w_benjamin

Actually, since no inertia has been built up from the plane moving, the wheels might not even be able to get up to the top speed of the plane.

David Cutler

Erm, obviously not!



-David C.

David Cutler

>>Since the conveyor only reacts to wheel speed, it does not generate any wheel speed on its own<<

You're still thinking it's a car.

The wheels are free to turn, from any inputs. The belt, therefore, drives the wheels, and if the plane is moving relative to the ground that the belt is bolted to, so will that movement.

(argh!)

-David C.

w_benjamin

"This is where your logic breaks down.

Look at it this way, the plane flies due to the airspeed ... OK?

Airspeed isn't affected by the belt, whatever it does, as it's not connected.

Therefore whatever the belt does, doesn't affect the ability to fly.
"
Ever hear of Newton's Cradle?

David Cutler

>> since no inertia has been built up from the plane moving<<

Of course it is. It's being driven by the engine/prop. Where do you think all that energy from the engine is going to? It can't just be lost!

-David C.

David Cutler

Yes.

I have also heard of Newtons laws of motion. One of these laws says that something will either remain in a state of constant velocity, or remain stationary until acted on by an external force. Since the wheels are freewheeling (and therefore cannot affect the speed of the plane) and they are affected by the belt, then the motion of the belt does not affect the speed of the plane.

-DC

w_benjamin

(sigh) The belt does NOT turn the wheels. The original question states:"The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." It is REACTING to the wheels, NOT acting on them. Just because the wheels are touching the conveyor does not affect its response. If you lifted the tire off the conveyor and spun it by hand, the conveyor would move backward at the same rate, slowing down as the tire does until both come to a stop.

David Cutler

OK, but you are still being tied up by what makes the wheels turn.

If I may ask a question (disregard the wheel bearing friction please) ...

What mechanism is connecting the aircraft's speed to the wheel speed?

-David C.

CHassan

My Feedback: (13)

The wheels on the bus go round and round, round and round...............

The forward movement of the plane and the friction of the wheel to the surface of what ever it is on. So is the plane moves at 5mph and the conveyor moves at 5mph in the oposite direction you get a "wheel speed" of 10 mph.

Since the objects are moving in the oposite direction at the same speed the total speed between them must be twice that of either individual object.

example. 2 cars moving towards each other at 10 mph if they have no other referance other than the other vehicle how fast does one car look to the other?Like it's moving at 10mph.

Now the cars are going away from each other. How fast does one car appear to move away from the other? 10mph.

Now the cars are facing each other, nose to nose not moving. If one moves one direction the other moves the oposite direction, you say one moves at 10mph and the other at -10mph right? That means to one car the other does not look like it is moving. Only problem is you now have 2 cars going the same direction. 10mph --> (that way) -10mph <-- that way.

The problem is based on absolute values for speed. It can not be 1-1=0. Then something is moving backwards in the oposite direction (same direction as the other object) The formula is 1+l-1l=2. To move in oposite directions at the same speed the total speed cannot be zero. It must be the sum of the speed of the objects.

Adding the conveyor in does not mean the plane will sit still. just means the wheels will spin at the sum of the absolute values for the speed of the conveyor/and plane.

David Cutler

Precisely, and since the speed of revolution of the wheels and the speed of the belt, whatever they are, does not affect the speed of the plane, whatever is going on down there on the ground will not affect the plane's speed, and therefore its ability to gain enough airspeed to take off.

-David C.

gtmattz

WAIT A MINUTE...


I think I just had an epiphany...


If the conveyor is set to match the 'rotational speed' of the wheels.. as the airplane moves forward.. the belt will in turn mov forward to keep the wheels rotational speed at 0rpm, as that is the 'current speed of the wheel'. remember the conveyor is set to match the rotation of the wheel.. the wheel is not rotating unless the belt moves in relation to the wheel.

The plane will take off as normal, but the wheels will be moving at 0rpm and the conveyor will be moving at the speed of the aircraft.


<dances like a loon> I figgeted it ouuuutt.. I figgered it ouuuutt!!!

David Cutler

Yaaaaaay! Absolutely right!

-David C.

mr_matt

My Feedback: (10)

David, trust me if you disregard friction, the plane will take off fine, no question. Of course you would also need to disregard the polar moment of inertia of the wheels, but once you have ignored friction, the moment of inertia is nothing.

but remember, if you ignore friction some interesting things happen......

Imagine if the plane were sitting on the conveyor. With no friction, when you start the conveyor, the plane would just sit still. If you ran the conveyor up to 20,000 RPM, the plane would sit still....if you ran the conveyor up to 10,000,000 RPM, the plane would sit still.

No friction between the belt and the planes wheels, she takes off fine! ( I have been saying this now for almost a week on these various threads)

dfalcon

My Feedback: (76)

nothing more to add, just have to unsubscribe. i left town for two days and had over 200 notifications about this post in my inbox.

David Cutler

I agree with every word you wrote, and I too have been saying this since the 14th century when this question was first asked.

The plane will take off.

The parrot is dead.

-David C.

mr_matt

My Feedback: (10)

If you ignore gravity it takes off even easier.

w_benjamin

"What mechanism is connecting the aircraft's speed to the wheel speed? " Here's where we come to Newton's Cradle. It doesn't matter if the force turning the wheels is directly connected to it. The fact is it is still acting upon it. The thrust from the planes turns the wheels. Some of the thrust is used to 'push' the wheels forward. Since it it easier for them to roll then skid, they rotate. The whole time you are moving down the runway, you are trying to overcome the force of the tires that hold you back. Because the ground doesn't move that force stays constant during takeoff, while you add energy to the plane in the form of stored interia, meaning the even though the amount of thrust stays the same you pick up speed, 'till your speed reaches the point where it lift the plane. If you have a headwind, the speed of the air going across your wings is greater than your groundspeed and you take off sooner. With a tailwind, longer. With no wind you must achieve a groundspeed equal to the airspeed needed to achieve lift. All the while you are on the ground you are pushing against the ground, and it is pushing back. But what if the force pushing against the tires were increased? More force would be necessary to counter it. Where does the energy come from? The same place it came from in the first place: the thrust. If you put on the brakes just a little, could you still move the plane? Now you've got two forces acting on it: the push from the ground and the drag from the brakes. The plane still want to push the wheels forward. but now they don't roll so easy. Will it roll first, skid first, or not move at all? If the energy needed to turn the wheels is greater than the energy need to simply skid them, then they will still turn. If the egergy needed to skid them is greater, they would skid. Maybe the plane doesnt have enough energy to do either? then it doesn;t move at all. The point is the wheels have a direct response to the thrust of the plane, and some need more energy than others. (747's really don't as well as a piper cub) If you have a direct response the the energy being transferred to the wheels the plane is never going to move. And since your not building up that extra inertia in the form of stored energy, it makes it that much harder to turn them.

David Cutler

And if you ignore the existence of the plane it becomes a question for Sartre or Heidegger.

-David C.

David Cutler

and

Absolutely not, and this is where your theory fails.

Except for a tiny bit of friction and inertia stopping the wheels from turning there is NO force the aircraft is experiencing from the ground, so it doesn't matter what the ground is doing.

-David C.

gtmattz

Guys, stop obsessing on the fact that the wheels will turn... they WONT! think about my solution in relation to the original question for a moment and you will see!

w_benjamin

since the tires don't hold you back, let's try this. We'll take two identical planes. One, we'll put on a nice flat runway made of asphalt. The other, on a nice flat runway made of nice soft sand.., the kind you sink in when you step. Since pushing the wheels has no effect on the takeoff of the plane, they should take off in the same distance, right?

w_benjamin

gtmattz, before I ask you my next question, does your plane take off?

ArmedZagi

My Feedback: (1)

I had talk with my physics professor today, and I showed him this page. He has a post graduate degree in physics, and he's been a private pilot for 25+ years... He says, "Theres no way the plane could take off if the conveyor matched the wheels speed. It wouldnt be moving relative to the ground."

rmh

Well -- better change schools --fast!

gtmattz

Are we talking about a plane taking off on sand? Are we talking about a plane taking off on a normal runway? NO!

we are talking about a plane taking off from a conveyor that is:

PLEASE read my solution in regards to the above statement!?

and to answer your last question YES! ground speed does NOT affect the speed at which an aircraft takes off, so a plane with the wheels not rotating will take off!


maybe my understanding of what makes airplanes fly is all wrong... maybe, just maybe.. an airplanes wing can only work when the wheels are rotating fast enough for the wing to generate lift...

or maybe, if there is sufficient air moving over the airfiol shape of the wing, it will generate lift regardless of wether its groundspeed is 0kts or 1000kts

a plane moving THROUGH THE AIR at sufficient speed WILL fly, regardless of wether the wheels are moving at all.