Airplane and conveyor belt thread...
Discussion
hugoagogo said:
This results in NO forward motion
it's like galileo innit
"Eppur si muove" "and yet it moves" we've had all this many times over, wheels make no difference to a plane, they jsut spin freely, if it was on skids on ice it would still take off. we'll suffice to say you are wrong, and move on
It really, really bugs me when people insist that their junk science is correct. And you managed to drag Galileo into it. Poor fellow must be turning over (in place) in his grave!!!!
Hugobubbo, you are right about the skids/skis. You are dead wrong about the wheels. Powering up a plane results in a "push forward" on the plane by the air (Newton, remember) BUT that results in the wheels being pushed an rotating, causing the plane attached to them to move forward.
Now put it on a conveyor belt that matches the wheels' speed at all times. The wheels rotate, but the plane does not move forward. No movement, no airflow over wings, no lift, and therefore, no takeoff. Simple. Jet engines push really hard. So the wheels will end up spinning very quickly. That's all. Since it is a given that the conveyor belt that the plane is sitting on is always exactly matching the wheel speeds at all times, the plane will never get to move forward, ie. it will not accelerate from its initial speed of zero. No change from speed = zero means zero speed relative to the air at all times.
Would you agree that the following experiment is a valid illustration of the principles involved?
You put a fan on a skateboard and put it on a treadmill. Fan is on setting 1. Treadmill can be set to a speed that causes skateboard to sit in place, wheels rotating freely.
Now you switch fan to setting 2 (higher speed). And you switch treadmill to a higher speed. Again, if you can get the skateboard to sit still, wheels rotating, then it proves that if wheel speed is matched by conveyor belt speed, the skateboard does not move.
The only thing left is to think about the change in speed from setting one to two. You can deduce logically that if beginning and ending speeds are matched, with no forward motion, then if acceleration is matched at all times, that would also result in no forward motion.
If you do not agree with the logical deduction, then you will need a bicycle with computer to figure out the treadmill's acceleration rate from one speed to another, and set up the skateboard to match it. To slow the skateboard's acceleration, you can add ballast (eg, pounds of flour), to speed it up you can set the fan to setting 3 or higher.
Would you agree that this experiment is valid? If so, I can try to reproduce it and post a video here.
If you don't agree with the experiment, then pray tell what you think is a proper experiment.
>> Edited by orgasmicliving!! on Tuesday 23 May 17:12
mechsympathy said:
Have I missed a handbag session??
orgasmicliving said:
It really, really bugs me when people insist that their junk science is correct.
Do you annoy yourself often??
The subject came up in a different thread, and I didn't want to derail that one. Apparently this has been discussed here before?
Bliarout, instead of just insisting on your (incorrect) viewpoint, why don't you back it up with science/evidence/experimentation?
motco said:
like this? 
That's exactly it. They just need to match two parameters--the skateboard wheel speed and the "belt" or paper speed. The moment they do, I bet you the skateboard would appear to be sitting in place, wheels rotating merrily.
pdV6 said:Last I heard, skis did not rotate? So, you push on them, it results in translational motion.
Look, when 1,000,000,000 better qualified people explain why your warped view of physics is totally wrong, its usually time to give up!
Wheels do rotate. When you push on them about their pivot point, it resuts in rotational motion. Objects attached to the axle experience translational motion. Except when the wheels rotate in place. If you have an airplane hanging by cables in the air, and rotate the wheels, does it move forward? Of course not.
If the plane is resting on the ground, and the wheels rotate on top of a belt rotating in the other direction, again, there is no forward motion of the axle. And the plane attached to the axles also does not move forward.
>> Edited by orgasmicliving!! on Tuesday 23 May 17:21
mechsympathy said:Whether the propulsion mechanism acts directly on the wheels or is attached by rods (in this case, the plane's body) has NO bearing. No matter how, ultimately, they push on the pivot point of the wheel. This causes the wheel to rotate. Anything attached to the axle moves forward horizontally as the wheel rotates along the surface it is on. If the wheel rotates in place, obviously it does not move forward horizontally.
Apparently this has been discussed here before?
I have no idea, but I don't imagine we have. It just amused me that you slagged off "junk science" while talking such utter bollox.
As far as I understand your POV, you think that planes have driven wheels and therefore placing one on a conveyor belt would mean that it wouldn't move. In which case what happens when the wheels leave the ground? Why doesn't the plane slow down again?
In a regular takeoff, this is what is happening. The airplane's engines' thrust is pushing it along the ground via the wheels. If there were no wheels, it would still experience the push of the engines and slide forward. If there was a conveyor belt, the wheels would accelerate but the plane would not move if the belt matched the wheel speeds in the opposite direction.
Once it takes off, it is directly pushing against the air. The wheels' rotation against the surface do not come into play. Its forward motion is a direct reaction to the thrust of the engines (actually, the engines move forward, the plane attached to them simply hangs on and ends up going along for the ride).
On the ground, the plane's forward motion is a result of the wheels being rotated. They rotate because the plane is pushing on the axles. The plane is pushing because the engine, in turn, is pushing the plane.
ATG said:
Must say I don't really understand the calls for censorship/pleas to lock it down. You don't want to talk about it, don't talk about it. You don't want to read it, don't read it. Why censor others who do?
The original thread's original question suggested to me that you were supposed to assume that there was no airspeed over the wings. Therefore there is no lift, therefore it doesn't take off.
Now in reality that is a slightly silly thought experiment because however fast the conveyor belt ran backwards, it wouldn't stop the plane from moving forwards and gaining some airspeed. This is because in the thought experiment the only thing opposing the thrust of the engines are the frictional losses in the wheels (a combination of the rolling resistance of the tyre and the friction in the bearings). In order to stop the plane pushing itself off the conveyor belt, the conveyor belt would have to be running backwards at an enormous speed and the heat generated in the tyres and the bearings would be equivalent to the heat generated by the engines ... they wouldn't last very long. In any case, I doubt the tyres are capable of generating enough grip to overcome the engine thrust, i.e. if you put the plane on a runway and locked the wheels the aircraft would probably skid forward under full thrust. But all of this is "in reality", and is not what the original question was considering.
First good response. Yes, you are absolutely correct. The question is a theoretical one, although they have made wheels for the SR71 that go up to several thousand miles per hour. F= ma (force = mass X acceleration). The force of the engines' thrust has to be dissipated in the acceleration of the wheels, (mv^2/r), so, given enough wheels of large enough diameter and mass, you could get fairly high levels of thrust to be absorbed in the wheels' acceleration before the whole thing goes up in flames from bearings seizing, etc. Because the wheels are still very, very light, relatively speaking, they would experience very high acceleration and would spin up to very high speeds.
The question is theoretical. If all things held, then the plane would not take off.
ATG said:
Hugobubbo, you are right about the skids/skis. You are dead wrong about the wheels. Powering up a plane results in a "push forward" on the plane by the air (Newton, remember) BUT that results in the wheels being pushed an rotating, causing the plane attached to them to move forward.
Indeed, it is not about the wheels braking the aircraft in some manner to prevent it from taking off. Rather, they can spin up as much as they like. BUT, no matter how fast they spin/rotate, the conveyor belt under them matches their speed in the opposite direction. As long as this situation exists, the plane will not move forward.
For it to move forward, the condition of the question has to be broken. Either the wheels skid, in which case they are not functioning as normal wheels in a normal takeoff roll. Or the conveyor belt does not match their speeds. The question poses a different situation, one where the speeds are always matched. For the condition posed by the question, the wheels rotate in place, and so the aircraft does not move forward.
ATG said:Saying the same thing, really. If the pilot stands on the brakes, the plane will stop, until the thrust of the engines overpowers and breaks the landing gear assembly. The whole point of the wheels is to "slide" the plane forward smoothly on the ground. The "sliding force" comes from the engines. It is transmitted to the airframe and the landing gear. The landing gear acts, via the axle and axle bearings, on the wheel, which acts on the rubber tyre. The tyre is "tipped forward due to the forces acting on it. Since it is circular, the tipping is continuous, and we see it as rotation. This results in the jumbo moving down the runway, and I am not talking about the occupant of seat 32C.
On the ground, the plane's forward motion is a result of the wheels being rotated.
mechsympathy said:Would you agree that the skateboard with the fan is set up similarly? The fan powers the skateboard. Agreed?
If I can be pedantic, I think you'll find that the wheels rotation is a result of the plane's forward motion rather than the other way round.
The wheels are freewheeling. Agreed?
If the paper/conveyor belt underneath matches the speed of the wheels at all times, then the fan-skateboard assembly would still move forward, according to you. According to me, it would not. Agree?
jay gti said:
Read my lips:
It - would - take - off.
Read my arse:
No - it - wouldn't
...
Thanks jay gti. Let's try to keep it civil, though.
Everyone, can we please stop talking about boundary cases of seized bearings and flaming wrecks? The question assumes that things are working as normal. When they don't, obviously the situation/simulation would change too. That is not the thrust of the question, though.
ATG said:No, I am not. I am saying they are freely allowed to accelerate and spin up to whatever equilibrium speed they need to get to. The wheels might apply a very, very slight braking force due to friction, but that is negligible.
Indeed, it is not about the wheels braking the aircraft in some manner to prevent it from taking off.
I'm afraid it is. In the thought experiment this is the only mechanism that is generating a force on the aircraft that opposes the thrust of the engines. I agree with you entirely about the wording of the question, but I think you're a attributing some magic powers to wheels that they don't posess.
The plane is not held in place due to braking. Rather, the acceleration thrust provided by the engines is transferred down to the wheels, and dissipated completely in the massive "spinning up" of the wheels to very high speeds. Typically this results in a forward roll. However, it's sitting on a belt moving in the opposite direction, matching the wheel speed at all times. So, they accelerate to massive speeds, but rotate in place. The plane ends up sitting still.
madazrx7 said:Afraid you are the one who has the relative motion slightly mixed up. Relative to the surface of the belt, whizzing along backwards, the tyres of the plane are screaming forward at very high speed (they have matching speeds, but in the opposite direction). Relative to the air, the plane is sitting still. Just like you being on a treadmill. You could spring along merrily, relative to the treadmill, but relative to the rest of the room, you are not moving forward.
OL, Just for a moment I will assume that you aren't taking the piss.
You seem to be confused in terms of relativity? If the 'conveyor belt' runway matched the speed of the wheels, then you are right, the plane would not be moving... RELATIVE TO THE CONVEYOR BELT. It would still be moving relative to the air, and everything else. Therefore, it would take off.
DieselJohn said:
The correct physics has been explained so many times now that there is no point doing it again.
The plane takes off.
The wording of the question is rubbish. What exactly does the conveyor belt do? It goes at the same speed as the plane? Speed realtive to what? If the plane is staying still than so is the conveyor belt, so what is stopping the plane going forwards? It's totally meaningless gibberish.
Actually, incorrect physics has been applied. You are wrong, I am afraid. Are you saying you can't run in place on a treadmill? Of course you can. So, too, can a plane sit on a conveyor belt, tyres spinning, conveyor spinning in the opposite direction at the same speed, and the body of the plane not moving. What is stopping the plane moving forwards is the fact that no net force is being applied to push it forwards. Any force from the engines' thrust is going to accelerate the wheels, which spin faster and faster. But so does the conveyor, always exactly matching the tyre speeds in the opposite direction.
everitj said:Yes, that is exactly what I am saying. You missed a small, but important piece though. If you turned on a jetpack, it would cause you to accelerate. You would pump your legs harder to keep your balance. If the treadmill also accelerates to exactly match your speed at all times, you would run faster, your legs would be pumping harder, but you would still be running in place.
Yes but your missing the point, imagine running on a treadmill in the gym, then turning on a jet pack on your back... what your saying is... If you ran fast enough you would stay still....
Because the treadmill also accelerated. As long as the speeds match, you ain't going anywhere. -v + v = ? (Yup, zero).
The question says that the treadmill matches the wheel speed at all times.
Nic Jones said:I have already said that it does not matter how the wheels are connected to the motive force. You could have a direct engine coupling through halfshafts, indirect coupling through an airframe, or have something act on a column of air which then pushes on the body. All of them result in one thing, A force. If that force is then dissipated, there is no more force left.
the tyres of the plane are screaming forward at very high speed (they have matching speeds, but in the opposite direction). Relative to the air, the plane is sitting still. Just like you being on a treadmill. You could spring along merrily, relative to the treadmill, but relative to the rest of the room, you are not moving forward.
Thats where your problem lies you can't compare a runner on a treadmill as the runners legs are powered, to an aeroplane-unpowered wheels.
You need to think laterally rather than just at what 'seems' to be happening.
I am saying that, in this case, the force is dissipated by the acceleration of the wheels, even though they are not directly coupled to the jet engine.
Nic Jones said:Yes, the wheels are not directly driven. However, they are being accelerated. By the thrust of the engines. Their rotation causes the plane ot move forward on the ground. Put them on a treadmill, going the other way, and the plane will not move if the speeds match. Even at full power, if a fast enough treadmill could be built.
The thrust of the engines forces air backwards, pushing the plane forward, resulting in it taking off.
Newtons laws of physics. 'Every action has an equal and opposite reaction'.
You really need to remember the WHEELS ARE NOT DRIVEN, they are free rolling.
DieselJohn said:No, that would result in the body acting like a rocket. Rockets don't fly, they hurtle.
Been following this from afar so...
the thrust of the engines forcing the air backwards (understand this bit) but is enough to get lift without the air flow over the wings?
No. But there is air flow over the wings. The idea that the plane can be made to remain stationary relative to the air is not sensible.
We are talking about lift caused by airflow over wings. Good point though.
SamHH said:It absolutely does. 100%.
Been following this from afar so...
the thrust of the engines forcing the air backwards (understand this bit) but is enough to get lift without the air flow over the wings?
There is airflow over the wings because the plane is moving forward at its normal takeoff speed. The conveyor does not stop the plane moving forwards.
Does a treadmill allow you to run in place?
Does it allow you to bicycle in place?
Does it allow you to drive a car in place (think dyno)?
So what if a plane's engine does not act directly on the wheels. It does it indirectly, via the airframe.
Same end result. It pushes on the wheels. Or, to be more clear, it pushes on the airframe which pushes on the wheels.
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