You have a plane and a conveyor belt.

[kevinthenerd]

Originally posted by: JujuFish

Originally posted by: NanoStuff
Yes but you CANNOT entirely depend on this static friction of the wheels if they are moving, nor can you apply sliding friction to wheels that have traction with their surface. But just per example, if you lock the wheels and try to pull the plane forward you can realize just how much of this static friction is between the wheel and the belt. This friction is what gives the tire traction, and this traction is what inhibits the plane's momentum on a moving belt.

Have you taken any physics course?

The cog example is pretty damn good as far as I can tell. All you have to do is imagine one cog rotating on top of another (very comparable scenario as both have 'traction'). If the cog at the bottom is rotating in reverse of the cog at the top and is set in place on it's axis, the cog at the top will not progress forward. It MUST rotate faster than the bottom cog to advance forward. This is not possible because the rotation of the two is correlated, and one rotates exactly as fast as the other.

You're right about the cog example being a good one. However, you're wrong in the part I bolded. With two different sized cogs, one will always be faster than the other. It shows the inconsistency with your interpretation of the OP.

I think they mean outer linear speed.

 

[kevinthenerd]

Originally posted by: NanoStuff

Originally posted by: JujuFish

Originally posted by: NanoStuff
Yes but you CANNOT entirely depend on this static friction of the wheels if they are moving, nor can you apply sliding friction to wheels that have traction with their surface. But just per example, if you lock the wheels and try to pull the plane forward you can realize just how much of this static friction is between the wheel and the belt. This friction is what gives the tire traction, and this traction is what inhibits the plane's momentum on a moving belt.

Have you taken any physics course?

Nope, and I'm glad I can understand such a concept without having taken one.

One of the most important things that I learned throughout my education so far is that I'll always know nothing in comparison to the great amount to be learned.

 

[SilentRunning]

1. Enough with the talk about wheels. In the spirit of the original question I am hereby deeming that the plane on the conveyor belt has only snow ski gear. So back to the original incarnation, the belt increases in speed opposite the thrust of the plane to keep the plane from moving forward relative to the ground, and the engines provide no direct airflow over the wings. (the coefficent of friction is high enough that the plane can't move relative to the belt without flipping :laugh: )

Discuss

Or

2. The plane is on a conveyor belt which is mounted on the catapult of an aircraft carrier which is going full speed into the wind. At the moment the belt starts to move the catapult launches the belt with the plane on it.

Discuss

 

[jagec]

Originally posted by: kevinthenerd

That assumes that the belt doesn't compensate for the rotational movement of the wheels. We need to stop here and realize that this problem's two interpretations have two different answers.

After thinking about it a bit more, I have to agree that the second interpretation (matching angular velocity) isn't actually impossible. Just a really weird control system, and yes, it keeps the plane from taking off.

I STILL think the first interpretation (linear velocity) is the correct one, however.

Originally posted by: SilentRunning
Enough with the talk about wheels. In the spirit of the original question I am hereby deeming that the plane on the conveyor belt has only snow ski gear. So back to the original incarnation, the belt increases in speed opposite the thrust of the plane to keep the plane from moving forward relative to the ground, and the engines provide no direct airflow over the wings.

Discuss

Is the conveyor covered in snow?

When you say "...to keep the plane from moving forward relative to the ground", are you meaning that the conveyor ATTEMPTS to do so, or is success implied?

 

[SilentRunning]

Originally posted by: jagec

Originally posted by: kevinthenerd

That assumes that the belt doesn't compensate for the rotational movement of the wheels. We need to stop here and realize that this problem's two interpretations have two different answers.

After thinking about it a bit more, I have to agree that the second interpretation (matching angular velocity) isn't actually impossible. Just a really weird control system, and yes, it keeps the plane from taking off.

I STILL think the first interpretation (linear velocity) is the correct one, however.

Originally posted by: SilentRunning
Enough with the talk about wheels. In the spirit of the original question I am hereby deeming that the plane on the conveyor belt has only snow ski gear. So back to the original incarnation, the belt increases in speed opposite the thrust of the plane to keep the plane from moving forward relative to the ground, and the engines provide no direct airflow over the wings.

Discuss

Is the conveyor covered in snow?

When you say "...to keep the plane from moving forward relative to the ground", are you meaning that the conveyor ATTEMPTS to do so, or is success implied?

Caught me before edit. Flypaper belt and plane is the fly

 

[skace]

Kevin,

"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. "

I linked this once, please actually read it this time:
http://www.avweb.com/news/columns/191034-1.html

Even in the paradox scenario, the plane would take off unless the tires blew out first. But if you are going to build the scenario around a paradox of an infinitely increasing treadmill, then I think you can build some tires that can sustain the speeds.

If anything, this thread showed just how ignorant many people on this forum are. Infact, I'm kind of shocked at the people who displayed poor judgement in this thread, there were some names I would never have imagined. Not because you do not know physics, but because you cannot even read links or understand common analogies. Reading comprehension is the only thing this thread required.

 

[kevinthenerd]

Originally posted by: skace
Kevin,

"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. "

I linked this once, please actually read it this time:
http://www.avweb.com/news/columns/191034-1.html

Even in the paradox scenario, the plane would take off unless the tires blew out first. But if you are going to build the scenario around a paradox of an infinitely increasing treadmill, then I think you can build some tires that can sustain the speeds.

If anything, this thread showed just how ignorant many people on this forum are. Infact, I'm kind of shocked at the people who displayed poor judgement in this thread, there were some names I would never have imagined. Not because you do not know physics, but because you cannot even read links or understand common analogies. Reading comprehension is the only thing this thread required.

I didn't take the time to read the nearly 1000 replies. Sue me.

 

[kevinthenerd]

Originally posted by: jagec

Originally posted by: kevinthenerd

You see, if a car was on a belt, and if you assume that its torque band is constant for any given speed and that it experiences no friction and that the wheels weigh nothing, its real acceleration (in relation to the ground) would be the same regardless if it was a on a belt or not. It's F=ma. If the belt travels backwards, you can just subtract the belt speed from the spedometer and you'll find the real speed.

I hope when you say "ground" you mean "conveyor".

Nope, I mean ground. Without friction, a constant velocity belt would apply no force.

 

[AMCRambler]

Nanostuff, you're saying that the engine provides no airflow over the wings. In order for the plane to lift off, there has to be air flow over the wings correct? My question to you is, where is the airflow coming from? The conveyor belt does not move the air. The conveyor belt is stationary relative to the earth, and because the plane is matching the speed of the conveyor, so is the plane. So the plane does not move forward through the air so it doesn't lift off. There is no interpretation of this that I can find that gets that plane to fly.
Take a tread mill for instance. When you run on a tread mill, you match the speed of the treadmill just like in your theoretical situation here. You're running on the treadmill, do you feel a breeze in your face? Is there air flowing over your body? Nope. No air flow, no lift. Aliencraft is correct. Cracks me up that you guys have been discussing this all weekend.

Edit: Nevermind. I think I've got my brain around what you're asking here. In instance #1 the conveyor will always be matching the forward rotation of the wheels unless the wheels were slipping along the belt. So unless the wheels are skidding, which you specifically state is not happening, #1 and #2 are not exclusive. They are tied together. The movement of the plane #2, can not occur without the rotation of the tires #1. For these two to not be tied together, would violate the physics you are trying to use to prove the plane would take off. You are telling us to disregard one law, but all others apply? Your question is horribly malformed.

 

[skace]

AMC how long after your post until you read the thread and realize how incorrect you are? I'm pretty sure this thread can hit 1000 replies with people who still incorrectly think the plane will not move or take off.

 

[AMCRambler]

Originally posted by: skace
AMC how long after your post until you read the thread and realize how incorrect you are? I'm pretty sure this thread can hit 1000 replies with people who still incorrectly think the plane will not move or take off.

Read my edit. I didn't understand what he was trying to ask. Now that I do, I realize, it's not my answer that is wrong. It is his question that is incorrect.

 

[kevinthenerd]

Does anyone here do any sort of computational fluid dynamics that could be used to prove or disprove what I'm about to say? I'm starting to wonder whether the plane would take off in a stationary position (i.e. the rotational assumption) just from the fact that it might induce its own airflow over the wings at wide open throttle. When you spit stuff out of the back, you're inducing a vacuum at the intake because it requires oxygen to burn, and some of that air that goes into the intake or that comes out of the back might pass over the wings. Planes usually have very powerful motors, so this might be a real enough effect to decrease the normal force on the wheels just enough to make them slip. I don't know.

 

[Don Vito Corleone]

This thread has long since gone off the deep end. It's really strange to see so many apparently bright people, arguing so strenuously, in favor of a proposition that is so obviously wrong. I think this is how things like the Maginot Line happen. Holy moly . . .

<---- Understands that a jet is not driven by power to its wheels, so the conveyor belt is irrelevant to the question.

 

[Armitage]

Originally posted by: kevinthenerd
Does anyone here do any sort of computational fluid dynamics that could be used to prove or disprove what I'm about to say? I'm starting to wonder whether the plane would take off in a stationary position (i.e. the rotational assumption) just from the fact that it might induce its own airflow over the wings at wide open throttle. When you spit stuff out of the back, you're inducing a vacuum at the intake because it requires oxygen to burn, and some of that air that goes into the intake or that comes out of the back might pass over the wings. Planes usually have very powerful motors, so this might be a real enough effect to decrease the normal force on the wheels just enough to make them slip. I don't know.

CFD is completely irrelevant. No plane ever takes off from air blown/sucked ovber the wings by the engines - whether on a conveyor or not. And the wheels do not have to slip on the conveyor - they simply roll - just like on a normal runway, except twice as fast because the runway is moving backward at the same speed as the plane is moving forward. Which adds a trivial amount more drag because rolling resistance is nearly independent of velocity. So the plane takes off.

 

[PurdueRy]

Originally posted by: kevinthenerd
Does anyone here do any sort of computational fluid dynamics that could be used to prove or disprove what I'm about to say? I'm starting to wonder whether the plane would take off in a stationary position (i.e. the rotational assumption) just from the fact that it might induce its own airflow over the wings at wide open throttle. When you spit stuff out of the back, you're inducing a vacuum at the intake because it requires oxygen to burn, and some of that air that goes into the intake or that comes out of the back might pass over the wings. Planes usually have very powerful motors, so this might be a real enough effect to decrease the normal force on the wheels just enough to make them slip. I don't know.

The question states the air flow over the wings is not produced by the engines being in front of them. It says the engines must be in the back.

 

[Don Vito Corleone]

Originally posted by: PurdueRy

The question states the air flow over the wings is not produced by the engines being in front of them. It says the engines must be in the back.

The location of the engines is irrelevant. The engines, not the wheels, drive a plane, so the conveyor belt has precious little effect on the plane's ability to build speed for takeoff. The only effect it has is that the wheels must spin faster than they normally would as the plane takes off.

 

[wvtalbot]

wow you guys put way to much into this.

 

[PurdueRy]

Originally posted by: DonVito

Originally posted by: PurdueRy

The question states the air flow over the wings is not produced by the engines being in front of them. It says the engines must be in the back.

The location of the engines is irrelevant. The engines, not the wheels, drive a plane, so the conveyor belt has precious little effect on the plane's ability to build speed for takeoff. The only effect it has is that the wheels must spin faster than they normally would as the plane takes off.

If you would read the thread then you would know I know the answer as I have been explaining to to people forever. Kevin was asking about the case where the conveyor matches the rotational speed, things get dicey here and his theory of how it might be able to take off in that case could not occue and I was pointing it out to him.

This situation is a stupid one because it introduces a paradox where the speed of the conveyor goes to infinity VERY fast,,,so I usually have said to ignore it. But I wanted to point that out to Kevin.

 

[kevinthenerd]

Originally posted by: Armitage

Originally posted by: kevinthenerd
Does anyone here do any sort of computational fluid dynamics that could be used to prove or disprove what I'm about to say? I'm starting to wonder whether the plane would take off in a stationary position (i.e. the rotational assumption) just from the fact that it might induce its own airflow over the wings at wide open throttle. When you spit stuff out of the back, you're inducing a vacuum at the intake because it requires oxygen to burn, and some of that air that goes into the intake or that comes out of the back might pass over the wings. Planes usually have very powerful motors, so this might be a real enough effect to decrease the normal force on the wheels just enough to make them slip. I don't know.

CFD is completely irrelevant. No plane ever takes off from air blown/sucked ovber the wings by the engines - whether on a conveyor or not. And the wheels do not have to slip on the conveyor - they simply roll - just like on a normal runway, except twice as fast because the runway is moving backward at the same speed as the plane is moving forward. Which adds a trivial amount more drag because rolling resistance is nearly independent of velocity. So the plane takes off.

No, I was assuming problem version #2 in which the rotation was matched, not the forward speed of the fuselage.

 

[CaptnKirk]

For an aircraft to fly, there MUST be airflow over the wings.
The entire function of aerodynamic lift is the pressure differential of that airflow.

I don't care how fast the wheels spin, if the entire vehicle doesn't reach a velocity where the wings can provide lift
there is no mechanism to create the lift.

A wing is a 'passive' machine, it needs the speed of airflow to make it work.

Coefficient of lift

 

[kevinthenerd]

Originally posted by: CaptnKirk
For an aircraft to fly, there MUST be airflow over the wings.
The entire function of aerodynamic lift is the pressure differential of that airflow.

I don't care how fast the wheels spin, if the entire vehicle doesn't reach a velocity where the wings can provide lift
there is no mechanism to create the lift.

A wing is a 'passive' machine, it needs the speed of airflow to make it work.

Coefficient of lift

I was asking earlier if engines at extremely high speeds would induce such an air flow.

 

[CaptnKirk]

Originally posted by: kevinthenerd

Originally posted by: CaptnKirk
For an aircraft to fly, there MUST be airflow over the wings.
The entire function of aerodynamic lift is the pressure differential of that airflow.

I don't care how fast the wheels spin, if the entire vehicle doesn't reach a velocity where the wings can provide lift
there is no mechanism to create the lift.

A wing is a 'passive' machine, it needs the speed of airflow to make it work.

Coefficient of lift

I was asking earlier if engines at extremely high speeds would induce such an air flow.

Wouldn't that be a Wind Tunnel?

Here's the one at Embry-Riddle in Daytona Beach, FL

 

[kevinthenerd]

Originally posted by: CaptnKirk

Originally posted by: kevinthenerd

Originally posted by: CaptnKirk
For an aircraft to fly, there MUST be airflow over the wings.
The entire function of aerodynamic lift is the pressure differential of that airflow.

I don't care how fast the wheels spin, if the entire vehicle doesn't reach a velocity where the wings can provide lift
there is no mechanism to create the lift.

A wing is a 'passive' machine, it needs the speed of airflow to make it work.

Coefficient of lift

I was asking earlier if engines at extremely high speeds would induce such an air flow.

Wouldn't that be a Wind Tunnel?

Here's the one at Embry-Riddle in Daytona Beach, FL

I'm familiar with wind tunnels. The problem with comparing this to a wind tunnel, however, is that I'm assuming this is an external flow, and the plane is the only source of energy to move air. There would be a LOT of places for the air to go when it shoots out of the back of the airplane, and when the vacuum comes into the intake, it's coming from a very wide spread of angles, meaning that, at a fair distance from it, it's coming in very slowly. If you constricted the flow to a tunnel, I'd have less of a problem thinking of this as a wind tunnel.

 

[JujuFish]

Originally posted by: kevinthenerd

Originally posted by: Armitage

Originally posted by: kevinthenerd
Does anyone here do any sort of computational fluid dynamics that could be used to prove or disprove what I'm about to say? I'm starting to wonder whether the plane would take off in a stationary position (i.e. the rotational assumption) just from the fact that it might induce its own airflow over the wings at wide open throttle. When you spit stuff out of the back, you're inducing a vacuum at the intake because it requires oxygen to burn, and some of that air that goes into the intake or that comes out of the back might pass over the wings. Planes usually have very powerful motors, so this might be a real enough effect to decrease the normal force on the wheels just enough to make them slip. I don't know.

CFD is completely irrelevant. No plane ever takes off from air blown/sucked ovber the wings by the engines - whether on a conveyor or not. And the wheels do not have to slip on the conveyor - they simply roll - just like on a normal runway, except twice as fast because the runway is moving backward at the same speed as the plane is moving forward. Which adds a trivial amount more drag because rolling resistance is nearly independent of velocity. So the plane takes off.

No, I was assuming problem version #2 in which the rotation was matched, not the forward speed of the fuselage.

No, you were assuming condition #1, which is an impossibility, and one cannot determine whether it takes off or not.

 

[CaptnKirk]

Originally posted by: kevinthenerd

Originally posted by: CaptnKirk

Originally posted by: kevinthenerd

Originally posted by: CaptnKirk
For an aircraft to fly, there MUST be airflow over the wings.
The entire function of aerodynamic lift is the pressure differential of that airflow.

I don't care how fast the wheels spin, if the entire vehicle doesn't reach a velocity where the wings can provide lift
there is no mechanism to create the lift.

A wing is a 'passive' machine, it needs the speed of airflow to make it work.

Coefficient of lift

I was asking earlier if engines at extremely high speeds would induce such an air flow.

Wouldn't that be a Wind Tunnel?

Here's the one at Embry-Riddle in Daytona Beach, FL

I'm familiar with wind tunnels. The problem with comparing this to a wind tunnel, however, is that I'm assuming this is an external flow, and the plane is the only source of energy to move air. There would be a LOT of places for the air to go when it shoots out of the back of the airplane, and when the vacuum comes into the intake, it's coming from a very wide spread of angles, meaning that, at a fair distance from it, it's coming in very slowly. If you constricted the flow to a tunnel, I'd have less of a problem thinking of this as a wind tunnel.

I'm seeing this like it was an engine run up in an open area, which is done all the time & it doesn't matter if 1, 2, 3, or 4 engines are running at the same time -
if the plane isn't up to take off speed as an entire vehicle nothing happens.
Wheels spinning at any speed are nothing more than spinning wheels.

I've stood at the blast fence behind airplanes as an 'observer' several times including watching a compressor stall
and the associated fireball from the re-ignition of the mixture as it leaves the hotstream turbine.
They don't fly, no matter how much air is being sucked through each engine or the collection of engines,
there is insufficent airflow to create the dynamic environment required for 'flight'.

On the otherhand, I've seen pilots called to the cockpit to run the engines and turn the airplanes into the headwinds
to minimize damage to the aircraft during some very adverse weather conditions when 40 MPH and higher gusts are expected.
Now in that senario, a light aircraft like a Piper or a Cessna could encounter a burst of wind that may make it 'kite' off the ground,
but that would in most conditions upset the airplane and just blow it like debris accross the field.
The plane would tend to 'nose' into the wind, although a side shear could result in the leading side wing lifting (asymetrical) and turning the plane over.
This happens all the time along the squall lines associated with severe weather & tornados.