Mr Evil,
It is I who looks over your shoulder.
The speed of the wheels have no bearing at all. The only thing that matters is the speed of the air mass around the plane with respect to the plane. The force is applied to this air mass, within reason it is not affected at all by the wheel speed.
You missed your flight.
-Chris
It is I who looks over your shoulder.
The speed of the wheels have no bearing at all. The only thing that matters is the speed of the air mass around the plane with respect to the plane. The force is applied to this air mass, within reason it is not affected at all by the wheel speed.
You missed your flight.
-Chris
Hi rfbrw,
Okay, we had to develop other compounds to melt the ice. But it's our secret, okay?
The temperature here today was the coldest since the early thirties.
Now you know why we build things indoors.
There are colder places. Really.
-Chris
Okay, we had to develop other compounds to melt the ice. But it's our secret, okay?
The temperature here today was the coldest since the early thirties.
Now you know why we build things indoors.There are colder places. Really.
-Chris
Dear eVITAERC,
Well, interesting thought experiment, but it will not work. The plane in that case can only apply forces to the earth and the atmosphere rotating with it. Translating your results CORRECTLY would mean the aircraft could only apply forces to the conveyor belt (earth including atmosphere in your thought experiment). As stated before, this would be the case for any wheel driven vehicle, where your belt works perfectly fine.
The REAL plane does not move forward by applying forces to the ground (or the belt it its standing on), its engines create a force on the surrounding atmosphere, that means net forces in YOUR reference frame. Translating reality back to your thought experiment as correctly as possible, imagine a large strut construction on top of the plane, reaching up into space (where you are floating around in your spaceship). On top of this construction, we mount a large rocket engine, firing in the aircrafts longitudinal axis. If we now fire this rocket engine (which creates a net force in YOUR reference frame) you can speed up the old little earth as much as you want, the plane will no longer remain stationary to you...
Think about it..
Andreas
Well, interesting thought experiment, but it will not work. The plane in that case can only apply forces to the earth and the atmosphere rotating with it. Translating your results CORRECTLY would mean the aircraft could only apply forces to the conveyor belt (earth including atmosphere in your thought experiment). As stated before, this would be the case for any wheel driven vehicle, where your belt works perfectly fine.
The REAL plane does not move forward by applying forces to the ground (or the belt it its standing on), its engines create a force on the surrounding atmosphere, that means net forces in YOUR reference frame. Translating reality back to your thought experiment as correctly as possible, imagine a large strut construction on top of the plane, reaching up into space (where you are floating around in your spaceship). On top of this construction, we mount a large rocket engine, firing in the aircrafts longitudinal axis. If we now fire this rocket engine (which creates a net force in YOUR reference frame) you can speed up the old little earth as much as you want, the plane will no longer remain stationary to you...
Think about it..
Andreas
Mr Evil said:
There are limits to this system. The belt cannot exceed the take-off speed of the plane.
The wheel speed does have an effect. Imagine wearing roller-skates and standing on a conveyor belt moving backwards. Unless the wheels are perfect, you will be pulled backwards by the belt. In order to remain stationary you would have to exert a force (you could do that by somehow pushing air backwards like a plane). In order to move forwards, you would have to exert a force greater than the wheels, but if the conveyor belt accelerates to try to cancel that, you have to keep exerting more and more force.anatech said:Mr Evil,
It is I who looks over your shoulder.
The speed of the wheels have no bearing at all. The only thing that matters is the speed of the air mass around the plane with respect to the plane. The force is applied to this air mass, within reason it is not affected at all by the wheel speed.
You missed your flight.
-Chris
I don't see why there should be such a specific limit on conveyor speed; it's not stated in the original question. But as I said, you can reach any number of conclusions depending on what assumptions you make.rfbrw said:
Anymore? 😉Mr Evil said:
-------------
Understanding that the plane can propell itself forward through engine thrust regardless of whether it is sitting on a counteracting conveyor relative to its free wheels is an easy enough conclusion to reach (for some of us anyway).
What perplexes me is what happens to the conveyor and wheel rotation. First of all I thought it would increase in speed to inifinty (or the plane takes off 😉 ), but now I'm not so sure...
Well actually it would break and the tryes would probably be knackered by the time the plane's in the air, then there'd be some scared passengers...but hold on, there was no mention of passengers either
OK firstly I know a bit about aerodynamics, airfoils, lift etc and categorically know that the wheels have nothing to do with the plains thrust..... though if you strapped some wings onto a car and drove it fast enough it would eventually lift off the ground it just wouldn't stay in the air for very long, because it would have no thrust once the wheels leave the ground.... as many have already pointed out the wheels just support the plane whilst it is getting enough air speed to generate enough lift to take off.
I've attached a simple diagram of what I believe from the question the movement is of the wheel and the conveyer.
One thing I think that a lot of people are forgetting is that the plane has significant weight. This weight is pressing down on the wheels holding them quite firmly against the said conveyer. IF the jet engines were not running and the conveyer belt was running backwards the plane would go backwards.
Let’s turn the question upside down and say that the jet engines keep generating enough thrust to stop the conveyor from pulling the plane backwards, this would keep the plane stationary with respect to its surroundings no???? In this case the wing has no airflow as it is not moving through the air so it can't take off.
This is a relative motion question as far as I can see. provided the conveyor is always producing exactly the same amount of rearward motion to counteract the forward motion that would normally be produced by the thrust of the engines then the plane will remain stationary to an observer not on the conveyor (and to the surrounding air) and can therefore not take off.
The big difference with rockets which someone pointed out earlier is A, the amount of thrust, and B the thrust is directed at the ground opposing gravity. In this case there is one force pushing the plane forward, i.e. the thrust of the engines, and another force trying to pull the plane backwards, i.e. the conveyor belt, provided these forces are equal, the plane doesn't move. (Remember for every action there is an equal and opposite rection, in this case the equal and opposite reaction is the conveyor belt).
Think about it another way. If you tied a cable to the back of the jet which had a higher breaking strain that the thrust of the engines, and put them on full thrust, the plane wouldn't move, there would be no airflow and it wouldn't take off.
If the conveyor always matches the speed of the wheels, then how can there be any forward motion, relative to the ground outside of the conveyor??
Just my take on it
Tony.
edited to correct shocking spelling 😉
edit2: changed force to action and reaction, been a while 😉
I've attached a simple diagram of what I believe from the question the movement is of the wheel and the conveyer.
One thing I think that a lot of people are forgetting is that the plane has significant weight. This weight is pressing down on the wheels holding them quite firmly against the said conveyer. IF the jet engines were not running and the conveyer belt was running backwards the plane would go backwards.
Let’s turn the question upside down and say that the jet engines keep generating enough thrust to stop the conveyor from pulling the plane backwards, this would keep the plane stationary with respect to its surroundings no???? In this case the wing has no airflow as it is not moving through the air so it can't take off.
This is a relative motion question as far as I can see. provided the conveyor is always producing exactly the same amount of rearward motion to counteract the forward motion that would normally be produced by the thrust of the engines then the plane will remain stationary to an observer not on the conveyor (and to the surrounding air) and can therefore not take off.
The big difference with rockets which someone pointed out earlier is A, the amount of thrust, and B the thrust is directed at the ground opposing gravity. In this case there is one force pushing the plane forward, i.e. the thrust of the engines, and another force trying to pull the plane backwards, i.e. the conveyor belt, provided these forces are equal, the plane doesn't move. (Remember for every action there is an equal and opposite rection, in this case the equal and opposite reaction is the conveyor belt).
Think about it another way. If you tied a cable to the back of the jet which had a higher breaking strain that the thrust of the engines, and put them on full thrust, the plane wouldn't move, there would be no airflow and it wouldn't take off.
If the conveyor always matches the speed of the wheels, then how can there be any forward motion, relative to the ground outside of the conveyor??
Just my take on it
Tony.
edited to correct shocking spelling 😉
edit2: changed force to action and reaction, been a while 😉
Attachments
Mr Teal said:
The airplane is not moving!
It is the forward motion of the airplane that causes the air to flow over the wings and because of the design of the wings there is a pressure differential from over & under the wing. It is this pressure differential that causes airlift. Airlift is a technical term that means flying.
If you could imagine that with the questionaires scenerio that the aircraft should get airborn... then what. The airplane has no forward thrust. No forward thrust means no airflow. No airflow means no airlift. No airlift means no flying.
Man oh man. I can't beleive I just read 9 pages of this.
Ok, so the belt is only moving when the wheels are rotating. And I'll take the description to mean that the belt would rotate in the dirction which would keep the plane stationary if were driven by its wheels (ie belt speed matches wheel rotation). A plane is of course not propelled by wheel rotation, it is pulled through the air by its propeller.
So, turn the propeller on, the plane starts to move forward, the wheels start to rotate, the belt matches the circumfrential speed of the wheels, the wheels just keep spinning faster and faster.
I don't know how fast those wheels will be spinning, but the plane will take off.
Max
Ok, so the belt is only moving when the wheels are rotating. And I'll take the description to mean that the belt would rotate in the dirction which would keep the plane stationary if were driven by its wheels (ie belt speed matches wheel rotation). A plane is of course not propelled by wheel rotation, it is pulled through the air by its propeller.
So, turn the propeller on, the plane starts to move forward, the wheels start to rotate, the belt matches the circumfrential speed of the wheels, the wheels just keep spinning faster and faster.
I don't know how fast those wheels will be spinning, but the plane will take off.
Max
OK maxro, so with your reasoning, there is a difference between a stationary wheel and a spining wheel (with regards to the amount of force being exerted in a rearward motion on the plane). If once the wheel is turning, there is less force required to keep the plane stationary, than the force applyed by the belt to a stationary wheel (ie one that can't turn). in this case the plane WILL be able to accelerate forward, and the wheels and belt will just keep spinning faster and faster as the planes airspeed increases to the point where it can take off.....
I'm afraid my physics is too distant in my past for me to remeber the equations required to work this out... seems reasonable, but I still think that when you take into account there is a thrid force Ie gravitational pull which is pulling the plane down against the belt, the plane will remain stationary if both the thrust and the belt force are applied equally from the start. but now I'm starting to wonder, the only friction when the wheel is turning (relative to the motion of the belt), is the friction in the bearings.....arrrggghh I don't want to think about it 😉
Tony.
I'm afraid my physics is too distant in my past for me to remeber the equations required to work this out... seems reasonable, but I still think that when you take into account there is a thrid force Ie gravitational pull which is pulling the plane down against the belt, the plane will remain stationary if both the thrust and the belt force are applied equally from the start. but now I'm starting to wonder, the only friction when the wheel is turning (relative to the motion of the belt), is the friction in the bearings.....arrrggghh I don't want to think about it 😉
Tony.
maxro said:
Oh com'on, please, I can't believe I'm still explaining this:
As the wheel spins faster, SO WILL THE CONVEYER BELT, and in such a way that the two motions cancel out!
so this whole thing about wheels spinning faster and faster IS IRRELEVANT
wintermute said:
Thank you
So far you're the only person who's displayed any knowledge about the physics of rolling things.
eVITAERC said:
What do you mean cancel out? Cancel out what, in relation to what?
The wheels are not what is propelling the plane, they are simply along for the ride. If we assume no friction from the wheels (and I think we can when we are to imagine a conveyer belt capable of any speed) then they simply do not impede the planes progress forward and the plane takes off.
So yes, wheel speed is irrelevant.
just to review:
"Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation."
Sorry my explanation sucks, I'm not a scientist.
Max
maxro said:
Like wintermute said, it is easy to be decieved into thinking that in this situation the wheel speed doesn't come into play. HOWEVER, if you analyse the system methodolically like a physicist, you will discover that the kinematics wheel is actually a critical part of this system.
The plane is held down by gravity, just like a speaker cab builder who doesn't have a glue clamp and is forced to apply pressure to the joints by putting Nelson Pass style huge @$$ transformers on it. The speaker is pressed against the floor by weight, and one cannot push it along the floor at will without sliding it against friction.
Here the plane is in the same situation, held to the ground with its own weight. Except that it contacts the ground via a wheel instead of a rigid part of its body.
Now it's too simple-minded to say that the engine pushes the fuselage along, and the wheel just comes along for the ride. In a sense this is true, because the engine is what provides the forward push. So the engine is pushed forward, the fuselage is pushed forward, the landing gear, which is attached rigidly to the plane, is pushed forward, and the wheel (imagine the breaks are on), which is attached rigidly to the landing gear, is pushed forward.
All is well. Not. The wheel is rigidly held to the ground by STRONG friction, because the plane is uber heavy. The wheel's forward motion is resisted, and this transfers to the entire plane, much like chaining someone's feet to the wall.
The wheel has two options. It can refuse to roll and simply overcome the friction. In this case it'll slide and grind along the floor, and the plane will eventually come to a painful takeoff.
Or it can roll. Rolling is such a miraculous motion that it allows horizontal movement without overcoming friction in any way.
So there's only two ways that the plane can move forward. Either the wheel has to roll along or it has to slide.
Think about this. REALLY think about this.
The wheels actually have everything to do with this question.
wintermute said:
Is that what I was saying? Hmm... I'm not sure I understand your interpretation of what I wrote.
Maybe someone should go to an airport and try this out.
Items needed:
2 people
skateboard
fan
battery
potentiometer
At the airport, find one of those moving sidewalk things with a walkway next to it. Get a fan and hook it up to some batteries and put it on a skateboard. Have a friend ride the moving sidewalk. Adjust the fan with a potentiometer until it moves on level ground at the same speed as your friend on the moving sidewalk. Once you reach the end of the moving sidewalk, turn the fan-powered skateboard around and put it on the moving sidewalk. Now what will happen?
This of course only tests at one belt speed and I'm sure there are a whole bunch of variables not accounted for.
max
Mr Evil said:
The limiting factor is the take off speed. Once the plane is in the air, its all over.
sss said:
Haven't really read what I've written, have you.
rfbrw said:
Well, luckily I don't 😉 Obviously the plane moves forward because the the engine providing linear propulsion.
My whole argument is that the plane can't move unless the wheels are rolling. Or else it must slide.
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