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#31
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Why are low-revving, high torque engines preferred?
wrote in message ... On Fri, 28 Nov 2008 08:30:06 -0500, Dana M. Hague wrote: On Thu, 27 Nov 2008 09:37:54 -0500, "Peter Dohm" wrote: IIRC, snowmobiles are/were derived from chain saws--which required an engine light enough for a man to carry and use as a hand held tool. That made them an obvious choice for another application were light weight was the most critical factor--even at the cost of reliability and maintenance intervals... I've never heard that, seems unlikely... they are really very different, except for both (along with outboard motors) being 2-strokes. All for the same reason of light weight. -Dana Some of the first ultralights DID run chainsaw motors - just like the go-cart crowd. Some of the small aircooled outboard engines were also used - and some early snowmobiles also used a converted aircooled outboard. (horizontal twin Johnson Snow Cruiser and OMC) I seem to recall that my first powered ultralight was an Easy Riser powered by a 10 HP West Bend Chain Saw engine with a belt reduction made and marketed by our own Chuck Slusarcyk. :-) Highflyer Highflight Aviation Services Pinckneyville Airport ( PJY ) |
#32
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Why are low-revving, high torque engines preferred?
I think if you look at the engine cutaways or blueprints, you will see
in the drive train a torsion shaft that functioned as a torsion bar to keep the gears engaged in the same direction and soak up some of the inevitable torque variations around the revolution of the crank. Engines that didn't have that were a bit fussier to run and had some blanked out RPM ranges where you got extreme gearbox wear. That was a problem with the ill fated Cessna 175 with its GO-300 engine. It performed a LOT better than the 172 that it shared an airframe with, but got a reputation for eating gearboxes. The gearbox also got a reputation for being as expensive as the engine to overhaul. The main problem was running them in the same RPM range as the O-300 they were used to from their 170/172. The GO-300 was good from 2800 to 3200 RPM. Below 2800 it was best to go all the way to idle. Then the gearbox only transitioned once from driving to driven. People tried to run them at 2400 RPM, which just "felt" better to an old 172 pilot. At that RPM the gearbox would beat back and forth from driving to driven every time a cylinder fired. A few hundred hours of that would destroy a set of gears! :-) Highflyer Highflight Aviation Services Pinckneyville Airport ( PJY ) "Morgans" wrote in message ... "Charlie" wrote You don't often see that because any torsional resonance problems get amplified at an even-multiple ratio, like the cam's 2:1 ratio. Note the final drive ratio of almost any reduction gear train & it will be some odd number like 2.17:1, 2.85:1, etc. Perhaps an even larger factor is to get different teeth meshing together in a cycle, each time around. it is better to not have the same mesh, time after time, although what you say about harmonics is also a factor. In the really big "warbird"engines, the gearboxes had to be built so hell-for-stout to deal with the HP levels, the harmonics were not as much of an issue because the resonance was hard to achieve with the components being so stiff. -- Jim in NC |
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