will it take off?

[755inNY]

O.K. Last try (of course, I'm lying )....

Read these articles at the straight dope:


The Straight Dope: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?
The Straight Dope: "A plane is standing on a runway. . ." No, it's not. Here's why.

In the question originally imposed, it seems like the plane will fly. I thought it would, too. But no one takes into account the force that the conveyor will exert on the plane. Read the first article... it supports the IT WILL FLY train of thought(even mentions roller blades, ropes, etc...). Then read the second article. It shows where most folks forget about the force exerted by the treadmill. If the treadmill was built to continue to accellerate to an infinite speed, the plane stands still.

As the referenced article states often happens, you have changed the conditions to establish a result where the plane won't fly. The conveyor matches the SPEED of the plane. The conveyor does not accelerate at a rate to match the force the propeller is applying to the plane. Why not just change the conditions to "The plane is tied to the ground with a rope" if you want the answer to be "the plane won't fly?

 

[RayH]

Its fun. I love a debate, it will be sad when its over.

The inner workings of the human brain always amazes me. How in some cases, its limited by our experiences and in others, it has no limit.

 

[kwolfe]

HaHahahaha
That post is maddening.
The plane is not stationary to the ground, its not tied down, it rolls. The wheels turn. The plane can go forward or backward in relation to the ground or the conveyor.

Imagine if you will. You are standing on a treadmill with rollerskates on. Neither you nor the treadmill are moving because its matching your speed, which is 0.
A buddy shoves you from behind or pulls you from the front (acting just like a plane prop). Will you move forward on the treadmill? Of course you will. Even if the treadmill instantly matches your speed, you will still move forward because you are being pushed/pulled.
If you cant see that, I give up.

Ray,
You are right. I was not thinking of the push factor. The air from the props is pushing against other air in order to create thrust. The wheels are simply there to reduce friction. I hate being stupid. SHould have thought about it more before I started typing.

 

[shvl73]

Ray,
You are right. I was not thinking of the push factor. The air from the props is pushing against other air in order to create thrust. The wheels are simply there to reduce friction. I hate being stupid. SHould have thought about it more before I started typing.

The prop is actually a modified airfoil which will create low pressure in front of each blade and high pressure behind each blade similar to a wing. The push factor more accurately relates to a jet engine with thrust.

 

[toolaholic]

I was a helo mech. in the usmc. simple answer NO. an airfoil[wing] has more surface on top than the bottem. air travels faster over the top ,reducing the pressure, causing lift. the plane would not be moving,no lift.
How many 3 cent stamps in a dozen? he he 12 dummey

 

[RayH]

I was a helo mech. in the usmc. simple answer NO. an airfoil[wing] has more surface on top than the bottem. air travels faster over the top ,reducing the pressure, causing lift. the plane would not be moving,no lift.
How many 3 cent stamps in a dozen? he he 12 dummey

The prop is also an airfoil. When its spinning, it is creating lift, not vertical lift but horizontal lift. If you have horizontal lift that overcomes the small bit of drag of the wheels spinning, the plane will move forward, thus creating vertical lift and off into the wild blue.

 

[NewToy]

Ray,
You are right. I was not thinking of the push factor. The air from the props is pushing against other air in order to create thrust. The wheels are simply there to reduce friction. I hate being stupid. SHould have thought about it more before I started typing.

How many more no flyers to go? Once you seperate those wheels from the prop driving the plane it seems so easy. You will sleep much better tonight.
John

 

[MossRoad]

As the referenced article states often happens, you have changed the conditions to establish a result where the plane won't fly. The conveyor matches the SPEED of the plane. The conveyor does not accelerate at a rate to match the force the propeller is applying to the plane. Why not just change the conditions to "The plane is tied to the ground with a rope" if you want the answer to be "the plane won't fly?

Re-read the second post. The Straight Dope: "A plane is standing on a runway. . ." No, it's not. Here's why.

As soon as the plane starts to accellerate from a stop, the conveyor would attempt to match its speed. This would apply more force against the tires in the form of friction. Just like his roller blader example, the plane would feel the tug of the conveyor on its wheels. The plane would try harder to overcome that force and the conveyor would apply more force to keep it there. The thing would zip up to the speed of light in the blink of an eye. The forces on the tires and conveyor would be incredibly high. Something will fail before the plane can move foreward and we all know it won't be the magic conveyor because it was built to match the speed of the plane, no matter what. Hey, I didn't build the magic conveyor. The original problem stated it already existed.

The original question states the conveyor will match the plane's speed and makes no mention of force. But the force is there and has to be dealt with. Even if the wheel bearings were magical mag lev frictionless bearings it wouldn't matter because that is not where the friction is. It is between the surface of the tire and the surface of the conveyor. The tires are attached to the wheels. The wheels are attached to the bearings and the bearings are attached to pins that are attached to the struts that are attached to the plane. The plane feels the force of the conveyor pulling it backwards. That occurs where the tires meet the conveyor. The plane will go nowhere as the tires will be vaporized by the magic treadmill about a millisecond after we turn the thing on. Then the plane will fall on its wheels, those will vaporize, so will the struts and finally the prop then plane itself. I hope it was unmanned.

 

[RayH]

Not much force there. The plane would not feel much tug on the wheels at all. Not enough to matter.
How much force or resistance do the wheel bearings and tires have when you land a plane. Not enough to stop it. How much do they have when you take off, not enough to hold it back. How much would they have on a conveyor, not enough to hold it back.
The conveyor would not reach anything near "light speed" unless the plane was able to reach lightspeed because the conveyor will only go as fast as the plane can go and the wheels will only go twice as fast as either. Most small planes takeoff with around 70-80 knots of airspeed.
So, the plane sits still on the conveyor. The pilot starts the engine and releases the brakes, the plane starts to roll because the prop is pulling it forward through the air. As it starts to roll forward, the conveyor starts backward. The plane rolls forward at 5 knots of airspeed, the conveyor rolls back at 5, the wheelspeed is 10. This goes on up until the plane reaches 80 knots of airspeed, the conveyor is going 80 knots backwards, the wheels are going 160 knots. These figures assume no headwind.
How hard is this anyway.
Not to complicate it more but if the question was formed to mean that the conveyor matched the wheelspeed of the plane and not the airspeed or groundspeed of the plane, then the conveyor would never move. So I guess it is true that if the conveyor matched the "wheelspeed" of the plane, the plane could not ever move because as soon as it started to roll, some unknown, magical, question generated force would stop it. As a matter of fact, the whole question would be null because if the plane isnt able to move, the conveyor would not move either and it would be nothing more than a plane sitting still on the runway, not moving. This is where the second explanation in the link starts to break down. Why would the conveyor start "tugging" in the first place. Its only job is to match the speed of the plane. If the planes wheels arent rolling, the conveyor isnt moving. If the planes wheels where to ever start rolling, guess what, the whole plane is moving, now we are back to the plane moving and it will take off.
Heres the question as it is stated "as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction." It clearly states that the conveyor matches the PLANES speed.

 

[MossRoad]

How come when you put a plane at the end of the runway with the engine at idle and you take the brakes off, it still doesn't go anywhere? The propeller is turning. It is forcing air behind it. The brakes aren't on. But it won't move. Something is holding it in place even though air is moving past the propeller towards the rear of the plane. You have to power up the engine to get it to force enough air through the prop to overcome the resistance to the forces that are holding it in place. If there was no resistance to the forces holding it in place, as soon as you turned the propeller, the plane would vanish into the distance with a blinding flash, probably similar to the Enterprise jumping to warp speed. Wooosh BANG! We'll find it circling that planet next to Neptune searching for Klingons.