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Jock football player or not, he's right. The way in which wings create lift is complicated, but VERY briefly, the wings divert air down (action), which forces the wings up (reaction) in accordance with Newton's law.
not sure whats gotten up everyones arse lately.....i was simply giving a lamens description of the principle.....so those that are not involved with science would have an idea what the bernoulli principle states (wrt the question).....
so good job correcting me when i made no reference to the amount of effect, or otherwise the other factors, that contribute to lift. hope that degrees workin for ya....
Wow, just saw this thread. Amazing how impassioned some get with opinions based on lack of knowledge and/or understanding.
Now for you smarties: take your average passenger aircraft and round off the take-off speed to 150mph to keep it simple. The treadmill would thus be spinning 150mph the opposite direction, causing the wheels to spin at ~300mph (depending on diameter hahaha). The amount of friction generated would most likely cook the grease right out of the wheel bearings thus smoking the bearings and locking up the wheels.
The plane will take off, but how the heck are they going to land?
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According to the requirements of the original version of this thought experiment the plane, by definition, cannot take off. Why? Because if the treadmill is moving at the same rate as the wheels then the plane is not moving forward, resulting in an airspeed of zero.
So what happens when the engines provide thrust? The treadmill has to speed up to such a speed that the drag on the plane equals the thrust of the engines. Otherwise the requirements of the problem are not met.
In reality we don't have any treadmills which can go fast enough, so the plane would take off. But that doesn't meet the problem's requirements.
Jock football player or not, he's right. The way in which wings create lift is complicated, but VERY briefly, the wings divert air down (action), which forces the wings up (reaction) in accordance with Newton's law.
Well....no.
The shape of the wing causes air to travel faster over the top of the wing than the bottom. The faster moving air over the top creates a low pressure on the top of the wing where the pressure remains the same on the bottom. Since the pressure on the bottom of the wing is greater, it has the tendency to create lift.
Also the downwash off the back of the wing and just plain old thrust can help create the lift needed for takeoff.
According to the requirements of the original version of this thought experiment the plane, by definition, cannot take off. Why? Because if the treadmill is moving at the same rate as the wheels then the plane is not moving forward, resulting in an airspeed of zero.
So what happens when the engines provide thrust? The treadmill has to speed up to such a speed that the drag on the plane equals the thrust of the engines. Otherwise the requirements of the problem are not met.
In reality we don't have any treadmills which can go fast enough, so the plane would take off. But that doesn't meet the problem's requirements.
Think outside the box real quick. The speed of the airplanes wheels have no effect on the airspeed of the plane. Say a plane has to go 50 knots to take off. While accelerating to 50 knots, the treadmill will accelerate in the opposite direction. Once the plane gets to 50 knots, it will take off with a GROUND speed of 100 knots since the "ground"(treadmill) is matching its speed in the opposite direction.
Think outside the box real quick. The speed of the airplanes wheels have no effect on the airspeed of the plane. Say a plane has to go 50 knots to take off. While accelerating to 50 knots, the treadmill will accelerate in the opposite direction. Once the plane gets to 50 knots, it will take off with a GROUND speed of 100 knots since the "ground"(treadmill) is matching its speed in the opposite direction.
I did, even if it is an overused (and useless) expression. Thinking inside the box results in the conclusion that the plane will take off.
By default, there are many assumptions in these kinds of problems. One being that the air is still. This means that ground speed and air speed are the same.
The original version of this thought experiment refers to the rate at which the wheels spin versus the rate of the treadmill. If these match, then the axis of the wheels does not move. They stay in one place. If the wheels are staying in one place, this means the plane's ground speed is zero--which means that its air speed is zero.
In your example, the speed of the wheels and the speed of the treadmill do not match, which breaks the rules of the problem. It's not asking if a plane can take off on a treadmill which is going the opposite direction; but if the plane can take off on a treadmill for which the surface is moving in a direction opposite the plane's and at the same speed as its wheels. As, by definition, this means that the plane is not moving (either because the engines are shut off, or because the drag from the treadmill against the wheels equals the thrust), then this also means it cannot take off.
Contrary to the apparently popular opinion, the wheels of a plane and their contact patch with the ground is not perfectly free of drag.
Think of it this way:
You have a plane parked on a huge treadmill.
Scenario 1: The treadmill starts to move. So what happens? The plane starts moving backward because of the force of friction both in the hub and at the contact patch of the wheels/tires. At this point the treadmill is moving faster (say, for example, 1 MPH) than the wheels (not rotating at all.) This does not fulfill the requirement that the treadmill and wheels move at the same rate.
Scenario 2: The pilot powers up the plane and provides just enough thrust to reach 1 MPH in relation to the treadmill. Now the treadmill and the wheel speeds match. The result? A groundspeed of zero. The plane is no longer moving in relation to the ground and, as a result, in relation to the air. Thus: no lift. This does fulfill the requirement that the treadmill and wheels move at the same rate.
Scenario 3: Now the pilot wants to take off. He provides full thrust and the plane overcomes the drag in the wheels. The plane speeds up in relation to the ground and air and will eventually be able to take off. However, in doing so the wheels have to spin faster than the treadmill's 1 MPH. This does not fulfill the requirement that the treadmill and wheels move at the same rate.
Scenario 4: So the treadmill operator speeds up the treadmill to whatever speed is necessary to provide enough drag to keep the plane from moving. In real life this would destroy the gear and the treadmill. In a thought experiment, nothing breaks. So the treadmill moves really fast, holding the plane back by counteracting its thrust with friction. As this means the plane isn't moving in relation to the air/ground, it does not take off. This does fulfill the requirement that the treadmill and wheels move at the same rate.
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