[SoftTalker]

The fuel burn of take-off and climb substantially lightens the aircraft for cruise. Electric batteries have no such effect, you're carrying all that dead weight for the rest of the flight. This reduces the passenger or cargo capacity of the aircraft, which reduces potential revenue.

And what if you need two go-arounds?

[repiret]

Don’t conflate airplanes with rockets.

On an airplane, most of the energy in cruise is spent overcoming parasitic drag, not induced drag. It’s spent pushing the airmass out of the way as it moves forward, not creating lift to stay aloft.

For that reason, a change in weight does not significantly change cruise fuel usage.

Weight is still precious, but that’s because airplanes’ load are more often weight constrained than volume constrained, and capital and operating costs are such that you want to maximize the load.

[arijun]

> you're carrying all that dead weight for the rest of the flight

If you're recharging the batteries for extra go-arounds during landing, they are as dead weight as the fuel you would otherwise reserve for that purpose. And if you have 30% more efficient engines, meaning less fuel and smaller engines, it's possible you could come out ahead, weight-wise.

> what if you need two go-arounds

I assume that a go-around requires less sustained power output than a full climb from takeoff, so you will probably get more than one go-around anyway, and we don't know how much over-capacity they're designing for. In any case, any design will require tradeoffs in safety, and having more engine-out capabilities might improve safety enough to overcome the higher risk with go-arounds.

Not saying this project is will work out or that you're even wrong necessarily (this could be the equivalent of a concept car for Pratt & Whitney).

[vablings]

Due to various penalties, wind resistance. gear down and aircraft configuration. A go-around consumes a huge amount of fuel, not as much as climbing to cruise but its alot

[dmitrygr]

> I assume that a go-around requires less sustained power output than a full climb from takeoff,

No.

Source 1: PE = mgh

Source 2: am pilot

[fransje26]

So, for a Dash 8-100, at 13,000 kg, disregarding drag, engine efficiency, etc, to take-off and climb to 1000m and accelerate to 150 knots (77 m/s), you will need:

- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude

- 0.5 * 13000 * (77)^2 = 38.5 MJ, to accelerate to your climbing speed.

Total: 127 + 38.5 = 166 MJ, or about 46.11 kWh

For a go around, re-accelerating from 1.3 * stall speed (85 knots / 44 m/s) to your climbing speed, and going to your missed approach altitude of 1000 m, you will need:

- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude

- 0.5 * 13000 * (77^2 - 44^2) = 26 MJ to accelerate back to your climbing speed.

Total: 127 + 26 = 153 MJ, or about 42.5 kWh

[usrusr]

Nice to see some numbers. So for the peak load situations, a Dash 8-100 would not require a battery bigger than that a short range BEV ("city", though in reality the short range BEV use case is more for the rural equivalent of stuff that would be walkable in a city setting). And that's even before considering the energy contributed by the fossil fuel engine.

"13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude"

Presumably quite a bit of that would be harvested back during the descent that follows. The conventional engine would still need some excess power (relative to cruise load) to fill the gap left by drag and imperfect circle efficiency of the electric motor/generator, but mass x altitude is stored energy, not lost. (I'm still talking about the "what if we need a second abort" of the root post)

[arijun]

You don't do a full climb after a go-around, so the heights are not equal, and the mass is less since you've expended fuel. You also retain some kinetic energy but I assume that is closer to a negligible effect.

Also, PE = mgh is probably an not a great formula for energy cost of takeoff/go-around, as there are probably large costs it ignores (gravity loss, less efficient engine use, maybe less efficient turbines?).

For your source 2 I have no rebuttal so will have to defer to you, but would ask for an explanation.

[dmitrygr]

> You don't do a full climb after a go-around,

an IFR missed approach can have you climb quite high, especially in areas with serious terrain. Example: https://aeronav.faa.gov/d-tpp/2607/00346IZLZ17R.PDF airport is at 4400 feet over sea level, but missed approach says: climb to 13,000. Also, some go arounds will lead you to have to divert to an alternate airport, getting there may require climbing high to clear terrain or gaining required engine efficiency to fly the distance.

> And the mass is less since you've expended fuel

In our theoretical aircraft with batteries, mass is the same.

> You also retain some kinetic energy but I assume that is closer to a negligible effect.

Negligible indeed.

[arijun]

> you climb quite high, especially in areas with serious terrain.

Interesting, thanks.

> In our theoretical aircraft with batteries, mass is the same.

The fuel that's expended during cruise reduces the mass.

[SoftTalker]

Not a pilot, but on approach for landing you bleed off a lot of energy. For a go-around you need to reverse your descent and build up enough energy to fly away again. Take-off/Go-around tends to be the same throttle setting, AFAIK. Of course it also depends on how early you decide to throw away the approach and go around. Doing it at 1000 feet is different from bouncing it off the runway.

[serf]

but the point they were making is that it inevitably takes less energy to get to a level flying state (in similar weather conditions) due to fuel consumption.

so, unless the pilot is fighting weather it would make sense that equal throttle levels and equal pitch plans in equal weather conditions would require less and less fuel burn until the tanks are empty.

[card_zero]

Time to invent regenerative air brakes, like fold-out windmills.

[dmitrygr]

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

[buildsjets]

And besides that, there is the looping parachute APU.

https://patents.google.com/patent/US9452721B2/en

[Melatonic]

Would imagine these are significantly more useful on heavier and faster aircraft - surely the weight and whatnot to retract move them is less worth it for smaller planes ?

[buildsjets]

They are commonly used on small single-engine fighter jets as well. They typically cannot be retracted once deployed, and free-fall using their own mass, so there is no actuation system weight to account for.

[0cf8612b2e1e]

Energy density of liquid fuels cannot be beat by batteries, so this is not competitive if you are looking to maximize cargo. However, there are plenty of short haul flights: private jets, island hopping, regional routes where you need to move little mass.

[usrusr]

"And what if you need two go-arounds?"

Easy: you don't try the second landing approach before the battery is sufficiently recharged to contain enough energy for the second abort. Chances are this does not take any longer than going through the pattern anyways.

The saving is not just the dead weight of the bigger engine you'd need to do take-off, climb and abort without electric assist, it's also the fuel saved during cruise from running an engine that is completely designed for efficiency at cruise load instead of for some compromise between cruise efficiency and sufficient peak power for start and abort.

[xattt]

Exactly. A hybrid passenger car can tolerate unpredictable power output that may come with an auxiliary power setup that may or may not be available when stronger dynamics are called for.

A plane doesn’t have this luxury and needs predictable output. The fossil fuel engine either needs a sacrificial “overboost” mode for emergencies (at the cost of wear/long-term longevity), or has to be sized for full power at the ultimate cost of efficiency.

[Melatonic]

So theoretically if the electric motors fail (or battery is dead) an engine could be sized and designed smaller (for cruise efficiency) but have some sort of boost mode that still ensures safety ? At the cost of increased maintenance or wear or something if it must be used

[Tade0]

On anything but very short flights most of the fuel is spent on cruising.

[dmoy]

I would classify 290 miles as a very short flight, that's like 1-2 hours or something?

[hobonation]

Valid.

Perhaps it's not all negative: the electric portion could give a pilot a bit more glide than the gas portion dies.