Airplane on a Conveyor conundrum

[Sancho]

Oh surely not? Do people like that truly exist?

I think so. I started off thinking it was all to do with airflow (like the people mentioned in the 'airplaneonatreadmill' blog) but I think I understood at about post #350 (a record in comprehension for me).

[dieselboy]

sorry Diesel boy I edited my post, but you can see the changes luckily as you quoted me.

but I'll briefly repeat that you have to remember the question states the plane IS MOVING FORWARD.

If you're on the sideline, the plane is moving away from you at 10mph, and the conveyor is moving away (opposite) also at 10mph, the plane relative to the conveyor is travelling at 20mph. This 20mph is irrelevant - what matters is the 10mph that the plane is moving away from you. Change this speed to 500mph and the plane of course will take off (N.B it's wheels would be spinning at 100mph of course but only the 500mph matters to gt the lift).

sorry Slugsie.

ok please quote where it says that the plane is moving forward, and there is a person on the sideline measuring things and that the conveyors speed is measured from the ground.

this is why i say the plane takes off.

but they conveyor is the same speed as the ground.

[MattF]

people like what,
i made two valid points, because some people dont understand doesnt mean they are being wound up :/

Right. Just working on the assumption that you are actually having trouble comprehending the theory, I'll try a simple version. Imagine you have a sheet of paper on a table top. You place upon that piece of paper a model airplane with free wheeling wheels, (which all planes have). Now, if you hold onto the body of that airplane and pull the sheet of paper backwards, (without pressing downwards upon the body of the plane, but merely holding it stationary), you can pull the paper from underneath with minimal effort and the planes wheels will rotate.

Now, the important factor there is your hand. Imagine that the plane has in fact either a propellor or turbines, (which will be taking over from your hand). Either of those will be propelling air backwards past the airplane, thereby creating the thrust which pushes the plane forward. As you pull the paper backwards, the wheels themselves will be rotating due to the movement of both the plane in a forwards direction and the movement of the paper. The wheels, however, are only serving the simple purpose of stopping the planes axle bedding into the ground. The plane itself is propelling itself forward by pushing on the air behind it, so whatever happens on the ground level is actually completely and utterly irrelevant, other than for minor friction related properties.


Is that any simpler?

[TonyN]

If the plane produces enough power to go 500mph on solid ground, then you take this plane and put it on a conveyor belt so it's wheels freewheel, it'll stay stationary relative to the ground!

again the plane CANNOT move forward.

It wouldn't, its wheels still freewheel, what its sitting on isn't isn't important (leaving aside mythical friction on the wheel bearings stuff). as the planes forward momentum is indepentant to it wheels, as it gets it thrust by pushing air.

If the air was on the conveyer somehow, going backwards at the same speed the engine was pumping thrust against it out, the plane wouldn't go anywhere.

[Snorkel]

airflow IS everything. Either a plane sit's still and air is blown over the wings at 500mph, or the air is still and the plane has to travel through it at 500mph. Either way lift will occur.

As for the conveyor speed, one needs to know if it's speed is the same as that of the plane's speed relative to it, or that of the plane's speed relative to the ground. but I think this point has been stated already. However it was never stated in the original question.

[MattF]

I think so. I started off thinking it was all to do with airflow

It's completely to do with airflow. There'd be no forward momentum if not. Methinks some have been missing the fact that there are in actual fact two independent sets of airflow to consider.

[TonyN]

airflow IS everything. Either a plane sit's still and air is blown over the wings at 500mph, or the air is still and the plane has to travel through it at 500mph. Either way lift will occur.

As for the conveyor speed, one needs to know if it's speed is the same as that of the plane's speed relative to it, or that of the plane's speed relative to the ground. but I think this point has been stated already. However it was never stated in the original question.

OK, ignore the airflow over the wings, I was pointing out that whole trick to the question is its about how the plane moves to get to take off speed, it pushes on air, so the whole converyer belt/wheels thing is irrelevent, its there to catch you out.

[dieselboy]

Right. Just working on the assumption that you are actually having trouble comprehending the theory,
WHAT! i pointed out the theory in the first place!
I'll try a simple version. Imagine you have a sheet of paper on a table top. You place upon that piece of paper a model airplane with free wheeling wheels, (which all planes have). Now, if you hold onto the body of that airplane and pull the sheet of paper backwards, (without pressing downwards upon the body of the plane, but merely holding it stationary), you can pull the paper from underneath with minimal effort and the palnes wheels will rotate.
well done, you have realised that you need a force acting on the plane with your hand to keep it from going backwards. you provide a forwards force with your hand. this would be the thrust from an engine

Now, the important factor there is your hand. Imagine that the plane has in fact either a propellor or turbines. Either of those will be propelling air backwards past the airplane, thereby creating the thrust which pushes the plane forward. As you pull the paper backwards, the wheels themselves will be rrotating due to the movement of both the plane in a forwards direction and the movement of the paper. The wheels, however, are only serving the simple purpose of stopping the planes axle bedding into the ground. The plane itself is propelling itself forward by pushing on the air behind it, so whatever happens on the ground level is actually completely and utterly irrelevant, other than for minor friction related properties.


Is that any simpler?

ay? we have been over this numerous times. i completely understand both situations. the situation above, all your saying is that no matter how much force the plane and the conveyor produces, the plane will some how magically be able to get more thrust from somewhere else and take off.

noboddy as commented on my previous post where another scenario is the plane is already moving backwards.

you even state that you have to put your hand on the plane to stop it going backwards with the piece of paper.

the only way you wouldnt need to use your hand is if you did it really quickly. but even so, you will see the plane move backwards a tiny bit.
then if you had an infinate ream of paper and kept pulling it from underneath the plane, it would eventually be going the same speed as the paper. regardless of how well the wheels were made.

all the difference is, the better the wheels were made, the longer it would take for this plane to reach the speed of the paper. but it would still get there eventually.

[Antshiel]

In laymens terms, a plane gains speed by thrust(pulling air into the enging and forcing it out through the back) and not by direct drive to the wheels.

[MattF]

you even state that you have to put your hand on the plane to stop it going backwards with the piece of paper.

I'm now working on the simple assumption that you are indeed just taking the p1ss.

[dieselboy]

As for the conveyor speed, one needs to know if it's speed is the same as that of the plane's speed relative to it, or that of the plane's speed relative to the ground. but I think this point has been stated already. However it was never stated in the original question.

precisely. and as it is the factor that was left out,
if it had been included then the answer would be that the plane will not take off.

[Sancho]

precisely. and as it is the factor that was left out,
if it had been included then the answer would be that the plane will not take off.

read this again, it can't be put any simpler.

http://www.hiluxsurf.co.uk/forums/sh...&postcount=597

[dieselboy]

I'm now working on the simple philosophy that you are indeed just taking the p1ss.

not at all.

how can science be taking the p1ss. you state in one post that you need to use a force just to keep the plane stationary. this is to counteract the force of the paper.

then in another post you say that it doesnt matter about any backwards force, the plane will still take off.

if anyone is taking the mick its you isnt it?

[Sancho]

It's completely to do with airflow. There'd be no forward momentum if not. Methinks some have been missing the fact that there are in actual fact two independent sets of airflow to consider.

I should have shut up about that.....What I meant was I thought it was about the engines moving air over the wings through thrust (or suction to create thrust), whereas the point is actually that the plane moves forward almost entirely irrespective of the conveyor.

[dieselboy]

read this again, it can't be put any simpler.

http://www.hiluxsurf.co.uk/forums/sh...&postcount=597

yarr.
that would mean that there was a total friction limit acting on the plane.

if this was the case then all you would need is 0.1 higher than the total friction of the planes wheels, and then the plane would continue to accelerate infinately.
of course this doesnt happen cos, and we know why.

this is why i mentioned space, and crafts in space.
but that sort of confused again.