will it take off?

[NewToy]

New Toy,

I understand what you are saying and I agree that if the plane moves, it takes off. In your scenario it moves. But to quote the original question:

"a plane is standing on a movable runway( something like a conveyor).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."

I interpret this to mean that the magic conveyor is propelling the plane backwards at an identical rate to its forward movement. So yes, your thrust is moving the plane forward but the "thrust" of the conveyor is exactly countering that. Again, this is just like the plane sitting there with brakes on.

Now take the winch example and apply it to a plane with its brakes on - what happens? Either the cable breaks or the winch burns out or the wheels slide on the ground. It is different from propeller thrust - the propeller can apply thrust when the wheels are braked and nothing may happen. So adding a winch or other mechanical pull to this scenario negates the whole thing.

The thrust is moving the plane forward resulting in the conveyor moving backwards at the same speed. The wheels will FREELY spin at 2x the speed of either the conveyor or the plane. The thrust has to move the plane forward in response to the rearward thrust. What becomes of the thrust energy in your scenario? The plane moves forward with the gear wheels going double time.
John

 

[RayH]

The thrust has to move the plane forward in response to the rearward thrust. What becomes of the thrust energy in your scenario? The plane moves forward with the gear wheels going double time.
John

Ok good, If the brakes are released and the wheels are free to move, how much rearward thrust is the conveyor producing? Keep in mind that the wheels are not locked, they are free to rotate as the conveyor moves backwards under them. There wont be much if any rearward thrust. Start up the engine and the prop will produce quite a bit more forward thrust than the conveyor has rearward thrust. The plane moves forward.

 

[DieselPower]

Well I thought I would ask a real pilot today. I was going to the gun store and thought I would stop by the local Fairfield Airport and asked this question. The 4 piolt's standing around me got a good laugh out of it if nothing else. They all agreed that the plane could not lift off if it had no movement through space. No movement through space, no air movement over the wings, no flight. All they did was verify what I said the other day. If the plane can not move forward in space due to the "magic" convyer there is no air movement over the wing's to create lift.

 

[dillo99]

okay - this is my last post on this topic.

I fully agree that if the plane moves it flies. It all comes down interpretation of the conveyor...if it is matching the planes speed (not the wheels...but the speed) then I would argue that it is stopping all movement of the plane. Naturally, this is impossible

So what happens to all the thrust if the plane isn't moving? Well what happens to it if the brakes are on? The thrust can't overcome the friction of the brake/tires.

So...if the thrust moves the plane and the magic conveyor is NOT able to exactly counter forward speed of the airplane then it will fly.

If this fictional and magic conveyor can exactly counter the speed of the airplane then the airplane does not move nor fly.

Based on all the posts here - I think everyone would agree with those last two statements - it all comes down to what this fictional conveyor is capable of!

 

[DieselPower]

Well one of the people (pilot's) I spoke with earlier today just gave me a call on the phone. Being the salesman that I am I had left a couple of business cards with them so they had my number. In any case they apparently changed their minds after I left. Here's why and it does make sense the plane WILL take off so I stand corrected.

He told me to check out this guy's webpage for the details. The Straight Dope: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

Here's the important part.

"First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly."

So, in laymans terms it means that no matter how fast the convyer moves backwards the plane WILL move forwards due to it being drawn forward through space/air. No matter how fast the convyer goes and how fast the wheels spin the plane will gain forward movement through the air at it's normal speed and will take off.

I think my head hurts, fizzle, fizzle, pop!

 

[shaley]

Just one or two more questions so I can get this straight.
Have you ever tried to push your tractor by hand on level pavement? I know it took 6 guys just to budge mine. I'm guessing 300 Lbs of force. Arent these rubber tires just like the ones on aircraft.
How much force do you think it would it take to push a Cessna 150, DC3, or 727 on pavement?

 

[BTDT]

Just one or two more questions so I can get this straight.
Have you ever tried to push your tractor by hand on level pavement? I know it took 6 guys just to budge mine. I'm guessing 300 Lbs of force. Arent these rubber tires just like the ones on aircraft.
How much force do you think it would it take to push a Cessna 150, DC3, or 727 on pavement?

I'm sure others will jump in here also, but this is nowhere close to the same thing. The airplane wheels are just tires and wheels on a spindle. The tractor tire/wheel is connected to axles, gears, heavy oil, etc. A closer analogy would be pushing a grocery cart and a riding mower. One just rolls, the other has drag due to mechanical makeup.

 

[MossRoad]

Mike,

Look at it this way...

The props pull the plane forward slower than the wheels are spinning on the conveyor. All the wheels and conveyor do is keep up with each other.

Another way to look at...

Model airplane and small conveyor. You start the conveyor and hold the model airplane in place with your finger. You can push it forward with your finger (airspeed). The only thing changing is the speed at which the tires and conveyor are spinning. Replace your finger with a prop or jet engine and they'll do the same as your finger did. The plane will move forward which will create airspeed.

Nope. As soon as you push the model plane forward with your finger, its wheels are turning faster than the surface speed of the small conveyor. You, in effect, have cheated.

 

[MossRoad]

For those having trouble with this, you should try tackling the flight pricipals of a helicopter.

Attached is a picture I took at the Henry Ford museum of a quote from Sikorsky regarding the first helicopter.... enjoy!

 

[MossRoad]

...

So...if the thrust moves the plane and the magic conveyor is NOT able to exactly counter forward speed of the airplane then it will fly.

If this fictional and magic conveyor can exactly counter the speed of the airplane then the airplane does not move nor fly.

Based on all the posts here - I think everyone would agree with those last two statements - it all comes down to what this fictional conveyor is capable of!

I agree completely.