Did we ever discuss it here? One of the pre-wipe topics I assume?
Its not even a brain teaser. No airflow across the wing no lift. Was rehashed yet again on "Ask a Pilot" on salon.com this week.
Didnt someone do a video for this with scale models?
A harrier jump jet would have no probs. But for a normal plane wouldnt it move forward regardless? Its pushing against air, all the wheels are doing is spin around, they dont contribute anything to forward motion, on a converyor belt theyd just be spinning much faster than normal. The engines push against air, not the runway.
edit: thinking about it, Id put a tenner on it taking off.
Also, you could just point the plane backwards, brakes and flaps on full, and spin the belt up to 300mph.
Wheels on planes are unpowered but have brakes.
Just to be completely off topic there was a couple of reports of people being stung by scorpions on US planes recently.
I wonder if they are going to make a movie about that.
not bitten by snakes then? lol
yeah of course it would take off, engines produce thrust and the wheels freewheel
Two conflicting opinions already......
:mrgreen:
All the conveyor belt is going to do is make the wheels spin a bit more - the plane will still take off.
In fact the conveyor belt can spin backwards at twice the speed that the plane moves forwards - the plane will still move forwards - its wheels will just be spinning a bit faster than normal.
(we do have to assume that the tires dont burst etc.. )
Quote from: maximusotterIts not even a brain teaser. No airflow across the wing no lift. Was rehashed yet again on "Ask a Pilot" on salon.com this week.
Dude its got you assuming the wrong answer - how is there going to be no airflow across the wing - its a plane not a car with wings.
Chris can you edit you first post to include the question - Im assuming it is this one....
Quote"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"
Im not sure youre right there m8.
There has to be airflow across the wings for the plane to lift. The airflow is normally generated by forcing the plane to move through the air at a certain speed.
If the plane is on a runaway that is moving in the opposite direction at the exact same speed as the plane then the relative travel of the plane to the air will be zero; the plane will be motionless (think running on a treadmill) and there will be no airflow across the wings to generate lift from.
how is a moving floor going to stop the plane moving foward when the thrust is genereated by the engines moving air on the wings?
As you state a planes thrust comes from its engines and not the friction/drive of its wheels.
If the conveyor is travelling at an equal speed to the plane but in the opposite direction (treadmill) then no matter how much thrust the plane uses the conveyor will cancel out any movement.
how? the wheels are freewheeling, like a shopping trolley rolling down a hill
the belt could be doing mach 2 for all it matters, the plane will still accelerate and take off, yeah the wheel bearings might cook but thats a minor detail
when youre running on a treadmill do you get "wind blowing" into your face due to the speed at which youre traveling ?
Let me put it another way...
If you put the plane on a big "car" jack so that the wheels were not touching for the floor/conveyor or anything and stuck the engines on...would you take off ?
when your on a tread mill your not using your arms to move are you
Quote from: NimrodLet me put it another way...
If you put the plane on a big "car" jack so that the wheels were not touching for the floor/conveyor or anything and stuck the engines on...would you take off ?
youd open up the throttles it would pull the jacks over and get rolling else rip the engines off the wings
this is like asking if sea planes really take off form water ffs
I see what youre getting at; the plane is free wheeling, the thrust/drive is not dependant upon the friction/force of the wheels and ground.
If this is true why do planes need a runaway at all ? Why dont they all take off from a static start ?
Quote from: NimrodIf the plane is on a runaway that is moving in the opposite direction at the exact same speed as the plane then the relative travel of the plane to the air will be zero; the plane will be motionless (think running on a treadmill) and there will be no airflow across the wings to generate lift from.
Imagine a plane on ice with its wheels locked up - it can still take off - the speed at which the wheels turn around is irrelevant.
thinking running on a treadmill is the trap people fall into - if the plane was a car with wings then youd be right - however planes move by using a propeller or jet engine not by powering their wheels.
Supposing the plane produces enough thrust to move it forwards at 200mph - the theoretical runway therefore moves backwards at 200mph - all that is going to happen is the plane wheels will spin around at 400mph. The plane will still move forwards regardless as it isnt driven by its wheels it is driven by its jet engines.
The point of forces have no connection, unless there is a direct friction between the wheels and the struts that connect them to the plane then the conveyer will have effect on it.
The plane thrust will just push the plane forward, as i said, unless the wheels create a negative force due to friction with the conveyor there would be no negative force
Quote from: NimrodI see what youre getting at; the plane is free wheeling, the thrust/drive is not dependant upon the friction/force of the wheels and ground.
If this is true why do planes need a runaway at all ? Why dont they all take off from a static start ?
the need runways for a smooth flat surface to get up to speed on and a long run to slow down on when landing as they deffo need the friction then
Quote from: Binary ShadowQuote from: NimrodI see what youre getting at; the plane is free wheeling, the thrust/drive is not dependant upon the friction/force of the wheels and ground.
If this is true why do planes need a runaway at all ? Why dont they all take off from a static start ?
the need runways for a smooth flat surface to get up to speed on and a long run to slow down on when landing as they deffo need the friction then
also they tend to use their engines to slow down - if a pilot simple locked up the wheels upon landing hed just burst the tyres
reverse thrust indeed but even so they need a surface to apply brakes on as well, and a Cessna wont be using reverse thrust anytime soon
ok, I just want to come at this another way as I cannot see it myself...
Do you know those big roller things they use to test cars on? The car drives its wheels but the the rollers just spin so that the car stays stationary...
Am I right in thinking that youre proposing that if you were to put a plane on those rollers the plane would not be hindered because its thrust is not directly linked to the movement of the wheels ?
is that not the truth?
imagine a car spinning along on the rollers the rollers exactly matching the speed of the cars wheels...
...Now imagine the mechanic crashing another car into the back of it at 100mph :mrgreen: - the first car will fly forwards off the rollers as a force was applied to it pushing it forwards
wouldnt make a blind bit of difference.
As in order to get airflow over the wings, the plane needs to move.
If the road underneath is moving at speed that the plane will be doing over the ground, then there will be no net airflow over the wings. the plane will be stationary.
Put a rocket car on a treadmill... just because the force is applied elsewhere doesnt mean that it has no effect.
Push a car... now push a car on a treadmill that is so well lubricated theres next to no resistance, as the wheels roll, the road will move backwards. Keeping the car where it is.
You can push that car as hard as you like so long as the rollers match your speed perfectly and it aint going anywhere.
A plane... moving along the ground still has to overcome the resistance of its undercarriage on a road. if the road is giving way underneath it (moving backwards as it attempts to move forward) then it wont move.
Equal and opposite reaction and all that.
plane has to overcome resistance of road and wheels to move on a road, it applies a force equal to the force its applying forward, backward on the road, the road spins... the wheels spin, the plane goes nowhere.
Put a rocket on wheels horizontally and then put on the conveyor belt. The rocket is gonna move forward. The engine pushes against the air. The wheels add hardly any friction so it might as well be flying through the air.
Rollers wouldnt work cos the plane would move forwards and come off them.
A force diagram would explain all.
edit: Its been a long time since I did one of these, hope the arrows are pointing the right way;
(http://www.palmer934.plus.com/force.jpg)
Quote from: M3ta7h3adPush a car... now push a car on a treadmill that is so well lubricated theres next to no resistance, as the wheels roll, the road will move backwards. Keeping the car where it is.
Unless you the driving force is on the treadmill as well that is crap, if your pushing the car from a solid bit of ground the fact the treadmill is moving will make no difference the car will have to move the way your pushing it.
No it wouldnt.
a rocket on wheels horizontally (rocket car), the rocket will attempt to move forward, in order to do so the wheels WILL NEED TO ROTATE! (assuming that they arent siezed or something equally abstract).
The MOMENT the wheels rotate... the treadmill rotates. From then on in, those wheels will be spinning like crazy, but that treadmill in theory will keep on right with em.
yeah and all the time the car will be accelerating as the force is applied from the jet not the wheels on the road
Quote from: Binary ShadowQuote from: M3ta7h3adPush a car... now push a car on a treadmill that is so well lubricated theres next to no resistance, as the wheels roll, the road will move backwards. Keeping the car where it is.
Unless you the driving force is on the treadmill as well that is crap, if your pushing the car from a solid bit of ground the fact the treadmill is moving will make no difference the car will have to move the way your pushing it.
no... your seeing movement as a floating car.
The car in order to move forward, will need to roll its wheels.
When it rolls its wheels...
THE ROLLERS WILL ROLL!
Therefore any forward movement completely gets cancelled out. You just come flat up against a non-moving car whilst trying to push it. You are pushing against a wall to all intents and purposes, on wheels... on rollers... therefore on solid ground. only the wheels are spinning realllllllllyyy fast.
get a treadmill, jump on it with roller skates on, then pull yourself forward with your arms, you will move forward
Quote from: Binary Shadowyeah and all the time the car will be accelerating as the force is applied from the jet not the wheels on the road
No... the wheels will be the only things moving on the car.
How do bearings work mate?
that and the turbine in the jet engine pushing it all fowards doesnt matter if the wheels are spinning at mach 2 itll go and go
Quote from: Binary Shadowget a treadmill, jump on it with roller skates on, then pull yourself forward with your arms, you will move forward
Only because your roller skates have less resistance than a treadmill.
Put a nice loosely moving treadmill there and I could quite happily get towed while wearing roller skates if I wanted.
When I say towed, I mean hold onto a bar mounted to the front of the treadmill much like the practice watersking units :)
Quote from: Binary Shadowthat and the turbine in the jet engine pushing it all fowards doesnt matter if the wheels are spinning at mach 2 itll go and go
your saying that a jet engine will defy the laws of physics.
TO GO FORWARD do you admit in a normal situation, the plane is required to make its wheels rotate, in order to get sufficient airflow over its lift surfaces?
to move foward the plane has to apply a force in the direction that overcomes the near frictionless wheel bearings.. oh yes i can see that being an issue for 4 olympus engines capable of dragging an airliner to mach2 and beyond
QuoteOr
Speed of Wheels - Speed of Conveyor = Speed of Plane
In the statement of question, Francisco stated:
"The conveyer belt is designed to exactly match the speed of the wheels
at any given time, moving in the opposite direction of rotation."
He did not state the speed of the aircraft, but the speed of the wheels.
Given the formula above the forward speed of the plane should always be
zero and can not generate enough lift to takeoff.
is the viewpoint im taking on this. (googled it).
If the treadmill is intended to match the speed of the airplanes wheels at every moment, then frictionless wheel bearings mean naff all. Wheels will be spinning... treadmill will be spinning just as fast. To someone standing outside the treadmill plane system it will appear stationary, onboard ground speed will be sky high, yet airspeed will also be 0.
Airspeed is what allows the plane to take off. NOT GROUND SPEED. So... there.
Edit:
Or in your words, in relation to the ground (treadmill) the plane will be moving at mach 2. in relation to the air, itll be stationary.
what a load of crap, whoosh it will be hurting down that runway and you better hope the treadmill is able to travel at take off speed without blowing up
theres no resisting force!!!
The plane takes off. Now move along folks.
Id explain it, but in all honesty trying to get simple physical concepts through to a certain person on these boards is like slamming ones head in a vault door.
All Ill say is this: The wheels are freely rotating. Their rotation doesnt exert a force on the plane itself. The plane is "pushing" itself forward off the air around it, not the road.
let me put it simply
concorde sat on end of convayor, has 4x35190 lbf thrust being produced, whats going to stop it? theres no way the wheels spinning will slow the aircraft down as the friction is minimal
There has been no physical constraints put on the treadmill other than "it has been designed to match the speed of the wheels at every moment".
At least thats the viewpoint im taking it from, as thats the traditional method of discussing this.
Youve just clarified for me that the wheels would have to turn in order for the plane to move forward.
Ive just told you the treadmill would move accordingly.
So... the wheels will move forward, the treadmill will move backwards.
Meaning that the wings stay where they bloody well are, and the plane doesnt take off.
Do you really need me to spell it out for you some more?
no cos your talking crap, doesnt matter what the wheels do, could roll 1mph or 60000mph the fact remains that the treadmill will accelerate as the plane does and it will lift off no worries
Quote from: Binary Shadowlet me put it simply
concorde sat on end of convayor, has 4x35190 lbf thrust being produced, whats going to stop it? theres no way the wheels spinning will slow the aircraft down as the friction is minimal
the 4x35190lbf of treadmill moving in the opposite direction.
Quote from: Binary Shadowno cos your talking crap, doesnt matter what the wheels do, could roll 1mph or 60000mph the fact remains that the treadmill will accelerate as the plane does and it will lift off no worries
IN THE OPPOSITE DIRECTION.
ACCELERATION + ACCELERATION IN AN OPPOSING DIRECTION DOES NOT EQUAL TWICE THE ACCELERATION. IT EQUALS 0!
Quote from: M3ta7h3adThere has been no physical constraints put on the treadmill other than "it has been designed to match the speed of the wheels at every moment".
At least thats the viewpoint im taking it from, as thats the traditional method of discussing this.
Youve just clarified for me that the wheels would have to turn in order for the plane to move forward.
Ive just told you the treadmill would move accordingly.
So... the wheels will move forward, the treadmill will move backwards.
Meaning that the wings stay where they bloody well are, and the plane doesnt take off.
Do you really need me to spell it out for you some more?
:lol:
You never fail to amaze me. A true bastion of bloody-mindedness :D
the treadmill cannot place any force on the aircraft unless the brakes on the wheels are applied.. do you not get that?
Heres a test for metalhead.
Draw me a force diagram of the plane. Show me where the force exerted from the conveyor belt on the plane is. Resolve forces.
I shall mark your work, points will be deducted for poor presentation and grammar.
pmsl
Quote from: funkychicken9000Quote from: M3ta7h3adThere has been no physical constraints put on the treadmill other than "it has been designed to match the speed of the wheels at every moment".
At least thats the viewpoint im taking it from, as thats the traditional method of discussing this.
Youve just clarified for me that the wheels would have to turn in order for the plane to move forward.
Ive just told you the treadmill would move accordingly.
So... the wheels will move forward, the treadmill will move backwards.
Meaning that the wings stay where they bloody well are, and the plane doesnt take off.
Do you really need me to spell it out for you some more?
:lol:
You never fail to amaze me. A true bastion of bloody-mindedness :D
Why because my opinion counteracts the almighty one?
I am simply stating what I think will happen.
Only way that plane would take off is if the force applied to the air was enough to cause the air to move faster over its wings (completely irrelevant to tyre speed), but in my head the volume of air removed from the front of the turbines would not cause a net movement of air over the whole surface area of the wing.
Same thing can and has been modelled in a wind tunnel.
Put plane on a flat surface not moving, and lock the wheels (or remove and place on a pole for arguments sake) so the plane is not moving relative to the ground around it (same state that the plane is in on the treadmill).
You will need to add a flow of air over the wings of the plane to cover the entire wing surface to get lift. Sooooo wind tunnel turned on.. voila.. plane takes off without moving its wheels, but it required a socking great big fan sucking air over the wings.
Relying upon the planes engines to create enough of a volume of air to pass over the wing surface (when its infact a highly localised thing in my mind, i.e. directly in front of the turbines, and behind the turbines, no discernable directed airflow over the wings) will not create enough lift to take off.
Lets see, this comes from a Physics graduate who is now doing a PhD for Rolls Royce (who make the bloody jet engines so should know how they work).
The planes wheels spin freely, so no force is transferred from the ground to the plane. Its like saying an airborne plane cant go forwards if theres another plane directly beneath it flying in the opposite direction. No matter how fast you spin the wheels it wont affect the movement of the plane.
What youre basically arguing is that if you suspend a plane in the air and spin its wheels really fast, itll move forwards.
In simple terms, the engines push the air to make the plane move. The wheels are there solely to stop the bottom of the plane scraping along the ground making a horrible noise and lots of sparks.
edit, you just beat me: the amount of air sucked in by jet engines is humungous, hundreds of times more than the fan in a wind tunnel moves. Were talking tonnes of air every few seconds here (why do you think jet engines are so bloody loud?)
The engines pull the plan forwards with lots of force, causing it to speed up (forget about flying for now). Engines=lots of power=lots of acceleration = lots of speed. Once the plane, from this acceleration, is moving at 200mph RELATIVE TO THE AIR (the wings dont care about the ground) the air is passing over them at 200mph, so lift is generated.
Another way to see this is if a plane was stood still and wind was blowing at it at 200mph (ignore friction here) it would take off. This is how you see some birds hovering in teh wind near cliffs looking for prey.
Fair cop someone whos opinion means something other than almighty one over there.
QuoteWhat youre basically arguing is that if you suspend a plane in the air and spin its wheels really fast, itll move forwards.
Wasnt really suggesting this at least in my thoughts at least.
You mean to say that shove a plane on a pole, turn the engines on... itll take off? Cause in my mind im still just seeing a big assed airplane, not going anywhere... making a nice racket. :D
Quote from: M3ta7h3adTO GO FORWARD do you admit in a normal situation, the plane is required to make its wheels rotate, in order to get sufficient airflow over its lift surfaces?
erm nope....
imagine a plane on ice
Quote from: ShakeyLets see, this comes from a Physics graduate who is now doing a PhD for Rolls Royce (who make the bloody jet engines so should know how they work).
The planes wheels spin freely, so no force is transferred from the ground to the plane. Its like saying an airborne plane cant go forwards if theres another plane directly beneath it flying in the opposite direction. No matter how fast you spin the wheels it wont affect the movement of the plane.
What youre basically arguing is that if you suspend a plane in the air and spin its wheels really fast, itll move forwards.
In simple terms, the engines push the air to make the plane move. The wheels are there solely to stop the bottom of the plane scraping along the ground making a horrible noise and lots of sparks.
edit, you just beat me: the amount of air sucked in by jet engines is humungous, hundreds of times more than the fan in a wind tunnel moves. Were talking tonnes of air every few seconds here (why do you think jet engines are so bloody loud?)
lol cause they are burning fuel in a very loud manner? :D lol. Id no idea about the volume of air theyd push back. in that case id happily agree itd take off :)
As for the original problem - the plane takes off
it is not a car - it does not take off by spinning its wheels - the treadmill has very little effect - the big jet engines or propeller or whatever you want to have on the plane does have a big effect.
Quote from: DaveQuote from: M3ta7h3adTO GO FORWARD do you admit in a normal situation, the plane is required to make its wheels rotate, in order to get sufficient airflow over its lift surfaces?
erm nope....
imagine a plane on ice
so it uses skis instead of wheels? point?
and for the people who still dont get it - imagine a plane coming into land on the conveyor belt.
suppose it it flying 3 feet above the conveyor belt
now suppose its wheels touch down on the conveyor belt - does the plan come to a complete rest instantaneously?
right Ive done a quick google & there is a fairly good explanation here:
http://mouser.org/log/archives/2006/02/001003.html
no it doesnt as its moving relative to the stationary ground surrounding the treadmill.
Thunderbirds once did a rescue mission like that though.
involved 3 cars and them matching the speed of the plane, and the plane landing, on their roofs, and the cars slowing down. :|
Shakey:
QuoteThe engines pull the plan forwards with lots of force, causing it to speed up (forget about flying for now). Engines=lots of power=lots of acceleration = lots of speed. Once the plane, from this acceleration, is moving at 200mph RELATIVE TO THE AIR (the wings dont care about the ground) the air is passing over them at 200mph, so lift is generated.
aye which is what I was getting at.
Lots of power to pull it forwards along a nice runway to generate that 200mph airspeed, but the treadmill in my mind had lots of power to let it pull itself forward on the runway/treadmill, which pulled it "backward" by the same amount (doesnt happen on a real runway of course). Which meant it was in fact a stationary airplane, having never actually moved. so.. full power with brakes on so to speak. Soooo couldnt see how it could take off.
Quote from: Daveright Ive done a quick google & there is a fairly good explanation here:
http://mouser.org/log/archives/2006/02/001003.html
Good find :)
Quote from: M3ta7h3adLots of power to pull it forwards along a nice runway to generate that 200mph airspeed, but the treadmill in my mind had lots of power to let it pull itself forward on the runway/treadmill, which pulled it "backward" by the same amount (doesnt happen on a real runway of course). Which meant it was in fact a stationary airplane, having never actually moved. so.. full power with brakes on so to speak. Soooo couldnt see how it could take off.
because it can still move forwards at 200mph while the runway moves back at at 200mph
wheels simply move along at 400mph
the treadmill can move backwards at 1000 miles per hour in theory (wed have to assume very strong wheels here) & it wouldnt matter.
imagine if you tied a rope to the plane & didnt turn the engines on but spun the treadmill - the plane would stay still - it doesnt matter how fast you spin th treadmill the plane will stay still and the wheels move faster to compensate - now imagine if you apply a big force to it - i.e. from the jet engines - it will move forwards - the wheels are free moving so the fact that the ground is moving has relativly little effect.
The main force a plane has to overcome on takeoff is the air resistance.
lol dave. I get it.
Was missing the vital part that your link gave me :)
Read as far as "2m/s in total" and went ahhhhh yea.. :D
sorry dude - was writing that previous post before I saw your reply ;)
So would the conveyor belt have to be the same length as a normal runway? Im guessing itd be the same length, but it would be better for crash landings cos its bouncy.
edit: dinny question, but hey..
Quote from: M3ta7h3adlol dave. I get it.
Was missing the vital part that your link gave me :)
Read as far as "2m/s in total" and went ahhhhh yea.. :D
The penny drops. You assumed the plane stayed stationary, when clearly it doesnt. That was your stumbling block; anything you dreamed up beyond that point was consequently irrelevent.
i was pretty certain it was going to take off, but wasnt gonna but in cause i cannot resolve forces to save my life :D
So basically:
IF the belt could actually manage to hold the plane stationary, it would never take off. No lift would occur (lift is due to lower pressure over the aerofoil wings made by air rushing over the top faster than the bottom).
BUT the belt simply couldnt do that, as the plane is pushing on air, not on the ground, and so will reach desired take-off speed anyway, belt or no belt, wheels or no wheels.
Here ya go, morans:
Quote from: ask a pilotJan. 5, 2007 | Last month, New York Times technology columnist David Pogue set the Web abuzz by rekindling an old brainteaser about whether a theoretical airplane would be able to take off from a theoretical treadmill. The puzzle was "ripping around the Internet" (Pogues words), and appeals for clarification quickly reached Ask the Pilots in box. Will it or wont it fly, people wanted to know, imploring me to weigh in.
Belatedly, and grudgingly, I will now do so. Such topics tend to induce the rapid closure of my eyelids, and while Id like to tell you this is the kind of shop talk that keeps aviators engaged and alert in those quiet midnight hours high above the ocean, nothing could be further from the truth. (Mostly theyre just bemoaning the loss of their pensions and talking about movies.) Nevertheless, here goes ...
"Imagine a plane is sitting on a massive conveyor belt," poses Pogues Dec. 11 Times blog, "as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"
When I last checked, more than 860 people had posted their opinions, split about 50/50 between those who say the airplane will fly and those who insist it cant. If you look carefully you can locate my own contribution, flatly declaring that no, absolutely not, the aircraft will not get off the ground. "If this is truly ripping around the Internet," I snarked, "then heaven help us. The plane will not fly. Of course it wont fly."
And why should it? How can it fly if its not moving? For an aircraft to get and stay aloft, it needs lift; it needs air passing above and below its wings. And for that it needs to move. For a cursory lesson on how this works, simply shove your arm out the window of a speeding car. Shape your hand into an approximation of a wing, angle it slightly into the oncoming wind, and voilà, its flying.
Now, imagine you are in that same car, on a treadmill. The cars wheels are spinning -- be it at 60 mph or 600 mph -- but when you put your hand out the window, does it rise up? Of course not. Your hand wont fly, and the plane wont fly either for exactly the same reason: because for all its efforts, the vehicle isnt moving. You have zero relative speed and zero lift.
This seemed so obvious that it needed a caveat: "On the other hand, if you were able to generate a tremendous enough amount of thrust," I noted in a follow-up post, "and redirect the vector of that thrust downward, you could, conceivably, lift the plane off like a rocket. Heck, you can make anything fly if you stick enough power under it. But that isnt fair to the spirit of the premise."
Except, wait a minute, what is the premise?
Go back and read it. "Imagine a plane is sitting on a massive conveyor belt," it says, "as wide and as long as a runway." Id glanced right over those key words, "as long as a runway." I was so caught up in the image of a motionless plane on a regular old treadmill -- like the kind you might see at the gym -- that I missed the whole question. Looking back, that does seem a dull and senseless riddle: Can a plane fly if it cant move? Obviously not. There has to be more to it.
And there is. At heart, this has nothing to do with the principles of lift but, rather, with those of friction and acceleration. The gist of the question is better understood as follows: Will an airplane, under its own power, remain motionless on a 10,000-foot-long treadmill, or will it roll forward? Will it accelerate and fly?
Turns out the answer is yes. A distinctly theoretical yes, for reasons well get to shortly, but for all intents and purposes of the puzzle, thats yes enough.
A car wont accelerate on a treadmill; the belt will always match the rotation of the tires. You will not accelerate on a treadmill; the belt always runs in sync with your footfalls. But an airplane is different. An airplanes wheels are not powered by gears or a drive train. They hang inertly below, and are free-spinning. The thrust force that moves the plane along couldnt care less about the ground. The engines are not fighting against the surface, they are fighting against the air.
Confusing, I know, and in the interest of full disclosure, physics was the one class in high school that I outright failed and had to take twice (you try ciphering out equations while listening to Minor Threat on a pair of clandestinely strung ear buds). And some of you might remember what happened the last time I combined things aeronautical and mathematical. So lets get somebody else to explain.
According to Paul J. Camp, a professor in the department of physics at Spelman College, its all pretty simple. "At first, the conveyor will hold the plane still. But only to a certain point, after which, driven by thrust from its engines, the craft will accelerate."
But the problem clearly states: The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction.
"The key is in the behavior of friction," Camp says. "Friction is a peculiar force in that it has an upper limit. For instance, push an object on your desk, but not hard enough to move it. Why doesnt it move? Because the friction force exactly balances the force of your push. At some point you push hard enough to set the object in motion. This is the point where friction has topped out and is not capable of growing any larger."
With the airplane and treadmill, there is, at the outset, friction force capable of rotating the tires at the proper speed to keep the plane stationary. However, as the thrust is increased, that force eventually maxes out. (Two separate frictions are at play here, actually, one between the tires and belt, the other between the planes axles/bearings and its wheels. The first will max out before the second.)
"And at that point the wheels no longer roll, they slide," says Camp. "Or rather, they roll and slide at the same time. Tire motion is now decoupled from the belt motion. No matter how much you whiz up the treadmill, you wont add any more rotational velocity to the wheels because friction is already doing everything it is capable of. The plane skids toward takeoff -- likely accompanied by much smoke and a powerful rubbery stink."
And there you have it, at least on paper. Bear in mind that for a plane to reach that point of decoupling would require two things above and beyond the pale of normal engineering. First, a remarkable amount of power -- far more than any jetliner, and probably any military plane, is capable of developing. The illustration on Pogues blog is of an Airbus A320; some sort of rocket plane would be more appropriate. Second, no existing aircraft tires could take such abuse. The rotational velocity required before reaching the friction limit would have them bursting within seconds, causing the plane to be flung backward. Believe it or not, landing gear isnt engineered with giant treadmills in mind, and pilots need to adhere to maximum groundspeed limits, lest their tires wind up like this. These limits occasionally present problems during tailwind operations or in the case of flap and slat malfunctions -- scenarios dictating the need for unusually high takeoff or landing speeds.
For good measure, the treadmill itself, as described, could never be built. It cant "exactly match the speed of the wheels," because the wheels will turn at the speed of the treadmill plus the speed of the plane relative to the ground. When the speed of the plane is greater than zero (which it is the moment its wheels start to spin; otherwise they would never move), then the problem becomes impossible. By definition, the wheels have to be turning faster than the treadmill.
Whose idea was this crazy problem?
Meanwhile I cant decide if this is good or bad news for the conveyor belt industry and treadmill enthusiasts worldwide. Though, as they say, any publicity is good publicity.
And just to repeat: gosh what a pointless question. :roll:
Quote from: maximusotterHere ya go, morans:
Mor
ons
(http://memewatch.com/thelist/archives/pix/morans.jpg)
:D :P
(http://dictionary.cambridge.org/assets/cald/shotCALD-med.jpg)
youre a bit late max - the problem has already been debated and explained over 4 pages
lol i dont believe that it would require as much thrust as they make out, im betting a 747 could get off a conveyor system with maybe some issues with the tyres but with enough power of its own
Quote from: Daveyoure a bit late max - the problem has already been debated and explained over 4 pages
What a waste of time. Its such a dumb question to begin with. Ya want riddles, go to the riddle boards. :roll:
yep but you still managed to get it wrong on page 1 of this thread ;)
The explanation of the tyres was interesting.
all tyres have a speed limit, cars, buses, planes lol
Quote from: Daveyep but you still managed to get it wrong on page 1 of this thread ;)
Depending on the brand of bearings, I may very well have gotten it right.
Quote from: Nimrodwhen youre running on a treadmill do you get "wind blowing" into your face due to the speed at which youre traveling ?
Let me put it another way...
If you put the plane on a big "car" jack so that the wheels were not touching for the floor/conveyor or anything and stuck the engines on...would you take off ?
Sorry to say it but you are being dim :P
I thought exactly the same as you at first, but then when you think about it you realise you are wrong.
Unlike a car the power is not put through the wheels on a plane. You will get a little resitance due to friction in the wheels bearings, but that would not be enough to stop the plane taking off.
When you are running ona tread mill do your feet roll or slip on the belt? No, they stay fixed in position relative to the treadmill.
Think of it this way - if you stand on a treadmill with rollerskates on i could push you forwards relative to the machine, overcoming the small amount of reistance from the friction in the wheels.
Quote from: maximusotterQuote from: Daveyep but you still managed to get it wrong on page 1 of this thread ;)
Depending on the brand of bearings, I may very well have gotten it right.
Ah you mean all new "Grit in a metal casin yarhgh" (TM) brand bearings?
Quote from: Nimrodok, I just want to come at this another way as I cannot see it myself...
Do you know those big roller things they use to test cars on? The car drives its wheels but the the rollers just spin so that the car stays stationary...
Am I right in thinking that youre proposing that if you were to put a plane on those rollers the plane would not be hindered because its thrust is not directly linked to the movement of the wheels ?
Obviously you missed something, forget the conveyor belt, you missed something very basic.
The wheels on a car power the car, it goes because the car wheels are in frictional contact with the ground. Lift the car off the ground, it now wont go anywhere cause it has no friction to push it along.
Put the car back on the conveyor belt, switch the car into neutral and take the brakes off, it will now tend to stay where it is because of inertia. Eventually the friction of the bearings will have some effect but that is minimal in this.
OK can a plane fly through the air? If you get any other answer than YES you need to see a brain surgeon. It fles because the thrust of the engine throws atmosphere back giving forward motion.
If a plane flies through the air then having its wheels touching a conveyor belt wont have any real effect. IT WILL still travel no matter what speed your conveyor belt is going at because the motion of its wheels dont come into the equation.
If the plane moves fast enough then it will fly,
For the wheel problem you could still use solid metal wheels
TBH I have no idea why this has gone on for over five pages...
Quote from: maximusotterQuote from: Daveyep but you still managed to get it wrong on page 1 of this thread ;)
Depending on the brand of bearings, I may very well have gotten it right.
No maxi, unfortunately dave has you over the proverbial barrel on this. the conveyor belt would burn before the plane bearings had any real effect on the plane.
When you run on a Treadmill do you feel a breeze across your body like you would when not running on a treadmill.
Surely its not creating any pressure for air to flow over the wings & generate lift?
Quote from: SeriousQuote from: maximusotterQuote from: Daveyep but you still managed to get it wrong on page 1 of this thread ;)
Depending on the brand of bearings, I may very well have gotten it right.
No maxi, unfortunately dave has you over the proverbial barrel on this. the conveyor belt would burn before the plane bearings had any real effect on the plane.
What if it was a very small plane on a very large conveyor Belt?
Quote from: EggtasticoWhen you run on a Treadmill do you feel a breeze across your body like you would when not running on a treadmill.
Surely its not creating any pressure for air to flow over the wings & generate lift?
are you on a wind up?
this has been explained already
Quote from: EggtasticoWhen you run on a Treadmill do you feel a breeze across your body like you would when not running on a treadmill.
Surely its not creating any pressure for air to flow over the wings & generate lift?
Same thing as the car though, lift your feet off the floor and you go nowhere anyway. How many times do we have to point out that jet engines dont push against the ground?
Size of plane makes no difference. A model plane on a big belt would have the benefit of the belt pulling air along towards it so it might be able to take off while remaining stationary.
Quote from: DaveQuote from: EggtasticoWhen you run on a Treadmill do you feel a breeze across your body like you would when not running on a treadmill.
Surely its not creating any pressure for air to flow over the wings & generate lift?
are you on a wind up?
this has been explained already
Only in Theory....
Quote from: SeriousQuote from: EggtasticoWhen you run on a Treadmill do you feel a breeze across your body like you would when not running on a treadmill.
Surely its not creating any pressure for air to flow over the wings & generate lift?
Same thing as the car though, lift your feet off the floor and you go nowhere anyway. How many times do we have to point out that jet engines dont push against the ground?
Size of plane makes no difference. A model plane on a big belt would have the benefit of the belt pulling air along towards it so it might be able to take off while remaining stationary.
I havent said they push against the ground. Surely if everything was going at an equal speed, then plane would be motionless? So there isnt any Airflow being created (except maybe a small bit at the wheels) to create Lift..
1st Law of Motion
If an object is not moving, it will not start moving by itself.
If an object is moving, it will not stop or change direction unless something pushes it.
Can someone glue some wings to a Hamster & see if he takes off?
OMG how the hell people cant see how this work i do not know!
the wheels do nothing NOTHING
all the power is at the engines on the wings, the belt cannot stop the plane as theres not even close to enough friction created at the wheels with the brakes off
and yes the hamster would take off if you taped a rocket to it and lit it
can someone please lock this thread, I am rapidly loosing my belief in the intelegence of some people around here.
This was a bloody stupid question in the first place. The most important flaw with it is that the conveyor which the problem postulates is impossible
For ease of thinking, we ignore the possibility of the tires exploding, conveyor snapping, the bearings siezing and any of 100 other things which would happen very quickly if you tried to do this. To prevent these things from happening, there must be no friction in the bearing/tire/conveyor system except that which maintains the wheels turning at the rate of the conveyor plus the speed of the aircraft.
The jet engine is pushing the plane forward. This provides a thrust of several tens of thousands of pounds.
To prevent the aircraft from moving forward, an equal and oposite force would need to be exerted. It cannot be exerted through the wheels and bearings since there is no friction, and no other backwards force is stipulated in the question. Therefore there is no restraining force, therefore in accordance with Newtons Laws of Motion the aircraft will move forwards, gain lift and take off.
Furthermore, since the aircraft is moving forwards, no matter what you do to the conveyor the speed of the wheels will always be the speed of the conveyor plus the ground speed of the aircraft. The postulated conveyor violates the laws of physics.
Here speaketh an MSci in Physics, now please stop talking about this total pile of horse manure.
damn this thread greq quick :o
Think thats going to be the only answer as I cant be arsed to read through all the drivel I missed reading earlier :lol: