World-record electric motor for aircraft
phys.org
phys.org
The top advantage of electric motors for aircraft is that they can fly at much higher altitude, therefore much lower air resistance, reducing energy requirements. Fuel-burning jet engines require a certain oxygen density to operate[1]. Electric motors don't need oxygen, and propellers/fans can operate even with very low air pressure.
[1] http://en.wikipedia.org/wiki/Jet_engine:
"The limit on maximum altitude for engines is set by flammability- at very high altitudes the air becomes too thin to burn, or after compression, too hot. For turbojet engines altitudes of about 40 km appear to be possible, whereas for ramjet engines 55 km may be achievable. Scramjets may theoretically manage 75 km."
How does that work exactly? Of course, my intuition is that lower air pressure means less to "push off of" but also less drag. And you've got certain constant overhead, like internal friction. Is there some sweet spot of air pressure where you'll get the best efficiency?
A candidate plane for this engine would be the Cesna Turbo Stationair.
-It is a six passenger plane with a base operating weight of about a ton.
-It's engine delivers 231 kW, which is slightly less than Siemens electric engine. It's mass is approx. 200 kg, so the Siemens engine frees up 150 kg of weight.
-It's fuel capacity is 237 kg of 100LL avgas. With an energy density of 44.0 MJ/kg, this gives the plane a stored energy of 10.4 GJ. The plane's max range is 1300 km.
-Tesla's best battery (the Smart) has an energy density of 132 WH/kg, which is 475.2 kJ/kg. That means you'd need 21.9 thousand kilograms of battery to store the same energy that's in the Cesna's tank when full.
-If we assume the plane's range is linearly related to it's stored fuel (an approximation not really accounting for take-offs), 237 kg plus the 150 kg saved in engine weight gives 0.051 GJ of stored energy, which would reduce the plane's range to 5% of normal, or about 63 km.
Based on this, don't expect an EV passenger plane anytime soon. Battery technology just isn't good enough yet. A hybrid with a gas-turbine generator is much more likely in the near future. This engine may also have applications where range is not important, such as training, stunt and racing planes.
it's not btw: http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/...
You also failed to read the word "hybrid". But not to worry, battery tech will continue to improve too. Fossil fuel efficiency? Not so much.
Insolation is approximately 1hp per square meter. The Cessna in question has about 16m^2 of wing, so that's 16hp of incoming sunlight. Solar cells are maybe 20% efficient, so that's about 3hp of actual electricity generated. That's not even counting cosine error, which will hurt you badly any time you're not flying at high noon in the tropics.
But apart from drones, perhaps a specifically designed aircraft to target this could help: lighter, smaller, larger surface area, slower cruising speed would reduce air-friction so increasing efficiency. Mightn't be practical to get down to self-fueling, but at least make a significant contribution to range. NB: longer flights collect more fuel.
Human-powered flight is possible using only 0.35 hp (0.26 kW), though superslow at 18km/hr. See Gossamer Condor http://wikipedia.org/wiki/Human-powered_aircraft
EDIT: here's the real deal: human flight, pure-solar, no batteries at all http://wikipedia.org/wiki/Solar_Challenger
EDIT2 Solar Impulse 2 is currently on a circumnavigation of the globe (they store power to continue to fly at night) http://wikipedia.org/wiki/Solar_Impulse At the moment waiting for ideal weather in the China leg
Next, the Stationair is a really old airplane, the basic design is over 50 years old. If we take the 2004-ish Cessna 400 instead, which has a 60% greater maximum range than the Stationair... perhaps we can claim that 500km+ is achievable, without calling upon radical new designs or further technology improvements.
And an airplane with ~500 km range is pretty sensible. (frankly, long trips in small planes are a bit of a trial, though probably less so in a smooth and quiet electric plane).
OTOH, you do need to consider reserve range, that does not work in favour of electrically powered aircraft. But the effect might be limited to having to add some battery capacity, sacrificing some performance along the way.
In conclusion... It does not even matter if the first commercially available electric airplane matches the petrol airplane on range. It would be nice to have a useable range, but ~500 km is enough for many applications. The electric airplane does not need to dominate the market from the start... just enter it.
The whole point of hybrid cars are the fact that you're only on the throttle periodically. You start, stop, speed up, slow down all the time (especially in S. Florida!). Normally when you're slowing down, you're burning off the energy as heat (friction from the brakes), etc. Adding an electric motor/generator to this allows you to recapture energy that is normally wasted slowing back down, so it can be reused to re-accelerate back up to speed. This is why hybrids (unlike non-hybrid cars) get better fuel mileage in town than on the freeway.
Aircraft, on the other hand, are at full-throttle on takeoff, and close to 3/4 throttle during the vast majority of the flight. There is no slowdown to recoup the energy until coming in for a landing. @beloch has already done the math showing the electric range would be downright miniscule, so there's very, very little to be gained here.
Until we have some utterly astounding advances in battery (or more likely, supercapacitor) tech (probably via graphene), this won't really be a cost-effective solution.
However, this would be an absolutely incredible motor for electric car conversions. Imagine a pair of these (each powering an axle)... it would be considerably more powerful than even the Tesla Model S P85D.
FTA: "This innovation will make it possible to build series hybrid-electric aircraft with four or more seats," said Frank Anton, Head of eAircraft at Siemens Corporate Technology
Things more annoying than airplane noise: * People who voluntarily choose to live near airports (which predictably have associated airplane noise), then complain about it. * The ubiquitous, intentionally loud motorcycles (at closest approach, much louder than any small airplane, due to power law d^-2), which all get a free pass for some bizarre cultural reason, and are largely inescapable.
Some of the airplane noise perception problem is jealousy, and is easy to fix: go down to the airport, chat someone up, and they'll give you a ride.
1. The engine can be sized for average output, not peak output, which makes it more efficient.
2. The engine can stay closer to its optimally efficient RPM.
3. Because RPM variation is smaller, and because power requirements are smaller, you can use more efficient engine cycles like the Atkinson cycle.
I'm doubtful that the advantages will be worth the extra weight. #1 is not going to be as big of a deal for an airplane, since as you note, power output is a decent fraction of the maximum during cruise. (In contrast with a car, which might have a 200hp engine but only use 30hp of that in cruise.) But it should count for something. #2 might make a nice difference paired with a variable-pitch propellor. #3 gives a decent efficiency boost. Put it all together and it doesn't seem likely to be worth the extra weight, since that counts for so much in an airplane, but it's not completely absurd.
Note for the Tesla comparison that the limiting factor in the P85D's power output is the battery, not the motors. Lighter motors would certainly be good, but it'll be a small effect. If you really want more power, you either need a bigger battery, or a battery chemistry that can discharge faster.
A quieter, more efficient aircraft with full auto-pilot could make general aviation attainable for far more people.
http://en.wikipedia.org/wiki/Lockheed_YO-3
The largest source of propeller noise is transonic flow at the tips, so turning the propeller slowly helps significantly. The tradeoff is a larger propeller and more expensive engine installation due to the required gearbox.
Imagine if for take off and landing the pilot can fire up the existing jet engines, then whilst cruising fire up the electric engines and wind back the output from the jet engines (leading to a slower burn rate). You would still be using both, but you would have some additional boost from the electric engines. Put some lightweight solar panels on the roof of the plane, charging up the batteries while the plane sits on the tarmac and readies for taxi. Take the output of the existing engines and use it to charge the batteries in flight (like an alternator in a vehicle, but on steroids).
Of course, weight is the limiting factor of all of this. I am more thinking out aloud here, when it comes to reducing our reliance on fossil fuels, I think anything (even a marginally small improvement) is a great step forward.
The future is looking great.
Does the thrust generated by a turbine engine hit some point of diminishing returns that converting the engine energy to electricity and powering this engine be more efficient?
I know Boeing have made advancements in the efficiency of drawing electric power from a turbine engine with the 787, the generators are now directly connected to the transmission of the engine.
I'm sure there are details other than power and weight which makes a direct comparison unfair.
I really expected this to be about high-temperature superconductor motors. Whatever happened to those?
[1] http://en.wikipedia.org/wiki/Autorotation
[2] it's probably theoretically possible that a quad with reversible motors (or reverse pitchable blades) could be built to survive a single motor failure, but so far as I can tell nobody, at least in the hobby size quadcopter world, has done so (thoughI'd be surprised if the KMel Robotics and Pennsylvania University research teams don't know exactly how to do it).
It's not the motors that are the problem. They can spin freely any direction. It's programming the ESCs to be reversible.
Anyway it's already been done a hundred times over:
- Fixed pitch props but reversible ESCs https://www.youtube.com/watch?v=UoReJULASkw
- Variable pitch prop styles exist such as HobbyKings Reaper https://www.youtube.com/watch?v=mAFxkHAiimY and Curtis Youngblood's StingRay http://curtisyoungblood.com/V2/products/quadcopters/stingray....
Autorotation on quadcopters is possible with the variable pitch configuration.
> It's not the motors that are the problem. They can spin freely any direction. It's programming the ESCs to be reversible.
True - poor wording on my part. And even the ESC programming is a "solved problem" - all the brushless RC cars run ESCs which understand how to run brushless motors in both directions.
So actually reversing the thrust is, as you say, already happening. What I've not seen (yet) is a controller board with software designed to use that ability to stabilise and safely land a quad with one motor out.
https://www.youtube.com/watch?v=Scj8_XEEL1A
http://curtisyoungblood.com/V2/products/quadcopters/stingray...
By the way, it's worth noting that there are also quads with 4 electric motors that can reverse so you can fly upside down: