4 ms·

You get that energy back on descent, no?

• bagels 1y ago
  Some of it, but much is lost to drag. They do have to limit speed at all times.
• tamcap 1y ago
  Not really. While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel. One of the challenges of aeronautics is the efficient disposition of the potential energy without converting it all into kinetic energy (ie speed) so that the landing happens at an optimally low speed - thus giving you a chance to brake and slow down at the end.
  • Reason077 1y ago
    > "While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel." An electric fan aircraft absolutely can recharge it's batteries on descent. The fans simply act as turbines, creating drag to slow the aircraft and electricity to charge the batteries. Large commercial airliners already have a small turbine that works this way, the Ram Air Turbine (RAT) which is used to generate electrical power in emergencies.
    • tamcap 1y ago
      You can use a turbine to generate electricity, so yes, you are converting potential energy into electrical potential. However, no real mass produced passenger plane today can use that electricity for flight (thrust). RAT is only used when sh*t hits the fan. Even then, it can help you power some hydraulics / electrical, not “store” energy for further flight. The OP asked - in a low fuel situation, can the energy spent on a climb get effectively recovered - and the answer is not really. We convert as much as we can into unpowered (low-powered) descent. But once you are at a spot where you make a final decision to land or not, you are by design low and slow - and all that energy you had 15m ago is gone. If you need to keep flying, those engines need to spool back up. And that takes fuel.
      • rkomorn 1y ago
        > RAT is only used when sh*t hits the fan. Isn't it when air hits the fan, technically? (Sorry.)
      • Reason077 1y ago
        > "no real mass produced passenger plane today can use that electricity for flight (thrust)" Such aircraft do exist. For example, the Pipistrel Velis Electro trainer. And more recently, the Rhyxeon RX4E became the first electric aircraft to be type-certified for commercial passenger operations. It's likely that we'll see many more electric fan aircraft in the coming years/decades, whether powered by batteries and/or hydrogen fuel cells, or hybrids with both conventional turbofan and electric propulsion in order to improve efficiency and environmental performance.
        • tamcap 1y ago
          > It has a cruise speed of about 200 km/h (\(124\) mph), a payload of \(350\) kg (\(770\) lbs), and a range of approximately 300 km (\(186\) miles) While it’s a real plane, I think we can both agree this is more of a prototype than what the general public would consider to be a passenger plane. Yes, energy recovery is possible. It’s not happening currently in commercial aviation.
• epcoa 1y ago
  How? On descent you can trade some of your altitude (potential energy) for kinetic energy, but then you can’t land the plane. For descent on an approach you’re going from low energy to even lower energy. In emergencies and with enough runway you can futz around with this some, but wiggle room on an airliner is not great, negligible to what will be expended on a go around.
• recursive 1y ago
  As someone who has ridden a bike up a big hill, and then down it, I don't think you get it back.
  • krisoft 1y ago
    That is perplexing. Of course you get the potential energy back. It turns into kinetic energy as you descend. That is why you need not pedal downhill, and often even need to brake to prevent the bike from speeding up too dangerously.
    • anonymars 1y ago
      > often even need to brake to prevent the bike from speeding up too dangerously. Indeed, which is what the airplane would have done on its way down to land. So it's more like riding the brakes on your way down the hill, and now at the bottom when you realize you need to abort the landing, you are at low speed and it's quite an exercise to get back uphill to try again
      • howard941 1y ago
        The glider guys would always suggest a forward slip. It's a lot of fun to do. It's not taught often enough during primary training for powered airplanes.
        • anonymars 1y ago
          Yes, but that also doesn't get any energy back on descent, quite the opposite, that is "riding the brakes on your way down"
          • howard941 1y ago
            Well it's not all lost otherwise it'd be a stall spin accident caused by performing the maneuver with too little airspeed. And that's hard to do. It's a noisy maneuver, the air slamming against the fuselage makes itself heard. Once performed it's not easily forgotten. More dangerous than inadvertently spinning with too little airspeed is the possibility of shock cooling when relying on a forward slip for too much altitude loss. It really does need to be well-controlled.
• robotresearcher 1y ago
  No, and you don't want it. You want to be on the ground and stopped. In the lowest energy state. It's not currently feasible to harvest it into fuel. It's (very very nearly) all lost to drag, on purpose.
• MaxikCZ 1y 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.
  • jacquesm 1y ago
    (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.
    • FireBeyond 1y ago
      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.)
      • deleted 1y ago
• mrguyorama 1y ago
  Some of it. The air density is an important part of efficiency at higher altitudes, so every moment spent under like FL320 is wasted fuel. So the entire climb "up", you are also wasting energy fighting the thick air. On the way back "down", that air again fights you, even though you are basically at idle thrust. Your fuel reserves are calculated for cruise flight, so time spent doing low altitude flying is already at a disadvantage. "Two hours of reserves" is significantly less than that spent holding at a few thousand feet. Fuel efficiency while climbing is yet again dramatically worse
• csours 1y ago
  Wow this has a lot of replies! Yes, you get a lot of the energy back, BUT there is a huge problem! Large airliners incur a LOT of additional drag to slow down while landing. Some of that is entirely intentional, some is less intentional. It is highly preferred to deploy the landing gear before touching down. Failure to do so may lead to a hard landing and additional paperwork, so airlines do not allow the captain to exercise their own discretion. Extending the flaps maintains lift at lower speed, and higher flap settings allow even lower speed. The highest flap setting generally also deploys leading edge slats. If the wheels of the airliner touch down and detect the weight of the plane then spoilers kill the lift of the wings, air brakes fully deploy, as well as thrust reversers. All of these things add drag, which uses up all that energy you've been converting. The upshot is that each landing attempt uses a LOT of energy, and you have to use fuel to replenish that energy after every attempt. In other words, yes you get it back, but only for one landing attempt.
• throwaway173738 1y ago
  The problem isn’t getting the energy back, it’s doing so more slowly than gravity. Planes are somewhat limited in their ability to glide.