Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Launch HN: H3X (YC W21) – High power density electric aircraft motors

news.ycombinator.com

Post reply on HN
Print view View original

91–100 of 169 posts

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#91

Post by avidiax »

mliben wrote: post #59
serjester wrote: post #32

Earlier quoted context omitted.

Keep in mind that an electric motor isn’t limited by the amount of oxygen in the air. As a result it can fly significantly higher where there is far less air resistance. Since air density is proportional to the square of the elevation this can lead to significant efficiency gains. Believe it or not, partly as a result of this, the SR-71 had it’s best mpg at peak speeds.

A simple physics-based plane model (like the one we made to understand vehicle-level impact of our technology development) dictates that the range-optimal cruise speed is proportional to 1/sqrt(air density), so it makes sense that the blackbird was more efficient at high speed when at high altitudes (admittedly, this simple model is subsonic, and there are a lot of other factors for supersonic flight). Since having l…

> there are significant high voltage insulation challenges at higher altitudes

What are these challenges? How does having a near vacuum cause trouble with ~1kV potentials?

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#92

Post by elihu »

quickthrowman wrote: post #70
elihu wrote: post #65

Earlier quoted context omitted.

A simpler alternative could be to just have an electrified runway. The plane draws power from power rails embedded in a runway, or something like that. So, it doesn't switch to batteries until it's in the air. You could even have a long cable that hangs behind the plane and keeps an electrical connection until you're a few hundred feet up. (I'm picturing it connected to something like a slot-car that travels in an el…

> You could even have a long cable that hangs behind the plane and keeps an electrical connection until you're a few hundred feet up. I can assure you this will never ever happen. It’s wildly impractical, improbable, and sounds extremely unsafe. Sure, it’s theoretically possible, but that’s about it. The NFPA is not going to add a code section in the NEC for hundreds of feet long live electrical conductors being pull…

Tow planes and gliders routinely fly with a disconnecting cable between two aircraft, and that seems at least as impractical and unsafe (or it would if you were proposing it as a new idea). Though maybe that's the sort of thing that's "grandfathered in" from earlier, more permissive days of experimental aviation.

I think the strongest argument against using a power cable during takeoff is just that it's not worth the effort and complexity just for a slight increase in range, except in rare situations or planes that normally make very short flights and don't want to be weighed down with extra batteries (like the aforementioned glider tow planes).

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#93

Post by outworlder »

elihu wrote: post #65
mercurywells wrote: post #8

Earlier quoted context omitted.

> One aircraft we looked at required 30-35MW during takeoff and ~10MW during cruise. Do you envision some airframes to include assisted take-off technology? (JATO and the like, even catapults)

A simpler alternative could be to just have an electrified runway. The plane draws power from power rails embedded in a runway, or something like that. So, it doesn't switch to batteries until it's in the air. You could even have a long cable that hangs behind the plane and keeps an electrical connection until you're a few hundred feet up. (I'm picturing it connected to something like a slot-car that travels in an el…

As a rule, whenever one feels tempted to say "just do ", it's time to wait and think. Because, if it's "just" about doing something, why isn't it being done already?

In this case: let's say it's feasible to retrofit runways to use this system (it probably isn't) and look at a few issues.

For instance: "the cable releases from the plane". No system is fail safe. What happens if the cable does NOT release from the plane? What happens if it snags during the takeoff roll? What happens when there's wind gusts?

If there's no cable, and it's "just" a rail, presumably the plane is taking off aligned to the rail. What happens if the alignment is off? Or is the 'rail' supposed to keep the plane straight? If so, what about the force distribution on the plane's landing gears or (if a specialized system is installed), in the fuselage?

So say you have such a system and everything has been retrofit. What happens if there's an issue with the land-based generator during the take off roll? Would the aircraft still have enough power to perform the take-off from the onboard batteries? If so, this is just about range and the system would never be installed, as aircraft would be certified with the lower range instead. If not, it's a disaster in the making.

> I keep wondering if there could be a way to re-charge in flight so that battery range/weight wasn't such an issue, but that's a hard problem.

There isn't unless you can transfer power from elsewhere. In-flight "refueling" from another plane is out of the question. You are essentially left with beamed power from ground stations (or orbital if we are really forward thinking). That might theoretically be feasible (planes don't have a very large surface area so the power delivery system would probably look like a weapon and mostly behave like one). Engineering it is another matter, not to mention practicality.

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#94

Post by mliben »

LeifCarrotson wrote: post #85
mliben wrote: post #77

Earlier quoted context omitted.

Aluminum has the best conductivity/mass out of all of the common conductors. Motors are actually more volume constrained than mass constrained for the windings, which is why copper is typically used there instead.

Copper is also 60% better in the thermal conductivity properties as well, which is another critical property. Power density in an electric motor is really based on how fast you can remove heat from the motor. I'm involved in sizing industrial servomotors, but even there you have 1s/10s/60s power ratings. I wonder if H3X can post higher power levels for takeoff, assuming it starts cold and the flight plan calls for th…

Yes, thermal conductivity is of the utmost importance at the continuous current densities we are designing for.

That being said, typically the effective thermal conductivity of the winding (perpendicular to the axis of current flow) is limited by the insulation (strand and/or turn) and the encapsulation/varnish. As a result, changing the thermal conductivity of the conductors themselves will have much less impact on the total thermal resistance (from winding hotspot to coolant) than changing the insulation and encapsulant thermal conductivities.

At these very high power densities, the thermal RC time constants inside the motor are very short (small motor = small thermal capacity, low thermal resistance by design). Therefore, even for a "short" 10 minute takeoff, most of the motor will have already hit thermal steady state. As such, the motor needs to be able to run at takeoff power continuously. There has been a lot of fun discussion elsewhere in this thread about how to tackle that aspect of the problem (given that takeoff power is typically 3x cruise power).

I will say that we are working on developing a high thermal conductivity (> 1 W/m-K) and high temperature (> 300 C) insulation system.

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#95

Post by DaniFong »

exciting work! i'm curious has to how you think about hte tradeoff between power and efficiency? I guess you need to design for very high specific power for peak power requirements (takeoff / landing manoeuvres) but can still achieve good efficiency in a cruise

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#96

Post by 5600 »

cjbprime wrote: post #89
elihu wrote: post #65

Earlier quoted context omitted.

A simpler alternative could be to just have an electrified runway. The plane draws power from power rails embedded in a runway, or something like that. So, it doesn't switch to batteries until it's in the air. You could even have a long cable that hangs behind the plane and keeps an electrical connection until you're a few hundred feet up. (I'm picturing it connected to something like a slot-car that travels in an el…

Or for an even simpler alternative, just get towed into the air as motorgliders are (the ones with sustaining motors but not auto-takeoff). I wonder if this tech might be better suited to self-launching motorgliders than GA.

It certainly seems like something that will be towed.

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#97

Post by LeifCarrotson »

The main technical surprise to me from your pitch is the integration of power electronics and motor in one package. Your windings, at 96.7% efficiency and 200 kW, need to dissipate 6.6 kW of heat. Your SiC FETs are more efficient and therefore produce less heat, and while they can take high temps, are more efficient at lower temperatures. Why not connect the inverter to the motor with a short cable, and package those separately?

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#98

Post by mliben »

Retric wrote: post #90
mliben wrote: post #59

Earlier quoted context omitted.

A simple physics-based plane model (like the one we made to understand vehicle-level impact of our technology development) dictates that the range-optimal cruise speed is proportional to 1/sqrt(air density), so it makes sense that the blackbird was more efficient at high speed when at high altitudes (admittedly, this simple model is subsonic, and there are a lot of other factors for supersonic flight). Since having l…

Unless I am missing something obvious, lift-induced drag is largely independent of altitude. However, parasitic drag is significantly reduced by lower air pressure. Thus the advantage from high altitude flight.

Less dense air -> higher angle of attack required to produce required lift -> true lift vector is more offset from vertical -> horizontal component of lift is actually producing drag

Like I said in the other comment, if the plane is operating at the range-optimal speed, I think the air density does not impact the range capability (it cancels out) but it does increase the range-optimal speed, allowing for faster travel.

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#99

Post by outworlder »

elihu wrote: post #92
quickthrowman wrote: post #70

Earlier quoted context omitted.

> You could even have a long cable that hangs behind the plane and keeps an electrical connection until you're a few hundred feet up. I can assure you this will never ever happen. It’s wildly impractical, improbable, and sounds extremely unsafe. Sure, it’s theoretically possible, but that’s about it. The NFPA is not going to add a code section in the NEC for hundreds of feet long live electrical conductors being pull…

Tow planes and gliders routinely fly with a disconnecting cable between two aircraft, and that seems at least as impractical and unsafe (or it would if you were proposing it as a new idea). Though maybe that's the sort of thing that's "grandfathered in" from earlier, more permissive days of experimental aviation. I think the strongest argument against using a power cable during takeoff is just that it's not worth the…

> Tow planes and gliders routinely fly with a disconnecting cable between two aircraft

This is true. However, the cable is not carrying KW or megawatts of electricity, it's just there for tension, to transfer forces from something else to get the glider airborne.

Technically, you don't even need the tow plane, some places perform winch launches (or car launches!) exclusively. This is very common where general aviation is not as common.

Should the cable not detach (extremely rare), it can be cut at the other end. Cutting a live cable should be much more interesting. Other issues, the glider can release it. The glider will most likely be fine, even if the flight is now cut short.

Gliders are very light and still the cable weights a lot. That's probably the limit of what's practical. There are some gliders with electric motors, they don't need all that much power, by definition. Some can even self-launch.

Re: Launch HN: H3X (YC W21) – High power density electric aircraft motors

#100

Post by mliben »

avidiax wrote: post #91
mliben wrote: post #59

Earlier quoted context omitted.

A simple physics-based plane model (like the one we made to understand vehicle-level impact of our technology development) dictates that the range-optimal cruise speed is proportional to 1/sqrt(air density), so it makes sense that the blackbird was more efficient at high speed when at high altitudes (admittedly, this simple model is subsonic, and there are a lot of other factors for supersonic flight). Since having l…

> there are significant high voltage insulation challenges at higher altitudes What are these challenges? How does having a near vacuum cause trouble with ~1kV potentials?

The phenomenon is due to Paschen's law (there is a good wikipedia article on it). The breakdown potential of a gas is minimized at some pressure, and in the case of air, that pressure is I can't go into much detail, but we are working on addressing this in a couple different ways in our insulation system design.
Post reply on HN