>Wonder how they made it light enough to be able to fly...

You get a massive weight savings in the engines and all related components. DC motors are multiple times smaller and lighter than the equivalent powered turbine engine when you consider the entire system. On top of that you totally eliminate the need for variable pitch props and the related transmission parts since electric motors can adjust power instantly. You also negate a lot of hydraulic systems (and their redundant backups) without the need for things like fuel pumps, thrust reversers, etc.

It also seems they are not using anything like standard li-po tech. From their site:

>Utilizing industry-leading lithium-ion batteries and a proprietary Aluminum-Air system for range, we’ve surpassed the 400Wh/kg mark.

For reference this is nearly double Tesla's energy density. Of course the cells will be much more expensive, but that matters a lot less for a low volume, high usage application that can be amortized.

Slight quibble: you'd probably still want variable pitch props. Variable pitch props give you a lot of options for noise, speed, efficiency, etc that I wouldn't want to give up just because I have a different power plant. They're also extremely useful for engine-outs (you can feather them which reduces drag significantly) and for landing (you can reverse them which helps which reduces load on your brakes and lets you do questionable things if you need to get into a small strip.)

> DC motors are multiple times smaller and lighter than the equivalent powered turbine engine when you consider the entire system.

Airplanes this size can use cruise missile engines, which are very light. See the F-5 for more info (7:1 thrust to weight ratio.)

> On top of that you totally eliminate the need for variable pitch props

Why is that? Variable pitch props have nothing to do with engine type. They're variable to be more efficient depending on RPM and speed.

> You also negate a lot of hydraulic systems (and their redundant backups) without the need for things like fuel pumps, thrust reversers,

Not really. A lot of flight controls are boosted with hydraulics, and thrust reversers are for increasing drag (descending faster or slowing down.) None of these depend on engine type.

The real issue with electric planes is that they're a fire hazard. Already people have been killed when their electric plane's battery caught fire mid-air:

https://www.flyingmag.com/two-people-killed-in-first-crash-e...

The JAL 787 battery fire was a near-tragedy and resulted in interruption of service to SJC for 2 months:

https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_...

I find the most erroneous posts on HN to be about airplanes and databases.

>Why is that? Variable pitch props have nothing to do with engine type. They're variable to be more efficient depending on RPM and speed.

Variable pitch props have everything to do with engine type. They exist because turbine and piston engines have a very specific power/torque curve which requires them to operate at certain RPMs for certain conditions to achieve peak efficiency. With electric motors you have no need for this because you get a constant increase of efficiency across the entire power curve.

>Not really. A lot of flight controls are boosted with hydraulics, and thrust reversers are for increasing drag (descending faster or slowing down.) None of these depend on engine type.

Thrust reversers have nothing to do with descending faster or slowing down in flight, they are used solely for landing retardation. But an electric aircraft wouldn't even need thrust reversers. You literally just reverse the propeller immediately at full power when touching down. You can even save on less flaps required because the motors will be able to "windmill" on the descent and slow the aircraft as well as regen power.

Variable pitch props are useful for other design conditions. The prop RPM is typically limited to a speed that prevents the prop tip from reaching supersonic speeds, for noise certification. On fixed-pitch props at this fixed RPM, there's a tradeoff between climb and cruise performance, because these operate at two very different advance ratios. Constant-speed props allow the blade to adjust some of this difference.

There's also the engine-out design case, it's much easier to add prop feathering than add more power to the operating engines.

This design doesn't make a lot of sense, the props are located at literally the maximum arm from centerline, which requires bigger control surfaces for engine-out design (heavier and more drag) and gives very little ground clearance in crosswind landings. On the economics side, fuel costs are a small part of operating light turboprops/jets. The biggest costs are airplane depreciation/capital and crew. The real breakthrough would be an autonomous turbine plane that could eliminate crew cost and achieve 2000 hours/year utilization (versus ~200 for some privately-owned jets).

Your post exaggerates the danger from batteries. Tesla's billions of miles with fewer car fires than gas cars per mile driven is at the very least evidence that they are no more dangerous than conventional gas cars. The infamous boeing battery fire came about from poor design, and the battery wasn't used to run the engines. Tesla heats and cools their batteries and also tolerates batteries failing; boeing's design was a single large cell and did not tolerate failure, ie it was poorly designed.

Tesla has shown battery tech is a safe and reliable energy source. However, this doesn't mean there can be no problem - of course there can! Airplanes also have had many problems with catching on fire, crashing, burning in the air.

900KWH / (400WH/kg ) =2250kg > 2 ton for battery ?

That seem to be non trivial amount of weight.

I am curious how that compare to weight of typical fuel + engine for a small plane.

A pt6a runs 190-225kg dry. But I have no idea how much the combined weight of all engine systems, fuel, etc weigh together.

Also the fuel required to carry the remaining fuel goes down over the course of a gas powered flight, but obviously will not over the course of an electric flight. This would serve to further reduce the range. Curious too!

> 400Wh/kg [..] this is nearly double Tesla's energy density

Slight quibble: that's specific energy (ie, per mass). Density is per volume. (For airplanes, specific energy is the hugely important metric)

The airframe of airplanes is surprisingly lightweight. Engine(s) can easily be around 25% of the (empty) weight, and on anything with decent range, the fuel dominates. Both necessitates a lot of supporting infrastructure.

The airplane can probably pack battery cells much denser than Tesla. Tesla uses active, liquid-based cooling; an airplane could employ the airstream for that. Plus I expect a lot less of battery heating; while the capacity is 9x of Model 3, the engine power (thus electrical drain) is around just 3x of Model 3. [1] Also the metal shield under the battery can probably be done away with.

As others pointed out, electric engines are much different beast than any hydrocarbon burning ones. You need less support structure (for starters, less vibrations) and also much less auxiliary gear; think of all the cooling systems (either liquid cooling or flaps & openings & shrouds to direct air around); fuel tankage, pipes, valves, pumps & booster pumps; electric generator & possibly hydraulic pumps, and lots and lots of wires to both send electricity and control signals. As electric engines are essentially low-maintenance, you'd save a bunch of weight on monitoring instrumentation & servicing access panels. Maybe i'm reading too much into the renders, but the engine- and prop cowls seem to be very tight fit; both lessening drag and also simply being smaller, lighter parts.

Lastly, electric engines can typically be over-loaded way over their continuous power for well understood short stretches of time, facilitating extra power for prompt & safe take-off. Not so with piston or jet engines; the take-off power is typically maybe a dozen percent more than the rated continuous power, and that's about it. You end up carrying through the whole flight engines over-sized (and thus over-weight) just for the sake of take-off power.

Small weights savings all over the airframe add up.

--

[1] https://en.wikipedia.org/wiki/Tesla_Model_3#Specifications - 258 kW for Model 3 AWD, vs 3x260kW for the airplane.

That's an interesting statistic. According to Wikipedia, the Tesla's battery is around 500kg, so the aircraft is carrying around 4500kg in batteries. For comparison, the 10-seat Cessna 404 Titan weighs just 2200kg when empty and has a max take-off weight of 3800kg. So the battery alone weighs more than a similar Cessna, full of pax, luggage and fuel. Hm.

They list a MTOW of 6350 kg. Less 800kg for 10 fully clothed adults and the weight of the battery, there's only 1000kg left for the plane. Maybe that's possible?

About 3.8 tons they say for the battery. They're relying on removing some excess structure from the battery.

The thing is about 60% battery by take-off weight. That's not as crazy as it sounds. A 777 is about half fuel at max fuel load.

It burns that fuel as it flies though, reducing weight which is important for landing safely. The EV aircraft will weigh the same landing as taking off, which presents some complications.

...but those complications can be overcome. Partially because the airframe is so efficient that the landing speed should be tolerable even with a high landing mass.

Materials have improved significantly in the last few decades. There's a lot more possible now than in the past.

Good question. The Model 3 battery[1] weighs 478 kg so 9x = ~4,300kg. 11 people on board is another 1,000kg (not including the golf clubs). MTOW[2] is 6,350kg so the airframe needs to weigh in at ~1,000kg.

I'm sure they can make a battery lighter than a Model 3 battery (optimized for weight rather than cost), but how much lighter???

Unlike liquid fueled aircraft, you won't be able to trade off fuel weight for passenger weight to stay under MTOW.

[1] https://evannex.com/blogs/news/tesla-s-battery-pack-is-both-...

[2] https://en.wikipedia.org/wiki/Maximum_takeoff_weight

Excellent point. Often times only enough fuel for the route is taken so more payload can be taken. In a battery setup your max payload is always the same. No trade offs for range.

I would imagine that planes could have much higher battery cell per battery pack utilization, because the protective shell for the battery packs is less necessary.

Some back of the envelope math for the Model 3 battery: it has 4416 2170 cells, 66g each, is 291.5 kg, and the total battery pack weight is 478 kg. So only 60% of the battery pack weight is the actual battery cells.