SpiceWeasel
Tre Kronor
It's kind of funny that I got warned fairly quick when I took a perceived stab at someone, but someone calls you an idiot indirectly and gets off with nothing.
And instead of merely stating one is an idiot, provide facts to help shape a better understanding of the question:
The original question can be found on the physorg forums.
This myth, and the debate it has caused, has highlighted multiple failures, both in communication and in understanding.
One popular modification to the question states that the speed of the belt must match the speed of the plane's wheels. In this scenario, the speed of the plane's wheels is being measured in the belt's frame of reference, while the speed of the belt is being measured in the ground's frame of reference. This modification has only led to confusion, as it disallows the plane from moving in the ground's frame of reference. Since there is no wind in this situation, the ground frame of reference is the same as the airspace frame of reference, and so the plane is disallowed from gaining airspeed. There is no debate that with this restriction in place, the plane will never generate lift, and will never take off. This was a failure in communication.
A common failure in understanding is that, when measuring and comparing speeds, all measurements must be made using the same frame of reference. Measuring the speed of the wheels in the belt's frame of reference, but the speed of the belt in the ground's frame of reference, violates this rule. A common frame of reference, in this case the ground around/under the belt, or the airspace around the plane and belt, must be established. Once this is accepted, it should become clear that unless the plane moves forwards, the belt will never move backwards. If the plane is not moving forwards, there is no speed to be measured and matched.
Another common failure in understanding occurs when people attempt to use the analogy of a car or a human on a treadmill. This is an incorrect analogy for two reasons.
In order for the conveyor belt to prevent the plane from accelerating, it must apply Drag equal to or greater than the Thrust generated by the plane's engines. Since the only point of contact between the conveyor and the plane is the plane's wheels, this force must be applied through the wheels. However, since the wheels of the plane are specifically designed to minimize the application of force from the ground to the plane, the conveyor is unable to apply enough Drag to prevent the plane from accelerating, and so the plane accelerates forward, gains airspeed, and takes off.
The Mythbusters succesfully demonstrated that it is impossible for the conveyor to apply enough Drag to prevent the airplane from accelerating.
Summary: The myth does not ask "If an airplane is prevented from moving, can it take off?" - it asks "Can a conveyor prevent an airplane from moving?" The Mythbusters demonstrated that a conveyor cannot prevent an airplane from moving.
The answer to the question is easy - if you pose it right. I will bet that some people who dismiss those who question the airplanes ability to take of as idiots didn't fully understand the question themselves.
Fact: If the airplane is allowed FORWARD movement, ie the thrust created by the propeller allows the airplane to begin moving, it can take off. The wheel spin will have no effect at this point.... the wheels will obviously have a higher groundspeed however since there is airflow the aircraft can take off.
Fact: If an aircraft is forced to remain stationary with full thrust, it cannot take off. In that vein, though the wheels are free-spinning, if the aircraft cannot begin to move forward, it will not take off.
Easy.
And instead of merely stating one is an idiot, provide facts to help shape a better understanding of the question:
The original question can be found on the physorg forums.
This myth, and the debate it has caused, has highlighted multiple failures, both in communication and in understanding.
One popular modification to the question states that the speed of the belt must match the speed of the plane's wheels. In this scenario, the speed of the plane's wheels is being measured in the belt's frame of reference, while the speed of the belt is being measured in the ground's frame of reference. This modification has only led to confusion, as it disallows the plane from moving in the ground's frame of reference. Since there is no wind in this situation, the ground frame of reference is the same as the airspace frame of reference, and so the plane is disallowed from gaining airspeed. There is no debate that with this restriction in place, the plane will never generate lift, and will never take off. This was a failure in communication.
A common failure in understanding is that, when measuring and comparing speeds, all measurements must be made using the same frame of reference. Measuring the speed of the wheels in the belt's frame of reference, but the speed of the belt in the ground's frame of reference, violates this rule. A common frame of reference, in this case the ground around/under the belt, or the airspace around the plane and belt, must be established. Once this is accepted, it should become clear that unless the plane moves forwards, the belt will never move backwards. If the plane is not moving forwards, there is no speed to be measured and matched.
Another common failure in understanding occurs when people attempt to use the analogy of a car or a human on a treadmill. This is an incorrect analogy for two reasons.
- The thrust that moves a car or a human runner is derived through contact with the ground. If the ground is moving away from that thrust, the thrust is negated and no forward velocity is obtained. An airplane, however, derives thrust from the air. The treadmill moving does not negate that thrust. A small amount of additional friction and rolling resistance is generated by the wheels spinning twice as fast, but it is important to remember that wheels are designed to minimize friction - to minimize the transmission of force from one surface to another. If the friction and rolling resistance in the wheels were great enough to be significant, you would see planes at airports quickly coming to a stop once their engines are throttled down while taxiing. With only an insignificant amount of friction (much lower than the thrust of the plane's engine(s)) connecting the plane to the belt, it is impossible for the belt to counteract the plane's thrust.
- It is also important to note that the treadmill is not actually matching the speed of the car or the runner. Remember that all measurements must be taken from a common frame of reference - the car or runner is held stationary relative to the air/ground, and the belt is moving relative to the air/ground. In other words, the belt prevents the forward motion of the car or runner relative to the ground.
- Thrust: The airplane generates thrust by accelerating a mass of air backwards. Newton's Third Law of Motion ("For every action, there is an equal and opposite reaction") dictates that the engine, and thus the airplane, is thus accelerated forwards.
- Drag: The two main sources of drag are the air and the friction with the runway.
- Since the plane is starting from a standstill, and it is assumed that there is no wind, the drag due to air is 0 until the plane starts moving.
- Friction with the runway is mitigated by the wheels of the airplane. From a standstill, the plane must overcome the rolling resistance of the rubber over the concrete or asphalt and the rolling friction of steel over steel in the bearing of the wheel. Once the initial rolling resistance is overcome, The amount of friction in these components drops significantly, and increases slightly as the wheel accelerates.
In order for the conveyor belt to prevent the plane from accelerating, it must apply Drag equal to or greater than the Thrust generated by the plane's engines. Since the only point of contact between the conveyor and the plane is the plane's wheels, this force must be applied through the wheels. However, since the wheels of the plane are specifically designed to minimize the application of force from the ground to the plane, the conveyor is unable to apply enough Drag to prevent the plane from accelerating, and so the plane accelerates forward, gains airspeed, and takes off.
The Mythbusters succesfully demonstrated that it is impossible for the conveyor to apply enough Drag to prevent the airplane from accelerating.
Summary: The myth does not ask "If an airplane is prevented from moving, can it take off?" - it asks "Can a conveyor prevent an airplane from moving?" The Mythbusters demonstrated that a conveyor cannot prevent an airplane from moving.
The answer to the question is easy - if you pose it right. I will bet that some people who dismiss those who question the airplanes ability to take of as idiots didn't fully understand the question themselves.
Fact: If the airplane is allowed FORWARD movement, ie the thrust created by the propeller allows the airplane to begin moving, it can take off. The wheel spin will have no effect at this point.... the wheels will obviously have a higher groundspeed however since there is airflow the aircraft can take off.
Fact: If an aircraft is forced to remain stationary with full thrust, it cannot take off. In that vein, though the wheels are free-spinning, if the aircraft cannot begin to move forward, it will not take off.
Easy.
