Electric Planes Usher in the Second Great Age of Aviation • Hacker News Board

Re: Electric Planes Usher in the Second Great Age of Aviation

#21

Post by sokoloff » Tue, Jul 28, 2015, 2:48 PM UTC

masklinn wrote: post #10

sokoloff wrote: post #2

I drive an electric car and fly piston airplanes. I don't see electrics replacing internal combustion in aviation in my flying lifetime. Why? Weight. The specific energy (energy per kg, edited per masklinn) of batteries sadly lags the specific energy of gasoline or kerosene, and the airplane carries that weight for the whole flight for an electric airplane. (My gas airplane gets lighter and lands as much as 1000 poun…

> The energy density of batteries sadly lags the energy density of gasoline or kerosene Nitpick but the energy/mass density is usually called "specific energy" to differentiate it with the more usual energy/volume "energy density". And while a battery's energy density is pretty bad, they pretty universally have a higher density than petroleum fuels and thus even worse specific energy: the best batteries have 2 orders…

Thanks for that. Yes, I was concerned with "specific energy" (energy per kg, not per m^3).

Re: Electric Planes Usher in the Second Great Age of Aviation

#22

Post by sokoloff » Tue, Jul 28, 2015, 2:56 PM UTC

fnordfnordfnord wrote: post #18

sokoloff wrote: post #2

I drive an electric car and fly piston airplanes. I don't see electrics replacing internal combustion in aviation in my flying lifetime. Why? Weight. The specific energy (energy per kg, edited per masklinn) of batteries sadly lags the specific energy of gasoline or kerosene, and the airplane carries that weight for the whole flight for an electric airplane. (My gas airplane gets lighter and lands as much as 1000 poun…

I think we might see gasoline/jet-A over electric airplanes, perhaps with or without battery storage. That would allow a few neat optimizations. You'd have more control over CG. You could have a smaller high efficiency engine that always ran at peak efficiency (or an IC engine with several modes). You could have more control over noise in urban areas. With a battery for peak/boost power, you could have a tiny engine…

Once you get past about the 6200#/3000kg MGTOW mark, it's very common to permit takeoff weight to exceed landing weight.

I suspect all GA turbojets and most GA turboprops are in this situation.

Even the Beech Baron (a light twin) has a 100# (~16 gallons) difference in max takeoff vs max landing weight.

Re: Electric Planes Usher in the Second Great Age of Aviation

#23

Post by nacnud » Tue, Jul 28, 2015, 3:04 PM UTC

yread wrote: post #8

nacnud wrote: post #4

Earlier quoted context omitted.

Surely the landing gear, wing spar attachments, and brakes all have to be able to cope with an unexpected turnback and landing with most of the original weight? (not to mention heavy landings in adverse weather conditions)

No see http://www.caa.co.uk/default.aspx?catid=60&pagetype=90&pagei... or http://www.boeing.com/commercial/aeromagazine/articles/qtr_3... They are only certified to the maximum landing weight which typically is lower than the maximum takeoff weight. When an overweight landing (ie. the weight is higher then maximum landing weight) occurs engineers have to perform a thorough inspection of the plane structure - landing…

I guess I was thinking more about smaller GA aircraft, but thanks for the detailed reply.

Re: Electric Planes Usher in the Second Great Age of Aviation

#24

Post by sokoloff » Tue, Jul 28, 2015, 3:05 PM UTC

srb- wrote: post #20

simonsquiff wrote: post #5

I read a piece by Elon Musk, who was saying that electric planes - whilst very far away - are the future of air travel. Why? Because they will allow planes to fly higher - increasing efficiency and speed. Apparently the normal 30,000 feet cruising altitude is not higher because the reduced oxygen levels at higher altitudes impact the gasoline combustion too much. Even at 30,000 feet the air is thick as soup at cruisi…

Like most, I originally assumed that because batteries have poor specific energy compared to fossil fuels, there was no point in considering them for aircraft. But Musk's assertion has made me reconsider. Has anyone here done the math? What I think he's talking about is NOT a conventional plane with a battery simply replacing the gas tank, but instead essentially a flying battery, where 90% of the plane's mass is bat…

Electricity is not especially cheap compared to jet fuel.

Jet Fuel has about 128kBTU/gal and is available around $2/gal, so 64kBTU/$.

Electricity is about 3400BTU/kWhr and a kWhr is about $0.05, or about 68kBTU/$.

Re: Electric Planes Usher in the Second Great Age of Aviation

#25

Post by thearn4 » Tue, Jul 28, 2015, 3:27 PM UTC

At NASA, we're very interested in hybrid electric propulsion. As an applied math guy, it's a very interesting and surprisingly non-trivial design problem to tackle. Even addressing the "does this really make sense?" question is tricky.

Re: Electric Planes Usher in the Second Great Age of Aviation

#26

Post by termain » Tue, Jul 28, 2015, 3:32 PM UTC

6stringmerc wrote: post #17

sokoloff wrote: post #2

I drive an electric car and fly piston airplanes. I don't see electrics replacing internal combustion in aviation in my flying lifetime. Why? Weight. The specific energy (energy per kg, edited per masklinn) of batteries sadly lags the specific energy of gasoline or kerosene, and the airplane carries that weight for the whole flight for an electric airplane. (My gas airplane gets lighter and lands as much as 1000 poun…

I entered the Local Motors and ARPA-E contest for the Lightweight Sedan challenge and proposed a three part method to reduce weight: Revised chassis, bio-composite materials, and a hybrid drivetrain using micro-turbines to produce electricity to power the vehicle. The contest parameters stressed that the proposal should be "research for the next 10 years." So, I cited the efficiency and extremely light-weight of micr…

Exoskeleton? You mean monocoque construction (aka load bearing skin)?

Re: Electric Planes Usher in the Second Great Age of Aviation

#27

Post by coryrc » Tue, Jul 28, 2015, 3:33 PM UTC

sokoloff wrote: post #24

srb- wrote: post #20

Earlier quoted context omitted.

Like most, I originally assumed that because batteries have poor specific energy compared to fossil fuels, there was no point in considering them for aircraft. But Musk's assertion has made me reconsider. Has anyone here done the math? What I think he's talking about is NOT a conventional plane with a battery simply replacing the gas tank, but instead essentially a flying battery, where 90% of the plane's mass is bat…

Electricity is not especially cheap compared to jet fuel. Jet Fuel has about 128kBTU/gal and is available around $2/gal, so 64kBTU/$. Electricity is about 3400BTU/kWhr and a kWhr is about $0.05, or about 68kBTU/$.

Except, electricity will be about 85-90% efficient and a turbofan is about 30% efficient [1], roughly tripling the cost advantage.

Fuel costs are about 35%[1] of operating costs, so only a ~20% cost reduction is possible for fuel change, so your point is still mostly valid.

[1] https://en.wikipedia.org/wiki/Jet_engine#Energy_efficiency

[2] http://www.ajc.com/news/business/airlines-keep-adapting-to-h...

Re: Electric Planes Usher in the Second Great Age of Aviation

#28

Post by coryrc » Tue, Jul 28, 2015, 3:37 PM UTC

sokoloff wrote: post #2

I drive an electric car and fly piston airplanes. I don't see electrics replacing internal combustion in aviation in my flying lifetime. Why? Weight. The specific energy (energy per kg, edited per masklinn) of batteries sadly lags the specific energy of gasoline or kerosene, and the airplane carries that weight for the whole flight for an electric airplane. (My gas airplane gets lighter and lands as much as 1000 poun…

Electric planes could stop GA poisoning everyone with lead. You could also choose a different fuel.

Toy planes (general aviation) are responsible for the majority of lead pollution:

www.emagazine.com/earth-talk/lead-in-aviation-fuel

Re: Electric Planes Usher in the Second Great Age of Aviation

#29

Post by 6stringmerc » Tue, Jul 28, 2015, 3:38 PM UTC

termain wrote: post #26

6stringmerc wrote: post #17

Earlier quoted context omitted.

I entered the Local Motors and ARPA-E contest for the Lightweight Sedan challenge and proposed a three part method to reduce weight: Revised chassis, bio-composite materials, and a hybrid drivetrain using micro-turbines to produce electricity to power the vehicle. The contest parameters stressed that the proposal should be "research for the next 10 years." So, I cited the efficiency and extremely light-weight of micr…

Exoskeleton? You mean monocoque construction (aka load bearing skin)?

No, I mean like the chassis components were on the outside of the car, like a bug. That was the crux of their entry, mollified by some talk of aerodynamic efficiency. From what I recall, the rendered images of their concept didn't even have a method of entry for the driver/passengers. No doors. It was...well, frustrating, as my tone probably conveyed.

Re: Monocoque - my entry cited that format to give additional strength to the chassis/panels mating. Bio-composites are approaching the tensile strength of carbon fiber, and could potentially be much easier to mass produce and at a lower price point. I thought the implication that in 10 years, with research, such an avenue would be viable. Instead, the second place entry was some proprietary kind of new metal that could be used to replace body panels...that's it. I remember calling that entry a "body kit" because that's what it reminded me of overall.

Re: Electric Planes Usher in the Second Great Age of Aviation

#30

Post by masklinn » Tue, Jul 28, 2015, 3:41 PM UTC

srb- wrote: post #20

simonsquiff wrote: post #5

I read a piece by Elon Musk, who was saying that electric planes - whilst very far away - are the future of air travel. Why? Because they will allow planes to fly higher - increasing efficiency and speed. Apparently the normal 30,000 feet cruising altitude is not higher because the reduced oxygen levels at higher altitudes impact the gasoline combustion too much. Even at 30,000 feet the air is thick as soup at cruisi…

Like most, I originally assumed that because batteries have poor specific energy compared to fossil fuels, there was no point in considering them for aircraft. But Musk's assertion has made me reconsider. Has anyone here done the math? What I think he's talking about is NOT a conventional plane with a battery simply replacing the gas tank, but instead essentially a flying battery, where 90% of the plane's mass is bat…

> What I think he's talking about is NOT a conventional plane with a battery simply replacing the gas tank, but instead essentially a flying battery, where 90% of the plane's mass is battery.

Weight is the premium on planes, so the 10% of non-battery you're talking about is a full plane minus fuel tanks. An E-170 is 21t of empty plane, up to 9t of fuel and up to 36t total (MTOW). Given batteries have 1% the energy specificity of aviation fuel (so you need 100 times the weight in batteries for the same stored energy), the math doesn't quite work out.

> This sounds ridiculous at first

And at second, and at third.

> when you consider how cheap electricity is compared to jet fuel

Doesn't compensate the problem of needing a plane 10 times as heavy with 1/10th the payload.

> as well as the other benefits the OP mentioned

Which don't really make sense, lower air density means your turboprops have nothing to work with, there's no such thing as turbofans let alone turbojet so the gains in drag are killed by the inability to actually go forward.