A brain challenge that people get pretty passionate about

[JohnsonJones]

Vort's answer did get me thinking on something...not fully fleshed out yet, still thinking about it stage...Something else that occurred to me in relation to this part of the question...

Quote

The conveyor belt is perfectly programmed to roll the opposite direction the plane wants to move, in order to keep it stationary.  The conveyor belt is large, wide, and long enough to remain under the airplane and fully functional no matter what the airplane does. 

If it was rolling the opposite direction of the way the airplane wanted to move, which way are the wheels actually turning on the airplane?

In mythbusters they are rolling counterclockwise...because they are accounting for the ground...right?

But on a conveyor belt programmed to roll the opposite direction the plane wants to move....which way is the conveyor belt really moving and which way are the wheels turning?

If the conveyor belt is moving from front to back of the aircraft, it is not necessarily keeping an aircraft stationary, but if it is going from back to front at a fast enough speed to keep it stationary (meaning it overcomes it's forward momentum), which way are the wheels turning...clockwise.

It's counter intuitive...because our minds are made to think about the wheels like a car in relation to movement.  However, it is the body of the airplane that is being countered, not the wheels, which may mean, intuitively, the difference between the two questions (mythbusters and the original) is which way the conveyor belt actually needs to be turning.

The wheels are independent of the airplane, as Vort pointed out, which means that the airplane thrust really isn't going to be the effect it has on them that we think.  In order to keep the plane stationary, it may be more effective for the conveyor belt to actually be going from back to front (which is impossible as per the question), which means the wheels, which are independent of the actual aircraft would be moving as if in reverse, even as the aircraft engines are trying to go forward.  Hence the effect is basically countering the normal force of the jets on the wheels (the jets with enough force cause the plane forward which has the side effect of causing enough force for the wheels to move counterclockwise, so in theory, the conveyor belt would need to exert the opposite force to cause them to go clockwise???)

Just something that caught my thoughts when thinking on Vort's answer.

 

PS: To understand WHY this would work in this manner, you have to first realize the wheels in the question have no connection to the aircraft other than to connect it to the ground and reflect the forces that are acting upon them.  In this, the conveyor belt has to be moving at a speed that is fast enough to counteract the speed of the aircraft, which is why it would be moving quite quickly from tail to front to act upon the wheels which are turning in relation to the conveyor belt rather than the forces of pressure from the aircraft.  It is the AIRCRAFT that is being countered in this example, rather than the groundforce that is countered in that of mythbusters.  Because we are geared to thinking of it like a car to a small degree, it seems counter intuitive.  We think of it as if it were standing still, but if the wheels are the vehicle of conveyance to show force, than in fact, it would seem to me to keep the plane stationary if it were reacting to the plane rather than the ground speed and friction of the ground, in order to keep the PLANE stationary, rather than the groundforce/groundspeed, it would need to be acting in the manner of tail to front to counter the force acting upon the wheels which would move it from front to tail as it normally would.

The reason then why we might not see this as a practical physical example is because it would require twice as much energy to pull this off on a conveyor belt than it does now, or using a conveyor belt that worked like the mythbuster's idea instead.

Edited December 5, 2017 by JohnsonJones

[mordorbund]

The people advocating that the plane will fly are saying the treadmill is a red herring, so let's get rid of it just so we can consider the principles (if we get the same conclusions, then we can re-introduce the treadmill). Consider this:

Instead of a treadmill to counteract the wheels, I strip the plane of all landing gear. I then suspend the plane so that it hangs like a pendulum 100 American feet off the ground. I wait for it to get perfectly still and start the propellers or jets or whatever thrust-inducer I have.

1. Is there a force pulling the plane forward or does it remain still?

2. Is there a force pulling the plane up or does it remain still?

[dahlia]

My first thought is that there's no 'lift,' so the plane's not going anywhere.  

[JohnsonJones]

26 minutes ago, mordorbund said:

The people advocating that the plane will fly are saying the treadmill is a red herring, so let's get rid of it just so we can consider the principles (if we get the same conclusions, then we can re-introduce the treadmill). Consider this:

Instead of a treadmill to counteract the wheels, I strip the plane of all landing gear. I then suspend the plane so that it hangs like a pendulum 100 American feet off the ground. I wait for it to get perfectly still and start the propellers or jets or whatever thrust-inducer I have.

1. Is there a force pulling the plane forward or does it remain still?

2. Is there a force pulling the plane up or does it remain still?

Not positive I understand your questions...

As a NON-scientist, my thoughts would be...

Well, for the question you'd still need a force to replace the conveyor belt to provide an equal and opposite force to keep it stationary in regards to some of the considerations.  That's going to be a difficulty.  The original question was in relation to the airplane and not the wheels of the airplane having that force keep it stationary (relative to the aircraft, rather than via groundspeed, which is a pretty big difference).

without that equal force to keep it stationary, with enough thrust the plane 1. will go forward and with lift and speed it will 2. ascend.

At 100 American feet off the ground, the speed depends on the wingspan.  Ground effect normally affects the plane to around twice the distance of the wingspan on average, so unless it builds enough speed, it will speed up to move forward, start to climb, and then disastrously crash into the ground, unless the suspension remains in effect to prevent it from crashing until it meets minimum airspeed to meet rotate regulation for that airframe (we normally call for rotation, which is where we move the rudder to climb at a certain speed where it can climb and maintain altitude, ground effect means we can actually climb sooner than that, but it is not fast enough to really keep the plane afloat AFTER we leave ground effect)..

The way jets work is that it takes a whole bunch of matter and presses it into an itty bitty space and then pushes it out causing thrust.  It is thus pushing against the force that moves it forward, aka...it is being pushed forward.  This moves the aircraft or object forward.  Without resistance to it's movement (keeping it stationary, such as ground friction via the wheels or otherwise) it will move forward of it's own accord.  If you have enough thrust, you get a rocket (And it is on this principle that rockets move, except they normally have fuel inside of them and thus as it exerts energy out behind it, the equal but opposite principle moves it forward). 

Once it has the required amount of airspeed or airflow over the wings, the plane would indeed become airborne, if it has enough speed, it keeps on flying.  If it falls below the airspeed needed with the shape of the wing (which you can change to affect whether you climb or descend) than you descend. 

I suppose an answer could be, if the conveyor belt is of no consideration to the point that there is no friction or resistance to it whatsoever, it would be unable to actually keep the aircraft stationary at all.  We could say he wheels wouldn't turn, but the aircraft wouldn't move until it got enough thrust to suddenly burst up (sort of like what the navy does with it's navy catapults, but even moreso).  At that pressure though, at that sudden acceleration, with the speeds required to do that, it may tear the airframe apart...if you could get the speed up to do that.  It's an airframe afterall, not a rocketship. 

 

[Vort]

1 hour ago, JohnsonJones said:

Awe, I finally figured out what you did there.  I had to hit the quote button and make it all black to see it though.  Is there an easier way (I am an individual that is not quite as tech literate as many younger individuals may be, or even some older ones that are my age perhaps!)

As I wrote:

12 hours ago, Vort said:

Yes. To see my answer, select the text below.

My bad for not originally making the text bold, underlined, italic, and in purple.

1 hour ago, JohnsonJones said:

That isn't the answer to the question asked though.  That's the answer to the Mythbusters question in regards to ground speed....

On the contrary, that was exactly the question asked. To wit:

12 hours ago, NeuroTypical said:

An airplane sitting on a conveyor belt.  The airplane has either propellers or jet engines - it doesn't matter.  The airplane's wheels are just wheels - no brakes or motors on the wheels at all.

For purposes of this thought experiment, assume the airplane has sufficient gas and the conveyor belt has sufficient power, to reach a conclusive end to the experiment and arrive at a clear answer.

The conveyor belt is perfectly programmed to roll the opposite direction the plane wants to move, in order to keep it stationary.  The conveyor belt is large, wide, and long enough to remain under the airplane and fully functional no matter what the airplane does. 

The airplane's engines start working, and begin to do their thing, and the conveyor belt begins to do it's thing.  Does the airplane fly?

If you maintain that my answer did not answer NT's question, kindly point out exactly which part of the question I didn't answer. If you review the above problem closely, I'm confident you'll see that I responded to NT's question.

1 hour ago, JohnsonJones said:

The question has it worded so the conveyor belt is responding to the engines and airflow rather than the wheels, which as explained, means that this would move the plane backwards...thus meaning the plane needs to continually accelerate which makes it stationary, which creates an infinite loop.

Look again. The actual wording is, "The conveyor belt is perfectly programmed to roll the opposite direction the plane wants to move, in order to keep it stationary." As I already explained, the "in order to keep it stationary" part is irrelevant; physics does what physics does, regardless of the intent of a system designer. The conveyor belt can be programmed to sing the national anthem, for all the good it will do. Your interpretation of the question is flawed.

1 hour ago, JohnsonJones said:

That said, I would be delighted to be shown that my thoughts are wrong.

You are welcome, friend.

1 hour ago, JohnsonJones said:

It's like asking...if a car could accelerate at one mile per hour on an endless highway, could it go an infinite speed?

The speed of light: It's not just a good idea. It's the law.

[JohnsonJones]

@Vort since the Stationary part is part of the question, it cannot be tossed out...regardless of whether that's an impossibility as posed by the question or not.  In fact, when one considers it actually should move the same direction as the airplane wants to move, but in the opposite direction the wheels normally move when the plane moves them...it becomes even a bigger impossibility to a degree that one has to change the entire question to something else (like mythbusters did, where they made it regarding groundspeed rather than the aircraft itself).

The difficulty is the question relates to the airplane directly, rather than what it is trying to infer to, which is the groundspeed of the airplane.  The wording of the question specifies it is to keep the airplane stationary, when in fact, as it is stated, this is an impossible thing as per the question's wording, or we go into the infinite loop thing.

It is possible if the conveyor belt is going the same direction as the plane, but fast enough to move the wheels in the opposite direction of which the plane is directing them to move (in order for the conveyor to keep the plane stationary, rather than keeping the wheels at a stationary spot...it would need to counteract the forces on the wheels themselves, which is the force of the aircraft.  The assumption from your question, points out the flaw of the question to begin with. 

Many assume that because the wheels are turning counterclockwise, it works like a car.  However, when we realize that the question isn't about the wheels at all, you have to view it as what is causing those wheels to move in that direction.  It is a secondary effect of the real forces acting upon the airplane itself.  That force causes the wheel to turn in a counterclockwise motion. 

If we put it on a conveyor belt that simply reflects that motion, it may prevent the wheels from moving forward (but still rotating) but it is not actually countering the forces on the aircraft itself (the thrust, which is greater than the wheels, hence, even if the wheels are stationary, the aircraft itself may actually be moving forward in the mythbuster's example).

It may mean the wheels are actually moving at twice their normal speed though. 

To counter the actual force of the aircraft, you need the conveyor belt moving the same direction as the airplane if it's actions are upon the wheels...as it is the wheels that are the conveyance of that force/friction.  Hence, counter intuitive to what we see in a car, the belt needs to go in the same direction as the airplane at double the speed, thus moving the wheels in the opposite direction (aka with the opposite force) than what the aircraft is directing upon them if we are to even hope to dream of seeing the conveyor belt do as the question asks.  Of course this all presupposes that friction is going to be effective in keeping all this going on.

But the question makes it impossible to do this...

The question itself is flawed though...

Much like the car question...because when we discount physics the simple answer to the car question in any situation would be...just add +1...as in the speed of light+1...OR if one says it is infinite...ask at what point does it become infinite then?

Edited December 5, 2017 by JohnsonJones

[Vort]

One final attempt to explain this to JJ and anyone else who might not believe the given answer:

Let's pretend we have a small Cessna with a takeoff speed of 50 mph, sitting on the tarmac in perfectly still (windless) weather. In this case, the tarmac is actually a gigantic treadmill or conveyor belt, set up to run in the opposite direction to the aircraft's direction of travel. (Or the same direction, if you think that makes a difference. Whatever.) Please note the following two properties of the Cessna:

1. It is propeller-driven. That is, it has a propeller on the front that turns fast, pushing air back over the craft. This is an action-reaction force the moves the plane forward.
2. The wheels on the Cessna are neither powered nor braked. They're just there, spinning freely as the aircraft moves relative to the tarmac.

We also note, in passing, that obviously the aircraft will fly when the air passes over its wings at a rate of at least 50 mph. Slower than that, the airplane doesn't fly. Faster than that, the airplane flies. Simple. Because there is no wind at all, the only air passage over the wings that can take place will happen when the aircraft moves relative to the ground -- NOT relative to the treadmill tarmac, but relative to the ground beside the tarmac. (Or relative to the air, if you prefer.)

So then, what happens as the Cessna attempts to take off? Let's investigate it, step by step:

1. The aircraft is motionless on the tarmac, pointing (let's say) east. The tarmac, which remember is a treadmill, is also stationary.
2. The aircraft starts up its propeller and begins moving eastward at, let's say, 5 mph. In response, the tarmac moves westward at 5 mph. The aircraft is now traveling eastward at 5 mph, but its wheels are spinning at the rate they would spin to travel at 10 mph. That is, the wheels are spinning twice as fast as the airplane is traveling.
3. The aircraft speeds up to approach takeoff speed. It is now traveling at, let's say, 30 mph. The tarmac is traveling the opposite direction at 30 mph. The aircraft wheels are spinning at an equivalent of 60 mph.
4. The aircraft reaches 50 mph, with the tarmac moving backward at 50 mph and the wheels on the Cessna dutifully spinning at an equivalent rate of 100 mph. At this point, the aircraft lifts off.

Hope that's clear. If not, I'm afraid I don't have much else in the way of explanation.

Edited December 5, 2017 by Vort

[Vort]

3 minutes ago, JohnsonJones said:

The wording of the question specifies it is to keep the airplane stationary,

Yes. That was the intent of the conveyor belt programmer. But his intent is stupid and ultimately irrelevant. The conveyor belt cannot keep the aircraft stationary, no matter what the programmer was trying to accomplish. As I have already said a couple of times, the question is deceptively worded, but once you get around the deceptive wording, the correct answer is clear.

Equivalent question:

What if you wanted to drive away in a car, but whenever you pressed the gas pedal, I blew my nose so that your engine wouldn't work. Would you ever possibly be able to drive away? Or would my nose-blowing keep you stranded?

As mordorbund (I think) pointed out, the whole treadmill thing is a red herring. The treadmill cannot keep the aircraft stationary in the given situation. It's irrelevant. The only reason this seems like a brain-teaser is because if you substitute "automobile" for "airplane", then the car really and truly won't get anywhere on the treadmill. But it's not a car, pushing its wheels against the ground. It's an aircraft, using action/reaction with its propeller or jet engine shoving air backward. The treadmill or conveyor belt has nothing to do with it.

[JohnsonJones]

26 minutes ago, Vort said:

One final attempt to explain this to JJ and anyone else who might not believe the given answer:

Let's pretend we have a small Cessna with a takeoff speed of 50 mph, sitting on the tarmac in perfectly still (windless) weather. In this case, the tarmac is actually a gigantic treadmill or conveyor belt, set up to run in the opposite direction to the aircraft's direction of travel. (Or the same direction, if you think that makes a difference. Whatever.) Please note the following two properties of the Cessna:

1. It is propeller-driven. That is, it has a propeller on the front that turns fast, pushing air back over the craft. This is an action-reaction force the moves the plane forward.
2. The wheels on the Cessna are neither powered nor braked. They're just there, spinning freely as the aircraft moves relative to the ground.

We also note, in passing, that obviously the aircraft will fly when the air passes over its wings at a rate of at least 50 mph. Slower than that, the airplane doesn't fly. Faster than that, the airplane flies. Simple. Because there is no wind at all, the only air passage over the wings that can take place will happen when the aircraft moves relative to the ground -- NOT relative to the treadmill tarmac, but relative to the ground beside the tarmac. (Or relative to the air, if you prefer.)

So then, what happens as the Cessna attempts to take off? Let's investigate it, step by step:

1. The aircraft is motionless on the tarmac, pointing (let's say) east. The tarmac, which remember is a treadmill, is also stationary.
2. The aircraft starts up its propeller and begins moving eastward at, let's say, 5 mph. In response, the tarmac moves westward at 5 mph. The aircraft is now traveling eastward at 5 mph, but its wheels are spinning at the rate they would spin to travel at 10 mph. That is, the wheels are spinning twice as fast as the airplane is traveling.
3. The aircraft speeds up to approach takeoff speed. It is now traveling at, let's say, 30 mph. The tarmac is traveling the opposite direction at 30 mph. The aircraft wheels are spinning at an equivalent of 60 mph.
4. The aircraft reaches 50 mph, with the tarmac moving backward at 50 mph and the wheels on the Cessna dutifully spinning at an equivalent rate of 100 mph. At this point, the aircraft lifts off.

Hope that's clear. If not, I'm afraid I don't have much else in the way of explanation.

You just ignored the question though.

The question wasn't what you just asked.  The tarmac in this instance is simply reflecting rather than counter acting the wheels.  There is NO FORCE acting upon the aircraft itself.  It is acting upon the wheels, and in this instance, it is not actually presenting a countering force to keep the aircraft stationary as the question requires.  The wheels may be in a stationary position, though rotating, but the aircraft is moving forward (technically...this is all hypothetical).

In an airplane, this principle can also be applied, but it's different than what everyone thinks.  A plane is easier to take off when it has a headwind, aka...a force moving the opposite direction of the plane itself...however...it becomes much harder at times with a tailwind.  Just as a headwind will reduce the speed needed for rotate speeds, a tailwind can increase it.  It works the opposite of what it seems like.

The conveyor belt is not a red herring though, as per the question, as it is required to keep the plane stationary.  If does NOT keep the plane stationary, then the question is flawed, as that is specified as part of the question.

By tossing it out, effectively one ignores the question and creates their own question (as mythbusters did) rather than trying to adhere to the question in the first place...flawed as it may be.

From what I understand/have seen, that said the best chance one may have of showing this idea in principle would be via a plane taking off on a river and showing the effects, but as the seaplanes do not have wheels and the ensuing forces upon them to act upon them in the way an airplane or conveyor belt does, it's a very different approach.  HOWEVER...if one is flying a seaplane against a strong river current, it is possible that the some planes might not be able to take off unless their is a strong headwind to aid them...which is more of what actually happens in real life as opposed to this fantasy question.  On the otherhand, a plane going downstream with a strong tailwind may have similar problems (in which instance, it is better to wait till the wind dies and then do a downstream takeoff), but going downstream will make it so the plane can gain speed faster and hopefully get the airspeed it needs to take off which the upstream takeoff can affect.

When one sees the effect a river current (aka, replacement for a conveyor belt) has in real life on take offs and landings, one can see that something that actually can reduce the forward movement of a plane and the air flow over it's wings has upon takeoff and landing, the answer would be in a true situation where a plane has no forward momentum, it will not be able to take off as per the river trying an upstream takeoff which doesn't have the engine power to build up the airspeed to take off.  However, that same airplane may be able to take off with a downstream takeoff (of course, one should be cognizant of obstacles in the water, that the distance may be further than on still water, etc...).

Of course, in such an instance the best place to plan landings will be on a lake...where you don't have to worry as much about upstream or downstream...etc.

Edited December 5, 2017 by JohnsonJones

[Vort]

5 minutes ago, JohnsonJones said:

The tarmac in this instance is simply reflecting rather than counter acting the wheels.

The problem never specified that the tarmac counteracted the wheels (or anything else). It simply said that the tarmac ran in the opposite direction to the airplane in an effort to counteract the airplane movement. That effort is futile, so it's irrelevant.

"The conveyor belt is perfectly programmed to roll the opposite direction the plane wants to move, in order to keep it stationary."

Note the final phrase in bold. It does not specify that the aircraft is actually kept stationary, only that the conveyor belt is programmed to go in the opposite direction in order to keep the plane stationary. It fails. Your requirement that the airplane actually be kept stationary goes well beyond what the problem specifies.

In fact, your interpretation of the question is purely imaginary; as you yourself admit, it's impossible. So as long as we're making up purely false things, we might as well say that the airplane creates a wormhole and travels to another galaxy.

Edited December 5, 2017 by Vort

[JohnsonJones]

Here's a beautiful clip of a plane taking off of a moving tarmac...aka...a river...

 

[JohnsonJones]

8 minutes ago, Vort said:

The problem never specified that the tarmac counteracted the wheels (or anything else). It simply said that the tarmac ran in the opposite direction to the airplane in an effort to counteract the airplane movement. That effort is futile, so it's irrelevant.

 

Quote

The conveyor belt is perfectly programmed to roll the opposite direction the plane wants to move, in order to keep it stationary.  The conveyor belt is large, wide, and long enough to remain under the airplane and fully functional no matter what the airplane does. 

. It's part of the question...

Ignoring that it is part of the question only changes what you are asking, not the question itself.  I'd say it's the vital part of the question because it makes it impossible for the question to actually be really completed as asked.  It's easy to toss it out, but then it's no longer the same question anymore.

I agree the question is flawed, and thus it creates an impossibility...but if one changes it so that it's no longer an impossibility, it is no longer the same question.

Of course, I'm always open to see physical evidence of airplanes taking off in a situation as posed by the question, thus far, though I've posted multiple evidence of planes being able to take off under the current laws of physics, I haven't seen one that is posted in regards to the ones who say the plane would take off regardless.  I've seen the mythbusters solution, but there were several problems in that (question was different, ultralight plane that could have taken off with mere inches with how much thrust the pilot was giving it, doubling the thrust takeoff relavant to what was needed...etc...etc..etc)

 

Edited December 5, 2017 by JohnsonJones

[Rob Osborn]

I think you both agree on the same thing. I think we all agree that in order for a plane to take off it has to have enough forward movement to generate enough airspeed around its wings to create lift iregardless of what the ground is doing.

[Vort]

1 minute ago, JohnsonJones said:

I'd say it's the vital part of the question because it makes it impossible for the question to actually be really completed as asked.  It's easy to toss it out, but then it's no longer the same question anymore.

JJ, the question must be interpreted in a manner that it can be answered. To interpret the question as you have done not only does violence to the wording, it renders the question itself meaningless. A moving tarmac could not affect an airplane rolling on frictionless wheels, as you yourself have already agreed. Thus, your answer is as wrong as my answer, because no answer is correct -- the question is unanswerable, because it's meaningless. You might as well be asking about how a blue genie might solve a Rubik's cube and then claim that YOUR answer is the REAL answer. Nonsense.

My interpretation of the question does no violence to the wording of the question, and it does result in a meaningful and answerable question. Thus, I submit that my interpretation is the correct interpretation of the two, if for no other reason than my interpretation allows the question to be processed.

[Just_A_Guy]

17 minutes ago, JohnsonJones said:

You just ignored the question though.

The question wasn't what you just asked.  The tarmac in this instance is simply reflecting rather than counter acting the wheels.  There is NO FORCE acting upon the aircraft itself.  It is acting upon the wheels, and in this instance, it is not actually presenting a countering force to keep the aircraft stationary as the question requires.  The wheels may be in a stationary position, though rotating, but the aircraft is moving forward (technically...this is all hypothetical).

In an airplane, this principle can also be applied, but it's different than what everyone thinks.  A plane is easier to take off when it has a headwind, aka...a force moving the opposite direction of the plane itself...however...it becomes much harder at times with a tailwind.  Just as a headwind will reduce the speed needed for rotate speeds, a tailwind can increase it.  It works the opposite of what it seems like.

The conveyor belt is not a red herring though, as per the question, as it is required to keep the plane stationary.  If does NOT keep the plane stationary, then the question is flawed, as that is specified as part of the question.

By tossing it out, effectively one ignores the question and creates their own question (as mythbusters did) rather than trying to adhere to the question in the first place...flawed as it may be.

From what I understand/have seen, that said the best chance one may have of showing this idea in principle would be via a plane taking off on a river and showing the effects, but as the seaplanes do not have wheels and the ensuing forces upon them to act upon them in the way an airplane or conveyor belt does, it's a very different approach.  HOWEVER...if one is flying a seaplane against the current, it is possible that the some planes might not be able to take off unless their is a strong wind to aid them...which is more of what actually happens in real life as opposed to this fantasy question.  On the otherhand, a plane going downstream with a strong tailwind may have similar problems (in which instance, it is better to wait till the wind dies and then do a downstream takeoff), but going downstream will make it so the plane can gain speed faster and hopefully get the airspeed it needs to take off which the upstream takeoff can affect.

When one sees the effect a river current (aka, replacement for a conveyor belt) has in real life on take offs and landings, one can see that something that actually can reduce the forward movement of a plane and the air flow over it's wings has upon takeoff and landing, the answer would be in a true situation where a plane has no forward momentum, it will not be able to take off as per the river trying an upstream takeoff which doesn't have the engine power to build up the airspeed to take off.  However, that same airplane may be able to take off with a downstream takeoff (of course, one should be cognizant of obstacles in the water, that the distance may be further than on still water, etc...).

Of course, in such an instance the best place to plan landings will be on a lake...where you don't have to worry as much about upstream or downstream...etc.

What threw me at first, was the assumption that a plane would handle like a car—viz., that the forward thrust comes primarily from the friction of the spinning wheel with the ground.  But in a plane the thrust comes from the props/engines; and the wheel is just “along for the ride”—any friction from a reverse-operating conveyor belt would be relatively minor.

I’m not sure a seaplane is a valid comparison here; because until the seaplane gets up to hull speed and comes up on its plane, it’s basically a boat and factors like current and wave action will act on the boat in ways/magnitudes that the ground would never act on a free-wheeling tire.  Once the craft comes up on its plane and stops acting like a displacement hull, it won’t care about the direction/velocity of the water currents beneath it any more than a water skier does.

Edited December 5, 2017 by Just_A_Guy

[JohnsonJones]

18 minutes ago, Vort said:

JJ, the question must be interpreted in a manner that it can be answered. To interpret the question as you have done not only does violence to the wording, it renders the question itself meaningless. A moving tarmac could not affect an airplane rolling on frictionless wheels, as you yourself have already agreed. Thus, your answer is as wrong as my answer, because no answer is correct -- the question is unanswerable, because it's meaningless. You might as well be asking about how a blue genie might solve a Rubik's cube and then claim that YOUR answer is the REAL answer. Nonsense.

That is probably the REAL correct answer...as the question is basically a nonsense question probably originally asked by someone (not NT, I'm talking whoever originated this question) who doesn't seem to understand anything about aircraft to begin with.

However, as I pointed out, I'm open to be shown I'm wrong.  I've shown myriads of takeoffs of various aircraft with the current laws of physics (including one on a "moving"  tarmac as one could put it...or a river).  If the answer to the question is that indeed, a plane could take off under those conditions, as a pilot, I am interested in seeing the video of a heavy plane that normally requires a substantial distance to take off, completing such a takeoff in such a circumstance (as opposed to the ultralight aircraft or models that mythbusters utilized).

If it is such an easy thing as people propose, we should have many examples of this already to match the equal number of debaters who think it is possible.  I think my side has a TON of examples of how take offs normally work, as I've posted several of these clips from youtube as examples.

18 minutes ago, Just_A_Guy said:

What threw me at first, was the assumption that a plane would handle like a car—viz., that the forward thrust comes primarily from the friction of the spinning wheel with the ground.  But in a plane the thrust comes from the props/engines; and the wheel is just “along for the ride”—any friction from a reverse-operating conveyor belt would be relatively minor.

I’m not sure a seaplane is a valid comparison here; because until the seaplane gets up to planning speed it’s basically a boat, and factors like current and wave action will act on the boat in ways/magnitudes that the ground would never act on a free-wheeling tire. 

True, but if you look at the question it is not about the wheel position, it specifies it is to keep the AIRPLANE stationary.  It is not there to keep the wheel position stationary. 

As it is a force upon the airplane, I'd think if anything was possible, it would be the river as a replacement for the conveyor belt.  It's not a perfect example though, as I've already admitted, the question itself is deeply flawed.  There is a remarkable difference between the question and the river as an example (as I've already noted, the entire reason the wheels move counterclockwise is because they are reacting to the plane's thrust upon them as a side effect, meaning they are rolling to accommodate the airplanes forward movement, not moving the plane themselves.  Hence, a counter force would therefore need to move them the opposite direction, or clockwise, to actually counter the plane's force upon them to keep the plane's force itself stationary, but this is also impossible as per how the question is worded).

 

PS: Since many utilize a car to understand the force of the engines reactions on the wheels, rather than seeing them differently...I just thought of a car example that may explain somewhat the airplane principle.  This is from me, a non-car type guy though.  Imagine that you have a frontwheel drive.  The rearwheels thus react to the force applied to them in reaction to the front wheels moving counterclockwise.  This front wheel tire movement causes the car to move forward.  The rearwheels also move forward.

What direction would the rearwheels need to move to counter act the movement of the frontwheels and keep it stationary?

This can hopefully illuminate why, to create an opposing force for the aircraft wheels, if one is using that medium to keep the aircraft stationary (rather then the wheel position) one needs to move the conveyor belt in the direction from tail to front rather than from front to back.  Not a perfect example, I admit, but maybe one people can grasp a little easier???

Edited December 5, 2017 by JohnsonJones

[Just_A_Guy]

2 minutes ago, JohnsonJones said:

True, but if you look at the question it is not about the wheel position, it specifies it is to keep the AIRPLANE stationary.  It is not there to keep the wheel position stationary. 

As it is a force upon the airplane, I'd think if anything was possible, it would be the river as a replacement for the conveyor belt.  It's not a perfect example though, as I've already admitted, the question itself is deeply flawed.  There is a remarkable difference between the question and the river as an example (as I've already noted, the entire reason the wheels move counterclockwise is because they are reacting to the plane's thrust upon them as a side effect, meaning they are rolling to accommodate the airplanes forward movement, not moving the plane themselves.  Hence, a counter force would therefore need to move them the opposite direction, or clockwise, to actually counter the plane's force upon them to keep the plane's force itself stationary, but this is also impossible as per how the question is worded).

I read that the conveyor belt was *programmed* to keep the plane stationary; but programming can’t make magic clamps sprout of the belt and seize up the tires; not can it disable the thrust generated by the props/jets themselves.  All the programming can do is adjust the speed of the conveyor belt in an attempt to counteract the plane’s forward motion—but all that does, in turn, is cause the plane’s wheels to spin faster as the plane itself moves forward unchecked.  If plane moves forward at 100 mph and belt spins backwards at 100 mph to compensate, plane continues to move forward at 100 mph and the wheels are forced to spin at 200 mph to compensate.

[JohnsonJones]

1 minute ago, Just_A_Guy said:

I read that the conveyor belt was *programmed* to keep the plane stationary; but programming can’t make magic clamps sprout of the belt and seize up the tires; not can it disable the thrust generated by the props/jets themselves.  All the programming can do is adjust the speed of the conveyor belt in an attempt to counteract the plane’s forward motion—but all that does, in turn, is cause the plane’s wheels to spin faster as the plane itself moves forward unchecked.  If plane moves forward at 100 mph and belt spins backwards at 100 mph to compensate, plane continues to move forward at 100 mph and the wheels are forced to spin at 200 mph to compensate.

You're still looking at it from a groundspeed perspective though, which is forces that are acting upon the wheels.  In that light, if it is going to actually be a force to keep the PLANE stationary rather than simply the wheel positions, it will need to spin counter the direction of the wheels when they normally would accommodate aircraft ground movement.  However, if the plane is trying to move them counterclockwise at 100 mph, and the belt is trying to move them clockwise at 100 mph, it then falls to how much friction is on the belt.  With enough friction, the wheels might not move at all if they can directly be channeled into utilizing all of the aircrafts force to try to move them.  Of course, that's nonsense, but if we are talking about keeping an aircraft stationary via a conveyor belt, then we should assume the power of friction is perfect in this instance, and the forces acting upon the wheels by the plane to move them one direction, are equally matched by the conveyor in the equal but opposite force for the other direction.

Aka, the situation in this case would resemble a plane on the ground which has no tire movement at all...at which point will it take off?

[Just_A_Guy]

15 minutes ago, JohnsonJones said:

You're still looking at it from a groundspeed perspective though, which is forces that are acting upon the wheels.  In that light, if it is going to actually be a force to keep the PLANE stationary rather than simply the wheel positions, it will need to spin counter the direction of the wheels when they normally would accommodate aircraft ground movement.  However, if the plane is trying to move them counterclockwise at 100 mph, and the belt is trying to move them clockwise at 100 mph, it then falls to how much friction is on the belt.  With enough friction, the wheels might not move at all if they can directly be channeled into utilizing all of the aircrafts force to try to move them.  Of course, that's nonsense, but if we are talking about keeping an aircraft stationary via a conveyor belt, then we should assume the power of friction is perfect in this instance, and the forces acting upon the wheels by the plane to move them one direction, are equally matched by the conveyor in the equal but opposite force for the other direction.

Aka, the situation in this case would resemble a plane on the ground which has no tire movement at all...at which point will it take off?

So you’re saying that instead of the belt traveling backwards, it would need to travel forwards at some precisely-calibrated rate.

The trouble is that even if you were able to lock the wheels that way, the net motion of the whole works would still be forward (the wheels are locked to a specific spot on the conveyor belt, not to a fixed point in space), and (assuming you aren’t in a wind tunnel) you’ve still got airflow over the wings; so as the engines continue to build thrust and the conveyor belt has to move forward faster to keep the wheels locked up, the plane eventually reaches takeoff speed whether the wheels are spinning or not.  So if I’m interpreting right, we’ve just come up with a high-tech, low-efficiency catapult launching system.

Edited December 5, 2017 by Just_A_Guy

[JohnsonJones]

39 minutes ago, Just_A_Guy said:

So you’re saying that instead of the belt traveling backwards, it would need to travel forwards at some precisely-calibrated rate.

The trouble is that even if you were able to lock the wheels that way, the net motion of the whole works would still be forward, and (assuming you aren’t in a wind tunnel) you’ve still got airflow over the wings; so as the engines continue to build thrust and the conveyor belt has to move forward faster to keep the wheels locked up, the plane eventually reaches takeoff speed whether the wheels are spinning or not. 

Does it?  How many planes that are not moving have you seen take off?  Or, that have absolutely stationary wheels that are not moving in the least...take off from the ground?

It's counter intuitive to those who look at it the same as a car, but a plane does not MOVE like a car.  The wheels are merely reflections of the force being acted upon them (much like the rear wheel of a car).  The ONLY reason they are rolling counterclockwise when a plane moves forward is because of the forward movement of the plane itself.  It is not the wheels that are causing this.  It is a reflection of the plane's force upon them. 

Hence, if you move the conveyor belt in that same direction, all it is doing is amplifying this.  This has an effect on the wheels and groundspeed, but NOT on the aircraft itself or at least that would be the theory (closest anyone's come to proving that is the mythbusters...and there are still problems with their example), and hence the aircraft could still take off. 

However, to actually keep the aircraft stationary, you need something that counteracts the force of the aircraft itself.  Hence, if you are using the wheels and conveyor belt as a reflection of that force, it needs to be a force that keeps the wheels with an equal force the opposite direction.  Hence, as the aircraft tries to move the wheels counterclockwise due to it's force, you would have an equal but opposite force moving them clockwise.  Since they are not directly controlled by an engine (unlike the front wheels of a car in a frontwheel drive which ARE directly controlled), they are freely able to spin one way or the other.  Hence, in THEORY, an equal but opposite force upon them to move the opposite direction than the direction of the force the airplane is trying to make them move is doable to the point of basically having wheels that do not turn one way or the other, for as soon as the force of one acts upon them to spin one way, the force of the other acts upon them to spin the exact opposite way...hence, they are kept stationary...reflecting that the aircraft itself is at a stationary movement of force.

As I said, how many planes that do not have their wheels moving have you seen take off?

I admit, I have seen a harrier and a helicopter and even an osprey...but for normal aircraft that do not have jets or props that go vertical, I haven't seen a single one...yet...at least.  Not even seaplanes.

 

PS: Of course, it should be noted, the above example would require perfect friction in the situation, which is improbable, but would be required to see if this would actually hold up.  Or we could simply see the normal situation where we have the wheels as motionless but the engines at full throttle (this happens regularly on with some aircraft during the preflight checks) and yet the plane doesn't fly to see what would happen.

PPS: In regards to the mythbusters thing, where it was NOT countered by an equal force on the conveyor belt but an additive force, all this means is that the wheels may turn twice the speed before take off 2x=x rather which actually means the aircraft would be moving forward at an even great speed by some theories, rather than being stationary at all.  This is the big difference between what they did and what the question asks.  The mythbusters kept the wheels stationary relative to the ground, but the aircraft still was moving as expressed by the wheels as they were moving 2x or more faster than normal.  This is why you'd need something that acts as a counter to the aircraft's force to actually keep it stationary, rather than adding to it's momentum that it already has.

Even then, this is probably even more hotly debated (as most seem to understand the question as a CAR works rather than an airplane...), if the treadmill was able to actually move fast enough to also account for the airplane's thrust...would the plane be able to take off?  It's actually somewhat based on this idea that with the additive extreme force idea that navy catapults work (but they still need a little runway...though it is lessened).  This can also go into various areas, but normally devolve to several postions.  One where people basically say the test is impossible as no machine will ever be able to match the plane exactly, or if it does you go into the infinite loop scenario.  Others where they use mythbusters (which is actually a rather poor example, but one of the only ones out there except for the navy catapults) as an example and proclaim it would work.  Then you have others that take the stance that it only worked for reasons such as I did, where it doesn't take into account the thrust vector of the aircraft, only the reflective vectors of the secondary nature, and hence there is no equal but opposing force to the takeoff.  If the force could take into account the full force of the aircraft rather than just the reflective vectors of the wheels, and truly stay equal, the aircraft would not be able to take off, but it may end up, once again in that infinite loop that I discussed because the question itself is flawed, even to that degree. 

That's the one that most people normally hotly debate, but ignore that the question is about the airframe and aircraft being stationary, not simply the wheel position relative to the ground.  In fact most of this thread views it more of a car in action than an airplane in action, where as the question is relative to stopping the aircraft motion...not it's wheels.  The wheels though, are a reflection of the aircraft momentum, not what is actually moving it.  Hence a stationary aircraft will have stationary wheels.  Wheels that are rotating...are a form of movement/force and a reflection of an aircraft that has movement rather than being stationary.

PPPS: Of course, the best evidence is putting practice into reality.  If one truly believes the conveyor belt will work, then put it into practice (if one could even build it) and prove it with reality rather than postulating about it!  Thus far, I have read debates on this, but no one who says...yes the plane will fly...in either of the questions really have put it to the test other then the mythbusters, and their experiment was full of problems (which is par for the course...mythbusters in many ways try to show science, but aren't very scientific at times with how exact or precise their measurements need to be in some instances...this would be one of them from what I saw, they are still fun to watch though).

Edited December 5, 2017 by JohnsonJones

[pam]

17 hours ago, JohnsonJones said:

I'm betting  anyone who claims an airplane can go up in the air while at a standstill is also not a pilot...I could be wrong...

May I introduce  you to the V-22 Osprey?   Or the F-35B?  

[JohnsonJones]

6 hours ago, pam said:

May I introduce  you to the V-22 Osprey?   Or the F-35B?  

Good one, I've seen the Osprey (that's what you meant...right?).  It operates on the horizontal lift principle.

Of interest, the mythbusters answered a question, but most pilots I know question the results.  The know fact is that without ground movement, there is normally no air movement.  This is because airflow is needed in order to get lift.  If you have no lift, you cannot fly.  This is the simple reason for the aircraft moving forward.

In the Mythbusters, they accounted for groundspeed, or ground movement, but not the thrust of the aircraft.  Hence, rather than being at a standstill (or stationary), the airplane actually is moving forward and this is how the airflow is generated (on a plane that needs very little airspeed to actually take off).  That said, it still needs and showed it needed the airspeed or airflow across it's wings, even if that airflow is very light and not that heavy.

The Osprey on the otherhand, is definitely an interesting craft.  It CAN act like an normal aircraft that needs that airflow over it's wings, but it also can act very differently.  In the question of this thread, when asked what would happen, the Osprey pilot would probably say they would simply rotate their rotors and take off vertically!!!

Even if they didn't even have wheels moving, and were being dragged along by the car (if the vehicle was strong enough to drag an Osprey)...they could simply rotate the rotors and take off vertically!!!  I've actually never seen one take off in any manner except vertically now that I think about it.

That would definitely make the answer very simple!

I've seen a Harrier, but never seen the F-35 replacement that supposedly they got.

[Guest]

15 hours ago, Grunt said:

Of course, it will take off.  Why wouldn't it?  The medium upon which it sits is irrelevant.

THANK YOU!!!  Oh, I just used all caps.  I lose.

@Vort's explanation is the correct one in the real world.  The conveyor belt wouldn't matter.

The only point here that @JohnsonJones has here is the statement about the conveyor belt being programmed to match the speed of the wheels.  If it is what we all assume it means, this is an impossibility.  It cannot do so.  Thus it is pitting the immovable object against an irresistible force. 

HOWEVER, that's not how wheels behave.  There is an interesting paradox about wheels that confuses people.  The true velocity of wheels is zero.  In fact, if it were otherwise, we would either never move, or move very inefficiently.  That is the magic physics of wheels. So no matter how the conveyor belt ran, the wheel velocity is always zero.  Thus the wheels would not effect the plane's speed.

Edited December 5, 2017 by Guest

[zil]

Can someone clarify something for me?  (I will then go ask my brother to explain it or confirm / correct my understanding (if I manage to think I found some) - on Saturday maybe, when I have a few hours free.)

Going back to @Vort's explanation:

9 hours ago, Vort said:

So then, what happens as the Cessna attempts to take off? Let's investigate it, step by step:

1. The aircraft is motionless on the tarmac, pointing (let's say) east. The tarmac, which remember is a treadmill, is also stationary.
2. The aircraft starts up its propeller and begins moving eastward at, let's say, 5 mph. In response, the tarmac moves westward at 5 mph. The aircraft is now traveling eastward at 5 mph, but its wheels are spinning at the rate they would spin to travel at 10 mph. That is, the wheels are spinning twice as fast as the airplane is traveling.
3. The aircraft speeds up to approach takeoff speed. It is now traveling at, let's say, 30 mph. The tarmac is traveling the opposite direction at 30 mph. The aircraft wheels are spinning at an equivalent of 60 mph.
4. The aircraft reaches 50 mph, with the tarmac moving backward at 50 mph and the wheels on the Cessna dutifully spinning at an equivalent rate of 100 mph. At this point, the aircraft lifts off.

Hope that's clear. If not, I'm afraid I don't have much else in the way of explanation.

We have the airplane with wheels on the conveyor belt at the start of things (#1).  Beyond the conveyor belt we have the ground, which obviously won't be moving at any time during 1 through 4.  Let us say that at #1, we have a marker of some sort on the ground, even with the wings of the aircraft.  When we reach #4, will the wings of the aircraft still be even with the marker on the ground, or will they have moved relative to the marker?

Please don't try to explain, or at least not until after you give this simple answer: "they will still be even with the marker" or "they will have moved relative to the marker".  (I'm assuming that if the wings have moved relative to the marker, they'll be east of the marker - if I'm wrong, please let me know.)   Once I know this, I can try to puzzle out for myself how that all works - which will work better for my brain than trying to understand someone else's explanation.

[Vort]

4 minutes ago, zil said:

I'm assuming that if the wings have moved relative to the marker, they'll be east of the marker

You are correct.