Engineering question
-
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?
Only if you have a really big fan blowing at a speed at least that of the aircrafts V2 (take off) speed. :laugh:
-
Only if you have a really big fan blowing at a speed at least that of the aircrafts V2 (take off) speed. :laugh:
Then the aircraft will go and hit that fan :-)
-
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?
-
It obviously can since propulsion for airplanes is provided by i/e a jet engine. Airplane doesn't accelerate using wheels, like i/e a car. Conveyor belt would only make the wheels spin faster, and that's it.
Actually ... no. The conveyor will have an effect and can increase lift - slightly - since it will also drag the air along with it and if that interacts with the wings it effectively act like the plane is going faster. (That's why aircraft carriers always launch steaming full ahead into the wind when launching and recovering aircraft - it adds a few MPH to their airspeed and reduces the chances of a stall.)
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony "Common sense is so rare these days, it should be classified as a super power" - Random T-shirt AntiTwitter: @DalekDave is now a follower!
-
Actually ... no. The conveyor will have an effect and can increase lift - slightly - since it will also drag the air along with it and if that interacts with the wings it effectively act like the plane is going faster. (That's why aircraft carriers always launch steaming full ahead into the wind when launching and recovering aircraft - it adds a few MPH to their airspeed and reduces the chances of a stall.)
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony "Common sense is so rare these days, it should be classified as a super power" - Random T-shirt AntiTwitter: @DalekDave is now a follower!
Your answer is confusing. First, you say: actually no, then continue with a reasonable explanation on why some extra lift would be achieved. So, airplane WOULD take off, which I assume was the point of the q. Whether it'd have more lift or no due to conveyor belt is a minor detail. There's also the matter of how long the conveyor belt is. If it's as long as the runway, then yea, but if it's as long as the airplane, airplane could fall off and probably crash in front of it.
-
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.
-
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.
-
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.
-
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.
-
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.
-
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
-
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.
-
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.
-
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.
-
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
-
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.
-
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
-
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.)
-
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.
-
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).
