With 4-stroke engine, we basically have to answer two major questions:
- Air-cooled vs water-cooled?
- Aero-engine or Auto-conversion (or Industrial engine)?
Air-cooled vs water-cooled
It all boils down to (excuse the pun – I couldn’t resist it) what is available. The Spitfires (etc) during the war were all water-cooled – but since then, it seems that the only water-cooled engines available today are auto-conversions. Aero engines almost exclusively rely on air cooling.
And the reasons for this are simply that air-cooled engines are lighter (no radiator) and simpler (no plumbing). Water-cooled engines can have closer engineering tolerances, however, and are immune to such things as shock cooling.
Keeping a water-cooled engine cool
Simply having a radiator is insufficient. Air has to be able to pass through the radiator fins. And you’d be surprised at just how many builders seem to completely ignore this simple fact. I’ve seen radiators shoved up hard against the firewall, with no POSSIBLE way air could pass out the back. Placing the radiator out in the free stream of air is also not the solution. Apart from extremely high drag, the fact is that air does not easily pass through radiator fins, and actually finds it easier to veer off to the sides and go round it.
So we need to place the radiator in such a place where it is out of the free stream of air (to reduce drag as much as possible), to slow the air down as much as possible (so that it can absorb the maximum amount of heat before escaping out the back) and finally, ensure that there is a good low-pressure at the rear (to suck the air through). Here’s a schematic of the water-cooled Razorback F1
Keeping an air-cooled engine cool
Essentially, air passes through a system not because it is forced in (close the exit and you can force in as much air as you want – it won’t go in), but because it is sucked through. Even a small opening – so long as there is a good suction at the exit – will allow in as much air as you need. So what we have to identify is where the areas of high pressure are (the air wants to get IN) and where the LOW pressure areas are (ie a relative vacuum).
The pressure distribution on the cowl of an aircraft is a bit unexpected to be honest. It looks like this…
Notice the arrow? Arrows radiating OUTWARDS (eg: top of the cowl behind the spinner, top of the windshield, bottom rear of the cowl) indicate areas of relative LOW pressure – ie a suction. The only two areas where air is trying to get IN are on the bottom of the cowl, and at the bottom of the windshield.
And so here’s the surprising thing. By far the greatest suction is on top of the cowl just behind the spinner. And it is HERE that we need to vent the warm air.
So if the Razorback were to have an air-cooled engine (eg the VW), here’s the best way to keep it cool:
Bottom line – if you opt for an auto-conversion, you are into radiators and if you choose an aero-engine (or industrial) you are wedded to air cooling. So it’s really not that hard to decide. So the REAL question is really….
Aero vs auto vs industrial
Purpose-built AERO engines:
These come in two flavours. Pre-war designs and modern engines. I wouldn’t drive a car with an engine designed before the war, and neither will I fly a plane equipped with one of these engines. Those in favour of these dinosaurs go on about how they have stood the test of time, how they are tried and true etc. But I’m not convinced. I wouldn’t put one in my car. Bottom line. What engines am I talking about here? Your Lycomings and Continentals. Besides, they cost a FORTUNE. (Did you see the upper case letters in “fortune”?). Like $28k So, let’s not hear anything more about them.Fortunately, there are some excellent MODERN aero engines on the market today (also big bucks, but not horrendously so). HKS: http://www.hks-power.co.jp/hks_aviation/
In all my searching of the Internet, I have NEVER come across a single bad word about these engines. Except, possibly their price (about $12k). They come in normally aspirated (60hp, 121lbs complete with gearbox and exhaust) and the new turbo version, which costs considerably more, and doesn’t produce that much more power. Rotax: http://www.rotax-aircraft-engines.com/a_engine_912.htm
Mmmm despite their much-vaunted reliability, I just don’t LIKE them. They sound terrible (like little sewing machines whirring away), they are far too fiddly for my liking, and they cost almost as much as the Lycomings/Continentals of this world. I think there are better engines than the 912 (80hp) and the 914 (100hp turbo). About four times the cost of some other aero engines, like the VW conversions).Thunder Aero Engines:
These little 4-stroke engines (850cc) offer outstanding value. A true aero engine at a fraction of the price you’d expect. And they are fuel injected and water-cooled, to boot. The engine comes fully ready (ie exhaust, radiator, gearbox, starter etc) to bolt onto your airframe, add some gas, strap on a propeller and go flying.
After an exhaustive search of everything from tiny industrial engines to large, powerful (120hp) auto-conversions, I have decided that this is the engine of choice for the little Razorback. It is a perfect match for the airframe.
Water-cooled, 121lbs, 80hp. In-line twin only 9 inches wide. In fact, you can thank the distinctive bump at the top of the Razorback cowl on this engine, which is much higher than it is wide. I think this is the engine smaller aircraft have been waiting for. Certainly I have…
This Australian designed and built engine comes in 4 cylinder (85hp, 132lbs) and 6-cylinder (120hp, 178lbs) and is one of the engines of choice for the Sonex. I’d take the 4-pot Jabiru over the Rotax any day of the week. But they are also not cheap. (About twice the price of some other nice engines).
Smaller AUTO conversions (under
If you’re looking at smaller engines, there is only (in our opinion) one engine to consider: the Geo/Suzuki G10
- Raven Redrives are another source for this fine little engine. They’re based in the US, and are very active on the small engine forum at Yahoo Groups. Again, not a cheap option.
You can find them here: http://www.raven-rotor.com/
Larger auto conversions (80 to
I know I’m going to be shot down in flames for this, but although there are literally scores of companies offering auto conversions of every description, from every manufacturer imaginable, for me it boils down to a single engine type – the tried and tested VW. And there are only three places to get your VW conversion – again, my personal opinion. The first is Aerovee (a subsidiary of Sonex Aircraft), the second is Revmaster, and the third is Great Plains Aircraft.
This is a lovely engine, and in our opinion, the best on the market today for the home builder – bar none. 80hp, 161lbs. You buy it in parts, and assemble it yourself, working from an assembly manual, and a video. Even I can do it.
Aerovee also offer all sorts of add-on goodies, like throttle quadrants, optional Nikasil Cylinder upgrade (saves 10lbs) and there is an active online support group at Yahoo Groups. One of these will set you back about $6,500 (USD)
Theyoffer three variants, so you get to choose between front drive, reduction drive and flywheel drive versions. Prices, power and reputation about the same as Aerovee. Only hassle is you can’t just buy an engine, You have to work your way through their options and “build-up” your engine bits from all their options. I found it quite confusing. Theyoffer three variants, so you get to choose between front drive, reduction drive and flywheel drive versions. Cost? About the same as the Aerovee.
Not QUITE as pretty as the AeroVee, but this is a very sweet engine. There is almost nothing of the original VW engine left in this built-from-the-ground-up aero engine. Special crank, extra bearing at the prop hub, custom pistons, conrods, compression chamber etc etc. AND you can use any prop you like, including composite or even metal props. The more I think about it, the more I like this engine. 85hp take-off power, 80hp continuous. 170lbs. Cost: about $7,000 (USD)
And a very fond word reserved for the BMW motorcycle conversions…
Both the R1150 and R1200 series horisontally opposed engines are frequently used in aircraft. Mainly because you just can’t break them. They have dollops of torque, gobs of HP and they will run forever.
The K-series engines are also great little powerplants. The K-75 (750cc) weighs in at 185lbs fully oiled, and fitted with a Rotax gearbox. It produces 73hp, and is just about bullet-proof. Great engines to turbo, since they will take 8lbs of boost without blinking an eyelid, and suddenly you have a reliable turbo aero engine, capable of cruising at its full 110hp all the way up to 10k feet. One of the problems with these motors is that they can’t be bought off the shelf. These are all one-offs, and you need to know what you’re doing. This is not an option if you want an easy solution.
Now this is where it gets REALLY interesting. If you don’t need a lot of power (ie under 40hp) then one of these might just do the trick. The idea is relatively new, but there are already two companies which specialise in aero-converting these little workhorses. (Valley Engineering (US-based) and Soloflight (in the UK).
Valley Engineering use the Generac 40hp engine in their conversion, which tips the scales at 112lbs (the included 8lb redrive brings the weight to 120lbs. All for $4995 (USD) which includes a custom made prop, AND the PSRU. Nice. Very nice.
And the latest development is that they now have a newly ground cam, together with high compression pistons enabling the engine to produce 50hp. This costs an extra $750 – but I think this is a bargain.
One of the nice things about the B&S or the Generac industrial engines is that these motors are designed to run flat out all day, taking terrible punishment. They are simple, rugged and while not the most powerful engines for their size, make up for it with almost unrivalled reliability. And low cost, of course. You can buy a brand new engine for about $1,500 (USD). An engine with redrive, and other mods to make it suitable for aircraft use will cost you more (about $5,000) from either Valley or SoloFlight. But then it is a bolt-on-and-fly affair, and both of them produce lovely looking conversions.
And it only drinks about 6 litres an hour. That is under $7.50 for an hour’s flying. Very impressive.
A new purpose-built aero engine from
If you’re looking for something in the region of 40hp, then you have just GOT to check out the new Verner JCV-360. It is a purpose-built aero engine, 4-stroke, water cooled boxer. Weight = 26kg. Wow! Talk about a great pedigree. At 35hp, however, it just doesn’t quite have the oomph I’m personally looking for, but if you can live with 35hp, then this one might be for you. Cost? Apparently (I have this 3rd hand, however… 2,600 Euro)
Here are some details from their web site: (http://www.vernermotor.com/index.asp?sec=41)