Electric planes

[daniel]

I think that 2030 is way over-optimistic for a commercial electric jetliner, but I sure hope I'm wrong. First, new battery technology must be developed. Then it has to be extensively tested for safety. Then mass-manufacturing techniques have to be developed. Then the plant has to be built. A fourfold increase in energy density is a very big deal. I think it can happen, but I don't think it can be brought to market in that short a time line. Again, I really hope I'm wrong on the time line, but futurists tend to be wildly optimistic.

As for the present generation of batteries being adequate for surface transportation, I'd say, just barely. The bare-bones Model 3 will be affordable for a middle-class working family, but still out of reach for the majority of working-class folks, and cars like the Leaf are adequate as the short-distance car in a two-car family, but still far from being suitable for the majority of drivers. I still have my Prius because once a year I drive about 330 to 350 miles (if there are no detours and I don't take any wrong turns) to places I go hiking. I go on secondary roads through small towns. No prospects for superchargers any time soon. Using an RV park for a charge-up would add 3 or 4 hours to an otherwise 6-hour drive. With a 500-mile EV I could sell the Prius, but present battery cost and weight means nobody is going to put such a car on the market until there are big advances.

So it's not just airplanes that need better battery tech. Cars need it too for EVs to really go mainstream.

But I am still fascinated to read what you've written about electric planes, because even though I'm less optimistic about the time frame, I would love my next trip to Maui to be on an electric jet. -- And I don't think you mentioned noise. Jets are noisy. I imagine an electric jet would be quieter. Or does the fan make more of the noise than the combustion? When I drove a tractor on the farm, the transmission made more noise than the engine.

 

[macpacheco]

If you think jets are noisy, you should really fly on a Cessna or Piper and feel the vibration and hear the noise. That's noisy.
Modern jets are really quiet.
Even on takeoff the latest gen jets are getting quieter.
Some airlines might still be flying 80s/90s planes, maybe that's what you're used to. A 737MAX/A320neo/A350/B787 are very quiet planes.
That revolutionary plane can be designed and tested with the batteries that are almost there, that might give it 75 minutes range instead of 2 hours. Every other challenge resolved. And before certification, they replace the battery pack, much like you can change a Model S from 75kWh to 100kWh pack.
Even a 75 minute range electric jet would sell, but it would still be short of the mass market appeal.

 

[Lloyd]

.
Even a 75 minute range electric jet would sell, but it would still be short of the mass market appeal.

That would never meet commercial safety standards where you have to be able to go to your destination, climb back to altitude and make an approach at an alternate airport up to 200 miles away with a 45 minute reserve on landing at the alternate.

 

[macpacheco]

That would never meet commercial safety standards where you have to be able to go to your destination, climb back to altitude and make an approach at an alternate airport up to 200 miles away with a 45 minute reserve on landing at the alternate.

Its a bit more complicated than that.
First an aircraft that has the energy to climb to FL510 and cruise for a total of 2 hrs plus reserves, that way up is giving the aircraft a whole lotta potential energy.
But on the way down, instead of gliding, do regen instead, higher vertical speed, similar forward speed, by the time the aircraft makes it to an approach attempt it likely has an extra 30 minutes worth of just regen energy.
The regen is a great deal cause the aircraft gets to cruise for another 20 minutes or something like that. Going faster using the same energy.
When I say 75 or 120 minutes I mean 75 minutes + reserves or 120 minutes + reserves, reserves means another hour of flying.
The big deal is the energy spent to get up to FL510. I mean just the energy invested towards potential energy (altitude).
Its like having an airplane with 4 hour autonomy if its all flown at 10000ft.
But at FL100, the cruising speed would be less than half at FL510 !
The altitude gained would make all the difference.
Like I said, I didn't do any calculations, but the basic concept of flying really high makes a huge difference.
The other advantage of an electric aircraft over a turbofan is turbofans are very sensitive to outside air temperature efficiency wise. Half the advantage of cruising an A350 at FL430 is the much much much colder air which leads to a higher thermal efficiency.
Electric planes don't have that effect at all. Same thrust levels consumes the same energy/hr. A turbofan aircraft must avoid hanging around below FL300 absolutely as much as possible, so when it alternates, it would be spending a lot of energy going up again. The electric aircraft can cruise at a lower altitude with solely the air density penalty. It doesn't matter if OAT (outside air temp) is +40C or -60C, same thing, as there's no fuel being burnt.
ATC loves aircraft that can climb and descent really fast. Makes their job much easier as the aircraft spends less time going in between each vertical ATC layer.

 

[daniel]

If you think jets are noisy, you should really fly on a Cessna or Piper and feel the vibration and hear the noise. That's noisy.
Modern jets are really quiet.

I flew a Piper Tripacer circa 1965. And I know from my own experience that jetliners are a lot quieter than they used to be. But they're still annoyingly noisy. On my last flight home from Maui (last month) I was trying to listen to a podcast, and the noise in the cabin was loud enough that while I could hear the speakers on the podcast, it was hard to really listen to them.

... on the way down, [...] do regen ...

How does that work on a jet? In my car, the wheels are in contact with the road. With a very sticky contact, it's easy to use the car's momentum to turn the motor into a generator. But with a jet, all you have is air moving past it. Is there really enough air pushing through the jet engine for it to recover significant energy? In my car, during re-gen, the motor-generator is providing nearly all the resistance to slow the car, and it can turn that into energy. But in a jetliner, most of the resistance is due to air friction on the airframe, and very little is going through the engines.

 

[nativewolf]

I flew a Piper Tripacer circa 1965. And I know from my own experience that jetliners are a lot quieter than they used to be. But they're still annoyingly noisy. On my last flight home from Maui (last month) I was trying to listen to a podcast, and the noise in the cabin was loud enough that while I could hear the speakers on the podcast, it was hard to really listen to them.



How does that work on a jet? In my car, the wheels are in contact with the road. With a very sticky contact, it's easy to use the car's momentum to turn the motor into a generator. But with a jet, all you have is air moving past it. Is there really enough air pushing through the jet engine for it to recover significant energy? In my car, during re-gen, the motor-generator is providing nearly all the resistance to slow the car, and it can turn that into energy. But in a jetliner, most of the resistance is due to air friction on the airframe, and very little is going through the engines.

I was wonderign the same thing, how would you recapture that spent energy?

 

[macpacheco]

How does regen works in the electric motor ? Just reverse the polarity of the motor, isn't it ?
The same idea.
Of course its not going to be an aggressive regen like breaking a car.
But the system is reversible.
A turbofan/turboprop/prop engine when turned off will windmill, aka extract energy from forward motion into spinning movement.
You just go one step forward and convert that spinning energy into electrons.
The slower the fan is spinning, the higher the force airflow is applying to the fan.

 

[deonb]

New venture in this area - haven't seen it posted yet:

Zunum Aero’s electric passenger plane hopes to offer cheaper flights by 2020

 

[daniel]

How does regen works in the electric motor ? Just reverse the polarity of the motor, isn't it ?
The same idea.
Of course its not going to be an aggressive regen like breaking a car.
But the system is reversible.
A turbofan/turboprop/prop engine when turned off will windmill, aka extract energy from forward motion into spinning movement.
You just go one step forward and convert that spinning energy into electrons.
The slower the fan is spinning, the higher the force airflow is applying to the fan.

I don't buy it. Yes, the theory is correct. But the actual amount of re-gen available would be negligible. Again, in a car using re-gen, all the stopping force goes through the wheels and is available to the motor-generator. In a plane, nearly all the stopping force is applied to the airframe and does not pass through the fans.

This does not mean you can't have an electric fanjet. It does mean that you cannot plan on getting much re-gen when you descend from altitude.

New venture in this area - haven't seen it posted yet:

Zunum Aero’s electric passenger plane hopes to offer cheaper flights by 2020

They are making claims about what their plane will do, and how it will change travel, before they've even begun to actually design the plane. This, IMO, is bad journalism. The actual news there is that yet one more company has plans to begin design work on a short-haul hybrid commuter jet. This is good news. But everything after that is ungrounded, wild speculation. Note also that the idea of operating regional planes between commuter airports does not require electric planes. Small turbo-props are just fine for this. So that whole part of the article is just B.S. filler having nothing whatsoever to do with the subject. It's there to distract from the lack of substance and get people to say "Wow!"

Note also that a hybrid is not an electric plane. A hybrid (presumably a series hybrid, like a freight train, not a parallel hybrid like the Honda hybrid cars, or a series/parallel hybrid like a Prius) will still burn fuel for energy. The potential upside is that it would demonstrate the properties of a real-world electric propulsion system for a jet, so that when battery technology advances far enough, half the work will have been done.

But this is still just a dream. Lots of companies are founded with great dreams, only to collapse without ever accomplishing anything. I'm thinking of Aptera, which was going to give us a two-seat electric car long before Tesla; and EEStor, which promised electric energy storage density ten times that of lithium at a tenth the cost. I'd have given my eye teeth for an Aptera back then. But I saw EEStor for the scam it was from the beginning, when they were announcing that they were six months away from selling super-capacitors in quantity, before they'd even demonstrated a prototype. EEstor convinced the Zenn car company to invest big in them, and the result was that Zenn, which was building a very nice low-speed electric car, went under. A friend of mine also lost money investing in EEStor. Big dreams, no substance.

 

[macpacheco]

Have you ever put your hands outside a car at 120mph (180km/h) ?
Now imagine how much force your entire body would feel against a 180km/h wind ?
That normal terminal velocity in skydiving.
A person's body will feel quite a strong force at that speed.
Now increase that to 500km/h of apparent wind, the resulting power of that drag has increased 15.6 fold.
Because drag/lift POWER is proportional to speed cubed. Double speed = 8x more energy.

Now look at the front of a turbofan. Realize that the entire front surface inside the nacelle is covered with low pressure blades.
If the fan can provide massive thrust then it can recover that thrust too. Worst case half as much recovery or something in that magnitude.
Its true that if the aircraft is going slowly, there would be little regen available, but as long as its speed apparent to the wind is high, regen power will be very high too.
If you took physics, you'd know:
Power = Force x Velocity
The relative velocity of the incoming airflow is very high.
The force exerted on the blades is also very high.

I'm not a physics teacher. If you can't believe, go ask one.

 

[daniel]

Yes, I skydived (skydove?) once. Yes, it's like standing in a hurricane except that you're horizontal. I had to hold my mouth wide open in order to breathe, and the photos look like I'm screaming, even though I was not. Yes, it would push hard on the fan blades. But it's a mistake to imagine that there's a relationship between the amount of thrust an engine can develop, and the amount of energy it can recover. The thrust of that engine is sufficient to move a whole airplane. But the regen is only the amount of air resistance hitting a minuscule portion of the airframe. The vast proportion of the kinetic energy is dissipated against the frame, not the fan blades. At most you could maybe recover one or two percent of the total energy.

There's also the question of passenger comfort. Commercial jetliners accelerate more slowly than they're actually capable of, because the passengers would be uncomfortable with too much acceleration. You suggest a very steep descent rate in order to capture re-gen energy, but a steep descent rate would be uncomfortable for the passengers, as well as unsafe for anyone not wearing their seat belt, and would therefore be rejected both on grounds of comfort and safety.

Electric airplanes are a wonderful idea, and may be the future, if we can develop adequate energy storage. But re-gen is not realistically a significant factor, due to the different nature of airplanes and wheeled ground vehicles.

 

[JRP3]

It really has nothing to do with how much potential regen a plane might be able to recover with the motors, it's all about how many actual opportunities to do so exist while flying. Regen works in cars because we often need to decelerate faster than simple air resistance and road friction provide, but even so there is very little regen happening during long distance steady state highway driving. How often when flying do planes need to decelerate more than simply reducing thrust and gliding?

 

[daniel]

It really has nothing to do with how much potential regen a plane might be able to recover with the motors, it's all about how many actual opportunities to do so exist while flying. Regen works in cars because we often need to decelerate faster than simple air resistance and road friction provide, but even so there is very little regen happening during long distance steady state highway driving. How often when flying do planes need to decelerate more than simply reducing thrust and gliding?

I believe his claim is that by means of a steep descent the re-gen recovers the gravitational potential energy of the plane's cruising altitude. I argue that most of that energy is lost to air friction on the airframe, and only a tiny part through resistance against the fans.

 

[macpacheco]

You guys aren't physicists nor engineers and insist on using layman's though process to dismiss something that requires technical expertise.
I'm not going to insist as the discussion will be pointless unless you understand physics well.
I stand by my basic point.
You don't need a massive regen surface for this to work.

I believe his claim is that by means of a steep descent the re-gen recovers the gravitational potential energy of the plane's cruising altitude. I argue that most of that energy is lost to air friction on the airframe, and only a tiny part through resistance against the fans.

In flight there are several energies acting on the aircraft.
Potential, kinetic, thrust/regen, drag.
While descending, potential energy is decreasing, adding energy to the other energy types (conservation of energy). Drag is consuming some energy. Regen is consuming some of that too. In a normal cruise descent, indicated airspeed is constant (keeping lift/drag constant), but as air is getting denser, true airspeed is decreasing, so there's some more energy available for regen (kinetic energy also decreasing).
On the way up, indicated airspeed should also be constant after achieving cruise climb speed, but as air gets thinner with altitude the aircraft is constantly accelerating (gaining kinetic energy).

I'm not going to discuss this further. Its quite clear I don't have the teaching technique to explain this in writing and you guys think you understand the physics behind this but you don't by a mile. No intent to hurt anybody's feelings, but you guys are wrong.

There are several ways in which I could be wrong, but the arguments presented do not disprove me.

 

[JRP3]

If you can't explain something you probably don't understand it yourself. You didn't at all address the point I brought up in my post, i.e. what percentage of time do aircraft require greater deceleration than is currently provided by air drag in normal operation? I assume that in normal operation it's not required to reverse motor thrust. If that is accurate then there is little potential for regen.
Now, if it's common during descent to cause increased drag by increasing flap angle to further slow the plane then maybe it's possible with electric motors to capture some of that, though that may cause uncomfortably steep descent angles. I would still question the significance of recoverable energy available.

 

[daniel]

You guys aren't physicists nor engineers ...

Care to cite your credentials? You stated earlier that you think a technology that does not even exist yet could be in commercial use in 17 years. To my comment that re-gen would be ineffective due to the minuscule surface area available to react with the passing air (the only source of possible re-gen energy) your reply is that I'm not a physicist or an engineer.

If you cannot explain why that tiny surface area could capture a significant amount of energy, when the only thing slowing the plane is the air impacting on the plane, then I think we're justified in rejecting that portion of your thesis. Note that I'm in agreement with the major thrust of your posts. The notion of an airplane using re-gen effectively is the only point I cannot accept without some evidence or explanation.

 

[LargeHamCollider]

I was at CAFE 2014 which is a major electric airplane conference. Tesla's filled the parking lot.

I spoke with Tine Timovec, lead engineer at Pipestrel, and many others. In flight regen is definitely going to be a thing in electric GA aircraft. It was mentioned in most presentations. A high percentage of GA operations are flight training, a high percentage of flight training is pattern work. Re-gen will likely replace flaps on many GA electrics.

 

[evp]

How does that work on a jet? In my car, the wheels are in contact with the road. With a very sticky contact, it's easy to use the car's momentum to turn the motor into a generator. But with a jet, all you have is air moving past it. Is there really enough air pushing through the jet engine for it to recover significant energy? In my car, during re-gen, the motor-generator is providing nearly all the resistance to slow the car, and it can turn that into energy. But in a jetliner, most of the resistance is due to air friction on the airframe, and very little is going through the engines.

Did you ever look at a modern jetliner from the front? The turbofans are not much different in area from the cabin. Most of the air that goes thru a jet engine these days is just ducted straight thru, only a small percentage is actually used for combustion. The bypass ratio is between 5 and 10 to 1 in a modern engine. Those fans will work almost as well generating energy as they do blowing air.

So, how much energy is available? The glide ratio of a jetliner can be between 15:1 to 20:1, meaning that it loses 1 unit of altitude for every 15 units of forward progress. From a cruising altitude of 7 miles, you could glide 105 to 140 miles without power. This implies that if you could extract all the potential energy of the plane somehow, it could be used to fly the plane over 100 miles -- in fact, that's exactly what you're doing during a glide.

The manual for a Mooney (light plane) states that the glide ratio is 10.3:1 with the engine dead but propeller windmilling, 12.7:1 with the prop stopped. The prop passes for a generator in this scenario, turning 20% of the potential energy of the plane into heat by pumping air through the engine. Real-life experiments have managed to get the glide ratio down to 9.9:1. This suggests that you could reasonably expect to convert at least 20% of your potential energy into battery charge.

Of course, you can fly the plane the same distance just by gliding at a shallower angle at a slower speed. Probably farther, if you take into account generator and battery charging inefficiency.

 

[daniel]

Thanks to LargeHamCollider and evp for the above explanations.

 

[bxr140]

I'm not a physics teacher.

We got that.

You guys aren't physicists nor engineers

If it helps, I'm an Aerospace Engineer...

Slightly more serious, you're right that electric aircraft regen is possible, but there are some basic limitations to the concept. Aircraft typically don't have a lot of 'wasted' energy that can be recovered by regen, because they operate in an explicitly balanced dynamic environment. You fall out of the sky if your basic Thrust vs Drag and Lift vs Mass equations don't square up. Its not the same as a car, where the vehicle has [presumably] already fallen out of the sky and is resting safely onto the ground.

Random thoughts:
1. Aircraft on descent still typically operate at some higher-than-idle thrust--meaning they're still putting energy into the system. So...a first order comparison against today's technology suggests there's little opportunity to take energy out of the system during descent via regen.
2. Controlled airspace is just that--there are regulated limits on airspeed and descent rates, partially for safety and partially for passenger comfort, so bouncing off #1 there's not a lot of regen opportunity while still adhering to current regulations. In the far future when every aircraft is electric we could see a bit more of a dive into the destination which would facilitate regen a little more, but at some point you hit the practical limit of passenger comfort. People don't typically like the sensation of falling.
3. The ability to generate more energy on descent is actually pretty low value added. During that phase of flight an aircraft is headed to its destination where it can charge up again. There's not a whole lot of reason to design appreciable regen into that flight mode since electrons on the ground are always going to be cheaper than electrons in the air. The aircraft would also still have its required reserve, so its not like you can really make the safety argument that the regen would facilitate more flight time.
4. As LargeHam notes, the real benefit to aircraft regen could be in displacing other aircraft systems, like spoilers and airbrakes. Some obvious short term barriers aside, removing those systems removes mass, complexity, and cost, and that's A Good Thing.