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The age of electric flight is finally upon us

bbc.com

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Re: The age of electric flight is finally upon us

#221

Post by deepspace »

dreamcompiler wrote: post #201

I know nothing about Eviation other than what this article says. They're claiming 650 miles of range with nine passengers on batteries, and to that I will not say I'm merely skeptical. I'm saying they are flat-out liars selling bullshit, and I'll stake my engineering reputation on that statement.

I was reading the article with stunned disbelief at the BBC's lack of fundamental feasibility verification. And then I was unpleasantly surprised that I had to scroll down so far here on HN to see someone call out the bullshit.

From the exaggerated numbers for aviation fuel cost to the ridiculously low cost of electricity all the quoted numbers are obvious lies.

Not to mention that a back-of-the envelope calculation shows that the aircraft would never be able to take off due to the weight of batteries required for a 650 mile range.

You would be absolutely right in staking your engineering reputation on calling bullshit and I would happily join you.

Re: The age of electric flight is finally upon us

#222

Post by dmitrygr »

Robotbeat wrote: post #160
namirez wrote: post #106

As others have noted, electric flights require a major breakthrough in the battery technology. Here are energy density numbers for gasoline, jet fuel, and lithium-ion batteries: Gas and jet fuel specific energy density ~= 45 MJ/kg; Lithium-ion specific energy density Even worse, a jet engine is much better at generating power. A modern high-bypass turbofan engine can generate 10 kW/kg whereas an electric motor is aro…

This is constantly repeated, but it's not true. I work in the field of electric motors for electric aircraft, and the amount of nonsense constantly repeated on the Internet about how electric flight cannot be done without a breakthrough is breathtaking. Electric motors can achieve FAR greater than 1kW/kg. Here's one used sometimes on electric aircraft capable of 10kW/kg, same as a jet engine: https://emrax.com/produc…

That engine may have good power density, but it is quite weak in general. Site quoth: "Continuous power: up to 110 kW"

That is basically a negligible amount of power

Re: The age of electric flight is finally upon us

#223

Post by adminu »

FabHK wrote: post #197
namirez wrote: post #192

Earlier quoted context omitted.

Your numbers are not quite right. > jet fuel's USEFUL specific energy is more like 10-15MJ/kg due to needing to be burned. New turbofans have efficiencies close to 50%, so it's more like 20 MJ/kg. Also, you need to take into account the prop efficiency for electric planes. Beyond Mach 0.5, the efficiency degrades very rapidly. So it's not just electric motor vs jet engine. > more glider-like L/D of 25-35 (the best gl…

> > improved modern materials means you can just have most of your take-off mass be battery, compensating for the poorer specific energy > Again, this can be done for any aircraft, not just the electric ones. Not only that: you burn the fuel, so for the second half of the flight you only need to carry half the fuel. But you have to carry the full battery mass (modulo E=mc^2) all the way.

Well, that’s in part a point for the batteries, though. You spend a lot of energy to lift all that fuel that you just blow out with all its potential energy up in the air, whilst the batteries provide you with that very potential energy on the way down as well. Glider planes quiet commonly are filled with water to make them heavier so they can make more use of the potential energy they gain from the externally powered take off and so that they can bridge longer distances without lift generating upwind.

Re: The age of electric flight is finally upon us

#224

Post by ansible »

waterhouse wrote: post #209
FabHK wrote: post #197

Earlier quoted context omitted.

> > improved modern materials means you can just have most of your take-off mass be battery, compensating for the poorer specific energy > Again, this can be done for any aircraft, not just the electric ones. Not only that: you burn the fuel, so for the second half of the flight you only need to carry half the fuel. But you have to carry the full battery mass (modulo E=mc^2) all the way.

Dumb idea: If the weight of the batteries is significant, then dump out the empty ones with parachutes partway through the flight?

They do this for the 2nd stage of the Electron rocket, though the batteries are not recovered. Scott Manley video:

https://youtu.be/U5k1mlu6A7I?t=106

Re: The age of electric flight is finally upon us

#225

Post by rootusrootus »

audunw wrote: post #206
namirez wrote: post #106

As others have noted, electric flights require a major breakthrough in the battery technology. Here are energy density numbers for gasoline, jet fuel, and lithium-ion batteries: Gas and jet fuel specific energy density ~= 45 MJ/kg; Lithium-ion specific energy density Even worse, a jet engine is much better at generating power. A modern high-bypass turbofan engine can generate 10 kW/kg whereas an electric motor is aro…

I think the biggest point that you and others that bring this up is missing, is not just how electric designs gives you higher efficiency and close the gap (as others have mentioned). The main thing you're missing is that modern jet planes have absolutely ridiculous range, and a huge number of flights use only a fraction of that range. Electric planes don't need to match modern jet planes to be viable. They just have…

Even medium range flying is tough to accomplish with near-term electric technology. A flight across the United States would involve 10 hours of flight time, assuming you did it in a single leg, which of course you cannot. Three stops, so probably 15+ hours total. People do seem willing to take one-stop jet rides across the U.S. now, but the gold standard is about 5.5 hours for a non-stop flight.

Re: The age of electric flight is finally upon us

#226

Post by erobbins »

mannykannot wrote: post #21

A battery fire is a scary prospect unless the fire can be effectively contained and the hot gases vented (I don't suppose extinguishing one is likely to be an option.) There has been at least one crash involving fire, though I have not been able to find out if it was a propulsion battery fire. [1] In Eviation's Alice, the failure or shutdown of one of the wingtip's motors would require the shutdown of the other, on a…

Assuming battery swaps are the aviation "norm", it might be possible to design the aircraft so that the batteries could be jettisoned in an emergency. This would also give much better glide performance for the emergency, and make the possibility of parachute recovery a lot higher since suddenly there's half as much weight to bring down.

Re: The age of electric flight is finally upon us

#227

Post by sandworm101 »

verelo wrote: post #119
sandworm101 wrote: post #100

Earlier quoted context omitted.

>> off-field forced landing that went badly. Parachutes. They are now a very practical option for GA aircraft. More than a few cessnas have been saved. The concept of "off-field forced landing" should too be going away. https://brsaerospace.com/cessna-faqs/ " Q. How many aircraft saves are credited to a BRS Whole Aircraft Rescue Parachute System? A. As of August 2017, 376 ."

"Q. How much damage will be done to my plane if I land it with a parachute? A. In all likelihood the aircraft will suffer some significant damage. The terrain where you land will affect this greatly. Though the extent of damage has varied from plane to plane, most GA aircraft that have come down under a BRS deployment have eventually (or will soon) fly again." Call me a skeptic until they provide hard numbers and det…

How many are written off is irrelevant. What matters is passenger lives. Success is measured in total number of not-dead people because of not crashing as violently.

Re: The age of electric flight is finally upon us

#228

Post by Gibbon1 »

sokoloff wrote: post #90
ac29 wrote: post #66

Earlier quoted context omitted.

No, that's exactly backwards -- jet engines are far less efficient at low speed than at cruise speed. Various numbers I found say 100-300 kg of fuel used taxiing for single aisle planes like 737 [0][1] all the way up to two tons of fuel for large aircraft like 747 at large airports like JFK [0]. This can be extremely relevant for shorter flights: "[taxiing] aircraft fuel burn which is estimated to be as high as 27% o…

The taxi fuel is not primarily used for motivation of the airplane around the airfield, but just from idling the engines. (It's about minutes of ground operation more than miles of ground operation.) Electric drive wheels don't eliminate that unless you don't start the engines until near the hold short line (in which case, you don't have air conditioning or electric power, meaning you end up running the APU [itself a…

That's an another win for electric and hybrid electric designs. For the latter if you can safely take off on battery power you don't need to idle the engine before take off. Saves fuel, reduces noise, and localized pollution.

You can also turn the turbine off before landing.

Re: The age of electric flight is finally upon us

#229

Post by emanuensis »

dredmorbius wrote: post #195
emanuensis wrote: post #162

Earlier quoted context omitted.

How 'bout Ammonia fuel cells?

Very bad news in event of loss of fuel containment.

Haha, not so bad as loss of H2 containment (as seen IRL in Norway) ... and of course farmers deal with it every day.

Re: The age of electric flight is finally upon us

#230

Post by Gibbon1 »

opportune wrote: post #113
Havoc wrote: post #112

Surprised at the hybrid comment. I would have expected electric for the cruise phase not takeoff and landing

Electric is more burstable so it makes more sense for periods of rapid acceleration/deceleration. Hybrid cars work the same way

Lack of turbine lag with an electric powered fan is probably a big deal. Means when the pilot decides he needs power now he gets it now. Not in a few seconds.
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