air filter and exhaust. 16 hp might be enough power for the Mosi, but I'm aiming to get at least 18 hp peak power out of it at 3,600-4,000 RPM while hopefully preserving its high degree of reliability. Whether that's a realistic goal or not, the relevant fact is that the 4A032-equipped Mosi will require much less power than the KFM107-equipped Moni due to the Mosi's larger span and more efficient propeller. A 40x30 prop on a 4A032 engine should be able to match the thrust output of the KFM107 with 33x18 prop across the whole speed range of interest.. that is, if the 4A032 can rev up to 3,400 RPM or so in a static test with that prop.. which ought to require about 17 hp out of that cute little engine... maybe it can do it, maybe not. See the yahoo groups on this and similar government surplus engines:
http://groups.yahoo.com/group/4A032/
http://groups.yahoo.com/group/surplusengines/
There's no shortage of vehement shouts of "that won't work" from "experts," including those that have never even seen a 4A032 engine. In spite of the fact that there is almost no relevant test data on this engine, I'm fairly confident it can be made to work in this application. If it can't, my next choice would be an industrial V-twin like you see on lawn mowers & such. Briggs & Stratton makes a 23hp 627cc (38cid) V-twin that some have used to power airplanes.. see Colomban's MC30 Luciole blog (in French) and the Davis DA-11 which flew with the smaller 18hp B&S V-twin :
La Luciole MC30 ULM Colomban
http://www.aircraft-spruce.com/da11.html
Other more powerful (and heavier) V-twin options include Kohler's CH745, a 725cc 28 hp engine that has electronic fuel-injection and ignition, and Generac's 990cc model for 32+ hp. More info at the small 4-stroke yahoo group:
http://groups.yahoo.com/group/Small4-strokeEngines/
The V-twins generally operate in the right power and RPM band for a direct drive prop, but there are other issues and no easy answers. Potential problems include the vibration, uneven firing sequence (sounds like a Harley motorcycle), and low crankshaft position, which is a challenge for thrust line, cowling and/or prop ground clearance.
So, without the perfect engine waiting on the shelf for me to buy it, the struggle to get the right engine for the Mosi continues... for right now, I plan to finish modifications on the 4A032-4 with a 40" prop. There are some other mods that may improve power & reliability, including turbocharger, new carb/fuel injection, removing the governor, porting and/or shaving the heads, and grinding a new camshaft, but I don't want to get too carried away with trying a bunch of new things until I get some idea on static testing whether or not this engine will put out anything close to enough power to work. Even if it doesn't go on the Mosi, the 4A032 may find it's way onto lighter and "spannier" aircraft, like my favorite ultralight, the Kasperwing... wish I hadn't sold mine... but I digress.
As always, your comments & suggestions are welcome... Happy landings!!
Hi Dan, have you looked into the NST Mashinenbau NS 650 4 stroke for a possible powerplant ? It weighs 38kg developing 38hp at 4200 rpm. Price about 2,240 Euro. The shape maybe a bit tall for our Moni's, but have a look if you havn't already. I have a similar flying background I think to you - 1000hr hang gliding, 1000 ultralight, and 1000hr modeling and UAV pilot for AAI. I reckon that 30hp + would be good if possible on this ship, although if you have super long sealed runways everywhere you go maybe you don't need a good climb angle like the rest of us. Once airborne I guess 12hp would keep us up there. One idea we are trying with the uav's, is to use a really good brushless motor that we need anyway for a generator, to use as short term boost or take off power. Maybe the idea could be scaled up if you are going to use a small engine.
ReplyDeletecheers Warren
Hi Dan, I've been looking for a Moni (two out of the six in the UK have just changed hands, and I missed out...!), and came across your blog. Very interesting -- looks like you're doing what a lot of people would like to do. On engines, you mention that Briggs & Stratton, as used on the Luciole. This would be very high up my list. M Colomban has designed a neat engine mount to damp vibrations, and a good exhaust system. I have it on first-hand authority that the engine is very, very quiet. Also helping both noise and efficiency is the prop that has been designed specially to go with the engine. There was a good article about the Luciole and its engine in the LAA magazine ealier this year -- let me know if you want a copy; happy to send one.
ReplyDeleteThe NST looks to be a souped up version of the B&S 627cc V-twin.. You can always squeeze more power out of them with custom cam, ported intakes, superchargers, etc., but at the expense of engine longevity & reliability.. That's particularly important with a fast flying Moni, as opposed to ultralights. With powered paragliders, flexwing trikes and 3-axis ultralights, you have more options for emergency landing areas in the event of engine failure.. not so with a Moni.
ReplyDeleteColomban has the right idea with a direct drive B&S V-twin without sporty performance enhancements, but I've heard he isn't yet satisfied with the vibration levels, though his motor mount does appear to be better than most.
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