# Electric helicopter

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Hydrogen fuel cells would bring big advantages to helicopters.

Fly long! Airborne 1t including passengers and fuel takes ~100kW with 78m2 rotor(s), say mean 130kW with the manoeuvres: that's two Honda Clarity fuel cells of <100kg each. 48kg hydrogen keep it in flight for 8h - that's 100kg with the tanks I described there
http://www.scienceforums.net/topic/73798-quick-electric-machines/#entry738806

Electric motors save maintenance, expensive and long with gas turbines.

Electric motors are easily spread among many rotors, leading naturally to quadtoror-like designs. This is way easier than the pitch of common helicopter blades, which changes over a turn.

Swift start and stop. Important to a businessman who pilots himself.

Fuel cells bring range and flight duration that enable the classical missions of gas turbine helicopters, beyond city taxis and sightseeing tours. Electric multi-rotor copters must also be more silent and cleaner, hence better accepted.

Marc Schaefer, aka Enthalpy

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Sounds like a good idea.

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3* 100 kg - - - One pilot and two passengers, with luggage, dress, seats
3* 100 kg - - - Three Honda fuel cells of 100kW each
150 kg - - - 70kg of liquid hydrogen in one tank
6* 100 kg - - - Six rotors of D=3.8m in stators, with motors and electronics
70 kg - - - Cabin armour and floor
30 kg - - - Windshield
150 kg - - - Truss
80 kg - - - One parachute for the aircraft
40 kg - - - Shock absorbing gear
80 kg - - - Unaccounted
==========
1800 kg - - - Take-off mass

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Six rotors permit one to fail. The opposing one is also stopped or reduced, the remaining four get all the power and steer like a quadrotor does. Four D=3.8m 60% efficient rotors accelerate together 1046kg/s air to 18.5m/s at sea level, lifting 19400N or 1.1g.

Stationary flight (17700N) at sea level needs six rotors (68m2) to accelerate 1213kg/s to 14.6m/s, consuming 214kW. Two fuel cells shall provide this briefly to land if the third fails.

The fixed-pitch rotors can't brake a powerless fall, hence the parachute.

Mean 250kW provided by 60% efficient cells consume 70kg hydrogen in 3.8h. The single tank is lighter, three would add redundancy.

Panels of extruded AA5083 sandwich (t1=t2=1mm, a=45°) welded together separate the cabin from the tank

(more detailed previous description http://www.scienceforums.net/topic/60359-extruded-rocket-structure/ )

More of this material makes the tank's outer shell, as I described for aeroplanes
http://www.scienceforums.net/topic/73798-quick-electric-machines/#entry738806

The passengers sit at the pilot's sides just ahead of the craft's middle, exiting to the prow, and the tank and fuel cells lay just aft. Metal panels make a wall and the cabin's floor, and wrap a bit the tank below, above and at the sides, leaving much open area to the aft.

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For silence, each rotor has eight 1.8m long fixed blades of 0.5m chord, running at variable 3.4Hz or 40m/s. Made of foam covered with 560+300g/m2 composite, each shall weigh 3kg. The outer streamlining stators weigh each 19kg of the same material.

30°/s craft roll or pitch rate result in peak 2m/s and 43m/s2 at the blade tips. The gyroscopic moment is 81N*m peak per blade or just ~12MPa in the composite skins, and 324N*m at the shaft or just 0.15m times the lifting force.

2*15° roll or pitch in 2*0.5s need at each rotor 8.4m/s2 or 838N change over 2950N. Accelerating a rotor from 3.4Hz to 3.85Hz in 0.5s takes 245N over mean 1670N.

Each fast 50kW motor and its reducing belt, plus the shaft and bearings, shall weigh 57kg. I didn't check a slow motor. Stator blades don't look so useful.

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A truss of AA6082 D=150mm e=1mm tubes holds the rotors together and with the cabin.

The landing gear's shock absorber might use my viscoelastic elements or not
http://www.scienceforums.net/topic/71234-deployment-brake/

A sketch is to come. The aircraft is a simple assembly like toy quadrotors are, but optical features suggesting a high-tech aeroplane promote passengers' confidence.

Marc Schaefer, aka Enthalpy

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Example of a rotating-wing craft powered by fuel cells (click for full-scale). This one has six rotors to transport three people, according to the previous message; other numbers and patterns are possible.

The surrouding truss includes a landing sledge which holds the cabin by its floor. The truss has healthy angles and offers some protection to the tank and cabin.

For sight, access, aspect, the truss would better lift the cabin by the top.

The six rotors and the truss suggest a compact de-assembled craft, easing transport and storage.

Marc Schaefer, aka Enthalpy

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Electric motors start quickly, an advantage for an ambulance hexrotor with fuel cells. Here some figures.

Three rotors at each side take length, so the body shall accommodate two beds, plus four medics and a pilot. Mass estimate:

2* 110kg - - - Patients, dress, beds
1* 100kg - - - Medical apparatus and supplies
5* 90kg - - - Four medics, one pilot, dress, seats

250kg - - - Cabin
100kg - - - Truss
50kg - - - Armour
120kg - - - Parachute
100kg - - - Infrared vision, terrain radar

6* 125kg - - - Rotors, stators, motors, electronics
6* 100kg - - - Fuel cells
200kg - - - 100kg hydrogen in tank
==========
2850kg - - - Take-off mass

Six fuel cells provide more power to lift more mass with still D=3.8m rotors.

The degraded mode with 4 rotors active accelerates 4*326kg/s air to 23.5m/s with 60% efficiency to lift 30.7kN or 1.1g.
The degraded mode with 5 fuel cells lifts 1.1g at no cell overload.

Stationary normal flight accelerates 6*254kg/s air to 18.3m/s, consuming 425kW.
If flight consumes 550kW as a mean, 60% efficient fuel cells use 100kg hydrogen in 4.3h.

A sketch is coming.
Marc Schaefer, aka Enthalpy

Edited by Enthalpy

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Oops, I'm late here... Here a suggestion of how the ambulance could look like (sections are odd, don't get fooled):

It's tall enough that the medics can stand, and permits to walk around the beds.

The doors could slide to the roof instead, if durable enough - check how fire engines do it.

On this sketch the rotors lift the cabin by its floor. A dense truss to the landing sledge would strengthen the floor but prevent shock damping.

As the rotors are not synchronous, they should be made as quiet as possible.

Marc Schaefer, aka Enthalpy

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The Ehang 184, one more electric helicopter design, with secondary batteries instead of fuel cells:
http://www.wired.co.uk/news/archive/2016-01/07/ehang-184-personal-drone-car("View gallery")

Opinions should expectedly vary over its general design choices. My personal dislikes are:
- It knows only a fully automatic mode. As an old engineer, I claim that reliability results from humans in the loop.
- It has four rotor sites instead of six and apparently no parachute.

But its numbers for mass, power, energy, flight duration add up. Maybe this one breaks through, and if not, an other will

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Fuels cells make progress, and quickly. Three years ago they provided 1kW/kg, now the one powering the Toyota Mirai claims 2kW/kg
https://en.wikipedia.org/wiki/Toyota_Mirai
which eases electric helicopters quite a bit.

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I had suggested for fixed-wing aircraft to produce electricity by a few combustion engines and use it where many propellers or fans are more efficient. Several companies and agencies work on it presently.

This is even more interesting for multi-rotor helicopters like hexacopters. Much simpler than the cyclic pitch of single-rotor helicopters, they use fixed-pitch rotors whose independent speeds let pilot the craft. Electric motors are cheaper and more flexible to run the rotors. A combustion engine driving a generator is an alternative to limited batteries and to cold hydrogen.

The combustion engine can be a gas turbine, with which an alternator cooperates nicely
http://www.scienceforums.net/topic/73798-quick-electric-machines/#entry737931
an airliner Apu would fly a big hexacopter.

It can also be a turbocharged Diesel burning kerosene, or pretty much any combustion engine.

Marc Schaefer, aka Enthalpy

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Many companies and groups have flown drones and quadcopters from a fuel cell and hydrogen stored as a compressed gas or a liquid, as you guessed. As one example, Energyor Technologies Inc. from Montreal flew a quadcopter for nearly 4 hours in 2015 - videos available on the Internet.

They imagined oilfield operations, powerline inspection... as their first customers. But build a bigger hexacopter (October 05, 2013 here), and this flight duration is fantastic for rescue and search operations.

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The explosion hazard with liquid hydrogen isn't as critical as I had believed, at least in air. Arthur D. Little Inc. experimented it in 1960
The first video shows from 2min50 to 6min18 spillages of 1 to 5000 gallons ignited late, and in the open air they didn't detonate, but deflagrated instead. Other questions were investigated, for instance the evaporated spillage remains close to the ground over 200m and propagates a flame over 100m. But with liquid oxygen (second video), detonations do occur, said to be less bad than with kerosene.

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I think improved MHD generators could be more helpful here as they could be made flexy fuel and offer much higher power density than fuel cells. A much more appropriate thing for a helicopter.

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On 13.06.2013 at 1:22 AM, Enthalpy said:

If flight consumes 550kW as a mean, 60% efficient fuel cells use 100kg hydrogen in 4.3h.

I see you just mention amount of Hydrogen everywhere, omitting how much you will need Oxygen..

100 kg Hydrogen = 100 000 grams / 4.3h / 3600s = 6.46 grams of Hydrogen burned/reacted per second.

6.46 grams of Hydrogen requires 51.27 grams of Oxygen, from air, if you don't take it in tank.

There is 0.3 grams of Oxygen in Liter of air at sea-level (and less at higher altitude).

51.27 g / 0.3g/L = 170.9 L of air must be sucked in by fuel cells per second. Approximately 10.26 m^3 per minute. Looks quite a lot.

Wouldn't it require powerful air compressor.. ?

Edited by Sensei

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Hi Sensei and all,

oxygen comes from the air, yes. This is what makes hydrogen much lighter than kerosene.

Agreed with 51g/s oxygen needing some 0.2m3/s air. But this is little, especially in an aircraft where air moves already. Take 20m/s under the rotors, it's a D=0.1m intake.

An other logic is that the helicopter uses car fuel cells, which get enough air when used in a car. The helicopter has 6 cells instead of 1, and it suffices that each cell has an intake as big as on a car.

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This quadcopter or, for redundancy, hexacopter shall extinguish fires, optionally in cities too.

The multirotor is simpler and cheaper than a helicopter, and easier to (help) pilot by software. Its rotors are easier to surround by protective stators. It adds advantages specific to firefighters:

• The pipe and jet can aim up and down between the rotors, if no frame element interrupts the movement there, to tackle the braze from the side instead of fanning it.
• This lets tank in flight from a river, a fountain... with a reversible pump or two pumps.
• The tank is accessible from the top and the bottom.
• The jet creates a smaller torque and the rotors compensate it better.

Mass estimate:

--------------
6*5kg   Rotors with electric motors
30kg   Gas turbine and generator, 70kW
3kg   Empty tank
10kg   Frame
10kg   Pump
8kg   Pipe
2kg   Electronics
7kg   Undetailed
200kg   Water
--------------
100kg   Empty
300kg   Full

Six D=1m rotors accelerate together 131kg/s air from 0 to 23m/s to hover. At 70% efficiency, this needs 50kW, but climbing at 7m/s takes 70kW, and hovering with a defect rotor needs power too. Batteries don't suffice here, a piston engine would reduce the payload. A fuel cell does the job but hydrogen may awake superstitions. A gas turbine is light, possibly from a small APU. Get inspiration there
http://www.scienceforums.net/topic/73798-quick-electric-machines/?do=findComment&comment=737931
for the gearless light generator and for the geared motors.

The pump isn't easy. 60% efficiency need 3.3kW for 2 bar and 200L in 20s. A centrifugal design is compact and the motor too, but it sucks water badly when air is in the pipe. Maybe a fast screw pump
https://www.scienceforums.net/topic/73571-rocket-engine-with-electric-pumps/?do=findComment&comment=734835
or a second centrifugal pump at the lower end of an optionally separate pipe. Have floats under the frame?

The pipe is preferably of graphite composite, possibly a sandwich. Metal would need intricate reinforcements. Software would usefully stabilize it against the frame's pitch.

The sketches and figures let the copter fit in a lorry. 2-4 people carry it empty. It can still manoeuvre in Parisian streets, and possibly arrive by flight from the fire station. Scaling up and down is possible.

Marc Schaefer, aka Enthalpy

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Multirotors use to have all rotors in the same plane. To translate longitudinally or sidewise, they must first pitch or roll, which takes time. The reaction time matters if flying in the wind near a moutain slope for instance, or near a building...

I propose to tilt some or all rotors inwards, both length and sidewise. Increased thrust at some rotors and decreased at the opposite ones then creates immediately a net in-plane thrust and acceleration, even before the pitch or roll builds up.

The tilt sketched here wastes 1% of the lift force for each tilt direction.

Marc Schaefer, aka Enthalpy