Airplanes and Lift

  • #26
sophiecentaur
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yeah thats where my question basically lies is if the plane is not moving there is not as much air flowing over the wings therefor less thrust is created so
This is a complete non-sequitur. There can be as much thrust as you like when the plane happens to be stationary. You are totally confusing cause and effect. The thrust is not 'because of' airflow so your "therefore" is the wrong word.
yeah thats where my question basically lies is if the plane is not moving there is not as much air flowing over the wings therefor less thrust is created so I was wondering if the plane could still take off even though it's wings are not gliding through the air creating that downward thrust
You mean lift? "Thrust is conventionally taken to be the force that the engines produce. The only way you can get lift with aircraft not moving through the air is to use vectored thrust. VT has not been mentioned so far in the thread so it would be better not to move the goalposts by introducing it.

I think it's time for you to re-read this thread and to see how so many of your posts have not addressed the other comments. What do you actually want out of this exercise? You are not going to change the theory. Why not learn the theory and then you will be able to answer the questions yourself.
 
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  • #27
rcgldr
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Going back to the original question.

if a jet plane is sitting on a treadmill and the treadmill moves backwards at the same rate the plane is moving forward will it take off?
The planes tires will spin twice as fast, which won't have much effect on the plane accelerating forwards due to thrust, and the plane will be able to take off, only using bit more distance to compensate for the relatively small increase in angular kinetic energy of the wheels.

The TV show myth busters did an actual demonstration, by pulling a "conveyor belt" backwards:

 
  • #28
russ_watters
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Going back to the original question.

The planes tires will spin twice as fast, which won't have much effect on the plane accelerating forwards due to thrust, and the plane will be able to take off, only using bit more distance to compensate for the relatively small increase in angular kinetic energy of the wheels.

The TV show myth busters did an actual demonstration, by pulling a "conveyor belt" backwards:
This is fine for your, my and Mythbusters' assumptions, but fails for the OP's assumptions, which define the plane to be at rest and include other necessary assumptions like an engine at idle.
 
  • #29
CWatters
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If the aircraft has wheels there is very little or no friction between the aircraft and the conveyor belt. So the moving conveyor belt applies very little rearward force on the aircraft. This force is easily overcome by the thrust from the engine so the plane accelerates and takes of normally.

The only difference is that the wheels rotate faster.

If in doubt draw a free body diagram of the aircraft showing the force acting on it.
 
  • #30
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  • #31
Brad Jensen
To reduce this problem to its simplest terms, imagine that the velocity of the airplane and the velocity of the treadmill are equal, at zero. What happens?

The answer without all the nitpicking is no, the plane will not take off unless it is moving relative to the air it is in. If it is on a treadmill moving backwards at the same rate the plane is moving forward, it will not lift into the air.


Unless it is a Harrier jump jet., or AV8, with thrust vectoring. Or an Osprey. They can take off with thrust vectoring in still air.
 
  • #32
jbriggs444
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If it is on a treadmill moving backwards at the same rate the plane is moving forward
That's the ambiguous condition this thread started with. Is the plane moving forward relative to the ground or relative to the treadmill?
 
  • #33
berkeman
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  • #34
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If it is on a treadmill moving backwards at the same rate the plane is moving forward, it will not lift into the air.
The best way to visualize this problem is to imagine that you've taken your model airplane (with properly rotating wheels) into a grocery store, set it down on the conveyor belt, and are pushing it. Can you push it against the direction of the belt? Yes, whether the belt is moving or not. What do the wheels do when you push it against the direction of the belt? They turn. If the belt is moving while you're pushing they turn faster, but that's all.

The thrust from the engines is no different than the force of your hand pushing the toy airplane: the thrust is pushing the airplane forward relative to the air. Is there any force acting on the airplane in the opposite direction to resist the engine thrust? There is if the airplane is tethered to the treadmill or if the wheel brakes are locked so that the wheels cannot turn. In this case the airplane remains at rest relative to the treadmill and its wheels do not turn. There is if the airplane is tethered to the ground; in this case the airplane remains at rest relative to the ground while the treadmill moves and the wheels turn.

However, if the airplane is not tethered and the wheels are free to spin, then there is nothing to oppose the thrust of the engines so the plane accelerates forwards relative to the air, the ground, and the treadmill. The wheels turn faster because the airplane is moving relative to the belt, that exerts no force on the airplane.
 
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  • #35
russ_watters
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I think we've beaten this to death. We'll keep it locked.
 
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