Discussion: Plane on a Conveyor Belt

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GrossHam GrossHam	Plane on a Conveyor Belt Jan 30 2008, 11:18 PM EST One MAJOR flaw with the testing of this myth, If your plane is moving forward (relative to the ground) then it is NOT traveling at the same speed as your conveyor. For example, the conveyor was moving at aprox. 25mph, however the plane moved foreward on the belt with a speed over 25mph because relitave to the cones and ground it was still moving foreward. Therefore, if a speed over 25mph was needed to lift the plane off of the conveyor (and 25mph is the "take off speed" of your no conveyor control flight) you have yet to prove that a plane traveling the same speed as its conveyor can take flight. I am not doubting the myths outcome HOWEVER I will probrably not be satisfied until I see a plane on a conveyor "standing still" relitave to the ground, this means that if flight is acheived the distance necessary to take flight relitave to the ground will be ZERO!!! Post reply 48 out of 126 found this valuable. Do you find this valuable? Do you?
Ratbert- Ratbert-	False Assumption, False Premise Jan 30 2008, 11:40 PM EST You're acting like the force interaction between freely spinning wheels on the bottom of the plane(read: lubrication) and the ground the wheels rest on matter just as much as the force interaction between the air and the engine. This isn't an automobile and there's no drivetrain. A conveyor moving backward "at matching speed" isn't nearly enough to stop a vehicle that isn't pushing off it... unless you manage to get the wheels blow out and the plane to crash into the runway. The whole myth is based on a false premise in the first place but the reason it's been so popular is because a conveyor belt can stop an automobile and it's counterintuitive to say that it can't stop an airplane. Post reply 23 out of 27 found this valuable. Do you find this valuable? Do you?
GrossHam GrossHam	RE: False Assumption, False Premise Jan 30 2008, 11:55 PM EST I understand what you are saying however in relation to the conveyor, the plane wouldn't take off only going 25 mph, 50 mph would be needed in relotion to the conveyoy (so 25mph to the ground). if there was no friction with the weels and the plane was not on, the plane wouldn't move at all in relation to the ground. Post reply 5 out of 20 found this valuable. Do you find this valuable? Do you?
nthalj1 nthalj1	RE: False Assumption, False Premise Jan 31 2008, 12:55 AM EST not quite. see, the myth does not deal with forces. the myth deals with speeds. these are two quite significantly different things. having read the generally excepted statement of the myth, it is quite clear that the myth answers itself. the plane is moving at a particular speed with respect to the conveyor belt. the belt is moving at the exact same speed in the opposite direction, with respect to a stationary object on the ground. by the definition of speed and velocity, there is no net movement of the plane with respect to a stationary object on the ground. since wind speed is generally lower than take off speed at low altitudes, the plane cannot generate lift and thus will not fly. also, the notion on the show that the propulsion systems of planes and cars are inherently different is ludicrous! the exact same forces are involved. if you draw a free body diagram of all the forces involved in the propulsion of an aircraft, this is quite clear. now, if there were absolutely zero friction in the wheels of an airplane, this would be different, but the presence of friction means that they are the same situation. anyway, no matter what, this was poorly done and there is no possibility of flight if the terms of the myth are followed, unless it's in a wind tunnel Post reply 6 out of 30 found this valuable. Do you find this valuable? Do you?
ljasonl ljasonl	RE: False Assumption, False Premise Jan 31 2008, 2:16 AM EST The reason this myth is so prevalent is because a whole lot of people, including most of the people posting here, do not understand what's going on. Yes, the forces ARE different. A car generates forward motion through frictional interaction with the GROUND. If the ground is moving backwards, the car must overcome the backwards motion before it generates any net forward motion. A plane is different, it generates forward motion through THRUST, an interaction with the AIR. The motion of the ground is (mostly) irrelevent. You can pull the conveyor belt backwards at 1000mph if you want, the THRUST is still acting against the stationary air. It does not need to overcome the rearward motion of the conveyor belt. This is exactly what they mean when they say the car powers the wheels and the plane powers the propeller. It's two fundamentally different ways of generating forward motion. A free body diagram does not show this because a FBD shows the object itself (plane) and not the surfaces it's interacting with. Yes, if you hold the plane still in relation to the air column (clamp the tail down to the ground), it will not take off because there is no airflow over the wings. This will never happen on a conveyor belt, however, because the thrust is always acting against the air column and not the belt. Post reply 25 out of 27 found this valuable. Do you find this valuable? Do you?
mikeisright mikeisright	RE: Plane on a Conveyor Belt Jan 31 2008, 2:52 AM EST I've read though the posts and it looks like most of what I'm about to say has already been said in different forms. Here's some things to think about: The concept at hand is relative motion. The metrics of concern air speed and ground speed. Planes fly due to lift caused by air speed - the speed of the plane relative to the air. Planes do not fly due to ground speed - the speed of the plane relative to the ground. A plane in an air tunnel has zero ground speed, yet can fly because it has non-zero air speed. A plane flying into a head wind has ground speed < air speed A plane flying with a tail wind has ground speed > air speed As you can see, there is a correlation between air speed and ground speed, but ground speed isn't relative to the mechanics of flight. (if this doesn't yet make sense reconsider the point on the wind tunnel). If the test was executed as it should have been, the planes ground speed would have been zero . The planes airspeed would have been nearly zero, since there probably was some breeze even if minimal. So why did the plane fly? Look at the facts: The planes ground speed was not zero, it moved forward relative to stationary cones. In this case, the planes airspeed was nearly equal to the ground speed The plane achieved air speed of 25mph, generated lift > the planes weight (378 lbs or so), and that's why it flew The pilot was correct. He wouldn't have a pilots license (and perhaps his life) if he didn't understand the basic mechanics of lift, airspeed, and ground speed. My two cents: Given the budget of the show ensuring scientific accuracy is not a practical issue. Their own staffer Grant could get this correct. I therefore assume the producers accept the occasional practice of bad science since it generates conversation about the show, even if it is an academic disservice to the viewers. Post reply 15 out of 32 found this valuable. Do you find this valuable? Do you?
Darklingza Darklingza	RE: Plane on a Conveyor Belt Jan 31 2008, 12:44 PM EST The science was good, the plane flew as expected. Reworking the question and then claiming the science is bad is just sad. Of course the plane flew, and it did so because it only had to overcome the minimal friction acting on its wheels which it probably could have done at a low idle. I am a pilot with a degree in physics, and I am happy with the results. Perhaps the argument should now be moved to a linguistics board because there is no arguing the science anymore. Post reply 26 out of 30 found this valuable. Do you find this valuable? Do you?
EnderGT EnderGT	RE: Plane on a Conveyor Belt Jan 31 2008, 3:24 PM EST "If the test was executed as it should have been, the planes ground speed would have been zero . The planes airspeed would have been nearly zero, since there probably was some breeze even if minimal. " Why? This isn't, as you claim, an issue of relative motion. This is an attempt to test if the treadmill will prevent the plane from moving. They proved it quite will that the treadmill has very little effect on the plane, certainly not enough to keep it from flying. The reason people are still arguing is that there are two different interpretations of the question being tested. Interpretation 1 is "Can the plane take off if it IS NOT ALLOWED to move" relative to the ground (and thus the air, given that there's not supposed to be any wind). In this case, it is obvious to all of us that the plane cannot take off. Interpretation 2 is "Does the treadmill prevent the plane from moving". In this case, it should be obvious that it does not, and that the plane can and will take off. Post reply 35 out of 36 found this valuable. Do you find this valuable? Do you?
GrossHam GrossHam	RE: Plane on a Conveyor Belt Jan 31 2008, 4:29 PM EST ok i see where everyone is going and i came up with this, the entire myth depends on your interpretation of the conveyor, and more importantly the speed bu which it is traveling. first of all at 25mph the plane took off, overcame the friction of the weels etc. NOW suppose you had that conveyor going so fast that the force generated by friction between the weels and conveyor was so great that the plane could not overcome it (say at 400ish mph), then the plane wouldn't take flight. Therefore, the answer to the highly debeted question is both yes and no. Post reply 5 out of 10 found this valuable. Do you find this valuable? Do you?
EnderGT EnderGT	RE: Plane on a Conveyor Belt Jan 31 2008, 5:05 PM EST "ok i see where everyone is going and i came up with this, the entire myth depends on your interpretation of the conveyor, and more importantly the speed bu which it is traveling. first of all at 25mph the plane took off, overcame the friction of the weels etc. NOW suppose you had that conveyor going so fast that the force generated by friction between the weels and conveyor was so great that the plane could not overcome it (say at 400ish mph), then the plane wouldn't take flight. Therefore, the answer to the highly debeted question is both yes and no. " But for the belt to be moving at 400 MPH, the plane would have to be moving at 400 MPH (remember, they have to be moving at the same speed) thus the force has already been overcome and the plane still takes off. The real question that was tested, and the real question being asked by the riddle, is "Can the conveyor prevent the plane from moving?" No, it can't. The plane will take off. Post reply 6 out of 15 found this valuable. Do you find this valuable? Do you?
GrossHam GrossHam	RE: Plane on a Conveyor Belt Jan 31 2008, 5:13 PM EST if the bent was moving 400+ (or whatever speed is necessary for the force of friction to be greater than the force exerted by the plane) it will not have foreward motion, simple physics, subtract the forces Post reply 7 out of 10 found this valuable. Do you find this valuable? Do you?
EnderGT EnderGT	RE: Plane on a Conveyor Belt Jan 31 2008, 8:13 PM EST "if the bent was moving 400+ (or whatever speed is necessary for the force of friction to be greater than the force exerted by the plane) it will not have foreward motion, simple physics, subtract the forces" ok. Thrust of engines (25,000lbs) - force of treadmill on plane (10lbs) = 24990 lbs of thrust. plane takes off. Sure, those numbers are pulled out of my ass, but I guarantee you the relative magnitudes are pretty close. The simple truth is the treadmill CANNOT exert enough force on the plane to prevent forward motion, and so the plane will take off. Post reply 12 out of 12 found this valuable. Do you find this valuable? Do you?
GrossHam GrossHam	RE: Plane on a Conveyor Belt Jan 31 2008, 9:04 PM EST Im not talking about your rinky dink treadmill set to "fat burn" or peice of cloth on the ground, im talking fast conveyor bents, even though the plane's wheels are free spinning, they still create friction with the ground (the exact force can be found using the formula Ff=uFn {Ff[force of friction]=u[coeficient of friction aprox. .68 for rubber on concrete]Fn[normal force]}) so if you could get the belt moving to a velocity where the frictional force was high enough as to negate the planes thrust, it wouldn't generate lift and so would not fly. I admit it would be impractical to make such a conveyor, BUT it is possible. Post reply 8 out of 14 found this valuable. Do you find this valuable? Do you?
mikeisright mikeisright	RE: Plane on a Conveyor Belt Jan 31 2008, 9:15 PM EST Boy do I feel foolish. You are correct. I retract my fatally flawed analysis which stupidly neglected to consider that the wheels of a plane spin freely. Somehow my brain was mired in the drive train of an automobile. The only forces introduced by the conveyor are the rolling resistance of the tires, plus the frictional resistance of the wheel bearings. And of course. the magnitude of these resistive forces are minuscule as compared to the propellers forward force. Perhaps the test could be retried with a winged automobile. Post reply 14 out of 15 found this valuable. Do you find this valuable? Do you?
GrossHam GrossHam	RE: Plane on a Conveyor Belt Jan 31 2008, 9:27 PM EST I agree with you, they wouldnt even need to test it, a calculation showing how fast the conveyor would actually need to be spinning to prevent the plane from taking off would be sufficient. And perhaps a better explanation of the car vs plane propulsion systems which seems to be a big topic on many thread discusions Post reply 11 out of 11 found this valuable. Do you find this valuable? Do you?
spammenot spammenot	RE: Plane on a Conveyor Belt Feb 1 2008, 12:04 PM EST Sure, it's simply a trick play on words and not anything scientific. If I were the pilot I would have kept the throttle down so as to have the plane truly remain stationary and show the plane "staying in place like a brick". The "Mythbusters" who've kept their day job options open should be happy given I suspect the show is getting close to doing a test of whether someone can clear a tank of sharks on a motorcycle, so to speak. The show lost me since their first trick play on words bait and switch test--windows open or A/C on while driving on the highway. Remember, the one where they compared a most unaerodynamic vehicle (the largest SUV made) at sub-highway speed with windows down with running the A/C flat out at 50F degrees (note the coats and hats they put on when they got into the A/C vehicle). Post reply 5 out of 13 found this valuable. Do you find this valuable? Do you?
veshiy0leg	RE: Plane on a Conveyor Belt Feb 1 2008, 12:47 PM EST think it would be much easy to make the point to bust that myth my changing the plane from front-propelled to tail-turbine, which would create much less air moved under the wings. Post reply 4 out of 17 found this valuable. Do you find this valuable? Do you?
kaycoe kaycoe	RE: Plane on a Conveyor Belt Feb 1 2008, 1:54 PM EST did you see the episode about is yawning really contagious?? do you know the results? Post reply 7 out of 13 found this valuable. Do you find this valuable? Do you?
hohon3 hohon3	RE: Plane on a Conveyor Belt Feb 1 2008, 8:05 PM EST The myth was not busted. Watch the video and a blind man can see the plane move forward in reference to the orange cones, which proves the plane's ground speed and the conveyor belt speed were not matched. One important point which most people are missing is the difference between the speed of the planes wheels and the speed of the air over the plane's wings. Landing or taking off in a plane that needs 60 knots of air speed means that in a 60 knot head wind you could take off from a dead stop, or land and not move forward. Jamie, I think you mixed your apple and oranges. As in air speed and ground speed. The myth wasn't busted, the test of it was just poorly structured. Post reply 5 out of 19 found this valuable. Do you find this valuable? Do you?
abrucks abrucks	RE: Plane on a Conveyor Belt Feb 1 2008, 9:24 PM EST There is one more thing Adam and Jaime can do to help people understand the forces here. Take a fan and blow it over a sensitive scale that weighs items to the nearest gram. Then put Grant's plane on the scale and weigh it, then start the fan again with the plane on the scale. The weight of the plane will decrease, as it is "starting to fly". No, a plane can't take off from a stop, which is why people are getting confused. But this is why light planes must be tied down, if they are not in a hangar. Even a light wind of 20 mph will start to lift single engine plane. Post reply 7 out of 18 found this valuable. Do you find this valuable? Do you?

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