Electrifying flight: very different aircraft may end up taking to the sky • Hacker News Board

Re: Electrifying flight: very different aircraft may end up taking to the sky

#51

Post by Animats » Fri, Sep 25, 2015, 7:13 PM UTC

erikpukinskis wrote: post #20

Animats wrote: post #15

A related possibility is multi-rotor helicopters where the main power source is a jet engine, but the power distribution is electrical. One of the huge headaches of the Osprey is the mechanical linkage by which one engine can power both props. There are flexible shafts, U-joints gears, and a big clutch. This might provide a way to get to VTOL without insane mechanical complexity. NASA has built a big model tiltrotor…

Elon Musk has said he would just put one big engine on a gimbal and just ditch the ailerons and everything. No clutch necessary.

One-engine VTOLs need an ejection seat.

NASA built several "one big engine on a gimbal" lunar landing trainers for Apollo. Over the short life of those special-purpose craft, three pilots had to eject - two astronauts and a test pilot.

Re: Electrifying flight: very different aircraft may end up taking to the sky

#52

Post by credit_guy » Fri, Sep 25, 2015, 8:00 PM UTC

Shadow6363 wrote: post #42

rflrob wrote: post #2

I would be curious whether electric planes significantly reduce the carbon footprint of flying. My intuition is that you wouldn't be dumping CO2 and H2O (both fairly potent greenhouse gases) directly into the upper atmosphere, so it would be a huge win, but I am neither a climate scientist nor an aerospace engineer, so maybe I'm missing something.

Water is a greenhouse gas?

Not only water (vapor) is a greenhouse gas, it's by far the most important. Per wikipedia, its contribution to the greenhouse effect is 36-72%, while the next contribution, of CO2, is 9-26%.

https://en.m.wikipedia.org/wiki/Greenhouse_gas

Re: Electrifying flight: very different aircraft may end up taking to the sky

#53

Post by nkoren » Fri, Sep 25, 2015, 9:12 PM UTC

mrfusion wrote: post #37

nkoren wrote: post #35

The real breakthrough in electric aviation will come when it is combined with beamed energy. Rather than storing power onboard the vehicle, you have ground-based stations which power the vehicles with tracking microwave tightbeams. That way, you only need enough onboard battery power to take off, land, and bypass one broken ground station -- maybe 15-20 minutes of total flight time. This would allow electric aircraft…

What about the beams killing birds? Maybe have three beams that don't converge until the plane?

First, you can place the beaming stations on towers -- most birds stay within a few hundred feet of the ground (which is why skydivers don't have to worry about smacking into them). That'll rule out the majority of bird conflicts. Second, put a high-definition video camera aimed down the barrel of the beam, with enough field-of-view that you get a second of warning before a bird crosses the beam. With automated image recognition you should be able to switch off the beam within a few hundred milliseconds of detecting a bird. Switch it back on again when the bird leaves the beam path. Because the plane isn't being directly powered by the beam -- it's just topping up its onboard batteries -- the disruption should not be consequential.

Re: Electrifying flight: very different aircraft may end up taking to the sky

#54

Post by thedrbrian » Sat, Sep 26, 2015, 11:28 PM UTC

alkonaut wrote: post #25

ams6110 wrote: post #5

Li ion batteries still have a LONG way to go in energy density compared to Jet A kerosene. Jet A, about 43 MJ/kg. Lithium ion, <1 MJ/kg.

You have to consider the system as a whole. The kerosene tank with kerosene, plus the motor and gearboxes = X kg. The battery + The electrical motors and driveline = Y kg. Example take a small aircraft as an example: - Combustion: Motor 150kg (1kW per kilo) fuel 100kg (43MJ/kg) = 250 kg, 4300MJ - Electrical: Motor 20kg (5kW per kilo) batteries 230kg (1MJ/kg) = 250kg, 230MJ The amount of batteries carried is more than…

On the other hand batteries don't get that much lighter as you use the energy stored within. Also you're going to have to design quick capacity change battery packs or always fly with the maximum weight penalty.

Re: Electrifying flight: very different aircraft may end up taking to the sky

#55

Post by alkonaut » Tue, Sep 29, 2015, 9:05 AM UTC

thedrbrian wrote: post #54

alkonaut wrote: post #25

Earlier quoted context omitted.

You have to consider the system as a whole. The kerosene tank with kerosene, plus the motor and gearboxes = X kg. The battery + The electrical motors and driveline = Y kg. Example take a small aircraft as an example: - Combustion: Motor 150kg (1kW per kilo) fuel 100kg (43MJ/kg) = 250 kg, 4300MJ - Electrical: Motor 20kg (5kW per kilo) batteries 230kg (1MJ/kg) = 250kg, 230MJ The amount of batteries carried is more than…

On the other hand batteries don't get that much lighter as you use the energy stored within. Also you're going to have to design quick capacity change battery packs or always fly with the maximum weight penalty.

That's true, the average weight of the fuel is probably 60-70% of the takeoff weight or something, varying wildly depending on aircraft type and flight. Takeoff weight is probably the limiting design factor, so there you can compare the batteries to the full tank, which is what I have above.

On the other hand jet airliners are designed to be lighter when they land, landing gear and brakes don't have to manage a landing at full weight (which is why they have to dump fuel before making an emergency landing immediately after takeoff). An electrical plane would have to have brakes and landing gear designed for the heavy batteries, as it will be as heavy at landing as it was at takeoff. This might add quite a lot of weight.