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Comment by taf2 | Hacker News Reader

I wonder if with electric we can fly higher like much higher maybe 80k to 90k since we don’t have to worry about the engine stalling. Could this also make the flight times comparable or faster? For sure less turbulence. As someone who lives on a flight path I’m eager for the change in noise pollution too- though I’m worried the transition will take decades...

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You can't really go above the Armstrong limit (~60k feet) in a passenger airplane without adding huge complexity in handling the event of cabin pressure loss.

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

I see that does making exceeding 60k pretty scary. I was from London to Denmark and we had cabin pressure loss I think we were maybe at 30k and dropped pretty fast to 20 if I recall correctly. The sensation that I remember was slow motion and shock from the flight attendant as the masks did not deploy. Later after landing the captain told us during preflight they set the cabin pressure incorrectly such that as we went too high an emergency system kicked in the prevent the cabin from exploding... I was maybe 12 so the details are fuzzy but definitely the take away is cabin pressure loss is no joke

Hard to puzzle the details there without knowing the aircraft type, but in the pressurized piston plane I fly, there is a main outflow valve used to regulate pressure that's under my (indirect) control and a safety outflow valve that's set just over the rigged max pressure differential (such that it partly dumps the cabin if that pressure is exceeded).

Given that your flight didn't get the "rubber jungle" of deployed masks, the cabin pressure probably never dropped precipitously low, but rather started to oscillate as the safety valve dumped and closed, but never dumped enough to deploy the masks and the crew declared and descended in order to sort things out.

A small number aircraft (non-airliners) are lost each year due to pressurization issues. It's a serious business, even in the high 20s and 30s (of 1000' MSL).

Single pilots are required to continuously wear O2 mask at/above FL350 (35000') and one pilot of a two pilot crew must continuously wear O2 over FL410. Between FL350 and FL410, two pilot crews may rely on quick-donning masks. This is believed to be a commonly violated regulation (in that air crews regularly do not wear the required mask when things are operating smoothly).

> This is believed to be a commonly violated regulation

I believe that is an understatement. I've heard it described as the most commonly violated rule in aviation. There are a number of youtube pilots who commonly violate this rule seemingly without too much worry.

Pressurized cabins are going to be essential for electric flight to have any kind of reasonable speed (and range).

It shows that the most impressive electric aircraft specs are for the pressurized Eviation Alice (1000km, minus margin for contingency, and 250 knots cruise). If you stay low enough to not require pressurization, then you have to compromise the lift to drag in order to have a decent cruise speed or you have to tolerate a really low cruise speed.

Altitude is essential. Bite the bullet and build a pressurized cabin so we can get on with replacing fossil fuel aviation with full electric. https://www.eviation.co/alice/

Doesn't have to be above the Armstrong Limit, but it sure does help to be above 10,000 feet.

Eventually we'll have supersonic electric aircraft. To have sufficient efficiency, they'll need to be at or above the Armstrong Limit, like Concorde. (And perhaps higher, like the 96,000 feet record holder for horizontal powered flight, NASA's Helios... which just happens to be electrically powered. https://en.wikipedia.org/wiki/Helios_Prototype .)

EDIT: High altitude enables you to use an extremely efficient airframe with sailplane-like lift to drag but STILL achieve high cruise speeds. For instance, the Perlan II glider actually has no engine and is able to soar higher than any towplane, above 76,000 ft where it flies at about 250 knots (actual airspeed). Without any engine at all. https://www.youtube.com/watch?v=NnpE5xS1g80

And here's a solar electric aircraft aiming for similar goals: https://www.solarstratos.com/en/challenge/

All passenger jets have pressurized cabins, but they are working at lower pressure differentials than what you need above 60k feet, and they sometimes fail, with oxygen masks being the fail-safe.

If you go above 60k, and the plane experiences a rapid loss of cabin pressure, you have 60 seconds to restore cabin pressure before the passengers start dying. So the failsafe system will have to be massive. That increase in weight and complexity isn't worth the efficiency gains.

You mention Concorde, and indeed it had a very substantial failsafe system even though it only touched the lower end of the limit. Concorde had really small windows, so even with two windows gone it took some time to equalize pressure. The pilots had positive pressure oxygen masks, and the plane had the ability to drop altitude immensely fast in an emergency.

Concorde flew above 60,000ft. You may just have to demonstrate loss of cabin pressure is rare enough that it's worth doing.

"Engines stalling" is not really the factor that limits altitude. The problem is that the thinner the air, the faster you have to go to stay up. Eventually the required speed enters the transsonic region, and you start needing way more power. There comes a point where your engines just don't have the oomph to push you fast enough to stay up. If you want to go higher, there are basically two ways - more oomph, and better wing loading (i.e. less weight or bigger wings.)

Electric aviation has many merits, but being a lightweight source of plentiful oomph is not currently among them.

I imagine planes could fly a bit higher, but aeroydynamics are still going to be a big factor. See also coffin corner.

Engines don't stall, planes do. Plane stalls are about aerodynamics, and nothing to do with the engine.

Current engine designs operate poorly at higher altitudes for a number of reasons: less air density meaning less oxygen to burn and less air to push against. Turbine engines can flame out, which maybe that's what you were referring: https://en.m.wikipedia.org/wiki/Flameout

Stalling is a term also used with engines, and it's pretty clear he's talking about engines not working at altitude so "stall" is probably the correct term

Only with layman and in the media when incorrectly describing an aerodynamic stall and conflating two.

https://en.wikipedia.org/wiki/Stall_(engine)

That's about manual transmission automobiles. I've never heard it referenced in aviation other than non-pilots or people who know airplanes.

Yeah I’d imagine most of the reasons sited from this hat wiki article don’t apply to an electric plane. But yeah air density would definitely be an issue but I thought the nasa Helios flight showed 90k to be the upper limit? So figured pushing to bear that altitude could be achieved and possibly beneficial since it would avoid bad weather and reduce drag... I guess was imagining the bulk of the engine power would be consumed at the start of the flight to reach altitude and then from there mostly glide until descending to land... but I’m not an aviation engineer at all just dreaming of quieter sky’s and less pollution

In my estimation, the shortest practical path to less pollution (at least less net carbon emission) is to use electricity to synthesize jet-fuel. Take yesterday's carbon dioxide, turn it into tomorrow's jet fuel, making the fuel carbon-neutral.

Turbine compressors can also 'stall': https://en.wikipedia.org/wiki/Compressor_stall

Higher means lower air density, meaning less lift, meaning higher power usage, which is even more critical with electric

Lower air density also means both lower drag and more slippage; would definitely need adjustable pitch.

Although electric motors are capable of very high power density. If the flight is substantially shorter than the full range, flying at a faster speed, or higher altitude may be practical.

I'm doubtful about passenger aircraft but the Facebook solar plane was supposed to go to 90k feet https://en.wikipedia.org/wiki/Facebook_Aquila

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