Because I'm bored, here's a brain teaser for your 4th of July weekend:

A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. Can the plane take off?

Update: Answer on page 2 (and also below).
Last edited {1}
Original Post
quote:
Originally posted by TheMeInTeam:
Because I'm bored, here's a brain teaser for your 4th of July weekend:

A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. Can the plane take off?


Yes, the conveyor/runway would double the ground speed of the wheels. The real airspeed is generated by the prop thrust, not the runway speed.
I remember seeing this on Mythbusters, but I don't remember the results. It seems like the flow of air over the airfoil(wing) would have to have sufficient velocity to create lift. That means the speed of the wing moving through the air would need to be sufficient to generate that lift,so the speed of the wheels turning would be irrelevant. I could EASILY be wrong but it looks real good in print. Big Grin Again, I may be wrong but I thought the propeller was like a screw to pull the plane through the air.
And, what if the propeller was in the rear called a pusher prop?
quote:
Originally posted by Jugflier:
quote:
Originally posted by TheMeInTeam:
Because I'm bored, here's a brain teaser for your 4th of July weekend:

A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. Can the plane take off?


Yes, the conveyor/runway would double the ground speed of the wheels. The real airspeed is generated by the prop thrust, not the runway speed.


It seems that if this were true when a plane were on the runway with it's brakes applied and the pilot throttled up his engine the plane would take off just sitting still.
quote:
Originally posted by TheMeInTeam:
Because I'm bored, here's a brain teaser for your 4th of July weekend:

A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. Can the plane take off?

Nope
quote:
Originally posted by thomaswayne0907:
not even remotely possible.


LOL Silly aeronautical engineers and their 1000+ foot runways. Why not just use conveyor belts Roll Eyes
It also must include the type of aircraft in the equation. For instance on a large airliner, there is absolutely no way the plane will leave the ground. Lift of the wings is created by airflow over them, and if the plane is not moving, there will be no airflow. All you are going to get is lot of noise. The most interesting thing about this equation would be if the conveyor belt stopped suddenly....LOL
Now take a smaller airplane such as Piper Cub, with a big engine (ie, SuperCub) and put it on this runway. The engine thrust alone, if there is not much weight in the plane will be enough for the plane to break the drag and gravity and slowly fly away. It would take a helluva skilled pilot to do it however.
Like Scro says, if this principle worked there would be no need for long runways, just build a conveyor belt and have at it.
quote:
Originally posted by leo:
quote:
Originally posted by Jugflier:
quote:
Originally posted by TheMeInTeam:
Because I'm bored, here's a brain teaser for your 4th of July weekend:

A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. Can the plane take off?


Yes, the conveyor/runway would double the ground speed of the wheels. The real airspeed is generated by the prop thrust, not the runway speed.


It seems that if this were true when a plane were on the runway with it's brakes applied and the pilot throttled up his engine the plane would take off just sitting still.


I do believe applying brakes would creat drag and prevent the prop from pulling the aircraft. Me thinks your principle is in reverse. { I could be wrong}.
The propeller would pull the aircraft forward thru the air, irregarless of how fast or slow the wheels turn.
In order to create lift, there has to be air moving above and below the wings. If a plane is on a treadmill and is not making any forward progress (aka no wind through the wings), it's not getting off the ground.

In order to take off in this situation, the plane would have to reach it's minimum speed it needs to take off, PLUS the speed of the treadmill. Of course, the speed I'm talking about is relative to the treadmill, not the actual speed of the plane relative to the ground.
quote:
Originally posted by Jugflier:
Yes, the conveyor/runway would double the ground speed of the wheels. The real airspeed is generated by the prop thrust, not the runway speed.


I think you may have read that wrong. The ground speed of the wheels would be zero, not doubled. The belt is moving in the opposite direction.
quote:
Originally posted by Scro:
In order to create lift, there has to be air moving above and below the wings. If a plane is on a treadmill and is not making any forward progress (aka no wind through the wings), it's not getting off the ground.

In order to take off in this situation, the plane would have to reach it's minimum speed it needs to take off, PLUS the speed of the treadmill. Of course, the speed I'm talking about is relative to the treadmill, not the actual speed of the plane relative to the ground.


Scro and company,
Check this out.
http://www.straightdope.com/co...n-the-plane-take-off
See, I never said I was an aero engineer. LOL Just shootin' from the hip. That explanation does make sense. So a plane will take off from a treadmill, but the treadmill would still have to be as long as a normal runway in order to reach takeoff speed.
It seems that if this were true when a plane were on the runway with it's brakes applied and the pilot throttled up his engine the plane would take off just sitting still.[/QUOTE]

By this I meant a plane sitting on a real runway not a conveyor.
ok now I get it. The plane will still move forward its just that the wheels will be "over worked". Hope those bearings and lube hold up. Big Grin
The plane takes off.

I think this is a good brain teaser because the answer that seems so obvious to many people is so completely wrong. The key to understanding why the plane takes off is realizing that there is absolutely nothing the conveyor can do to keep the plane from moving when it fires up its engine(s). The question (and the use of the word treadmill in the title) is supposed to trick you into thinking that the conveyor will hold the plane stationary, when in reality it can't. An airplane's propeller or jet engines generate thrust through the air, not by pushing against the ground like a car, and the plane's wheels are free-spinning, meaning they're basically frictionless other than the negligible friction in the bearings. The airplane in the problem will take off just exactly like on a normal runway, only with it's wheels spinning twice as fast, like Jugflier said in the first reply.

Now, in the real world with some large jets with high takeoff speeds, that double wheel speed would probably blow out the tires before liftoff, but as a thought experiment, the plane always takes off.
I totally miscalculated that the engines are not having to overcome drag. There would be no drag if the treadmill is moving, but I still beg to differ that any plane could do this. Most large aircraft fly on the wing not the the engine alone, they would have a hard time producing as much thrust to weight ratio as a fully loaded 747 or C5A.
?

Every plane relies on its wings to take off and fly. The point is that the conveyor can't and won't do anything to prevent a plane of any size from moving forward (thus moving air past its wings) and taking off just like normal.
The deception in this question is the relationship of the wheels and the conveyor. As others have pointed out for an airplane to take off is dependent on the air speed across the wings. Most people tend to relate the ground speed with take off speed, but in reality it is dependent on air speed. The thrust of the aircraft pusses against the ambient air not the ground.
You might look at it in reverse and conciser landing on a convenor, how much roll out would it need.
There is a story of a P3 aircraft landing at Keflavik, Iceland where the wind was coming directly down the runway at near the landing speed of the aircraft. The result of which allowed the aircraft to have a very short roll-out (less than 100 yd, yes that is one hundred).

Now I'll give you a new one to ponder, If a rope was warped around the earth, in a perfect circle, at the equator, and you wanted to move it one foot further away from the earth all the way around. How much more rope would you have to add?
Correct. I would have said 6.28, but then I rounded of pie more than you. For those that need reminded, 2*pie*radius=circumference
You are right,I didn't realize until later that you actually only increased the earth's diameter by 2 feet and so increased it's circumference by 2xpi.

I did it the hard way, I looked up the diameter of the earth and figured it that way and probably didn't carry the decimals out far enough which is probably the reason for my slightly off answer. The answer sounds unbelievable at first.

if you wanted to raise the rope 2 feet you would need 4xpi more rope.

Good one.

Add Reply

×
×
×
×
Link copied to your clipboard.
×