Can the airplane take off?

[Estrella]

No.

 

[BriGy86]

I don't see how it could take off, no one has mentioned the force of gravity that holds the plane to the ground, if your on a convayor belt with roller skates (never mind the rope) and you turn the belt on you will start to go backwards, because there is friction in bearings and gravity is pulling you down.

the rope idea I see as being flawed, you're pulling on a stationary object.

I also looked over at the mythbusters forums to see if this is covered, they have a 53 page thread, why the hell hasn't it been aired yet?
one good way for them to test this would be with a small model airplane (that can actually take off and fly) on some sort of long tread mill

 

[msparish]

It makes me weep that so many people in HT are saying the plane won't take off.

 

[JonTom]

More to the point - can a floatplane take off from a moving river?

 

[BriGy86]

Originally posted by: JonTom
More to the point - can a floatplane take off from a moving river?

if the flow of the river doesn't speed up then yes

 

[JonTom]

why would the flow speeding up change anything?

 

[BriGy86]

Originally posted by: JonTom
why would the flow speeding up change anything?

because the flow carries it backwards

 

[Horus]

Ok.

Let's say the maximum speed of the plane is 300mph. You place the aircraft on the belt, and set it in reverse for 300mph. With the engines off, the plane will fall off the back of the belt rather quickly.

Turn on the engines, put them to full power. The speed of the plane FORWARD is 300mph, but that's being counteracted by the speed of the belt BACKWARDS of 300mph. Include the friction of the wheels, and you see that there is NO WAY FOR LIFT TO FORM UNDER THE WINGS! Therefore it cannot take off! Simple physics.

 

[Gibsons]

Originally posted by: Horus
Ok.

Let's say the maximum speed of the plane is 300mph. You place the aircraft on the belt, and set it in reverse for 300mph. With the engines off, the plane will fall off the back of the belt rather quickly.

Turn on the engines, put them to full power. The speed of the plane FORWARD is 300mph, but that's being counteracted by the speed of the belt BACKWARDS of 300mph. Include the friction of the wheels, and you see that there is NO WAY FOR LIFT TO FORM UNDER THE WINGS! Therefore it cannot take off! Simple physics.

You're assuming the friction of the wheels exceeds the thrust of the engines. Even if you type in caps, that's probably not the case.

 

[Toastedlightly]

Originally posted by: JonTom
More to the point - can a floatplane take off from a moving river?

Yes, thrust > reverse flow. Keep in mind, the friciton in the wheel bearings probably isn't that great. The wheels spin at least 100 mph at landing, so Im sure they can take over 200 easily.

 

[pcy]

Hi,

Originally posted by: beansbaxter
Alright, this is NOT a homework question. In another thread on the Anandtech forums, I saw this mentioned deep within a thread but it was never addressed. I am still curious so I ask it here:

You have an airplane on a conveyor belt - when the airplane moves forward, the conveyor matches its speed in reverse. Can the airplane take off?

The effect of the conveyor belt is to double the speed of rotation of the aircraft's wheels.

The Aircraft is driven by the trust of it's engines, not through its wheels. Apart from a tiny amount of friction in the wheel bearings the copnveyor belt has no effect on the aircraft's speed.

So the aircraft still moves forward, accelerating, and when it reaches the speed at which the airflow over it's wings will lift it's entire weight it takes off.



Peter

 

[BrownTown]

The airplane takes off due to the afformentioned reason

/thread

 

[Born2bwire]

Originally posted by: Horus
Ok.

Let's say the maximum speed of the plane is 300mph. You place the aircraft on the belt, and set it in reverse for 300mph. With the engines off, the plane will fall off the back of the belt rather quickly.

Turn on the engines, put them to full power. The speed of the plane FORWARD is 300mph, but that's being counteracted by the speed of the belt BACKWARDS of 300mph. Include the friction of the wheels, and you see that there is NO WAY FOR LIFT TO FORM UNDER THE WINGS! Therefore it cannot take off! Simple physics.

Hahha.

The wheels on the plane are nothing more than casters. They are freely rotating and while there does exist a small amount of friction between the motion of the wheels and the bearings interfacing them with the plane, for all intents and purposes, this is negligible to the amount of force provided by the jet engines.

It's like if you held a matchbox car ontop of a running belt sander (which they did in Mythbusters in one episode to see how fast they could go). Even if the motion of the wheels are at very high speeds, it takes little force to push it forward.

 

[dkozloski]

Originally posted by: JonTom
More to the point - can a floatplane take off from a moving river?

Speaking as a floatplane pilot with nearly 40 years experience the airplane takes off either upstream or down stream on even a fast moving river. This is not a good analogy as the water provides much more resistance to the floats than rolling resistance does to wheels. I doubt seriously that the rolling resistance of the backwards traveling conveyor belt would lengthen the takeoff run even 10%. The moving conveyor belt is little more than an annoyance and has absolutely no effect on the lift provided by the air moving over the lifting surfaces. You can't over rule the laws of nature with an act of congress.

 

[TheoPetro]

I used to respect HT...now with so many of you saying it doesnt take off im kinda losing that

 

[imported_Seer]

Originally posted by: Aflac

Originally posted by: dkozloski
The conveyor belt is of little consequence. It's just noise in the problem. The only way it would come in to play is if it's too short for the aircraft to reach flying speed. The velocity of the aircraft relative to the ground is of no consequence. The only thing that matters is the speed of the aircraft relative to the air. The wheels and/or tires may be oversped and fly apart by contact with the belt. This is no different from an aircraft being launched from an aircraft carrier except for the relative speed of the belt. There really is no serious question here.

Excellent answer.

Think of this situation in another form: you're standing on a treadmill, wearing roller skates, while holding onto a rope attached to a far wall in front of you. Ignoring friction, while you're standing on the treadmill, the wheels counteract the pull of the treadmill itself, so that you stay in place. But when you start pulling on the rope, you begin moving forward. This is akin to the airplane's engines "grabbing" ahold of the air and pulling the plane forward. The conveyor belt has no way to counteract this force and thus the plane will move forward, allowing it to take off.

It is erroneous to assume that the plane does not move just because there is a conveyor belt in the equation.

Originally posted by: Howard
I forgot that the wheels are free-spinning...

Originally posted by: kpb

Originally posted by: Shaftatplanetquake

Originally posted by: kpb
The problem with a plane is that the movement is generated by the jet engine or propeller and is not dependant on the wheels at all.

I disagree. Unless the plane doesn't have wheels (one of those crafts that is stored on a body of water) then the plane is completely dependent on the wheels for moving forward if it is grounded. If you take the wheels off the plane either doesn't move at all or moves completely unpredictably.

No it's accurate to say that they don't depend on the wheels for movement. The fact that you can use skys, or floats or what ever just proves that. The wheels on a plane are like the wheels on a roller skate. They are not hooked up to any motor and can not move the plane forward on thier own. They do allow it to easily roll but it's the jet engine that actually causes the motion to occur.

++

 

[dkozloski]

If you want a real world application of the problem, reflect on the case of a take off with a tail wind. The takeoff still happens but with a much longer takeoff run to accelerate the aircraft to both the wind speed and then the additional takeoff speed required. In the case of the conveyor belt the whole deal is much easier because the belt starts when the aircraft starts. Relative to the starting point, the takeoff run is less on the conveyor belt than with a tail wind. If the flying speed is 45MPH(typical small plane) and you start with a 45mph tailwind you have to accelerate the plane to a ground speed of 90MPH. On the conveyor belt you only have to accelerate to a ground speed of 45MPH but the wheels are turning 90 MPH, which is nothing. Think of jacking up the drive wheels on your car and how easy it is to make the speedometer read anything you want up to redlining the motor.

 

[BrownTown]

Originally posted by: TheoPetro
I used to respect HT...now with so many of you saying it doesnt take off im kinda losing that

I agree, this is not something which is up for debate, and we don't think your *clever* for manipulating the problem to support the wrong answer. This question is like a hook designed to get people tricked into saying the plane won't take off. And so many of the people in this thread are bitting at that hook and making fools of themselves. You don't look smart, or clever, or imaginative when you argue against a fact. If you really want to look smart and got the problem wrong to begin with (this happens to the best of us), then the best way to look smart is to simply admit you are wrong. I got this problem wrong the first time i saw it, it was explained to me, and I admit I was wrong. It is especially annyoing becasue the answer was already stated several times, and is all over the web, if you arent gonna at least read the thread, or do a little research you probably should not post in this forum.

 

[dkozloski]

I think this thread reflects how far the education system of the U.S. has sunk. Even the supposed best and brightest are sadly lacking in education of the sciences. If anyone had brought up this question in my H.S. physics class in the '50s they would have been "laughed outa town". This isn't even up to bonehead General Science standards.

 

[smack Down]

It depends on how the question is asked. If the treadmill tracks the turn of the wheels such that as the wheels move forward the treadmill rolls them backward then the plane does take off. If the treadmill tracks the planes speed then it will take off.

 

[dkozloski]

It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

 

[beansbaxter]

If you read the original wording of the question in the first post...it says "when the airplane moves forward" which means isnt the airplane moving relative to the ground?

In other words, if something is on a conveyor belt and is rolling to stay in one place, we would not describe it as "moving" because it would be rolling in place. Since the airplane is actually moving forward (regardless of the force), it could possibly attain enough speed to take off. The wheels would just be turning twice as fast.

Thoughts??

 

[smack Down]

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

 

[Born2bwire]

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

Why? The plane's only contact with it's wheels are free rolling bearings.

 

[smack Down]

Originally posted by: Born2bwire

Originally posted by: smack Down

Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.

That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.

Why? The plane's only contact with it's wheels are free rolling bearings.

I stole this from the other thread because he says it better and even includes the FBD.

Originally posted by: jagec

Originally posted by: dxkj
Correct, a small amount of energy is used to rotate the mass of the wheels. but wouldn't that energy be generated by the super fast super powered tread mill, and not by the plane? As long as the friction is zero on the axles the wheels could move infinitely fast with the tread mill, and the plane wouldnt move at all even under 0 power.


edit: IE, the wheels have a frictioned connection to the conveyer belt with the rubber touching rubber, but the ball bearing connection to the plane is frictionless, thus the energy and angular momentum provided to the wheels comes completely from the surface the wheels are touching.

If you draw the FBD, you find that the conveyor exerts a linear force on the base of the wheel, and the engines exert a linear force in the opposite direction through the axis of rotation. This creates a torque. If the force of the engines is not balanced perfectly by the force of the conveyor, you'll get linear as well as angular acceleration. This would be the case in any real-world scenario, since it's nearly impossible to exert the same force on the wheels with the conveyor, as you can on the plane with the engines. F=ma; when you're working against the large mass of the plane, that's fine. When you're working against the small rotational inertia of the wheels, the acceleration term becomes exceedingly large.

But in magical physics land, where we can accelerate the conveyor as fast as we bloody well please, we're able to equalize the forces. Thus, there is no net lateral force, just a huge torque that's spinning the wheels faster and faster, until the plane runs out of fuel.

Even with frictionless bearings, the wheels are able to exert a lateral force on the plane through the angular momentum term. That's the secret. It's easier to think of a massively weighty wheel on a treadmill...if we exert no additional forces on it, the wheel will begin to slowly turn and slowly move backwards at the same time when we turn the treadmill on. If we add a force through the axis of rotation, we're able to keep the wheel in one place, but it builds angular momentum that much faster.