Comment by recursive
Comment by recursive 4 days ago
As someone who has ridden a bike up a big hill, and then down it, I don't think you get it back.
Comment by recursive 4 days ago
As someone who has ridden a bike up a big hill, and then down it, I don't think you get it back.
> 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
100%. You are correct on that. You can’t use your kinetic energy to go around after a landing attempt.
But not because “you don’t get the energy back”. (As recursive suggested about a downhill bike ride which is the part i am disagreeing with.) You do get it back, but because you want to land you bleed it away to drag. And once it is bled away you don’t have it anymore.
So we don’t disagree about the practical implications for flying. I’m disagreeing with recursive’s particular statement about downhill cycling and what it implies about the physics of the problem.
Imagine a hill with 500 feet of elevation descent, followed immediately by 500 feet of ascent. No curves.
If you coast all the way down the first part, you'll get about 20 feet up the other hill before you need to start pedaling. This is a direct analogy to "getting your energy back" by losing elevation.
That is exactly what a rollercoaster does and it doesn’t start “pedaling” after 20 feet. Of course real systems have losses and you can’t practically use all the energy.
But you don’t have to believe me. Look at the video of this glider doing an unlicensed airshow: https://youtu.be/QwK9wu8Cxeo?si=L-0Mfmu8wk1ZlQU7
It is a glider so it can’t “pedal”. You can see it steeply descending from 5:13 to 5:30 while it is speeding up and then the pilot picks up the nose and trades some of his speed for elevation again. And then he does it again around the 7 minutes mark.
You have two buckets of “water”. One bucket is kinetic energy and the other is potential energy. You can trade one for the other. You can also “lose” from the total volume of “water” due to drag (or friction in the case of the bike or roller coaster). Or you can add more “water” to your system by pedaling or thrusting with your engines. This is just simple physics 101. Also simple lived experience if you ever have the opportunity to fly an airplane.
This is because bikes cost you about 50% more energy going uphill than walking[1]. You get back everything you don't lose from having to pedal too slowly, hunch over the front wheel, and maintain constant torque on the pedals.
Just as with bikes, it will depend on how slow it is descending. On "right" trajectory engines could technically be basically idle, and you save fuel flying high so it might not be all that huge loss.
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.