- Oct 18, 2003
- Saline Michigan
Well, if you wanted single speed out of the CVT, you still need the engineered surfaces on the sheaves, a method to maintain the grip forces, cases, seals, etc. With a fixed speed ratio, I suspect one sheave can be made solid, but the other still has to be loaded - factory pump was around 1000 psi, and the hydraulic diameter of the sheave was large, so the forces are big indeed. You may be able to come up with enough force with a BIG Bellville washer and a big nut to close the thing. If you keep the hydraulic actuation, you will also need to keep the 1000 psi oil pump. It can be made to work with enough effort and time and money. I doubt you will save any weight and it will be way more complicated than one of the existing PSRU with good service history.My original motivation for this topic is to explore the idea if CVT can replace a geared PSRU, simple setup and one speed only. OK, getting back to what you guys discussing. IMO, a CVT can keep the power to a fixed pitch propeller more constant. If you selected certain engine output, you can use engine power more effectively during different phases of a flight. Higher prop rpm in cruise, and lower rpm in climb. This is for a fixed pitch prop only of cause.
Now start changing gear ratios in search of getting more power over a wider airplane speed range? Nope, prop mechanics are working against you. Torque you can actually apply through a prop goes with rpm squared, power with rpm cubed. You can not slow down a fixed pitch prop without greatly reducing the torque and power that gets expressed. Likewise, you can not speed up a fixed pitch prop without greatly increasing the torque and power applied to it. Change prop blade angles and sure you can shift to another place on the engine's speed vs torque map with out changing gear ratios. All this stuff was considered back in the early 1930's and again with turboprops (it is jet with a power turbine and a gearbox, so adding another gear set, a clutch and a one-way-clutch is easy), and they all went with constant speed props...
The advantage of CVT's in cars was they had more range of gear ratio than four speed automatics and let you run right on the fuel island, both improving fuel economy. Downside was the fuel used to maintain the 1000 psi to grip the belt/chain was significant, but still a net gain over four speeds. From the advent of six speed automatics, wider ratio range became available with lower pumping loss, and the fuel island has become wider with direct fuel injection too, the CVT's era passed. With the now current 8-9-10 speeds, even manual trans vehicles are not as good as automatics. Sigh. I like my own gear change.
Anyway, the big measure for which engine and PSRU to run in my mind is how big is the fleet and how well is the fleet doing for reliability? Pick from among the best, and cry over the cost only once.