[aerodude]

The first paragraph is incorrect. The wing has to be built to withstand lift, which far exceeds the weight of the engines. The weight force of the engine typically counteracts the lifting force, which means -- if you're clever with your design -- you can build a lighter wing structure with a bigger engine. This is why the engines are typically located on the wing in the first place -- aside from being easier to access for maintenance, it reduces the weight of the aircraft.

The second problem with battery-electric aviation is that most large aircraft aren't designed to land at their take-off weight. Landing gear typically comprises some of the heaviest materials on the airframe, so landing at a lighter weight than you take off at is a big advantage. This also manifests in the range of the vehicle. Burning fuel reduces the power needed to maintain lift, which give you additional range with your remaining fuel. By comparison, range with battery-powered aircraft would be linear.

The last hurdle is just the sheer energy density required. A 747 requires 50-60MW just to stay airborne at cruise, and will produce on the order of 250MW during take-off. Even a business jet produces ~10-15MW at max power, and will cruise at around 1.5-3MW. Over the course of a 10 hour flight, the lower bound is somewhere around 10-15MWh, which would be 37-56 metric tonnes of battery mass using the upper limit of current battery energy density (265Wh/kg). Keep in mind, it's not enough just to get to your destination, you also need a reserve while you're in the holding pattern, plus an additional safety factor.

There might be interesting new markets opening up in the personal air-taxi space, but I'm skeptical. Personally, I think the new wave of short-haul rooftop-hopping electric aircraft will prove to be a black hole money-pit, and a regulatory nightmare. The aerospace industry is littered with broken dreams.

[state_less]

Could we rethink short haul flights somewhat? What if instead of a 400 mph aircraft, we had a 200 mph aircraft with high aspect ratio wings? In other words try to make a more efficient aircraft to compensate for the lack of energy aboard.

I suppose people want to get there in a hurry, but a flight that goes from 30 min to 1hr or 1 hr to 2hr might not be too bad.

[aerodude]

Maybe, though you probably still want to fly above the weather, which puts a lower bound on your power requirements. To get above the weather, the energy needed is mgh plus whatever kinetic energy your aircraft still has (i.e. it's velocity); if you trade that for speed, you won't have a 200mph aircraft anymore.

Customers are definitely willing to trade time for cheaper flight (Boeing made this gamble and won in the last airframe generation), but there are certain practicalities that need to be met.

[state_less]

I agree getting up over the weather would be a good thing. And as long as we're dreaming, may as well dream big. Maybe we can do something like they did with jato bottles on the c-130 at takeoff.

Attach some batteries with wings to the aircraft that you release after you get up to altitude. These days you can put an autopilot on the batteries and fly them back to the airport for recharging. You'd save some weight once at altitude and could get a 'free' lift up to altitude. Sort of like how a sailplane can get a tow. Maybe you could take them up to 30-40k ft at a good speed and let them trade some height for speed along their path to the destination.

Clearly there is a cost for the added complexity, but it might be worth it given the cheap computation and improving autopilot software.

[pasabagi]

That sounds absolutely terrifying. Iirc, the first rule of building rockets is, always assume it will explode - not even starting on all the stress it would put on the frame.

[NickNameNick]

Rather than JATO bottles, a better option may be to have an 'electric runway', that allows you to draw ground-power while accelerating down the runway, which might save a non-trivial amount of battery capacity, and may allow you to run your motors at a higher power output than the batteries would normally allow. As an added bonus, the runway could be shorter.

[njarboe]

If you mean the airlines when you say customers, I would say you are correct. Planes are flying slower these days because it saves fuel and one can offer a lower fare. If you mean the people who fly, when you say customers, I would say that is incorrect. People have no idea that flights can have different lengths and the only way to search for fares is by destination, number of stops, and price. It would be great if airline ticket buyers had search parameters for leg room and flight time so that those very important parameters could be selected for. Otherwise evolution will continue to force slow flights and smaller seats.

[aerodude]

You're right, this is an important clarification. Airlines like smaller airframes that are easy to fill, have fewer engines, and they're okay with breaking journeys up into multiple smaller hops. Passengers don't necessarily have a choice.

[soperj]

I always look at flight time since I'm flying with kids.

[sandworm101]

Slowing down does not make an aircraft more fuel efficient. Best efficiency, as in fuel per passenger-mile, occurs with large aircraft at the very high altitude where air resistance is least. And to fly up there you need to move quickly. An a380, almost supersonic at mach 0.85, is more fuel efficient than Cessna at down low at mach 0.2. Even at lower altitudes, there is little advantage in flying slower. You just end up spending more time/energy keeping yourself up rather than moving forwards. A high-lift wing, great for maximizing time aloft, becomes a big draggy airbrake when you want to actually go somewhere.

In the extreme, the most efficient place to fly is in the most cold and dry air. That occurs at the tropopause, which is generally between 35,000 and 50,000 feet (-60*C). You don't get to play around at that altitude without a large complex aircraft.

https://www.windows2universe.org/earth/Atmosphere/tropospher...

[jillesvangurp]

You're taking current batteries as the benchmark and comparing them to a 747. A 747 takes about 50K gallons of kerosene. Or roughly 150 tonnes; according to wikipedia the maximum takeoff weight for a 747 is around 500 tonnes. That's also a good explanation of why these big planes are being replaced by smaller ones; using that much fuel is stupendously expensive.

The reason why the engines and fuel are in the wings has more to do with the center of gravity. This needs to be where the wings are for the plane to be stable because of the non trivial amounts of fuel that get burned and the weight of the engines. If you'd store this in the back or front of the plane, the plane would get progressively harder to control through the flight.

With batteries, this is a non issue. You can put them wherever as long as the center of gravity is in the right place. It won't shift as you deplete the batteries. But still, the wings are probably going to be a popular place for this.

Comparing with a long haul 747 is of course a bit unfair. It is worth noting that these planes are being replaced by more efficient two engine planes that are lighter and much more efficient. But I agree it will be a while before we see cross Atlantic flights at that scale. But we're talking short haul here; 1-2 hour flights.

[aerodude]

I didn't just compare with a 747 -- the battery numbers I ran were for the business jet. The 747 obviously looks far worse. If you want mass for a 2 hour journey in an electric business jet, divide those numbers by 5. Also the battery numbers I ran were for the upper range of current lithium-ion tech. If there are batteries on the market that have an energy density of 265Wh/kg, I haven't seen them. I'm no battery expert so I could be wrong, but from what I understand, most current batteries sit at around 60-70% of that.

A non-shifting static margin is certainly a benefit for electric aircraft, but we already successfully design aircraft that do have a shifting static margin. The interesting question is whether or not a constant mass would let you design airframes that would be impossible with fuel and ICE.

1-2 hours looks possible using current and near-future technology, but has anyone actually done a cost analysis on flying electric aircraft that are always at "full fuel" weight versus standard aircraft doing the same journey with half a tank? I can't imagine it's completely cut and dry, because the aerospace industry has had its eye on various forms of electric propulsion for decades.

Also, what's the turnaround on these things? Airlines want to make money, so they want to minimize the amount of time the aircraft spends on the ground. That's going to be a major hurdle, since even charging 2MWh over the course of an hour or so requires 2MW of power going into the vehicle. That's not an ungodly amount, but it's still non-trivial.

[Kimm0n0]

Most domestic flights in Norway last less than an hour, like Bergen - Oslo and Stavanger to Oslo.

[lukeschlather]

> The first paragraph is incorrect. The wing has to be built to withstand lift, which far exceeds the weight of the engines. The weight force of the engine typically counteracts the lifting force, which means -- if you're clever with your design -- you can build a lighter wing structure with a bigger engine.

But couldn't you achieve the same effect by locating batteries in the wing? The point of the previous comment is that comparing fuel weight with battery weight is deceptive because electric engines are lighter. That doesn't mean an electric engine system is lighter weight than a conventional jet engine system. (So the structural design you're describing sounds feasible.)

[aerodude]

Yeah you could; you would probably place the batteries in the wing where fuel is located, which potentially has benefits.

I was pointing out that the reasoning was flawed. The engine is not the limiting factor in the design of the wing, and in fact can let you reduce the mass of the wing. An electric motor and airframe might be lighter than an engine and airframe depending on the design. However, you do need to compare the battery weight with the fuel weight, because the energy has to come from somewhere, and an airframe is designed with that reduction in weight in mind. It's hard to stress just how important energy density is for an aircraft.