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# Quick Electric Machines

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Hello, heirs of James Clerk, Nikola and the others!

It is well known, but by too few people : in an electric machine, only the force means losses and heavy parts, the speed comes for free. When a motor or generator runs quickly, say 50 or 100m/s at a power plant, it is smaller than a turbine.

Quick machines with rotating permanent magnets use to hold them in a tight sleeve of strong steel to counter the centrifugal force. I propose to wind a composite of graphite fibres around the magnets instead of the steel sleeve. Graphite fibres are lighter than steel and produce less eddy current losses where they cross the stator's windings; better, while the accurate diameter of a steel sleeve is difficult, fibres are commonly wound tight over varied cores, even with a pre-tension useful here. A unidirectional composite looks best here, and pre-impregnated graphite is usual at wound pressure tanks for instance - other fibres may emerge. Some thin elastic material below the magnets can prevent cracks.

To run at 200m/s, 5mm thick magnets weighing 7500kg/m3 need 1.5mm of graphite composite withstanding 1000MPa - or scale both thicknesses. Neodymium magnets like Thyssen-Krupp's 300/110 still achieve 0.78T through the graphite plus 1.5mm radial gap.

I already described a small electric motor turning a rocket engine pump, there
http://www.scienceforums.net/topic/73571-rocket-engine-with-electric-pumps/#entry734225
and http://saposjoint.net/Forum/viewtopic.php?f=66&t=2272&start=80#p41298

the following one outputs 2083kW like the PW127M gas turbine that moves the ATR-72 and other successful planes.
http://www.pwc.ca/en/engines/pw127m
http://de.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PW100
http://en.wikipedia.org/wiki/ATR_72

The motor rotates at 255Hz, so a gear drives the propeller at 20Hz - but a turbofan would need none. 5mm thick magnets at D=250mm run at 200m/s. The 355mm long stator has 3 phases and 18 poles. The windings are one turn of square 5mm*5mm copper that makes 36 passes through the shared 54 slits. The induced coil voltage is 1726Vpk (reduce the length if I botched a cos30°...) and the current 804Apk, nice for an inverter supplied with 3kVdc maximum. Coil resistance is 13mohm (or a bit more as the skin depth is 1.4mm at 2292Hz) so ohmic losses are 13kW or 0.6%; core losses are small with the proper material.

This electric motor weighs ~120kg, is ~400mm long and 312mm wide , while the PW127M is 660mm wide and ~1.2m long without the gear, and weighs 481kg with the gear. Direct retrofit, though we still lack proper fuel cells.

Marc Schaefer, aka Enthalpy

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The gear can be avoided. Again 2082kW as the PW127M, but the wide ring motor rotates directly at 20Hz.

At D=1000mm hence 62.8m/s, the 3mm thick Neodymium magnets (happen to be Thyssen-Krupp 340/88) need only 0.15mm graphite winding and achieve 1.02T in 1mm radius gap. With 66 poles and 132 passes 205mm long, each of the three phases gets 1726Vpk induced in the single turn. The 6mm*3mm wires get 1mm thick insulation; the slits may need some potting in addition, especially at altitude.

Diameter, mass and losses can be optimized against an other. At this speed, a squirrel cage would have been possible. The present design is wider than the gas turbine but only as tall. The motor's ring weighs about 280kg alone. Its ohmic losses are 42kW or 2%, but as it saves the gear's mass, maintenance and 1.5% losses , I prefer it.

Marc Schaefer, aka Enthalpy

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If an inverter that produces sine waveforms of MW at kHz is too difficult, I figure it can make nearly square waveforms.

In that case, I'd drop the refinements that induce a quasi-sine voltage at electric machines: split coils, shared slots, variable gap... One coil per pole, one slot per pole and phase, wide magnets shall induce a nearly square voltage. Keep maybe some slot skew.

Then the power components can switch once per output period to apply the rail voltage when the induced voltage has reached its plateau. During the voltage transitions, current flows through the flywheel diodes. 2 phases from 3 (or 4 from 5, or 6 from 8...) provide power at any time.

The sizing advantages of three-phase current are lost, but the square waveform compensates them. Optimized timing between one phase switching off and the next switching on can reduce noise. If the slot inductance hampers the current rise, drive the switches shortly before the voltage plateau.

The rail voltage must adapt to the rotation speed and the required torque - indicated by "Buck" on the sketch. At least near cruising speed, I'd synchronize the voltage regulator with the phase switches.

The nearly-square induced voltage applies also - more easily - to a generator that feeds an input rectifier.

Marc Schaefer, aka Enthalpy

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The unducted fan or propfan has two counterrotative propellers so the downstream air doesn't rotate. A bigger blade pitch then remains efficient, so the propeller can be used for faster flight.
http://en.wikipedia.org/wiki/Propfan
Electric motors combine with counterrotative propellers more easily than turbines do:

The version at the fuselage has concentric shafts. The detached version needs the pylon between the propellers, but helps dynamic stability and the bearings; I expect propellers wider apart to be less noisy.

Electric motors can have the rotor outside - not depicted here, please look at a processor cooling fan. This enables more layouts that combine well with propellers but are less favourable to the bearings nor the protection against dust and rain.

Marc Schaefer, aka Enthalpy

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Turbofans fit faster flight, and an electric motor can rotate them too. The example shall be a Trent WXB, please: D=3.0m fan receiving 70MW at 2683rpm (taken from varied versions, hence not fully consistent).

The 40mm thick neodymium magnets (here Thyssen 300/110) on OD=1.0m move at 140m/s. Thin narrow bands of permeable hard steel wrap them, totalling 7mm thickness to limit to 3mm the radius variation due to
centrifugal force. The bands can be brazed (especially if covered with nickel) or glued; chemical etching can bevel their ends. The air gap is 6mm outside the magnets at idle; some elastomer or springs are required at the magnets' inner side. If feasible, pre-stressing the steel bands when winding improves that. Magnetic leaks in the bands widen the transition between the poles but loose no induction at the plateaux.

The motor is 404mm long, its 16 poles have 7 active slots of 8 at any time, each 11mm wide and 44mm deep to host one phase that comprises 4 turns of square copper wire, 9mm*9mm large with a 3mm*3mm central hole. Induction at the gap is flat 1.09T, current per slot 10.1kA, the inverter's supply is 4kV.

Ohmic losses total 730kW or 1%. Fe-Co laminations at 2.0T loose 70kW. 18dm3/s cooling oil flow at 5m/s in parallel through all wires, and some more through iron.

The bare electric motor weighs 2030kg, a complete original Trent XWB 6600kg. The stator has OD=1.33m, or the mean diameter of the Trent's primary air inlet.

Huge strong magnets are a real danger for mechanics; a squirrel cage design would improve it. Wrap the aluminium or copper cage in steel or graphite composite to resist the centrifugal force, including the shorting ends.

Well, as soon as we have 70MW electricity on board...

Marc Schaefer, aka Enthalpy

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Some cars with piston engines have an integrated starter-generator
http://en.wikipedia.org/wiki/Integrated_starter-generator_system
As the name tells, this electric machine does both, and the difficulty is that it runs together with the piston engine all the time (and at the same speed), so torque is needed as a starter but the resulting big diameter results in a high linear speed after starting, when the piston engine runs fast.

Imagine a D=300mm starter-generator that must survive if the engine runs at 10,000rpm: that's 157m/s, which produces a brutal centrifugal force.

For them as well, winding graphite composite around permanent magnets - or possible around a squirrel cage or rotor coils - is a simple means to withstand the centrifugal force, simpler than a steel sleeve. Several turns of thin steel band, possibly as wide as the magnets or rotor, is an other one, where gluing or brazing is easier than slipping a sleeve on and can cumulate more thickness.

-----

The integrated starter-generator saves a few parts; it also starts the engine quicker, easing features like the start-stop system
http://en.wikipedia.org/wiki/Start-stop_system
with more steps possible towards the hybrid electric vehicle
http://en.wikipedia.org/wiki/Hybrid_electric_vehicle

One little ambitious step I'd enjoy would replace the mechanical reverse gear by the electric motor. An other one would complement or replace the first gear with the electric motor so the car has no minimum speed in a traffic jam. Smooth control of low speed in both directions would also help to park a car.

Marc Schaefer, aka Enthalpy

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Auxiliary Power Units feed aeroplanes with electricity and traditionally compressed air when the main engines don't do it. This alternator design shall rotate at 813Hz like the gas turbine, suppressing the gear, to provide 1270kW. Click the sketch for full scale.

10m thick Nd-magnets create 0.9T at the gap. 3mm of wound graphite composite hold them at 200m/s. A single strand per output passes once at each of the 6 poles to produce flat ±136V; three 120° outputs in a hexaphased rectifier provides +270Vdc to the aeroplane (the 15kHz filter is not represented). This saves heavy power electronics. A second set of 3 phases share the slots and provides -270Vdc. Or put the second set in distinct slots, or have dodecaphased rectifiers...

Six 3.5mm*3.5mm wires make the "Litz" strand. They swap their positions and the ends of the stator, where they spread to dissipate 24W per wire in a strong wind; each wire could be subdivided. The wires evacuate the heat produced at the middle, dropping 54K. Alternately, oil can remove heat at the wires' turns, or (easier to seal?) oil can flow through each wire. I evaluated ohmic losses to 7kW=0.55% with turns too short. Mass is around 16kg but without casing, cooling, shaft...

Air cooling conflicts with the slot inductance, and both are marginal here. This lets the +270Vdc and -270Vdc interact if they share the slots.

The flux' permanent return path at the rotor is designed too small, and a complementary plunger adjusts the flux and output voltage in real time.

Marc Schaefer, aka Enthalpy

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An electric motor can propel Burt Rutan's LongEZ: the "Long ESA" does it (search keywords).
http://en.wikipedia.org/wiki/Rutan_Long-EZ
Glider-like aircrafts like Yuneec can fly for an interesting time on battery power, but not the LongEZ meant for general aviation.

A 10-week student project at the TU Delft checked hydrogen and fuel cells to supply an electric motor on LongEZ:
http://www.tudelft.nl/en/study/undergraduates-bachelors/undergraduate-programmes/aerospace-engineering/degree-programme/third-year/design-synthesis-exercise/ds-exercise-2012/taking-the-next-step-towards-zero-emission-general-aviation/
with excellent flight duration and range.

I wanted to check a quick electric motor, but a slow 86kW motor is light and saves the gear. To rotate at 45Hz or 36m/s, its 2mm thin Nd magnets are wrapped with little graphite; they achieve 0.89T in 1mm gap, which is 52mm long and has 260mm diameter. 28 poles contain each 3 slots, 2 being active at any time, each fitting one phase. Each slot contains 2*3 wires of 2mm*2mm, in series for a flat-top 282V drive. The 4,5% losses could improve if wanted, and the wires' spread half-turns dissipate them easily in air. The motor looks similar to the previous APU and weighs 10kg.

A single Honda FCX Clarity fuel cell supplies the electricity. Does its price fit this aeroplane? Its mass, plus the motor and the inverter, replaces the original Lycoming O-235's 109kg.

3235km range at 232km/h, or 50,200s (14h!) at 34kW (40% power) need 1.85GJe. That's only 10.7kmol or 21.7kg hydrogen with a 60% efficient fuel cell. At 298K and 300b it takes 1.1m3, light with graphite composite but hard to accommodate. Liquid at 20K and 1b it takes 0.31m3, a D=0.84m sphere or ellipsoid which fits in a longer aft. More later about the insulation.

Marc Schaefer, aka Enthalpy

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Piaggio's P180 Avanti is a business aircraft:
http://en.wikipedia.org/wiki/Piaggio_P180_Avanti
and it could fly right now with fuel cells:
- Reported 5.3M€ price allows 2*6 fuel cells from Honda's FCX Clarity, as the whole car is said to cost ~140k\$, and this saves the turbines.
- 2*6 fuel cells weigh ~1000kg but replace 1200kg kerosene and 2*170kg turbines with light hydrogen and light electric motors.
- No more turbine exhaust through the propeller. Swept blade tips should help as well.

2800km cruise at 644km/h and 80% power or 507kW take 2*8GJ, obtained with 60%*95% efficiency from 2*98kg hydrogen carried liquid at 1atm: the tanks are smaller, lighter and supposedly safer than for gas under pressure. Then, hydrogen fits in the volume of the original nacelles, together with the fuel cells, motors and propellers, and can be kept away from the cabin.

Hydrogen takes 2*1.4m3, for instance ellipsoids 1m wide and 2.8m long. The nose fairing protects against impacts, with fibre and foam like a bulletproof vest. Adjust the center of mass by the compact fuel cells. Bigger tanks would increase the range.

The neodymium electric motors (2*65kg) can drive the propellers directly, resembling the PW127M equivalent already described. Unoptimized, the gap can have 480mm diameter and 105mm length. 30 poles and square wire let lose ~6kW, maybe air-cooled at the spread half-turns of the wires.

I propose to hold hydrogen liquid at 20K and 1atm in thin steel, superinsulated and hold in a vacuum vessel by polymer straps; this design may also fit cars and others. Ask your usual satellite designer, or Wiki.

• Here 200µm Maraging steel (11kg, per nacelle here under), coated with nickel and brazed together, resists 10bar.
• 30mm foam (10kg) let pass only 1.5kW if vacuum is lost. This leaves 1 hour to reach 10bar, or lets 3.4g/s evaporate. That's only half the engine's consumption, so the plane can go on flying, but it's a strong flame on the ground, so foresee a safe purge.
• 80 plies multilayer insulation (10kg) in vacuum let 0.6W pass through. That's 3.3kg/month evaporation, which can optionally feed a fuel cell that power a cryocooler keeping hydrogen liquid, as Nasa proposes.
• Polymer belts (<1kg) hold the steel envelope in all directions. Cumulated 1cm[super]2[/super] are more than enough, 10cm free length leak <0.1W. At the proper (3D!) angle, they let the steel envelope shrink unhindered.
• Extruded aluminium profile is welded together to constitute the vacuum vessel (74kg). Details there:
http://www.scienceforums.net/topic/60359-extruded-rocket-structure/
1mm walls (less if possible) of AA5083 for 40mm sandwich thickness resist jumping on and offer successive airtight walls. The profile can be round, maybe in the extrusion direction as well. Someone else shall design the airtight opening, preferably as lower and upper half-shells...

Marc Schaefer, aka Enthalpy

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Hydrogen and fuel cells carry more energy per kg, which increases an aeroplane's range.

This example combines ATR 42's short body (even the tail?) with ATR 72's wider wing to carry the <4000kg fuel cells providing 4000kW electricity.

7000km range (not the limit) at 554km/h and mean 2*1400kW with 60% *95% efficiency take only 1574kg hydrogen carried in 4 tanks under the wings. A geared motor eases the overall design. I prefer to spread the fuel cells among the tanks and distribute only electricity. In a new design, consider several bodies like Rutan's Voyager had.

The chimera ATR 72-42 frame shall take-off at 22500kg but weigh 12100kg empty. Minus 4000kg fuel cells but plus 300kg from lighter electric motors leave 6700kg. Four tanks built as previously take 1484kg. Hydrogen for the full range and two pilots leave 3.4 t transport capacity.

London - Dubai - Hong Kong or Tokyo - Anchorage - New York - Amsterdam at 554km/h here is too long for passengers but parcel or freight companies can like the improved flexibility or fuel savings over airliners' hold. A fast business aeroplane would also benefit from hydrogen's range.

Marc Schaefer

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• 2 months later...

High frequency spindles for machine tools shall offer the highest angular speed for a given torque or power. Even if not required simultaneously, bigger maximum speed and torque from the same motor let the spindle accommodate more varied tool diameters - but this demands a higher possible linear speed from the motor.

For instance the TCV-2 from Peronspeed illustrates it:
http://www.peronspeed.it/scheda1.asp?idserie=6
for 10kW and 40,000rpm, the motor's gap of D=64mm (drawing there) runs at 134m/s already, so its stainless sleeve must be the limit, and winding graphite fibres instead, or a thin long steel sheet, improves it.

Would friction stir welding benefit from higher speeds? This paper investigates 24,000rpm:
http://www.niar.twsu.edu/researchlabs/ajt_presentations/Widener%20-%20Paper%2038%20-%20Presentation.pdf
to reduce the forces.

Marc Schaefer, aka Enthalpy

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• 8 months later...

A quick electric motor-alternator, hopefully wounded with graphite fibres as I suggested, would be nice at the turbocharger of a piston engine. Rotating that fast, the motor-alternator is tiny.

Coupled with a battery or supercapacitor, it permits to accelerate the turbocharger quickly when the driver requests power, and bring more air to the piston engine than the exhaust turbine permits. When braking, it regenerates energy from the turbine.

A similar function exists with pressured air, but electricity is more flexible, and a battery uses to be lighter than an air tank.

Splitting the turbine and compressor on two shafts, with an alternator and a distinct motor to transfer power, may bring some more flexibility.

This is foreseen at Formula 1 race cars in 2014, if I interpret properly newspaper articles. Useable on any turbocharger car, and the fibre-wounded magnets reduce the mass and magnet costs over a steel sleeve, through higher azimutal speed.

Marc Schaefer, aka Enthalpy

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• 6 months later...

To prototype a vacuum-insulated hydrogen tank as described here on 14 April 2013, one doesn't have to pay the special aluminium profile right from the beginning. Existing aluminium profile is heavier but demonstrates the balloon in an evacuated shell.

One example of existing profile, among many from varied suppliers:
http://www.bso-gmbh.de/pdf/produkte/alu-profil-nut-8-40x160-4n-leicht.pdf
its flat face is easy to weld and 160mm width limits the weld work.

----------

The production shell, sphere-like or more ellipsoidal, can also consist of cast elements, for instance two shell parts with seals and bolts, or 20 triangles welded as an icosahedron, possibly subdivided in smaller triangles, and the many variants
http://en.wikipedia.org/wiki/Icosahedron
http://en.wikipedia.org/wiki/Pentakis_dodecahedron
http://en.wikipedia.org/wiki/Truncated_icosahedron
cast elements commonly have integral stiffeners.

Marc Schaefer, aka Enthalpy

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• 2 weeks later...

For the vacuum vessel of a hydrogen tank, cast shell elements make naturally a single wall. I prefer sandwich double-walled shell elements, as they resist shocks and deformations far better and offer redundant airtightness. Diffusion bonding permits it; not as cheap as casting, but aeroplanes afford it.

Among other providers, a doc with nice examples, sandwiches on page 9 and 10:
http://www.formtech.de/download/formtech-short-presentation.pdf
and here is how the vacuum vessel may look like:

The external skin with stiffeners in several direction, for instance as an isogrid, is cast or rather machined. It can provide the smoothly curved outside aerodynamic shape, while its inner face is machined flat for bonding. The inner skin is a sheet here, though ribs are possible. Diffusion bonding makes a sandwich shell element of both, with manageable size and shape.

Then the elements are assembled by Tig, Mig or other welding to make a shell. Milled keys bring accurate positions when welding.

Alternately, two thin skins and a (bidirectionally?) corrugated spacer could be superplastically formed before diffusion bonding, if the shells are accurate enough for diffusion.

Seemingly, magnesium alloys still don't resist deep bumps. This would leave aluminium (AA5083) and titanium (little alloyed).

Cylindrical tanks at rockets can keep my cheaper extruded profiles.

Marc Schaefer, aka Enthalpy

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• 4 weeks later...

Vacuum or low-pressure vessels are often needed, so the already described extruded profiles, the cast shell elements, the sandwich shell elements (possibly by diffusion bonding) have uses beyond hydrogen storage. Few examples:

• Some alloys are better cast under vacuum.
• Vapour condensers and the turbine that uses them. This one may prefer an Al-Mg alloy or titanium.
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• 9 months later...

Hydrogen and fuel cells bring less to a supersonic airliner. Fuel cells are too heavy in 2015 for this power, and hydrogen volume creates more drag at high speed.

This example is to cruise at Mach 1.3 - but M=0.85 over the continents. Despite the swept wing, I've kept the lift-to-drag ratio L/D=9.3 from Concorde at M=1.3 and added Cd=0.23 drag from two D=4m liquid hydrogen tanks, so L/D=7.6.

(Click to see full-sized)

The airliner with D=5m body shall weigh 255t with tanks half-full, needing 329kN and 127MW for 387m/s at 17000m. The 85%, 98% and 99% efficient fan, motor (speed makes it easy) and electronics take together 154MWe from the fuel cells. Present car cells weigh 100kg/100kW, so the plane's 154t fuel cells are prohibitive - this is hence prospective. >3 times lighter fuel cells are needed; they improve quickly and have further potential, but through a big effort.

The drawn tanks contain 40t together, from which 60% efficient fuel cells permit to fly 7300km at M1.3. Hydrogen needs less mass than kerosene, but the volume wouldn't let carry so much more at high speed. If pyrolyzing to 2*H2, CH4 would cost slightly less than kerosene for the same flight presently; since propane and butane are still torched at oil wells, they too could provide much hydrogen, in addition to propene for plastics and butene for alkylates.

Design options and variants:

• The tanks and cells spread the weight nicely over the wing.
• The cells can move to the fore and aft to balance the airframe. The tanks need compartments.
• Between the three hulls is a nice location for stacked wings whose interaction minimizes the wave drag.
• The F-111's variable-sweep wing would bring low drag at all speeds and improves take-off and landing.

In constrast to slower aircraft and helicopters that can fly right now with hydrogen and fuel cells, reduce their noise, gain autonomy and range, operate promptly, save taxiing fuel, supersonic aircraft need more power and must await lighter fuel cells.

Marc Schaefer, aka Enthalpy

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• 8 months later...

The fuel cells for the previous estimates provided 1kW/kg, but they have progressed to 2kW/kg:
https://en.wikipedia.org/wiki/Toyota_Mirai
which eases everything at aeroplane design.

At the 255t supersonic airliner, fuel cells weigh 77t instead of 154t. Still not reasonable, but it becomes feasible.

The subsonic frames were already feasible, they improve. The Atr72-42 chimera improves its freight capacity from 3.4 to 5.4t. The Piaggio 180 Avanti gains 500kg.

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• 5 months later...

Hydrogen and fuel cells make good aeroplanes... Other people believe it strongly enough to have developed some.
http://www.aviationpros.com/press_release/12264576/hy4-maiden-flight-of-the-hydrogen-powered-airplane
This four-seater is still rather slow (150-200km/h) and resembles more a motor glider, but it claims already 750-1500km range, much for this plane category, and a strong advantage of fuel cells.
No word about sales, so it must be a demonstrator rather, but to my opinion, its performance is market-ripe. And the members of the team (a wide consortium) speculate loudly about commercial transport soon.

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Sorry, I have not read entire thread carefully,

but I only see you mentioning Hydrogen,

and completely ignoring Oxygen issue.

On sketches there are just Hydrogen tanks, and you're providing liquid Hydrogen calculations.

Am I correctly expecting that Oxygen will be taken from air?

If so,

should not you make calculations also for Oxygen.. ?

And show how, and how fast, Oxygen will be taken from air on various altitudes.. ?

You mentioned 17 km altitude in post #16.

If we use this calculator with 17000 m

"At 17000m, the standard barometric pressure is 10 kPa (75 mmHg). This means that there is 10% of the oxygen available at sea level."

Let's assume that at standard conditions there is 0.04464 mol/L

21% is Oxygen, and 10% of that is at 17km.

So it gives 0.04464 mol/L * 0.21 * 0.1 = 0.00093744 mol/L of Oxygen O2.

To split 1 molecule of water to H+ OH- there is needed 1.23-1.5 eV,

so for 2 Hydrogen atoms (1 H2), it's 2.5-3 eV.

Multiply by e and it's 3.94e-019 J.

Let's assume that it'll be given back in reverse reaction, without loses.

In post #8 you mentioned motor has 86 kW = 86000 J/s

Divide 86000 J/s / 3.94e-019 J = 2.1827e23 Oxygen atoms reactions per second.

/2 = 1.09137e23 O2 /Na = 0.1813 mol O2 needed per second.

0.1813 mol / 0.00093744 mol/L = 193 L per second needed.

In post #16 there is mentioned 127 MW.

And it would be 285000 L of air needed per second.

Edited by Sensei
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• 3 months later...

You should check what air throughput a turbofan needs.

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• 3 months later...

Tesla has improved its batteries and you get a 14kWh Powerwall-2 for 120kg, 1.12m*0.74m*140mm and 5500usd. A battery-powered aircraft looks now less like a demonstrator and more like a vehicle.

The following example is adapted from DG Flugzeugbau's DG1001T, thanks
http://www.dg-flugzeugbau.de/wp-content/uploads/Flyer-DG-1001T_d.pdf
http://www.dg-flugzeugbau.de/en/maintenance-service-aircraft/scale-drawings-gliders

Modifications bring the 750kg to:
475kg frame
200kg two people with luggage
480kg four batteries
40kg propulsion and wheels
-------------------------------
1195kg take-off mass

The frame mass is kept by giving up the full +7g -5g aerobatic capability. Two main wheels let take-off autonomously; powering them for take-off would be easier than at airliner speed.

A longer wing chord keeps the speed. This reduces the lift-to-drag from >46 to estimated 30 at 40m/s = 144km/h = 78knt, so 391N provide the cruise speed. 75% efficiency need 20.9kWe, fitting the Powerwall-2 unit 5kW mean and 7kW peak.

The capacity lets fly 2.6h for 370km = 200nm range.

Marc Schaefer, aka Enthalpy

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• 10 months later...

The quick machines need AC in the kHz range, and sometimes MW power. PWM inverters are then difficult.

I had suggested to provide square voltages rather than sines to reduce the number of lossy transitions per cycle
http://www.scienceforums.net/topic/73798-quick-electric-machines/?do=findComment&comment=736205
but now there is an alternative, with my waveforms that suppress harmonics using few transitions. In the thread "quasi sine generators", for instance there
https://www.scienceforums.net/topic/110665-quasi-sine-generator/?do=findComment&comment=1041409

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• 5 months later...

The Norwegian government wants electric airliners for all flights under 90min in 2040
bbc.com and dailymail.co.uk

In 2018, I see how to fly for 90min with batteries, but to divert to an airport 100nm away then wait 45min in the air, as safety demands, a plane doesn't resemble a profitable airliner.

With liquid hydrogen and fuel cells, such a design is easy. The Atr42-72 modification I described on Apr 18, 2013
scienceforums.net
does much more than that.

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• 2 weeks later...

20 to 30 passengers, said Norway's minister, and 90 minutes electric flight. Here you are, with fuel cells, not batteries. It takes limited development, essentially the superinsulated tank I described earlier. The fuel cells exist at least for cars, the motors are nearly banal.

With the main gear at the fuselage and engine nacelles under the high wing, the Dornier 328 adapts to fuel cells easily, and its size fits better than the ATR-42.
de.wikipedia and (other variant) en.wikipedia
Can just the nacelles be retrofitted?

2*1400kW need 2*700kg fuel cells with the Toyota Mirai's performance
en.wikipedia

Estimated energy needs: 262kg liquid hydrogen in two ellipsoids D=0.9m L=4.6m.

2.2MW*5400s Flight
1.0MW* 600s Taxiing
1.6MW*1000s Divert 100nm @540km/h
1.4MW*2700s Waiting to land
Zero        Descent compensates ascent?
----------
17.9GJ      Energy at shafts
31.4GJ      Chemical energy in 130kmol *60% *95%

The motor is a ring D=0.8m gap and nearly 0.2m length, because gears need maintenance, can fail and they weigh too. With some 2*30 poles and sine three-phase, each motor weighs 250kg roughly.

This is the estimated mass change from kerosene to hydrogen:

 -800kg     Two turboprops
+500kg     Two electric motors
+1400kg     Fuel cells
-1300kg     Kerosene
+260kg     Hydrogen
+250kg     Two hydrogen tanks
----------
+310kg

Keep the airframe, take 3 or 4 passengers less. 32 had too little room anyway.

The hydrogen and its tanks being so light, more range is seducing. Extra-silent propellers would make sense.

Marc Schaefer, aka Enthalpy

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How would burning hydrogen in an ICE or turboprop compare?   More noise, but efficiency?

Also, SOFC output is so hot, it is used to run a turbine to increase efficiency in generator situations.  Could some version of this work to combine electric and thermal power?

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