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Discussion Starter · #1 ·
You guys know where this is going. I'm much too lazy and disheartened over the other thread to look into this myself.

*disclaimer* while perfectly capable of understanding the physics behind it, my INTUITION would tell me that it won't take off unless the props are actually throwing air around the wings to create lift. I know that this is somehow not the case.

I understand that it has the speed necessary compared to the ground it's on. But I don't understand how wings can generate lift in still air.

Someone please explain to me why my first reaction is wrong. I will ship 100 rounds of federal 45 to the first answer that makes me understand.

That is all.
 

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clever quip
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The wheels of the plane are not the plane's propulsion source, therefore the treadmill can be taken out of the equation. The plane can and will take off.
 

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Discussion Starter · #5 · (Edited)
I hope you win. I'd save on shipping. But that just showed the pressure differences and how air creates lift travelling around the top and bottom of a wing. Unless I missed something, where is this stating that the same lift occurs without movement through the air? If that were the case, planes would take off while sitting in the hangar :dunno:
 

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Discussion Starter · #6 ·
The wheels of the plane are not the plane's propulsion source, therefore the treadmill can be taken out of the equation. The plane can and will take off.
I realize that the wheels have nothing to do with anything. The engines overcome the otherwise rear movement, EXACTLY. But no air is being forced under or over the wings.

I SWEAR I'm not trying to argue black is white, but am simply looking for an explanation that I understand.
 

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The props pulls the plane through the air, causing airflow over the wings. Higher pressure on the bottom and lower on top, causing the aircraft to rise. Being on a treadmill has no bearing, since it is relative to the air and not groundspeed.
 

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In order to understand how wings produce lift, you first have to understand Bernoulli's priciple = As Velocity increases, pressure decreases



The air pressure in your car will change when you are going down the highway and roll down a window. This is because the high velocity of air rushing by the open window actually creates a low pressure area and many people feel their ears pop a little.

As air passes over and under a wing, the air moving over the top of the wing moves faster creating a low pressure, the air passing under the wing moves slower due to the shape of the wing creating a high pressure. The high pressure pushes the wing up as the low pressure yields to the wing.

You will notice that when landing in a plane, the pilot will lower the flaps of the plane to change the dynamics of the wing. This allows the plane to increase it's ability to create lift at lower speed, by the added effect is that it increases drag. This is why many times, you will hear the engines roaring, but you are still traveling at a slow speed.

Lift is generated based on Airspeed regardless of ground speed. Therefore, if you were traveling 200mph in an airplane on a very long runway with a 200 mph tailwind, you would not create any lift and depending on the aircraft, you maybe exceeding the speed that the landing gear are Max rated to take.

Conversely, if you were sitting in a plane with a 200mph headwind, your ground speed could be 0mph, but due to air under your wings, you could create lift. Of course, if the wind stopped blowing, you would fall out of the sky.

A good way to illustrate this point is to make a paper airplane and place it on a table in front of you. Then blow really hard. The plane should take flight and then fall as airspeed decrees.

Does this help?
 

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An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

February 3, 2006
Dear Cecil:
Please, please, please settle this question. The discussion has been going on for ages, and any time someone mentions the words "airplane" or "conveyor belt" everyone starts right back up. Here's the original problem essentially as it was posed to us: "A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"
There are some difficulties with the wording of the problem, specifically regarding how we define speed, but the spirit of the situation is clear. The solution is also clear to me (and many others), but a staunch group of unbelievers won't accept it. My conclusion is that the plane does take off. Planes, whether jet or propeller, work by pulling themselves through the air. The rotation of their tires results from this forward movement, and has no bearing on the behavior of a plane during takeoff. I claim the only difference between a regular plane and one on a conveyor belt is that the conveyor belt plane's wheels will spin twice as fast during takeoff. Please, Cecil, show us that it's not only theoretically possible (with frictionless wheels) but it's actually possible too.
— Berj A. Doudian, via e-mail
Excuse me--did I hear somebody say Monty Hall?
On first encounter this question, which has been showing up all over the Net, seems inane because the answer seems so obvious. However, as with the infamous Monty-Hall-three-doors-and-one-prize-problem (see The Straight Dope: "On Let's Make a Deal" you pick Door #1, 02-Nov-1990), the obvious answer is wrong, and you, Berj, are right--the plane takes off normally, with no need to specify frictionless wheels or any other such foolishness. You're also right that the question is often worded badly, leading to confusion, arguments, etc. In short, we've got a topic screaming for the Straight Dope.

First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.
A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.
As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.
— Cecil Adams


http://www.straightdope.com/columns...the-opposite-direction-can-the-plane-take-off
 

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Get off my lawn
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I realize that the wheels have nothing to do with anything. The engines overcome the otherwise rear movement, EXACTLY. But no air is being forced under or over the wings.

I SWEAR I'm not trying to argue black is white, but am simply looking for an explanation that I understand.
The plane will not stay still with the throttle on. The wheels-on-the-treadmill don't matter. The sucker will move fwd on said-treadmill and take off in the same horizontal fixed distance it usually does. If it takes off in 30' and you only have a 20' treadmill, it'll run into the end and die. If you have a 50' treadmill - a la the HUGE one that Mythbusters built, you'll take off before reaching the end. You could pull that treadmill at 4x the takeoff speed, it don't matter. . . until you burn out tires.

Stop thinking that the plane needs the same propulsion to keep up with a 10mph treadmill as it needs to go 10mph on fixed ground. They are <>.
 

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From what I understand, even though the treadmill is moving, the pilot would have to actively try to stay in the same spot even though the wheels aren't motorized.

Kind of like running on a treadmill. You can stay in the same spot, move closer to the front of the treadmill, or move towards the end.

So, in a plane, the pilot can still propel the plane forward and take off.

Maybe I'm wrong though. :supergrin:

Also - http://mythbustersresults.com/episode97
 

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Discussion Starter · #12 ·
Does this help?
Nope. I understand lift and your examples perfectly. But there's zero wind on this treadmill, and none generated by movement of the plane. Where is this high and low pressure from oncoming airflow coming from is my question.
 

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Smartass Pilot
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Short version, the treadmill won't stop the plane from accelerating, the free wheeling landing gear will just spin faster than they would normally. The plane will accelerate because of thrust from the prop and wind will move over the wing and lift will be created as usual. It's been proven at full scale with a real airplane (Mythbusters).
 

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Discussion Starter · #14 ·
However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.
— Cecil Adams
So, correct me if I'm wrong:

The plane doesn't take off because it has generated lift while "sitting still".

The plane takes off because it's impossible for a treadmill to spin fast enough to overcome the plane's propulsion THROUGH THE AIR? (which is obviously true)

So the answers is it flies, but only because the question itself is an impossible scenario???
 

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So, correct me if I'm wrong:

The plane doesn't take off because it has generated lift while "sitting still".
Yes
The plane takes off because it's impossible for a treadmill to spin fast enough to overcome the plane's propulsion THROUGH THE AIR? (which is obviously true)
Treadmill is not relevant, but you can say yes.
So the answers is it flies, but only because the question itself is an impossible scenario???Yes
:wavey:
 

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Luggage
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The plane's wheels are freewheeling. The treadmill's speed and direction of travel, therefore, don't matter at all; the interaction of the treadmill and the wheels impart no force on the airplane itself (assuming no rolling resistance from the tires, bearings, and treadmill surface).

No force being imparted on the airplane could also be demonstrated by an airplane resting on perfectly waxed ski landing gear on a wet ice lake. It's easy to see how the propeller would pull the airplane along the ice until the wings generate enough lift to take off.
 

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Smartass Pilot
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The plane doesn't take off because it has generated lift while "sitting still".
The plane won't sit still, the treadmill can't stop it from moving forward. The wheels on an airplane are free-spinning. They'll just spin a bit faster during takeoff.

The plane takes off because it's impossible for a treadmill to spin fast enough to overcome the plane's propulsion THROUGH THE AIR? (which is obviously true)
Sorta, the treadmill's speed is simply irrelevant. It has no way of imparting any significant friction on the aircraft therefore it can't stop it from moving forward. It helps if you realize that a plane is not a car. It is not accelerating because of force applied through a transmission/axle assembly. A plane moves because of the air moved by the prop.

So the answers is it flies, but only because the question itself is an impossible scenario???
The question isn't impossible, you can put a plane on a treadmill, it just makes no differrence on the outcome.

[Edit: It is impossible for a treadmill to match the speed of the planes wheels. This would result in the treadmill accelerating into infinty as the plane moved forward even a mere inch. Is that what you meant?]
 

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Forget about the treadmill. It is confusing you.

The plane, or the wings actually will only generate lift if air is moved over its surface begining with its leading edge. It is not the props that "throw" wind over the wing as someone else has posted. The props pull the plane (using the same principle as the wings). If the plane moves along the ground fast enough it causs air to pass over and under the entire surface of the wing. At a certain speed proper lift is achieved.

In fact a plane could achieve lift while sitting still if the wind were to blow hard enough in the correct direction relative to the wings.
 

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transmogrifier
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The plane's wheels are freewheeling. The treadmill's speed and direction of travel, therefore, don't matter at all; the interaction of the treadmill and the wheels impart no force on the airplane itself (assuming no rolling resistance from the tires, bearings, and treadmill surface).

No force being imparted on the airplane could also be demonstrated by an airplane resting on perfectly waxed ski landing gear on a wet ice lake. It's easy to see how the propeller would pull the airplane along the ice until the wings generate enough lift to take off.

Exactly this...
This is one of the best, most succinct explanations I've seen.


Forget about the treadmill. It is confusing you.

The plane, or the wings actually will only generate lift if air is moved over its surface begining with its leading edge. It is not the props that "throw" wind over the wing as someone else has posted. The props pull the plane (using the same principle as the wings). If the plane moves along the ground fast enough it causs air to pass over and under the entire surface of the wing. At a certain speed proper lift is achieved.

In fact a plane could achieve lift while sitting still if the wind were to blow hard enough in the correct direction relative to the wings.
Also a very good explanation.
 

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Does time slow down as the plain approaches take off?
 
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