A brain challenge that people get pretty passionate about

[NeuroTypical]

Here's a brain challenge.  There is a right answer and a wrong answer.  In my experience, bright people arrive at both answers, and tend to dig in ferociously and start yelling.  So here it is - remember to be nice and not yell at people - first person to use all caps loses.

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?

Answer Yes or No, and explain why.

Edited December 4, 2017 by NeuroTypical

[Vort]

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

The question is phrased in order to guide you to making a wrong assumption; that the airplane speed is controlled by the conveyor belt speed. This is false. If the wheels roll with negligible friction, the airplane speed will have nothing to do with the conveyor belt speed.

Note that this assumption would be true with an automobile or any vehicle powered by driving its wheels. In that case, if the conveyor belt matches the speed of the wheels but in the opposite direction, the vehicle remains stationary with respect to the ground. But an airplane is not powered by driving the wheels; it is powered by a propeller or jet engine, which acts on the air, not on the ground.

Once the airflow over the wings is sufficient, the plane will take off. How fast the wheels are or or not rolling is irrelevant. Consider the same situation, but with the plane on a sheet of ice and fitted with ice blades on the bottom. (And pretend the ice is on some kind of conveyor belt or some such thing.) It doesn't make any difference how fast the ice is moving; as a frictionless surface, it will not hold back the airplane. Same with the treadmill idea.

[JohnsonJones]

I believe as per the laws of physics, and as a private pilot myself, the answer would be no.  I do not know why smart people would even debate this.

The reason a plane is able to fly is due to the shape of the wing and the airflow over that wing.  If the wind is strong enough, the airplane does not even need to get faster, it will fly if it is pointed into that headwind.

In fact, the headwind or tailwind is calculated into the takeoff calculations on when you need to pullback/takeoff, as well as the landing calculations.  Normally, the air at ground level is not strong enough for takeoff.

Simplifying the calculations

However, if I were in a jet and I need 135 KPH to takeoff, with that airflow over my wings with a full tank, that's a base level.  However, if their is a groundwind that I'm taking off in, and it's a direct headwind, that is 10 Knots per hour, than my takeoff speed will be 125 Knots.  However, if I had the runway countering my groundspeed to 0 Knots, then I'd need to add 135 knots to that to beat that runway/conveyer belt in order to counter that.  I need the speed of the air over my wings to equal that 135 knots, or I am not going to be able to take off.  An aircraft going at 0 miles per hour on the ground, will go nowhere, unless their is a strong enough headwind that makes the airflow over the wings equal the amount needed for a takeoff speed.

In otherwords, it does not matter how fast your wheels are going, or how fast your ground speed actually is, what matters is the AIRFLOW or airspeed on your wings.  If that airspeed is not fast enough, you will never fly. 

This is because it's due to physics in what allows the airplane to fly, NOT your ground speed, or even your wheels.

This is very basic stuff.

[zil]

Wait, is this conveyor belt inside a building like most conveyor belts?  Cuz if it is, I want off the plane before you start experimenting - just in case.

[NeuroTypical]

Vort has given the correct answer, and JohnsonJones has given the incorrect answer - even though every single thing he says about the laws of physics is accurate and relevant.  Two smart people - one of them is actually a pilot.  

(And Zil, of course, helps make the whole thread more palatable to everyone.)

 

Edited December 4, 2017 by NeuroTypical

[Vort]

So this sort of makes up for how insistent but wrong I was regarding the white and gold dress. Right?

[Rob Osborn]

Honestly, theres some very bad logic involved here. Even mythbusters (the tv show) bombed on this one and got it wrong.  Its easily debunked-

If the conveyor is really able to move fast enough to actually keep the the plane stationary to its environment it will never be able to fly because there is no air movement across its wings. An airplane's wheels that are chalked so they absolutely will not move on stationary ground will prevent the plane from flying no matter if the engines were at full thrust. Sure, it will or may be pressing hard against the chalked system that keeps it locked but the plane will never start to float in the air because there isnt enough air movement around its wing to create lift. The mythbuster show got it completely wrong because they were never able to create a test to actually keep the planes stationary with its environment. What little air movement that did exist in order for it to lift off was due to it moving forward and create a difference in pressures over and under its wings. It doesnt take much air movement to create lift but it does require actual air movements over and under the wings. The forward thrust of the engine provides the power to sustain enough forward movement of the plane through the air to keep it from falling.

[Rob Osborn]

47 minutes ago, Vort said:

So this sort of makes up for how insistent but wrong I was regarding the white and gold dress. Right?

Thou shalt not bring up the gold and white dress.

[zil]

14 minutes ago, Rob Osborn said:

Thou shalt not bring up the gold and white dress.

Let alone post negative images of it.

[Rob Osborn]

The problem with this mind experiment is that in practicality the conveyor belt could never move fast enough to provide the sufficient resistance on the axles to overcome the thrust. If one were to tether the axle in the opposite direction of takeoff to its stationary environment and ran the test the plane would never lift off. 

[zil]

I'm pretty sure my 11th grade science class was Chemistry.  I'm pretty sure 12th grade would have been Physics.  (It could be that Chemistry was 10th grade, and Physics 11th grade, but then what would 12th grade have been?  Pretty sure they had something slotted for each year, so unless they offered 2 years of Physics, I have my numbers right.)

Aaaaaanywho, I enjoyed Chemistry, but Physics was not required (almost nothing was required in 12th grade).  So, one day, as I sat pondering what to do with my 12th grade, and whether to take Physics, the thought came to me: "If I take Physics, I'll learn just enough that one day, when someone on the internet¹ asks a trick question, I'll think I'm smart enough to answer it, and display my stupidity to the whole wide world - or at least the world wide web¹."  So, I scratched that out and decided to take typing and shorthand instead because typing seemed useful (my dad could really type fast) and shorthand seemed fun - like a way to send coded messages to friends.²

Meanwhile, as you can see, my premonition was correct, and I was smart enough not to learn enough to look stupid...

¹ This part of the story clearly isn't true, because we didn't even have Compuserve when I was in 11th grade.  It's possible the entire story is made up just for fun.

² This didn't work out as I forgot the shorthand before I got friends - let alone friends who could read shorthand.  For some reason, I promptly forgot shorthand rather than using it to take notes in college.  Not sure what was wrong with me.

Edited December 4, 2017 by zil

[JohnsonJones]

1 hour ago, NeuroTypical said:

Vort has given the correct answer, and JohnsonJones has given the incorrect answer - even though every single thing he says about the laws of physics is accurate and relevant.  Two smart people - one of them is actually a pilot.  

(And Zil, of course, helps make the whole thread more palatable to everyone.)

 

Trust me, there is NO way to get an airplane afloat without air movement over it's wings...or don't.

Your right, one of us is a pilot and knows the basic things needed to fly, stall, or actually get lift on an airplane.

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...but that would be just weird once you know how a plane actually takes off and flies.  Ground movement plays very little, but if you have no air movement on the wings...the rest really doesn't matter at all.

Who came up with this question anyways?

If you can show me proof of this in action, I might deliberate about it (The mythbusters episode was already shown to be false as the airplane was still moving through the air at twice the speed they were pulling the little conveyor belt...aka...it was still moving across the ground at twice the takeoff speed needed as per it's wheels [which ironically meant it was actually moving forward and not at a standstill]...who actually relies on Mythbusters for science???  In order to match the plane, they would have to be going at double the speed of the plane at an accelerating rate that exactly matches the plane at a 2x=x forumula, which they didn't do).

That said, as I said before, the ground speed actually has very little to do with anything, it has everything to do with the airspeed over the wings.  If there is NO airspeed over the wings...it is not going to go anywhere.  For a plane taking off at a general 200 Knots, you would have to have the conveyor belt going at close to the speed of sound.

For a small plane like in mythbusters, you'd still have to be accelerating faster than the vehicle could accelerate and faster than the car might be able to go.

But what you are dealing with is AIRSPEED vs. groundspeed. 

If the conveyor belt thing worked, we'd actually use it on aircraft carriers rather than the catapults to spin the wheels and the engines to build up thrust because the less space needed to takeoff (and land) the better on one of those.  However, you can spin the wheels there, but until it's released by the catapult, you aren't going to fly.  That's because it's due to airspeed over the wings, and very little to do with the actual groundspeed (Which is what I tried to demonstrate with the headwind idea when taking off).

The spinning wheels but no takeoff is demonstrated daily by the US Navy from what I hear...

On the otherhand, airflow over an airplane is demonstrated daily in other arenas.  For example, a helicopter (which needs no groundspeed to take off, but generates airspeed in other ways) also needs airspeed over the appropriate arenas.

However, if you simply move the airplane on wheels on a conveyor belt, with no thrust over the wings...it's going to be stationary just like it is at other times.

Edited December 4, 2017 by JohnsonJones

[Rob Osborn]

8 minutes ago, JohnsonJones said:

Trust me, there is NO way to get an airplane afloat without air movement over it's wings...or don't.

Your right, one of us is a pilot and knows the basic things needed to fly, stall, or actually get lift on an airplane.

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...but that would be just weird once you know how a plane actually takes off and flies.  Ground movement plays very little, but if you have no air movement on the wings...the rest really doesn't matter at all.

Who came up with this question anyways?

If you can show me proof of this in action, I might deliberate about it (The mythbusters episode was already shown to be false as the airplane was still moving through the air at twice the speed they were pulling the little conveyor belt...aka...it was still moving across the ground at twice the takeoff speed needed as per it's wheels [which ironically meant it was actually moving forward and not at a standstill]...who actually relies on Mythbusters for science???  In order to match the plane, they would have to be going at double the speed of the plane at an accelerating rate that exactly matches the plane at a 2x=x forumula, which they didn't do).

That said, ss I said before, the ground speed actually has very little to do with anything, it has everything to do with the airspeed over the wings.  If there is NO airspeed over the wings...it is not going to go anywhere.  For a plane taking off at a general 200 Knots, you would have to have the conveyor belt going at close to the speed of sound.

For a small plane like in mythbusters, you'd still have to be accelerating faster than the vehicle could accelerate and faster than the car might be able to go.

But what you are dealing with is AIRSPEED vs. groundspeed. 

If the conveyor belt thing worked, we'd actually use it on aircraft carriers rather than the catapults to spin the wheels and the engines to build up thrust because the less space needed to takeoff (and land) the better on one of those.  However, you can spin the wheels there, but until it's released by the catapult, you aren't going to fly.  That's because it's due to airspeed over the wings, and very little to do with the actual groundspeed (Which is what I tried to demonstrate with the headwind idea when taking off).

Maybe the whole point of the experiment is to sell sell this idea to the North Koreans so that they will spend all their resources on aircraft carriers with conveyor belts and then we can bomb them easier while they are trying to figure out why they cant get their planes off the ship.

[JohnsonJones]

Hmm, upon reflection, the above answer may be too complex for one to understand.

Let's put it this way, the original question is flawed. (the main mistake of the question is this The conveyor belt is perfectly programmed to roll the opposite direction the plane wants to move, in order to keep it stationary ). There are some things to understand about Aircraft in general...

1. It needs thrust and airflow in order to take off.  This airflow has to be generated in some fashion.

2.  An item at rests...stays at rest.

3.  A force acting in one direction has an equal and opposite force acting in the opposite direction.

4. If a plane truly stays stationary, it is NOT going to be moving.  In order for it takeoff, it MUST be moving forward.

(example, using the mythbusters episode as an example...they keep the aircraft stationary until it moves at twice the rate they are trying to keep their belt moving in the opposite direction.  Once that occurs, the plane (and that type need VERY LITTLE thrust to move it forward) suddenly starts moving forward faster than they can move, it no longer remains stationary and in fact is moving forward with the airflow over it's wings being as such...if it was kept stationary, it would not have any airflow over it's wings and it would not move forward, even if it somehow got airborne, this imaginary conveyor belt would reach up and drag it back down motionless or it would crash, but if you watch the video, the plane actually flies forward).

5.  NOW, if you have no engines working at this point, the plane will not take off unless you have a headwind that causes the airflow high enough to cause takeoff speed.

6.  There is a caveat, but which requires a plane to NOT remain stationary, and it is exhibited by the Navy catapults.  What is happening at the same time the wheels are moving, is that the propeller or jets are creating thrust.  This is SEPARATE from the ground movement.  It has nothing to do with the question really, nor with the ground movement.  All ground movement is in your takeoff roll is to create airflow over the wings.  Thrust is used to promote this ground movement, but it is this thrust which is vital to the takeoff, not the actual ground movement, as ground movement and airflow are two different things (As I've stated).

7.  When you reach an appropriate thrust, it will force your aircraft forward.  AT that point, what matters is how much airflow you have over the wings.  With enough thrust, you can take off in a very short area...

(example, now using the Navy Catapults as an example.  They create it so that the plane has the thrust needed to take off in a very short distance.  A regular plane may need a half mile runway to work on the ground to get the thrust needed to get the airflow over it's wings in order to takeoff ,but a Navy Catapult reduces this distance to a VERY SHORT runway.  Hence, with the thrust, you can takeoff almost instantly, shown daily on Navy Aircraft.  One could also point that this is demonstrated by the Mythbusters video, though they do not understand it.  It has very little to do with their runway belt, and more to do with the fact that at a certain point that aircraft generates enough thrust to counteract anything that may be happening on the ground, and once it hits that 2x=x ratio, takes off in a very short manner, as this is a similar idea to what is happening in the navy Catapults when thrust is being generated).

8.  However, as per the question, if an aircraft was actually kept stationary, meaning even the thrust was managed to be kept at an absolute standstill...then no, the plane would not take off.  It would be impossible unless there was another way to generate airflow over the wings.  This is due to friction and the friction between the wheels and the conveyor belt...but this is considered impossible to actually pull off, and if one did, in theory the belt or the wheels would eventually break down.  If it were possible however, it would prove interesting.  The reason the mythbusters plane actually was able to take off is that they changed the parameters of what they did compared to what the question actually asks. 

Adding...

9. The big thing to remember is there is a DIFFERENCE between groundspeed and airspeed, or the speed the wheels turn on the ground and the speed that the air is moving over the wings. 

10.  The wheels do NOT represent airspeed...hence a conveyor belt that matched their speed is only utilizing one conveyance of power and friction.  Once thrust is enough, it overcomes the need for that and takeoff is inevitable.  HOWEVER...if there ever was a belt designed to exactly match the thrust of an aircraft immediately (instead of responding to wheels which is an indirect medium), the answer as per the question...as I stated before, I believe is that the aircraft could not take off.  It requires the airflow, and if there is no airflow it cannot fly.  Anything thus far though, has not been aimed at the airflow factor, but more the groundspeed factor.  In regards to the wheels turning, it is the THRUST which overcomes that obstacle...regardless of the runway belt that one is dragging behind their car.

Hoepfully that explains what I was thinking a little more clearly.

 

PS: One other item that was brought up that I might be corrected upon.  IF the thrust was high enough, it COULD go airborne even if kept stationary...like a rocket.  This would be independent of airflow over the wings though, but in this matter (not demonstrated in the mythbusters scenario though, that was an airflow-thrust thing instead) a rocket could indeed become airborne, even if held stationary (an example if you wish to try it, blow up a balloon and then if you can keep it stationary against a wall, have it go against a ceiling and a wall corner...it will stay afloat until it runs out of fuel/air).  on this, I am corrected from my above answer I suppose.

Edited December 4, 2017 by JohnsonJones

[NeuroTypical]

And JohnsonJones wins the "first to start yelling via ALLCAPS" award!  You lose!   And your answer is still wrong - the airplane will take off and fly.  There is a hidden, critical, wrong assumption that changes everything you are saying.

And Rob Osborn has the wrong answer too.

Edited December 4, 2017 by NeuroTypical

[NeuroTypical]

 Consider the thrust from the airplane engines in a different way.  Think about it as a pole on the rear end of the plane, being held by a guy who is off the conveyor.  Or maybe a rope tying the front end of the plane to a telephone pole in front of the whole experiment.  

When the engines produce a little thrust, the guy pushes the pole one inch.  Or someone tugs on the rope one inch.  

No matter what the wheels or the treadmill do, the plane will move forward one inch.  

That annoying little fact is as much a part of the laws of physics, as everything JohnsonJones keeps painstakingly explaining.

Edited December 4, 2017 by NeuroTypical

[Grunt]

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

[Vort]

To be clear: This is a cleverly misleading phony word problem.

5 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.

Pay close attention to the bolded words above. They are the crux of the problem.

5 hours ago, NeuroTypical said:

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 bolded words quoted above are the misleading part of the description. Specifically, the italicized part of the bolded sentence describes, not the result of the conveyor belt operation, but its intent. The guy who programmed the conveyor belt wanted it to keep the airplane stationary, and so programmed it to move in the opposite direction of the airplane.

But his intentions are irrelevant. Physics is king, not intent. Whatever the programmer of the conveyor belt wanted or intended, the fact remains that the wheels are just wheels, not driven or braked. So when the prop starts spinning or the jet engine fires up, the plane will move forward -- irrespective of what the conveyor belt is doing. And when the airplane is moving fast enough, the airflow over the wings will lift it off the conveyor belt, which will be spinning madly in a vain effort to keep the airplane stationary.

[Traveler]

 

The problem with the scenario given is that it is confusing two disconnected conditions and assuming that they are connected – a problem with many superficial logical challenges or arguments.  The engines work to create forward movement by moving the air.  Thus, one platform is the air in which the plane exists.  The other platform is the ground, which is the conveyor and wheels.  The conveyor is trying to keep the plane stationary relative to the ground by moving the conveyor backwards.  But the plane engines and propellers are trying to move the plane – not on the ground but through the platform in which the air is initially stationary to the ground.

The only convergence between the two platforms is the wheels connecting the plane to the conveyor and the only thing to keep the plane from moving forward is the friction in the wheels – but that was not given as a component of the problem.  If the wheels are allowed to move freely over the conveyor without accounting for friction then the only force acting on the plain is the engines and propellers moving the plain through the air.  The motion of the conveyor become irrelevant and the assumption that the conveyor would or could counter the speed of the plain is a false or bad assumption.  Because the air will not remain stationary in relation to the plane and ground.  Once the propellers create enough air movement over the wings the plain will leave the ground (conveyor) – which according to the statement of the problem is not actually a factor of force anyway. 

I believe the answer is that the problem is unsolvable in that any possible solution (the plane flying or not) will contradict at least one of the conflicting assumptions. 

 

The Traveler

[NeuroTypical]

7 minutes ago, Traveler said:

I believe the answer is that the problem is unsolvable in that any possible solution (the plane flying or not) will contradict at least one of the conflicting assumptions. 

A third answer!  Hmm...

"The conveyor is trying to keep the plane stationary relative to the ground by moving the conveyor backwards."

So what does physics care about what the conveyor is "trying" to do?  If the engines make air flow over the wings by moving the airplane forward, what do the wheels and conveyor belt have to say about it?

 

[Rob Osborn]

56 minutes ago, NeuroTypical said:

 Consider the thrust from the airplane engines in a different way.  Think about it as a pole on the rear end of the plane, being held by a guy who is off the conveyor.  Or maybe a rope tying the front end of the plane to a telephone pole in front of the whole experiment.  

When the engines produce a little thrust, the guy pushes the pole one inch.  Or someone tugs on the rope one inch.  

No matter what the wheels or the treadmill do, the plane will move forward one inch.  

That annoying little fact is as much a part of the laws of physics, as everything JohnsonJones keeps painstakingly explaining.

Perhaps its in the understanding, or lack of, the question but I am right, perfectly so, in that a plane that stays exactly stationary with its surrounding environment will not fly no matter how much thrust is applied to the engines. Its impossible to create a conveyor belt system that would keep the plane actually "stationary" without actually tethering the plane, otherwise it would move forward and once it moved forward would begin creating lift and fly. If it was tethered though to prevent it from moving forward so that it truly did stay statinary with its environment it would never lift off the ground as there being little to no air flow over and under the wings. The planes propellers or jet engine may be generating some slight air movement around the plane if it was stationary but in nowise even close enough to ever create any lift.

Edited December 4, 2017 by Rob Osborn

[Grunt]

13 minutes ago, NeuroTypical said:

A third answer!  Hmm...

"The conveyor is trying to keep the plane stationary relative to the ground by moving the conveyor backwards."

So what does physics care about what the conveyor is "trying" to do?  If the engines make air flow over the wings by moving the airplane forward, what do the wheels and conveyor belt have to say about it?

 

Nothing, beyond the friction of the hubs on the wheels.  You would have to make assumptions not presented to assume the plane would not fly, ie:  hubs generate enough friction to retard forward movement or engines aren't powerful enough to overcome the minor increase in friction generated through increased wheel speed.  The scenario assumes and takes strides to convey, that unnatural impediments at the wheels don't exist.

[zil]

(I just want to thank you guys for making me more glad I never took Physics.)

[Traveler]

12 minutes ago, NeuroTypical said:

A third answer!  Hmm...

"The conveyor is trying to keep the plane stationary relative to the ground by moving the conveyor backwards."

So what does physics care about what the conveyor is "trying" to do?  If the engines make air flow over the wings by moving the airplane forward, what do the wheels and conveyor belt have to say about it?

 

 

The conveyor does not apply force but rather only maintains velocity.  But acceleration is directly associated with force and can only be countered with friction in the wheels but the friction in the wheels is not dealt with.  Not in regards to ether keeping the plane from moving forward or the conveyor moving the plane backwards (remaining stationary).   According to the scenario the only force on the plain is the air movement created by the prop.  The force to run the conveyor can only be transferred to the plain through friction in the wheels.   But the wheel friction was not part of the initial assumptions.

 

The Traveler

 

 

[bytebear]

No, because it's not the movement of the wheels, but the air flow, and if the plane isn't moving forward, there is no air flow.  You can actually see lift in wind tunnels, where the plain is stationary, but the wind is flowing.