I have to admit when I first read this I put my pilot hat on and quickly thought it would not fly because as a pilot the first thing you think is that to generate lift you need relative airspeed over the wings and if the plane is not moving there will be no relative airspeed (assuming no wind).

But then when I put on my engineer's hat, I realized the plane's thrust is being reacted by the air, not the conveyor, so it will move forward through the air no matter how fast the conveyor is moving (and eventually take off). The key is that the wheels on a plane are free spinning so it doesn't matter what the conveyor is doing as long as the jet engines' thrust is much greater than the friction in the wheels. The plane's jet engines are pushing against the air so it will move forward relative to the air and fly.

Another way to think about this is to ask yourself, would a submarine move through a tank of water even if it was connected (through free spinning wheels) to a track underneath it that was moving in the opposite direction? The answer is yes because the submarine's propeller is pushing against the water which is independent of the track. This pushing will propel the submarine forward through the water no matter what the track is doing. Now replace the water with air and you have the same thing with the airplane.

What pushes the plane back? If you are talking about the belt, it is just turning the wheels.

Mike

It makes for a fun discussion

IF the pilot locked the brakes, then it would be interesting, since outside of military aircraft which can climb straight up, the thrust of the engines is not equal to the weight of the aircraft. This is why you can stand on the brakes and rev the engines and not move. IF the wheels are not locked, then as stated, they have no effect since they are there simply to reduce friction and the plane would take off.

Sharyn

So why don't we just build conveyor belts on carriers and at airports ? And if I have my 747 sittign with lock brakes at full throttled, assuming planed doesn't rip apart, it should fly lift straight up because of the air flowing through the engines ?. I air speed over the wings it what gives lift, not air being sucked through engines. That provides the speed to overcome inertia to get the speed needed.

And to take that concept one further, suppose you are on a sheet of ice and stand on the brakes. The wheels won't turn, but with a lack of friction between wheels and the ice, the plane could conceivably slip forward and get enough airspeed to fly.

As to the water analogy made earlier, if you had an air boat like they use in the swamps, you could face the boat into the current then apply just enough throttle to hold your position relative to the stationary landmarks on either side of you on dry land (zero groundspeed). Or, put yourself in the water and swim. If the water current is moving against you beyond what thrust you can generate with your body, you will be swimming like mad but getting swept downstream none the less. If you were in still water and started to swim forward and someone could measure your progress and apply a current opposite to your efforts, you will swim ever faster trying to move forward, but you wil go nowhere.

Hehe, we can have a lot of fun with vector math. Just as every pilot knows how to calculate a wind correction angle to maintain a straight path over the earth. Or like when I use the trolling motor on my bass boat to face into the current and turn the prop just ehough so that I don't move relative to the landmarks (again, zero ground speed).

I kind of agree with Chris in that as thrust tries to push the aircraft forward, the conveyor belt will increase its opposite direction to keep the aircraft from moving relative to a stationary landmark (zero ground speed). Or, let's say the engines on this jet are shut off. What would happen to the aircraft if you turned on the belt?

-gb-

If you assume the perfect wheel and tire with no friction and perfect conveyor, then you might have a point, but science tells us there is no such perfect machine...

Because the conveyor is not reducing the takeoff roll. The plane still needs to move forward through the air to generate lift. It is doing this and it is moving forward relative to the air and the earth (i.e. the airport) because the jet engines are reacting its thrust against the air. The wheels are just free spinning. They could be spinning Mach 2 and the airplane would still move forward because the thrust of the engine is way more the friction in the free spinning wheels. As long as the force trying to make the plane go forward is more than the force trying to hold it back (friction and drag), it will go forward.

If you locked the brakes and assuming the brakes were stronger than the thrust of the engine and the plane does not skid, then the plane wouldn't move at all and neither would the conveyor (the forces holding it back are now equal to or greater than the force trying to make it go forward). But that's not what the riddle says.

The riddle says that the conveyor speeds up in opposite direction, every time the plane does speeds forward. I assume a balancing of forward motion against the reversing motion. The plane stay in one place relative to the air around it, therefore no air speed for lift.

A plane going down a run way, just the same as one on the conveyors is subject to many variable impeding its progess including downward force of gravity, and its affects on bearing, tire flexion, etc.

Chris, in accordance with Newton's third law, what propels an airplane? Is it the:

a) wheels passively spinning against the ground
or
b) engine thrust against the air?

Answer is b. You don't get to add your own set of variables to a thought experiment, so considering wheel friction, etc. is a foul. Try thinking about it this way...what if instead of a giant treadmill it was wet ice and instead of wheels the plane had giant skates. (Sorry, I tried really hard to resist posting to this thread, but I obviously I succumbed. Aerodynamics 101 for non-aerodynamicist: the four primary forces acting on an airplane are thrust, drag, lift, and of course, gravity). Jet propulsion 101 for non-mechanics: all you need to know is "suck, squeeze, bang, blow."

Chris.

The Plane DOES react to the conveyor belt moving backwards. It reacts by rotating it's wheels TWICE AS FAST as it's forward momentum.

Another thought.

A plane is landing at 175 miles per hour. It lowers it's landing gear. The wheels are not rotating. (Or possibly even free rotating in the OPPOSITE direction of travel)

The plane touches down. The wheels do not STOP the plane. Because they are standing still. Yes, there is a great deal of friction, take a look at the end of the runway sometime.

In the scenario described, the wheels will spin TWICE AS FAST as they normally do on takeoff. That is the sum total of the effect of the conveyor belt runway on the aircraft. More wear and tear on the tires and bearings? Sure, but not enough to prevent takeoff.

And for the record, the engines do not "push against the air" the thrust within the engines pushes against the engines, escaping out the back. The total force vector in the engine translates to forward thrust. The engine pushes against the wing/fuselage. The plane moves forward. When it moves fast enough, lift is generated.

LIFT + THRUST must overcome GRAVITY + DRAG for ANYTHING to fly. We know from experience that the engine and wings of the 747 will easily overcome the drag of it's wheels spinning. THAT is the only change in the equation.

Now can I take off my P.I.C. hat and get back to editing?

Oh alright, I suppose so. I had my PIC hat on also, but chose to wear it backwards. (grin)

At least we finally pulled Bauer into the thread. And for all the pilots, what are the four left turning tendencies of a single engine, propeller driven aircraft?

-gb-

This propulsion will be countered by the conveyer belt.

The plane will not be moving through the air, no lift will be generated.

As the free spinning wheels are countered by the spinning conveyer we can remove this factor from our calculations, the plane would effectively be sat still with its brakes on - it would not lift into the air, it would not fly.

Lee.
The wheels, are in effect, bearings between the plane and the ground. They have NOTHING to do with the propulsion of the aircraft.

The aircraft gets it's thrust and momentum from it's engines.

The aircraft will move in response to the thrust, just as suredly as if there were a giant, NOT STANDING ON THE RUNWAY, pushing the jet along with his hands. He cares not if the wheels are spinning twice as fast as they normally would.

You must resist the assumption that the wheels add to, or subtract from the speed of the aircraft. They are not the drive force, like in an automobile.

(I swear this will be my last post. I can't think of any more analogies to make it plain.)

First pick any six cubes and put three on each side of the scale, this shows you which of the three groups the heavy cube is in.

Repeat the process with the chosen three cubes, one on each side of the scale one stays on the table.

From the original post: And there is also a device on the plane that communicates with the conveyor belt to tell it how fast the plane is traveling, which would then make the conveyor belt match the speed IN REVERSE.


I am presuming that if the plane is moving at 286.3 mph the conveyer belt is spinning at 286.3 mph in the opposite direction and therefore, relative to the surrounding air and ground the plane is static.

In this scenario the plane will not lift into the air.

Like this >> http://img223.imageshack.us/img223/6784/fffsg3.jpg

I understand this and make no claims that the wheels would add or subtract from the speed of the aircraft.

Richard, I agree the engine pushes against the wing/fuselage and this creates the forward momentum, but the engine is pushing against the air to create this forward reaction thrust. Each blade on the engine is sucking in all the little air molecules and basically pushing it back (as it compresses the air, combusts it and propels it out the back). Because of Newton's third law, as you push on each air molecule, the air molecule pushes back (the reaction force) on the blades and this force then goes to the engine which then goes to the wings and fuselage. The sum of all the reaction forces is the forward thrust.

It would be different if this was a rocket engine. A rocket engine pushes propellent out the back. The propellent pushes back and creates forward thrust. Thrust is not created by the propellant pushing on the air (when the rocket is still in the atmosphere).

But a jet engine take air and pushes it out the back. The air pushes back and creates forward thrust.

Torque, P-Factor from the blades, Spiral Slipstream...and I forget the last one....dangit.

I'm no pilot, but this analogy just popped into my head, thought it might be useful:

I'm standing on a nice, long treadmill. I start to walk, and some sort of fancy sensors watch my speed, turning the treadmill's belt so as to counter me. I, therefore, remain stationary as far as the surrounding environment is concerned. As I begin to jog, the sensors speed up the belt, producing the same result: I go nowhere. Even as I run top speed, I'm accomplishing nothing. A rather depressing metaphor, I must say.

This, I imagine, is what would happen with a car driving along this hypothetical motorized runway of Gene's.

However, let us now assume that the treadmill I'm running on top of is equipped with two waist high handrails, something you might find a gymnast practicing on. If I grab these with my hands, and pull myself forward, I will indeed move forward. The belt may match the speed of my feet, but since my grip is on the handrails, it makes no difference.

This is what I understand to be happening when we're dealing with an aircraft; the propulsion of the vehicle is conceptually disconnected from the machinery used to rest said craft on the ground. The jet's engines are "grabbing" the air, so to speak, and so long as they have a good grip, the thing will want to move.

That is my point, as long as the plane is in touch with the ground it doesn't seem to matter what propulsion is being used. It still has to overcome the inertia and gravational forces that connect it to the conveyor belt. That connection of gravity and inertia is all centered in the wheels. And with the 200 tons a 747, that is a lot to overcome. Once it reaches a ground speed/air speed, it can transition to flight. But the way I reed the scenario, with the belt in reverse running to counter act the forward motion of the plane, it never can get there.

When the plane's engines are fired up, it will move forward on the treadmill, with the only impediment being the relatively small amount of extra drag on the wheels from the treadmill's backward motion. If this is only a theoretical proposal and the treadmill is as long as a full runway, the plane would be able to reach flying airspeed in just a bit longer distance than it would normally. If this is a real experiment, then the treadmill would probably be very short and the plane would quickly roll past its front end and that little problem would be over. The tires and wheel bearings on this aircraft would be able to withstand a doubling of the normal takeoff speed of about 142 knots for the short time involved.

In addition to all the previous remarks about aerodynamic lift and the Bernouli Effect that produces it, don't overlook the major aspect of "ground-effect" lift that occurs when the plane is within a few hundred feet of the surface. This contributes a large portion of the total lift during landing and takeoff. On landing, the deployment of large slats and flaps on aircraft, creates a cushioning "ram-wing" effect, which is an extreme type of lift that is similar to that of ground-effect.

The Russians have developed large air tranport planes that fly across the Black Sea at a typical altitude of about 50 feet, primarily using ground-effect lift. This results from a compression of the air between the aircraft and the surface. Boats have a similar effect on them in shallow water, that is caused by increased pressure between their hulls and the bottom. By flying in the ground-effect zone, these Russian air transports get more than twice as much fuel-efficiency than those flying higher with aerodynamic lift. Their ratio of lift to drag is much higher with ground-effect. However, with boats, the "bottom-effect" slows them down, as the increased pressure speeds up the flow of the non-compressible water around them, raising the drag.

The Human-powered aircraft, such as the Gossamer Condor and others, flew almost entirely on ground-effect lift, staying within a few feet of the surface. Their airspeed was not much more than about 12-15 knots, which wouldn't generate much aerodynamic lift. For a Human to power an aircraft at an altitude that used only aerodynamic lift, it would require a significant extra amount of thrust.

The principle of thrust is the same between a jet engine and a rocket engine. The thrust is genrerated through different MEANS, but the thrust vector in the compression chamber is the net same as the thrust vector in the ignition chamber.

As you stated the 'air' or 'compressed gasses' are PUSHING AGAINST the engine, and escaping out the back.

Independent of what is happening to the wheels.

I've used the example of someone standing on a treadmill, with the rope in his hands, attached to the far wall. As he pulls himself forward, the treadmill spins backwards,at ever increasing speeds, but he's STILL MOVING forward. Why? because his method of propolusion is independent of the connection to the treadmill.

You can also imagine the person standing on the tread mill, and with super human strength, PUSHING AGAINST THE WALL behind him. He would move forward, even as the treadmill moves backwards. (At least untill the force of the intitial push was overcome.) But the engines continue to push as long as you power them.

The airplane will fly.

(It would fly even if it were a prop driven aircraft as well.)

Hehe Mike, I know all of this. I was quoting someone else and asking Why? :)

What everyone can't seem to understand is that a plane isn't driven BY its wheels. A car IS driven by it's wheels. A human is driven by it's LEGS!! Both the car and human are completely different than a Jet which is NOT driven by it's wheels. Sinking in yet? Ok one more... Jet = free spinning wheels. Car = NOT free spinning wheels.

All this question means is that the wheels will be spinning exactly twice as fast when the plane takes off. That's all this means.

If you replaced Jet with Car in the original question then yes, it wouldn't move anywhere. And if that car had wings on it, it would NEVER take off. The difference is that the car is driven by it's wheels. Get it now?

Cheers

Yes, I got it a long time ago; that was supposed to be the point of my example. Perhaps my choice of a human walking on a treadmill was poor, it may have been better to visualize someone on rollerskates, but the example still works.

The human in my scenario is only "driven by its legs" as long as he's walking or running. As soon as this imaginary person grabs the handrails and pulls himself forward, he's powered by his arms, and the means of forward movement is separated from his connection to the ground.

I agree with you, Gene, and I was trying to provide an analogy to help explain why I believe you're correct. Maybe I wasn't clear about that when typing my last post. It was late and I was tired, what can I say?

Ah sorry Robert I didn't mean to direct my comment at you. I just happened to use the person walking on a treadmill as an example.

Cheers

Maybe it would be easier for some to picture a skier on skis, on an ice covered belt, and a rocket backpack. He may not fly with out the wings, but he will surely get going fast if he can stay on his feet, er skis!

Mike

Try this experiment
 

Here is my proposed experiment to prove my position:

1. Go to your kids toy box and get a "free wheel" tonka truck out.

2. Get a long rubber band. Tie it to the front bumper.

3. Go to your variable speed tread mill. Point the the truck in the opposite direction of the tread mill direction. Mark the position

4. Pull the rubber band forward enough that the truck starts to pull forward. That should be your independent forward thrust not connected to the free spinning wheels

5. According to the scenario, the conveyor belt instantly senses the forward motion, and adjust by moving opposite direction. So turn on the variable speed until the truck "just due to friction and down ward force of gravity takes the vehicle back to it original position by stretch rubber band back. You are at static position, with no air speed for lift.

6. Pull tighter on the rubber band, and the tonka truck will move forward again, but with "instantaneous" adjust per the scenario, if you adjust the speed of the tread mill, you are again back at static position, still no air speed.

7. Of course if you suddenly shut of tread mill off, the tonka truck should lurch forward with the thrust that is being applied, approaching air speed required.

So that is the way I imagined the the scenario, and that analysis is why I think it can't reach lifting air speed. I think those who are saying other wise are failing to account for 200 tons of weight, inertia, and friction on tires and wheels. I have been accused of adding something to the equation, but I did not. The scenario indicates a 747. I assume those characteristics.

Where I can be wrong is with respect to the jet engine itself. I am acting under the understanding that it does produce its own airs speed for lift. It uses air pulled throught itself to produce propulsion which in turn produces air speed as the jet is propelled down the runway.

Also, I may be misunderstanding the what is proposed in the first place. I think some here are assuming the conveyor belt is free wheeling, and doesn't have its own motive force. That is not what I read into the proposition, and a motorized conveyor belt such as seen on a motorized treadmill.

And that is where I disagree. I understand what you're proposing, but I can't possibly imagine it would be a perfect balance; just enough friction and gravity to pull it back to its starting point, but not enough to snap the rubber band and send the truck flying off of the conveyor belt.

I'm operating under the assumption that jets are not equipped with engines that can produce only the bare minimum thrust required to move the vehicle forward under ideal conditions. It's my understanding that most aircraft, especially commercial airliners, have more than enough power available to move themselves forward (they can lose engines midflight and keep going, after all), and pushed far enough could easily overcome any friction presented by the wheels, no matter how fast this belt is moving.

"...a very small rock?"

Nevertheless, the scenario proposed initially was that the conveyor would instantaneously adjust. And its not just friction, its the 90% downward thrust of gravity that has to be over come, even by a car driving down a perfectly flat surface. That why on a perfectly flat surface we can just push a car and expect it will roll forever...

That's something that's bothering me; what does gravity have to do with this? I mean, of course, it's always there, and has to be overcome, but how is gravity any different in this situation than it is on a regular runway? It's just as strong, isn't it? Planes overcome gravity all the time, right? They can go from dead stops, not moving at all, to the required takeoff speed without issue. Why would the force of gravity be harder to overcome here?

Gravity has nothing to do with the wheels. The wheels are simply the interface between the ground and the plane.

The sole purpose of the wheels, is to overcome the force of DRAG.

There are two 'negative' forces working on an aircraft. GRAVITY and DRAG. In order to overcome those forces, the aircraft must generate sufficient LIFT and THRUST.

The sole purpose of the wings is to overcome the force of GRAVITY.

LIFT is provided by the wings. THRUST is provided by the engines.

GRAVITY is provided by the earth. DRAG is provided by the friction with the air (as the plane moves through it) and THE GROUND as the plane moves along it.


In the scenario described, we know that the 747 is designed to provide MORE than enough lift and thrust to overcome GRAVITY, and the DRAG proudced by friction with the air. But what about friction with the ground?

Friction between the ground and the 747 is minimized, by employing very very efficient devices called WHEELS. On a ski-plane, they use teflon coated skis, on a seaplane, it's pontoons. But in the case of our aircraft, its wheels.

The ONLY element of takeoff that is being altered from a normal takeoff in our scenario is an INCREASE IN FRICTION between the interface of the aircraft and the ground. (Someone described and increase in friction between an airplane and a muddy field. In that case MORE power would be needed to overcome the clinging mud).

In OUR situation, there is no 'clinging' property to the moving runway. It is simply 'pushing' back against the surface of the wheel twice as hard as it normally would. Since NOTHING else changes - IE Airpseed, thurst or lift, then the wheels will accomodate the increase in friction with an increase in ROTATIONAL SPEED. (and yes, probably an increase in temperature as well, but certainly nominal.)

There is NOTHING to prevent the wheels from rotating at twice the usual speed of takeoff. (At whatever given altitude and air density our imaginary take off assumes)

So the aircraft uses the same thrust and lift it normally uses to take off, and accomodates the increase 'drag' of the runway interface, by allowing the wheels to turn twice as fast as they normally do.

The. Airplane. Will. Fly.

It isn't. Okay, you know how when you start to push a stalled car when you first push it, it is hard to move it at all. You are overcoming the downward pull of gravity, as well as the friction and flexion of the tires. Then, as you get in rolling it get easier if you roll at a constant speed, but if you want to push it faster to pop the clutch, it gets harder to pick up speed.

Hey Robert, maybe this is one for the the MythBuster to figure out. I love that show...

Okay, with the scenario we have here, the conveyor speeding up instantaneously seems to be bring us back, under my analysis, to that inertial state again, and we really are only standing still--- depending on how instantaneous the conveyor belt adjusts.....

Again, I am not MENSA, and this seems freaky to me, but I also can't figure it any other way... So I am looking for somebody to actually show me where I'm wrong rather than act like that Apple guy on the Apple v. PC commercials.

Okay. Here are the facts.

The 747 has four engines, that generate appx 58,000 llbs of thrust EACH.

There is NOTHING preventing the wheels from turning in our scenario. They are as free as they ever are, to turn at whatever rotational speed is required.

In order to prevent the plane from moving forward, the drag provided by the wheels turning 'twice as fast' as they normally would, would have to EQUAL the thrust of the four engines generating 58,000 lbs of thrust EACH.

You MUST supply equivelant thrust of the engines in the OPPOSITE direction to prevent movement. And the wheels rotate beneath the plane, in order to prevent that.

Not gonna happen.

The plane will take off.

It's pretty simple - the plane takes off as it is airspeed that is required to gain lift, the mass is accelerated by the jet turbines and airspeed increases until takeoff velocity is reached. Velocity differential between the conveyor and plane is largely irrelevant.

It is worth mentioning that the conveyor/runway would need to be just as long as a normal runway.

Kyle

Anyway, here’s another one:

A boat is floating in a contained, fixed volume of water – eg a pool. At no time does any part of the boat touch the sides or bottom of the pool, it is always floating freely in the water. On the deck of the boat is a large steel girder weighing 5 tons. The water level in the pool is marked. Now the steel girder is pushed off the deck and into the water. Obviously the girder sinks to the bottom of the pool, totally submerged.

Does the water level in the tank rise, fall or stay the same?

The water level falls. The amount of water volume required to support or bouy the beam, while in the boat, is much more than the water volume the beam itself displaces.

Mike

So what if we tie the beam on the bottom of the boat ?? (meaning in the water but understill tied to boat)

The water level falls, the beam in the boat in order to float has to displace more volume than the the beam in the water which sinks.

Sharyn

Water level falls.

To keep afloat, the boat displaces 5 tons + boat weight of water.
Once the girder is in the water, it doesn't displace so much water (which is why it sinks) and the boat only displaces its own weight of watter (much less than at the start).

As a result, the watter level falls.

If the beam hits bottom and layes there, it doesn't matter, same result.

If the rope/chain is short and the beam is held in the water without touching the bottom, the boat (and beam) still need to displace their combined weight in water, so in this case, no change in water level.

As opposed to the case where the beam is tossed loose into the pool, the water level rises, for complex reasons. The amount of flotation the suspended beam generates, equals the amount of water it displaces. The boat floats lower in the water, because although it is relieved of supporting the part of the beam's weight that the beam's flotation supports, the boat is still supporting part of the beam's weight, and is displacing more water, raising the level in the pool. If the beam is sitting on the bottom, its weight, minus its flotation, is being supported by the bottom and is not pulling the boat down. In this latter case, the pool level falls, as opposed to when the beam was in the boat.