Comment by MaxikCZ
4 days ago
4 replies and 3 are dismissing even the idea..
Yes, you get "some" back, and its not negligible amount. Typical modern airliner can descend on 15-20:1, giving you over 150-200km (90-120mi) range from typical cruising altitude of 33 000 feet even with engines off. Most everyday descents are actually done by maintaining altitude as long as possible, and then iddling the engines fully for as long as clearance allows. (Ofc you then use engines as you geat nearer, because its safer to be a little low when stabilizing on approach, than a little high)
Thanks to turbofans(edited from turboprops) better efficiency + less drag at higher altitude its actually more fuel economical to command full thrust and gain altitude quickly, than slower climb, or maintaining altitude (which goes against our intuition from cars, where if you wanna get far, you never give full throttle).
But theres still some drag, so you dont get everything back, so you generally want to avoid murking in low altitudes as long as possible. Full thrust repeatedly at lowest altitudes (from failed go arounds) is the least economical part of flight, so you want to avoid those if possible. But its true that the altitude you gain is equivalent to "banking" the energy, just not all of it.
(1) this was a jet, not a turboprop
Edit: changed turbofan into turbprop, which is what I meant.
(2) fuel burned stays burned, you don't 'get it back'
(3) the altitude gained may have been adjusted to account for the low fuel situation
(4) the winds are a major factor here, far larger than the fact that 'what goes up must come down', something that is already taken into account when computing the fuel reserve in the first place.
The it that they get back is not fuel, it's energy. Maintaining flight is energy management. They are getting the gravitational potential energy back, which is converted to velocity on descent, or bled off in drag by slowly losing altitude while maintaining airspeed.
So it seems. But because you want to land you then want to shed all that velocity. So you 'get it back' only to have to waste the bigger fraction of it. A go around is much like a mini take-off, you just miss the runway portion of it.
1 reply →
(1) The turbofan category of jet engine seems to inspire a lot of very pretty animated technical diagrams—here’s one set from a German manufacturer [0]. Now if only we could convince Bartozs Ciechanowski to take on such a subject… [1]
(2) I know glider pilots who fly without any fuel at all, once aloft… sounds not unlike the 150-200km glide range that @MaxikCZ mentions at idle from cruising altitude.
[0] https://aeroreport.de/en/good-to-know/how-does-a-turbofan-en...
[1] e.g. https://ciechanow.ski/airfoil/
Aircraft that are designed as gliders are much lighter and thus have much longer glide range than aircraft that aren't. They're so lightweight that they can climb on thermals. A 737 is not going to be able to do that, but a regular glider can't fly at 400 knots.
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Re: (2): There's a difference between sailplanes and gliders. Sailplanes are gliders that can “soar”, i.e. gain altitude just from the air that is moving up for some reason. Your friends have licence that says „Sailplane Pilot Licence”, not „Glider”.
The distinction is less pronounced nowadays, because there is no mondern aircraft designed as gliders-but-not-sailplanes, but historically there were planes that fit this niche, mostly military transport of WW1 and WW2 vintage.
Passenger jets (with engines turned off) are relatively decent gliders, but incapable of soaring. So no, you can't get more that about 20:1 glide ratio no matter how good is the weather (for sailplanes).
Regarding the turbofan and [0], above...if you're communicating to a non-engineer (me), how does the design get to the point of such complexity? I would love to learn the design story behind such an incredibly complex piece of machinery.
I am being serious, if you cannot tell.
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Yes, sorry, meant to write turboprop.
1 - a turbofan is a subset of jet engine, and there are no 738s running anything other than a turbofan.
Actually, nothing in civil aviation that has a "jet engine" has used anything but a turbofan (or turboprop) since the early 70s with the exception of Concorde and some older business jets.
(Turboprops are jet engines, too, to be precise, with the jet of exhaust gases powering the propeller.)
> Turboprops are jet engines
They are certainly turbine engines, but I thought "jet" was reserved for those engines that propel the vehicle solely by their exhaust stream and bypass air. I am willing to be told I'm wrong, though.
4 replies →
No, you don’t magically get the fuel back. But you do get a lot of the _kinetic energy_ back, and that energy keeps you flying without having to burn yet more fuel. You burn a lot of fuel while climbing, but then hardly any at all while descending. And that descent might cover 100 miles across the ground.
The 737-800 uses CFM56-7 turbofan engines.
[1] https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-...
1) Yea, sorry, turbofan, not turboprop nor a jet.
2) It stays burned, but the energy is banked in potential energy of the aircraft, namely in a form of altitude. If you run out of fuel 5 feet above ground, you dont get to fly far. When you run out of fuel 35000 feet above ground, you can still choose where to land from multiple options.
3) huh? I dont get what you trying to say, but: Its always more economical to climb, and the faster the better. Ofc you cant climb too high when you intend to attempt to land in 5-10 mins, but nontheless, every feet gained is "banked", and the aircraft is more economical to run the higher you are.
4) I am not saying the winds arent a factor, and in no way I was arguing about how fuel reserves are calculated. My only claim is that: yes, by spending more fuel to gain altitude, you can then "glide" down almost for free later. Its not 1:1, because of constant losses like drag, but its being compensated by higher engine efficiency and less drag at altitude, that its always worth it to climb if you can.
There was a flight that was low on fuel diverting to alternate between 2 islands. The pilot panicked and chose slower climb to intuitively save fuel. They had to ditch the plane in water because of it - if they initiated full climb, they would have made the jump.