Which is insane as it’s not that difficult to understand
I found it hard to understand because neither they nor any of the other sources I’ve seen explaining this even attempted to answer what I thought was an incredibly obvious question: at what point does this become true? A stationary aeroplane on a treadmill will obviously move with the treadmill. I assume an aeroplane moving at like 1 km/h still gets pulled backward by the treadmill. At what point does the transition occur, and what does that transition process look like? Why can’t a treadmill prevent the plane from taking off by pulling it backwards by never letting it start getting forward motion? Where does the lift come from?
I can understand how a treadmill doesn’t stop a plane that’s already moving, but how does it get lift if it is prevented from accelerating from 0 to 1 km/h of ground speed (relative to the real ground—relative to the ground it experiences, it is moving forward at the same speed as the treadmill is moving backward), since until it starts getting lift, airspeed and ground speed are surely effectively equal (wind being too small of a factor)?
A stationary aeroplane on a treadmill will obviously move with the treadmill. I assume an aeroplane moving at like 1 km/h still gets pulled backward by the treadmill.
so, every wheel or ball or any other kind of rolling-thing has rolling resistance, which is how we sum up the total drag on the system. A steel ball bearing on a steel plate will have a significantly lower rolling resistance than, say, a steel cube on that same plate. Tires have some- but not a lot- of rolling resistance.
You can see that in a car, just put it into neutral and watch as you slow down, even on flat ground. Plane wheels also have rolling resistance. it’s just the way our world works. But it’s generally ignored because it’s hard to model perfectly and in any case pretty negligible relative to the amount of acceleration being put out by modern aircraft engines.
A treadmill will only push an aircraft or whatever else along, with an acceleration that is equal to, or lower, than the rolling resistance. If you try to accelerate the plane faster, it’ll ‘slip’, and the plane will remain largely stationary- like the dishes in the tablecloth trick (if you want to try that at home… make sure the tablecloth doesn’t have a hem, heh.)
But, keep in mind you’re thinking about the plane relative to either the ground, or the treadmill’s belt.
the plane’s wings and it’s engines are ‘thinking’ about the plane relative to the air it’s moving through. It’s the airspeed that generates the lift, and the engine isn’t coupled to the wheels, they’re just rolling along doing their thing. (aircraft engines work by taking a volume of air and accelerating it. newton’s equal-and-opposite does the rest.)
Oh wow thank you. This is genuinely excellent and immensely helpful. I think this bit:
A treadmill will only push an aircraft or whatever else along, with an acceleration that is equal to, or lower, than the rolling resistance. If you try to accelerate the plane faster, it’ll ‘slip’
As well as this video that I found where a pilot explains how under specific but unrealistic conditions you could construct a treadmill that does indeed prevent an aeroplane from taking off,
Really helped solidify my understanding of the problem. So you end up with a situation where the wheels are going to be slipping, just like the slippage created when your hand pushes a toy car on a treadmill.
The air flows around it the same way it would any other kind of aircraft, though they have effectively zero ground speed.
They do differ in that, being tethered, they’re pulled through the air, with the wind providing the energy to stay up.
But they’re still moving through the air, and the airfoils are inducing drag to convert some of that energy into lift.
In both cases, the important speed is relative to the air, not the ground and not the treadmill. The wheels might impart some drag while they’re on the ground, but they’re never going to impart enough to overpower the engines- 747s typically take off at about 75% of their rated take off power, which means a longer take off roll, but less wear and tear.
The key insight is that the force a plane uses to move is independent of the ground, because planes push on the air, not the ground.
Imagine you put a ball on a treadmill and turn it on, what happens? The ball starts to spin and move with the treadmill. Now take your hand and push the ball backwards against the motion of the treadmill, and the ball easily moves in that direction. The force your hand put on the ball is exactly what planes do, since they push on something other than the ground (the treadmill) they have no problem moving, no matter how fast the treadmill is moving.
I found it hard to understand because neither they nor any of the other sources I’ve seen explaining this even attempted to answer what I thought was an incredibly obvious question: at what point does this become true? A stationary aeroplane on a treadmill will obviously move with the treadmill. I assume an aeroplane moving at like 1 km/h still gets pulled backward by the treadmill. At what point does the transition occur, and what does that transition process look like? Why can’t a treadmill prevent the plane from taking off by pulling it backwards by never letting it start getting forward motion? Where does the lift come from?
I can understand how a treadmill doesn’t stop a plane that’s already moving, but how does it get lift if it is prevented from accelerating from 0 to 1 km/h of ground speed (relative to the real ground—relative to the ground it experiences, it is moving forward at the same speed as the treadmill is moving backward), since until it starts getting lift, airspeed and ground speed are surely effectively equal (wind being too small of a factor)?
so, every wheel or ball or any other kind of rolling-thing has rolling resistance, which is how we sum up the total drag on the system. A steel ball bearing on a steel plate will have a significantly lower rolling resistance than, say, a steel cube on that same plate. Tires have some- but not a lot- of rolling resistance.
You can see that in a car, just put it into neutral and watch as you slow down, even on flat ground. Plane wheels also have rolling resistance. it’s just the way our world works. But it’s generally ignored because it’s hard to model perfectly and in any case pretty negligible relative to the amount of acceleration being put out by modern aircraft engines.
A treadmill will only push an aircraft or whatever else along, with an acceleration that is equal to, or lower, than the rolling resistance. If you try to accelerate the plane faster, it’ll ‘slip’, and the plane will remain largely stationary- like the dishes in the tablecloth trick (if you want to try that at home… make sure the tablecloth doesn’t have a hem, heh.)
But, keep in mind you’re thinking about the plane relative to either the ground, or the treadmill’s belt.
the plane’s wings and it’s engines are ‘thinking’ about the plane relative to the air it’s moving through. It’s the airspeed that generates the lift, and the engine isn’t coupled to the wheels, they’re just rolling along doing their thing. (aircraft engines work by taking a volume of air and accelerating it. newton’s equal-and-opposite does the rest.)
Oh wow thank you. This is genuinely excellent and immensely helpful. I think this bit:
As well as this video that I found where a pilot explains how under specific but unrealistic conditions you could construct a treadmill that does indeed prevent an aeroplane from taking off,
Really helped solidify my understanding of the problem. So you end up with a situation where the wheels are going to be slipping, just like the slippage created when your hand pushes a toy car on a treadmill.
Thanks!
So, another way to think about it is with Kites.
The air flows around it the same way it would any other kind of aircraft, though they have effectively zero ground speed.
They do differ in that, being tethered, they’re pulled through the air, with the wind providing the energy to stay up.
But they’re still moving through the air, and the airfoils are inducing drag to convert some of that energy into lift.
In both cases, the important speed is relative to the air, not the ground and not the treadmill. The wheels might impart some drag while they’re on the ground, but they’re never going to impart enough to overpower the engines- 747s typically take off at about 75% of their rated take off power, which means a longer take off roll, but less wear and tear.
The key insight is that the force a plane uses to move is independent of the ground, because planes push on the air, not the ground.
Imagine you put a ball on a treadmill and turn it on, what happens? The ball starts to spin and move with the treadmill. Now take your hand and push the ball backwards against the motion of the treadmill, and the ball easily moves in that direction. The force your hand put on the ball is exactly what planes do, since they push on something other than the ground (the treadmill) they have no problem moving, no matter how fast the treadmill is moving.
The point it occurs at is when the plane uses the air to propel itself.