Physics question.. can you figure it out?

[Epitome]

The easiest example I can come up with for people that think somehow the conveyor belt has anything to with holding the plane from moving is this. The plane is facing to the left and the conveyor is moving to the right. The thrust from the engines of the plane is pushing with, let's say 10,000lbs of push to the left. Conveyor belt CANNOT push back against this force with anything other than friction as it spins the free wheeling tires. The question clearly says the conveyor belt matches the planes speed, not spins fast enough to hold the plane in place (which also wouldn't really be possible). So, although the tires are spinning at double their normal speed....the friction wouldn't be enough to keep the plane from moving.

 

[EightSecV6]

some testing

http://users.pandora.be/bontinck/zijwindtest.wmv

 

[FlyinGN]

interesting. being an Engineer I found this intersting.. At first without really thinking about it I thought it would not fly. Then after thinking about how the wheels and thrust works, I recind and say it will fly..

 

[microELVIS]

So mathematically

Tell me if this is correct:

Wheel speed = plane speed + conveyor belt speed.

 

[GNandGS]

Ok, messing around last night with the treadmill I proved to myself the plan will fly (guessed so). While I didnt use a RC plane, I did use my RC truck (power off).

It was VERY easy to move the vehicle forward against the belt direction. Other than the initial "hit" of belt movement you would be hard pressed to notice. In fact, it "felt" easier to move forward because the wheels were already turning.

The "thrust" here was ME. Im kinda curious if I can link to my cameraphone vids so try the below link... sorry for the poor lighting, I didnt realize it was bad until I was done. If there is a demand for it I can do a better vid with reg camera. Only one of the clips shows what I wanted but Im too lazy to delete the others.

http://pictures.sprintpcs.com/share.do?invite=vEzr4nmVP7k6CYjYzhRL&shareName=MMS2

You might notice that when the belt is stopped the car doesnt suddenly surge forward.

 

[TType84]

So mathematically

Tell me if this is correct:

Wheel speed = plane speed + conveyor belt speed.

yes. if we're talking about a cessna that needs about 70mph to take off.. it will be moving, east, we'll say, at 70mph. the conveyor surface will be moving west at 70mph

the wheelspeed will be 140mph.

so, provided the craft in question can withstand double the wheelspeed, it will take off with no problem.

 

[1ARUNM]

This seems simple to me.

Air passing across the wing is what makes the plane fly. It does't matter how must thrust the engine makes or how fast the wheels turn. If there is no air passing across the wing, it won't fly. As long as your original conditions are kept, air is not moving across the wings.

 

[72firebird]

This seems simple to me.

Air passing across the wing is what makes the plane fly. It does't matter how must thrust the engine makes or how fast the wheels turn. If there is no air passing across the wing, it won't fly. As long as your original conditions are kept, air is not moving across the wings.

There is air passing over the wing since the plane is moving forward.

I think the easiest way to think of it is as to completely different systems. The conveyer is just turning the wheels because they're virtually frictionless (bearings). The other system is the turbine engines producing the thrust which actually pushes the air. Since the wheels are technically not connected to the plane (because of the bearing causing them to be frictionless) they don't affect the plane. So since the thrust is pushing the plane it will move foward and be able to lift off assuming the bearings in the wheels are good enough that the wheels wont cause too much drag and prevent it from reaching the speed to take off.

 

[TurboJunky]

A car moves by friction. No friction, no movement. That's why my car is having trouble moving down the driveway this morning. There is ice all over the driveway; the car doesn't have the required friction to move.

A jet or prop driven craft is different. It does not move by friction, it moves by air pressure and prop lift. That means it can move on my driveway, ice and all.

Because of this one simple fact, a plane and a car will react differently on the conveyor as well. If the car is set to move 40mph north and the conveyor is moving 40mph south, the car will remain motionless relative to the ground, and 40mph relative to the conveyor.

If a winged aircraft (jet or prop, it doesn't matter) were on the same conveyor, it would not behave the same way. If the aircraft is moving at an airspeed of 40mph north and the conveyor is moving 40mph south, the airplane will still go 40mph north, and the wheels will have a surface speed of 80mph.

The difference is in what they are pushing against. The car is pushing against the conveyor; the airplane is pushing against the air. The two will move relative to what they push, not what they are resting on.

To fully understand this, you have to take Newton's Third Law, and fully understand it. For every action, there is an equal and opposite reaction.

The car is acting against the conveyor, so it must move in relation to the conveyor. Movement of the air is irrelevant.

The aircraft is acting against the air, so it must move in relation to the air. Movement of the conveyor is irrelevant.

__________________________________________

Consider this:

A river flowing south at 20 mph.

A regular boat will need to maintain a speed of 20 mph northbound via it's prop in the water to remain stationary.

An airboat can easily remain stationary and MOVE UPSTREAM with very little power since it's deriving it's power against the AIR and not the water. The air is not moving 20 mph to the south like the water. Obviously the airboat has more drag against the water than an airplane on a conveyor with rolling wheels but the concept is exactly the same.

 

[72firebird]

A car moves by friction. No friction, no movement. That's why my car is having trouble moving down the driveway this morning. There is ice all over the driveway; the car doesn't have the required friction to move.

A jet or prop driven craft is different. It does not move by friction, it moves by air pressure and prop lift. That means it can move on my driveway, ice and all.

Because of this one simple fact, a plane and a car will react differently on the conveyor as well. If the car is set to move 40mph north and the conveyor is moving 40mph south, the car will remain motionless relative to the ground, and 40mph relative to the conveyor.

If a winged aircraft (jet or prop, it doesn't matter) were on the same conveyor, it would not behave the same way. If the aircraft is moving at an airspeed of 40mph north and the conveyor is moving 40mph south, the airplane will still go 40mph north, and the wheels will have a surface speed of 80mph.

The difference is in what they are pushing against. The car is pushing against the conveyor; the airplane is pushing against the air. The two will move relative to what they push, not what they are resting on.

To fully understand this, you have to take Newton's Third Law, and fully understand it. For every action, there is an equal and opposite reaction.

The car is acting against the conveyor, so it must move in relation to the conveyor. Movement of the air is irrelevant.

The aircraft is acting against the air, so it must move in relation to the air. Movement of the conveyor is irrelevant.

that's a really good explanation and puts this argument to rest if you ask me.

 

[1ARUNM]

As I read the original post it implies that the plane is moving on the conveyor in the opposite direction at the same speed as the movement of the conveyor. That means that the plane is not moving in relation to the ground or to the air, only in relation to the conveyor belt. Then no air is moving across the wing, and the plane will not fly.

 

[jpratt]

As I read the original post it implies that the plane is moving on the conveyor in the opposite direction at the same speed as the movement of the conveyor. That means that the plane is not moving in relation to the ground or to the air, only in relation to the conveyor belt. Then no air is moving across the wing, and the plane will not fly.

How would the plane engines not move it threw the air? What makes a plane move? The engines push air. THe engine push the plane, the conveyor moves the opposite, the plane flies.

 

[TylerDurden]

I will say this once, THE PLANE WILL FLY.
I am a pilot (appear to be the first one in this thread). In physics you have to get your terms correct, same applies here. Groundspeed (GS) has nothing to do with lift. True Airspeed (TAS) does. And since we are on the ground it will be equal to Indicated Airspeed (IAS). IAS is measured via air molecules flowing into a PITOT tube and is displayed in the cockpit. When I pull up to lift off, it is because I see the required IAS for flight.

Now for those who have discussed why it will fly I will not repeat it, but I do want those who think it wont fly to ponder this scenerio......

I am about to land on this same moving runway......when I touch down what will happen? Am I going to come to a complete stop right away? NOOO!!!!! the plane will continue to roll down the runway but my wheels will be spinning twice as fast as normal. Same aerodynamic principles apply for takeoff.....my engines will push the aircraft thru the AIR despite the movement of the runway. And if you think landing is not the same as taking off, then I give you this.....I am doing a touch and go.....when I touch down I will roll down the runway normally, and when I power up, I will speed up again and take off........

 

[jpratt]

I like the landing bit. Never thought of it like that, hope it clears up a few minds out there.

 

[justabuickv6]

Let me get this straight? The airplane and conveyor belt is moving at the exact same speed but in opposite directions correct? IE belt is going south at 40mph and the airplane is going north on the belt at 40mph. The speeds are matched and they airplane is stationary. Is this correct? If this is correct, the airplane has no motion in relation to the ground. No air over the wings, no lift.

Think of your tires on your car and photographs that you have seen of spinning tires on a car. The bottom, where the tire touches the ground, is hardly moving at all, (in relation to the ground). The part of the tire at the top is moving very fast in relation to the ground because it is not touching it. This is why photographs are usually blurred at thet top and at the bottom are in focus. Take a coke can or anything round and roll in on your desk. The part of the can not touching the desk it covering much more area than the part touching the ground (desk).

 

[TylerDurden]

Let me get this straight? The airplane and conveyor belt is moving at the exact same speed but in opposite directions correct? IE belt is going south at 40mph and the airplane is going north on the belt at 40mph. The speeds are matched and they airplane is stationary. Is this correct? If this is correct, the airplane has no motion in relation to the ground. No air over the wings, no lift.

no this is NOT correct, the wheels of the plane are spinning at twice what they normally would, the runway could move at whatever speed it wants, it is NOT going to keep the plane from pushing its way thru the air...the only thing affected here is wheel speed (which I will not call groundspeed)


one more way to think of it, I tie a rope to the front of the plane and hold on to it (this may have already been mentioned) lets say I start the conveyor but hold the plane still.....the force required to hold the plane back is the amount of friction by the wheels/bearings (not very much), now power up the engines and go full thrust, is the plane still going to remain still, or is it going to release the tension on the rope I am holding and come at me?

 

[justabuickv6]

If the plane is stationary in relation to the ground, it will not fly. There is no air to create lift.

If the plane is stationary, do this. Someone get on a treadmill and run as fast as you can on it. In relation to the treadmill you are going 10-15 mph for example. Are you going anywhere? Can I stand next to you and talk to you? Do you have wind in your hair? You have no speed in relation to the ground. No speed (knots) no lift.

 

[TylerDurden]

If the plane is stationary in relation to the ground, it will not fly. There is no air to create lift.

If the plane is stationary, do this. Someone get on a treadmill and run as fast as you can on it. In relation to the treadmill you are going 10-15 mph for example. Are you going anywhere? Can I stand next to you and talk to you? Do you have wind in your hair? You have no speed in relation to the ground. No speed (knots) no lift.

you are correct, if the plane remained stationary it wouldnt fly, the point here is that the plane WILL MOVE FOWARD, thus it will fly........the runway/wheel friction is not enough to hold the plane back...re-read my last reply, i added to it

 

[BLACK6PACK]

justabuickv6...

That isn't what the original question states... "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)." It just says the coveyer moves at the same speed in the opposite direction of the plane. Just by reading the question it says that it tracks the planes speed. So, all of those that say the plane wont fly... If the plane has no forward motion, then the conveyer wouldn't be moving because the speed of the conveyer is equal to the speed of the plane.

 

[justabuickv6]

If the plane is NOT stationary, then I take back everything I said. I thought that the plane and conveyor belt was traveling in opposite directions and the exact same speed. Meaning the the plane was stationary.