Thread: Something to think about

This problem is so flawed, it leaves room to to much interpretation ... I think the key point that is overlooked is that the treadmill matches the plane's speed, not the wheels' (for this problem to work, we also have to assume that there is enough friction between the wheels and treadmill to avoid any slipping). Also, whether the force is derived from the action/reaction effect of the engines or the direct rotation of the wheels' axes, there is no difference. In the end, it's still the wheels that make the plane go forward.

Originally Posted by Spartiate

This problem is so flawed, it leaves room to to much interpretation ... I think the key point that is overlooked is that the treadmill matches the plane's speed, not the wheels' (for this problem to work, we also have to assume that there is enough friction between the wheels and treadmill to avoid any slipping). Also, whether the force is derived from the action/reaction effect of the engines or the direct rotation of the wheels' axes, there is no difference. In the end, it's still the wheels that make the plane go forward.

If it matches the planes speed nothing will move, correct? That's the way I'm seeing this. Neither start out moving and the treadmill matches instantly so everyone is saying the plane won't move. Well if the treadmill matches the plane's speed and the plane isn't moving then neither is the treadmill.

Also am I correct in thinking that the speed of the plane refers to the plane with a reference point on the ground (not on the treamill) whereas speed of the wheels would refer to matching the wheels so that the wheels are spinning but the actual gears attached to the plane are stationery with respect to a reference point on the ground?

My god, isn't this a no brainer?

By all logical logic, the plane will not move...EVER under these circumstances

Originally Posted by ThePhobiaViewed

If it matches the planes speed nothing will move, correct? That's the way I'm seeing this. Neither start out moving and the treadmill matches instantly so everyone is saying the plane won't move. Well if the treadmill matches the plane's speed and the plane isn't moving then neither is the treadmill.

Also am I correct in thinking that the speed of the plane refers to the plane with a reference point on the ground (not on the treamill) whereas speed of the wheels would refer to matching the wheels so that the wheels are spinning but the actual gears attached to the plane are stationery with respect to a reference point on the ground?

Well the system will move, I mean think it out logically, how can it be at full thrust and not move ? It's just that the treadmill compensates. The term "speed" is misleading actually, we should be talking more about applied forece and energy.

Sorry, the plane would still take off. Propellor or jet engines, they are pulling/pushing the plane through the atmosphere. Driving a car, running, or whatever else on a conveyor belt would result in no forward movement - because the forward movement relies on the negative force caused by pushing against the ground (in this case the ground is moving backwards at the same rate resulting in no motion).

But with a plane, the driving force is not pushing against the ground (or in this case the conveyor belt), it is pushing against the air (which is not matching the plane's forward motion at all).

If the plane were travelling forward at 100mph, the conveyor would be travelling backwards at 100mph. Since the wheels are in freewheel with the brakes disengaged during takeoff and landing, the plane would move forward at 100mph, with the wheels spinning as if it were doing 200mph.

If you were in a car at 60mph on the clock and the conveyor was matching your speed of 60mph, your geographical speed would be 0mph since the car relies on exerting a force on the ground to propel itself, not the air.

Oh and here's a small experiment to prove the plane would still be able to propel itself forward regardless of the surface its on. Think of the opposite conveyor belt as an almost zero friction surface. That's basically what happens if a seaplane were to take off from water.

http://uk.youtube.com/watch?v=-EopVDgSPAk

Originally Posted by ouija

If the plane were travelling forward at 100mph, the conveyor would be travelling backwards at 100mph. Since the wheels are in freewheel with the brakes disengaged during takeoff and landing, the plane would move forward at 100mph, with the wheels spinning as if it were doing 200mph.

I agree.

I do find it quite amusing how, given the full explanation, people still disagree. It seems intelligent people find it difficult to believe that it would be able to take off, because they understand the principles of aviation and how to generate lift etc. But they fail to see that as long as the plane can propel itself forward... it can take off.

Haha! I just watched that video. That was a really cool experiment.

I agree that it's possible to take off from a treadmill. But, what I don't agree with is the way it's all being worded. We've been using the hypothetical that the conveyor belt is moving at such a speed that it counters any forward movement of the plane. With that scenario, no lift can be generated because the plane isn't moving forward.

All that experiment shows is that if you allow the plane to move forward faster than it's being pulled backwards then it will take off. That's the no-brainer part.

The question (as far as I've been understanding, and therefor the basis of my entire argument) is what would happen at a net forward movement of zero (and not "is it possible for a plane to take off on a treadmill", which is something I don't think anyone said isn't possible.). In that case nothing would happen.

But I just re-read the original question and I see that it actually did ask if it's possible to take off. So, I guess I got sidetracked by all the back and forth arguing going on.

All that guy's experiment shows is that there is indeed a limit to the hindrance the treadmill can impose. But the flaw in his experiment is that it fails to take into account the great weight of a real plane. And that weight will take a certain amount of thrust just to keep the plane stationary (to overcome the friction it takes to rotate the plane's wheels).

It only limits the max treadmill speeds we're considering. Granted, the ability of the treadmill to hinder forward movement is rather low. Beyond that point, the plane would be able to move forward. But, you can't omit the fact that there is a threshold of wheel friction that must be overcome first. Before that threshold is exceeded it fits into what I've been talking about (from my understanding of the original question) and no forward motion will take place - I think.

Either way, now understanding the original question, I have to say that it is possible and there is a (rather low) point where the treadmill can't possibly hinder the plane at all. That limit is the determined by the wheel rotation friction, whatever level that may be. Beyond that point, the momentum is greater than the friction can effect. Right??? Or am I still wrong?

So, basically the point is that it's not possible for the treadmill to hold the plane still past a certain amount of momentum. That I completely agree with in light of this new perspective.

Though it's hard to completely relate the scale of the model plane to a real plane (in terms of the plane weight to wheel/axle friction ratio), there is one element of his experiment that helps to bring that, at least a little bit, into the equation: The model plane is moving uphill, as it is on an inclining treadmill, yet it still moves forward, fighting the force of gravity along with the force of the treadmill.

And yeah, when the question was first asked, I was only thinking about the entire problem while having the assumption that the plane wouldn't have any forward momentum. It wasn't until Phobia chimed in that I realized that there was more to it than that. But, that's what I'd been tryin ta tell you, since then.

Originally Posted by Oneironaut

But, that's what I'd been tryin ta tell you, since then.

I see the err of my ways. Actually, I still think that the position I was in support of is valid but, it breaks down immediately once the critical speed is reached.

Originally Posted by ouija

I do find it quite amusing how, given the full explanation, people still disagree. It seems intelligent people find it difficult to believe that it would be able to take off, because they understand the principles of aviation and how to generate lift etc. But they fail to see that as long as the plane can propel itself forward... it can take off.

But it can't propel itself forward with the conveyor belt. Therefore it will stay grounded.

Originally Posted by CymekSniper

But it can't propel itself forward with the conveyor belt. Therefore it will stay grounded.

A plane doesn't use the ground (or a conveyor belt) to propel itself forward. It uses the air. You are thinking of a plane moving forward like a car moving forward, and they are completely different.

Right, it would only take as much energy as it takes to overcome the friction of the wheels. Then after that, it doesn't matter how fast the treadmill moves. It wouldn't take any more energy to hold the plane's position, so forward movement would be as easy as it is on a non-moving ground.

Judging by some of your comments, you guys make it seem that even if the plane was powered down and the treadmill was running, the wheels would spin freely. Remember that there is A LOT of friction going on, the plane's wheels move at a speed that is relative to the plane's speed (take it from the pilot). GH, is right, in the exact wordings of the original question, the plane cannot take off...

Originally Posted by Spartiate

Remember that there is A LOT of friction going on, the plane's wheels move at a speed that is relative to the plane's speed (take it from the pilot)

Pilot or not, I still disagree.

The wheels move at a speed that is more relative to the ground than to the plane's movement through space.

Think about it: (In an attempt to look at this from another perspective)

You're flying a jet plane and you're going 600 miles per hour. You power down to 300 miles per hour and are coming in for a landing, set to land on a treadmill that is going 300 miles per hour in the opposite direction. Your wheels touch down to the treadmill and - due to space-age (fictional) technology - your wheels grip the ground with a precision that causes absolutely no skid, but you hold the throttle at 300 miles per hour and do not power-down from there. Do you think your plane (because of the rotation of the wheels on the ground) is going to automatically just slow to 0mph through space? Or do you think the wheels are going to start spinning at 600 mph, because your plane is going 300mph one way, and the treadmill is going 300mph the other way?

If you choose the former, that's like saying that a plane coming into a landing, on a still runway, doing 200mph will power down to 100mph even if the throttle stays at 200mph upon landing. It won't. (Because the throttle in a jet would gauge how hard the jet engine is pushing, not how fast the wheels are turning) The plane is going to go 200mph until someone lets off of the throttle. It doesn't matter if the ground is standing still or going backward, the wheels are going to compensate, and the plane is going to move through space as fast as the jet engines are set to move.

I know you're a pilot, and I'm not discrediting you from knowing your shit, but I don't think you're looking at this problem the right way.

Originally Posted by Oneironaut

You power down to 300 miles per hour and are coming in for a landing, set to land on a treadmill that is going 300 miles per hour in the opposite direction.

Ahah, there's the problem...

The treadmill isn't spinning at 300 mph, it's spinning at a speed that will negate the plane's speed (which would be considerably faster), or at least this is how I perceive it... The original question leaves to much room for interpretation.

Originally Posted by Spartiate

Ahah, there's the problem...

The treadmill isn't spinning at 300 mph, it's spinning at a speed that will negate the plane's speed (which would be considerably faster), or at least this is how I perceive it... The original question leaves to much room for interpretation.

Heh. Not true. The treadmill is moving at a speed that matches the plane. So, as I said, if you are in a plane that is going 300 miles per hour one way, the treadmill is going 300 miles per hour in the other direction.

What if the treadmill breaks?

How much is this giant treadmill costing the taxpayers, anyways?

Originally Posted by CoLd BlooDed

What if the treadmill breaks?

How much is this giant treadmill costing the taxpayers, anyways?

Just tell them we're testing the "reverse-treadmill" technology for use in Iraq. You'll get unlimited funding. All the taxpayers will just have to deal with it.

Originally Posted by Oneironaut

Heh. Not true. The treadmill is moving at a speed that matches the plane. So, as I said, if you are in a plane that is going 300 miles per hour one way, the treadmill is going 300 miles per hour in the other direction.

Let's continue tomorow, please! I wan't to sleep !

Lol. Fair enough.

I didn't read all three pages, but it looks like this is all just a semantics argument. I didn't see any disagreements about what the airplane would do when everybody agrees on what the scenario is.

Originally Posted by ouija

The plane moves in one direction, while the conveyer moves in the opposite direction.

That does not look like it is asking what would happen if the wheels moved in such a way that the airplane would go a certain speed if it were on a concrete runway. It seems to involve an airplane that is actually moving in relation to the rest of the ground while the wheels just roll along. If that is the case, and it is the case that the conveyor belt will go the same speed in the "other" direction (There is no "other" direction if the airplane is staying still in relation to the rest of the ground. In that case, there would be a direction for the conveyor belt and stillness for the airplane.), then the airplane is moving in relation to the rest of the ground, so it is moving in relation to the air. If that is what is happening, the airplane will be able to catch air if it gets enough speed.

Correct

It's one of these questions that automatically makes you think "No. The plane doesn't move so it can't take off", but the conveyor belt is in fact a red herring because of the way the plane generates forward motion.

Yes there will be some friction in the wheel bearings, but given the amount of thrust the jet engines can generate - it won't be enough to cause a significant drag on the plane's forward motion. Granted the bearings might seize at the higher rotational speed, but that's not what the question asks

The question only states that the conveyor matches the speed of the plane. At no point does anyone say the conveyor keeps the plane stationary.