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Discussion in '2-Stroke Aircaft Engines' started by Jeff Higgins, Dec 9, 2019.
Slippage and breakage
Dude, I’m beyond qualified to machine and make my own redrive and prop hub. I don’t overlook steps and I know what’s needed for it to safely function and last. Prime example: people often try to use pillow block bearings and a collar with set screw but neglect to realize the amount of thrust force on the bearings, shaft and collar. I’m no dummy. I’ll be fine.
No offence, but you've not mentioned the issue that kills more beautifully and badly machined redrives than everything else put together...
Google 'torsional vibration'. It's not a simple task to design around. But it is possible. Machining a good design is the easy part.
I was an ASE certified master tech for 17 years so I am aware of it. It’s the reason that front crank pulleys have a large amount of rubber cast between the inner and outer pulley, to absorb “harmonics”.
Car engines, that is.
I measured 90 pounds static thrust with a 20hp 250cc Fuji Polaris engine. 30" diameter and wide chord prop, direct drive.
I think it weighs 30 or 40 pounds.
I guess I do not understand it.....all those PPG engines do they have more than just a couple of pulleys and a belt drive?
That’s typically all they have because they use such a short, robust stub of a shaft and everything is well balanced.
The Arctic Cat JAG 2000 275 Twin Free Air is 20hp@6000rpm with a Muffler, JAG 3000 340 is 30hp@6000rpm with a Muffler, and 440 was 40hp with a Muffler. A Twin is going to be Heavier than a Single. 20hp x 7cc = 140cc with a Tuned Pipe Designed for 6500rpm. Hirth makes a Direct Drive Hub for there Engines but I have never seen anyone using it, or a photo of it. A Kawasaki 292 Single was rated 20hp@5500rpm with a Muffler, and 22hp@6000rpm with a Muffler. Every Brand Name (25+) Snowmobile Engines made a 292 Single. Kawasaki, JLO, Sachs, others, had threaded holes on PTO Case to run a Belt Drive. Since your only using it for Takeoff of your Motor Glider. You could turn it 7000rpm for 2 Minutes for takeoff, then throttle back to gain your desired Altitude.
At 77F, using Calc: http://godolloairport.hu/calc/strc_eng/index.htm
A 32" x 8 (2) Blade can be turned 7000rpm and make 137.76 lbs Static Thrust! Needs 19.547 hp. 2 min Takeoff only!
A 32" x 8 (2) Blade can be turned 6000rpm and make 101.21 lbs Static Thrust! Needs 12.309 hp.
A 32" x 8 (2) Blade can be turned 5500rpm and make 85.04 lbs Static Thrust! Needs 9.481 hp.
A 32" x 13 (2) Blade can be turned 6000rpm and make 101.21 lbs Static Thrust! Needs 20.003 hp.
Do centrifugal clutches adsorb harmonics? (slippage?)
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