The net result I found to be very positive.
I was primarily interested to find out if the CVT (Scot you’re right it functions as a variable reduction unit) worked well with a propeller connected to it and could be substituted for the reduction units currently used. To test this idea I took the engine and CVT out of a snowmobile (a 1973 Raider 44TT which I have owned from new), I used a 3-blade 48-inch ground-adjustable Warp Drive propeller, and I had a flange machined to connect the propeller to the output shaft of the CVT. I selected this test equipment because that is what I have.
What I have found is that the CVT will adjust the prop RPM to maintain a consent maximum engine RPM. This means that in a low air speed situation, when a fixed drive will tend to lower engine RPM (usually below maximum torque and horsepower), the engine can maintain its high torque and horsepower output by the CVT slowing the propeller. But only enough to maintain maximum torque. This means that maximum thrust will be maintained. As airspeed increases the propeller RPM can increase without over speeding the engine.
This has benefits during take off and cruse. Because the CVT will adjust, a coarser pitch propeller can be used. During take off the engine with a CVT can still achieve maximum torque (the value that makes the prop give thrust) because its RPM is not limited by the propeller as with a fixed reduction unit. During cruse, the CVT will increase propeller RPM to keep up with the increased airspeed.
The propeller selection is still of importance. Normally the propeller selection is a trade off between low airspeed (take off and climb) performance and high airspeed (cruse) performance. With a fixed reduction unit, the propeller must be set to a low enough pitch to allow the engine to achieve a high enough RPM to achieve sufficient torque, and thus thrust, during take off. This lowering of the propeller pitch for take off is balanced against higher pitched propeller needed for faster cruse speeds without running the engine too fast. By using a CVT a coarser pitched propeller can be selected without necessarily losing low airspeed performance. Of coarse the propeller still has it’s own physical limits with respect to stall during low air speed with high power conditions and tip mach number during high RPM conditions.
So here is the experiment that I actually performed from which I have drawn the above conclusions.
Equipment
Engine--Canadian Curtis-Wright (CCW) 440 cc two stroke, forced air cooling, carbureted with high and low speed needle valves, electric start, CDI ignition, 12 volt charger
Fuel--87 octane mogas mixed at 50:1 Amsoil 2-cycle oil (in 1973 the manufacturer called for 20:1 mix of "high grade 2 cycle oil" as I recall)
CVT--Salisbury 820 series (as best as I can remember, I have to find the book. Does anyone know anything about these? Who makes them? How they are rated?)
Drive belt--1.25 inches wide by 46 inches long
CHT--one sensor on each cylinder
Propeller--3-blade 48 inch diameter ground adjustable Warp Drive rated at 4200 rpm ( to the best of my memory, I’ll have to check the paper work but it is between 4000 and 4500 rpm)
TEST
Sunny, 85 degrees F, winds light and variable, density altitude 1200 ft.
Engine with no propeller--The CVT engaged at 3500 engine RPM complete travel achieved at 6500 engine RPM (remember this is a no load situation). I had no instrument on the CVT shaft to know what RPM it was turning. This is the only time that the CVT shaft bearing felt warm. During all loaded tests the bearings remained at air temperater.
Engine with propeller set at 11 degrees pitch (the pitch I used with the 72 hp Mac to achieve 3600 RPM static)
Engine 1500 rpm (idle) the propeller turned 100 rpm due to friction of belt on engine shaft CHT 220 degrees F (propeller rpm was determined with a hand held light flicker prop rpm meter)
Engine 3500 rpm the propeller turned 1280 rpm CHT 260 degrees F
Max throttle--Engine 6500 rpm the propeller turned 2000 rpm (noted slight black residue on engine pulley after test. I suspect this is the result of relatively small area of belt contact during this high reduction ratio test. The residue was not present after any of the other tests) CHT 300 degrees F
Engine with propeller set at 6 degrees of pitch
Max throttle--Engine 6500 rpm the propeller turned 2700 rpm CHT 300 degrees F
Engine with propeller set at 3 degrees of pitch
Max throttle--Engine 6500 rpm the propeller turned 3200 rpm CHT 300 degrees F
Each test was continued for 10 or more minutes to allow CHTs to stabilize.
I do not have any thrust measurements yet. I have to set up a scale (probably a bathroom style because that is what I have) for it to push onto. I hope to do that next week. As with everything else, safety is important and I’ll make sure to have secondary restraint for the engine and prop in case the scale should slip. I have a 5-year-old son to whom I wish to say happy 70th birthday.
Any thoughts about other measurements I might take?
Boy this is fun!
Oh and by the way, in case anyone is concerned for my health, I am taking gyroplane instruction with Chris Bergess in Frederick MD. I have only 2 hours so far, but will be getting more. Distance and the large block of time required for each hour of instruction has been a cramp in my training schedule. I find him a very helpful instructor in case anyone is looking for instruction in the mid-Atlantic area.