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UPDATE: Well, that’ll teach me to blog about physics I don’t fully understand! I have researched the issue and edited the post below, now that I understand the explanation from The Straight Dope.
Last night the Discovery show Mythbusters settled a longstanding debate: whether an airplane on a conveyor belt (running in the opposite direction as the plane) can take off. The short answer, as liveblogged by Jason Kottke:
HELL YEAH THE PLANE TAKES OFF
It’s a curious problem. As a thought experiment, it seems (at least to me) like the plane shouldn’t take off, since it wouldn’t gain takeoff velocity relative to the ground. But according to, you know, SCIENCE, the plane will still reach takeoff velocity — the wheels will just spin twice as fast. This is because the wheels aren’t providing any thrust, it’s the engines (propellors) that are pulling the plane forward through the air. It’s the velocity of the air relative to the wings that counts, which is generated by the action of the engines pulling the plane forward. So the conveyor belt will only stop the plane from gaining takeoff velocity if it creates enough friction to counteract the forward thrust of the engines (or propellor).
Despite explanations of this sort by physicists, the issue wasn’t really settled until last night’s Mythbusters episode — they replicated the experiment on a small scale, then with a real airplane (albeit an ultralight), using a huge tarp dragged by a truck as the “conveyor belt.” Even the plane’s pilot thought the plane wouldn’t overcome the power of the conveyor belt, and thus wouldn’t gain takeoff velocity. When Jason Kottke first blogged about the issue last February, his comment thread was hot with controversy. So Kottke tuned in to Mythbusters last night and liveblogged the event, with results visible above. His exuberance over the plane’s liftoff has resulted in a “HELL YEAH THE PLANE TAKES OFF” tee-shirt available starting at $18. Wow.
Watch the Mythbusters clip in question below…. (Note: if this clip is pulled down, I’ll try to dig up another.)
Keep in mind that the issue here is partly semantic, and has to do with how you explain the theoretical problem. I explained it poorly in my first post, since I (like apparently many) assumed that air would flow over the wings as a result of the propellor spinning, and that would be enough to make the plane take off, even if it was stationary. This is not the case — the plane is going to need to move air over its wings in order to take off. The point of the experiment is simply that yes, the plane will move despite the conveyer belt underneath it. And that movement (at least in this experiment) provided sufficient lift to get the plane off the ground.
It seems like you still don’t get it. The plane is moving with respect to the ground. To get lift it must be traveling with some minimum velocity relative to the air. Since there was no wind in the experiment, the velocity relative to the air was identical to the velocity relative to the ground (the real ground, not the conveyor belt). The belt was moving at take-off speed in the opposite direction. Relative to the belt, the plane was moving at twice the takeoff speed and relative to the air it was moving at takeoff speed.
posted by Zach on 1-31-2008 at 4:48 pm
Yeah, this was not what I envisioned when I read what the experiment was supposed to be. If a conveyor belt is moving the same speed, opposite direction, the plane should remain pretty much where it started, not gaining momentum as it zips farther and farther down the runway. What would merit an all caps jubilant shout would be if it gained lift from a fixed relationship with the ground!
posted by Johnny Cat on 1-31-2008 at 4:59 pm
Zach - thanks…I’ll actually edit the post a bit to reflect this improved understanding.
posted by Higgins on 1-31-2008 at 5:02 pm
the plane was still moving relative to the ground.
Let’s see them try to do it with the plane in a stationary position relative to the ground. It won’t work because you need air flow over the wings.
It appears that the tarp had no effect on the planes WHEELS and the plane achieved lift because of it’s forward motion.
The tarp and plane were also accelerating at different speeds.
This is false exhuberation over a poorly conducted experiment.
Good tv for fools.
posted by barc on 1-31-2008 at 5:07 pm
Ditto. (Well, except for the “Good TV for fools” remark - that seemed a little harsh). Looked like the plane was able to accelerate a lot faster than the truck, and that it reached the required takeoff speed relative to the ground despite the effects of the tarp. Had the plane actually been still relative to the ground, I don’t think it would have taken off. The lift required for flight is provided by airflow over the wings; no airflow, no lift, no flight.
I’m no expert, though…any physicists out there care to weigh in?
posted by David on 1-31-2008 at 5:59 pm
One other thing…as far as the engine providing adequate airflow, that doesn’t seem possible to me. That little prop isn’t creating an airflow over the entire length of the wings. And what of planes where the engines are behind or directly below the wings?
posted by David on 1-31-2008 at 6:03 pm
Indeed, the clip didn’t prove anything other than letting an airplane sit on a “frictionless” surface does not prevent it from going forward.
The fundamental flaw in this situation is that the propulsion power does not come from the wheels, it comes from a fan pulling air in and pushing it out in the opposite direction. The wheels are merely a passive support to prevent the airplane fuselage from hitting the ground.
The proper experiment would lock the airplane to the ground and only allow motion in the vertical plane. This would properly test whether the wind pull that the propeller generates is enough air to cause significant lift. As it is now, allowing any forward progress simply tests the frictionless surface idea, which is similar to the situation with sea-planes.
A very poorly conducted experiment with faulty scientific reasoning.
The engine does not produce an air flow with a velocity needed for sufficient lift, it only produces a force to propel the airplane through faster and faster wind to produce the necessary lift.
posted by Piotr on 1-31-2008 at 6:10 pm
The plane takes off because the propeller is providing the forward movement, not the wheels. They wheels just act like ball bearings, allowing the propeller to pull (or push) the plane - that’s why the treadmill has no effect on a plane taking off.
To better understand it, imagine holding a grocery cart or other wheeled thing on a treadmill. When you push it forward, it moves forward on the treadmill. The engine of the plane works similiar to your hand.
There’s a great write-up about it on straightdope.com, but the comment system won’t let me link to it.
posted by xander on 1-31-2008 at 6:11 pm
David - I think your analysis is the most pertinent here. I went ahead and updated the post to reflect this.
I think the real issue here is that the problem being tested is much smaller than people make it out to be. The Mythbusters are testing whether the plane can achieve takeoff speed (yes, speed relative to the ground) despite the action of the conveyor belt — THAT’S ALL.
Any discussion of a theoretical ideal conveyor that exactly matches the speed of the plane (etc. etc.) is a completely different experiment, which you’re welcome to attempt. :)
posted by Higgins on 1-31-2008 at 6:21 pm
{Smacks Noggin} got it. Consider it grok’d. Thats actually quite cool.
posted by Johnny Cat on 1-31-2008 at 7:48 pm
Hmmm - if the engine alone is generating sufficient lift to make a stationary craft airborne I do believe you are flying a helicopter.
posted by paul on 1-31-2008 at 11:11 pm
Personally, you build a large enough conveyor belt for whatever size airplane you consider to be a real “airplane” and then we can discuss the accuracy of the experiment. Until then, most of the commentary here is just theory.
I’ll go back to watching my “t.v. for fools”; at least its not “American Idol” or “America’s Next Top Model”.
posted by Kate on 1-31-2008 at 11:31 pm
I think a lot of you are missing the point. There is NO SUCH THING as a “large enough” conveyor belt to make this “work” the way you expect it. The conveyor belt is INCAPABLE of preventing the plane from taking off. It doesn’t matter how fast, how strong, or how big the conveyor is - the laws of physics promise that the plane will achieve take-off speed no matter WHAT the conveyor does. For an extremely long-winded, and detailed explanation of all these points, see airplaneonatreadmill.com
posted by Chris on 1-31-2008 at 11:59 pm
I am both a physicist and a pilot so maybe I can put this in perspective that is easy to understand.
What gives the airplane lift is its speed relative to the surrounding fluid - air. It’s easiest if you stop thinking about a moving conveyor belt or the speed relative to the ground. Speed through the air is all that matters.
Piotr’s experiment would be useless as pilots do his experiment before every takeoff. We put on the breaks and gun our engines up to full throttle to check various settings and to make sure we’ll have full power before trying to head down the runway. You’ll never see an airplane rise off the ground during this time, however.
In short, the propeller provides the basic thrust to cause the airplane to move through the air, conveyor belt or not.
posted by David Gunter on 2-1-2008 at 8:26 am
Of course the plane takes off… the problem is that the treadmill is almost completely irrelevant to the plane’s speed because the plane’s wheels aren’t driving it forward. If the forward propulsion of a plane was dependent on its wheels, the plane would have to rely on momentum to get it from takeoff to landing, with only a bit of lift and turning to navigate, like a glider. A glider will eventually come to the ground because it has no means of pushing itself forward. A theoretical airplane with infinite fuel could fly forever (engine failure aside) because the engines are pushing the plane through the air.
The same process that pushes an airborne plane through the air pushes a grounded plane along the surface of the ground. The wheels, like someone said, are there to reduce or provide friction as needed. (Providing friction when the brakes are applied to increase the plane’s deceleration.) There is no treadmill that can hold a plane in place — the closest analogue would be a bit of ground that raced along underneath the plane, so the plane didn’t move relative to the ground. It would still be moving relative to the air, however, and it would take off. (Or perhaps - a wind tunnel - where the plane is immobile relative to the ground but the air runs past it at liftoff speeds - the plane would lift off of the ground in those conditions, but it would probably be a bad idea to try to fly in a real-world situation where the wind was blowing fast enough to produce lift. Unless you’re one of those storm-plane people.)
The flaw with Piotr’s experiment is that the propellor or engine are not creating the wind over the wings, they’re moving the plane, which is creating the relative movement of air over the wings.
posted by Deuce on 2-1-2008 at 9:20 am
David and Deuce,
I know that it would not work because of the reasons you laid out. I didn’t really claim that it would work, I knew it would be a failure because obviously the propeller doesn’t move that much air, just enough to start moving the airplane forward. The problem is that the perception from the MythBusters hosts was making it seem like the wind behind the propeller was enough wind to move over the wings to produce enough lift to take off. Which is of course ridiculous.
MythBusters simply proved a different point, that airplanes can move over frictionless surfaces, which has been known since sea planes and ice planes were invented.
posted by Piotr on 2-1-2008 at 11:21 am
Wow, I’m amazed at the lack of knowledge of the theory of relativity here….
But I’m going to make one point in particular on this, and let the age-old descussion of the fact that the weeld dont make a lick of diffrence to the physicist. To the people that keep saying this has anything to do with “the propeller being able to pull enough air over the wings to make it take off” are missing the point. That’s not what the issue is. The issue is relativity, inertia of the wheels, and friction.
I can say no, infact NO PLANE EVER MADE has enough raw thrust the propeller can push enough air across the wings to make it take off with no motion to the relative air. I personally fly a Cessna 172 w/180 HP engine swap, and on a “short field” takoff I lock the wheels and put the throttle all way way in to get ready for takoff. Does the place create any relative lift when this happens? NO. The additional airflow across the wings in this case might produce 50 lb of lift at most, and maybe another 25lb in the tail. Walter
Also, water is a HIGH FRICTION SUFFICE compared to wheels on a runway for airplanes. Your general seaplane can get off the ground in ~500 feet on a runway where it take half a mile on water. Landing takes longer due to the fact that breaks arn’t avalialbe after air-resistance (and also water resistance) becomes a factor.
posted by Walter Hooper on 2-1-2008 at 5:21 pm
The issue here is the motive force: In this case, the propeller generates the forward thrust — thus, whether the wheels are on a conveyor belt or not is moot. Imagine a glass on a table with a napkin on it. If you pull the napkin, the inertia of the glass keeps it stationary. With the plane, the conveyor belt may affect the rotational velocity of the wheels, but not the actual forward thrust of the plane — that system is held within the propeller. Imagine a plane with perfect friction on a conveyor belt in the same place as the conveyor belt travels in the opposite direction at x speed. Since the plane is stationary with a moving conveyor belt, the propeller CANNOT be moving — otherwise, the plane would be moving forward. capisce?
posted by Will R on 2-1-2008 at 7:53 pm
The experiment was prepared and executed perfectly by Mythbusters’, and proved that the plane would fly. And their success also proved my assertion that this stupid debate would rage on in spite of the evidence.
Think, people. Think. It’s not that difficult. Of course the plane is gonna fly because the prop is moving it through the air. As the plane moves through the air, air flows over the wing surfaces and creates lift. So it flies. It doesn’t matter a whit if the conveyor belt is moving forward or backward under the plane; as long as the wheels allow the plane to roll freely, the prop will move it through the air and create lift.
As fun as this is, I’m tired and I’m going to bed. See you tomorrow.
posted by Dave on 2-1-2008 at 11:57 pm
My personal favorite part of this video is that the Segway Jamie was riding falls over when he jumps off of it. Look at 3:40 in the video and it is lying flat on the tarmac. Not being a fan of them I find that positively brilliant!
posted by tim on 2-3-2008 at 10:41 am