That plane/conveyor problem - filmed solution
Discussion
Cotty said:
micky g said:
Car on rolling road doing 130 mph, whats the air resistance?
Doh
Doh
Zero
Until you shove a large metal bar in the rollers then it might create some
Actually there might be litte due to the cooling fans but that wouldnt be generated by the car
Edited by Cotty on Monday 11th December 21:25
So where is the air flow under the wings for the plane to take off?
Assuming that the plane, like the car, is going nowhere. (Unless its a Harrier)
Just got home and caught up on this. As soon as it was clear the conveyor belt is just a moving bit of ground and not some glue derived from magic bean juice.
I'm a believer - the plane will fly, no questions. The limits of the friction with the ground limit it, but I REALLY hope all planes are designed with wheels that can do at least twice their normal take off ground speed.
As a slight aside then - What if the friction at the ground/plane interface was sufficient that in normal (non-conveyor belt) conditions the plane could only just take off (once airbourne it's all ok). Would the build up in actual ground speed (air flow speed) be countered by the friction, limiting the velocity of the plane with the maximum given thrust it has available, so it wouldn't reach the critical air flow speed required to generate the lift to counter gravity's effect on it?
I do miss doing this all day every day for Uni!
I'm a believer - the plane will fly, no questions. The limits of the friction with the ground limit it, but I REALLY hope all planes are designed with wheels that can do at least twice their normal take off ground speed.
As a slight aside then - What if the friction at the ground/plane interface was sufficient that in normal (non-conveyor belt) conditions the plane could only just take off (once airbourne it's all ok). Would the build up in actual ground speed (air flow speed) be countered by the friction, limiting the velocity of the plane with the maximum given thrust it has available, so it wouldn't reach the critical air flow speed required to generate the lift to counter gravity's effect on it?
I do miss doing this all day every day for Uni!
The flaw in the question, and the reason people are unable to grasp that the plane takes off, is in the phrase "the plane speed". It is deliberately vague and the uninitiated think the speed of the wheels and the speed of the aircraft are one and the same. They aren't. The conveyor can only counter the speed of the wheels which have very little influence on the speed of the aircraft as they are undriven and free-wheeling.
It really is that simple.
(Hands up who knew I'd be drawn in eventually)
It really is that simple.
(Hands up who knew I'd be drawn in eventually)
micky g said:
Cotty said:
micky g said:
Car on rolling road doing 130 mph, whats the air resistance?
Doh
Doh
Zero
Until you shove a large metal bar in the rollers then it might create some
Actually there might be litte due to the cooling fans but that wouldnt be generated by the car
So where is the air flow under the wings for the plane to take off?
Assuming that the plane, like the car, is going nowhere. (Unless its a Harrier)
The car goes no-where because the wheels are driven by the engine. Aeroplane engines do not drive the wheels.
308mate said:
This is briiliant!! I wasnt privvy to the original thread but I want to ask those that sound like they know, a question. The lift bit I get (I think). To me, if you could pass air over a Jumbo fast enough, while it might not actually rise, surely if you took the ground away, it wouldnt fall? As the plane's wings attempt to occupy the area of low air pressure above the wings surface and therefore hold it there(?).
BUT, the need for the plane to physically move forward I dont get yet. How do they measure downforce/lift in a wind tunnel where the subject CANT move?
PB - be patient with me, learning and knowledge sharing is the original vision for the internet in any case....blah blah blah whatever..
BUT, the need for the plane to physically move forward I dont get yet. How do they measure downforce/lift in a wind tunnel where the subject CANT move?
PB - be patient with me, learning and knowledge sharing is the original vision for the internet in any case....blah blah blah whatever..
Edited by 308mate on Monday 11th December 16:59
In a wind tunnel, the object is stationary. You can measure its weight by putting it on a set of scales/weighing platform/whatever. Then you flow air past the object. If it creates any downforce or lift, the measured weight will be different.
moleamol said:
micky g said:
Cotty said:
micky g said:
Car on rolling road doing 130 mph, whats the air resistance?
Doh
Doh
Zero
Until you shove a large metal bar in the rollers then it might create some
Actually there might be litte due to the cooling fans but that wouldnt be generated by the car
So where is the air flow under the wings for the plane to take off?
Assuming that the plane, like the car, is going nowhere. (Unless its a Harrier)
The car goes no-where because the wheels are driven by the engine. Aeroplane engines do not drive the wheels.
I'm getting there....
So the plane moves irrespective of wether it's on a conveyor or not, the wheel speed being irrelevant to that of the plane.
Therefore uplift and take off!
Me, Jon- considering how often you quoted a post of mine in previous replies to folk who don't understand basic physics. Wishing you well..kmicky g said:
moleamol said:
micky g said:
Cotty said:
micky g said:
Car on rolling road doing 130 mph, whats the air resistance?
Doh
Doh
Zero
Until you shove a large metal bar in the rollers then it might create some
Actually there might be litte due to the cooling fans but that wouldnt be generated by the car
So where is the air flow under the wings for the plane to take off?
Assuming that the plane, like the car, is going nowhere. (Unless its a Harrier)
The car goes no-where because the wheels are driven by the engine. Aeroplane engines do not drive the wheels.
I'm getting there....
So the plane moves irrespective of wether it's on a conveyor or not, the wheel speed being irrelevant to that of the plane.
Therefore uplift and take off!
Absolutely spot on.
Sigh. Dead wrong.
If the conveyor belt's speed matches the plane's wheels' speed such that the wheels rotate in place (just like a car on a dyno) then the plane does not move forward relative to the conveyor (just like a car on a dyno). So, no airflow, no lift, no takeoff. This holds true regardless of whether the wheels are directly driven or not. It's just a matter of force. The wheels have a force acting on them, therefore they experience acceleration. This results in a certain speed at any time. If the conveyor matches that speed, then they are not moving with respect to the conveyor. Neither is the plane attached to them. It does not matter that the wheels are directly or indirectly driven. All that matters is the relative speed of the wheels and the conveyor belt.
The analogy about the person on a skateboard being pushed is a somewhat decent example. It is agreed that the person is staying in place with the skateboard wheels rotating before the push. When the push is given, the person experiences an acceleration. So do the skateboard wheels. They rotate faster than the treadmill surface going in the opposite direction. That's why the skateboard moves forward. IF the treadmill sped up to match the new speed of the skateboard, then the skateboard and person would still not move forward, they would sit there while the wheels and treadmill now rotating at the faster speed.
Here we go...
someone please lock this thread
If the conveyor belt's speed matches the plane's wheels' speed such that the wheels rotate in place (just like a car on a dyno) then the plane does not move forward relative to the conveyor (just like a car on a dyno). So, no airflow, no lift, no takeoff. This holds true regardless of whether the wheels are directly driven or not. It's just a matter of force. The wheels have a force acting on them, therefore they experience acceleration. This results in a certain speed at any time. If the conveyor matches that speed, then they are not moving with respect to the conveyor. Neither is the plane attached to them. It does not matter that the wheels are directly or indirectly driven. All that matters is the relative speed of the wheels and the conveyor belt.
The analogy about the person on a skateboard being pushed is a somewhat decent example. It is agreed that the person is staying in place with the skateboard wheels rotating before the push. When the push is given, the person experiences an acceleration. So do the skateboard wheels. They rotate faster than the treadmill surface going in the opposite direction. That's why the skateboard moves forward. IF the treadmill sped up to match the new speed of the skateboard, then the skateboard and person would still not move forward, they would sit there while the wheels and treadmill now rotating at the faster speed.
Here we go...
someone please lock this thread
Edited by orgasmicliving!! on Tuesday 12th December 01:59
Can you spot what he did there, kids? He said "if the conveyor matches the speed of the wheels". If you make that assumption, he's dead right, the plane remains stationary relative to the ground by definition. If however you say "the conveyor matches the fuselage's speed", then the plane moves and will take off (unless, for example, the tyres explode before they reach twice normal take off speed... which is not an unreasonable scenario). You can make the answer whatever you like by your interpretation of the question.
This might help.
However, for those insisting that the question actaully means that linear speed of the conveyor matches the linear speed of the plane in the opposite direction, it won't help. If that were the case, then the plane's linear speed would be matched by the belt, not the wheel speeds. As the wheel speed would be different from the surface speed, there would be motion relative to the belt, and, therefore, airflow past the wings.
I don't feel that this is what the question means. When you talk about a car on a dyno sitting still, you are talking about the drum/conveyor belt surface moving at the same speed as the wheel surface, not the car's linear speed forward (which is always zero).
However, for those insisting that the question actaully means that linear speed of the conveyor matches the linear speed of the plane in the opposite direction, it won't help. If that were the case, then the plane's linear speed would be matched by the belt, not the wheel speeds. As the wheel speed would be different from the surface speed, there would be motion relative to the belt, and, therefore, airflow past the wings.
I don't feel that this is what the question means. When you talk about a car on a dyno sitting still, you are talking about the drum/conveyor belt surface moving at the same speed as the wheel surface, not the car's linear speed forward (which is always zero).
Edited by orgasmicliving!! on Tuesday 12th December 01:56
I think there are some proper wind up merchants on here at times.
Let me see, if you ran the conveyer belt fast enough, due to the viscous nature of air, you could entrain air at a sufficient velocity to lower the static pressure underneath the plane to actually create downforce!
Or, um, how about the coriolis effect. With the entrained air flowing past the rotating object of the wheel, a lower static pressure would be generated on the top surface of the wheel to lift the them off the conveyer belt. Of course that depends on how soft the landing gear was sprung. You may just get a stumpy plane.
Let me see, if you ran the conveyer belt fast enough, due to the viscous nature of air, you could entrain air at a sufficient velocity to lower the static pressure underneath the plane to actually create downforce!
Or, um, how about the coriolis effect. With the entrained air flowing past the rotating object of the wheel, a lower static pressure would be generated on the top surface of the wheel to lift the them off the conveyer belt. Of course that depends on how soft the landing gear was sprung. You may just get a stumpy plane.
right. fed up now. i require: someone with a big flat garden/feild car park, someone with an rc plane, a pair of car batteries and starter motors, a few other random bits and bobs and something to make a converyor belt out of... and a video camera... if i show it working, will you all shut up?
To all the doubters, I need to know one thing that I've not read a single answer (or even theory) to -
(i.e. Orgasmic this is for you!)
If the plane remains stationary, where is all the additional energy from the thrust of the engines being expended? Are you saying that the engines make the wheels spin faster (as if the wheels were driven by its own engines) but the plane remains stationary?
(i.e. Orgasmic this is for you!)
If the plane remains stationary, where is all the additional energy from the thrust of the engines being expended? Are you saying that the engines make the wheels spin faster (as if the wheels were driven by its own engines) but the plane remains stationary?
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