Mostly because fans still argue about it and it’s hit the point they had to ban PoaT comments.
Which is insane as it’s not that difficult to understand. When a plane is on the ground, its gear/wheels will roll at ground speed, but the wings provide lift at airspeed.
If the ground is being moved under the plane (as on a treadmill,) the wheels will just roll faster.
Sure they’re not zero friction and some of that needs to be overcome; but this is something encountered on a daily basis all across the world- or rather, the opposite.
If the wind is coming from ahead, its airspeed is increased and the plane needs a lower ground speed to get into the air where if the wind is coming from behind, then they need more.
(This is why carriers set course into the wind when launching jets,)
At no point is ground speed and airspeed necessarily the same (i suppose you could have a calm day, but most days, the wind is blowing at least some.)
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
What do you mean? The plane has its parking brakes on and moves with the treadmill surface? If you don’t have parking brakes engaged and start up a treadmill under a plane, the plane’s wheels will spin and the plane will stay pretty much in one place. Because the wheels are free to spin, initially that’s all that will happen. The inertia of the plane will keep it in place while the wheels spin. Over time, the plane will start to drift in the direction the treadmill is moving, but it will never move as fast as the treadmill because there’s also friction from the air, and that’s going to be a much bigger factor.
I assume an aeroplane moving at like 1 km/h still gets pulled backward by the treadmill.
Moving at 1 km/h relative to what? The surface of the treadmill or the “world frame”? A plane on a moving treadmill will be pulled by the treadmill – there will be friction in the wheels, but it will also feel a force from the air. As soon as the pilot fires up the engine, the force from the engine will be much higher than any tiny amount of friction in the wheels from the treadmill.
but how does it get lift if it is prevented from accelerating from 0 to 1 km/h of ground speed
It isn’t prevented from accelerating from 0 to 1 km/h of ground speed. The wheels are spinning furiously, but they’re relatively frictionless. If the pilot didn’t start up the propeller, the plane would start to move in the direction the treadmill is pulling, but would never quite reach the speed of the treadmill due to air resistance. But, as soon as the pilot fires up the propeller, it works basically as normal. A little bit of the air will be moving backwards due to the treadmill, but most of the air will still be relatively stationary, so it’s easy to move the plane through the air quicker and quicker until it reaches take-off speed.
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 think the confusion is that the conveyor belt is running at a fixed speed, which is the aircraft’s takeoff speed. That just dictates how fast the wheels spin, but since the plane generates thrust with its propeller, the wheels just end up having to spin at double takeoff speed. Since they’re relatively frictionless, that’s easy.
The more confusing myth is the one where the speed of the conveyor belt is variable, and it always moves at the same speed as the wheels. So, at the beginning the conveyor belt isn’t moving, but as soon as the plane starts to move, and its wheels start to spin, the conveyor belt movies in the opposite direction. In that case, the plane can’t take off. That’s basically like attaching an anchor to the plane’s frame, so no matter how fast the propeller spins, the airplane can’t move.
Except it’s not like attaching an anchor. The plane isn’t physically attached.
The wheels will just roll double whatever the current ground speed is. If the plane has enough thrust to take off with the treadmill moving an inverse of its take off speed, then it has enough force to start rolling, too.
At most, the force applied by the treadmill would be sufficient over enough time to lengthen the take off roll, but given enough space to do so, the plane will take off.
To keep the plane from rolling forward; the treadmill would have to be able to apply an equal force as the engines, it can’t do that through the wheels- the wheels can only apply a force equal to their rolling resistance and friction in its mechanics.
If the conveyor moves at the same speed as the wheels, it is exactly like attaching an anchor. That isn’t the myth they were testing, but it’s a more interesting myth.
it can’t do that through the wheels- the wheels can only apply a force equal to their rolling resistance and friction in its mechanics.
It can do that if it can spin the wheels fast enough. Picture the ultra-light airplane from the episode with big, bouncy wheels and a relatively weak propeller. If the treadmill was moving 1000 km/h backwards, that little propeller could never match the force due to rolling resistance from the wheels.
Just to clarify; you understand that because the engines are pushing on the plane itself and not the wheels, by the time the wheels start moving, the plane is already moving relative to ground and air alike.
Which, said another way, this thought problem appears confusing because it’s being considered from otherwise irrelevant reference frames.
An anchor sufficient to keep the plane from rolling forward is different because the force it is apply is significantly greater.
Sure, you can deflate the tires and increase the rate of spin on the wheels. But at that point, you might as well ask “can we creat a scenario where planes can’t take off”
To which the answer is definitely “Yes”,
And as a side note, if we assume the wheels are indestructible, which I’d argue is only fair, then even if what you’re saying is true and we ramp up the drag induced by the wheels sufficient to counter the engines… then the wind generated by the rolling treadmill would be producing a sufficient headwind for the plane to take off. (Remember, the air resistance of the treadmill’s belt moving will accelerate the air some.)
But again, the wheels have almost zero drag to begin with, the speed at which the roll is independent of both the actual groundspeed and the airspeed of the airplane.
If it has the thrust to over come friction at take off speeds, and at standing, then it has enough power to get to take off velocity eventually.
On the other hand, this entire conversation assumes the thrust to weight ratio is less than 1. If it’s more than one, well they just…. Go straight up.
My favorite is planes on a treadmill.
Mostly because fans still argue about it and it’s hit the point they had to ban PoaT comments.
Which is insane as it’s not that difficult to understand. When a plane is on the ground, its gear/wheels will roll at ground speed, but the wings provide lift at airspeed.
If the ground is being moved under the plane (as on a treadmill,) the wheels will just roll faster.
Sure they’re not zero friction and some of that needs to be overcome; but this is something encountered on a daily basis all across the world- or rather, the opposite.
If the wind is coming from ahead, its airspeed is increased and the plane needs a lower ground speed to get into the air where if the wind is coming from behind, then they need more.
(This is why carriers set course into the wind when launching jets,)
At no point is ground speed and airspeed necessarily the same (i suppose you could have a calm day, but most days, the wind is blowing at least some.)
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)?
It’s always true.
What do you mean? The plane has its parking brakes on and moves with the treadmill surface? If you don’t have parking brakes engaged and start up a treadmill under a plane, the plane’s wheels will spin and the plane will stay pretty much in one place. Because the wheels are free to spin, initially that’s all that will happen. The inertia of the plane will keep it in place while the wheels spin. Over time, the plane will start to drift in the direction the treadmill is moving, but it will never move as fast as the treadmill because there’s also friction from the air, and that’s going to be a much bigger factor.
Moving at 1 km/h relative to what? The surface of the treadmill or the “world frame”? A plane on a moving treadmill will be pulled by the treadmill – there will be friction in the wheels, but it will also feel a force from the air. As soon as the pilot fires up the engine, the force from the engine will be much higher than any tiny amount of friction in the wheels from the treadmill.
It isn’t prevented from accelerating from 0 to 1 km/h of ground speed. The wheels are spinning furiously, but they’re relatively frictionless. If the pilot didn’t start up the propeller, the plane would start to move in the direction the treadmill is pulling, but would never quite reach the speed of the treadmill due to air resistance. But, as soon as the pilot fires up the propeller, it works basically as normal. A little bit of the air will be moving backwards due to the treadmill, but most of the air will still be relatively stationary, so it’s easy to move the plane through the air quicker and quicker until it reaches take-off speed.
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 think the confusion is that the conveyor belt is running at a fixed speed, which is the aircraft’s takeoff speed. That just dictates how fast the wheels spin, but since the plane generates thrust with its propeller, the wheels just end up having to spin at double takeoff speed. Since they’re relatively frictionless, that’s easy.
The more confusing myth is the one where the speed of the conveyor belt is variable, and it always moves at the same speed as the wheels. So, at the beginning the conveyor belt isn’t moving, but as soon as the plane starts to move, and its wheels start to spin, the conveyor belt movies in the opposite direction. In that case, the plane can’t take off. That’s basically like attaching an anchor to the plane’s frame, so no matter how fast the propeller spins, the airplane can’t move.
Except it’s not like attaching an anchor. The plane isn’t physically attached.
The wheels will just roll double whatever the current ground speed is. If the plane has enough thrust to take off with the treadmill moving an inverse of its take off speed, then it has enough force to start rolling, too.
At most, the force applied by the treadmill would be sufficient over enough time to lengthen the take off roll, but given enough space to do so, the plane will take off.
To keep the plane from rolling forward; the treadmill would have to be able to apply an equal force as the engines, it can’t do that through the wheels- the wheels can only apply a force equal to their rolling resistance and friction in its mechanics.
If the conveyor moves at the same speed as the wheels, it is exactly like attaching an anchor. That isn’t the myth they were testing, but it’s a more interesting myth.
It can do that if it can spin the wheels fast enough. Picture the ultra-light airplane from the episode with big, bouncy wheels and a relatively weak propeller. If the treadmill was moving 1000 km/h backwards, that little propeller could never match the force due to rolling resistance from the wheels.
Just to clarify; you understand that because the engines are pushing on the plane itself and not the wheels, by the time the wheels start moving, the plane is already moving relative to ground and air alike.
Which, said another way, this thought problem appears confusing because it’s being considered from otherwise irrelevant reference frames.
An anchor sufficient to keep the plane from rolling forward is different because the force it is apply is significantly greater.
Sure, you can deflate the tires and increase the rate of spin on the wheels. But at that point, you might as well ask “can we creat a scenario where planes can’t take off”
To which the answer is definitely “Yes”,
And as a side note, if we assume the wheels are indestructible, which I’d argue is only fair, then even if what you’re saying is true and we ramp up the drag induced by the wheels sufficient to counter the engines… then the wind generated by the rolling treadmill would be producing a sufficient headwind for the plane to take off. (Remember, the air resistance of the treadmill’s belt moving will accelerate the air some.)
But again, the wheels have almost zero drag to begin with, the speed at which the roll is independent of both the actual groundspeed and the airspeed of the airplane.
If it has the thrust to over come friction at take off speeds, and at standing, then it has enough power to get to take off velocity eventually.
On the other hand, this entire conversation assumes the thrust to weight ratio is less than 1. If it’s more than one, well they just…. Go straight up.