#### Aviator168

##### Well-Known Member

Is this going to work? And what is the efficiency lost?

- Thread starter Aviator168
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Is this going to work? And what is the efficiency lost?

The basic idea is sound. The implementation isn't. Long story short - doesn't work in aircraft. That is why we use variable pitch props.

Tires and tracks have near 100% linkage with the ground. Propellers, maybe 85% with the air.

Van Doorne belt and LuK chains require significant pressure in the piston chambers of both sheaves. Pump,seals, etc. Also the running surfaces on the sheaves are specifically engineered to each type. so, using a bunch of the tranny pieces would be needed, including ratio change software and controls, but with your own case, Ugh! Heavy and complicated...

Now if you selected a specific ratio, you might get by with spring loaded sheaves of specific sizes and much more compact set of cases. Still heavy and complicated.

The need for rpm change is handled way better with controllable props.

Now if you selected a specific ratio, you might get by with spring loaded sheaves of specific sizes and much more compact set of cases. Still heavy and complicated.

The need for rpm change is handled way better with controllable props.

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But then there's a limit to how fast it can spin. Not much faster than that 2600 RPM and the tips start going supersonic, where drag goes way up and thrust starts to fall off. Useless, that extra RPM.

And that's why we have constant-speed props. That prop stays in the ideal RPM range and its pitch changes to keep the angle of attack of its blades in ideal places for whatever speed the airplane is moving. The engine is able to reach redline, where its max power output is.

The airplane's drivetrain requirements are entirely different from that of a car. Propellers aren't wheels. The CVT has been tried in ariplanes, anyway. The first iterations of the Bede BD-5 used it, and it was abandoned.

What you're saying is equivalent to saying that somewhat lower rpm engines won't work well in aircraft. I don't believe it. Lower rpm engines have lower rpm's but more torque, just like an engine with a CVT geared down for climbing.

I can believe that getting a CVT to work with a propeller might be very difficult due to issues like torsional vibration, but that's another matter.

I believe he is correlating the engine rpms to the actual prop rpms, as in a direct drive. Relatively speaking, the engine rpms are irrelevant, he is comparing the prop efficiency at various rpms.Dan:

What you're saying is equivalent to saying that somewhat lower rpm engines won't work well in aircraft. I don't believe it.

You have propeller mechanics working against you. A fixed pitch props, fans, even centrifugal pumps all behave in the following way:P.S. I'm only talking about relatively small differences in gearing. Like maybe 2:1 for cruise and 2.5:1 for climb. Assuming an automotive engine with the CVT being used as a redrive.

The torque a propeller can react goes with the square of its rotation speed. Since power is torque times rotation speed, power then is with the cube of rotation speed. So let's doa quick thought experiment:

Size a prop to convert 100% power while running at 100% rpm , and it is running at 100% torque. Direct drive airplane engines are close to this, so the approximation is a decent example. Pull the throttle back to 95% rpm, and you are at 86% power. Come back to 90% rpm, and you are down to 73%, which is usually about where most of us will cruise. Get to 80% rpm, and you are at 50% power - this is approach power or the power setting while you are parked in a hold.

So, let's put your prop on the front, and run it at 100% rpm and torque for 100% power for takeoff and climb.

Then you get to altitude and want cruise power, so let's just say that we use a 10% higher gear ratio for cruise. If the engine is still making 100% rpm, the prop has to turn 110% rpm, consuming 133% power, but we only have 100%.

So let's let it slow down to 100% power, 100% prop rpm, but now the engine is making 91% rpm, so we only have 91% power to the system, so we have to slow the prop some. Let's say 97% prop rpm, which means the prop is using 91% power, but now the engine is now at 88% rpm. Even if it had a flat torque curve with 100% torque available there, that means only 88% power is available.

Well, pull the rpm back some more, say 95% prop rpm, which is 86% power, but now the engine is at 86% rpm. If the engine can make 100% torque here, you are just about there. But to do this, the engine throttle is still wide open, but you are still in mixture enrichment to keep the exhaust valves in the heads.

You can repeat this exercise at other gear ratios, but the same problem occurs. Then you can do the rest of the real world issues - as you climb the air becomes thinner and the engine can not make as much torque as it made at sea level, so you have to back off even more.

And to do all this, you need a gearbox of some sort. A straight gearbox is simple, but weighs something, has to have a shift mechanism, lube, cases, seals, etc. If instead the gearbox becomes a salvage job, the sheaves, pump, oil pressure control, speed sensing, a method to adjust the speed ratio, cooling... UGH!

A controllable prop is WAY lighter and simple and works great and you do not have to figure out everything in that CVT that Ford or ZF or VW or Porsche has inside that boneyard gearbox and ECU...

Billski

I was simply trying to get through the basics of why even modest gear ratio changes do not work well, and why it is just not done in airplanes nor in boats. Get into the reductions to the inflow velocity with decreased speed when the expressed power is reduced and yeah, things get even worse.

Simply put, it is a heavy, expensive, complicated way to get to a poor way to drive a prop. When you have a controllable propeller hung, you have a light, relatively simple CVT. Find a suitable hydraulic or electrically adjusted prop and be happy.

Billski

Anyway, in climb the advantage of using a CVT is the same as having a somewhat more powerful engine spinning the same prop.

Here's another way to say the above. Let's optimize the prop for cruise at 100 knots at 8,000 feet. Now we want to climb at sea level at 75 knots. The engine can't reach full power rpm. Now we gear things down a bit. Now the engine CAN reach full rpm and spins the prop faster than it would have. My experience is that if you speed up the prop, you get more thrust.

BTW, I tried to look at this with Javaprop, but so far I haven't figured out how to get the performance at slower speed and greater air density using the prop optimised for 8,000 feet. And now my PC is ill.

True and greatly different from our case...It's sometimes done in boats but for a different reason. Boats with surface drive propellors benefit from a change in reduction ratio when ventilated than when fully submerged.

lr27 is proposing that if we progress through the same cases starting from the opposite direction, the results will be better. Think about that... Can that work?

Anyway, in climb the advantage of using a CVT is the same as having a somewhat more powerful engine spinning the same prop.

Ran through some simple numbers anyway. Set up 75% power at 91% rpm (power goes with n^3, torque with n^2) and the resulting 83% torque at a base gearing. Now I drop the gearing to 1.1:1, let the engine run to 100% rpm, prop rpm goes to 91%, torque at the prop can be no more than 83% (rpm^2) and we are at 75% power... You can get to 100% power by letting the engine over-rev. With the 1.10 additional reduction, a 10% over rev and if the engine makes 91% torque at that speed, you have 100% power.

Cruise gearing only works in cars and trucks because we routinely run down the highway (level road) at modest power settings. We do not do that in airplanes, where we cruise level at 75% power and over 90% of WOT speed.

You can go get into all this using the more sophisticated equations, but they end up further reinforcing the same basics. If you really want to set rpm and torque independantly, go constant speed prop and be happy.

Billski