will it take off?

[JimParker]

Now, the original parameters state – “as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction”

I copied this just to frame the reference...

As thrust is applied by the engines, they begin to push the plane forward along the MCB.

The "no fly" crowd assumes that the "and matches it exactly in the opposite direction" statement means that the MCB can run at whatever speed it takes to counteract the thrust. From their viewpoint, the plane never moves, because the infinitely accelerating MCB can create enough drag (even with the low-friction bearings) to counteract the thrust generated by the engine/propeller or jet blast.

The "will fly" crowd assumes that the MCB instead can only turn at a speed (reactive to the plane's speed) that equals the plane's speed. In other words, if the plane is moving at 10 MPH, the MCB can only move at 10 MPH in the opposite direction. They view the problem from the opposite perspective - that the MCB can only respond after there is movement. Once movement occurs, the MCB accelerates to match that speed. However, the plane is already moving, and has more thrust than drag, and therefore will continue to accelerate. If it is moving, and accelerating, it will continue to accelerate to the point where it can fly.

I may be the only one in the entire world who can see this problem from both points of view. I don't know which is correct. If I look at the problem from each perspective, I can see clear evidence to support both theories (assuming the "magic" part to be true). Of course, in the real world, with real conveyor belts, the plane would unquestionably fly, because the large surplus of available thrust when compared to the drag produced by the wheels and bearings.

As a software engineer, if I were assigned to solve this problem, I would categorically state say that the problem definition is too ambiguous to allow for a definitive solution.

Kind of reminds me of an argument I witnessed in a calculus class in college: Professor challenged a brilliant student (not me!) to mathematically prove that parallel lines were possible. Student took the bait, and used the definition of parallel lines as his proof. From his perspective - case closed. The Professor, on the other hand, countered with calculus proof starting with two lines at 90 degree angle to each other. He measured off a point about 4 inches below from the intersection, and pivoted the vertical line clockwise around that new point. As he did so, he used calculus to measure the acceleration of the point of intersection as it moved to the right. He went on to show the detailed calculus proof that this point of intersection would accelerate to infinity, but that the lines would never uncross. As long as you measured from one point in time to the next point in time, there was an increment where the point continued to accelerate at even faster speeds... but you could never show a time interval where the lines magically uncrossed.

It was an exersize in exquisite mathmatics to the two of them, and an exercise in frustration for the rest of us, who just really wanted to know how to solve the nearly impossible homework problem that started the discussion! That was the day I decided I would never again take a class where the professor was a Jesuit monk...!

 

[jk96]

"Of course, in the real world, with real conveyor belts, the plane would unquestionably fly, because the large surplus of available thrust when compared to the drag produced by the wheels and bearings"

Jim, thanks. This is the problem I'm having. I'm operating in the real world, and the no flys keep adding "magic" to the original statement/question. It states the conveyor matches the speed of the plane. It will no doubt fly if applied in a real world application.

jk

 

[SPYDERLK]

Rback,

Yeah I know, the artwork could use a little work. Hopefully the illustration makes up for my stupidity on post #442 though.

One other note on this subject. One person I talked to had a good point. There have been several people converted from the "no fly" to the "fly side", but no one I know of has been converted from the "fly side" to the "no fly" side.

jk

That is because no one has been able to grasp the way in which the conveyor can apply counterthrust to the plane even tho its only connection to it is thru free spinning wheels.
Larry

 

[jk96]

Spyder,

I understand what they are trying to say. However, the plane will reach take off speed well before the speed needed from the conveyor to create this type of friction to counteract thrust. Again, the coveyor matches the speed of the plane. Are you trying to say that at 200 mph, the conveyor belt will produce enough counterforce to slow and stop the airplane.

jk

 

[RayH]

That is because no one has been able to grasp the way in which the conveyor can apply counterthrust to the plane even tho its only connection to it is thru free spinning wheels.
Larry

SPYDERLK, Do you think the plane will move forward?

 

[TomKioti]

Your on an icee lake and your trying to push your brother in a sled. Hard to do even by a strong fellow. Now move the ice in equal opposition to your push. Even harder.You will probably move backwards if you enter the yet un-applied downward effect of weight (mass) into the equation. Totally leaving flight out of the equation. I submit that you wold be hard pressed to have forward movement not to mention lift from the required V.

 

[RayH]

Your on an icee lake and your trying to push your brother in a sled. Hard to do even by a strong fellow. Now move the ice in equal opposition to your push. Even harder.You will probably move backwards if you enter the yet un-applied downward effect of weight (mass) into the equation. Totally leaving flight out of the equation. I submit that you wold be hard pressed to have forward movement not to mention lift from the required V.

Has nothing to do with it.
Does the plane move?
Your answer=NO.
If the plane doesnt move, the conveyor doesnt move, right?

I gotta go to bed.
Here it is in a nutshell (again).

If no plane movement = no conveyor movement (as per the question)
No conveyor movement = plain old normal runway
plain old normal runway + plane with engine running and brakes released = no forward movement.

It doesnt add up.

 

[TomKioti]

In the original crux the convyer keyed off the wheels of the plane. If I was wrong about my interpretation that I apolgize and the world is definately flat and NASA filmed the moon walk in Hollywood. Kidding. Other than that
I would submit... Why doesn't the Navy apply conveyers instead of catapults to invoke quick flight?
C'Mon Ray just one more!

 

[SPYDERLK]

SPYDERLK, Do you think the plane will move forward?

It depends on:
1) How much thrust it has, and
2) How much weight is left on the wheels at full engine thrust, and
3) How much power the conveyor has - 10x engine HP would do.

If there is too little weight on the wheels the conveyor will slip against them. Thus an effective counterthrust could not be developed and the plane would move forward.

With appropriate weight and thereby traction of the wheels against the conveyor surface, The plane could be forced to stand still or even move backwards until the wheels exploded.

 

[MossRoad]

Will-flys....

Read this article.
The Straight Dope: "A plane is standing on a runway. . ." No, it's not. Here's why.

IF you can build a conveyor that moves EXACTLY at the same speed as the airplane, but in the opposite direction, it WILL keep accellerating at the same speed as the airplane, but in the opposite direction. To quote the article:
"The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off. Welcome to BR #2."

Now I could say "what you fail to understand is..." but I won't. But I will say, open your mind to the possibility that if you could make a conveyor that would move EXACTLY at the same speed as the airplane, but in the opposite direction, it will keep accellerating to counter the planes accelleration in the opposite direction, preventing the plane from moving.

Read the article, please. Thanks.

Nice weather in the mid west for flying tonight, no? (rain, ice snow, wind, YIKES!!!)

 

[Egon]

That's freakin AWESOME!!!

Tom:

Not really. It's a pretty standard vehicle construction practice.

 

[patrick_g]

JK96, et al...

JK you are quite close to being a convert!

I have copied in all your assumptions even the ones about lengths and speeds and all are right on!

Here are the general ideas that I am assuming.
1. The plane is being propelled by thrust, i.e. jet engine, or by propellers.
2. The wheels on the plane are free spinning, no brakes applied, similar to a bicycle.
3. The plane is attempting to take off just like a normal takeoff, from a stop, full thrust applied until it begins to move with enough air speed to create the required lift to fly.
4. The plane needs 2000 feet to gain a speed of 200 mph, the required air speed on a normal runway for this particular model plane X to fly.
5. The MCB is 3000 feet long and the plane is at one end of the MCB.
6. The question “Will it take off, means will it leave the ground and remain in the air.

The following paragraph is also dead on!

Now, with these assumption made, the only way for our plane to take off and remain flying is for it to travel, or move, from the start of the MCB toward the opposite end of the MCB, all the while gaining speed until it reaches 200 mph with relation to the ground. If it remains in one place on the MCB, I think we can assume it will not take off and continue to fly.

Now, the original parameters state – “as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction”

The above is also correct.

As thrust is applied by the engines, they begin to push the plane forward along the MCB. The only way to stop the plane from moving is to apply an equal amount of force against the plane in the opposite direction. Now, lets factor in the movement of the MCB.

OK here is where you start to wander off course big time. This next paragraph of yours is based on unwarranted assumtions and assumes facts not in evidence and violates the conditions imposed by the original problem statement!

Our plane is moving forward at 10 mph using X amount of force from the engines.

How did the plane suddenly get to be going 10MPH???
Wouldn't it have to go 9MPH first and 8MPH before that and 7,6,5,4,3,2,1,,,0.0000001MPH before that? The MCB is a very good device and doesn't wait for the A/C to get to 10MPH before it starts to work. The MCB starts countering the planes attempt to move as soon as the motion is detected which can be at a very low value a tiny fraction of an inch per hour or whatever (it might use LASER interferometry or more advanced motion detectors)

If there were no traction between the plane's wheels and the MCB and if the plane's wheel bearings were frictionless, and if the tires had no rolling friction and the wheels and tires were massless (no weight at all) then the MCB could not counter the planes thrust.

I am not reproducing the rest of your argument because it does not matter since it assumes the plane can move forward and it can't.

NOTE: This is not a practical real world situation. If it were then of course the plane would take off (I'm a pilot, do you suppose I would want the plane to fail if it were avoidable???)

Apparently your statement regarding not understanding "MY THEORY" is a major part of the gap in our understandings. I will try to explain how the MCB can counter the thrust of those powerful Binford Belchfire 5000 Tim The Tool Man Taylor signature model engines!

As soon as the sensitive detectors of the MCB detect the smallest motion they start the conveyor moving in the opposite direction of the plane's desired motioin at just enough speed to match the plane so as to keep it motionless. You will agree won't you that it is possible to move the conveyor such as to stop the planes initial attempt to move, even a microscopic distance? If you have a hard time visualizing that then think that the pilot only gave just a little throttle to move just a very little bit. Then the MCB would move the conveyor just a little bit to exactly match the plane and the net result is the plane stands still with respect to the world.

Here is were the MCB borders on SciFi... As the pilot advances the throttle the MCB accelerates as required to prevent the A/C from advancing. OK, lets take this part slowly and carefully.

In the real world the conveyor would speed up as much as it could and if the tires didn't explode the A/C would roll forward and probably get enouigh speed and lift to take off.)

This however, is not the real world it is the world with an MCB in it.

Ever seen the parlor trick where you yank the table cloth out from under a fancy setting of fine china and crystal? The objects on the table move just a little bit in the directon of the moving cloth but their inertia holds them mostly in place. If you could yank the cloth the cloth fast enough (magic table cloth) you would not detect motion on the part of the table setting.

OK, I'm back... If you "yank" the conveyor belt backwards the plane tends to stand still but the wheels, due to friction between the conveyor and the tires, tend to rotate.

You CAN try this at home kids.

Place something with wheels on something you can yank out from under it. The wheels spin a lot more than the object moves! Why does the object move at all? Why don't the wheels just spin and the object stand still? The wheels have mass (inertia) and the energy required to spin them is supplied by the force on the conveyor you yanked out from under the wheeled gadget. This places a force on the (lets say toy car) and that is why the toy car moves in the direction of the moving conveyor. If the axles were frictionless, there were no roling resistance in the tires, and the whole rotating wheel assembly were massless (weighed nothing) then you could jerk the cloth out at any speed and the wheels would rotate but the toy car would remain motionless.

In your home experiment and in our thought experiment here the wheels are real wheels with mass, roling resistance, and bearing friction. When the MCB moves it puts a real force on the A/C tending to move it in the same direction as the MCB.

In your response you said something aboiut the friction not being too great. Well that depends on the speed of the MCB doesn't it? The faster you run the wheels the more force is transfered due to friction. Since there is no upper limit to the speed of the MCB, it runs as fast as is required to concel the thrust of the engines. All the frictions; roling resistance, bearing friction, whatever contribute to the force holding the plane back. The iinertia of the wheels helps hold the plane back too.

Real wheels have weight and spining them takes energy. Where does the energy come from and how is it transmitted to the wheels? Lets say the pilot firewalls the Binford Belchfire 5000 Tim The Tool Man Taylor signature model engines and produces X amount of lbs of static thrust. This levies a requirement on the MCB. It must continuously accelerate to faster and faster speeds at a rate that will impart a force on the A/C that tries to move it rearward. This force must equal the thrust of those powerful engines. This means for a constant thrust by the engines there must be a constant acceleration of the MCB.

Before the plane runs out of fuel the MCB will have accelerated to extreme speeds and real world tires and bearings and such will have long since failed but that is outside of the realm of the way the problem was stated.

If anyone has a SPECIFIC question they need answered to further their understanding of this explanation I would be happy to provide it here or by PM (your choice)

If someone can point out a serious flaw in this explanation, please do. I am equally pleased to lead, follow, or stay the **** out of the way! IF My description is not valid I sure would like a clear explanation of why not.

I do not represent all my explanation as being unique. Many other posters said essentially the same thing, over and over and over.

I will answer specific direct questions but there will be no additional cradle to grave analysis inflicted on the readers here.

I tried to do this straight forward. There is no tongue in cheek or otherwise humor intended (unless you laugh at the mention of Tim Taylor) Also I forgot to mention that the MCB is a Binford Industries model 7700 and coincidently is also a Tim Taylor signature model.

Pat

 

[HTWT]

There must be some politicians who take a simple statement and change it completely to make it sound complicated and then apply voodo to satisfy their belief. AS THE PLANE MOVES THE MCB MOVES IN THE OPPOSITE DIRECTION AT THE SAME SPEED. If the MCB moves to keep the plane stationary how can the MCB move and keep the original statement true that it moves in the opposite direction at the same speed as the plane. If the plane doesn't move the MCB cannot move. All the words in the world doesn't change the original fact that the MCB moves only if the plane moves. If the plane moves then it cannot be stationary. If it is stationary so is theMCB.

 

[jk96]

Hi Pat,

Thanks for the explanation on your reasoning. I don't dispute it at all. The only thing I am saying is that your changing the original question and statement.

Pat says - "Since there is no upper limit to the speed of the MCB, it runs as fast as is required to concel the thrust of the engines." and "Before the plane runs out of fuel the MCB will have accelerated to extreme speeds and real world tires and bearings and such will have long since failed but that is outside of the realm of the way the problem was stated."

The original statement and question says that

"the conveyor has a system that tracks the speed of the plane and matches it exactly"

not that the conveyor can accelerate at will to any extreme speed needed to counteract the movement.

Now, what is the speed of the plane? When I read "speed of the plane" in the original statement, I am assuming mph in relation to an object on the ground off to the side of the MCB, not the speed of the wheels. My assumption is that if the wheels are spinning 10,000 mph, but the plane is moving at 100 mph, then the actual speed of the plane is 100 mph. This is what a radar gun would show if we measured it. Therefore the MCB could not be reversing any faster than the "exact speed of the plane" 100 mph.

Because the conveyor must match the planes speed exactly, it cannot increase its speed infinatly. It can only move at the speed the plane is moving. A normal plane cannot go 10,000 mph, so if our conveyor has to match the speed of the plane in mph, it cannot reverse at 10,000 mph. If our plane has a top speed of 300 mph, the MCB will never be able to reverse faster than 300 mph. Not enough to counteract the thrust of the plane. This is my reasoning as to why the plane will indeed move.

I just tend to look at it in a real world application.

jk

 

[jk96]

Pat, I will give you this.
If the MCB can accelerate to infinity, then you are right. The plane will not move. However, if the MCB can move no faster than the top speed of the airplane, say 300 mph, then the plane will fly. Can we agree than we are both right -----and wrong, and put this thing to rest.

jk

 

[SPYDERLK]

Pat. Good. No problem with your thorough expanation the I can see worth noting. If someone picks up on a nomenclature issue it can be dealt with. The physics is all out there.
I do believe that a plane with conventional thrust to weight ratio could be held still by a very unconventional real world conveyor until the wheels or conveyor failed. This time would be short but infinitely longer than zero. The conveyor would cost more than the plane Im sure.
Larry

 

[TomKioti]

Yeah... what Pat said aaand Like i said before, "It ain't flying"
My head hurts.

 

[jk96]

All of the no fly theory suggests that the MCB can accelerate instantly to any speed and run faster than the plane, which cannot be the case if you stick to the original statement. You are changing the original statement. "The MCB matches the speed of the plane exactly" If you don't change this statement to match your justification, please explain to me why you are allowing the MCB to run faster than the plane.

Are you allowing the MCB to run faster than the plane? If so, how do you do away with "the MCB matches the speed of the plane EXACTLY" in the original statement.

jk

 

[patrick_g]

I*'m gonna go watch some icycles melt and bumpers rust. Someone PM me when this thread has been unused for a week.

Did I mention the problem statement was too loose and it requires unstated assumptions to support either contention?

Due to the higher than normal volumes of confusion we will respond to any inquiry within 480 to 720 hours. We apologize for the delay. Not to worry, specious answers are still being processed and will be completed as soon as possible. We will not continue to send out any updates via email.

Thank you for your patience.

Best wishes from Pat


Pat