Plane on a Conveyor Belt - Page 2

I did vote yes, but I really had to think hard about... not an easy question and one that I'll have fun asking to others later.

A flight surgeon got this right when the engineers got it wrong?  Scary thought.

Maybe I'm slow but I still picture the plane "running in place" in this scenario so I don't understand where the "lift" is coming from? Can someone slow this down so I can get it? Under my understanding the answer is definitely no, but I think I'm misunderstanding the scenario??

The plane is not "running in place." It is moving forward... the wheels on a plane do not propel the plane forward as they do on a car, the jets do that.

 

If you couldn't see the conveyor belt, everything would appear to be normal. It's not like the plane appears to be standing still and then all of a sudden is flying. The only thing the conveyor belt actually does is make the wheels spin faster (twice as fast to be exact).

If an object is moving forward 100 mph on a surface that is moving in the opposite direction at a speed of 100 mph wouldn't the object in question be stationary? And if said object was a plane, wouldn't it be incapable of liftoff? Not trying to argue just trying to understand why I don't get it....

 

No.... because the wheels are essentially just there to hold the plane up when it's on the ground... they just spin but they do nothing to move the plane forward, the thrust from the jets or propeller do that. The only thing that the conveyor belt does is makes the wheels spin faster, so the plane is going 100 mph forward, the conveyor belt 100 mph backwards, meaning the wheels are spinning at 200 mph.

The airport I frequent (ironically) has a moving sidewalk, it is possible to walk against the "flow" in a way that makes you stationary, if you were to do this and then leap into the air you would not be "leaping" forward, you would just be hoping up and down no? What difference does it make whether the "thrust" is created by the friction drive of the wheels or the propulsion drive of the jets or propellers? Again, not trying to be an ass and this is obviously and old riddle but I can't quite grasp the logic behind it and I keep thinking that there is a flaw somewhere....

Ahhhh, did a little research on this question via the interweb and the pretense is that, if the conveyor belt were moving "backwards" at 100 mph (or any other arbitrary speed) the plane would be stationary as the wheels would just spin, I am not sure I buy that. I think that if you were to put a model plane on the surface of a record player (for example) the plane would spin around with the record and not sit stationary with the wheel spinning. If anyone has a record player and a model plane handy I would like very much to see a video of the result. 

 

I suppose if you were to introduce the thrust of the jets or propellers the plane would be able to remain stationary BUT in order to take off the thrust would necessarily be greater than ( and not equal to) the opposing force of the moving runway in order to generate the aerodynamics required for liftoff...

Of course, I am not a scientist by any stretch of the imagination so maybe I just don't get it.

Imagine you are wearing roller skates on that moving sidewalk, going against the direction of the belt. Now imagine there is a long rope tied on to a pillar beyond the length of the sidewalk. Can you pull yourself through the conveyor belt with the rope? That's how thrust independent of the ground works.

Also if you are stationary on a conveyor and jump straight up, you would land behind your original position. That might be the piece you are missing. While the plane may appear stationary (relative to our perspective) it is actually moving forward (per its perspective). Remember that engines on planes will move air regardless of whether they are on wheels or affixed to a specific position.

This is gonna take some serious "Bong Time" lol.

Pulling on the Long Rope attached to the pillar would mean I'm travelling faster than counter-motion of the belt no? I don't disagree that if the planes forward thrust is greater that the belt's "backwards" thrust then the plane can take off, but if they are equal...????

 

Would the planes ability to take off not require some degree of differential??????

I guess I didn't explain it as well as I thought about it in my head...

 

The plane takes off via air pressure differential on its wings. The engines propel air, causing air around the plane to move faster than the surrounding atmosphere. As long as the air movement is fast enough to create lift, the plane will take off. Therefore, as long as the plane's engines are creating enough thrust upon the plane, it will fly.

 

Maybe try thinking about a mattress strapped on top of someone's car moving down the highway. The mattress is stationary in relation to the car (much like the plane is to the conveyor) but the air movement caused by the propulsion of the car lifts up the mattress. The relation of the mattress to the earth is irrelevant.

That's because the model airplane has very little mass and therefore little inertia, which means it is easily moved by the record player. A plane, having more mass, is more likely to remain stationary, causing the wheels to spin.

 

My wife tells me that my mattress example isn't very good, either, so I apologize if you're still confused. I guess the important thing here is that a plane's movement is through the air and independent of the ground.

I have to disagree with the consensus of this thread in three areas...

 

If the conveyor belt is going 170 mph in a backwards direction, and the plane engines are providing enough thrust for the the plane to go forward at 170 mph, then the plane would actually be sitting still and would have no forward movement with regards to other stationary objects.  There would also be no air movement over the wings, unless the wind was blowing.

 

[quote] The only thing that the conveyor belt does is makes the wheels spin faster, so the plane is going 100 mph forward, the conveyor belt 100 mph backwards, meaning the wheels are spinning at 200 mph.[/quote]

In the example I gave above, the wheels of the plane would be moving at 170 mph, not 340 mph.  There is no force that would double the speed of the wheels.  The conveyor belt is going backwards at 170 mph, and the plane wheels are going forward at the same speed.  170 - 170 = 0, which is the speed of the aircraft.

 

 

[quote] I'm a pilot, and the answer is yes.  Forward movement of an aircraft is caused by the thrust of its propeller or jet, not by it's wheels like that of a car. Therefore, the movement of the conveyor in no way affects the aircraft's ability to move forward relative to its position through the air which is what generates lift and allows for flight.[/quote]

No offense, but since the plane is sitting still relative to the air surrounding the wings, there would be no lift, and this plane has no chance of taking off.  The rotation speed required for takeoff must be in relation to the air moving across the wings.  I think this is called "indicated airspeed".  So with a 20 mph headwind, you'd only need 150 mph of actual land speed.  But with no aircraft speed, you might need ~ 170 mph headwind to take off.

 

This would be a completely different story if we were talking about a plane specifically designed for vertical takeoff...

Edited by Chipless - 11/22/12 at 11:04am

OK.  Looks like I misread the initial post, and now have my tail tucked between my legs.