I would also think that the cost of buying and maintaining these engines would quickly outweigh the cost of some new rubber occasionally.
I would also think that the cost of buying and maintaining these engines would quickly outweigh the cost of some new rubber occasionally.
I've already invented a solution. But I don't really know what to do with it now... Perhaps I need to speak to a lawyer 
And the fact that if something goes wrong with them during a landing, like the gear that spins the wheel being stuck on, could cause an accident.
perhaps have a reginatitive braking system (ie a dynamo/motor) that can also spin up the wheels before landing? Or perhaps a little motorised "wheel" that jump-starts it a lá the things that start the MotoGP bikes?
You could even use a little propeller (which iirc are fitted to 747's to maintain Hydraulic power if something fails) to generate the power for the motor, which could be deployed on decent and be another way of slowing the plane down via drag.
You could even use a little propeller (which iirc are fitted to 747's to maintain Hydraulic power if something fails) to generate the power for the motor, which could be deployed on decent and be another way of slowing the plane down via drag.
Jakg and Tristan, I understand where your stubborness comes from, since at start I had the same opinion with you. But then I saw people really believing it wouldn't take off, and decided to take a second look at the problem. I don't say your opinion is necessarily wrong, but at least take the time to think with another logic.
The problem is in the wording. There are 2 options:
1) The conveyor matches the planes speed = No logic in this phrase, since the planes speed is supposed to be zero. There is a logical gap in this phrase, can't really express it though.
2)The conveyor matches the plane's wheel rotation speed = That's where the whole problem lays, and that is what needs a bit of thinking. THIS is a brain teaser. You guys think along the first phrasing, that's why it's so easy for you.
Think about it, but please take the actual time to think...
*The fact that the power comes from the engines and not from the wheels is absolutely irrelevant. From the moment the plane itself has not reached at least 50 knots relative to the air, the wings are useless.
It really IS irrelevant. The plane has to overcome friction between the wheels and the runway generated from it's weight before starting to move on a runway.
Similarly, a plane with full brakes (good ones) on, and full thrust, won't move forward. From the moment the only contact between the plane and the surface has 0 speed, it won't move. In the following example I'll show why the only contact between the plane and the surfact remains always 0.
*The following example is assuming that the refresh time of the conveyors belt sensor for speed = 0 secs.
Mechanical components are not affected by physics. This means conveyor belt can reach faster than light speeds.
Similarly the plane cannot run out of fuel, or overheat it's engines etc.
This means this can't be tested in real life.
Plane tries to accelerate ( Wheels turn at 5 km/h (not actually 5 km/h of rotational speed but lets say the speed that a car's wheel turns when it runs 5 km/h), Conveyor turns at 5km/h, Plane speed relative to air AND ground beneath = 0, assuming there is no wind that day)
Plane continues trying to accelerate ( Wheels turn at 100 km/h, conveyor turns at 100 km/h too, Plane speed = 0 again)
Plane engages afterburner, still tries ( Wheels turn at 800 km/h, conveyor turns at 800 km/h too, Plane speed = ZERO)
Plane is stil stubborn, ( Wheels turn at Mach 2, conveyor turns at Mach 2 too, Plane speed = Yep you guessed it, 0)
Theoretically in a world where physics don't apply on mechanical components, the wheels and the conveyor belt could reach infinite speed, till disruption of the space/time fabric...
This of course leads to a paradox. This is why it's an unsolvable riddle, like many of them exist, and that is why it has long been discussed.
Not if you think about it. If the conveyor works properly as the problem says, the more you pull, the more the speed of the conveyor will accelerate, leading again to the paradox.
It's a type of paradox like the problem with the Turtle trying to reach the finish line, but never reaching it, because to reach it, it first has to reach the middle, but to reach the middle it has to reach 1/4, and then 1/8, 1/16th and on and on. The turtle logically never even moves. But it the real world, a turtle of course can reach any distance.
I know I won't convince you, nor am I trying to, just trying to explain you why there is such a gap between the two different opinions here. Just different schools of logic.
Btw I'm really dissapointed by Cecil, usually she is more free thinking and less aggressive than this.
Sorry for the long post!
The problem is in the wording. There are 2 options:
1) The conveyor matches the planes speed = No logic in this phrase, since the planes speed is supposed to be zero. There is a logical gap in this phrase, can't really express it though.
2)The conveyor matches the plane's wheel rotation speed = That's where the whole problem lays, and that is what needs a bit of thinking. THIS is a brain teaser. You guys think along the first phrasing, that's why it's so easy for you.
Think about it, but please take the actual time to think...
*The fact that the power comes from the engines and not from the wheels is absolutely irrelevant. From the moment the plane itself has not reached at least 50 knots relative to the air, the wings are useless.
It really IS irrelevant. The plane has to overcome friction between the wheels and the runway generated from it's weight before starting to move on a runway.
Similarly, a plane with full brakes (good ones) on, and full thrust, won't move forward. From the moment the only contact between the plane and the surface has 0 speed, it won't move. In the following example I'll show why the only contact between the plane and the surfact remains always 0.
*The following example is assuming that the refresh time of the conveyors belt sensor for speed = 0 secs.
Mechanical components are not affected by physics. This means conveyor belt can reach faster than light speeds.
Similarly the plane cannot run out of fuel, or overheat it's engines etc.
This means this can't be tested in real life.
Plane tries to accelerate ( Wheels turn at 5 km/h (not actually 5 km/h of rotational speed but lets say the speed that a car's wheel turns when it runs 5 km/h), Conveyor turns at 5km/h, Plane speed relative to air AND ground beneath = 0, assuming there is no wind that day)
Plane continues trying to accelerate ( Wheels turn at 100 km/h, conveyor turns at 100 km/h too, Plane speed = 0 again)
Plane engages afterburner, still tries ( Wheels turn at 800 km/h, conveyor turns at 800 km/h too, Plane speed = ZERO)
Plane is stil stubborn, ( Wheels turn at Mach 2, conveyor turns at Mach 2 too, Plane speed = Yep you guessed it, 0)
Theoretically in a world where physics don't apply on mechanical components, the wheels and the conveyor belt could reach infinite speed, till disruption of the space/time fabric...
This of course leads to a paradox. This is why it's an unsolvable riddle, like many of them exist, and that is why it has long been discussed.
Not if you think about it. If the conveyor works properly as the problem says, the more you pull, the more the speed of the conveyor will accelerate, leading again to the paradox.
It's a type of paradox like the problem with the Turtle trying to reach the finish line, but never reaching it, because to reach it, it first has to reach the middle, but to reach the middle it has to reach 1/4, and then 1/8, 1/16th and on and on. The turtle logically never even moves. But it the real world, a turtle of course can reach any distance.
I know I won't convince you, nor am I trying to, just trying to explain you why there is such a gap between the two different opinions here. Just different schools of logic.
Btw I'm really dissapointed by Cecil, usually she is more free thinking and less aggressive than this.
Sorry for the long post!
And I repeat - where does the spare thrust go to?
Thrust from engine = big
Drag from wheel bearings, rolling resistance, hell even brake drag if you like = small
Lets say the wheels are turning with a linear speed of 1000m/s, so the belt must be moving at 1000m/s. Now lets say, hypothetically, that at 1000 the rolling drag is 1000N in the horizontal plane (no pun intended).
Also hypothetically the engine is capable of producing 10000N of thrust, at any air speed or altitude (etc with silly assumptions and simplifications).
10000 - 1000 = 9000. Where does this spare 9000N of thrust go? Because I think it will acclerate the plane; eventually to Vrotate, and thus take off.
Similarly (and oppositely) you could tie the plane still with long ropes that do not touch the conveyer belt, and speed up the belt to 1000m/s. The plane won't move, and the only force acting on the plane will be the tyre drag force in both directions (cancelling out, so the plane doesn't move). But with the engines going, acting on the air, there is a huge unbalanced force.
Thrust from engine = big
Drag from wheel bearings, rolling resistance, hell even brake drag if you like = small
Lets say the wheels are turning with a linear speed of 1000m/s, so the belt must be moving at 1000m/s. Now lets say, hypothetically, that at 1000 the rolling drag is 1000N in the horizontal plane (no pun intended).
Also hypothetically the engine is capable of producing 10000N of thrust, at any air speed or altitude (etc with silly assumptions and simplifications).
10000 - 1000 = 9000. Where does this spare 9000N of thrust go? Because I think it will acclerate the plane; eventually to Vrotate, and thus take off.
Similarly (and oppositely) you could tie the plane still with long ropes that do not touch the conveyer belt, and speed up the belt to 1000m/s. The plane won't move, and the only force acting on the plane will be the tyre drag force in both directions (cancelling out, so the plane doesn't move). But with the engines going, acting on the air, there is a huge unbalanced force.
If you consider that it's about wheel-speed, then yes it's a paradox - however a question like that where a variable affects the value that affects it will create a continual loop eventually melting the bearings and locking the wheels - upon which point the plane just needs funking big engines to push past this and take off :P
T'would take off.
If you imagine a car on the same conveyor, it puts its power to the conveyor through its wheels - in effect, its wheels are pushing the conveyor backwards. Hence, if the conveyor matches the wheel speed in the opposite direction, the car's forward movement is nullified
But a plane doesn't put it's power to the ground through the conveyor, or through its wheels. A plane's forward power comes through thrust pushing on the air, it bypasses the conveyor completely. The wheels don't provide any form of forward thrust, they only exist as a buffer to keep the plane off the ground. A plane's engine still provides exactly the same amount of forward thrust regardless if there's a conveyor belt there or not... the only difference is that the wheels will rotate faster before the plane lifts off the ground
(wow that's a lot of thrusting)
If you imagine a car on the same conveyor, it puts its power to the conveyor through its wheels - in effect, its wheels are pushing the conveyor backwards. Hence, if the conveyor matches the wheel speed in the opposite direction, the car's forward movement is nullified
But a plane doesn't put it's power to the ground through the conveyor, or through its wheels. A plane's forward power comes through thrust pushing on the air, it bypasses the conveyor completely. The wheels don't provide any form of forward thrust, they only exist as a buffer to keep the plane off the ground. A plane's engine still provides exactly the same amount of forward thrust regardless if there's a conveyor belt there or not... the only difference is that the wheels will rotate faster before the plane lifts off the ground
(wow that's a lot of thrusting)
Fun some people can't grasp the meaning of a paradox (aside from the fact that a paradox is exactly something that can't be grasped)...
Good point that actually.
As has already been said, attaching a small engine/motor to the wheels isn't really possible. So i guess the next thing to look at would be some sort of fixed fan arrangement on the wheel itself, although i can see problems with this too.
Just thinking while i'm typing, but how about a fixed fan on the wheel hub, with some sort of venturi tube type (or a pipe) thing to direct air onto the fan. But have a movable flap in the end of the tube to direct the passing air away from the fan when in normal flight. It would be light, wouldn't take up any extra room and is almost maintenance free, and very cheap.
Yunno, there could be a few bob in this if done properly. Might be worth checking the patent office to see if anyone else has done it before.
Edit: Oh yeah, on topic. I'm with Mr Rodgers on this one. Is been done to death and back on every single flight forum and website, whether sim/rc/real or otherwise on the net, so, we'll see how long it lasts here. I'm guessing not long..
Just looked it up - and would you know there are at least 2 patents for a fan-style device on the wheel to use the moving air to move the wheel.
No-ones using them though - perhaps because they weigh more than they are worth in Jet Fuel and Tyre Repairs, or perhaps because Tyre Resurfacers are running some massive Conspiracy to stop them :P
No-ones using them though - perhaps because they weigh more than they are worth in Jet Fuel and Tyre Repairs, or perhaps because Tyre Resurfacers are running some massive Conspiracy to stop them :P
Shit....might have guessed.
It does make you wonder why they're not using them though. I'm not sure the weight would be too much of an issue what with modern alloys and all. Maybe these previously patented designs didn't have a cutoff valve. Without it i can see that it may upset the aero a bit and effect the handling.
Oh well, guess i'll have to make my millions from my other invention, 'Soup Pie'....
Okay.. let's shift the riddle a bit. Say a plane needs 50kts to get airborne and has a max airspeed of 95kts, and you start off with the conveyor, not matching the wheel speed, but wheelspeed+5kts.. how long will it take the plane to get airborne?
Is this a trick question ?
To my mind it's impossible to answer with the information you've given. We'd need to know the acceleration figures of the plane, it's overall weight, the strength of the (presumably) oncoming wind. Which way is the conveyor belt moving ? is it on an aircraft carrier moving at 30kts ? When you say long, do you mean time wise or distance wise ?
Basically, i'm both stalling and giving excuses, coz i literally wouldn't know where to begin.
It's real easy to go off people yunno, grrrr
hehe.. okay, let's say that today is a still. The conveyorbelt is being pulled in the opposite direction to the plane's orientation (causing forward wheel rotation when > 0kts) as in the first experiment/riddle
your esplanation might be okayish from a mathematical perspective but physically its tosh
with your constraints as soon as the prop is able to overcome the rolling resistance of the tyres (which is more or less constant over speed... same goes for bearing friction) the plane would move forward and both the conveyor and the wheels would instantly reach infinite angular speed
unless you find some formula that states that rolling resistance and friction reach infinity at this point the plane will still happily take off as if nothing happened
This thread is unbelievable, I'm actually speechless at how people can't see that the plane can quite easily, and quite happily take off, regardless (assuming a few factors, as Kegtys also mentioned).
Gobsmacking.
Gobsmacking.
Well ok Mr Smarty-pants,
if you're so clever lets see you answer big old SammyH's teaser 
I'm timing you
If similar threads on other forums are anything to go by, you ain't seen nothin' yet.
You can't adequately answer Sam's 'riddle' without more information. Just having the plane's top speed, and it's takeoff speed... plus a statement of conveyor speed, makes it impossible to calculate
hehe.. okay, so it's impossible to know how *long* it will take, since there's not enough info. Let's say the plane can accelerate at a constant rate of 3kts/second up to 96kts. The conveyorbelt can accelerate instantly to any required speed, but maintains a speed that is precisely +5kts over wheelspeed.
[edit] The experiment begins with the conveyorbelt traveling at 5kts, and the plane's wheels are static. The plane is therefore traveling at -5kts at idle.
[edit] The experiment begins with the conveyorbelt traveling at 5kts, and the plane's wheels are static. The plane is therefore traveling at -5kts at idle.
Yeah thats what i told him too, but it seems as though he's a hard chap to please.......hang on, i've got an idea...
SamH, we've worked out the answer to your riddle, it took a while, but i'm absolutely certain we've got the answer. Infact i'm so certain it's correct that we won't believe you if you say it's not.
And behold after much 'deep thought', the answer to your question is: 42
Easy
edit: oh bollocks, too slow, again...
seriously though, i havn't a clue about this sort of stuff, so i'll have to leave it to the others. Infact, the only reason i'm posting in this thread is i can't sleep, and i'm a bit bored right now.
From a physics point-of-view, the essential question is whether the airplane accelerates through the air, to takeoff airspeed (sufficient to provide lift greater than gravity). Such acceleration is determined by the net force, acting upon the airplane; if there is a net forward force, on the airplane, then the airplane will accelerate forward.
The airplane engines apply a force to a mass of air, pushing it backward. According to Newton's 3rd Law, that air mass simultaneously pushes the airplane forward, with an equal and opposite force. So, there is a forward force on the airplane, from the air.
Is there a backward force acting upon the airplane, from the conveyor belt? Generally not a significant one, since any such force would have to be communicated to the airplane, through friction at the wheel bearings, which friction is quite small in magnitude. Instead, the conveyor belt would produce a torque on the wheels, and cause them to rotate unless there is a greater-than-or-equal, opposite torque on the wheels, produced by bearing friction - which, again, is very unlikely since the wheel bearings are designed to minimize friction.
Generally, the airplane could be noticeably pushed backward by the conveyor belt, only if bearing friction torque is equal to (or greater than, which is not, in itself, possible) conveyor belt torque, thus preventing the wheels from rotating and allowing the backward (linear) force of the conveyor belt, to be the predominant force on the airplane. This would require that the conveyor belt not push very hard, against the wheels.
It can be recognized, btw, that if such a conveyor belt support were spun, all of a sudden at high speed, the wheels would rotate and the airplane would remain almost still because of its inertia. This indicates that the conveyor belt cannot apply much of a backward force to the airplane; even if a great force is applied to the conveyor belt, accelerating it quickly, very little of that force is actually transmitted, by friction in the wheel bearings, to the airplane; the conveyor belt could - at best, and if acting alone on the airplane - accelerate it backward, only very slowly, thereby demonstrating only a small, backward force on the airplane.
Anyway, considering everything, there is a net forward force on the airplane, due to the engine's propulsive, forward force's being greater than the wheel bearings' frictional, backward force (and any air drag on the airplane). So, the airplane will accelerate forward, to takeoff speed.
P.S. - The speed of the conveyor belt wouldn't matter; as long as the wheels were rotating, the frictional force would be the same, regardless of the speed of rotation.
The airplane engines apply a force to a mass of air, pushing it backward. According to Newton's 3rd Law, that air mass simultaneously pushes the airplane forward, with an equal and opposite force. So, there is a forward force on the airplane, from the air.
Is there a backward force acting upon the airplane, from the conveyor belt? Generally not a significant one, since any such force would have to be communicated to the airplane, through friction at the wheel bearings, which friction is quite small in magnitude. Instead, the conveyor belt would produce a torque on the wheels, and cause them to rotate unless there is a greater-than-or-equal, opposite torque on the wheels, produced by bearing friction - which, again, is very unlikely since the wheel bearings are designed to minimize friction.
Generally, the airplane could be noticeably pushed backward by the conveyor belt, only if bearing friction torque is equal to (or greater than, which is not, in itself, possible) conveyor belt torque, thus preventing the wheels from rotating and allowing the backward (linear) force of the conveyor belt, to be the predominant force on the airplane. This would require that the conveyor belt not push very hard, against the wheels.
It can be recognized, btw, that if such a conveyor belt support were spun, all of a sudden at high speed, the wheels would rotate and the airplane would remain almost still because of its inertia. This indicates that the conveyor belt cannot apply much of a backward force to the airplane; even if a great force is applied to the conveyor belt, accelerating it quickly, very little of that force is actually transmitted, by friction in the wheel bearings, to the airplane; the conveyor belt could - at best, and if acting alone on the airplane - accelerate it backward, only very slowly, thereby demonstrating only a small, backward force on the airplane.
Anyway, considering everything, there is a net forward force on the airplane, due to the engine's propulsive, forward force's being greater than the wheel bearings' frictional, backward force (and any air drag on the airplane). So, the airplane will accelerate forward, to takeoff speed.
P.S. - The speed of the conveyor belt wouldn't matter; as long as the wheels were rotating, the frictional force would be the same, regardless of the speed of rotation.
it will take however long it would take to take off with 5kts of tailwind without the belt
take take take my english teachers will be proud... even more so my economics teachers
Ok... If the conveyor belt is only as long as the plane (which is how I first imagined it) and it (the belt) is stationary relative to the ground, the plane won't take off FROM the conveyor belt, it would just roll off the front. However, if the conveyor belt is as long as it takes the plane to take off normally, it WILL take off with twice the wheel speed, correct?
Another Brain Teaser
(115 posts, started )
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