To the people that say the treadmill matches the speed of the plane so that the plane doesn't take off...how is this a possible scenario?

If the plane is not moving with respect to the ground, neither is the treadmill. You understand this, right? So you must be talking about plane speed with respect to the treadmill, and the ground is irrelevant (no external frame of reference other than the air itself): this, my friend, is what is known as a runway, and the thought experiment fails.

Let's say they both start at rest. Spin the prop, the plane moves forward and the treadmill instantaneously rolls backward equally. Either:

1) The plane moves forward with respect to the ground, and the treadmill matches it in the opposite direction, acceleration and eventually flight occur

2) The plane does not move with respect to the ground and the treadmill thusly stops


THERE IS NO CASE FOR A POWERED PLANE STATIONARY WRT THE GROUND AND YET MOVING WRT THE TREADMILL. The only way this is possible is if you consider friction and the engine is used only to keep the plane steady as the treadmill rolls out from under it, but then this is no longer the same thought experiment.

1/31/2008 3:03:02 PM

Quote :

"You do realize that if you sit down with basic physics, all you have to do is match the acceleration of the belt to counter the force of the engines."

you lack basic physics skills

i'm no aero, so my terms might be fucked up, but you don't have to be an aero to figure this out:

the sum of the forces acting on a body is equal to the mass of the body multiplied by the acceleration of the body. there are 3 significant forces acting on the plane: drag (backwards, very small, changing with airspeed), friction in wheels (backward, very small, constant because friction=mass*acceleration_from_gravity*coefficient of friction), thrust (forward, huge). the only way the plane's velocity doesn't change (read no acceleration) is if the forces sum to zero (sum_forces=m*a where a=0 --> sum_forces=0). i just find it hard to believe that a constant friction force and an airspeed-dependent drag force that the plane can overcome under normal take-off can overcome the thrust force.

i haven't read the whole thread because i don't need anyone to explain to me what really happens, so is this guy just trolling or is he really this stupid?

[Edited on January 31, 2008 at 3:13 PM. Reason : why'd i let y'all get me into this?]

1/31/2008 3:04:36 PM

That right

how do you define force then

1/31/2008 3:09:30 PM

Quote :

".....the effect of the engine producing more windspeed allows for the plane to lift off without moving forward?"

Airplanes do not use the thrust from their engines to push air over the wings.

[Edited on January 31, 2008 at 3:11 PM. Reason : Wording.]

1/31/2008 3:09:33 PM

Quote :

"so is this guy just trolling or is he really this stupid?"

yes, and yes.

i think the most amazing this about this urban "myth" is the ability to suck unsuspecting bystanders into it, whereupon they get trolled to no end by losers that were also at one time trolled to no end by other losers who were....do you see where this is going? it's a fucking scam, people. LEAVE IT ALONE

1/31/2008 3:27:02 PM

Quote :

"i suspect if you had a seaplane that was designed such that the skis had some kind of baffle sticking down in the water so as to my very un-hydrodynamic, then eventually the material connecting the engines to the baffle would fail. i don't understand how you think this would be any different from, say, tying a plane down by it's rudder and trying to take off. of course it's not going produce lift, it can't go forwards "

Good thing I wasn't talking about designing a plane to be un-hydrodynamic. I was more talking about starting a sea plane on rapid and turbulent water moving anywhere up to its take-off velocity in the opposite direction. The turbulence of the flow is important by the way. Where the heck are you getting tying a plane down by its rudder? That's just silly.

Obviously I'm not talking about the treadmill myth anymore, but (as I said) something that would probably be more interesting to watch. You could have- engines burn out, the plane shook or torn apart, etc. Much more potential for fun tv. Of course the plane COULD take off, but remember I'm talking about having it start on water flowing some speed in the opposite direction. Wouldn't have to be take-off speed, in fact that would probably be impossible. Anyways, water would have made this "myth" more fun.

[Edited on January 31, 2008 at 4:04 PM. Reason : ]

1/31/2008 3:58:26 PM

you people need to get over the fact that it will not take off

1/31/2008 4:10:56 PM

I have a question for the people who think it takes off.

Are you assuming that if the conveyor belt was running, the plane's wheels would spin but the plane would not move backward? if so, then I see why you say it will take off.

They still didn't get the experiment right. The treadmill did not match the forward speed of the plane. If it had, the plane would not have moved forward.

they ran the truck 25 mph but the plane obviously went over 25 or it wouldn't have moved forward to take off.

1/31/2008 4:12:50 PM

you are a complete idiot.


THE PROBLEM SAYS THE PLANE WILL MOVE FORWARD



IT DOES NOT SAY IT WILL STAY STILL

1/31/2008 4:13:44 PM

Quote :

"they ran the truck 25 mph but the plane obviously went over 25 or it wouldn't have moved forward to take off."

lol

[Edited on January 31, 2008 at 4:15 PM. Reason : ]

1/31/2008 4:14:47 PM

^^^^ troll

^^^ supertroll

[Edited on January 31, 2008 at 4:15 PM. Reason : damn]

1/31/2008 4:15:28 PM

OMG BUT (+)X + (-)X = 0!!

1/31/2008 4:15:58 PM

Quote :

"You do realize that if you sit down with basic physics, all you have to do is match the acceleration of the belt to counter the force of the engines."

I addressed this earlier in my post (about what would happen if the bearings were frictionless and the treadmill was started while the plane was off):

The plane WOULD move. The lack of friction in the bearings means that the wheels won't exert TORQUE on the axle. Imagine if there were no plane attached to the wheel. The wheel would move down the treadmill. Attaching a plane to the wheel by frictionless bearing doesn't change the laws of physics. It still takes horizontal force to hold the wheel still.

What would actually happen is the plane would move backwards as the wheels accelerate to match the speed of the treadmill. As the wheels approach the treadmill speed, the plane would decelerate due to friction with the air until the plane was stopped and the wheels were spinning freely beneath it. The plane WOULD move. If at this point you slowed the treadmill, the plane would move FORWARD as angular momentum from the wheels is transferred to the forward momentum of the plane, and the plane would again come to a stop.

With frictionless bearing the plane WOULD move, but not very much and only as long as the wheel speed didn't match the treadmill speed. The plane would only stay completely motionless on an accelerating treadmill (engines off) if the wheels had a ZERO moment of inertia.

I think this is the argument SandSanta is making. However, I don't think he realizes that in order to counteract TENS OF THOUSANDS of pounds of thrust, the treadmill would have to be accelerating at hundreds or thousands of g's (and the wheels would explode from heat and centrifugal forces).

The engines in a plane or jet have enough force to accelerate the entire mass at around 3 g's. In order to maintain zero forward motion, the entirety of that force would have to be transferred into the angular momentum of the wheels. The treadmill could stop accelerating when the viscosity of the air provided sufficient torque in the wheels to counteract the thrust of the engines and the plane wouldn't move. If I had to estimate, I'd say this would arrive when the wheels are spinning at around a few hundred thousand RPM. If you could measure the energy dissipated in the air at the wheels during this state, you would have constructed the world's largest dynamometer and you could determine the power output of the engines.

[Edited on January 31, 2008 at 4:57 PM. Reason : ]

1/31/2008 4:56:35 PM

that's all correct and all, but I don't see the need in picking this simple thing apart, arbitrarily choosing where there will and wont be friction, adding moments and inertias and torques, etc. Getting more specific on this isn't going to help someone who doesn't understand the basic concept.


unless you're just trying to sound smart...which we all do. you've succeeded!

1/31/2008 5:01:43 PM

Can someone re-post the initial setup and variables, because I for one got confused by misinterpreting the setup and different conditions much more than the actual science. Can someone re-post it?

1/31/2008 5:06:07 PM

^^that's a post I made on another page where someone said if the bearings were frictionless, the plane wouldn't move.

however, the post also explains why a treadmill can't prevent a plane from taking off when its engines are throttled to normal takeoff thrust. in order to stop the plane, the entire thrust has to accelerate the wheels. On a 747, there are 18 wheels of a few hundred pounds each totaling a few thousand pounds of wheel, versus thrust at takeoff, nearly 250,000 lbs.

[Edited on January 31, 2008 at 5:07 PM. Reason : ]

1/31/2008 5:07:37 PM

the original wording is long lost i'm sure, but the concept is easy enough to reproduce:

"You have a plane on a long conveyor belt; if the conveyor exactly matches the speed of the plane in the opposite direction, will it take off?"

1/31/2008 5:13:12 PM

this myth is fucking stupid if the plane is thought to move forward, because if its moving forward, that means it can accelerate faster than the treadmill can pull it back.

basically the forces arent equal and thats the important thing. regardless of the speed the treamill and plane are making, the plane will move forward, which will eventually have lift and fly


but the problem with the myth is ive heard different stories as to whether the plane is supposed to stay still because of the treamill or if it is allowed to make a forward movement in relation to the ground.



SOMEONE, please find the myth and put it down as a quote.

like actually say, this is the myth...........no deviation from it.

1/31/2008 6:00:32 PM

Quote :

"whether the plane is supposed to stay still because of the treamill or if it is allowed to make a forward movement in relation to the ground."

hahahahha

what are you supposed to do?

tie down the god damned plane?

it moves and takes off

is that not the answer to the fucking question at hand?

1/31/2008 6:07:31 PM

WHY YES, YES IT IS SIR

1/31/2008 6:10:14 PM

Quote :

"this myth is fucking stupid if the plane is thought to move forward, because if its moving forward, that means it can accelerate faster than the treadmill can pull it back."

Dont people understand? It doesnt mother fucking matter what the myth says. Aside from destroying the plane, the treadmill CANNOT PULL THE PLANE BACKWARDS!!!!!!!!!

The treadmill has no mechanism by which to grab hold of the plane and pull it backwards. The wheels on the plane are free spinning. What part of this do you not get?

Give it the fuck up.

1/31/2008 6:12:15 PM

Quote :

"but the problem with the myth is ive heard different stories as to whether the plane is supposed to stay still because of the treamill or if it is allowed to make a forward movement in relation to the ground."

yeah i really don't understand where people get the idea that the plane isn't allowed to move

all the myth says is that the conveyor moves opposite to the plane's motion but at the same speed


where in that is the plane's motion restricted? if the plane doesn't move, the treadmill doesn't move, and there's no experiment.


THE QUESTION IS POSED TO TRICK PEOPLE THAT ASSUME THE PLANE IS LIKE A CAR AND WILL NOT MOVE BECAUSE THE CONVEYOR COUNTERACTS THE PROPULSION OF THE WHEELS

ONCE THE PERSON REALIZES THAT A PLANE IS DRIVEN BY ITS PROPELLER AND THE WHEELS HAVE NO BEARING ON THE SITUATION, NORMAL PEOPLE GO "OH, YEAH THAT MAKES SENSE, IT TAKES OFF. HEY MAN, YOU WANT TO GO GRAB A BEER?"

though I don't know why I expect to find normal people on the internet.



[Edited on January 31, 2008 at 6:19 PM. Reason : better]

1/31/2008 6:15:31 PM

well hell, if the wheel bearings are assumed frictionless, then you can just put the plane on the treadmill w/o the engines running, turn on the treadmill and the plane will sit still with the wheels spinning because the only forces acting on it are gravity and the normal force.

1/31/2008 6:15:44 PM

if nothing else, this thread shows who understands physics and who does not lol

plane will take off = understanding physics

plane will not take off = history major

here is a lil thought experiment for you non-physics folks....

with the plane engines off, assuming the brakes are not applied, start the conveyor belt....will the plane move along with the belt?

consider 2 cases, starting the conveyor belt slowly vs. turning it up to 11 right away

i'll grade the answers later

^hey, no answering before the question is asked

[Edited on January 31, 2008 at 6:28 PM. Reason : .]

[Edited on January 31, 2008 at 6:29 PM. Reason : spellering]

1/31/2008 6:27:54 PM

I was trying to watch the end of it last night, but passed out. It was almost 2am.

1/31/2008 7:22:48 PM

Though I beleive some people are arguing for the hell of it.

here is a picture with all the forces the plane is experiencing

The rotation of the wheels has no real effect on the plane body.

So, there. There is no arguing this. I am an engineer like everyone else in here. so uh can we ask these people to take it on faith?

1/31/2008 7:26:35 PM

yes, i know it should fly in certain ideas


so as far as the myth goes, i know why it works

i just didnt understand what the myth was trying to say
thats why i questioned it


its like those big swamp boats with the fan on the top.

so even if the waves and current are pushing backwards really hard, the boat will still go forward because the fan doesnt not compete with the water, just the air

1/31/2008 7:31:26 PM

Quote :

"I'm talking about having it start on water flowing some speed in the opposite direction. "

and how the fuck would that be any different from holding the plane by the skis? you're clearly a troll, so im going to ignore any future posts until you change your mind.

1/31/2008 8:38:14 PM

Quote :

"well hell, if the wheel bearings are assumed frictionless, then you can just put the plane on the treadmill w/o the engines running, turn on the treadmill and the plane will sit still with the wheels spinning because the only forces acting on it are gravity and the normal force."

FOR THE 4TH TIME, THE PLANE WILL NOT SIT STILL EVEN WITH FRICTIONLESS BEARINGS

You people call yourselves engineers! If a wheel sits alone on a treadmill, what happens? The treadmill drags the wheel along with it. Do you think attaching a plane negates the laws of physics? I've explained this scenario in detail already. The plane will NOT sit still until after the wheels reach the speed of the treadmill. Once the wheels have matched the speed of the treadmill, the plane would still move (REALLY SLOWLY) backwards. The viscosity of the air would impart a torque against the wheels' motion, thus applying force on the runway and moving the plane. Of course, this would equate to a few pounds of force, but a non-zero net force results in acceleration.


I had stated before that to hold the plane in place with the engines running, the treadmill would have to be accelerating at hundreds or thousands of g's. Now I realize There is no way the wheels would be able to maintain traction. Well, what does that mean?


Any plane that can initiate forward motion (skidding tires) with the brakes applied CAN NOT be held in place by a level treadmill, regardless of treadmill speed or acceleration.

I would imagine most jets would have no problem taking off with the brakes applied (even using air brakes in conjunction with wheel brakes it still takes more runway to land than to take off).

1/31/2008 8:53:56 PM

I believe that is all explained in detail with my picture



[Edited on January 31, 2008 at 9:09 PM. Reason : dfg]

1/31/2008 9:08:47 PM

I wonder how this myth would hold up with a old russian k-5... it literally took off with a backwards velocity at the cherry point airshow a few years back with no treadmill.

[Edited on January 31, 2008 at 9:11 PM. Reason : with no treadmill]

1/31/2008 9:10:53 PM

^^ your picture doesn't show shit. it doesn't even have the weight of the plane or the drag.

1/31/2008 9:21:43 PM

LimpyNuts is funny

1/31/2008 9:32:40 PM

Quote :

"e : "well hell, if the wheel bearings are assumed frictionless, then you can just put the plane on the treadmill w/o the engines running, turn on the treadmill and the plane will sit still with the wheels spinning because the only forces acting on it are gravity and the normal force." FOR THE 4TH TIME, THE PLANE WILL NOT SIT STILL EVEN WITH FRICTIONLESS BEARINGS"

Actually, it would sit still with frictionless bearings, with the ENGINES OFF, while the treadmill is moving.

1/31/2008 9:33:29 PM

You gotta zoom into the picture with the little magnifying glass tool in paint. You will see, because its science

1/31/2008 9:36:42 PM

if this was done in a perfect vacuum and not with wheels, but with a maglev, then yeah, it would kind of stay still.

1/31/2008 9:39:53 PM

1/31/2008 9:42:36 PM

Quote :

"Actually, it would sit still with frictionless bearings, with the ENGINES OFF, while the treadmill is moving"

No, that is not correct. Put a wheel with frictionless bearings AND NO PLANE ATTACHED on a treadmill. Tell me what happens to the wheel. The wheel does NOT sit still on the treadmill, because there is a net force in the direction of treadmill travel. This force is what causes the wheel to spin. When the wheel has reached the speed of the treadmill it still won't remain stationary unless it's in a vacuum. In air, the viscosity acts against the circular motion of the wheel. There is a reaction force at the treadmill because the treadmill wants to keep the surface of the wheel moving at the treadmill speed. The result is a wheel that's rotating at a slightly slower pace than the treadmill speed, and is moving sowly with the treadmill.

The principle is that same with a plane attached. The force exerted is the same in either case, but when the plane is attached it will move much slower.

1/31/2008 9:54:27 PM

no that is correct because it is a french made treadmill, and the coefficient of friction is non existent in france.

1/31/2008 10:04:44 PM

Quote :

"No, that is not correct. Put a wheel with frictionless bearings AND NO PLANE ATTACHED on a treadmill. Tell me what happens to the wheel. The wheel does NOT sit still on the treadmill, because there is a net force in the direction of treadmill travel. This force is what causes the wheel to spin. When the wheel has reached the speed of the treadmill it still won't remain stationary unless it's in a vacuum. In air, the viscosity acts against the circular motion of the wheel. There is a reaction force at the treadmill because the treadmill wants to keep the surface of the wheel moving at the treadmill speed. The result is a wheel that's rotating at a slightly slower pace than the treadmill speed, and is moving sowly with the treadmill. "

this example is not correct...think about the inertia of the wheel, including rotational inertia

then consider the much larger inertia of the plane and it becomes clear that the situation is different vs. just a lone wheel, which is of course intuitive to most folks

since so many of you will never be able to understand, let's do something different

list if you think the plane will take off or not and then list your credentials for making this decision....major, degrees, experience....common sense is a non-answer

i'll tally the results as we go

1/31/2008 10:32:17 PM

Quote :

" and how the fuck would that be any different from holding the plane by the skis? you're clearly a troll, so im going to ignore any future posts until you change your mind."

Haha, I'm not talking about starting at rapids. Obviously there's some current against which a plane can still take off in. The current that would cause it to fail spectacularly probably would be river-rapid like but I'd be interested to see. But hey, if you can't see a difference between a bizarre test of what it takes to shred a sea plane and bolting something down then I guess you're just no fun.

This is different from holding the plane by the skis because it's not contrived simply to stop the plane, you don't know exactly what's going to happen, AND it wouldn't be boring to watch. What I'm describing would be kind of neat and possibly have (in the later half when they typically do their overboard blow stuff up crap) some neat stuff on tv.

1/31/2008 11:22:23 PM

^^ god you're retarded. a wheel by itself moves because there is a NET FORCE applied to it. sitting a plane on top of that wheel does not change that fact no matter how the plane is attached.

the net force is a result of viscosity in the air (both inside the tire and out) resisting rotation. the force is a function of the treadmill speed. even on a treadmill moving 100mph, the force would be on the order of 10's of pounds, but it exists. on frictionless bearings, the plane is guaranteed to move under the application of ANY force.

Imagine the bearing is made of a zero-contact magnetic system. The axle floats inside the wheel because of like magnetic poles (this would create a nearly frictionless bearing (only friction due to air shear between surfaces). when the wheel tries to roll away because of the non-zero net force applied to it, it will move toward the axle, generating a non-zero magnetic force against the axle.

Once again, the force is miniscule compared to the mass of the plane, but if you put a 747 with frictionless bearings on a treadmill going 100mph, I GUARANTEE, you'd be able to notice the movement.

if i had the money for a shitload of really strong permanent magnets I'd demonstrate.

[Edited on February 1, 2008 at 12:57 AM. Reason : i've clearly stated over and over that the treadmill can't stop the plane]

2/1/2008 12:56:47 AM

Quote :

" THE QUESTION IS POSED TO TRICK PEOPLE THAT ASSUME THE PLANE IS LIKE A CAR AND WILL NOT MOVE BECAUSE THE CONVEYOR COUNTERACTS THE PROPULSION OF THE WHEELS ONCE THE PERSON REALIZES THAT A PLANE IS DRIVEN BY ITS PROPELLER AND THE WHEELS HAVE NO BEARING ON THE SITUATION, NORMAL PEOPLE GO "OH, YEAH THAT MAKES SENSE, IT TAKES OFF. HEY MAN, YOU WANT TO GO GRAB A BEER?" "

This cleared it up for me. Makes perfect sense.

2/1/2008 1:26:01 AM

Haha.. I used to think it wouldn't take off. Then I realized exactly what was stated in that quote. Fun reading the arguments though.

2/1/2008 5:59:24 AM

^^^aww, don't get frusrated and resort to name calling

think of it as me helping you be a better engineer

you almost have it right, but are missing why sitting the plane on the tire is a big effect, yet you include something about air viscosity

do the simplest fluid dynamics calc you can think of for this and it sould become clear that the viscosity effect is ~0, so let's neglect that

2/1/2008 8:21:50 AM

^ I thought about it and LimpyNuts is right.

If depends on the moment of inertia of the wheels. Different wheels have different moments of inertia. Not all of the force from the treadmill is transformed in to rotational energy, but most of it is (I think... some of it at least, i haven't ran any numbers,and it's very dependent on wheel design), but the rest is computed as a direct force on the center of mass of the wheel. So the wheels would be spinning pretty fast, but the plane/wheel is going to be moving forward a little bit.

Seriously, look up the formulas for moments and torque and it'll make sense.

2/1/2008 9:22:16 AM

^ Yeah, it's not a steady state system so you can't just think of it as a a statics problem. But yeah if you examine an angularly accelerating wheel assuming no-slip you can look at moments about the center of the wheel and the moment about the non-slipping point of contact. You do that about 30 times in dynamics (if you expect to pass that is.) Of course, that force will be tiny compared to the thrust from the engines- obvious since the plane took off just fine.

2/1/2008 9:48:51 AM

I stated that the force is very small. The friction in the bearings acts on the wheel the same way the air viscosity does. It applies TORQUE to the wheel. It is the reaction force of the wheel against the treadmill that causes it to move laterally.

My argument is simply that: if a plane with frictionless bearings sits on a treadmill and the treadmill is turned on, the plane will NOT sit still. The treadmill applies a net lateral force to the wheel therefore the wheel moves laterally unless you apply an equal and opposite force. The plane could weigh a trillion tons and it would still move unless the wheels had a ZERO moment of inertia (wheels can accelerate to treadmill speed with no applied force) AND ZERO surface area (air viscosity exerts 0 torque against wheel movement).

Quote :

"think of it as me helping you be a better engineer"

^ The no slip assumption turns out to be a big problem. At full throttle, small aircraft have thrust roughly equal to their weight. With a coefficient of static friction of u, the treadmill can exert a maximum resistive force equal to the weight of the plane * u, regardless of the mechanism resisting the rotation of the wheels.

2/1/2008 12:39:46 PM

I will say again-Kari would get it. Confirmed.

btw-this pic. on her discoverychannel.com profile is one of the better ones i've seen of her:\

2/1/2008 4:52:19 PM

^^

Quote :

"^ The no slip assumption turns out to be a big problem. At full throttle, small aircraft have thrust roughly equal to their weight. With a coefficient of static friction of u, the treadmill can exert a maximum resistive force equal to the weight of the plane * u, regardless of the mechanism resisting the rotation of the wheels."

Not really a problem at all. The friction force is always a reaction force and at worst case the assumption gives the static friction forces to be a fraction of the weight of the plane. Also, remember that a lot of that static-friction force on the wheels is going into accelerating the wheels, not the plane anyways. Yeah the no slip condition isn't exactly perfect and you will get some slipping at higher speeds, but it still doesn't change the relative magnitudes of forces you're dealing with or any of the conceptual stuff.

Why exactly do you think it's a problem though? No-slip just establishes that the movement of the wheels is purely rolling as opposed to skipping along the ground. It doesn't mean they're not turning.

Besides, when exactly that slipping occurs isn't relevent to the end conclusion that the plane would move forward. Complete no slip for the wheels is the best someone could hope for in terms of total force applied to the plane from the treadmill- and even that ends up being fairly small. When you consider slipping then that just makes the treadmill even that more laughable as far as stopping the plane from taking off.

[Edited on February 1, 2008 at 6:44 PM. Reason : ]

2/1/2008 6:37:01 PM