Engineering question
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
The ONLY way the airplane can take off is if the speed of air respect of the airplane's speed is equal to the minimum speed the airplane needs to take off when the wind is absolutely calm. That is, because the conveyor makes the plane to be static respect to the ground, the only way the plane will take off is if there is a really hard hurricane that accelerates de wind to the plane's take off speed.
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
What a plane needs to take off is lift which is created by the pressure difference between the top and bottom sides of the airfoils (wings). To generate that difference, we use high-speed air flow aimed at the leading edge of the wing. The shape of the wing (an airfoil) makes air pile up in a high-pressure zone under the wing and zip over the top to create a low-pressure zone above the wing. This crazy magic lifts the plane with all of its weight into the air. To generate the air flow, we generally use the easiest thing at hand, the velocity of the plane itself. On a conveyor belt, it won't move against the wind, but if it's facing into a gale strong enough, it could theoretically lift into the air and its jets would then be sufficient to make it go so long as the gale persists long enough for the jets to achieve enough speed through the air to maintain lift.
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
Has anyone posted the Mythbusters episode about the question?
I’ve given up trying to be calm. However, I am open to feeling slightly less agitated.
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
Yes. An airplane is not propelled forward by the wheels, but by the propeller(s) or jet engines. How fast the wheels are spinning is irrelevant. It may be a little trickier to steer but it'll take off.
If you think 'goto' is evil, try writing an Assembly program without JMP.
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Not a joke. A question asked by my friend, for which I am not aware of the answer.
Degree in physics, as if it mattered: No. The lift depends on airflow over the airplane wings. There will be none.
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I’d argue that it’s the airspeed that matters. If hurricane force winds start blowing during the experiment, the airplane might take off. However the original question didn’t mention anything like that and, under normal conditions, airspeed and ground speed are roughly equal.
Mircea
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Other thought experiment: a toy car is on a conveyor, you push it forward and the conveyor goes backward "at the same speed" (whatever that means, which is not quite clear). Can you push it forward? Whatever the answer, an airplane would do the same thing, because its thrust is applied in the reference frame of the air around it. The wheels are not driven, they spin freely except when the brake is applied.
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Not a joke. A question asked by my friend, for which I am not aware of the answer.
Yes it can, the wheels spin freely and have nothing to do with propulsion, even in normal takeoffs. The Mythbusters even did a show about it.
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Degree in physics, as if it mattered: No. The lift depends on airflow over the airplane wings. There will be none.
Certainly true - as long as you don't fire up the engines. But without the engines running, the plane won't lift even on a normal runway. The engines will push the plane up to speed, creating that airflow. The push is unaffected by those free-running wheels spinning like crazy - the plane accelerates just as much, wheels spinning or not.
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
wind speed is all that matters. Nothing else for take off. Not ground speed, not wheel speed. Only the speed with which the plane is moving in relation to the air. This is why planes prefer to take off into the wind. It reduces the amount of time on the ground before liftoff. But again ground speed doesn't matter. Air speed matters.
To err is human to really elephant it up you need a computer
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Certainly true - as long as you don't fire up the engines. But without the engines running, the plane won't lift even on a normal runway. The engines will push the plane up to speed, creating that airflow. The push is unaffected by those free-running wheels spinning like crazy - the plane accelerates just as much, wheels spinning or not.
The engines don't directly cause the airflow. The engines push the airplane, whose movement through the air causes the airflow over the wings.
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It is airspeed that matters, but how fast the wheels are spinning is irrelevant, they aren't what propels the airplane forward.
If you think 'goto' is evil, try writing an Assembly program without JMP.
Of course you are right. I realized (late) that engines push the air back and, hence, the airplane forward irrespective of wheels moving or not (or even not existing at all as in the case of seaplanes). Seems my brain was taking a day off yesterday :laugh:. Luckily it was a weekend day.
Mircea
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Not a joke. A question asked by my friend, for which I am not aware of the answer.
The answer is not without wind. An airplane is lifted off the ground, not because of the speed of the plane, per se. But because of the speed of the air moving above and below the wing. The shape of the wing leverages the Bernoulli affect. (High Pressure below the wing, lower pressure above), giving the plane "lift". In fact, during a strong wind storm. Planes that are stored OUTSIDE, and TIED DOWN. WILL Lift into the air, and pull against the ropes. Being in Florida, I have witnessed this first hand. It's wild. (And it only works if the plane is facing the wind! The other planes get pushed "down/away" as their wings are "reversed", or they get turned.)
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wind speed is all that matters. Nothing else for take off. Not ground speed, not wheel speed. Only the speed with which the plane is moving in relation to the air. This is why planes prefer to take off into the wind. It reduces the amount of time on the ground before liftoff. But again ground speed doesn't matter. Air speed matters.
To err is human to really elephant it up you need a computer
Yes. Unlike a car that uses it's engine to turn the wheels to propel it forward, on an airplane the wheels are not what propels the plane at all. Its the thrust being generated by moving air via propellers or jet engines that provides thrust for an airplane, and the medium they are moving in, and what that thrust is relative to, is a sea of air. Put a boat on wheels, place it so it sits on a conveyor in water, and turn on the propeller. As long as the wheels on the boat that are in contact with the conveyor are free wheeling the boat will move forward.
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
Ignoring the possibility of a head wind strong enough to take of on the spot (or very close to it): No, it will not take off. The engines will obviously apply a force to the aircraft. To match the speed of the wheels, the conveyer belt would have to keep the plane still - if the plane is moving forward, the wheels much move faster than the belt (ignoring maximum friction, so the wheels will not slip). This means the belt will have to apply enough reverse force on the tires that the increased tire rolling friction and bearing friction transferred to the landing gear is identical to the force applied by the engine. This would quickly require the wheels to spin so fast the centrifugal force will rip them apart - first the ties, then the wheel or bearings. Then anything remaining of the landing gear will be ripped off, and the aircraft will crash on its belly on top of a conveyer belt moving it rapidly backwards. Kind of hard to get in the air from that position. Any limit to the available friction between belt and tires will allow the tires to slip over the belt - this means the aircraft could be moving forward while the belt is still matching the speed (but not position) of the wheels. But any friction available will be "used" to accelerate the wheels - so anything but the most minuscule friction would not allow the plane to reach takeoff speed before the wheels collapse. To take of, you should basically be able to do it with the breaks applied (ignoring the pesky nose or tail wheel without breaks).
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
Yes.
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If an airplane is positioned on a conveyor belt as wide as a runway, and this conveyor belt is designed to exactly match the speed of the wheels, but moving in the opposite direction, ... Can the airplane take off?
In general, no. The movement of the wheels has no effect on lift. To take flight, a craft needs either sufficient airflow across the wings or enough thrust to overcome gravity. A plane can take off from a stationary position, without a conveyor, if there is enough wind and the engine has enough thrust to counteract the drag; the plane would simply rise straight off the ground. Additionally, air craft with very powerful engines, like an F-16, can accelerate vertically. In this case, the wings do not generate lift; the engine itself provides all the lift. Theoretically, an F-16 could take off from a stationary but vertical position. Again, the wheels would not be used for this.
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The ONLY way the airplane can take off is if the speed of air respect of the airplane's speed is equal to the minimum speed the airplane needs to take off when the wind is absolutely calm. That is, because the conveyor makes the plane to be static respect to the ground, the only way the plane will take off is if there is a really hard hurricane that accelerates de wind to the plane's take off speed.
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Ignoring the possibility of a head wind strong enough to take of on the spot (or very close to it): No, it will not take off. The engines will obviously apply a force to the aircraft. To match the speed of the wheels, the conveyer belt would have to keep the plane still - if the plane is moving forward, the wheels much move faster than the belt (ignoring maximum friction, so the wheels will not slip). This means the belt will have to apply enough reverse force on the tires that the increased tire rolling friction and bearing friction transferred to the landing gear is identical to the force applied by the engine. This would quickly require the wheels to spin so fast the centrifugal force will rip them apart - first the ties, then the wheel or bearings. Then anything remaining of the landing gear will be ripped off, and the aircraft will crash on its belly on top of a conveyer belt moving it rapidly backwards. Kind of hard to get in the air from that position. Any limit to the available friction between belt and tires will allow the tires to slip over the belt - this means the aircraft could be moving forward while the belt is still matching the speed (but not position) of the wheels. But any friction available will be "used" to accelerate the wheels - so anything but the most minuscule friction would not allow the plane to reach takeoff speed before the wheels collapse. To take of, you should basically be able to do it with the breaks applied (ignoring the pesky nose or tail wheel without breaks).
The conveyor stuff is just complication. The entire point of the conveyor is to prevent lateral movement. Mythbusters had no budget for a real conveyor and their facsimile wasn't a conveyor and didn't prevent lateral movement. The weight of the plane and the stretch of the material allowed for lateral movement. A dynamometer would have been better. Sure, the wheels don't matter. So take the wheels off and jack the plane onto cinder blocks. Same concept, only without the fake-conveyor nonsense that lets people think you can have lift in no wind with no lateral movement. Jet, plane, whatever... If you prevent it moving forward, it's not going to move upward just because you tilt a control surface. Tie a sea plane off by its rear to a dock. Throttle slightly to get all the slack/stretch of the line out, then push to full. It would remain more or less stationary. "Prop wash" lift is a thing in R/C aircraft where thrust-to-weight blows pretty much all real planes out of the water. You can hover some R/C planes like a helicopter. You might be able to do that with some real sport planes, but I doubt it.
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Of course you are right. I realized (late) that engines push the air back and, hence, the airplane forward irrespective of wheels moving or not (or even not existing at all as in the case of seaplanes). Seems my brain was taking a day off yesterday :laugh:. Luckily it was a weekend day.
Mircea
I hear you. My brain takes frequent breaks, and not just on weekends. :wtf: I just happened to have argued this same scenario a few years ago (and was on the wrong side at first) and recalled the facts.
If you think 'goto' is evil, try writing an Assembly program without JMP.