Airplane physics question

Wasn't it Galileo who looked into this?

 

Is the vacuum a Hoover or a Eureka?

 

Eliminating the element of aerodynamics in regard to mass, both would hit the ground at the same time.

I imagine next somebody will be asking which weighs more a pound of feathers or a pound of lead?

 

Monica Lewinski

 

Since apparently some of you aren't attracted to normal font sizes, i'll repeat what I have said a number times, earlier. The original question was not repeated to us properly. Somewhere along the line, someone changed it around. The original question is this:

"On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

Forget the wheels, forget the infinte speed conveyer belt. The god damned plane takes off according to the original question. And, if you reason the current question similarily, well, it takes off here too.

 

Sorry. It does not take off. The weels move, but the plane doesn't. Since it doesn't move, there is no airflow over the wings. So it does not experience any lift. Hence, no take off.

 

Wrong. Explain how the wheels could move if the plane is not.

 

The wheels move because they rotate about a fixed point, the axle. As they rotate, they cover ground...but the ground surface is moving in the other direction because it is a treadmill. Net result is that the wheels rotate, but they don't actually go forward in relation to any fixed object not moving with the treadmill. So the plane does not move forward in relation to any fixed object, but in relation to the treadmill, it is going forward faster and faster. Even though the wheels and the engine have no mechanical coupling, the thrust from the engine ends up driving the wheels. But because they are on a treadmill moving in the opposite direction, they spin in place.

Think of a water wheel. It stays in place and spins as the water goes past. If the water speeds up, the wheel spins faster. But it doesn't suddenly break from its mount and start rolling upstream. Here, instead of the water, we have a treadmill. And the wheel is fixed to the airplane and is being driven by the airplane's engine (indirectly). Simple as that.

 

Sorry, you're wrong. It's understandable. That's exactly what the question was intended to make people think about, and exactly where they are expected to make mistakes. I gave an example previously, but apparently you either didn't read it or didn't understand it, so I'll give a similar one.

You have the conveyer system in this question, and an airplane. The airplane has broken down. So you call a tow truck. The tow truck can't get onto the conveyer belt, so he pulls up beside it, but well ahead of the airplane (i.e he is an external source of thrust). The tow truck driver attaches a cable from the truck to nose of the aircraft. He then begins to pull on the airplane. Will the airplane move forward, desipite the fact that the conveyer will produce an equal and opposite conveyer speed? Of course it will.

Since the conveyer instantaneously matches any wheel speed, we can call this a steady state problem. So lets say the tow truck is moving at 5mph, which is pulling the aircraft at 5mph. Now, the conveyer will be moving backwards at 5mph, but why would that stop the plane from moving forward at 5mph? it wouldn't. It would simply cause the wheels to rotate twice as fast. The airplane freewheels, there is no way for the conveyer to produce a force through the wheels to counter the aircrafts acceleration. Yes, there is bearing friction, but it is negligible.

The force produced by the tow truck is analogous to that of the aircrafts engines.

If you have a hard time believing this, you can do a simple experiment. Get a freind, a pair of rollerskates/blades whatever, a treadmill, and some rope. Have someone stand on a treadmill, set it to a constant speed, and hold them in place with the rope. Then walk backwards at the same speed as the treadmill is moving. You will see, that they will move up/along the tread mill. It's analogous.

 

Do we agree that the plane needs to move forward in order to get airflow over the wings and take off?

If so, then the plane needs to move forward in relation to a fixed object on the side of the treadmill, right? For this to happen, the forward speed of the wheels has to be greater than the backward speed of the treadmill surface, right?

Anyone who claims that the plane does move forward, please explain how it can do this if the wheel speed is always matched by the treadmill surface going in the opposite direction.

1. The wheels can't go at the same speed as the treadmill and faster than the treadmill at the same time. They cannot have two speeds in relation to the same object.

2. The plane cannot move forward if the wheel speed is the same as the treadmill surface speed, right? Why would it not stay in place with the wheels spinning? If you fire the jets, or increase the prop speed, it will try to accelerate teh plane. That translates to an acceleration of the wheels. But the treadmill matches the acceleration in the opposite direction. The plane still does not move. It jsut sits there, generating more noise and heat, with the wheels spinning faster.

So how does the plane move forward to get airflow over its wings? Answer, it doesn't.

 

Pick up a Hot Wheels, and spin the wheel with your finger.

Sounds like a Science Fair project!

 

I'll address this before I read arts response. If there is a condition that my finger can only move if the hotwheel car moves forward, then how does your example prove anything? If the hotwheel is stationary, then so to must be my finger.

 

Here I go again.
Suspend a toy (or real plane, your choice) from string. Turn on its jets (or propeller, your choice). What happens? It will thrust forward and, but for the string, fly. What does the treadmill under it add or subtract? I submit, nothing. If you could hold a treadmill under this imaginary plane’s wheels, it will still go. A treadmill takes the place of string. Just as the string does, so too the treadmill just provides freedom from drag/resistance. That’s all.

 

Hah, got you! The plane will move forward if the conveyor does not speed up. If the conveyor speeds up to match the acceleration of the plane, then the landing gear or the tow-bar will break. Or, if it does not have enough grip, the tow-truck will spin its wheels and overheat!

You are right about the force, but that force is being expended in the plane's wheels spinning faster. So, the tow truck exerts more towing force. The plane's wheels again accelerate to move the plane forward, but again the treadmill speeds up in the opposite direction to counter the acceleration of the plane's wheels. The plane still stands still. Tow truck applies more power, etc., etc. until something gives. Probably the tow bar will go first. At that point, in the real world, the plane would have less force on it, and would shoot backwards. In our theoretical problem, the belt would instantly slow to match the deceleration of the plane and it would all sit there, still not moving, as the wheels and belt matched each other's deceleration back down to zero.

If the belt matches the wheel speed at all time, the plane ain't moving. Force equals mass times acceleration. The mass that "gives" first is the mass that is accelerated first. The wheel, being on greased bearings, is the mass that gives first. It is accelerated first from the force, whether it is a tow truck or a propellor or a jet engine, or a bunch of people on the ground pushing with long poles. But the wheel is fixed about a pivot point, the axle. So it "gives" by accelerating at an angle about the pivot point. This is angular acceleration, which results in circular motion.

Any additional forced exerted on the plane will translate into additional angular acceleration of the wheels, as they will continue to "give" first until they overheat and seize. The wheels' acceleration will always be countered with an equal acceleration of the belt in the opposite direction. So, the wheels will accelerate and decelerate, the belt will decelerate and accelerate, but the horizontal displacement of the plane will be zero. That's all there is to it.

 

You're confusing different reference frames. Lets say the airplane has a groundspeed of 50mph. That is, the speed which you and I, sitting in a ferrari next to the conveyer belt, expereince as it whizzes by. Now, the wheels have a translational groundspeed of 50 mph. So, if you picked the very centre of the wheel and watched it go by, it would be moving by at 50 mph, same as the airplane, which makes sense, it's attached. At this time, the conveyer would have a groundspeed of 50mph in the opposite direction. But the wheel speed that matters in this case is not the translational, it's the rotational wheel speed. Which, for the aircrafts 50mph groundspeed, and conveyers -50mph groundspeed, would be 100mph. So, it would be moving forward at 50 mph, but rotating as if it was moving forward at 100mph.

 

Art, this is a terribly flawed understanding of physics.

 

That is a great experiment. Hook the plane up to a variable dc power supply. Tape it to a matchbox car and put it on a treadmill. Get it to stay still while the treadmill is moving by giving it just enough current. Now have a friend slowly increase the speed of the treadmill while you increase the current to the motor. I bet you can get it to stay in one spot. It will not spit out the front or back of the treadmill. Try it and tehn tell me you don't believe me.

 

First of all, the plane is not suspended, it is on the ground (treadmill).

When a plane is on the tarmac, the engines fire, and the plane moves forward, ultimately building sufficient speed to take off (shorter distance in a headwind). BUT this plane is on a treadmill. So do this, go walk on a treadmill, then run, and let me know when you take off.

 

Sorry, no. The speed at the perimeter of the wheel is the translational speed times twice the radius of the wheel times pi (2*pi*R). The bigger the wheel, the faster it will be going. It will not be twice the translational speed. You are correct that if two objects are going in the opposite direction, then their speed relative to each other is twice the individual objects' speed.

 

I would have thought, by page 8 million and 53, people would see that a person walking on a treadmill is not the same system as an airplane on a treadmill. An anaolgy to walking on a treadmill, a car on a treadmill. Why? because the source of foward motion, is at the contact point of the two bodies. Not so with an aircraft. Refer to my treadmill, rollerblade, rope example a few posts above, infact, i'll repeat it, and, by all means, try it out.

Get a freind, a pair of rollerskates/blades whatever, a treadmill, and some rope. Have someone stand on a treadmill, set it to a constant speed, and hold them in place with the rope. Then walk backwards at the same speed as the treadmill is moving. You will see, that they will move up/along the tread mill. It's analogous.

 

wow just looking at this thread gives me nightmares about physics class....i get chills just thinking about it.

 

You have to do more than just tell me I am wrong. You have to prove why. Either show me where I am wrong, or where you are more right.

 

Right. That is my point. I have yet to have had a response to my hypo that shoots it down (so to speak).

 

Must resist...... can't stop...... kill me now!

See video below, which was posted earlier, it is a demonstration of the problem. Of course it flies!

http://videos.streetfire.net/Player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B

Rick

 

It is the same thing. How the wheels are driven does not matter. All that matters is that a force is exerted on the object. Force results in acceleration. That's it. The effects are just more visible wth a mechanically coupled drive system, as there will be lots of heat, noise, vibration, etc., when the engine accelerates the tires and the treadmill doesn't quite match the speed.

I get the part about the rollerblades. I get the part about frames of reference. I get how, if you walk backwards, a still person appears to be moving forwards. It's like houses whizzing by on the freeway. Actually, it's you whizzing by in the opposite direction.

How does all this prove that the person will move forward in relation to the wind? That's what needs to happen for the plane to take off. The plane is not moving in relation to the ground. It is not moving. Only its wheels are moving, on a surface that is moving at the same speed in the opposite direction.