Aircraft and conveyor belt (again)

[kevnurse]

Tim West asked what was the OP's question. Here is the exact question, copied from the OP:

An aircraft is standing on a very long runway that can move (a conveyor belt). The aircraft moves in one direction, while the conveyor belt moves in the opposite direction. This conveyor belt has a control system that tracks the aircraft's wheels speed and tunes the speed of the conveyor belt to be exactly the same as the wheels, but in the opposite direction. There is no wind. The pilot begins to add thrust to the engines...

Will the plane ever take off?

The question, as written, describes an impossible scenario, but we can only attempt to answer what we are asked.

I wasn't aware of the videos, but having now seen them, I now understand what the OP was trying to ask. The video of the car on the belt clearly shows what happens: the belt is moving at a constant speed, the car is moving forward along the belt and must therefore be going faster than the belt (due to thrust from anywhere but the wheels) and the wheels are moving faster than the belt and the car. When translated into a plane on a belt, the question should have been:

"With take-off thrust set, a plane will take off at 100 kts. The plane is sat on a conveyor belt which will automatically accelerate to a belt speed of 100 kts at the same rate as the aircaft will do, when the pilot selects take-off thrust. Will the plane take off?"

Answer: Yes. The belt will reach 100 kts, while at the same time the plane will reach 100 kts through the air due to the free wheeling tyres, which will be rotating at 200 kts. This is coarsely demonstrated by the video of the biplane on a tarpaulin. Unfortunately, we were not asked this question.

Megawatt, softus, et al. RTFQ.

 

[Softus]

Megawatt, softus, et al. RTFQ.

Well f**k you.

You were still in shorts when I answered the original question, on the original topic, and got it right first time.

 

[noseall]

Megawatt, softus, et al. RTFQ.

Well f**k you.

You were still in shorts when I answered the original question, on the original topic, and got it right first time.

i think you'll find "thank you" starts with "th" and not an "f".

 

[megawatt]

This is just a site classic.

All those who believed from the start that the aircraft would take off please take a bow and smile the wry smile which only comes from kicking ass on this forum.

All those who didn't and have accepted that they were wrong ... Hey, we all get it wrong on occasions guys.

The remainder (Blondini, KevNurse and JohnD for starters) who are now trying to hide behind semantics of the OP's question ... Get a life, hold your hands up, accept you got it wrong and move on.

The only thing which surprises me about this thread is how long its taken for people to see the light (and that stupid pilot in the video of course ... He should have his licence revoked by the FAA ... I wonder how many good pilots they asked before they found him ).

MW

 

[Trazor]

As a member of the yes it will take off club, I thought it was an interesting video.
But it did not, and could not perform the experiment as described in the original question.

What was very salient however, were the pilots words "we took off as normal". IE. their was no drag backwards, and no extra power was needed to take off.

Which proves that the wheel speed was irrelevant, Newtons third law of motion moves the plane forward, irrespective of what the wheels are doing.

So I believe you can now choose from 2 scenarios.

1) In a real life experiment, its just whether you think the wheels could survive the speeds generated.

2) In a purely theoretical argument, let the wheels do an infinite speed , the plane will still take off.

And johnD ,where I think your argument fails, about the impossibility of the question, is that you are considering a plane doing 1 mph and trying to hold station at 1 mph, and yes the wheel speed would build up very quickly.
Whereas in reality we are considering a rapidly accelerating plane where the conveyor will be playing catchup. The conveyor will have 15 to 20 seconds to impart as much speed as it can.

 

[megawatt]

What was very salient however, were the pilots words "we took off as normal". IE. their was no drag backwards, and no extra power was needed to take off.

I've been saying this from the start and any pilot would say the same ... Other than the clown they found for the video

MW

 

[empip]

1) In a real life experiment, its just whether you think the wheels could survive the speeds generated.

More to the point would the conveyor, reach or survive any sort of real speed??

 

[tim west]

ok reading the original question in its entirety, I don't think its a case of impossible scenario, rather a case of unnecessary dressing up of the scenario.

The wheels freewheel therefore the belt will drive them so they must be in synch at all times with the belt speed so what need for sensing?

the fact they freewheel means there is no relation to the aircraft itself so in theory the belt and wheels cancel each other out and are not relevant.

As there is no backward force acting against the thrust of the aircraft, then newtons law means there must be an equal and opposite reaction to the thrust unimpeded so therefore the aircraft must move forward as though it was a rocket in free air so movement will cause airflow over the wings and therefore lift will be achieved and so takeoff.

 

[Softus]

The wheels freewheel therefore the belt will drive them so they must be in synch at all times with the belt speed so what need for sensing?

Both of those statements are flawed.

the fact they freewheel means there is no relation to the aircraft itself so in theory the belt and wheels cancel each other out and are not relevant.

And that one.

As there is no backward force acting against the thrust of the aircraft, then newtons law means there must be an equal and opposite reaction to the thrust unimpeded

And that one.

so therefore the aircraft must move forward as though it was a rocket in free air

No - it will move like an aircraft through free air.

so movement will cause airflow over the wings and therefore lift will be achieved and so takeoff.

Correct.

 

[tim west]

glad you agree softus

 

[tim west]

come on only another five pages to go, dont fizzle out yet

 

[kevnurse]

Tim,

tim west wrote:
The wheels freewheel therefore the belt will drive them so they must be in synch at all times with the belt speed so what need for sensing?

if you watch the video of the model car on the belt you will see the wheels have black and white sectors to visually indicate wheel speed. While the car is not moving along the belt, the wheel speed and the belt are the same. But when the guy pushes the car, you will see the wheel sectors spin up. Therefore, the speed of a freewheeling wheel is proportional to the sum of the forces acting on it: one force is the belt and the other force is the hand pushing the vehicle.

Regarding your statement of the scenario being impossible: while the actual question posed on this forum described an impossible scenario (belt speed equals wheel speed), the video has proved, to some extent, that the concept is not impossible, provided that the wheels freewheel with minimal friction.

 

[megawatt]

As this thread has been quite clearly answered why is it still running?

Therefore, the speed of a freewheeling wheel is proportional to the sum of the forces acting on it: one force is the belt and the other force is the hand pushing the vehicle.

Errrmmm ... Yes, this was highlighted right at the start.

Where have you been Tim?

MW

 

[blondini]

come on only another five pages to go, dont fizzle out yet

Patience.

 

[tim west]

But its not finished yet!

if the wheel moves faster than the belt then they cannot be synched that would not be cconsistent with the belt matching the wheel speed.

if the wheel is faster then it must be skidding on the slower belt as the tyre has relationship with the belt due to friction between the tyre surface and the surface of the belt the wheel must stay in synch with the speed of the belt which happens to inrease in speed as you try to push the aircraft forward with thrust, in prac tice this doesnt happen but in the theoretical question if you splodged a blob of white paint on the tyre it should leave white spots on the belt evenly spaced out at the exact distance of the circumferance of the tyre.

bet it'll go the 5 pages now