Has anyone used a Polini Thor 250 on a MiniMax? With it's light weight and 35+ hp it looks like it might be a good option for going part 103 legal. I wonder how difficult it would be to mount and what mods would need to be made for mounting?
I saw a Polini Thor 250 at Sebring and it was mounted as a firewall style mount on a powered parachute. It is a interesting engine but it looks like it would be difficult to do a bed mount. The liquid cooling would be a benefit but it adds complexity to the mounting and service of the engine. I talked with James Wiebe (Belite) and he is enthusiastic about it.
I am building a himax. I research lots of engines (simonini, minari, hirth, and rotax) and I am not finding any engine that match better with the himax than the polini thor 250. Light, powerfull enoght, dual ignition, electric start, liquid cooled and cheap: the best option nowadays in my opinion. I am not finding any rotax 277, 477 or 503 with less than 25 years of manufacturing in my country (Brazil): and God knows how it was used. I will not fly an experimental airplane with an old 2 stroke engine. I will make an steel tube suport, or simply attach it to a reinforced firewall.
The only drawback I am seeing with the Polini is that it runs at higher RPM. I'm not even sure that is a drawback but I heard it runs at 7,800 RPM to get the 35hp. I also heard that the redrive might be on the weak side for larger props.
No, many don't rev this high. The Rotax 447 & 503 redline at 6800. My 625cc MZ202 only revs to 6000, but still makes 60hp.
You don't get something for nothing. One reason the Polini is so light is because it makes its power from rpm, not displacement. Horsepower is a function of torque & rpm. A large displacement engine typically produces a lot more torque than a smaller engine, so to produce the same power it doesn't need to rev as fast. Conversely, if you want a small displacement (cc or cu. in) to produce the same power as a larger displacement engine, then it must rev faster. And, all other things being equal (which they often aren't), a faster revving engine will wear more quickly than a slow revving one.
Hence the Polini at 250cc makes 35hp at 8000rpm, while the Rotax 447cc makes 40 hp at 6800 rpm.
I also heard that it idles at higher RPM and that is why it has a clutch. With the clutch the prop will windmill if you have an engine out and that creates a lot of unwanted drag. I don't know this to all be true, just what I have heard so do your own research if considering it.
With the clutch the prop will windmill if you have an engine out and that creates a lot of unwanted drag. I don't know this to all be true, just what I have heard so do your own research if considering it.
If the windmilling prop is running free and not turning an engine over it might have the same or less drag than a stationary prop.
Although it is a bit counter intuitive, it is the case that a windmilling prop is draggier than a stationary one. This is because the oncoming air sees the area of the propeller disc as a solid, but with a static prop only the area of the actual blades presents an obstacle.
Also, consider that to turn a propeller requires energy. That energy can only come from the forward motion of the aircraft, & thus detracts from the inertia of the craft. That is, it slows down to provide energy to turn the prop.
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