340 replies to this topic 3 votes

[RealFduch]

Slugsie, on Jan 29 2008, 18:58, said:

No-one has yet managed to provide a mechanism whereby a normal conveyor (albeit pretty big) is able to excerpt a force on a normal aircraft that can prevent the aircraft from moving, given that the wheels of a normal aircraft are free to spin in either direction and at any speed

Let's check it.
The conveyor exerts force on the PLANE (Because there is friction between the plane and the wheels). So the conveyor ~ slightly pressed brakes.
So the problem becomes: "Can the plane take off the runaway if the brakes are pressed a little". And the answer depend on the concrete parameters/situation.

[Slugsie]

RealFduch, on Jan 29 2008, 20:26, said:

Let's check it.
The conveyor exerts force on the PLANE (Because there is friction between the plane and the wheels). So the conveyor ~ slightly pressed brakes.
So the problem becomes: "Can the plane take off the runaway if the brakes are pressed a little". And the answer depend on the concrete parameters/situation.

Yup, that is pretty much spot on. The only effect of the conveyor is to increase the friction within the wheels, and is thus analogous to slightly applying the brakes. It'd contend that pretty much any aircraft would be able to do that. If you want to take it to an extreme to demonstrate the point. Imagine a fighter jet sat there with the pilot pushing moderately hard on the brakes, with the jet engines at 100% with full after burners. Anyone seriously believe that the aircraft is not going to move forwards?

[RealFduch]

Slugsie, on Jan 29 2008, 23:51, said:

Yup, that is pretty much spot on. The only effect of the conveyor is to increase the friction within the wheels, and is thus analogous to slightly applying the brakes. It'd contend that pretty much any aircraft would be able to do that. If you want to take it to an extreme to demonstrate the point. Imagine a fighter jet sat there with the pilot pushing moderately hard on the brakes, with the jet engines at 100% with full after burners. Anyone seriously believe that the aircraft is not going to move forwards?

He-he. Moving forward != taking off. The runaway may be not long enough to speed up when additional friction takes energy away.

[sundayx]

Can I argue that the wheels actually do not spin double it's speed? Wouldn't it be exponentially increasing in RPM?

[Minimoose]

Surely there wouldn't be any lift under the wings (unless the volume of air passing under the wings was the same amount as a normal plane speeding up, so u would need very powerful winds for it to take off like this?), and the forward motion would be countered by the treadmill equaling the thrust forward to an infinite speed. This is just my silly thought process, probably completely wrong

Also i know nothing of aerodynamics so i don't know how the dynamics of air affect the take off of a plane

[curme]

OPaul, on Jan 30 2008, 03:48, said:

There are around 140 replies to this thread, it would be impractical for me to go through them all. Where abouts was the point made and disproved? Just a page number will do.

Watch Wednesday's Mythbusters.... then tell me what happened!

[MGS4-SS]

Holy **** this thread is from 2006!

I think I was still in high school back then.

[gigapixels]

I think the reason people are disagreeing on this issue is that the experiment is somewhat unclear on the treadmill's speed... The way I understood it is that it goes as fast as the wheels do, not as fast as the plane does as some people are saying.

If it goes as fast as the plane does, then I do agree that it will take off. But if it goes as fast as the wheels do, then I say it cannot take off.

[monkey13]

Well I can't sleep and I've been thinking about this again. Damn it!

My theory is the plane will not take-off, regardless of if it can move down the conveyor. Here's why.

There's been a lot of talk about the wheels having no effect. This is impossible. For the wheels to have no effect the wheel bearings holding the weight of the plane would need to be entirely frictionless (impossible). The force required to compress the wheels as they rotate would have to be nil (impossible). Another way to think of it is; If the plane had these magic wheels and was in a vacuum (i.e no air resistance) the plane would move with any forward force applied to it (e.g. a small child could push it along with one hand) Another way to think of it is that the wheels behave the same regardless of if the plane is there or not. Ridiculous.There is a resistance which is why the engines have to be revved to get the pane moving when taxiing.

The next thing I was thinking about is; If the plane's engines were turned off and the conveyor belt it's on accelerated would the plane remain still? The answer is no. The forces, as mentioned above, that act through the wheels would still act moving the plane backwards. Therefore a force is required just to keep the plane in the same place. Meaning that more force is required to accelerate the plane on the conveyor than on the ground. Regardless of if the plane can move along the conveyor.

Now the question is; Does the plane have enough extra power to overcome these additional forces and move along the conveyor to reach take-off speed?

I'm going with no.

[Post-It Note]

monkey13, on Jan 29 2008, 18:05, said:

Well I can't sleep and I've been thinking about this again. Damn it!

My theory is the plane will not take-off, regardless of if it can move down the conveyor. Here's why.

There's been a lot of talk about the wheels having no effect. This is impossible. For the wheels to have no effect the wheel bearings holding the weight of the plane would need to be entirely frictionless (impossible). The force required to compress the wheels as they rotate would have to be nil (impossible). Another way to think of it is; If the plane had these magic wheels and was in a vacuum (i.e no air resistance) the plane would move with any forward force applied to it (e.g. a small child could push it along with one hand) Another way to think of it is that the wheels behave the same regardless of if the plane is there or not. Ridiculous.There is a resistance which is why the engines have to be revved to get the pane moving when taxiing.

The next thing I was thinking about is; If the plane's engines were turned off and the conveyor belt it's on accelerated would the plane remain still? The answer is no. The forces, as mentioned above, that act through the wheels would still act moving the plane backwards. Therefore a force is required just to keep the plane in the same place. Meaning that more force is required to accelerate the plane on the conveyor than on the ground. Regardless of if the plane can move along the conveyor.

Now the question is; Does the plane have enough extra power to overcome these additional forces and move along the conveyor to reach take-off speed?

I'm going with no.

The friction force isn't so huge that it can counteract any other force that is applied. Since friction is independent of speed or acceleration, it doesn't matter if the conveyor belt is going at 1 m/s or 100 m/s (assuming the wheels are built to withstand it. Certainly that isn't part of the question). Therefore, the plane is just trying to overcome the wheel's friction, which it always does normally. Static friction is greater than normal friction, so once the wheels are able to spin, they will keep on spinning until some other force makes it stop.

Even a person can pull a plane: http://www.ananova.c...sm_2672263.html . It doesn't require a massive amount of force to do so.

[OPaul]

monkey13, on Jan 29 2008, 21:05, said:

There is a resistance which is why the engines have to be revved to get the pane moving when taxiing.

The next thing I was thinking about is; If the plane's engines were turned off and the conveyor belt it's on accelerated would the plane remain still? The answer is no. The forces, as mentioned above, that act through the wheels would still act moving the plane backwards. Therefore a force is required just to keep the plane in the same place. Meaning that more force is required to accelerate the plane on the conveyor than on the ground. Regardless of if the plane can move along the conveyor.

I would think the thrust produced by the engines would be plenty to overcome the force of friction between the wheel bearings... and trusty enough to keep the plane accelerating no matter how fast the wheels may be spinning.

[dacris2000]

The force that causes the plane to take off is called "lift" and it's caused only when the wings are moving relative to the air. The plane on a treadmill would be stationary relative to the air. Ask yourself: Will you feel a headwind? If the answer is "No", then the plane will not take off. That's why pilots use the airspeed indicator, not the ground speed indicator, when they fly the plane.

Here's another example: A plane in a wind tunnel takes off, regardless of whether the wheels are moving. A very strong wind storm could actually cause a plane to take off with no engine power.

[Marcel T]

In the MythBusters preview of this piece, the pilot of the big test guesses that he won't take off, and bb quotes Adam Savage as saying "even the pilot guessed wrong".

I've always thought it won't take off because the problem is posed as "the conveyor matches the wheels speed" which means to me the conveyor is canceling any forward movement the plane makes which can only be generated by propeller/jet engine propulsion and without the forward movement to generate the air compression needed for take-off, no upwards-ness is going to happen.

But I suppose with the right model plane it could be possible by getting propulsion up to a high rate then somehow tipping back, getting leverage from a tail "mod" lifting the wheels off the ground for even a second or two and boosting off like a rocket, lol.

[I am Reid]

The plane is not doing any WORK against the air, so there will not be any lift, its as simple as that, some people are making this way to confusing.

[sundayx]

Guys guys guys guys guys the conveyor belt has nothing to do with the plane accelerating in space, I once made that mistake.