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Imagine a plane is sitting on a treadmill
A physics and engineering riddle
Oh shit. Of course. Bloody hell
courtesy of GOOGLE!
"A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily."
"A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily."
NOW SHAT AP!
Everyone instantly thinks about the way cars work. I reckon that's why so many people get it wrong at first.
And because planes are just so heavy, people can't imagine the wheels are actually free to spin.
Well done Pirulero. Your googling skills are far beyond ours
Another thing that I think people are imagining is this conveyer belt running at a hundred miles an hour under this plane who's wheels are going around at a hundred miles an hour but the plane is actually going no where.
This is of course wrong, because the conveyer belt is going at the same speed as the plane. The plane in the above scenario is going 0klm/h. Therefore this situation cannot exist. Only when the plane is moving forward (and therefore getting the airspeed required to lift off), is the conveyer belt going to move at all.
This is of course wrong, because the conveyer belt is going at the same speed as the plane. The plane in the above scenario is going 0klm/h. Therefore this situation cannot exist. Only when the plane is moving forward (and therefore getting the airspeed required to lift off), is the conveyer belt going to move at all.
Ok to end this arguement someone had to go and test it!! See result from AV aviation!
QUOTE
OK, let's figure out why the airplane will fly.
We'll use Manfred again. Although we're bringing him forward into the 21st Century, we'll still let him use the 65 hp J-3. It doesn't really matter what airplane he flies, but he got used to the J-3 while he was demonstrating downwind turns and this one happens to have lifting rings on the top of the fuselage. It's also been modified with a starter so no one has to swing the prop.
Manfred's in the airplane. Old Hack has the Army-surplus crane fired up and he's picking up the J-3 and Manfred and carrying them over to Runway 27, which has been transformed into a 3,000-foot conveyor belt. It is a calm day. The conveyor drive is programmed so that if Manfred can start to move in the J-3, if he can generate any airspeed or groundspeed, the conveyor will move toward the east (remember Manfred and the J-3 are facing west) at exactly the speed of the air/groundspeed. Because the wind is calm, if Manfred can generate any indicated airspeed, he will also be generating precisely the same groundspeed. Groundspeed, of course being relative to the ground of the airport surrounding the conveyor belt runway. So, the speed of the conveyor belt eastbound will be the same as Manfred's indicated airspeed, westbound.
Manfred does his prestart checklist, holds the heel brakes, hits the starter and the little Continental up front clatters to life. Oil pressure comes up and stabilizes and Manfred tries to look busy because the eyes of the world are upon him, but all he can do is make sure the fuel is on and the altimeter and trim are set, then do a quick runup to check the mags and the carb heat. He moves the controls through their full travel and glares at the ailerons, doing his best to look heroic, then holds the stick aft in the slipstream to pin the tail and lets go of the brakes.
Baron of the Belt
So far the J-3 has not moved, nor has the conveyor. At idle power, there's not enough thrust to move the J-3 forward on a level surface, so Manfred starts to bring up the power, intending to take off. The propeller rpm increases and the prop shoves air aft, as it does on every takeoff, causing the airplane to move forward through the air, and as a consequence, forward with regard to the ground. Simultaneously the conveyor creaks to life, moving east, under the tires of the J-3. As the J-3 thrusts its way through the air, driven by its propeller, the airspeed indicator comes off the peg at about 10 mph. At that moment the conveyor is moving at 10 mph to the east and the tires are whirling around at 20 mph because the prop has pulled it to an airspeed, and groundspeed, of 10 mph, westbound. The airplane is moving relative to the still air and the ground at 10 mph, but with regard to the conveyor, which is going the other way at 10 mph, the relative speed is 20 mph.
Manfred relaxes a bit because the conveyor cannot stop him from moving forward. There is nothing on the airplane that pushes against the ground or the conveyor in order for it to accelerate; as Karen -- one of our techies here at the Lounge -- put it, the airplane freewheels. In technical terms, there is some bearing drag on the wheels, but it's under 40 pounds, and the engine has overcome that for years; plus the drag doesn't increase significantly as the wheel speed increases. Unless Manfred applies the brakes, the conveyor cannot affect the rate at which the airplane accelerates.
A few moments later, the roaring Continental, spinning that wooden Sensenich prop, has accelerated the J-3 and Manfred to 25 mph indicated airspeed. He and the airplane are cruising past the cheering spectators at 25 mph, while the conveyor has accelerated to 25 mph eastbound, yet it still has no way of stopping the airplane's movement through the air. The wheels are spinning at 50 mph, so the noise level is a little high, but otherwise, the J-3 is making a normal, calm-wind takeoff.
As the indicated airspeed passes 45 mph, groundspeed -- you know, relative to where all those spectators are standing beside the conveyor belt -- is also 45 mph. (At least that's what it says on Manfred's GPS. Being brought back to life seemed to create an insatiable desire for electronic stuff.) The conveyor is also at 45 mph, and the wheels are whizzing around at 90 -- the groundspeed plus the speed of the conveyor in the opposite direction.
Manfred breaks ground, climbs a few hundred feet, then makes a low pass to see if he can terrify the spectators because they are Americans, descendants of those who defeated his countrymen back in 1918.
It's All About Airspeed
(Don't try this at home!)
While the speed of the conveyor belt in the opposite direction is superficially attractive in saying the airplane cannot accelerate, it truly is irrelevant to what is happening with the airplane, because the medium on which it is acting is the air. The only time it could be a problem is if the wheel speed got so high that the tires blew out
We'll use Manfred again. Although we're bringing him forward into the 21st Century, we'll still let him use the 65 hp J-3. It doesn't really matter what airplane he flies, but he got used to the J-3 while he was demonstrating downwind turns and this one happens to have lifting rings on the top of the fuselage. It's also been modified with a starter so no one has to swing the prop.
Manfred's in the airplane. Old Hack has the Army-surplus crane fired up and he's picking up the J-3 and Manfred and carrying them over to Runway 27, which has been transformed into a 3,000-foot conveyor belt. It is a calm day. The conveyor drive is programmed so that if Manfred can start to move in the J-3, if he can generate any airspeed or groundspeed, the conveyor will move toward the east (remember Manfred and the J-3 are facing west) at exactly the speed of the air/groundspeed. Because the wind is calm, if Manfred can generate any indicated airspeed, he will also be generating precisely the same groundspeed. Groundspeed, of course being relative to the ground of the airport surrounding the conveyor belt runway. So, the speed of the conveyor belt eastbound will be the same as Manfred's indicated airspeed, westbound.
Manfred does his prestart checklist, holds the heel brakes, hits the starter and the little Continental up front clatters to life. Oil pressure comes up and stabilizes and Manfred tries to look busy because the eyes of the world are upon him, but all he can do is make sure the fuel is on and the altimeter and trim are set, then do a quick runup to check the mags and the carb heat. He moves the controls through their full travel and glares at the ailerons, doing his best to look heroic, then holds the stick aft in the slipstream to pin the tail and lets go of the brakes.
Baron of the Belt
So far the J-3 has not moved, nor has the conveyor. At idle power, there's not enough thrust to move the J-3 forward on a level surface, so Manfred starts to bring up the power, intending to take off. The propeller rpm increases and the prop shoves air aft, as it does on every takeoff, causing the airplane to move forward through the air, and as a consequence, forward with regard to the ground. Simultaneously the conveyor creaks to life, moving east, under the tires of the J-3. As the J-3 thrusts its way through the air, driven by its propeller, the airspeed indicator comes off the peg at about 10 mph. At that moment the conveyor is moving at 10 mph to the east and the tires are whirling around at 20 mph because the prop has pulled it to an airspeed, and groundspeed, of 10 mph, westbound. The airplane is moving relative to the still air and the ground at 10 mph, but with regard to the conveyor, which is going the other way at 10 mph, the relative speed is 20 mph.
Manfred relaxes a bit because the conveyor cannot stop him from moving forward. There is nothing on the airplane that pushes against the ground or the conveyor in order for it to accelerate; as Karen -- one of our techies here at the Lounge -- put it, the airplane freewheels. In technical terms, there is some bearing drag on the wheels, but it's under 40 pounds, and the engine has overcome that for years; plus the drag doesn't increase significantly as the wheel speed increases. Unless Manfred applies the brakes, the conveyor cannot affect the rate at which the airplane accelerates.
A few moments later, the roaring Continental, spinning that wooden Sensenich prop, has accelerated the J-3 and Manfred to 25 mph indicated airspeed. He and the airplane are cruising past the cheering spectators at 25 mph, while the conveyor has accelerated to 25 mph eastbound, yet it still has no way of stopping the airplane's movement through the air. The wheels are spinning at 50 mph, so the noise level is a little high, but otherwise, the J-3 is making a normal, calm-wind takeoff.
As the indicated airspeed passes 45 mph, groundspeed -- you know, relative to where all those spectators are standing beside the conveyor belt -- is also 45 mph. (At least that's what it says on Manfred's GPS. Being brought back to life seemed to create an insatiable desire for electronic stuff.) The conveyor is also at 45 mph, and the wheels are whizzing around at 90 -- the groundspeed plus the speed of the conveyor in the opposite direction.
Manfred breaks ground, climbs a few hundred feet, then makes a low pass to see if he can terrify the spectators because they are Americans, descendants of those who defeated his countrymen back in 1918.
It's All About Airspeed
(Don't try this at home!)
While the speed of the conveyor belt in the opposite direction is superficially attractive in saying the airplane cannot accelerate, it truly is irrelevant to what is happening with the airplane, because the medium on which it is acting is the air. The only time it could be a problem is if the wheel speed got so high that the tires blew out
QUOTE (Pirulero @ Dec 18 2006, 8:05 pm) 
courtesy of GOOGLE!
"A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily."
"A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily."
Pulling a rope isn't the same as pushing a huge mass.
Why bring bier into the equation? OK, so now that you have mentioned beer then it opens up a whole load of possibilities...
QUOTE (eurovol @ Dec 18 2006, 8:45 pm)
Pulling a rope isn't the same as pushing a huge mass.
Maybe. But pulling a bird is different.
QUOTE (eurovol @ Dec 18 2006, 8:45 pm)
Pulling a rope isn't the same as pushing a huge mass.
especially when its full of beer
Add up your vectors, your potentials and your kinetics and realize that airplanes are at full throttle being held back by only brakes at the start of the runway. Now realize that you actually have to have a runway and most, if not all, runways are built on a downhill slant. Turn that runway into a conveyor working against the airplane and you'll start to get the picture. Of course, I am assuming that DW is refering to a real commercial jet type airplane and not some trick balsa wood deal or rocket propelled jobby.
I wonder at what maximum runway incline a plane could take off at and what amount of extra length would be required per degree of inclination? Seems to me that if a plane could overcome its weight and the movement of the conveyor against it, the above question would sort of add light on a real world situation where real results could be extrapolated to actually answer this question.
I wonder at what maximum runway incline a plane could take off at and what amount of extra length would be required per degree of inclination? Seems to me that if a plane could overcome its weight and the movement of the conveyor against it, the above question would sort of add light on a real world situation where real results could be extrapolated to actually answer this question.
QUOTE (eurovol @ Dec 18 2006, 9:11 pm)
Now realize that you actually have to have a runway and most, if not all, runways are built on a downhill slant.
Hmmm... Maybe you should check out Bristol Airport for one... It's like taking off going up a bloody mountain
What about if you cut the conveyor belt up into long strips and built a fuckin' huge catapult with the framework?
As they say in Chicken Run, aye, that would give it thrust.
QUOTE (eurovol @ Dec 18 2006, 9:11 pm)
Add up your vectors, your potentials and your kinetics and realize that airplanes are at full throttle being held back by only brakes at the start of the runway. Now realize that you actually have to have a runway and most, if not all, runways are built on a downhill slant.
And do they tilt up and down depending on which way the planes are taking off
Has anyone ever used one of those cheap model treadmills that only go as fast as you go? you speed up, the conveyor speeds up. Same with the plane's engines: any force which would normally propel the plane forward will be totally absorbed by the belt, hence no lift.
Except plane engines don't push on the treadmill, they push on the air which isn't moving backwards... Or was that a troll attempt?
Now that is a troll attempt. Air doesn't move backwards? Ha! Ever stood behind a plane taking off?
Lets also get rid of this fallacy that somehow jet engines are different because they "push" the air instead of turn a wheel. Both propulsion sysems are because of pushing unlike the trick rope thrown in in some post above. Lets take a fan-boat and put it on a river that matches the thrust exerted by the big ass fan which isn't attached to any wheels or propellar. Now, lets assume the maximum thrust is met with the same amount of water moving the boat in the other direction. Now add 350 passengers and their luggage and assuming the fucking thing doesn't sink, tell me what it is most likely going to do?
Oh, and please don't try to tell me that wheels on a plane have no friction and that increased weight doesn't somehow magically dissappear in those frictionless wheels.
Lets also get rid of this fallacy that somehow jet engines are different because they "push" the air instead of turn a wheel. Both propulsion sysems are because of pushing unlike the trick rope thrown in in some post above. Lets take a fan-boat and put it on a river that matches the thrust exerted by the big ass fan which isn't attached to any wheels or propellar. Now, lets assume the maximum thrust is met with the same amount of water moving the boat in the other direction. Now add 350 passengers and their luggage and assuming the fucking thing doesn't sink, tell me what it is most likely going to do?
Oh, and please don't try to tell me that wheels on a plane have no friction and that increased weight doesn't somehow magically dissappear in those frictionless wheels.
The air as a medium isn't moving. A small amount of air (relatively speaking) is propelled backwards. This is enough to move the plane forward.
Edit: that is, there's still plenty of stationary air to provide lift as the plane moves through it.
Edit: that is, there's still plenty of stationary air to provide lift as the plane moves through it.
QUOTE (Wheel @ Dec 18 2006, 9:31 pm)
The air as a medium isn't moving.
There's enough air in this thread to launch an entire squadron.
QUOTE (sGb27 @ Dec 18 2006, 10:22 pm)
Except plane engines don't push on the treadmill, they push on the air which isn't moving backwards... Or was that a troll attempt?
Why would you say that? I'm no engineer, but it seems logical to me!
Anyone who says the plane won't take off after page 10 or 12 is a complete and utter moron. People just stop embarrassing yourselves. If you think the plane won't take off, you should really go take an intro to physics course and quit posting the same stupid misconceptions over and over here.
THE PLANE WILL TAKE OFF.
THE PLANE WILL TAKE OFF.
QUOTE (Wheel @ Dec 18 2006, 9:31 pm)
Edit: that is, there's still plenty of stationary air to provide lift as the plane moves through it.
You're assuming that the planes engines can overcome the force exerted by the conveyor belt and the planes weight. We are working with "as wide and as long as a runway" and "Everything is normal save for the fact the plane is on a treadmill" and nothing else. That is why I asked the question about runway inclines and the extra length needed. There are many variables and little room for error even under perfect conditions and a conveyor belted runway doesn't sound close to being perfect. Hell, even slight downdrafts at the wrong time can be a problem for a plane taking off.
The wheels of a plane are not driven, they are free to rotate backwards or forwards as long as the brakes are off. So the conveyor won't be exerting much of a force.
I would even venture a guess that the friction between the runway-treadmill and the air would cause a pretty sizeable amount of air to move with the treadmill. This would actually ADD to the lift of the plane, allowing it to take off using less distance (measured from the ground). The extra friction of the wheels would be negligible. This is conjecture and depends on factors not specified in the question. There is still no question that the plane will take off.
During take off, the plane is moving on the ground, not through the air. The weight and the friction does matter and more than you are giving it credit for. At best, the runway would have to be extended significantly and at worse, the wheels would burn up. Jet engines are not rockets. If the weight and wheels were not a factor, then why are there runways in the first place and why are they longer for larger airplanes?
This is turning into a 14th century flat earth discussion.
We know the clacking plane will fly, so why do people keep crapping on about vectors and slopes and wind speed.
Leave it out guvnor. Does the weight of the plane change due to the conveyor belt? No. Does the friction between the wheels and the ground change? No.
We know the clacking plane will fly, so why do people keep crapping on about vectors and slopes and wind speed.
QUOTE (eurovol @ Dec 18 2006, 10:02 pm)
The weight and the friction does matter and more than you are giving it credit for
Leave it out guvnor. Does the weight of the plane change due to the conveyor belt? No. Does the friction between the wheels and the ground change? No.
At take off the plane is moving along the ground and through the air. The air is stationary, apart from a relatively small amount which is propelled backwards.
QUOTE (Freiheit @ Dec 18 2006, 9:59 pm)
I would even venture a guess that the friction between the runway-treadmill and the air would cause a pretty sizeable amount of air to move with the treadmill.
I would hazard to guess that this is an invalid statement.
QUOTE (Johnny English @ Dec 18 2006, 10:03 pm)
We know the clacking plane will fly,
Under the conditions of the question, prove it?
Sorry I cannot prove it. You are not part of the "we".
Excuse me a minute.
*bangs head on table*
*bangs head on table*
QUOTE (Wheel @ Dec 18 2006, 10:04 pm)
At take off the plane is moving along the ground and through the air. The air is stationary, apart from a relatively small amount which is propelled backwards.
The ground is stationary unless it is a conveyor belt working against the movement of the plane. Is it enough to stop the thing from flying with everything else being normal, I say yes. I would most assuredly not board such a plane with the arguments put forth so far and definitely not without a significantly extended runway.
Basic high school physics: jet engines do not "push against" air.
Thrust is the result of the engine throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result. Regardless of the presence of air (i.e., for every action there is an equal and opposite reaction).
With that cleared up, please, continue the discussion.
EDIT: the only thing that air is needed for is for combustion within the engine.
Thrust is the result of the engine throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result. Regardless of the presence of air (i.e., for every action there is an equal and opposite reaction).
With that cleared up, please, continue the discussion.
EDIT: the only thing that air is needed for is for combustion within the engine.
QUOTE (eurovol @ Dec 18 2006, 10:09 pm)
I would most assuredly not board such a plane with the arguments put forth so far and definitely not without a significantly extended runway.
Why the f*** not? According to your argument the clacking thing is not going anywhere is it ? - so not much point in the "extended frigging runway".
Or do we detect a weaking of your position?
Fook me, is this still going ?
QUOTE (eurovol @ Dec 18 2006, 9:11 pm)
Now realize that you actually have to have a runway and most, if not all, runways are built on a downhill slant.
Actually, thinking about it more, that statement cannot be correct as runways are used in both directions depending on the wind direction.
QUOTE (bluedave @ Dec 18 2006, 10:14 pm)
Fook me, is this still going ?
Yes, and so much better that I could have ever hoped for!
It's complete bullshit.
Edit: that the runway goes downhill. Or uphill. Whatever.
Edit: that the runway goes downhill. Or uphill. Whatever.
QUOTE (Anwalt @ Dec 18 2006, 10:12 pm)
Basic high school physics: jet engines do not "push against" air.
Thrust is the result of the engine throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result. Regardless of the presence of air (i.e., for every action there is an equal and opposite reaction).
Thrust is the result of the engine throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result. Regardless of the presence of air (i.e., for every action there is an equal and opposite reaction).
Depends on how you define "push". Two ice skaters, one pushes the other, they move equally in opposite directions. Some of that is going on when the jet engine "pushes" against the air.
And if there were no air, the jet engine would be worthless. That's what makes it different from a rocket engine (which have no air intake opening), most of what comes out the back of a jet engine is the air that was sucked in. Only a relatively small amount of fuel makes up the output compared to that of a rocket.
If you throw a ball on ice, you move backwards a little as you propel the ball. Newton's (third) law. Jet engines are the same. No pushing going on.
Hmmmmm. So does a jet engine generate less thrust, when there is less air? Obviously plane moves quicker 'cos there is less friction, and engine may have a problem due to lack of oxygen - but does it struggle with less to push against? (ooooppps - which it doesnt do of course).
But then spacerockets zip along pretty quick...and there ain't much air to push against up there?
But then spacerockets zip along pretty quick...and there ain't much air to push against up there?
But what if the airplane, the conveyor belt and the air were all wrapped in a big hermetically sealed box (so, therefore, the relative speeds are all zero)...
...and then the big box is accelerated to close to the speed of light (with all its weird and wonderful distortions of length, time and mass)...
what would the answer be then?
All answers on the back of a very large envelope to Pedants of the World Unite, Munich...
...and then the big box is accelerated to close to the speed of light (with all its weird and wonderful distortions of length, time and mass)...
what would the answer be then?
All answers on the back of a very large envelope to Pedants of the World Unite, Munich...
Eurovol, you're just making yourself look stupid now, and I really do hope you don't have a degree in Physics or Engineering... If after having read this entire thread, you still don't think the plane will take off then there's nothing more any of us can say really, apart from we hope you never get involved with anything technical!
Big spanner.
QUOTE (Johnny English @ Dec 18 2006, 10:25 pm)
Hmmmmm. So does a jet engine generate less thrust, when there is less air? Obviously plane moves quicker 'cos there is less friction, and engine may have a problem due to lack of oxygen - but does it struggle with less to push against? (ooooppps - which it doesnt do of course).
Yes - categorically so. In the exact same way that helicopters cant operate above certain altitudes because they just cant generate enough thrust to keep them up
(I recall a story about the US flying some helicopters to a disaster zone in the back of a Hercules type plane. Nobody had actually noticed that the altitude was so great that the helicopters were useless and so they had to be loaded back on to the plane and flown away...)
All of these arguments are fabulously put forward and well thought out, but the problem still remains that the plane does not move in relation tothe ground, air, people on the ground or anything else, it remains perfectly still (in the question we were posed), so how the fuck does a plane that is not moving (except wheels) get enough air under its wings to cause it to take off of the ground. Maybe a prop plane that has the air pushing back intot he wings (probably not, but maybe), but a fucking jet?? The air is not being pushed undser the wing and it heavy as hell. Explain that and Ill concede.
My point was the definition of "push". Thrust and throw are very much like pushing.
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