Airplane and conveyor belt thread...
Discussion
orgasmicliving!! said:The wheels only turn because the plane is being pushed down the runway by the engines. The wheels are not pushing the plane forward at all; what little force they apply to the aircraft is acting to slow it down.
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.
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.
[/quote]Why the distinction between on and off the ground? The thrust of the engines works in exactly the same way whether it's in the air or on the ground.
The wheels a simply there to avoid all the scraping and sparks that would result if they weren't there. Imagine if the (totally freewheeling) wheels were replaced by mag-lev units, does your theory work then? NO!
The wheels a simply there to avoid all the scraping and sparks that would result if they weren't there. Imagine if the (totally freewheeling) wheels were replaced by mag-lev units, does your theory work then? NO!
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.
In simple terms......The plane engines exert their forward force directly on the surrounding atmosphere, not via the wheels to the ground/conveyor belt. This means that the plane moves relative to the atmosphere, not the ground it happens to be on. Ground speed has nothing to do with whether a plane will take off or not (look at a wind tunnel as a good example)
If you can't understand that then you may aswell give up.
If you can't understand that then you may aswell give up.
orgasmicliving said:
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.
If you bear ATG's points in mind but simply had skis you could have the same effect. Sit the plane on skis on a conveyor and you would have the interface between the skis and belt doing the job of the wheel bearings with the same result - heat, smoke and a plane buggering off down the runway.
orgasmicliving said:
On the ground, the plane's forward motion is a result of the wheels being rotated.
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.
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.
jacobyte said:
Read my lips:
It - would - take - off.
Read my arse:
No - it - wouldn't
There will be some frictional losses, rolling resistance and inertia that would, if nothing else happened, roll the plane off the back of the conveyor belt as soon as the belt was turned on. You could overcome that force by applying a very minimal amount of forward thrust to compensate for the rearward movement.
However, apply full thrust and you'd easily overcome the drag of the wheel bearings, tyres, inertia and all that, so the plane would then would move forward, off the conveyor belt, and providing it had gained enough velocity in relation to the air around it, then it would take off.
This has all been said before, I am an engineer, it is the correct answer.
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?
orgasmicliving!! said:
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
FFS... The wheels are not driven therefore they rotate freely, therefore assuming they are not overly frictioned they will rotate at whatever speed necessary.
The plane will accelerate to the speed required as the thrust produced by the engines will power it forward, the conveyor belt pushing backwards will spin the wheels but the thrust caused will make the plane move forward.
I may have rambled here but the plane will take off.
From someone doing Aerospace Engineering at University, with the knowledge of my lecturers as well who I asked on the matter last time this subject was raised.
If you really want to ask pilots, engineers and more aeronautical folk ask the people on www.pprune.org they'll point you straight.
>> Edited by Nic Jones on Tuesday 23 May 17:51
orgasmicliving!! said:
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.
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.
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