the infamous Jet Question

[PolySciGuy]

Ok, this has been driving my friends and me ABSOLUTELY CRAZY!

Here is the question in its two forms

1) So there is a jet on a runway that is a conveyor belt. The conveyor belt is set to match the speed of the plane, but in the opposite direction. So assuming those conditions are true, could the plane (jet) take off?

2) essentially the same as #1, but instead of matching the PLANE speed it matches the speed of the TIRES on the plane

Honestly I don't know the answer to number one

Number two I think is a no, since the force acting on the tires is the SUM of the plane speed AND the speed of the conveyor belt, the conveyor belt would be moving at double the plane speed in the opposite direction

 

[jfuh]

Ok, this has been driving my friends and me ABSOLUTELY CRAZY!

Here is the question in its two forms

1) So there is a jet on a runway that is a conveyor belt. The conveyor belt is set to match the speed of the plane, but in the opposite direction. So assuming those conditions are true, could the plane (jet) take off?

Yes, because the jet's wheels - contact with ground, are not what propels the jet forward, it's the turbines that do the work against the air.

2) essentially the same as #1, but instead of matching the PLANE speed it matches the speed of the TIRES on the plane

Same as 1. The plane is still able to take off.

Now if the question had been that there is a giant fan behind the aircraft blowing in the direction that the plane is moving then no, the plane would not be able to take off.

 

[PolySciGuy]

Yes, because the jet's wheels - contact with ground, are not what propels the jet forward, it's the turbines that do the work against the air.

Same as 1. The plane is still able to take off.

Now if the question had been that there is a giant fan behind the aircraft blowing in the direction that the plane is moving then no, the plane would not be able to take off.

I understand what you're saying and will probably agree with you, but doesn't the amount of friction between the tires and the conveyor belt have anything to do with it?

 

[jfuh]

I understand what you're saying and will probably agree with you, but doesn't the amount of friction between the tires and the conveyor belt have anything to do with it?

The wheels spin freely and the friction would have little to no impact on the airflow over the wings of the aircraft.
Were it not for the need to stand up right, a plane would have no need for wheels.

 

[PolySciGuy]

whew, thanks for all of your input! Now I can sleep easy

 

[ashurbanipal]

No, the plane would not take off, in either case (the speed of the conveyor belt would be the same in either). The air over the wings is not being moved by the conveyor belt, and hence it does not create any lift over the wings.

I understand about the turbines, but any forward motion they create for the aircraft will be cancelled by the conveyor belt.

 

[PolySciGuy]

yea I have actually been thinking about this myself, since some of the energy of the turbines is lost in the wheel, it doesn't work out

 

[jfuh]

No, the plane would not take off, in either case (the speed of the conveyor belt would be the same in either). The air over the wings is not being moved by the conveyor belt, and hence it does not create any lift over the wings.

I understand about the turbines, but any forward motion they create for the aircraft will be canceled by the conveyor belt.

Hardly. Turbines push against the atmosphere that's where a plane's thrust come from. The conveyor belt could be moving twice as fast as the plane and it'd still take off. Why? because any motion from the conveyor belt is canceled out at the wheels that spin freely, thus the conveyor belt is exerting a net force of 0 on the plane itself.
The thrust of the turbines are completely free of the force of the conveyor belt. Even at an incline, as long as that incline does not cause the planes turbines to stall the plane will still take off.
Hence the force on the free spinning wheels has absolutely nothing at all whatsoever to do with the thrust of the turbines action on the atmosphere.
Thus the thrust from the turbines will still propel the aircraft forward creating an air flow over the wings thus creating lift and thus flight.

I'll close off with this thought for you.
How much energy will it take for you to keep up with a treadmill going at 50mph while wearing roller blades? Though it will take some energy to stay upright, but aside from that practically none as long as you have well lubricated wheels. Same thing here. The aircrafts wheels are the roller blades in this case.
The thrust is not coming from the wheels on the ground, they are coming from the force against the atmosphere.

 

[PolySciGuy]

Hardly. Turbines push against the atmosphere that's where a plane's thrust come from. The conveyor belt could be moving twice as fast as the plane and it'd still take off. Why? because any motion from the conveyor belt is canceled out at the wheels that spin freely, thus the conveyor belt is exerting a net force of 0 on the plane itself.
The thrust of the turbines are completely free of the force of the conveyor belt. Even at an incline, as long as that incline does not cause the planes turbines to stall the plane will still take off.
Hence the force on the free spinning wheels has absolutely nothing at all whatsoever to do with the thrust of the turbines action on the atmosphere.
Thus the thrust from the turbines will still propel the aircraft forward creating an air flow over the wings thus creating lift and thus flight.

I'll close off with this thought for you.
How much energy will it take for you to keep up with a treadmill going at 50mph while wearing roller blades? Though it will take some energy to stay upright, but aside from that practically none as long as you have well lubricated wheels. Same thing here. The aircrafts wheels are the roller blades in this case.
The thrust is not coming from the wheels on the ground, they are coming from the force against the atmosphere.

Umm..... negatory, have you ever been on a treadmill with Rollerblades on? You have to hold on to the handlebars and essentially pull yourself upright.

Perhaps the misunderstanding is in the word "free-spinning" are you saying that this is a hypothetical area with 0 friction affecting the contact between the plane, the axle, the wheels, and the conveyor belt?

 

[ashurbanipal]

Hardly. Turbines push against the atmosphere that's where a plane's thrust come from. The conveyor belt could be moving twice as fast as the plane and it'd still take off. Why? because any motion from the conveyor belt is canceled out at the wheels that spin freely, thus the conveyor belt is exerting a net force of 0 on the plane itself.

No--if the turbines are exerting some force along some vector x, given the conditions of the example, the converyor belt will be exerting force along a vector y such that y=-x.

The thrust of the turbines are completely free of the force of the conveyor belt.

Of course they are. The plane, however, is neither free of the force of the turbines, nor of the conveyor belt. The two equal and opposite forces cancel each other out such that the plane does not move. So long as air is not flowing over the wings at some speed above whatever the wings' stalling speeds are, the plane will not take off. The conveyor belt is not moving the air, it's acting to keep the plane stationary on the ground. Therefore, air is not moving over the wings.

Even at an incline, as long as that incline does not cause the planes turbines to stall the plane will still take off.
Hence the force on the free spinning wheels has absolutely nothing at all whatsoever to do with the thrust of the turbines action on the atmosphere.
Thus the thrust from the turbines will still propel the aircraft forward creating an air flow over the wings thus creating lift and thus flight.

No--until the plane reaches the speed of rotation, it still travels on wheels, which are not "free spinning" in this sense--they can't spin any which way while the plane is on the ground.

How much energy will it take for you to keep up with a treadmill going at 50mph while wearing roller blades?

Once I get going, very little. But this doesn't matter.

Though it will take some energy to stay upright, but aside from that practically none as long as you have well lubricated wheels. Same thing here. The aircrafts wheels are the roller blades in this case.
The thrust is not coming from the wheels on the ground, they are coming from the force against the atmosphere.

But this is a different situation. The analogous question would be how much energy it would take for me to travel forward at 50 mph while on the treadmill that is also going 50 mph. The answer is however much energy it would normally take to propel me at 100 mph. The plane must move forward through the air in order to take off (or stay aloft); the conveyor belt is not moving air over the wings--it is acting to keep the plane stationary.

 

[jfuh]

Umm..... negatory, have you ever been on a treadmill with Rollerblades on? You have to hold on to the handlebars and essentially pull yourself upright.

Perhaps the misunderstanding is in the word "free-spinning" are you saying that this is a hypothetical area with 0 friction affecting the contact between the plane, the axle, the wheels, and the conveyor belt?

Like I said, you need to exert some force to stay upright, but essentially it's not the same force that you would need to exert or even close to the force you would need to exert if you were to "run" up to 50mph.

 

[jfuh]

No--if the turbines are exerting some force along some vector x, given the conditions of the example, the converyor belt will be exerting force along a vector y such that y=-x.

Only the conveyor belt's force is canceled at the wheels. The reaction is negated at the wheels of the aircraft.

Of course they are. The plane, however, is neither free of the force of the turbines, nor of the conveyor belt. The two equal and opposite forces cancel each other out such that the plane does not move. So long as air is not flowing over the wings at some speed above whatever the wings' stalling speeds are, the plane will not take off. The conveyor belt is not moving the air, it's acting to keep the plane stationary on the ground. Therefore, air is not moving over the wings.

How is the conveyor belt even acting on the plane? The wheels move freely.

No--until the plane reaches the speed of rotation, it still travels on wheels, which are not "free spinning" in this sense--they can't spin any which way while the plane is on the ground.

Yes, they can spin any which way quite literally. As long as the plane is not anchored it will be able to take off.

Once I get going, very little. But this doesn't matter.

Actually it's the very core of the issue.

But this is a different situation. The analogous question would be how much energy it would take for me to travel forward at 50 mph while on the treadmill that is also going 50 mph. The answer is however much energy it would normally take to propel me at 100 mph. The plane must move forward through the air in order to take off (or stay aloft); the conveyor belt is not moving air over the wings--it is acting to keep the plane stationary.

Since when was a runway ever designed to move air over the wings of an aircraft? No, that is done with the force of the turbines pushing the aircraft to then move air over the aircraft. Just as with any runway anywhere in the world not moving air over an aircrafts wings, the conveyor belt has 0 to do with the airflow.

 

[Iriemon]

I have to agree with jfuh on this one. It is clear the jet has to be moving forward for the wings to create lift. The question is whether the jet will move forward based on thrust if the conveyor belt is moving in the opposite direction.

jfuh is correct that the planes forward motion is not based on force applied to the wheels (in which case the plane would not move) but based upon thrust against the atmosphere.

Unless the brakes were on, or there was some other friction that would offset the force of thrust create by the engine, the plane would move forward.

 

[ashurbanipal]

Yeah, I have realized that Jfuh is correct. The plane will achieve forward motion (discounting friction) regardless of the speed of the conveyor belt.

 

[jfuh]

I can see the makings of a myth buster episode already.
In fact I was about to submit the topic to myth busters until I saw that this same question had been talked to death on the site and even similar if not identical "analogies" raised.

Can we call this myth busted? I say yes.

 

[Thinker]

Hardly. Turbines push against the atmosphere that's where a plane's thrust come from.

Wrong. The turbines accelerate air (a mass) backwards. It is the equal and
opposite reaction to this (Newton) that accelerates the plane forwards.
That's why rockets work in a vacuum. This reaction moves the plane forward to
give an airflow over the wings and generate lift.

The conveyor belt is largely irrelevant; any frictional forces would be
insignificant in relation to the power of the jets. All the belt would do is make
the wheels turn faster without otherwise affecting the plane.

 

[jfuh]

Wrong. The turbines accelerate air (a mass) backwards. It is the equal and opposite reaction to this (Newton) that accelerates the plane forwards.
That's why rockets work in a vacuum. This reaction moves the plane forward to
give an airflow over the wings and generate lift.

Uh... ok that's exactly what I said. Only without the variance of rockets - which, do not accelerate air the way a jet turbine does.

 

[Morrow]

I haven't taken any physics stuff, but I don't see how the plane could take off. A wing needs air movement to get lift, how will it get air movement it it is staying in one spot? What am I missing here?

 

[Thinker]

I haven't taken any physics stuff, but I don't see how the plane could take off. A wing needs air movement to get lift, how will it get air movement it it is staying in one spot? What am I missing here?

It won't stay in one spot; that's what you are missing.

Think of the experiment with a large brick wall at the back of the plane,
preventing it from moving backwards. With the belt running, all that would
happen is that the wheels of the plane would turn. The force the plane exerts
on the wall would be fairly small - the result of friction in the system. It would
certainly be insignificant with respect to the force the jets could apply.
The plane is stationary with respect to the ground and air, even with the jets
OFF.

Now turn on the jets. The large forward force easily overcomes the tiny
frictional force and moves the plane forward relative to the ground and air,
creating an airflow over the wings. The speed relative to the belt goes up and
the wheels turn faster.

 

[Thinker]

Uh... ok that's exactly what I said. Only without the variance of rockets - which, do not accelerate air the way a jet turbine does.

OK. Your phrase "push against the atmosphere" led me to assume you were
using the old and wrong idea that a jet pushes against the air behind it.

There's actually very little difference between the underlying mechanisms of a
jet and a traditional rocket. Both essentially accelerate gas by heating it. The
real difference is in where that gas comes from.

 

[C.J.]

1) So there is a jet on a runway that is a conveyor belt. The conveyor belt is set to match the speed of the plane, but in the opposite direction. So assuming those conditions are true, could the plane (jet) take off?

Let's try this. Put on your favorite roller skates and stand on an inside treadmill set to instantaneously match your speed. Have a friend push you from behind (He is providing the thrust). He can push you as fast as he can, but if the treadmill is instantly matching your speed (In the opposite direction), you are not moving in relation to the treadmill frame or the floor are you? Zero ground speed.

Now, how much wind is hitting you in the face?

A problem with the question is that it doesn't define speed. If it refers to ground speed, the plane will not lift off as it is not going anywhere in relation to the air ahead of it. If it refers to the air speed of the plane vs the ground speed of the conveyor (which actually invalidates the question), then yes the craft would fly.

Just my thoughts

Regards,

"C.J."

 

[Morrow]

It won't stay in one spot; that's what you are missing.

Think of the experiment with a large brick wall at the back of the plane,
preventing it from moving backwards. With the belt running, all that would
happen is that the wheels of the plane would turn. The force the plane exerts
on the wall would be fairly small - the result of friction in the system. It would
certainly be insignificant with respect to the force the jets could apply.
The plane is stationary with respect to the ground and air, even with the jets
OFF.

Now turn on the jets. The large forward force easily overcomes the tiny
frictional force and moves the plane forward relative to the ground and air,
creating an airflow over the wings. The speed relative to the belt goes up and
the wheels turn faster.

I thought that the belt was keeping the plane from moving forward.

 

[Thinker]

Let's try this. Put on your favorite roller skates and stand on an inside treadmill set to instantaneously match your speed.
Have a friend push you from behind (He is providing the thrust). He can push you as fast as he can, but if the treadmill is instantly matching your speed (In the opposite direction), you are not moving in relation to the treadmill frame or the floor are you? Zero ground speed.

Now, how much wind is hitting you in the face?

A badly-formed example. If you are stationary wrt the ground, the treadmill
will stop (instantaneously matching your "speed").

You are missing the point; it's all about forces. Take your scenario. When you
are pushing the person on the treadmill, how much force do you have to
apply?


If there were no skates, you would have to apply exactly the opposite
force to that of the treadmill to stop him moving wrt the ground. That would
be a lot of force.

Now take the opposite extreme: assume the bearings in the skates are
completely frictionless (think wet ice). Now how much force would you need
to stop him moving? No force at all. Zero. There is no way for the movement
of the treadmill to be transmitted through the frictionless bearings regardless
of the speed of the treadmill. Now if you "push harder" (i.e. any force greater
than zero) there will be a resultant forward force and he will move forward.

Now give him the skates. The friction is not zero but very low (that's what
skates do). The force you apply to keep him still depends on the friction in
the bearings and so is very small.

What happens to that force as the treadmill speeds up? Very little! As long as
the friction in the bearings of the skates is low (we can assume they are
well-designed), the force you need to keep him still will remain low.

What happens if you now increase your force? No matter how fast the
treadmill goes, it will simply make the wheels on the skates turn faster, and
there's a resultant forward force to move him forward.

 

[PolySciGuy]

It won't stay in one spot; that's what you are missing.

Think of the experiment with a large brick wall at the back of the plane,
preventing it from moving backwards. With the belt running, all that would
happen is that the wheels of the plane would turn. The force the plane exerts
on the wall would be fairly small - the result of friction in the system. It would
certainly be insignificant with respect to the force the jets could apply.
The plane is stationary with respect to the ground and air, even with the jets
OFF.

Now turn on the jets. The large forward force easily overcomes the tiny
frictional force and moves the plane forward relative to the ground and air,
creating an airflow over the wings. The speed relative to the belt goes up and
the wheels turn faster.

Sorry about jumping in again, but:
The conveyor belt moves proportionately faster as the plane moves faster

 

[PolySciGuy]

A badly-formed example. If you are stationary wrt the ground, the treadmill
will stop (instantaneously matching your "speed").

You are missing the point; it's all about forces. Take your scenario. When you
are pushing the person on the treadmill, how much force do you have to
apply?


If there were no skates, you would have to apply exactly the opposite
force to that of the treadmill to stop him moving wrt the ground. That would
be a lot of force.

Now take the opposite extreme: assume the bearings in the skates are
completely frictionless (think wet ice). Now how much force would you need
to stop him moving? No force at all. Zero. There is no way for the movement
of the treadmill to be transmitted through the frictionless bearings regardless
of the speed of the treadmill. Now if you "push harder" (i.e. any force greater
than zero) there will be a resultant forward force and he will move forward.

Now give him the skates. The friction is not zero but very low (that's what
skates do). The force you apply to keep him still depends on the friction in
the bearings and so is very small.

What happens to that force as the treadmill speeds up? Very little! As long as
the friction in the bearings of the skates is low (we can assume they are
well-designed), the force you need to keep him still will remain low.

What happens if you now increase your force? No matter how fast the
treadmill goes, it will simply make the wheels on the skates turn faster, and
there's a resultant forward force to move him forward.

ok... I think I am getting it... but

The question in its second form has to do with the tire rotation
so...
assuming the skater is moving 0 relative to the ground speed, but the treadmill and wheels could be going infinitely fast and still not move him forwards... right?

But as far as the first form goes... isn't there a lot of friction on a jet because of its weight?