“Hybrid” helicopters

Helicopters and tilt-rotor aircraft
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Unread post17 Jul 2018, 02:39

I had a thought today, inspired by the idea of electrically powered compressors in jets. Would there be any gain in having the transmission of power between a helicopter’s engines and rotor being electric? Basically, the turbo shaft engines driving generators that in turn power an electric motor driving the rotors?
You could do away with the gear box, the multiple right angle turns, and the long shaft out to the tail rotor. Assuming you had a battery to even out the load, you might even get an improvement in the autorotation capability in case of engine failure.
Of course, it all comes down to weight and cost.
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jetblast16

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Unread post17 Jul 2018, 18:32

Basically, the turbo shaft engines driving generators that in turn power an electric motor driving the rotors


Like a diesel electric locomotive?
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zhangmdev

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Unread post17 Jul 2018, 21:45

A 400 ton electric drive dump truck has about half of the power of a Black Hawk.

https://www.cat.com/en_IN/products/new/ ... 34162.html

I don't think it could be light enough to fly.
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sprstdlyscottsmn

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Unread post17 Jul 2018, 23:00

Also moves at a fraction of the speed, and the 400-ton mass in not all electric motor and battery. It's steel body.
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southernphantom

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Unread post18 Jul 2018, 00:21

sprstdlyscottsmn wrote:Also moves at a fraction of the speed, and the 400-ton mass in not all electric motor and battery. It's steel body.


It's also only about a 195-ton truck unloaded; the 400-ton figure is fully loaded with ore. Still huge; the mine I work at operates trucks with only a 126-ton max gross weight. Those 400-ton ultra-class monstrosities don't get to go underground, though :wink:

zhangmdev, by your logic, because an Arleigh Burke is powered by gas turbines and displaces 8,000-10,000 tons, no turbine-powered aircraft can fly.
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Unread post18 Jul 2018, 01:02

So, the basic functional difference between a desel engine and a turbo shaft engine is that the former provides high torque at a low power to weight ratio, while the latter provides high power to weight at lower torque.
The real question is whether the electrical components (generators, power lines, motors) are comparable in mass to the mechanical driveshafts and gearboxes.
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Unread post18 Jul 2018, 01:05

southernphantom wrote:<snip>

zhangmdev, by your logic, because an Arleigh Burke is powered by gas turbines and displaces 8,000-10,000 tons, no turbine-powered aircraft can fly.


This is a straw man argument. The dump truck example is meant to show electric drive is too heavy (low power-to-weight ratio) to fly. So heavy that mobile land based application, while matching the output of a helicopter power plant, is limited to the heaviest machinery. The power generation package is likely less than 10% of the weight of the emoty weight, but still way more than the gross weight of a Black Hawk.

A 2000 kw power generation package is something to be moved on a trailer. There is no practical way to reduce weight/size while increasing the power output, even when you use gas turbines instead of diesel engines. Megawatt alternator is neither small nor light. The power package needs gearbox and cooling.

Gas turbine, alternator, induction motor, none of them are new technology. So why so far no smart people have figured out how to put them together to fly a helicopter or to propel an airliner?
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Unread post18 Jul 2018, 12:46

Phfffft. It's preposterous to believe electric motors can be used in flight.... /sarcasm

Main rotor size is a major limiting factor in forward speed. There is absolutely no reason electric drive props cannot augment a larger central propeller. Those same, smaller electric drives can use centrifugal force to deploy the props and turn in whichever direction is desired. The electric motors are much easier to swap. And your efficiency of pushing a generator for electric generation can be more fuel efficient than a direct drive counterpart if you engineer it for that goal from the beginning. Simply using the single original motor that was only meant to drive mechanical transmission and adding in electrical generators is not a good endeavor in most cases.
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Unread post18 Jul 2018, 23:15

It looks like a power to weight ratio of 10 kW/kg is possible for electric motors. That would be 300kg for a Blackhawk, doubling to 600kg if the generators are the same mass. Seems a lot, but I don’t know how that stacks up to the weight of the gearboxes and drive shafts. Maybe it’s only reasonable for the tail rotor.
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