Will it take off?

We did this experiment at school in science lessons. Our teacher was a physics freak, and he posed us this question one day. We all said it would take off. He assured us that it would not. So we tried it with a set of rollers and a small petrol engined plane. To cut a long story short, there is no air velocity change over the wings as in forward flight as the plane is actually stationary hence it cannot produce any lift. Dr A worked for NASA in cutting edge aviation for 20 years, and he sure as hell proved his point. Flight can only be achieved by forward motion through the air as controled by the laws of aerodynamics and physics working in conjunction with the wings and body of an aircraft.

nobody disputes that, its obvious. The argument is whether the plane will move or not, I think it will as the reversal of the belt will not act against the forward motion induced by the propeller, only against the rotation of the wheels, which are only castoring so will not retard the aeroplane. Anyway, thats my last post as what I think is now clear, people can only agree or diagree as they think fit.

So for anyone posting a counter view first please realise that I'm not for a second suggesting a vertical lift off by a stationary plane so that is not the argument you need to try and disprove. I am fully aware that a plane cannot take off without forward motion.

Heres a good one to have a look at. Its from NASA and its a good guide to begginers aerodynamics. The plane cannot take off as if you think it will have to go in a noe up attitude at some point...and the thrust will no longer be equal to the tredmill speed, and there is no air....rofl i love this post.

www.grc.nasa.gov...

Originally posted by ghost
1. Weight, inertia and friction will act on the plane as you try to accelerate.
-if the wheels are irrelivent, try this: (A)Sit in a chair that is anchored in place and strap youself in. (B) Have someone Anchor a steel cable to a 747. (C) Have the pilot make sure the breaks are off. (D) Pull at the cable and see if you can get the plane to move.

A better example would be to have the 747 kick on the engines. Will the chair move or will the chair stop the jet due to friction?

The chair is just as meaningless as the treadmill.

Originally posted by ghost
2. Lift comes from airflow. No airflow=no lift!

The simple answer: There will be airflow and lift.

Originally posted by ghost
Why is this so hard to understand?

Good question, I can only assume it's because many people have a hard time getting over the mental block of dealing with cars, where the wheels on the pavement are your driving force (which a treadmill actually can cancel out).

In a plane, the wheels can freely spin as fast as they want to and it won't change the forces of thrust involved (the prop pulling the plane through the air) to prevent takeoff.

Originally posted by waynos
Thrust and drag are not equal because there is no mechanical link between the wheels and the propeller, the wheels are completely independant and thats why they don't cancel out the thrust.

Exactly

If we say the take off speed is 160mph, the plane will still accelerate forward on the treadmill, (because the wheels are completely independant and the conveyer cannot cancel out the thrust like it can with a car) once the forward acceleration/speed of the plane (relative to an independant bystander) reaches 160, the treadmill will be going 160 in reverse, but, the actual wheel speed will be roughly 320 mph (because you're accelerating down the treadmill), and the plane will still takeoff.

The plane's forward acceleration/speed and it's wheelspeed are not dependant on each other, because the wheels spin freely and are not where the accelerating "thrust" comes from.

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction). Can the plane take off?"

The key to unlocking the puzzle is that the conveyer "tracks the plane speed", not the wheel speed, because the two are independant with a free spinning wheel system!

[edit on 2/16/06 by redmage]

Originally posted by MadGreebo
We did this experiment at school in science lessons. Our teacher was a physics freak, and he posed us this question one day. We all said it would take off. He assured us that it would not. So we tried it with a set of rollers and a small petrol engined plane. To cut a long story short, there is no air velocity change over the wings as in forward flight as the plane is actually stationary hence it cannot produce any lift. Dr A worked for NASA in cutting edge aviation for 20 years, and he sure as hell proved his point. Flight can only be achieved by forward motion through the air as controled by the laws of aerodynamics and physics working in conjunction with the wings and body of an aircraft.

What was the diameter of the rollers in comparison to the wheel diameter and how far apart were they spaced.

The aircraft will take off - or its tyres will explode before it does, but either way, it will move.

Originally posted by MadGreebo
Heres a good one to have a look at. Its from NASA and its a good guide to beginers aerodynamics. The plane cannot take off as if you think it will have to go in a noe up attitude at some point...and the thrust will no longer be equal to the tredmill speed, and there is no air....rofl i love this post.

www.grc.nasa.gov...

You not heard of minimum unstick speed or rotation speed then?


Suggest you look here

Originally posted by waynos
nobody disputes that, its obvious. The argument is whether the plane will move or not, I think it will as the reversal of the belt will not act against the forward motion induced by the propeller, only against the rotation of the wheels, which are only castoring so will not retard the aeroplane.

Waynos,

Unless I misunderstand you, I think I see why we can't seem to agree!

You are focused on the question of movement, which is about countering forces. The point of this thread, however, is will the plane be able to achieve flight.

The wheels of the plane are free spinning(as others have said). However, until the plane has enough forward speed to acheve lift, the wheels will be supporting the weight of the plane. Now, the engine is pulling (or pushing if it's a jet) the plane forward as the same speed that the treadmill is pushing the plane backward (because the weight of the plane is sitting on the wheels). Now according to the laws of physics, when two oppisite forces of Equal strength act on the same object, but in oppisite directions, they cancell each other out. This makes the Net Force equal to ZERO. With a net force of Zero acting on the plane, it will remain stationary(Newton's First Law of Motion, A.K.A Inertia).

We all know that if the plane is stationary, you will NOT get airflow over the wings (Assuming there is no external source of wind). No airflow, means there can be no lift. (Bernoulli's Principle proves this) Airplanes can't fly without lift, no matter how much thrust they have.

Tim

[edit on 16-2-2006 by ghost]

Ghost.

The planes force is acting on the air to provide thrust. The conveyor does not counter this in ANY way. Without it countering that the plane will move.

The plane is also acting on the wheels making them spin.

The Conveyor is acting on the wheels making them spin the SAME direction.

Therefore the net force is not 0.

Then we have the arguement that until the plane achieves a forward velocity of greater then 0. Gravity will cause enough friction on the wheels to let the conveyor hold it back. While I do not beleive this to be true. I will say that the conveyor itself will not move until the plane has a velocity of > 0. Therefore that force has already been overcame by the time the conveyor comes on.

OK, let’s look at it this way.

Lets consider two opposite extremes.

1) The is zero friction between the plane and the conveyor. Zero friction in the wheels, the wheel bearings, etc. Think of the conveyor belt as being made of ice and the plane as being on skates, if that helps. Under this condition, there would be NO interaction between the plane and the conveyor belt. In fact, if the belt started moving, the plane would stay still (an object at rest will remain at rest, etc.). Therefore under these conditions, the movement of the conveyor is immaterial and the plane will indeed take off. The plane will be perfectly capable of accelerating down the runway.

2) Let’s now consider the opposite extreme. Let’s take the wheels off of the plane and put it up on concrete blocks like a ’74 Chevy Impala. In fact we’ll even put a few more blocks in front of the gear struts so that the plane can not move forward in relation to the conveyor at all. It can, however, move up in response to lift. This plane will not take off regardless of how much thrust is being generated by the engines except under one condition. The conveyor belt has to be moving forward at the proper take-off speed to generate the necessary lift.

Are you all with me so far?

Those two conditions are on the opposite ends of the spectrum as far as the friction between the plane and the runway/conveyor belt goes.



What about if we have an intermediate, “real world” scenario?

Let’s say the wheels on our plane have a certain degree of friction, or rolling resistance if you will. The plane has to generate a forward thrust of value X just to overcome this resistance.

Is that different from either of the above scenarios? The answer is, no it isn’t. All the pilot has to do is to apply enough thrust to overcome the resistance X and he will start to move forward, independent of the conveyor’s speed. Once the plane begins to accelerate forward, eventually it will reach take off speed.

Originally posted by Bhadhidar

Originally posted by redmage
The best example is the wagon/handrail example.

If you're in a car on a treadmill, your wheels are your driving force so a treadmill can cancel your momentum out because the wheels "pull" against the ground/treadmill to move you forward.

However, if you are in a wagon(plane), and put the treadmill in a hallway with a handrail, your wheels are not your driving force, aka the wheels spin freely like on a plane.

Then your arms are the "prop" and the handrail is the air that you pull on to move you forward.

Your wheels can spin like crazy, but, it's your arms/prop and the handrail/air that pull you forward, regardless of the speed (or direction) that your wheels spin.

[edit on 2/15/06 by redmage]

I think your analogy is a bit flawed.

We seem to agree that lift can only be generated if air flows over the airfoil of the wing, right?

We also all seem to agree that on a plane, the wheels serve basically two purposes; to support the weight of the plane while on the ground, and to facilitate the plane's ability to taxi efficiently.

I think we can also agree that that neither function of the plane's wheels is related to the propulsion of the plane while it taxis or once it is airborn. And neither function is especially crucial to the generation of lift, which allows the plane to become airborn.

Your analogy does a fine job of pointing out the relative irrelevance of the actions of the wheels insofar as not enhancing the forward progress of the plane through the air mass in the given scenario, while at the same time underscoring the necessity of the plane to move relative to a fixed point on the ground, as would be required to generate the needed airflow over the wing, and thus lift the plane off the runway.

However, you may have overlooked one significant factor: The handrails, which in your scenario are analogous to the air mass the plane is to fly through, are solidly fixed to the walls of the hall housing the treadmill/runway. These walls are themselves solidly fixed to a point on the ground!

This detail, rather than proving that the plane can indeed take-off, only serves to move the moment of inertia from the non-stationary contact point between the plane's tires and the "conveyor belt runway" to a Stationary contact point between your hand and the Fixed (that is to say a point fixed relative to a point on the unmoving ground) contact point on the handrail.


The analogy would only prove your point IF you could show that air is both solid and , more importantly, fixed to a specific point on the ground.

Obviously, air is not solid, nor is the mass of air above a specific point on the ground fixed to that one spot. Therefore, I propose that your analogy might be more germain to the scenario if instead of grasping the handrails and pulling yourself along, you instead "grasp the air" and attempt to propel yourself forward by making pulling motions in the air, as it were.


Another point relating to the action of an aircraft's propulsion. This may be little more than a question of semantics, but nevertheless...

I believe that it has been stated that the purpose of an aircraft engine is to "pull/push against the air" thus driving plane forward.

As I understand it, however, the function of the enigine is actually to push/pull the mass of the plane through the air with sufficient velocity to allow the wings enough lift to counter-act the weight of the aircraft, vis-a-vis the pull of gravity. Heavier than Air flight is thus made possible.

So it might be more precise to say that, in keeping with Newtonian physics, the force generated by the plane's propulsion (whatever form it may take) is employed to push/pull against the airplane itself to counter its inertia and lend it suffcient velocity to fly.

Again, it may be just a matter of semantics.

It is semantics ... but that bolded statement is exactly what I meant. Your arms (the engines) are pulling the mass of the plane (you + the wagon) ... or your arms (the engines) are pulling the air (handrails) behind you. Doesn't really matter because the forces can be explained either way.

Even if the handrails aren't FULLY stationary (say they are moving in the direction of the conveyor belt at some slow speed and up and down) you will STILL be able to move forward (it will just take alot more arm strength). Aircraft engines are designed to easily produce that "strength" or thrust. It's really a pretty week analogy I guess ... anything that could cause thrust in the wagon (hands/rails, a rocket, blowing REALLY hard) can be used. The key is that the thrust/lift forces are totally independent of the wheels (because they are stated as an assumption to be frictionless).

Everything really can be explained with physics and simple free-force diagrams. If there are still non-believers I'll create some gif's with some math.

First consider a wheeled vehicle such as the family car on the same type of banded conveyor belt. The automobile transfers force directly onto the banded conveyor via the wheels.

The force applied (if greater or less rather not equal to the speed of the banded conveyor belt) accelerates or decelerated the vehicle relative to the speed of the banded conveyor belt. If the banded conveyor belt is moving equally in relation to the force applied by the wheels, then the vehicle stays in a fixed position

An airplane transmits force, not from its’ wheels, but from it ‘jets’ (or props) onto the air surrounding it and not to the banded conveyor belt. When thrust is accelerated the plane will move in relation to relative to the wind speed, and not the banded conveyor belt.

As mentioned as part of the original question, in this case the wheels provide a zero friction point. However, in reality, some friction would be realized, but nothing noticeable and would be more easily understood as a form of ‘drag’.

Considering either the mentioned ‘drag’ or ‘zero friction point’ the banded conveyor belt would not influence the take-off of the airplane.

It might be easier to conceptualize if you imagine and replace the wheels with skis or blades and replace the surface of the moving banded conveyor with ice.

mg

Park your car, and the car of your best friend on opposite slopes of a hill, then connect thier rear bumpers with chains before placing each vehicle in neutral.

Fortunately, both of your cars are relatively safe from rolling down the hill, as opposite forces are applied to each of them. However, there is no mechanical linkage between the aircraft and the conveyerbelt runway. Please cut the chain between the cars.

who ever suggested that the wheel speed is double has a complete lack of physics.

ITS A ROUND ROTATING OBJECT WITH EQUAL FORCE ON BOTH SIDES ( BU TIN OPPOSITE DIRECTIONS OF FORCE) OF THE ROTATION THEREFORE IT CAN EXERT NO POSTIVE FORWARD MOTION

ONLY IF THE CONVEYOR IS RUNNING IN THE SAME DIRECTION DOES THE WHEEL SPEED DOUBLE.

Check out a pitching machine tennis ball softball etc, which direction do the rollers move... opposing ? no they rotate one counter clock wise and one clockwise so that an object placed between the rollers has force actiing on it in the same direction from both sides.

Same principal for a roller coaster, if the rail has a drive wheel only on one side then you impart only that kinetic energy onto the car, if however you have a drive wheel on both sides you get double the amount of kinetic energy transferred onto the car.

Just draw a circle the top of the circle you are going to push forward and the bottom of the circle you are going to push backwards, is the circle going to roll or will the rotation be stopped by equal yet opposing forces

Just think really hard about the turning wheels in a pitching machine and what happens if both wheels are turning the same direction clockwise. A ball placed between the two will jam and it may plop out towards the side of which ever wheel as the least amount of surface friction or higher torque motor

stick your hands out fingers flat with your thumbs facing each other like you are superman, now put a coin between your thumbs and push out to "throw" the coin into the room. Now put the coin and push one thumb out and pull the other thumb towards your body... where does the coin go..



[edit on 16-2-2006 by robertfenix]

Originally posted by robertfenix
who ever suggested that the wheel speed is double has a complete lack of physics.

ITS A ROUND ROTATING OBJECT WITH EQUAL FORCE ON BOTH SIDES ( BU TIN OPPOSITE DIRECTIONS OF FORCE) OF THE ROTATION THEREFORE IT CAN EXERT NO POSTIVE FORWARD MOTION

ONLY IF THE CONVEYOR IS RUNNING IN THE SAME DIRECTION DOES THE WHEEL SPEED DOUBLE.

Check out a pitching machine tennis ball softball etc, which direction do the rollers move... opposing ? no they rotate one counter clock wise and one clockwise so that an object placed between the rollers has force actiing on it in the same direction from both sides.

Same principal for a roller coaster, if the rail has a drive wheel only on one side then you impart only that kinetic energy onto the car, if however you have a drive wheel on both sides you get double the amount of kinetic energy transferred onto the car.

Just draw a circle the top of the circle you are going to push forward and the bottom of the circle you are going to push backwards, is the circle going to roll or will the rotation be stopped by equal yet opposing forces

Just think really hard about the turning wheels in a pitching machine and what happens if both wheels are turning the same direction clockwise. A ball placed between the two will jam and it may plop out towards the side of which ever wheel as the least amount of surface friction or higher torque motor

stick your hands out fingers flat with your thumbs facing each other like you are superman, now put a coin between your thumbs and push out to "throw" the coin into the room. Now put the coin and push one thumb out and pull the other thumb towards your body... where does the coin go..



[edit on 16-2-2006 by robertfenix]

The problem with your analogy is that both wheels or rollers of the pitching machine are fixed at their axis of rotation, i.e., they spin at the exact same place in space.

Think of the speed of the wheels on a normal takeoff. Now imagine the airplane is fixed in space (harnessed to the ground somehow) but on a conveyor belt that accelerates at the exact same rate as a plane would on a normal takeoff. The wheels would spin at the same velocity at every point of takeoff (all you are doing is switching the point of reference for the velocity). This is the "fixed-fixed" situation.

However in our situation, the plane accelerates forward INDEPENDENT of the belt as we have proven earlier. So the conveyor's axis is fixed but the wheel's axis is now moving forward with a velocity. Since the plane and the belt have equal velocities at any given point (according to the given in the problem), the wheels would have twice the speed at any given point than they would on a normal takeoff.

HowardRoark

Great brain teaser

I have to admit, it took me till page 3 or 4 to drop all preconceived assumptions, evaluate it it objectively, and get past my own "mental block".

It's a bit of a good personality test too.

[edit on 2/16/06 by redmage]

Originally posted by redmage
HowardRoark

Great brain teaser

It took me till page 3 or 4 to drop all preconceived assumptions, evaluate it it objectively, and get past my own "mental block".

It's a bit of a good personality test too.

[edit on 2/16/06 by redmage]

I agree, props to you. It took me a few looks at it to get it right.

It's all about the physics ...

can we PLEASE stop arguing about this. It's getting on my nerves the number of people trying to 'scientifically prove that it will fly'. I won't bother trying to prove again that it won't fly because my responses have been over-simplified or over analysed. It's simple. the plane won't fly. take a step back, look at the bigger picture, the question is quite simple, as is the answer. CAN SOMEONE PLEASE CLOSE THIS TOPIC. it is going around in tight little circles with people who now what they are talking about being continuely bombarded with people adament to debunk well established laws and theories of physics and aerodynamics. face it people, this thread has copped way too much attention for what it is. PLEASE CLOSE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Originally posted by redmage
It's a bit of a good personality test too.

As the previous post clearly illustrates.

Originally posted by av8or
can we PLEASE stop arguing about this. It's getting on my nerves the number of people trying to 'scientifically prove that it will fly'. I won't bother trying to prove again that it won't fly because my responses have been over-simplified or over analysed. It's simple. the plane won't fly. take a step back, look at the bigger picture, the question is quite simple, as is the answer. CAN SOMEONE PLEASE CLOSE THIS TOPIC. it is going around in tight little circles with people who now what they are talking about being continuely bombarded with people adament to debunk well established laws and theories of physics and aerodynamics. face it people, this thread has copped way too much attention for what it is. PLEASE CLOSE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

We are not debunking physics ... the plane will move forward on the conveyor belt due to the thrust of the engines just like it would on a normal runway. There will be the same airflow and lift and it will take the same distance to reach the proper takeoff speed. The wheels are frictionless and do not matter. It WILL take off. You need to take another look at YOUR physics as well as what was stated as given in the original problem.

NOTE: THE PROBLEM IS NOT REAL-LIFE ... in real-life there is friction on the wheels and the plane would need a longer runway to overcome that force. Also typical wheels and bearings would not be able to handle the doubled speeds that are encountered in this situation and would likely break before reaching takeoff speed. But we are discussing the PROBLEM, not REAL LIFE.

EDIT: grammar and content


[edit on 16-2-2006 by Fiverz]

Originally posted by Mayet
Without looking at all the other replies, yes.. cos the plane could be like a jump jet with vertical take off...

It doesnt have to be a jump jet. Any kind of jet or propellor will get it moving forward. The conveyor belt increases friction to higher than normal levels because the wheels are moving twice as fast as usual but it has no effect whatsoever on thrust, and hence propulsion. People are getting confused because they are used to wheels being the source of thrust as in cars or motorcycles.

With so many getting confused by a simple brain teaser its no wonder that con artists are able to raise money for anti-gravity and warp drive schemes.