MythBusters
Junior Member
Registered: 01-30-08
Posts: 1
|
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.
|
Senior Member
Registered: 03-29-07
Posts: 3940
|
Finally the "No Flies" get a demonstration of the concept of the WHEEL. There is nothing you can do with a treadmill except make the wheels turn faster. As long as the plane can overcome the friction of the wheel bearings, it will accelerate to take off speed and fly. This will always happen until you reach a speed that the bearings burn out or the tires bl0w. Get over it!
|
Member
Registered: 01-31-06
Posts: 8
|
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.
Paul
|
Junior Member
Registered: 12-01-06
Posts: 3
|
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.
|
Junior Member
Registered: 01-30-08
Posts: 1
|
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
|
Junior Member
Registered: 08-23-07
Posts: 3
|
I'm sure that this has been said but it seems to me that if the treadmill or conveyor belt is moving at exactly the minimum speed for the plane to take off, then the plane should be restricted to the same minimum speed. Any overaccleration would allow forward momentum and therefore air over the wings and finnaly take-off. Both treadmill and plane would need to be able to have perfect speed syncronization in order to truly and accurately test this situation. The thrust from the prop does not move air over the wings it is the forward motion of plane. If prop thrust alone could do it a plane would be able to take off with the wheels chocked and/or braked. If I am wrong please tell me why. This episode only illustrated both planes ability to accelerate above the minimum speed for take off. That should have been obvious from the start. I think the staff knows this and is keeping it under wraps as an excuse to revist the most popular "myth" on the boards.
|
Junior Member
Registered: 01-30-08
Posts: 1
|
i do not think this was a correctly conduted scientific experiment because it was that a plane on a conveyor belt would not take off but this experiment did not properly conduct the conveyor belt part of the experiment. my hypothesis is that a convetor belt would be equivelent to the entire ground under the plane traveling in the opposite direction but this makeshift conveyor belt did not make for the ground to travel in any direction it was simply a burlap sheet that moved over the top of the ground but do to the weight of the aircraft it was asentually still on sold ground not traveling in any direction until the prop was started then it began to move forward. where if it were on a conveyor belt the belt would have pulled the aircraft backwards befor the prop started therein making this experiment uneffective
|
Senior Member
Registered: 01-01-06
Posts: 147
|
This is so very sad. So many people still don't understand the plane will fly, and the test was perfect.
To everyone - read the wording in the very first post in this thread. Does it mention - anywhere - that the plane remains stationary? It does not! The conveyor CANNOT KEEP THE PLANE STATIONARY. Repeat after me:
The conveyor CANNOT KEEP THE PLANE STATIONARY.
The conveyor CANNOT KEEP THE PLANE STATIONARY.
Understood? If it truly worked this way, running the conveyor faster than the airplane would cause the plane to go backwards, so that particular criticism is moronic.
|
Junior Member
Registered: 01-30-08
Posts: 4
|
quote: Originally posted by mythmod: http://www.youtube.com/watch?v=4owlyCOzDiEIf you want to see something that is ACCURATE and TRUE that proves the plane will fly - click that link. Even on a REAL TREADMILL a RC PLANE will TAKE OFF. Also, please listen to his EXPLANATION. The guy knows what he's talking about. The testing that Adam and Jamie did is VALID. This youtube video corroborates their findings - INDEPENDENTLY. MythMod
That YouTube has nothing to do with this myth. The guy even states his only purpose is to show that once he has overcome the rolling friction, he does not need to give any additional throttle to keep the plane stationary. This has nothing to do with taking off. Nor does the plane in that video ever take off. This has nothing to do with this myth! |
Senior Member
Registered: 10-28-07
Posts: 5966
|
Again, folks...
Your concept that the plane should remain stationary was wrong; it was an illusion created by your mind.
The plane moved this way --->
The belt moved this way <---
If you want the belt to match the relative speed, and not the plane's airspeed, then you run into all kinds of logic issues and impossibilities. The device you want tested can not exist, not even in a theoretical sense. It is physically impossible for the belt to "match" the relative speeds, and it is physically impossible for the plane to create a speed or change the speed, for the belt to match.
If you feel cheated, it is you who envisioned an impossibility, and are now demanding to see it.
|
Senior Member
Registered: 01-30-08
Posts: 107
|
quote: Originally posted by soc21lax40:
Like said before a plane needs lift to "take off". This lift is provided by air passing over the wings. I a plane is on a conveyor that accelerates at the same rate at the plane, keeping the plane stationary, the plane will not have the lift to leave the groung. The same scenerio can be done by tying a rope to the tail of the plane. the plane will remain stationary because of the rope holding it back, and it will not take off because it will not have any air passing over the wings except for the air from the propeller. Unless the air from the propeller is enough to provide lift, the plane will stay grounded...
That is a laugh. Of course tying a rope around the tail of the plane is NOT the same scenario, unless you specify that the brakes of the plane are set. Without the brakes set, the only force the conveyor belt can put on the airplane is due to the TINY amount of friction in the wheel bearings. If you put a force on the edge of a wheel, tangent to the circumference, it will simply turn. The only forces that will pass through to the axle will be due to the inertia of the wheel resisting being accelerated (torque, moment of inertia and angular acceleration are related just like force = mass * acceleration for straight line events) and the friction of the wheel bearings. Once the wheels accelerate to speed, the force from inertia goes away. Unless the wheel bearings are really bad, there is no way the conveyor belt can keep the airplane from accelerating and taking off. The rope you propose can exert exactly the force the propeller/jet engine can up to the breaking strength of the rope. The conveyor belt can only exert the friction present in the wheel bearings. If the wheel bearings are so bad that they provide as much or more friction than the engine can provide thrust, the airplane wouldn't be able to take off at all, conveyor belt or not!!! It is impossible by the laws of physics for the conveyor belt to keep the plane from taking off!!! |
Member
Registered: 03-29-06
Posts: 39
|
On behalf of the pilot community I would like to point out that the "pilot" on the show was actually an ultra-light pilot and probably has had little to no ground instruction. Please do not take his lack of understanding of the aerodynamics of an airplane to be an example of private pilots, or even sport pilots for that matter.
That being said I would like to say the Myth Busters conducted this test properly (not often I say that) and even if the conveyor was moving faster then rotation speed (take off speed) the plane would have still have taken off.
|
Junior Member
Registered: 01-30-08
Posts: 1
|
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.  |
Member
Registered: 01-30-08
Posts: 5
|
I gotta agree with most of the posters on this point - the guys blew it on this test - they did a crap test to confirm or deny the myth, then left the results ambiguous - as evident by some poeple on here still thinking it was possible.
THe closest they came was when Adam held the model with his fingers on the treadmill. That was the only time plane acceleration and treadmill acceleration matched equally. Any other time - model or full scale - the plane accelerated (in the opposite direction) providing relative airflow across the wing, and causing lift. The proof was when Adam was standing at the relative starting point of the plane, the first cone, and as the plane accelerated away from him, it took aff at the last cone on the runway. It was still several huindered feet down the runway - the normal takeoff distance of the plane. IT wouldn't matter if the plane wheels were going 600 mph - or not moving at all - ergo the possibility of float planes and ski planes...wheel speed doesn't matter, only relative airspeed.
If they go back to a device like the one on the street car verses matchbox car episode, when they put the matchbox car on the real tire and accelerated the real tire to 60 mph, and the model matched the speed, that would be the same apparatus needed to test this. To make the results obvious, no engine on the plane would be required to be running - just as it was when Adam held the plane on the treadmill. The results will be the same - forward speed 0 mph - coefficient of lift 0. The plane will not hover off the tarmac no matter how fast you spin the wheels.
This is the reason pilots are given wind speed and direction before takeoff. If the wind is blowing the same direction as the pilot is taking off, he has to accelerate that much faster to overcome the negative speed of the wind - in other words the wind blowing the wrong direction over the wing. If it is a steady 10MPH he has to expect a 10 MPH faster takeoff speed. That can be a serious problem if the runway is too short. It doesn't matter that his wheels are traveling 9 mph faster then normal or 90 mph faster, he wont take off until the wing hits 10 mph faster, enough to provide the necassary lift to the wing.
|
Junior Member
Registered: 01-30-08
Posts: 1
|
quote: Originally posted by bigtank52:
You have got to be kidding. How can anyone claim there is a "myth" here? Not to mention that it was busted. This is my first post, I just created the screen name, as I could not sit on my hands after this debacle.
First of all the "myth", in this case, has been stated in conflicting statements. The first is that the conveyor is moving at the same rate the plane is in the opposite direction. The second that the plane can take off from the said conveyor at a stand still because the speeds are matched. These two statements are not the same.
The plane in this case was capable of taking off at 25 mph. Now, what you ding bats need to under stand is that is "air speed" not "ground speed". In the first case above the plane can take off because: 1.) The conveyor is moving at 25 mph, 2) the air speed is 25 mph. In this case the plane can take off because regardless of what the conveyor is doing the plane is going 25mph. It doesn't matter if the dang thing was trying to take off on a pile of cranberry sauce. Relative to the groud and therefore the air as well the plane is going 25 mph. Relative to the conveyor the plane is going 50 mph.
Now if the second case is that of the plane not moving relative to the ground, also mentioned a number of times on the show. Then the conveyor is moving 25 mph. The plane however, is moving at zero (0) air speed. Meaning that from the viewers perspective, the plane is standing still and TRULY matching the conveyor speed, as it is not moving. To the plane the speed would be zero, because a plane measures air speed not ground. If they hooked up a speedometer to the wheels they would see that the plane is actually moving at 25 mph. Now this is critical for everyone out there. THE PLANE WILL NOT TAKE OFF IN THIS CASE!! I've seen things as inane as "oh well the propeller moves the air over the wings and it will take off" UM, you sir just created a hover craft, or vertical take off plane. And if you can warp physics to actually make that happen, the air force wants you right now.
Of these two cases only one even makes sense to test. Why would you test the one where the plane simply beats the conveyor for speed and eventually gets enough speed to take off? Why not just state "can a plane move faster than and conveyor"? The only myth that makes sense, though minimally at that, is can a plane matching the speed of a conveyor and not moving relative to the ground take off. However, someone with basic knowledge of physics could have answered that one before you waste perfectly good busting time on the show.
Really poor myth to try, and though the laugh at the arguments here are funny, the fact that the entire crew of the show just made a fool of themselves is kind of sad. Not to mention the taunting they did after they claimed "busted" at the end, not even realizing what they were trying to prove.
Well said, Sir. I'm DrScience and I approve this message. |
Senior Member
Registered: 01-16-08
Posts: 56
|
SPEED DOESN'T MATTER!
The only force acting on the conveyor is the planes weight- ONLY THE PLANE'S WEIGHT. If you take the average circumference of the tires, and multiply it by the weight of the plane, you get the total amount of force acting on the conveyor for each tire rotation- NO MATTER HOW FAST THEY SPIN!
And that is why the plane took-off.
|
Senior Member
Registered: 10-28-06
Posts: 357
|
Im really dissapointed at the "no flys" here. I havnt seen one person admit they were wrong. What "Greatnt249" said is exaclty right, the results didnt go your way, so you say "the results were not what i expected, therefore you did it wrong"
What part of "A treadmill cannot hold an airplane stationary" do you not understand?
Imagine this you "no flys"
If they myth was "can airplane is sitting on a cd player playing a song at the same tempo as the mph of the airplane, can the plane take off?"
They test the plane on a cd player myth and it takes off, but you "no flys" say "But the tempo of the song didnt hold the plane stationary like i thought! you did it wrong!"
Same scenario here. a treadmill does not have the capability to hold an airplane stationary.
|
Junior Member
Registered: 01-30-08
Posts: 1
|
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.
|
Junior Member
Registered: 01-30-08
Posts: 2
|
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.
I really do love mythbusters, but they got this one wrong!
|
Junior Member
Registered: 01-30-08
Posts: 1
|
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.
*The wing is flat on the bottom, curved on the top. It takes the air the same amount of time to move from the front of the wing to the rear, but it has a longer distance to travel over the wing, than under it. Since the air must travel faster over the wing, it essentially thins out and presses down with less force than the air under the wing is pushing up.
|
|
Forums