I have been experimenting with ultralights and engines for years now. At this point I have convinced myself that the 1/2 VW gives the most "bang for the buck" for an ultralight aircraft. The 4-stroke engine avoids many of the frustrations with 2-stroke engines, hard starting, mixing oil in gas, shorter Time Before Overhaul, overheating problems, etc. Most of the weight savings using a 2-stroke engine are used by the weight of the exhaust system and reduction drive needed to make the engine work. The 4-stroke engine can avoid these issues.
If you watch motocross motorcycle racing you see that the rules allow the 4-stroke engines to be twice the displacement of the 2-stroke engines. This is logical since the 2-stroke has twice as many power cycles as the 4-stroke so in order to produce equal power the 4-stroke power cycle must be twice as large, a bigger bang but fewer of them.
The beauty of the 1/2 VW is that in addition to the 4-stroke benefits, it is relatively easy and affordable to increase the displacement a lot. My 1/2 VW is something like 1138cc increased from the original 800cc. Compare this to the 2-stroke Rotax 447cc. While the Rotax makes 40hp at higher rpm, my 1138cc will make 40hp at less than 3600rpm. And since I do not need a muffler or a gearbox, my engine is about the same weight as the Rotax. Hand propping is possible with the 1/2 VW so no starter or alternator is needed. However, turning over 500cc by hand is similar to trying to kick start a Harley Davidson 1000cc Sportster. Doable but takes a little muscle.
I have experimented with a supercharger, a turbocharger, an industrial V-Twin, and studied the opposed 2 and 4 cylinder military surplus 2A042 and 4A084 engines. None of them can match the combination of displacement and weight I can get with the 1/2 VW. The classic VW engine is 1600cc with a 69mm stroke and an 86mm bore. I buy a new crank 82mm stroker, and big bore piston set 94mm and go from 800cc to 1138cc for less than $600. That is very unlikely with any of the other ultralight type engines. I must have the head bored out for 94mm and the engine case bored out for 94mm but these are simple machine operations. I must also have parts of the engine case machined to provide clearance for my stroker crank. This is also a straight forward machine operation. In fact, my 1/2 VW project simply started with a reconditioned engine case that was already machined for 94mm x 82mm and my source provides this case for less than $600 including core charge and shipping.
So what other affordable engine can give 40hp with under 90 lbs? Expected TBO is over 1000 hrs if the application includes adequate cooling. With a tractor prop, some simple sheet aluminum ducting to force the air around the back of the cylinder does a good job. And of course the prop is an excellent fan if you manage the air flow right. I am having so much fun building the 1/2 VW, I am ready to build another one. I have designed a simple low cost engine mount for my tractor engine ultralight that will be tested first on my engine stand real soon now. Note that there are two engine case bolts, one on each end at the bottom of the engine case. Using 4 x 3/8 in aluminum straps I mount two 1 in square tubes under the engine. With 4 bolts through the square tubes and using auto sway bar plastic discs I have a 4 bolt engine mount under the engine similar to the Rotax 447 and others.
I have made my own 1/2 heads by cutting a dual port head in half. Then I use a no weld modification to add some aluminum at the intake port so I can install a second stud and the intake port becomes the same configuration as the exhaust port. I then use exhaust flanges and tubing to make a two into one intake manifold with a tee at the center and mount a 32mm Mikuni carb. I plan to be offering my 1/2 heads for sale at $450 per pair which is a great deal compared to the available welded heads at $600.
Norm Heistand my .02
