I can't wait for electric/hybrid aircraft.

[IO540]

At the end of the day all the power to drive the car has to come from the petrol or diesel engine.

That's not quite so.

1) A car spends a lot of its time at low power, which is OK with electric but a petrol engine is inefficient, and would be better used running at a higher power setting charging the batteries. In fact the efficiency situation is so poor that a small gas turbine (which is normally a lot less efficient than a piston engine) is viable, just to run more or less nonstop, as the battery charger - e.g. here. (I know the people who designed that one).

2) You can do regenerative braking

3) You can do direct drive to the wheels, which avoids gearbox (and diff) losses

But I agree the technology is in its infancy; IMHO largely because nobody has yet been able to do a commercially viable design "properly" i.e. direct drive brushless motors on each driven wheel, and a turbine charging the battery.

There are some major long term issues. If all current liquid fuel vehicles were suddenly charged from the mains, one would need to build many more power stations and totally overhaul the distribution grid. Especially with the steam cycle limiting PS efficiency to around 50%, it would mak sense only with nuclear fusion, which has had a great future for about 50 years now And, because mains electricity is currently much cheaper than petrol (per kWh), electricity would have to rise in price massively. Not sure everybody would like that.

[AdamFrisch]

The fuel cell is looking a lot more like a non starter. For the huge cost (mainly due to the platinum needed in them) and difficulties with hydrogen production and storage. Very little advancement has been done recently.

I think power cells and batteries have a brighter future.

Yes, the microturbine running a generator is interesting as it delivers pretty good efficiency at max rpm and at altitude. It's also lightweight. And the turbine stage that goes out to a shaft to generate power can also be made a lot more efficient than if you would just take a turboprop type solution - the turbine stage at the end that converts the hot gases to shaft power could for instance be made as a Pelton turbine. Peltons are very efficient at capturing high speed streams and converting it to shaft power.

Or the free piston generator is also a very interesting development, albeit not as light or vibration free as a turbine. But probably a bit more efficient. Basically, it's just an opposed piston running in a tube, but instead of converting the linear motion to a circular motion in a crankshaft that then drives a generator (with all the losses that entails), the pistons themselves act as a magnets and travel back and forth inside a coil (AC current):

Free Piston Power Home

Here's a clip showing a simple free piston generator (ignore the Stirling bit):

YouTube - RESONANT STIRLING ENGINE GENERATOR

Or here:

YouTube - Ubiquitous Electric Power Generation Devices : DigInfo

[rahosi]

There aren't enough rare earth elements available for the requisite magnets in DC motors. If adopted, the price would go through the roof, way beyond what the auto industry could / would pay. Similarly there is a supply problem with Lithium, if that is the battery technology employed. See the Jack Lifton Report (sign up to read part 1)

In hybrids, one of the limiting factors is that electric motors overheat, killing the magnets permanently. It's one of the principal reasons that hybrids switch back to ICE drive. Thus no point in having large battery capacity!

Having hub motors might lead to serious unsprung weight issues in cars.

Employing a small ICE running at constant speed / load solely powering a generator which would both power the electric motor and/or charge a battery and/or other storage technology, would provide significantly higher MPG.

Wheeltug, as mentioned above, is based on the Chorus Motor which doesn't have magnets. It has automotive potential See ChorusForCars.


There hasn't been much news from Wheeltug recently, but things are pretty busy on the inside.

(I'm involved)

[horizon flyer]

New Battery technology

There is a new Lithium battery that is due out in under 5 years.
Lithium Air, this battery could have an energy density of up to
5kwhours/kilogram. IBM are working on it and Japan with some success.

As Avgas is 12Kwh/kg but engines are only 25% efficient then it means only 3kgwh/kg is used, as electric motors are 85% then any battery about 4kwh/kg or over is about the same as Avgas.

These batteries can be recharged or the lithium & electrolite with a cassette can be replaced to recharge them, 10 minute to refuel. So a rechargable or lithium fuel cell.

Please forget about hydrogen it will never happen, just to difficult to implement and lower over all efficiency never mind the cost.

So by 2020 nearly all cars will be direct electric drive and light weight in wheel electric motors/brakes, no mechanical brakes, will be normal, 120Kw at 66lbs. These are available to day, Volvo and Ford are building demo cars and trucks with them.

So electric aircraft will be feasible, with good range and carry capability.

[IO540]

There aren't enough rare earth elements available for the requisite magnets in DC motors. If adopted, the price would go through the roof, way beyond what the auto industry could / would pay

I don't think so. Brushless motors are used all over industry. Everybody and their dog makes them.

[worrab]

Brushless motors are used all over industry

They are, but more routinely using AC. Some of the more interesting DC motors use masses of electronics (eg the so-called "Switched Reluctance (SR) Drive) to improve efficiency, but possibly making them difficult to get certified for aircraft use.

The key issues for an electric aeroplane are likely to be energy density of power sources and airframe efficiency. Energy recovery, in-flight generation and reliability may have a part to play, but until the first two are cracked, routine powered flight of significant duration is inevitably limited (though motor-gliding is another matter).

I don't agree that the noted factors afflicting light aircraft petrol engines are

actually non-issues

. Too often do we see the probable cause of a crash as "probable carburetor icing" - something that modern automotive engines have long ago left behind. On the other hand, GA engineers are - quite rightly - a conservative bunch and it can be instructive to look at the more and less reliable LAA engine installations to see that car technology doesn't always transfer happily into the air.

[IO540]

Some of the more interesting DC motors use masses of electronics (eg the so-called "Switched Reluctance (SR) Drive) to improve efficiency, but possibly making them difficult to get certified for aircraft use.

I wasn't thinking of the common induction motors which are indeed everywhere and have been for ~ 90 years. I was thinking of 3-phase brushless motors which are controlled from electronic drives, which take in either DC, or AC mains power. These motors are big versions of the 3-phase brushless motors used in model planes, boats, etc. Industry is full of them - conveyor belts, cranes, you name it. One customer of mine has a production line chucking out a motor controller every 10 or 20 seconds, and each of these will be used with a 3-phase brushless motor, with an iron core. There are some fancier designs but they go into less common applications. For an aircraft you would have just a dead simple 3-phase brushless motor driven from a controller using IGBTs or some such.

The technology for this is several decades old and anybody could take an existing plane (my TB20 would do just fine) and stick a 250HP direct drive brushless motor in there (a suitably ruggedised version with a massive shaft and bearings because it has to carry the prop and the axial loads) and an off the shelf variable speed controller, plus a lump of concrete for W&B reasons because the assembly will be so much lighter than the old engine, and ..... sit there and stare at it because it won't be going anywhere

The battery, and how to charge it in practice, will be the major issues.

It goes without saying that certification would be a major project, regardless of what motor is used. The motor will be the easy bit. Demonstrating safety of the battery would be something else. All you need is a defect within the battery, or a short downstream, and you have a massive fire - unless the battery technology is very different to the present LIPO ones.

[AdamFrisch]

You will also have to contend and design for pretty high voltages as rpm is a function of volts in these motors. There are ways around this with big outrunner motors that turn slowly or with parallel windings. But as it is with the motors now used in electric experimental flight (often adapted from r/c), they're often running around 200V or more and that is a killer should you come in contact with it.

As mentioned, the brushless motors are not the problem from a certification standpoint, but the batteries and the power distribution is. We'll see how this develops in the coming years.

[worrab]

I don't think 200V per se is that big a headache - though 200V DC can get quite interesting when it comes to handling faults. I also suspect that a fuel cell - perhaps a methanol type - would not be an entirely dead duck. And it's quite conceivable that a novel fuel linked to a novel fuel cell coupled to a modern control system that manages a stock motor could be more reliable than a 40-year old LyConting. But all that new-fangled stuff requires R&D for GA application with certification on top and I suspect that there is your problem.

(It'd be one fabulous PhD project though wouldn't it!!)

[david viewing]

The micro APU idea has been a dream for a very long time. Nevil Shute (Norway) wrote about it in his lovely model engineering novel "Trustee from the toolroom". Isaac Asimov has a micro APU of prodigious power in "Foundation and Empire" that welds itself into a solid block of metal in the event of tampering. And Scott Crossfield writes eloquently about the tribulations of the micro turbine APU's in the X-15 that would seize solid inches away from his head in "Always another dawn". But despite this, for 50 years or more, the micro APU has remained elusive.

One reason is certainly materials science - bearing cooling being one and demagnetisation being another, both related to the immense energy density required in these hypothetical machines. Even the tiniest inefficiency, converted into heat as it must be, will push the machine past it's materials limits. Perhaps these restrictions will be overcome, but if they are, I suspect there will be many more pressing (and financially rewarding) applications than private aeroplanes.

I'd love to see a successful micro APU giving perhaps 100KW for maybe 20Kg or something in place of diesels weighing 1/2 ton or more that never seem to work when needed. There would be plenty of server centres, hospitals, and so in line for that.

[IO540]

Not sure. Diesels are dirt cheap and they work often enough to be useful, especially if you have more than one

There was a well known "innovative" company (my shares in it were down 99% when I sold them ) which tried to take on this market and totally failed. They did develop some generator technology which can work at the high RPM that you get out of a turbine and are trying to sell that, AFAIK.

I suspect the existing engines will see most of us out...

100kW in 20kg would, assuming a small package and even high efficiency (say 50%) mame an impressive space heater You'd need quite some airflow to get rid of that 100kW loss when trying to make a 100kW real output.

[A_Pommie]

Aren't DC brushless motors just normal 3 phase ac motors driven by half a variable speed drive?
If that is the case then the weight of a 100hp motor is going to be something else. We have a fair few at work and they come in at significant fractions of a tonne. Yes you could shed some weight from the casing but the copper windings are going to be a serious weight. Unless you intend to run at 400vdc then the current draw for full power will mean some pretty heavy gauge wire involved.

Also aren't LiPos prone to failure or worse unless charged very carefully? So not only do you have to lug a shed load of copper into the air you then need to drag along a complex battery charger as well.

I suspect that this sort of thing won't be certified anytime soon the market is far to small to recoup the cost. It may take off in a small way for permit types or more likely EAA experimentals but spam cans won't be electric without some major advancements in superconductors.

[AdamFrisch]

They're very light even with all that copper winding. I can get an off the shelf 15Kw (21hp) DC brushless r/c motor that weighs 2 kg. Add the controller to that and you're looking at 3-4 kg, tops. There is no combustion engine that comes close to that, not even a turbine can deliver that kind of power-to-weight ratio.

All one has to do is look at the revolution in the r/c field. The electric aircrafts shatter everything and are not even allowed to compete with the nitro/turbine ones because they cleaned up so much - they had to create an electric class just for them. Now, naturally they don't need to take much notice of endurance, so they can use light battery packs and overpower them, but still.

Electric makes sense. We just need a way to store the energy now. When that's licked, it's game over for the combustion engine.

[IO540]

Aren't DC brushless motors just normal 3 phase ac motors driven by half a variable speed drive?

Yes, usually. They have a very high power to weight ratio. IMHO a 250HP brushless motor would weigh ~ 50kg. It would draw of the order of 2000 amps at say 100 volts; this is relatively trivial in terms of the DC-AC inverter.

The higher the voltage is used, the more efficient the electronic inverter can be, but they are very efficient anyway - well into the 90s % region.

There really is absolutely NO issue with the motor. If not off the shelf (due to the thick shaft with a prop flange on the end ) it would nevertheless be trivial. The inverter would be trivial too; this is off the shelf technology made by many firms e.g. this.

The battery, and charging it practically (where from???), would be the fun bit But the charging process is again straightforward and established technology for LIPO or whatever.

In the RC model world, the funny thing is that the "poor" flyers use fuel. Electric stuff is better in every way but it is "less authentic" and most cannot afford to buy enough of the £100 batteries to last them the day out My son has had a few of these and smashed them up in the obligatory crashes. One of bis brushless motors had to be dug out from about 6" deep in the soil, but iw was fine after some cleaning up. It soon gets very expensive and now he flies mostly with the nitro engines. But the other day he blew a hole in a piston so that was his savings gone for a bit

The thing I find more puzzling in aviation is why the hell do autopilot servos use brush motors. What you are supposed to do - and this applies to a TB20 as to a TBM850 or a King Air - is to fly until the motor (i.e. the autopilot) fails because there is nothing left of the brushes or the commutator (the two seem to have a similar life). There is no precautionary maintenance, the brushes are not replaceable, and the "motor repair" is a whole new or overhauled servo at at least $2000. This application is crying out for either brushless or microstepped stepper motors but there seems to be a total lack of imagination in the industry which keeps turning out the same old garbage. I wonder if the new Garmin autopilot uses brushless motors...

[Charles E Taylor]

An Interesting Thread

Some photos of the current, flyable, state of the fuel cell art for small aircraft.


Not much room left for anything else!


Boeing Fuel Cell Work on Flickr - Photo Sharing!

Boeing Fuel Cell Work on Flickr - Photo Sharing!

Boeing Fuel Cell Work on Flickr - Photo Sharing!


This will not be easy or soon.





Charlie

[worrab]

Any idea what the fuel is ?

[Charles E Taylor]

The Fuel is H2O








Charlie

[Charles E Taylor]

My previous post was much to short!


This is a Hydrogen Fuel Cell / Lithium Ion Battery Hybrid Concept.

More Information on this project can be found here.

Boeing: Boeing Successfully Flies Fuel Cell-Powered Airplane







Charlie

[worrab]

The Fuel is H2O

- That's the waste product!! You can use (typically) hydrogen, methanol or ethanol to feed a fuel cell. Water can be used as a fuel, but only if you split it into H2 and O2 first - which obviously takes more energy that the fuel cell delivers in electricity.

Hydrogen is bulky and heavy to store and perceived as dangerous but makes for quite efficient power conversion. Methanol is easier to store but poisonous. It's difficult to get good efficiency. Ethanol is easy to get hold of (ferment anything you like) and is already part of the distributed fuel chain. However, ethanol cells are in their infancy. All can be environmentally neutral, ultimately simply releasing recently stored solar energy (unlike many so-called environmentally friendly electric vehicles which for the large part just move the point of pollution further up the chain)

Given the huge investment going in to fuel cells we should see some significant steps in the short and medium term. The problems (and I'm sorry to say I'm merely a bystander in this) seem to be around the catalyst that promotes reaction.

[worrab]

In the end, the challenge is energy density. You can get a lot of heat out of a kilogram of avtur and rather less from a kilogram of lead-acid battery. Battery technolgy is still at the wrong end of the scale.