Mythbusters to take on - PLANE ON A TREADMILL!!!

[JHines]

still dont get it...I understand aerodynamics but I am just sitting here picturing an airplane at full throttle on a treadmill with zero forward motion. If those wheels are going a thousand miles per hour it makes no difference providing there is no relative airflow over the wings to make the plane fly.

I do understand how the wheels work and that they are just free on bearings and not mechanically driven. I should probably stop posting because I think what I am envisioning is not what we are talking about here and I am making myself look stupid

One more try here...you're completly correct that if the plane were "held back", no way it can take off. But, if you're thinking the plane would be "held back" by the interaction of the tires with the treadmill, try to conceive exactly how that would happen.

Remember, rotating the free-spinning tires using an external force (i.e. treadmill) acting on the tread does not affect the position of the axle.

If you rotate a tire by applying torque to the wheel, the way a car works, the tread is coupled to the road/treadmill by friction, and a shear force is applied parallel to the surface. Think of the "bottom" tread as "pushing backwards" against the road/treadmill if you like. The required equal and opposite reaction is the axle (and thus the car) moving forward.

When the wheel is free-spinning, the equal and opposite reaction to the "bottom" tread motion is the opposite direction motion of the "top" tread as the wheel rotates. No net force is applied to the axle, except for a small amount attributable to friction in the bearings.

Here's another analogy. Load your light aircraft of choice on a flatbed truck with no tailgate. Leave the brakes off and don't chock it or tie it down.

1. Drive the truck away veeerrrry gently and slowly. What happens?

2. Drive the truck away with normal acceleration. What happens?

 

[darrenf]

If that's the actual myth.

Most people seem to assume the treadmill matches the airplane's speed and so would only need the 60mph or so it takes the ultralight to T/O.

Should need much less speed than 60mph since an ultralights max stall may not exceed 24 kts.

 

[darrenf]

Here's another analogy. Load your light aircraft of choice on a flatbed truck with no tailgate. Leave the brakes off and don't chock it or tie it down.

1. Drive the truck away veeerrrry gently and slowly. What happens?

2. Drive the truck away with normal acceleration. What happens?

Yes please, anyone that has any doubt that the airplane on a treadmill will takeoff, PLEASE do this experiment and report back here with your results!

 

[Berkut]

No net force is applied to the axle, except for a small amount attributable to friction in the bearings.

Well, there is another way the treadmill can apply a force to the axle besides friction. The wheels have mass, and the force applied by the treadmill to rotate that mass will be transmitted through the axle and push the airplane backward. However, that would require the treadmill to be constantly accelerating (not just moving), and at a rate sufficient to overcome the airplane's engines. Which would be a lot. A whole lot.

In other words, that ain't something the Mythbusters will be doing.

 

[SteveC]

No. I'm talking about the rotational speed of the tires.

Normally, they are the same, however, in certain situations they are not. On ice, for one example, and on our hypothetical treadmill on another.

Sorry. I think I know what you are trying to say, but it is a fundamentally faulty premise.

Let me start you down the path to (possible) enlightenment by saying that the surface of a treadmill only has a linear speed (defined as distance per time), not a rotational speed (revolutions per time). It cannot have the same (or opposite) rotational speed as the tires since that term doesn't apply. Try redefining what you think the myth author is saying about the treadmill speed (different that my premise that the treadmill is matching the airplane's speed) and we'll go from there.

 

[Clocks]

No. I'm talking about the rotational speed of the tires.

Normally, they are the same, however, in certain situations they are not. On ice, for one example, and on our hypothetical treadmill on another.

Why does the speed the tires are spinning affect the engine's ability to push off the air?

The tires could be spinning in any direction, at any speed, or not spinning at all, and the engines are still pushing off the air (and therefore moving the airplane forward relative to both the air and the ground).

 

[Chris_Ford]

Why does the speed the tires are spinning affect the engine's ability to push off the air?

The tires could be spinning in any direction, at any speed, or not spinning at all, and the engines are still pushing off the air (and therefore moving the airplane forward relative to both the air and the ground).

1) Because eventually the tires are going to explode and the airplane will most certainly not take off due to the increased friction

2) If the speed that the wheels are turning is instantaneously matched by the treadmill, the airplane will never be able to gain forward velocity, because if it DID, the wheels would spin faster thus the treadmill would increase.

A few examples:
An airplane on ice may have an aircraft speed of 5 m/s while the wheels are moving at 0 m/s.

These two numbers are independent of one another. Since the wheels are on a frictioned surface, however, the airplane won't ever be able to have a faster velocity than the wheels (especially on the treadmill)

So, if the treadmill matches the AIRPLANE's speed, the wheels would just spin twice as fast. However, if the treadmill matches the TIRES' speed, the airplane will never gain forward velocity since once the airplane moved forward, its wheels would have to speed up and so would the treadmill thus not permitting forward aircraft velocity.

 

[Clocks]

So, if the treadmill matches the AIRPLANE's speed, the wheels would just spin twice as fast. However, if the treadmill matches the TIRES' speed, the airplane will never gain forward velocity since once the airplane moved forward, its wheels would have to speed up and so would the treadmill thus not permitting forward aircraft velocity.

So what force is canceling out the X pounds of thrust from the engines? What I mean is the engines are sitting there pushing off the air with several thousands of pounds (or whatever number) of force. Something has to be pushing back in the opposite direction or else the airplane has to move.

 

[Chris_Ford]

There's no acceleration, there's no force.

 

[Clocks]

There's no acceleration, there's no force.

uh...what?

The engines are accelerating the air which produces a force on the engines...which are attached to the airplane.

 

[SteveC]

...

Chris, Chris, Chris. You can BS better than that.

Sheesh.

 

[Chris_Ford]

When someone runs on a treadmill, where does the force from moving their legs back and forth end up? On the treadmill. In reference to the runner, the person is moving, but in reference to the outside observer, there is no net movement.

 

[Clocks]

When someone runs on a treadmill, where does the force from moving their legs back and forth end up? On the treadmill. In reference to the runner, the person is moving, but in reference to the outside observer, there is no net movement.

A runner produces his "thrust" by pushing off the ground. That doesn't apply here.

If you can explain the force which is canceling out the thrust of the engines in the riddle, you will win my support.

 

[Chris_Ford]

Sure thing. Come back in 5 minutes, I'm making a paint drawing.

Okay. Here it is. Now, be noted, I am not addressing the question as it is asked, I simply said that if the tire speed were matched by the treadmill, it would not move.

 

[gevo]

Sure thing. Come back in 5 minutes, I'm making a paint drawing.

Chris, I hope you take a minute, sit back and look at what you are saying... The wheels can spin at zero revolutions and the plane can still accelerate, that is if the threadmill matches the speed the wheels would spin at any given time.. Or, the wheels can spin at 500000RPM, and the airplane would still not be affected...

I made this mistake when I was talking about the wind matching the airspeed of the airplane, it won't matter because the thrust is produced by pushing the air (whatever initial velocity it has) that is local to the plane..

unless you strap wings to a car, the speed of the wheels have nothing to do with the accelleration... now, if they do this experiment with a car trying to gain enough airspeed to go airborn where it uses the ground to accellerate, then sure the threadmill can match the car and the car can stay in one spot the whole time..

I thought this was concluded..

back to vibrations for now

 

[gevo]

Sure thing. Come back in 5 minutes, I'm making a paint drawing.

wait.. the tires won't move.. right?
cause the airplane WILL move.

 

[Chris_Ford]

Okay, get a toy airplane out, one with wheels.

Push it forward at the tail, on the ground.

Notice how when you do that, the wheels spin? The wheels aren't creating the acceleration, but they're certainly responding to it.

Now let's think of that toy plane on a treadmill. Let's assume the treadmill is already moving. There's a required force by your finger to keep the airplane stationary. Now, as you roll it forward on the treadmill, the wheels spin faster...

If the speed of the wheels were met by the treadmill, the whole system would speed up to infinity at the rate of acceleration, the tires would explode long before that, but either way the airplane wouldn't be able to overcome the treadmill with the engine thrust (as it is when the aircraft speed is being matched)

 

[Clocks]

but either way the airplane wouldn't be able to overcome the treadmill with the engine thrust (as it is when the aircraft speed is being matched)

What force is the engine incapable of overcoming? Is it friction? Is it drag? Is it gravity? What's holding the plane back relative to an observer?

"The treadmill matches the speed" is NOT a force.

 

[Chris_Ford]

The treadmill accelerates in the opposite direction that the plane is going. . .

 

[Clocks]

The treadmill accelerates in the opposite direction that the plane is going. . .

You must be doing this on purpose...but I'll play along anyways.

The engines are producing thrust, and as a result the airplane wants to move forward relative to the air. This is obvious...every airplane works this way. If you disagree with that then any discussion is meaningless. So hopefully we're still in agreement so far.

But the next step...you are certain the airplane will not move forward. So I want to know: what force is acting in a direction opposite of the thrust (which is produced by the engines even when the airplane is stationary), preventing the airplane from accelerating relative to an observer?