Hydrogen offers better prospects for civil aviation than batteries do. Airbus did a study[0] that showed it would be possible to power all civil aviation use cases with cryogenic hydrogen. They even showed that in the near term existing aircraft could be modified to run off of hydrogen for short and medium range. Getting longer ranges would require new aircraft though. They also believe that we could see short range and medium range hydrogen powered aircraft introduced as early as 2015. Of course it's 2018 and this hasn't happened, possibly due to the cost of producing hydrogen.
Hydrogen is worse than electric on almost all fronts. Others here have covered the storage issues, but there's also the issue that generating hydrogen is very inefficient compared with just storing the electricity in batteries.
And yet the japanese are still spending money on refuelling stations for hydrogen cars. I don't really understand it, given EVs are taking off by comparison.
Cursory googling claims you can generate hydrogen for $28/million-BTU with electricity @ 5c/kwh. 1 million BTU is about 8 gallons worth of jet fuel, so I would guess that jet fuel is much cheaper than $4/gallon?
Cost runs actually about the same, jet fuel is about $5 dollars a gallon probably cheaper wholesale the issue comes from even though hydrogen produces more 3x energy per kg than jet fuel the energy density is 3x less per unit volume. Meaning you would need bigger fuel tanks per distance travelled which would require modifying the airframe to be bigger in addition to having to add insulation.
Hydrogen offered better prospects for cars than batteries do... Toyota and others can show you many studies which support hydrogen for cars. Where are the hydrogen-powered cars?
I believe that you’re thinking of Helium. As @deelowe said, the issues with a Hydrogen are safe and stable storage, and the tendency of Hudrogen to penetrate metals and make them brittle. Producing hydrogen is trivial with electrolysis or water, which could be achieved using nuclear and renewable power sources. Of course burning coal to produce Hydrogen is currently cheaper and more common, and an environmental disaster.
The problem with Hydrogen is that you want to store it in big, strong metal containers, but it destroys the container and leaks out. So instead you store it in composite fiber containers, but then you have an explosive gas under pressure in a vessel that can be cracked.
This is part of the reason that research tends to focus on processes within fuel cells which produce hydrogen JIT, so you obviate the need to produce and store large volumes. The problem with that is the need for more power input than you get out, or the need for expensive catalysts like Platinum.
tl;dr
With current tech you need nuclear power to efficiently produce large volumes of Hydrogen, but that’s a political minefield.
Storing Hydrogen is a nightmare.
Hydrogen explodes, and peolle are rightfully wary of carrying and transporting it in large volumes.
The kinds of containers which are affordable and practical are made of materials Hydrogen readily attacks.
I've only seen Peter in his guest spots on Flite Test. One of the sweetest and coolest dudes on that channel. Always down to experiment and obviously thinks big.
I was just about to post this same video. The trouble they had with the big generator at the field site was interesting. I imagine that like a Tesla it can charge from a 120VAC 15A (or euro/world 230-240V, 10A circuit) overnight, but it will be much, much slower to reach full capacity. It's good that they honestly state the limitations that it's best suited for flight training applications where it will always return to the same field.
Imagine trying to fly that thing from Perth to Adelaide in individual 50-minute hops between outback airfields... Impossible.
Yeah, the reality check in that video was eye opening. Clearly it’s a deliberate strategy to roll these things out at flight schools at first, so they’re always circling within landing range.
Electric planes will definitely graduate to short hop commutes eventually though and not just be niche training vehicles, especially when you consider how much safer they are than jet engines: they have way less moving parts, require less maintenance, and most importantly, they don’t stress the airframe nearly as much.
And that’s just with current airplane designs retrofitted for electric (which is essentially what these training planes are) instead of the upcoming designs that are built with many small electric motors and not the giant jet engines.
Lilium is an example of the new style of electric plane, with motors distributed through the wing:
Yep, and quiet electric motors is why air taxis could be a real thing again in major cities. E.g. Uber partnered with NASA to try to bring them to Los Angeles before the 2028 Olympics, with 18 heliports scattered around the county.
Also, 30 minutes is long enough to just do a few take offs and landings to stay current on the cheap, or just for doing short lessons or flight reviews. I could definitely see a flight school having one of these around just to meet those needs as cheaply as possible.
Agreed! They didn’t even mention the manufacturer of the airplane but mentioned the relevant government initiative at least twice. Reads like a city hall press release.
Considering how hard it is to get new technologies pushed through municipal red tape, the framing of this as a “city hall press release” was likely the manufacturer’s idea, part of a deal they made in exchange for some consideration for changing municipal airspace regulations to allow testing.
Municipalities don't have airspace regulations. That's all managed federally. Also the Alpha Electro is already legal to test. It's apparently registered as an Experimental aircraft in the US currently, which means it can be used by owners but not commercially (e.g. for flight training). The market Pipistrel's aiming for is the Light Sport Aircraft category, but electric planes can't be registered as LSA in the US so Pipistrel would have to go through the standard airworthiness certification process. Which is long and expensive.
Looks a lot like a Cirrus SR20. But I imagine that for aerodynamics and efficiency reasons, 2 to 4-seat sized, fixed gear, low wing, light aircraft will generally converge on a similar design approach.
One of my friends has the Alpha Electro. It's an amazing airplane. I've had the awesome experience of flying it with him. Definitely excited for the future of flight. Here are some videos of him flying it:
I wonder though if you could do "regenerative braking" in electric planes, i.e. spin up a flywheel anytime you descend, and then tap that energy next time you want to ascend. Doesn't really make sense for planes that cruise at altitude, but could make sense for hobby craft.
This very same airplane mentioned in the top article does in fact capture energy from descent, and it's only viable because the application is assumed to be training, where the point of the airplane is to do a lot of takeoffs and landings.
I think there's plenty of viability for recovering energy during a descent. Any aircraft that has spoilers for use during descent could instead use the energy extraction from the prop windmilling to reduce the need for spoilers and thus get some "free" energy during descent. It isn't going to be a ton of energy, but it might be enough to get a "free" taxi to the gate/hangar/tie-down out of the deal.
It's not quite that direct. Many aicraft use flaps to boost lift during descent (better low-speed control) and steepen the fuselage pitch relative to the direction of flight (better visibility of the ground). Putting that energy into the propeller doesn't help with either of those. Also, some of the speed brakes are deployed so that the engine can stay warm by producing power instead of being "shock cooled". Otherwise stress builds in the engine and results in the engine wearing out earlier than it otherwise would if it were consistently cooled slowly. So this also doesn't directly translate; the pilot of the electric plane simply pulls back all the power and is coasting, which for many airplanes (not gliders!) results in a satisfactory sink rate and no need for additional braking. The pilot could engage more braking, but then the sink rate would be "emergency" class.
Flaps and speed brakes are separate. You wouldn't be using regenerative braking for the same purposes as speed brakes/spoilers. You use spoilers specifically when you want to kill speed, or more often when you want to quickly lose altitude. Sucking potential (and kinetic) energy up and putting it in the battery would accomplish a lot of the same goals.
You're not likely to have shock cooling issues in an electrically powered aircraft. Shock cooling is a piston aircraft thing, and is mostly an issue on turbocharged aircraft. You're not worried about shock-cooling the engine, you're worried about shock-cooling the hot side of the turbocharger. So in the use-case of spoilers being used to keep the engine spooled up, you're right that it wouldn't be necessary here.
And as far as pulling the throttles to idle and more-or-less gliding during the descent, that's the ideal, but in the real world spoilers are used to drop more quickly all the time for non-emergency reasons. If you have favorable winds up high, you might choose to stay up high until absolutely necessary and then quickly descend to make better time. In trainer class aircraft you don't typically have spoilers to help this, so you end up cross controlling the aircraft to increase the sink rate. If I could have a big ol windmill in front of me in a 172 to do that, it'd be pretty darn nice.
The problem is that you'd have to descend rather steeply for any sort of "braking" to work. In practice, you're unlikely to get more range by descending steeply and then using the power gained from that to fly level than you would get by descending at best glide angle.
The slower your plane move, the more difference those would make.
As people already commented, it wouldn't make much difference for the usual models, but you can design a plane that benefits from it. It may have some good uses (agriculture comes to mind), but probably won't be good for transportation.
Not for substantial cargo or human transport, the areal energy density is simply too low.
For surveillance, comms, and very small-scale payload delivery (possibly drugs, far more likely munitions), ultralight drones with battery + solar could offer loiter / time-in-air and / or modest speed longer-distance capabilities.
Not in any meaningful way with just the sun's light hitting the panels.
On the other hands, if you were to beam a few hundred kilowatts by (carefully!!) aiming at the panels with a massive laser, you could power it this way, yes.
Bonus if you are doing it from a satellite: you can power a plane even across an ocean and don't need many ground stations. On the other hand you just built an orbital death ray so expect protests from, well, everybody.
It might. Wings have a lot of surface area, so it makes more sense than putting solar on cars. Also, if a plane isn't being constantly used it could be useful to have it charge while parked (which is also a good argument for putting solar on cars even if it doesn't noticeably improve range).
It would also provide a charging option if you have to land at an airstrip that doesn't have an electrical outlet handy.
Wing area of a Cessna 172 (representative GA aircraft) is 16 m^2.
Best-case insolation on a sunny day with no clouds at high noon is around 1000 W/m^2.
Best commercially available solar cells are the triple-junction cells used on commercial satellites, at about 30% efficient. They are horrendously expensive (around $50-100k per square meter), but we will ignore economics for now.
16 m^2 * 1000 W m^2 * 0.3 = 4.8 kW.
Powerplant for a Cessna 173 is a 160 hp piston engine, or 120 kW. So, we're about an order of magnitude and a half off. The cells also aren't massless, so they will add weight and reduce range... you'd need to do a cost/benefit there.
There are a couple solar UAVs in existence (example: https://en.wikipedia.org/wiki/Qinetiq_Zephyr) which use super lightweight materials and have very large wing areas to support solar cells. Even then, they are really on the hairy edge of where physics works in your favor.
Most of it is still used while cruising. 100hp+ depending on cruise speed. The excess power for climb is very small, which is why weight/loading is so critical in aviation.
> The excess power for climb is very small, which is why weight/loading is so critical in aviation.
This is one of the reasons in favor hybrid electric aircraft. The marginal hp to weigh ratio of an electric motor is about 3 hp/lb or ~5.5 kw/kg. And partial load efficiency is high. Means you potentially have a lot more hp available on take off.
For commecial passenger aircraft hybrid turbofans wouldn't have spooling up lag like straight turbofans. Turbine engines take seconds to spool up. This big big problem with jet aircraft during landing. If you hit wind shear and/or need to go around you need to apply more power and sometimes the lag is fatal.
Aviation is an expensive market, so in some future world where a 75% efficiency of solar cells was available, it would be possible. And maybe even economically feasible at some point. Not any time soon though.
And the great thing about flying is that it's pretty much always a sunny day when you're in cruise.
For electric planes I'd like to see dual battery dual inverter/motor twin props for redundancy. Sure I trust a good electric more than gas, but why not offer more than just that?
Power-to-weight ratio. It's a major weakness of electric power with current technology, and a single engine with a big prop is more efficient than any other configuration.
Sometimes you see electric aircraft have multiple smaller engines and props, but that is mostly driven by structural or design concerns of their esoteric missions like needing to distribute the weight along a huge wing (Sunseeker) or needing to land in tight spots (various VTOL designs). Conventional planform electric aircraft don't have such extreme limitations so there is no point in sacrificing performance.
Power to weight ratio's of electric motors are higher than piston engines. But the usable energy to weight ratio of batteries is 5-10 times worse than a gasoline tank.
I seen some stuff that indicates that you can use small props at the wing tips to reduce drag. Works by canceling out the wing tip vortexes. Hasn't been done for a lot of reasons but mostly because of structural concerns.
I think one prop is more efficient than two for the same thrust. Multi prop airplanes are potentially tricky to fly.
Given the fuel density problem it's interesting we haven't seen any hybrid designs that generate electricity with an ICE engine (like the range-extenders in some PE cars like the Volt or the i3). I assume the added weight of the engine would cancel out some of the benefit.
I think Airbus and others are poking at hybrid turbo fan designs. One reason is that high bypass turbo fans are hitting limits due to fan diameter. If you use electric motors to drive the fans you can have multiple fans per engines. Like a 2:4 configuration, 2 turbines feeding 4 fans. Some advantages to this.
1. Much higher bypass ratios. Maybe close to double.
2. Ability to shift power around to keep the aircraft thrust balanced in case of an engine failure.
If you have a battery then you also get.
3. Potentially much faster throttle response. The slow throttle response of turbine engines is a real issue that kills people.
4. Ability perhaps to take off and land with minimal turbine power. Which reduces noise.
Thing is all this stuff has only recently gotten cheap enough and commercial aircraft probably take 10-15 years to develop and roll out.
There are hybrid designs, and you can also do tricks like charging the batteries from running a prop backwards when descending... similar to regenerative braking in electric and hybrid cars.
You'll note that hybrid cars aren't exactly winning in the market.
There isn't much point to a hybrid gas/electric aircraft. Most of the value in cars is converting kinetic energy back to electricity when stopping. There's a bit from running the engine at peak efficiency as often as possible - if that produces an excess of energy you charge the battery for a while and then run in pure electric mode for a bit (engine off). But aircraft tend to run at high power all the time and don't stop much. Same thing with boats.
This is a good idea though, it gives you some flexibility in trading off range and payload. Probably maximum take off weight is the limiting factor here. But if it's just me, and I have 200 lbs of extra payload, why not load that up with 20 gallons of fuel and a tiny ~25hp generator.
Apparently these guys are doing wing tip propellers for their electric plane. They are expected to make a first flight later this year, if I recall correctly.
I was under the impression wing tip vortices were an inevitable result of having air deflected downward under the wing. Do wing tip props just change the behavior near the center of rotation, or eliminate vortices completely?
One way is that the coefficient of lift drops as you approach the wing tips. The other is that energy used to generate/contained in the wing tip vortexes shows up as a decrease in efficiency.
A wing tip prop that rotates in the opposite direction cancels the vortexes. And the pressure produced by the prop increases the Cl of the upwash side of the wing. Negating the loss of Cl due to spill over at the wing tips.
This is from two papers I read one from 1967 and the other more recently. The result of looking at this thing.
I know that, for fixed winglets, there are a couple of (equivalent) ways of looking at them. The first way to look at them is that they resist the tendency of higher pressure air under the wing to leak around the wing tip to the lower pressure air on the top. The second way to look at them is that the winglets are flying in a circulating flow, and a portion of their lift is in the direction of travel of the airplane. (That one is trickier to see, for some.)
My educated guess is that tip propellers would operate counter to the vortex direction, so that their vorticity would tend to cancel the trailing vortices. Notionally, you could think of them as throwing air back under the wing, though that's just a heuristic I made up to help imagine the direction they'd spin.
Electric motors can also be stacked multiple motors to one shaft, a trick that has also been put to work to bond an electric motor to a gasoline engine to provide "boost" at takeoff.
Existing gasoline single-prop engines typically have dual redundant ignition systems with segregated cylinders powering a unified driveshaft and camshaft, and I think your split inverter/motor/battery path is entirely in line with that approach to single-prop internal redundancy. It depends on the weight of the components, of course.
Multi-engine planes are more complex to fly and require a different license. If an engine quits during takeoff, the plane is going to yaw toward the failed engine side, among many other issues. The plane doesn't just fall out of the sky when the only engine quits, anyway - you can glide quite some distance, provided you've got the altitude to do it.
I never really understood this. Sure, it's complicated to compensate for asymmetric thrust. But if you know how to fly a single-engine plane with no engines, surely you can figure out a two-engine plane with one engine? At worst you can turn off the functioning engine and be back in the situation you understand.
The FAA publishes a couple of books, including the Airplane Flying Handbook [0], that has a good intro chapter to multi-engine flying [1] that goes into some of the issues that come up.
If you put them in back closer to the centerline it helps minimize that yaw and roll tendency. On the other hand, I picture one on each wing to minimize the length of cable needed to deliver the current. Cables for 100-400A add a couple pounds per foot.
They mention it is for flight training but a big part of flight training is fuel mixture/engine management. It would be interesting if someone were to train for their pilot license in this if they would be type restricted to only electric aircraft.
> a big part of flight training is fuel mixture/engine management.
When I was taking my private pilot lessons, this was a very minor thing - maybe a couple of minutes spent on it every other flight? The only time we spent more than a couple of seconds adjusting the fuel/air mixture was when we were intentionally stalling the engine to practice recovery procedures.
And with a private pilot's license, before you would be allowed to rent any plane you generally have to show you've been checked off in one - that's one of the things your log book shows.
> intentionally stalling the engine to practice recovery procedures
Are you talking about stalling the aircraft or stalling the engine? As far as I know, intentionally shutting down the engine is considered too dangerous for PPL training purposes. Whenever a zero power situation is practiced (e.g. aerodynamic stall or simulated engine failure) it's done with idle power.
It's a big part of the written test, but gets pretty much a cursory glance come checkride time.
Examiners will likely ask about the FAA fuel minimums, but those are expressed in minutes of flight time, not gallons.
Also, the regulations tend to place a great deal of trust on pilot's judgement. According to FAA a person who just received his private pilot certificate in a Cessna 152 is eligible to fly a Pilatus with Airplane Single Engine Land PPC. Is it legal? Yes. Is it advisable one does so? No.
> but a big part of flight training is fuel mixture/engine management.
But aren't most pilots ultimately training to fly turbofan or turboprop planes?
Is fuel mixture and engine management for a piston-engine plane relevant to that?
I'd imagine that the jump from electric to larger planes is not much bigger than from a piston-engine flight training plane. But I'm not a pilot so I'm just speculating here.
Regardless, I'm sure it's easier to do everything but engine management first, and then do fewer sessions with just focus on engine management.
>> Pipistrel introduced an electric version called the Alpha Electro in 2015 at a price of 69,000 euros,[3] with technology from the Pipistrel WATTsUP proof of concept design, for short training. It has energy for one flight hour plus reserves, and can recharge in 45 minutes or have its batteries replaced in 5 minutes.[4] Instead of 78 lb (35.5 kg) of fuel, it has 277 pounds (126 kg) of LiPo cells, however the water cooled electric motor weighs 11 kg;[5] much less than the gasoline engine. It has a useful load of 380 lb, whereas a Cessna 152 has between 350-480 lb useful load.[6][7]
There are two packs, one forward and one aft, each 53 kg (~117 lbs) for a total of 106 kg (~234 lbs).
For comparison, the same weight of avgas (39 gal) will fly a Cessna 172 for just under 4 hours with reserves (assuming 9 gph), compared to the the Alpha’s 60 minutes.
These batteries don't get lighter as they fly though do they?
I heard that airliners can't land with a full load of fuel and they have to dump it to land in an emergency. I wonder if that means you can't even replace max fuel weight with the same battery weight because then it'd be permanently too heavy to land.
I wonder if airliners could drop exhausted battery packs by parachute over designated DZs as they fly across the continent or ocean?
Jettisonable batteries (or lift magazines of battery + motor) are one design that's been explored. Take-off and initial climb is a major fuel-demand flight phase.
Typically they are equipped with either a Continental O-300 series engine from the 40s, or a more modern Lycoming O-320/360 from the 50s. They hark back to an era of magnetos and manually adjusting the fuel mixture. On the plus side they're so mechanically and electrically simple that there's very little to break. On the downside they're pretty inefficient and carburetor icing is still an issue. Also, they need leaded gas still.
I'm not kidding about the efficiency either. The Lycosaur O-360 is a 5.9L engine that produces at best 225hp. You can get 245hp out of a 2L Ecoboost in an economy car today. This isn't an apples-to-apples comparison, but it gives you a sense of how far engines have come since the 50s.
A Cessna 172 retrofitted with a modern engine could probably get more than a 50% increase in range, and also cabin heat.
I did a bit of research on this about six months ago, after watching a video of some guys who ferried a Cessna 172 via Nunavut and Greenland to Western Europe.
Google "thielert diesel engine" and you'll find a lot of good info to start from.
They don't use magnetos because they're scared of modern tech, its because magnetos self excite and don't need an external power source to start or keep running. And that modern 2 litre engine isn't going to put out max power like the aero engine will.
The aero engine is also air cooled (so no water pump or radiator) and generally simpler all around. But at the end of the day it has a hilariously large displacement for the horsepower delivered. It's not like the thing is using fancy variable timing or partial charge ignition or cylinder deactivation to save fuel either, it has to fill those cavernous cylinders with a combustible mix on every fourth stroke.
I'm far from an expert on this but my understanding is similar. I think there are several technical and market reasons for this. First is the simple cost of developing and certifying a new general aviation engine. That cost must be covered by sales in an already cost difficult market. The second part is technical...plane engines are used very differently than car engines. They are not purely steady state but they are typically used in a near steady state way. This makes improvements such as fuel injection less additive beneficial. Ad in that most of the uses of the engines are non-commercial and non-life critical and you have a logic - why would I spend tones more to gain a few points of efficiency on something I use for fun that already costs me an arm and a leg that works. Its the same reason you don't see the most fuel efficient innovations start in sports cars.
example: look at the Lycoming O-360 and the list of planes it is used in.
Why is it that we use heavy solid/liquid electrolytes, anyway? It seems that gaseous electrolytes are possible; in an environment where volume doesn’t matter nearly as much as weight, wouldn’t a battery built on such an electrolyte be an obvious choice?
[0]http://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2004_...