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-

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.

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.)

A) You have nine cubes that look exactly alike. Eight have exactly the same weight, one weighs more than the others. The only tool you have to measure the weight of the cubes is a balance (like the Justice statue), but you are only permitted to use it two times.

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.

The aircraft will move in response to the thrust

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

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

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.

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.

And for all the pilots, what are the four left turning tendencies of a single engine, propeller driven aircraft?

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.

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.)

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

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

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.
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

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

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.

...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.

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?"

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.

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.

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?


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?

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

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.

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

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

Very interesting ideas bouncing around, but the plane would not get off of the ground, based solely on the scenario.

Thrust, although it creates a suction at the intake and moves air through the engine, that air in no way creates lift.

To cause a winged aircraft to rise there must be lift. Lift is created by the air moving over and under the wing. This is done (absent of other influences) by forward movement.

In the scenario, AS STATED, no adds or creative influences, there is nothing present to create lift. Not the engine thrust, suction or pressure, the engine isn't a player (an aircraft could be pulled forward with a tieline and it would have lift), Not the conveyor belt, it reacts or actually COUNTERACTS any forward movement created by the engine.

There is no mention of wind tunnels, updrafts, downdrafts or any other thing that is moving wind in order to create lift.

If it was possible, for the airplane in this scenario to take off, aircraft would not need any runways at all. They would crank there engines, throttle up and away they would go.

I have had my hands on a operating F-16 (on the ground) at full throttle while performing maintenance, and I never saw one starting to float upward beacause of the "thrust". They go airborne when they move forward.

In the given scenario, with the boundaries applied, and the conveyor counteracting any attempt at forward movement, it is scientifically, and physically inpossible for the airplane to fly.

The wheels counteract the reverse motion of the conveyor belt by rotating at twice the speed.

The airplane will fly.

The wheels counteract the reverse motion of the conveyor belt by rotating at twice the speed.

The airplane will fly.
Why would they rotate at twice the speed? The opening post states that the belt matches the speed of the plane. In other words, the plane is stationary and no lift is created.

Why would they rotate at twice the speed? The opening post states that the belt matches the speed of the plane. In other words, the plane is stationary and no lift is created.

The belt matches the speed of the plane, in reverse, (that is key)! If the plane moves the belt moves and simply spins the wheels faster! The engines propel the plane forward and the wheels get spun faster by the belt. The belt makes no change to the actual forward speed of the plane. In fact if the plane does not move, the belt does not move!

Remember that the belt just turns the wheels, it does not actually move the plane at all.

It will fly!

Mike

i think the plane would stay on the ground being that it would only reach half it's take-off speed before it runs out of runway.


Is there an actual answer?
Oh Yeah! Is there really a MENSA member in here?

i think the plane would stay on the ground being that it would only reach half it's take-off speed before it runs out of runway.


Is there an actual answer?
Oh Yeah! Is there really a MENSA member in here?

It would reach it's full take speed, not half speed. Only the wheels get turned by the belt, the planes speed is not affected. Wheels Roll!

Mike

I have had my hands on a operating F-16 (on the ground) at full throttle...Hi Joel and welcome to DVinfo! Always nice to meet a fellow servicemember, past or present. I recently retired from the USAF and have been HOTAS in many jet aircraft, including the Block 52 F-16, both on the ground and in the air.

Well, I thought this riddle had run its course a while ago even though not everyone was convinced of the correct answer: the jet will fly. Richard and others have given the answer, but here it is again just stated in different words. When the burner lights, it doesn't matter if the ground does start rolling backwards (a la our imaginary treadmill), the wheels will just spin faster and you'll be going somewhere fast as all that thrust propels the jet. There'll be weight on wheels until flying airspeed is attained and enough lift is generated by the wings to climb, but the wheels spinning ever faster against the receding ground would not (in the theoretical world) impart drag to the jet to counteract the thrust driving it forward.

OK .... the plane takes off, it reaches full speed in the same distance and takes off. It will do this even if the belt/runway moves at double the speed of the plane, because the belt exerts no force on the plane to hold it back. The faster the belt moves, the faster the wheels spin, but the engines keep pulling the plane through the air. As they do, they bring the airframe (and wings) to take-off speed, the wings create lift, the plane lifts off.

The wheels are spinning like crazy, because the plane is moving forward and the "ground" is moving backward, but the wheels are not holding the plane back, the plane is not prevented from moving forward like it would be restrained for a maintenance situation.

To demo this for yourself...
Tie some string to the front of a toy car (or toy plane with wheels). Bring this to the nearest treadmill.
Put the toy on the belt, hold the string and crank up the treadmill speed.
Note that regardless of how fast the belt is moving, you can still pull the toy forward as fast as you like by tugging on the string. You pulling on the string is the same force as the engines pulling/pushing air.

Yes, there's a little extra friction to overcome, but it's really almost nothing in relation to the thousands of pounds of force that these engines are capable of producing.


PS: Just wanted to add a very heartfelt thank you to Pete, Joel and every other past and present service member here!

I don't know how to make it any clearer.

The engines generate forward thrust ON THE AIRFRAME.

In order to counteract the forward thrust, and 'keep the plane stationary'... the 'conveyor belt' runway must exert EXACTLY THE SAME THRUST in the oposite direction ON THE AIRFRAME..

The belt only touches the airplanes wheels.

The wheels ROTATE in order to eliminate the drag of the runway. They do this at whatever speed the plane is moving forward and the ground is moving 'backwards' in relation to it.

IF the belt were moving backwards at three times the forward velocity of the airplane, it would still not exert enough 'drag' to stop the aircraft, the wheels would simply rotate faster. Sure, a little more heat and friction in the wheel bearings, but not enough to stop the aircraft.

The plane will fly.

And this is absolutely my LAST posting since I first posted the answer.

Wheels Roll!

Mike


is it that serious? lol. I thought the Conveyer belt was turning in the same direction as the the plane was to be moving.

I think this needs to be sent to "mythbusters."

Guys Guys Guys!

Come on! It is not that hard. The plane has wheels to roll freely on. If the belt goes one way the wheels roll twice as fast, if it goes the other the wheels don't roll at all. The plane still moves and takes off.

Now I'm like Richard, I'm done posting! :) :) :) And, don't say thank God! :)

Mike

The wheels counteract the reverse motion of the conveyor belt by rotating at twice the speed.

The airplane will fly.

Exactly! Had to think this through for a few minutes.

The wheels are, in effect, bearings that decouple the airframe from the conveyor belt.

Now, if the 747 had skids instead of wheels, that would be different....

I think this needs to be sent to "mythbusters."


Them guys will actually try that thing out too!

In the scenario as stated, the conveyor belt reacts to changes in speed of the aircraft, not the speed of the wheels turning.

The folks that have this fictitious airplane flying, (in this fictitous situation), create forward motion by adding outside influences.

If the conveyor belt adjusts it's reverse speed based on the speed of the wheels, or ground speed, thrust would absolutely overtake the actions of the conveyor, forward motion of the aircraft would occur, lift would be created, and the aircraft would fly.

If the conveyor belt adjusts it's reverse speed based on the speed of the aircraft, or airspeed, thrust would not overtake the reverse action of the conveyor, the plane would remain stationary, no lift would be created, and the plane would not fly.

So, depending on how you want this conveyor belt to measure speed determines whether or not it will fly.

The data provided in the scenario is incomplete. It is a hilarious read though, and a great way to kill some time. Although... I've got 2 one hour videos to produce, and a 2 minute commercial to finish before Friday...

Maybe I am not smart enough to understand the complexities of this very simple scenario as I am not a MENSA member, but I did ride the short bus to school on occasion.

Joel:

That has been my argument from day one on this thing.... but then I am not mensa either.... but I did stay at a Holiday Inn Express last night.....

Joel:

That has been my argument from day one on this thing.... but then I am not mensa either.... but I did stay at a Holiday Inn Express last night.....

I just think the discussion is fascinating. I love hearing different arguments and theories for or against the plane being able to fly.

It is amazing to me which question, riddle or whatever will become a widespread discussion on a forum or other internet sites.

It is apparent to me however, that this forum is attended by some extremely bright and intelligent people. This is an off topic discussion, the really amazing ones are the discussions about true applications in video and it's associated technologies that are accomplished on this forum.

The results in that arena are demonstrated hourly by the members of this forum.

I procured, tested, setup and began a video ministry at my church totally from the information gleaned from the amazing folks here at DVi. From cameras, to tripods to spider braces, steadicams, NLE's plug-ins or whatever, the best in the industry are represented right here. The invaluable contributions by you folks in other threads here saved me countless hours and lot's of dollars. (you can see the results after only 3 weeks at kbbc.org)

So all that don't see eye to eye on planes and conveyors, thanks for your other contributions on this forum.

Maybe I am not smart enough to understand the complexities of this very simple scenario as I am not a MENSA member, but I did ride the short bus to school on occasion.

Joel, it's not really about being super smart but just being familiar with physics. And physics is tricky because sometimes your intuition does not always match the physics. For example, if you asked someone what would fall faster in a vacuum, a hammer or a feather, most people would say the hammer because their intuition says it's heavier. But the physics say they will fall at the same rate and this was even proven by one of the Apollo astronauts on the moon.

In this case, it doesn't matter whether the conveyor is measuring the speed of the wheel or the plane. If you're interested, here's a scientific explanation:

To scientifically prove this, a physicist or an engineer would draw a "free body diagram". This is a diagram showing all the forces acting on the plane and in which direction. In this case, the forces in the horizontal direction are what we are interested in and there are only three forces acting in this direction: friction between the conveyor and the wheels, drag from the air and thrust from the jet engines. Friction and drag forces "point" backwards and thrust "points" forward.

So for the plane to remain stationary, the sum of the friction and drag forces would have to equal the thrust force. We already know thrust can overcome drag--a plane does this all the time otherwise it can't move forward. So the question is, can thrust overcome friction? Yes, it can. The plane already does this every time it moves on the ground. The friction is no more than if there was no conveyor there at all or if the conveyor was moving like crazy in the opposite direction. The wheels are just spinning faster. In fact, the friction gets less as the wheels start rolling (just like how it's easier to push a car after you get it going). The equation for friction is f = µN , where f is the friction force, µ is the coefficient of static or kinetic (rolling) friction and N is the normal force to the direction of the friction force (in this case basically the weight of the plane).

Hi All,

this thread is a good way to waste some time. Here's my two cent's anyhow.


In short, it depends.


If there really is no rolling resistance with these wheels then the conveyor has no effect as many have stated. The plane flies (assuming it could fly in the first place).

If there is rolling resistance and this is constant with the wheel speed, again the conveyor has no effect and the plane flies.

If for what ever reason the rolling resistance resistance is not independent of wheel speed (i.e. the wheels have tyres or any such real-world features) then it depends again.

Scenario 1), the increase in rolling resistance due to the wheels rotating at double speed is sufficient that the maximum engine thrust balances the overall resistance (rolling + air) before the plane can reach a ground speed (relative to still ground NOT conveyor and assuming no wind) prior to take off. Plane does not fly but carries on travelling a constant speed.

Scenario 2), increase in rolling resistance is such that the plane has sufficient thrust to reach lift off speed. Plane flies.

Both off these scenarios are depicted on the attached link.
http://www.physics.ox.ac.uk/users/smithdmd/planes.bmp


regards

David Smith

Not sure if I understood what you are saying (you must be MENSA), but I have contended that the Gravity force down, creates an a drag on wheels that has to be overcome. I think those that are saying the plane will fly are ignoring the tremendous force of 200 tons of weight on tires and bearings, etc., and the what is need to overcome that. When a plane is sitting on a tarmac, it takes a tremendous thrust just to get the plane rolling. It certainly doesn't. With the plane on a "moving" tarmac, you still have to overcome the same forces. The scenario states that when plane speeds up, conveyor goes faster to counter act. So the way I saw it, at rest, all force on plane is straight down, by virtue of gravitational pull. Then, as engine starts, the forces exerted change the general vector of forces on the plane forward, but then the belt speeds up to counter act the forward thrust, and the forward vector thrusts are counter acted, due to the reverse vector force applied by the sped up conveyor belt... According to the scenario then, at the point the engines reach maximum thrust, the conveyor belt will be counteracting that forward thrust using the downward force of gravity, and the reverse thrust of the conveyor. Airs speed required for take off should never be reached.....