Airplane on a Conveyor Belt

Tomorrow night on Mythbusters, Adam and Jamie are going to tackle this question:

“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?”

That one question has been debated across the internet, in airports, over drinks all across the country. 

Well, I'm going to give you the answer early, in my own special way.

OF COURSE THE DAMNED PLANE TAKES OFF.

First let's look at it as a thought experiment:

When you run on a conveyor belt tuned to the same speed you're running, you won't move forward relative to the ground.  When you drive a car on a hypothetical conveyor belt tuned to the same speed as the car, the car won't move forward relative to the ground.

What you're supposed to think is that the plane won't move forward relative to the ground if it's on the conveyor belt.  But the plane has an important distinction compared to your feet and your car.  Both you and your car propel yourself along because of the force you exert against the ground.  The airplane exerts force, through propellers or jet engines against the air, the tires are not powered - not in any way. There is no motorization tied to wheels on any aircraft I've worked on or around.  Those wheels spin freely and are there just to reduce friction against the ground.  The skis on amphibious aircraft are proof of that.  A plane can take off on a relatively calm river either upstream or downstream.  What's more important is the direction of the wind, not the direction of the water. 

Said another way: The plane doesn't care what's below it, so long as there's free airflow to create lift through the pressure differential on the two wing surfaces.

What's going to happen is that the conveyor and wheels will keep going faster and faster in free spin, until the plane leaves the ground, but that plane is going to be propelled down the conveyor runway by it's engines, not by its wheels.

Comments

[Tim Mulcahy]

It's a function of airflow over and under the wings. That's what generates lift, as long as you have it the plane flies.

Old aircraft carriers - before catapults, the ships went to 35 knots and turned into the wind to help the planes take off on the short runway. with the really old light planes (old biplanes) sometimes the planes took off practically standing still. Yup, it will work.

[Tim Mulcahy]

Okay now that I'm thinking about it, you have the jets running but are you moving air over the wings if the plane is standing still. It's actually different than the aircraft carrier scenario. Actually, the more I think about I'm thinking that the plane will not take off.

[CV Rick]

because you are thinking that the ground serves a function. It does not.

[Tim Mulcahy]

I'm not thinking about the ground at all. I'm thinking about the physical movement through space which creates wind over the wings.

How do you get that movement over the wings to create lift if the plane isn't moving.

[CV Rick]

You must be thinking of the ground, if you believe that the plane isn't moving. The plane moves because of the acting force of the air, with either the propeller or the jet compression.

Think about it another way.

Picture a rocket on a launch pad. Now, next to this rocket, vertically. Put rows of small wheels against the conveyor, mounted to the rocket. Now turn on the conveyor and ignite the rocket. Does it launch?

[NFlanders]

I've got to disagree, CV Rick. Of course, I know nothing about jet propulsion, but since when has that ever stopped someone from expressing an opinion on the internet?

First question, is the plane moving relative to the air around it? Slightly. The engines are sucking a hell of a lot of air and pushing it back, but there is no other air flowing over and under the wings. I suppose if you had unthinkably powerful engines, you could suck enough air to get some push on the wings, but probably barely enough to lift a fighter.

The rocket analogy is incorrect, I think. A rocket doesn't have wings. I think a better analogy would be a plane trying to take off on a frictionless plane, like a perfectly icy lake. No matter how hard you crank those engines, the lift can only come from airflow. Airflow (neglecting winds) can only come from motion relative to air, which you ain't got.

Thus endeth the uninformed physics ramblings from Ned. I guess I'll have to watch tomorrow night to see if you are right.

[CV Rick]

"is the plane moving relative to the air around it? "

Yes. Why not? In the case of the F-16s I worked on with the GE F110 Engines, that would be 29,000 pounds of thrust. That's a lot of thrust - all through the exhaust with exactly zero thrust diverted to the wheels.

The rocket analogy is exactly, precisely accurate. I'm trying to get the idea across that the runway or conveyor is just a clear, obstruction-proof pathway, not a place where the plane exerts any positive forces (gravity produces some forces, but not the plane or the powerplant, or the engine, or the propellers)

Let's compare apples to apples - - a car on a conveyor is the equivalent to a plane in a wind tunnel. Now, I can foresee some instances where a plane in a powerful wind tunnel will stay still. But that's irrespective the ground, conveyors or even it's wheels.

[NFlanders]

This is probably more than you wanted to discuss this topic, but at the risk of wearing out my welcome, I'll give it another go.

I think I have discovered the root of our misunderstanding. The problem is that you're imagining an F-16 on a treadmill instead of *any* other plane. The problem is that those engines are basically rockets, and will move the plane forward no matter how fast the treadmill is moving beneath. That negates the whole premise of the question, which posits that the plane is going to stay on the treadmill. So, the answer to the question is that a fighter jet will be able to take off from any surface, but a 747, or a bi-plane, or a Learjet will not, not unless they can produce enough thrust to overtake the conveyer belt and start moving with respect to the air.

A treadmill is indeed just a clear, obstruction-proof pathway. Unfortunately, it is also a pathway where your tailwind will always equal your exact speed.

"[A] car on a conveyor is the equivalent to a plane in a wind tunnel." I disagree. When you run on a treadmill, do you feel wind on your face? No, because you're not moving relative to the air in the room. A wind tunnel is the exact opposite; you're only moving with respect to the air. A plane in a wind tunnel WILL take off. A (non-fighter) plane on a treadmill will not.

Sorry to go on and on. I will wait to see what they say tomorrow, but I imagine that they probably won't address the physics of an F-16 or a Harrier jump jet.

[CV Rick]

I just used the F-16 as an example because it's an extreme. I've also worked on F-15, F-106, F-4, T-33, T-36, KC-135, and Cessna. The physics is the same.

Come back after the show.

[Jane]

I don't believe in flight. There is just no way it works. Come ON.

Oh, right, airflow over the wings, blah, blah, blah. Sure, pal.

There is no way that big heavy jet plane I fly in is flying. I can't explain how I get to France, but it most certainly is not because that chunk of steel, etc. is flying.

[NFlanders]

You are right; I am wrong. That's what I get for not Googling it first!

For some reason, I just couldn't understand the fact that the conveyor belt cannot move the plane backward in space, since the wheels are not part of the propulsion. I envisioned the plane standing still and lifting off the ground (which obviously it can't do). In reality, it would look like any normal plane taking off.

Thank you for tolerating my excessively long comments.

[CV Rick]

Not a problem. I trust we'll be watching the show at the same time to see it play out.

I want to know how they're going to fabricate a plane width, runway length conveyor. Unless they're going to do it with an R.C. plane.

[CV Rick]

Okay .. . who watched it?

[NFlanders]

You know I did. I was impressed that they got a real plane, but I'm sure that show costs about 5 bucks to shoot, so they can splurge a bit.

I always wondered about the roaches too. Now, if we could only send them to Mars.

[Chris]

It is funny that you all are so wrong...rotational inertia is why you cannot ignore the conveyor belt.

It is not as convenient as "ignoring the ground".

Of course if they belt went THAT fast it would probably create a headwind anyhow the same way a tesla turbine works.

[CV Rick]

It's funny that they proved my assertion to be correct both mathematically and experimentally, Chris. Thanks for playing, I'm sure we'll have a lovely parting gift for you.