Posted January 31st, 2008 @ 11:13am by Erik J. Barzeski
The (in)famous "Plane on a Conveyor Belt" experiment was run last night on MythBusters and, as physics dictates, the plane takes off.
That was a terrible experiment. The whole basis for the myth was that the airplane could take off from a treadmill traveling at the same speed as the aircraft. Unfortunetly during their testing they failed to keep the aircraft and the treadmill at the same speed. It is clearly evident from the video that the aircraft is accelerating faster than the treadmill is being pulled. If they were moving at the same speed the aircraft would not be passing all the cones while on the treadmill...it would remain in a stationary position relative to the cones.
The physics are totally wrong, an aircraft doesn't fly because of a propeller, ie a glider. It flys because of the air flow over the wings creating lift. If you were to run 20 mph down a road your hair would blow in the wind stream being created. If you run 20 mph on a treadmill your hair will look just as pretty as when your standing in place. Its the same principle with a wing.
The myth is a scam. They need to retry the myth with a pilot that can keep the plane at the same speed as the treadmill.
- It's not clearly evident.
- The aircraft would still pass all the cones (just as it would if it were on ice). It most definitely would not remain stationary.
- The physics are correct. Lift is generated by airspeed. Forward motion is generated because the engine (jet or propeller) acts against the air, not the ground.
- A person running has nothing to do with it. That's the "car" problem. Strap a jet pack on a person running on a treadmill and you've got the equivalent version.
Why can't these idiots get it through their thick skulls that the plane moves relative to the ground? They seem to be basing all of their opinions on the assumption that the plane stays still due to, I dunno, magic??? Even the most basic version MythBusters ran - the actual plane on the treadmill - proves them otherwise. The propeller pulls the plane through the air.
The only shocking thing to me in the entire episode was that 400 pound planes exist and they can take off going only 25 MPH!!!
More quotes below, because, well, why not? It's like spectating a car accident. In the voice of Haley Joel Osment: "I see stupid people."
Please note: all the quotes below are incredibly stupid and WRONG. I've left the names off to protect the stupid, but they can all be found at the MythBusters forum link above.
I think they blew this one. The whole object of the conveyor belt was to keep the plane from moving. The plane still moved. Thus not busted.
I think they would have better luck tying the plane with a long rope to keep it from gaining forward motion while still allowing it to gain upward motion and trying that instead of a treadmill.
They needed to account for the forward motion of the plane. They did not measure how far it took the plane to take off while on the conveyor belt. I'm betting it was the same distance as off the belt.
Excuse me, but as the videotape proves, the planes (both scale and lifesized) were moving. As the tape clearly shows, the plane was NOT traveling at the same speed of the treadmill, as the plane was moving FORWARD in relation to the pylons that were on either side of the runway. Therefore, air WAS moving under the wings by the action of the movement, NOT THE PROPELLOR.
Do you REALLY think that the NAVY would spend BILLIONS of dollars on catapult systems for aircraft carriers, if they could have just gotten away with having a treadmill instead.
GET WITH THE PROGRAM PEOPLE. It is all physics, just like the pilot Mark said: If I move the air provides lift, if the treadmil matches my speed, then the plane will be a brick.
This has NOT been proven at all.
Presuming the 'conveyor belt' was moving at 25 mph and the plane was not powered, it would be moving backwards at 25 mph. When the prop turns sufficiently to pull the plane forward (without the tarp) at 25 mph it will have lift off, but when on the tarp, the forward speed is cancelled by the reverse speed of the 'conveyor belt'. Therefore the plane moving forward at 25 mph means the engine is revving sufficient to overcome the tarp plus 25 mph (total 50 mph) and creating lift off at its required forward speed.
The plane and the conveyor belt were NOT traveling at the same speed. If they were, the plane would be stationary. It wasnt hard to see the plane was moving forward pretty fast, obviously faster than the belt. So its ground speed was still fast enough to reach takeoff speed to produce enough lift on the wings to fly. The prop doesnt make the plane fly, the wings do. Shut an engine down during flight, what happens, you glide. Cut your wings off in flight, see what happens.
I have lost all faith in humanity. I am truly disappointed in the MythBusters on this one. This myth should never have made it past the so-called "science content" segment. A plane is able to fly because air is passing over and under the wings at a certain velocity. This myth is a question of relativity. If the plane and the conveyor belt are moving at the same speed but in different directions, the plane is still NOT moving relative to the air. The ultralight plane in the episode took off because it was moving fast enough relative to the air to create lift under the wings. The conveyor belt was not moving fast enough to keep the plane stationary relative to the air. If you can't understand this concept, I would suggest opening a physics or calculus III book. Books can usually be found in libraries. If you cannot find a library, try google searching "how a plane flies."
Is this the only way to make a formal critique of the method used to test this myth? I just want to make a point in the hopes of clarifying a few things.
First, if you watch the footage from the show you can see that the plane is moving relative to the cones on the ground which, i assume, are stationary. Therefore, the plane is moving relative the ground and the basic premise behind the myth (that the plane is not moving relative to the ground because it is traveling on the belt of the treadmill at the same speed that the belt is moving in the opposite direction) is not what is going on in the experiment. To test the myth properly, the plane must remain stationary relative to the ground.
Second, the speed of the truck pulling the treadmill being higher than the take-off speed of the plane means nothing. The take-off speed is the speed relative to the ground required to give the plane enough lift to take off. The plane's wheels could make it travel much faster than that if the plane would stay on the ground. The treadmill just increases the speed the wheels have to go to get the plane to the take-off speed relative to the ground.
The episode proved that a plane can take off with a 25 MPH tail wind. It did not prove that iit could take off if it was standing still in relationship to the ground/air.
The proper test for what they were trying to prove is almost as simple as what they did, but requires a bit more preflight testing.
What they need to determine is the propellor speed at which the plane will reach 25 MPH. (It will work because this is a fixed pitch prop)
Get the truck/tarp moving at 25 MPH and increase the airplane's throttle until the proper RPM is reached, if the plane does not take off, keep increasing the throttle until it does.
If it requires 20% or more rpm then I would say that a plane cannot take off on a conveyor belt that continues to increase its speed porportionately to the increase in power of the plane.
This guy seems to think the Mythbusters pulled the plane forward with the conveyor belt instead of in reverse:
Wow! I can't believe the Mythbusters folks messed up so bad on this episode. Air has to move over and under the wings to create lift. Newton's Third Law: For every action there is an equal and opposite re-action. There is NO air current (Action A) against the wings to push back (Action B) and create lift. Sorry guys, the truck pulled the airplane forward to create a lift.
This guy is adding words to the problem to justify his misconception. The "wheel" speed is never mentioned in the original problem, and in fact, the wheels go 2x as fast as in a normal, non-conveyor takeoff:
The Myth was, Can a plane take off on a conveyor belt when the "wheels" on the plane are going the same speed as the "conveyor belt" when both are moving the same speed in opposite directions. In that case no it cannot since both will cancel each other out and plane should stay in the same spot on the conveyor belt and the plane will have no forward movement to build airspeed to create lift. In order to the plane to take off, it will need to make up the speed that it needs to take off. So technically the wheels on the plane were going 50mph in that myth and the plane was going 25mph forward to build enough lift under the wings in order to take off. So yes, technically a plane can take off on a conveyor belt but if the conveyor belt and the plane's wheels were going to the same speed, then no it cannot. Myth is still not busted
I'm afraid the only thing they busted was my love for the show. Had they truly matched the speed of the plane (ie 25mph) to the belt and not have throttled the engine (thus prop) to more than a 25 mph pull it would have never took off. What a shame they didn't realize this in time to do a decent show.
I would just like to point out that, if in fact, the plane was indeed going as fast as the "treadmill" tarp pulled by the truck, there would be no forward motion. The plane has forward motion and is indeed going faster than the tarp is being pulled. Therefore, the plane is able to get air under its wings and thus has the ability to generate lift. When you understand that air speed generates lift than you can see that if in fact the plane and the treadmill were going at the exact same speed, there would be no forward motion, momentum or wind speed generated by the plane moving. It would be as if the plane were static, that is if they were the same speed (the "treadmill" and the plane) then the plane would have no lift generated by movement. Hence it would technically not be moving. It is impossible to lift if the plane has no forward mmotion. I know this is excruciating detail but it just not possible if the plane was static like the car in the episode. It only took off because it was going faster than the tarp. Point blank. If you watch the episode, the plane moves past the cones which would be impossible if both the plane and the tarp were going the same speed.
Ok I'm new, But it seems most of the later posts got it correct. The aircraft moved past the trafic cones (it had ground speed), the wing had lift. Had the aircraft not had foward movement, but relied only on engine thrust (coveyor and aircraft speed REALLY match); (provided thrust made enough lift over the wing). The aircraft would lift vertically before it would move foward.
An airplane's propeller pulls the plane through the air, causing lower pressure on top of the wing than under it*, which is what lifts the plane.
The propeller does not push enough air across the wing surface to cause a great enough pressure differential for the plane to overcome gravity.
If the full-size test were actually done precisely, the plane would have stood still with respect to the ground.
Instead, the plane accelerated down the runway, passing cones all the way.
The same is true of the small-scale test: if the speeds were exactly the same, the plane would have remained still with respect to both the ground and the frame of the treadmill.
If the "forward motion" of the plane exactly equaled the "backward motion" of the treadmill, there would be a net-zero wind passing over the wings, and therefore no lift. This should be true even if moved at double or triple the minimum takeoff speed because you still have a net result of no wind over the wings. Less physical strain on the hardware, but really no different than mounting the plane on a pole - the plane can not move forward through the air, therefore it can not generate any lift.
Let me try to explain this slowly: If the aircraft does not appear stationary then the speeds are not matched.
Let me try to explain it slowly: If the aircraft is stationary, it has no speed TO match.
This was a huge oversight! The ground camera clearly showed that the plane had forward motion. Anyone that knows anything about how a plane flies knows that the wings must have air flowing over them in order to produce lift. Spinning the wheels does nothing to produce lift. The experiment proved nothing. I don't know why the pilot didn't contest the validity of the test!
The plane moved forward. It was beyond the cones when it took off. The plane was accelerating faster than the treadmill was moving...thus it had airflow over the wings, which provided the lift for take-off. Most aircraft cannot accelerate straight up...they need airflow over the wings to to produce "lift". Only a few, mostly military and acrobatic aircraft...and most R/C models...can accelerate vertically...which means the thrust is overcoming the weight. Do this "busted" myth again with a camera placed 90 degrees from the plane. Keep the plane in that position and pull the conveyor belt as fast as you want...the plane won't take off. The propeller won't produce enough airflow over the wings to produce lift.
I'm a pilot, I'm solidly in the no fly group. In fact it is hard to believe there is a "fly" group. Then mythbusters says "busted". Puhleeze... All this myth proved is that the airplane( in both cases) has more than enough thrust to fly faster than its take off speed (11 and 25 MPH). In fact the yellow plane probably can fly at 80 mph. So unless the conveyor also sped up to 80 the plabne would generate lift and fly. To prove this myth the planes must be limited to ONLY their minimum take off power and speed. This can be verified by manifold pressure and tire speed. At any time that the power exceeds the minimum, clearly the plane is going to start to go forward (shown initially as tire speed increasing). If the plane is allowed to accellerate it is clearly going to generate a bit of lift. At this point the wheels will no longer be in firm contact with the conveyor, the (relative) conveyor speed will diminish (as shown by decreasing wheel speed) The (relative) aircraft speed will increase by this corresponding amount and the aircraft will IMMEDIATELY fly. Basically as soon as either plane moves forward in the frame and generates a bit of lift, then the myth is over as the conveyor and planes are no longer moving at the same speed.
There should have been a science content here, but then there would have been no show. To call it busted is just a way to generate letters and NO ONE could be that stupid.
I love the people who say "it was not supposed to move because moves at the same speed." Do they not see the blatant disregard for simple logic?
Just because you saw it on television doesn't make it true! Granted the mythbusters are pretty awesome but they're not perfect. And in this case, boy were they wrong. Anyone who has taken a physics class knows that a plane needs air passing over and under it's wings to take off. And in order for the air to pass under/over the wings it has to be moving. From what the mythbusters said the plane was not supposed to move because it was moving at the same speed that the conveyer belt was moving but in the opposite direction. I really think they need to revisit this myth or at least explain what happened and why their "busted" really wasn't.
I agree with all those who say that this myth needs to be redone. The plane and the conveyor belt weren't going the same speeds, if they were, the plane would have stayed stationary. They show the speed of the ultra-light on the practice run, but don't show it during the test and they never show the speed of the truck Jamie is driving. As a pilot (and I hope all other pilots know), you need airflow over the wings to make an airplane fly. A stationary plane with no wind will not create any lift so the plane will not fly. The plane in this myth was moving down the conveyor belt so it was able to create lift by flying faster than the conveyor belt and was able to take-off. If the idea of this myth was just to see if a plane could take-off from a conveyor belt, then yes it is busted, because all you have to do is get the planes speed up to conveyor belt speed plus take-off speed and the plane will take off. But if the myth is that a plane can take-off while matching the conveyor belt speed, this myth is far from being busted. If they decide to redo this myth, I don't see why they can't build an actual conveyor belt to place the plane on. According to the show, they were using an ultra-light aircraft that only weighed 300 lbs, are you telling me that after all the experiments that they've done, they can't build something like that. It would be able to show exactly how fast the belt is actually moving.
This guy talks (correctly) about a small issue with the host's explanation: the engine (and propeller) pulls the plane through the air, it doesn't blow air over the wings.
I think the guys got it right on the show. The plane should take off from the conveyor. However, their explanation of the physics was completely wrong. The plane does not take off because of the propeller moving air over the wings. The plane takes off because the propeller pulls the plane through the air, this movement of air over the entire wing allows the plane to fly. If the propeller were enough to keep the plane aloft, then why are the wings so large? By this theory you would only have to have enough wing directly behind the propeller. This can also be proven by testing with a "pusher" style aircraft, one with the propeller behind the wing. It would also take off with this test. To further clarify this whole thing, they should have been measuring air velocity at the wings, not ground speed at the wheels.