Hybrid or electric theory?

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Dan Thomas

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@Dan Thomas

An ICE engine requires the lower pitch to get to max power. Electric does not.

Tim
Horsepower remains a function of torque times RPM. Low RPM means low power. A fixed-pitch prop on an electric motor won't let that motor reach its rated RPM. If the pitch is low enough that it does reach RPM in the takeoff roll, it will overspeed as the airspeed increases.

An electric motor generates torque that turns the prop, exactly like an ICE, only smoother. It will generate max torque at low RPM, unlike the ICE, but without RPM it still won't generate rated HP. If it did, we could make direct-drive electric helicopters.
 

vhhjr

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I did some experiments some years ago using a variable speed drive, like those in a snowmobile or scooter, as a reduction drive. There is a small weight penalty over fixed ratio pulleys, the variable ratio units do solve the problem that fixed pitch props have as described in the posts above.

Have a look at the CVT PSRUs file first.

Vince Homer
 

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rv7charlie

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Unfortunately, a variable transmission won't help, when you're turning a prop. The power required to turn a prop varies with rpm, and is non-linear, to boot.
Consider the typical situation of a Lycoming that produces rated power at ~29.92" MAP and 2700 rpm. With a typical fixed pitch prop, the engine will turn ~2300 rpm during WOT takeoff/climb. The prop is absorbing (transmitting to the air) roughly 70-75% of total engine power. Now, if you add a gear box so that the engine can turn 2700 rpm while the prop is turning 2300, the prop is *still* only absorbing 70-75% of total power, because you will have to reduce manifold pressure (MAP) to keep the engine from overspeeding. The torque load on the engine is reduced, which means its power output is reduced even though it's turning 2700 rpm.

Remember, the prop itself is a transmission; it transmits power of the engine to the air. But unlike a set of gears driving tires on the road, its efficiency varies based on the relative speed of the air approaching the prop. The aero guys will probably crucify me for this, but maybe it will help to compare a prop in air to car tires on an iced up road. If you're stopped, and floor the gas pedal, the tires just spin. But once you're moving, the tires can couple to the ice better, because there's less difference between the speed the tread is trying to turn and the ice moving under it. Again, an ugly analogy, but closer to reality than a direct couple like tires to pavement.

Charlie
 
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BBerson

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Now, if you add a gear box so that the engine can turn 2700 rpm while the prop is turning 2300, the prop is *still* only absorbing 70-75% of total power, because you will have to reduce manifold pressure (MAP) to keep the engine from overspeeding. The torque load on the engine is reduced, which means its power output is reduced even though it's turning 2700 rpm.
The prop rpm would increase to say 2500 rpm to absorb the 100% rated power. That's the reason for the gearbox.
 

Dan Thomas

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I pointed out earlier that propeller have a very narrow useful RPM range. A little too slow and they're not generating much thrust. They're like wings that way, with lift increasing with the square of the increase in airspeed. And too much RPM results in massive losses to drag as the tips get past Mach 0.8 or so.

The variable-speed drive doesn't address either problem. On a fixed-pitch prop on takeoff, it will let the engine spin up to redline RPM, but that propeller's blades are at a higher than efficient angle of attack due to the low forward speed, so much of the power is lost to drag, and maybe some of the inboard blade is stalled. As the forward speed increases, the AoA decreases and the prop RPM will rise, but the tip speed soon puts a stop to that, too, and the prop's pitch is now too low to absorb the HP and turn it into a higher cruise speed. All you get is lots of noise.

That variable-speed drive was tried in one of the the prototype Bede BD-5s in the early 1970s. It wasn't satisfactory and they removed it and never went back to it. The fact is that propellers are not wheels, and treating them like wheels does not work. There is no substitute for a variable-pitch propeller, normally seen as the constant-speed prop. This has been known at least since the 1930s, when the Supermarine race plane had a fixed-pitch prop. That airplane became the famous Spitfire, which had wooden fixed-pitch two-blade propellers with some serious performance limitations due to that. In 1939 and 1940 they got constant-speed props, making them truly capable fighters. And in the 80+ years since then, no matter what fancy variable-speed drives or other doodads have been tried, nothing has replaced that constant-speed prop. Nothing.
 
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BBerson

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Those examples are max speed. An ultralight or motorglider pilot might want to putter around in level flight at a quiet reduced engine rpm and a higher prop rpm. Not all missions are the same.
 

Dan Thomas

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Those examples are max speed. An ultralight or motorglider pilot might want to putter around in level flight at a quiet reduced engine rpm and a higher prop rpm. Not all missions are the same.
Are there any such airplanes using variable-speed prop drives?
 

BBerson

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Are there any such airplanes using variable-speed prop drives?
The Boeing A160 helicopter has a two speed transmission for efficient loitering. Boeing A160 Hummingbird

[The A160's rotor includes blades whose stiffness and cross-section vary along their length. Their low loading and rigid, hingeless design allows for changing RPM between 140 and 350[17] using a two-speed transmission[15][18] to optimize efficiency at different speeds and altitudes, a technique called "optimum speed rotor technology", invented by Abe Karem.[1][19] It is primarily because of these features that the Hummingbird can fly with less power - and thus use less fuel - than comparable conventional helicopters,[11] which only adjust blade pitch and keep the rotor at constant rpm.[20][]
 
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