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Comment by david-given | Hacker News Reader

I dunno. Remember it's got wings, which means once it shifts into forward flight mode, the engines are used solely for propulsion and not for lift, which is vastly more efficient.

Playing with the numbers here:

http://web.mit.edu/lochie/www/erange/main.html

...shows it can be made to work, provided the thing has very low drag (I was using a battery specific energy of 200 for a LiPoly cell). Given it's a streamlined blob with stubby wings, that seems vaguely plausible.

Anyone with a better grasp of the principles want to comment?

Update: I found their tech page (see link upthread). They use lithium ion. They have about the same specific energy as lithium polymer.

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Look at the power requirements for similar aircraft to get an idea of range. An SR-20 (similar size, speed and composite) needs 180hp and probably 140 for the roughly 300kmph cruise. That's 104 kW for an hour plus the FAA 30min minimum remaining at maybe 80 kW (higher for commercial ops) that's 720kg of batteries or roughly the the empty weight of a cessna 172

edit: this is also very generous because it assumes you don't need to takeoff and climb. Max cruise on the SR-20 is 155kn so the power requirement is as much as 20% greater

I wonder how head-wind factors into that as well, I know on our EV I can easily see 10-15% consumption difference if I run into a strong one at freeway speeds.

Wind will always affect flight speeds regardless of configuration. To use physics terminology, the air around you becomes your frame of reference. 10kt tail wind and 100kt True Air Speed = 110kt ground speed

A SR-20 has a single engine at the front of the plane rather than 36 scattered over the wings. They claim, at least, that that makes a big difference in aerodynamic efficiency.

It does in some cases (see NASA X-57 where the fan is used to accelerate air over the wing, leading to a smaller wing with lower drag overall) but I'm not convinced in this case. Having a system that is efficient at high airspeed and high thrust generally requires a variable geometry propeller (constant speed prop in GA terms) because a 'lift fan' at high relative airspeed, would have to spin much faster than a 'cruise fan' like you would see on a regular aircraft meaning airflow over the blades is higher leading to higher drag on the blade than necessary.

I'd also emphasize that they have 9 years on their runway (pun intended) until they plan to have it operational for consumers. That's a long time for battery tech to get better.

Not necessarily. Don't be fooled by the improvements in semiconductors and storage in the last decades: not everything doubles every 2 years. Battery specific energy seems to grow at around 5% per annum, doubling only every 14 years.

You can source Li-ion/Li-Poly fairly easily (as a company, almost impossible as an individual) with 250 Wh/kg, I believe I read somewhere that Tesla hit about 260/270 with their latest power walls. Even so, I still have to doubt they'll reach more than 200km range with current technology.

Their mission is to release to the public in 2025 however, which makes the goal much more likely to be hit with improvements in technology.

Lift and drag are inherently connected; you can't have one without the other. So it doesn't really make sense to say that engines are not used for lift, when the drag that the engines are countering at least partially is induced by lift.

https://en.wikipedia.org/wiki/Lift-induced_drag

The problem though is with safety. This kind of design can work fine for drones, because if the power fails and a drone is lost it's not a big deal. But for a passenger carrying aircraft, you must have some semblance of aerodynamic control and glide in a dead stick situation. This thing would be a complete deathtrap.

I'm not in the aerospace industry, but couldn't it deploy a parachute in the case of power loss? Or are the batteries too heavy?

Also, what about helicopters? I guess the pilot might still have control after losing power because the blades would still be spinning?

Good luck falling from 100 m or so while deploying your chute. That's the problem zone, high enough and a chute would work, closer to the ground and there just isn't time enough for the chute to slow you down. Even a crash into water from that altitude and you'd likely be dead.

Weight and altitude are the problem with the chute. This thing would result in a fatal accident at low altitude with power loss (total, they say it can operate with some systems lost) that are too low for a chute to deploy before you hit the ground. A similar problem exists with many ejection systems for military aircraft (pilot will escape the aircraft if it's on the ground, but not reach sufficient altitude for the chute to help when they return to the ground). Helicopters can auto-rotate, allowing for degraded performance during the descent but still controllable and survivable.

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