I have been looking into re-drives for an ultralight trike. I will likely end up building my own. Is their a good reason why all of the redrive's use belts ? With a motorcycle back ground, all of my dirt bikes that I raced in open class desert were chain drive (duh) and worked just fine. Why not use a chain instead of a belt ?
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Hmm, we have talked a lot about PSRU. I suppose and open chain could be used. Maintenance and oil on stuff would probably be the biggies. Weight might also be an issue. WEIGHT IS THE ENEMY in all things that fly. Cog belt drives are reasonably light and sturdy. Why not design one to suitable sturdiness and compare weight, cost, and maintenance?
Chains and spur gears can also create vibration issues. Silent chain and helical gears can help but have their own issues. I would prefer the cog belt, personally. I didn't like the poly-v because they slipped on me once. It was not tight enough, my fault.
Martin W
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Keep in mind a chain driven motorcycle wheel only turns at 400-700 rpm .... by comparison a prop turns 2000-3000 rpm.
Chains do not like high rpm .... and when used require a more exotic design ... such as automotive timing chains etc.
Belts have some downsides with frictional power losses etc but advantages are vibration damping and they will tolerate some pulley misalignment .
GM had success with high power high rpm chain drives in early front drive Cadillacs and Toronados and some truck 4X4 transfer cases ... practically indestructible but heavy heavy heavy.
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rotax618
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When we were kids (a long time ago) we built a ply boat and powered it with a small 4 cyl car engine, we mounted the engine in the rear and made a Vee drive using two Motorcycle chains driving the prop shaft which went under the motor. Well it worked for about an hour, at full throtle the chain became so hot it disintegrated and shot through the bottom of the boat promptly sinking it. I did some research after the disaster and found that for the power and revs we needed about six 1/4” pitch roller chains bathed in oil as a minimum. Maybe better types of chains have been developed in the last 60 odd years that may make a chain drive viable - do your homework.
Actually, a quick check shows an older Honda 750 running @ 70 MPH with a 17:48 chain reduction and the large sprocket turning around 1800 RPMs. Cycle chains don't run in an oil bath either. Hear me correctly, I am NOT a fan of chain reduction drives for aircraft use. I threw the Kohler thing out there because I had previously come across it and the original poster was asking. Dennis
Martin W
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Maybe check your math .... the tire turns the same speed as the large sprocket ... a 32 inch tire moves about 8 feet every revolution ... 5280 feet in a mile tire turns 660 times .... at 70 mph it turns 46200 revolutions per hour .... divided by 60 minutes in an hour = 660 rpm
best wishes in your experiments
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I stand somewhat corrected. I took information from a motorcycle thread. I was based on THEIR quote for RPM and sprocket combinations (apparently THEY had something wrong). On the other hand, MY cycle has a rear tire diameter of only 25", and like most cycles, the rear tire is a smaller diameter than the front. Never seen a 32" diameter tire on a cycle
REAL point: while a chain drive has worked on cycles for what? a hundred years, I don't think it is the best (or lightest) choice for aviation. From first seeing the Kohler chain drive PRSU, I wondered about the reliability. (BTW that was an UNSUPPORTED engine shaft - also not a good choice. I've seen too many small engine crankshafts break off.
I am still in between the cogged drive and the multi V belt. I have seen much of both in industrial applications, and there are advantages to each. I still remember when the first guy showed up at Oshkosh with the Ford Escort engine and a cog belt redrive on a Pietenpol. I remember the guy who threw a used (2 cycle) belt reduction on a used Honda Civic engine and put it on his own design light plane (Little Red). A lot of this is what EXPERIMENTAL was all about. Dennis
REAL point: while a chain drive has worked on cycles for what? a hundred years, I don't think it is the best (or lightest) choice for aviation. From first seeing the Kohler chain drive PRSU, I wondered about the reliability. (BTW that was an UNSUPPORTED engine shaft - also not a good choice. I've seen too many small engine crankshafts break off.
I am still in between the cogged drive and the multi V belt. I have seen much of both in industrial applications, and there are advantages to each. I still remember when the first guy showed up at Oshkosh with the Ford Escort engine and a cog belt redrive on a Pietenpol. I remember the guy who threw a used (2 cycle) belt reduction on a used Honda Civic engine and put it on his own design light plane (Little Red). A lot of this is what EXPERIMENTAL was all about. Dennis
Kiwi303
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48 on the big (wheel) sprocket and 17 on the engine side is 2.82
660 x 2.82 = 1863 rpm.
Maybe Hawk switched Wheel and Engine?
Posts crossing, Hawk81A posted again while I was typing.
That's an interesting looking little reduction unit. I wonder how it compares weight wise to something like a Rotax box. I am not aware of any popular planetary redrives, but I have been pretty much out of all of this for years. Another consideration for PRSUs is prop rotation. Many redrives (gear) reverse the rotation. Dennis
Martin W
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Yes ... my Bell 47 helicopter uses a planetary gear-set between the 240 hp engine and the rotor mast .... reduction is about 9 to 1 .... practically problem free.
Planetary also handles high rpm .... turbine auxiliary power units have a built in planetary that reduces 30,000 rpm impeller speed down to 6000 rpm at the output shaft ... never hear of failures ... lots of modern kit helicopters use them
Planetary also handles very high torque loads ... such as in the Sikorsky S92 heavy lift helicopter ... planetary final drive to rotor mast ... low rpm and huge huge torque load.
I do not know of a planetary drive targeting home-built aircraft but it would likely be well accepted if the price was reasonable .... one design could handle 50 to 200hp very well ... simply have various mounting plates to bolt on to various engines .
I do believe that a piston engine should have its own crank mounted flywheel to smooth out engine impulses .... mounting a propeller on the end of the planetary unit may cause harmonic issues because the prop is also a flywheel ... I do not know .... all opinions welcome .... maybe there is a reason we dont use planetaries more often .
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rotax618
Well-Known Member
Another down side of planetary gears is that the output shaft stays concentric with the crankshaft making it difficult for forward vision in tractor configurations with upright engines, unless the engine is laid over at 90deg or inverted as Steve Whittman did.
What "disqualifies" planetaries is the LACK of any. Years back, (IIRC) there was a unit offered. I guess lack of demand caused it to go out of business (like so many other aviation ventures). Again, IIRC, some, or most GA engines with reduction used planetary units.
I mean if you have to reduce RPMs, it seems there are 3 "more simple" options: belt redrive (cog or Vee), chain redrive (what REALLY started this thread), and ADAPTED existing gear box redrive (Rotax, etc.)
I mean if you have to reduce RPMs, it seems there are 3 "more simple" options: belt redrive (cog or Vee), chain redrive (what REALLY started this thread), and ADAPTED existing gear box redrive (Rotax, etc.)
jedi
Well-Known Member
I believe you are referring to the NSI reduction on the Subaru engine. The lack of demand was because of faulty design and development which the producer refused to remedy. The reduction unit would overheat and fail. The Eggenfellner Subaru with his PSRU had/has a much better fire wall forward conversion.
IMHO The NSI planetary reduction was a good basic design that needed a little more development to make it work. The shiny, nice looking, CNCed housing needed some cooling fins and a few other details to make it a success. The sprag clutch was the source of many of the failures.
I believe the rights to the design are currently for sale.
This discussion should include consideration of the following:
These guys have by now gained considerable experience and insight into chain redrive in an ultralight aircraft application.
Seems to me that if a chain drive were suitable, its housing could be made out of a short length of extruded rectangular cross section aluminum tubing, say 2"T X 6"W X 12" H, with, say, automotive timing chain and gears from, say, a BMW 3.5L car engine, of which I have several. One such was removed with its sprockets from a 3,500 lb. car after 300K+ miles, still in good shape.
Also useful may be the BMW guide rail and chain tensioner bits, and all parts would fit inside the 2X6 extruded aluminum tube, with an oil bath at the bottom such that the chain constantly runs through and flays oil around dissipating heat to the aluminum walls, so add a sealed cap to each end of the tube.
These guys have by now gained considerable experience and insight into chain redrive in an ultralight aircraft application.
Seems to me that if a chain drive were suitable, its housing could be made out of a short length of extruded rectangular cross section aluminum tubing, say 2"T X 6"W X 12" H, with, say, automotive timing chain and gears from, say, a BMW 3.5L car engine, of which I have several. One such was removed with its sprockets from a 3,500 lb. car after 300K+ miles, still in good shape.
Also useful may be the BMW guide rail and chain tensioner bits, and all parts would fit inside the 2X6 extruded aluminum tube, with an oil bath at the bottom such that the chain constantly runs through and flays oil around dissipating heat to the aluminum walls, so add a sealed cap to each end of the tube.
