I just watched the episode about the plane and I don't think that the myth was demonstrated correctly. The idea was to put the plane on a conveyor belt to keep the plane in one spot and see if the plane could lift verticaly.
If you watch the episode you can see that Jamie's truck did not match the speed of the plane because the plane was moving forward on the ground and then it took flight. Watch the episode again and you can see that the plane is moving forward on the ground because it passes several cones before take off.

I don't see a difference between freezing, cutting off the head or puncturing it, and letting it sit and just spraying it all around the car, although the second may be more scary.

the one with AB foam was nuts.

quote:

I just watched the episode about the plane and I don't think that the myth was demonstrated correctly. The idea was to put the plane on a conveyor belt to keep the plane in one spot and see if the plane could lift verticaly.
If you watch the episode you can see that Jamie's truck did not match the speed of the plane because the plane was moving forward on the ground and then it took flight. Watch the episode again and you can see that the plane is moving forward on the ground because it passes several cones before take off.

What does this have to do with shaving cream?

Their version of the tredmill is ridiculous, it applies forward motion to the airplane causing lift to the wings. On a conveyor belt there is no such applied motion. It is effect no different than attaching a glider to Jamies truck and dragging it down the airstrip untill liftoff, and this with no onboard propulsion whatsoever. The explanation that "How force is applied" is different from a plane to a vehicle has no bearing whatsoever on a treadmill. Matched velocities are just that, matched velocities.

A) What does this have to do with shaving cream?

B) Wrong. "It applies forward motion"??? Huh?

I've done a similar prank when I was in college. But I didn't take the top of the can. I froze the shaving cream, hung it from the ceiling of another dorm room, poked a hole in the side and let it go.
It becomes a winter wonder land in no time.

The way I heard the Myth is, you freeze a can of shaving cream (the method was not specified) and then you are supposed to cut the can in half and throw the two halves into the car of the person you hate. Something to that effect.

Seems pretty dangerous to me, cutting the can in half and all.
I wasn't aware that they did an episode on this myth already...

Jeeze, why did I capitolize myth?

personally i just really want to see them use the gel.... i really think it's gonna expand a lot more than foam.

So is it safe to assume that something magical happens by cutting open the can as opposed to just holding down the trigger and emptying it? Expansion is expansion in my mind. If anything, I'd think that the freezing process and slow thaw would decrease they amound of total expansion possible and just holding the trigger down would get you a better result.

Here's a safe for work link that shows 'how much is inside' -

Shaving Can - how much inside???

I just watched the plane on a conveyor belt episode for the first time today (a bit behind, I realize). I think what they found was accurate with the plane, a plane on a conveyor belt is going to be able to overcome the opposing force created by the conveyor belt and take off. The demonstration with the toy car explaining how the conveyor would affect the plane was also good. However, what they didn't explain is exactly what the conveyor belt is going to do to the plane. The conveyor belt is going to pull the plane backwards with a force equal to the mass of the plane. Therefore, for the plane to have forward motion, it is going to have to overcome not only it's own mass, but the negative force applied by the conveyor belt. However, this additional negative force is still easily overcome by the positive force caused by the propeller. The takeoff speed will still remain 25 mph for the ultralight because that is what is required for the right amount of air to pass over the wing to create lift. What I wish they had presented, and would have made the myth much more comprehensible, I think, is if they showed the RPMs of the engine at takeoff. I would not be surprised if the RPMs were higher on the conveyor takeoff. I'm sure this discussion probably started over the issue of whether a conveyor belt could shorten takeoff time or something like that. The answer is that it doesn't, all a conveyor belt does is make the plane work a little bit harder to get going. If you want a plane to stand still on a conveyor belt, you have to match forces, not velocities per se, and for that plane to actually stand still, it would require enough force from the propeller to overcome twice the plane's inertia. If I'm wrong, call me on it, but that's how I see it.

Wall of text time!

I have been a long time fan of your show. Usually, I love the absolutely crazy ways you find to test your various myths, often going for function over form to make something that looks awful but performs beautifully. Rarely, if ever, do I find fault in your proceedings, at least those that are aired. However, with regard to the plane on a conveyor belt myth, I have to say, you got it totally wrong.

If a plane is “moving forward” at exactly 25 mph, while rolling on a conveyor belt that is moving exactly 25 mph the opposite direction, assuming no slippage, as well as no bounce and therefore constant tire contact, the plane WILL be stationary. It doesn’t matter how the plane puts its power into thrusting, 25 mph is 25 mph. Dan, the Producer, made a reference to a skateboarder on a treadmill in a forum post:

“But now tie a rope around your waist and throw it to your gym buddy who is standing in front of the treadmill. Get them to hold the rope taught and walk forward at 1 mph. You will be pulled along the treadmill until you fall off the front.”

That may be true, but you’re not looking at it from the right perspective. In that case, the skateboarder is actually moving at 11 mph. Let me give YOU an example, so we don’t think about this totally in physics. Put a plane on a conveyor belt. Just let it sit there. Now turn on the belt. What does the plane do? It goes with the belt, just like anything else would. A small rock, a church, bread, even a plane will be pulled backwards. It’s only after you start the engines and begin thrusting with them that you could start moving “forward” with relation to the belt, but even then, until you matched the speed of the belt, you would still be moving backwards in relation to the air in the room. And THAT is where your analogy fails. That rope you tied to your buddy is acting in lieu of 10 mph worth of thrust, so of course, adding the one more mph worth of thrust will move your forward, assuming he keeps the rope taught. Or if YOU add one more mph worth of thrust, as long as he also moves forward with his 10mph equivalent of force to keep the belt from pulling you backwards, you’ll move forward. But remove his taught rope 10 mph worth of thrust, and you’re back to 1 mph, and falling off backwards. In this case of the plane, you can put it on the treadmill, tie the rope, and sure, you can crank it up to 1000 mph and move forward with a taught rope + 1 mph worth of thrust. But take off that rope, and you go flying backwards at 999 mph. But your analogy is entirely false with regards to the episode, as there were no ropes, taught or otherwise, attached to the plane in the experiment!

The problem with your method of testing in the episode was that you were using two wholly different forms of acceleration. Like Jamie said, the car and the plane DO accelerate differently, the former using the parallel (to the direction of travel) torque through the tires, while the latter uses perpendicular torque through the propeller to move air backwards. However, this air is not the air that is passed over the wing. Look at any modern military jet: the intakes are generally on the side or bottom of the fuselage, with the output at the rear of the aircraft. The purpose of the jet engines is to get the aircraft moving forward, not up (not taking into consideration thrust vectoring, a feature on some military jets). Under normal circumstances, thrust would create motion forward. The aircraft must move forward in relation to the air. The result is that air will pass over the wings, generating lift. Now, I’m sure you know this already, but that is significant. The LIFT and the THRUST are two wholly different aspects of flight. But without thrust, and the resulting motion forward, an airplane cannot generate lift. As such, were the plane to be on a moving belt, sitting still (much like Adam’s demonstration with the RC car), and therefore lacking any motion relative to the air, there is no air moving over the wings, and therefore no lift being generated. Liken this to the skateboarder. Adam’s car’s “rope” was his tires. Now, put the little plane in that same spot. If its engines are off, and there’s no rope, then it will crash to the back of the treadmill. It has to use its prop as its rope, much like the car uses the tires, in order to stay in place. This is true for ANY speed, regardless of takeoff or not. For the plane to sit still like the car did, the props have to drive air backwards at a rate that would push it forward at a rate equal to that of the treadmill. Add in a rope, constantly taught, and sure, it would now move forward with any thrust, but take that rope away, and your 11 mph plane now goes backwards at 9 mph.

The problem is, your test with the truck/tarp didn’t create anywhere near the ideal situation. There is no way for the two vehicles to have matched each other pace for pace in acceleration. This is due partially to having two separate people controlling two separate vehicles. The solution to this problem would be to create a large scale treadmill, on your own if you must, to allow for actual acceleration control of both bodies. By syncing up the acceleration of the treadmill to the acceleration of the plane, you could truly say, FOR SURE, that both the plane and the conveyor belt were traveling at exactly the same speed. As the myth stands now, you can’t say that. Don’t even try to say that the light weight plane and the heavy truck accelerated at the same rate, especially when both of them would be dealing with very different coefficients of friction. If you want to have some numerical proof, try this: get a plane to taxi just below its takeoff speed. As it maintains this speed, get an rpm count of its tires. Then, when you put that plane on the belt, accelerate the belt until the tires match that rpm. THEN you could say that the plane is “traveling” at just below its takeoff speed (this would be best performed in conjunction with the suggestion in the paragraph below).

Additionally, should you find it difficult to keep the speeds the same between the belt and the plane, you could make one addition to the apparatus, should you choose to retest this myth (which I STRONGLY urge you to attempt). Putting in a forward/backward stopper of some sort that prevents the plane from going more than the speed of the treadmill would alleviate this problem (much like a taught rope would keep it from going backwards, but would lose the taught-ness when the plane started accelerating, ensuring that the plane was moving forward totally on its own volition, but NOT faster than the belt it was rolling on!). Making it so that the plane has, for example, a couple feet margin to move forward and backwards as it accelerates would keep the speeds of the two surfaces (wheels and belt) exactly the same, regardless of how much difference there was in the thrust between the two. That was another problem Is aw, as I doubt the pilot was going to sit there with his throttle right at takeoff speed; rather, he would be flooring the accelerator, like any other pilot does, to get up to and well past takeoff speed as fast as possible. Were Jamie and the pilot matching each other for speed, there would have been no forward motion of the aircraft; but since the aircraft accelerated faster than the truck did, it did move forward, generated lift, and took off (that is NOT what the original myth was about). But while you limit the forward and backward motion of the plane with this addition, you would put no limit on its up and down motion, which would allow you to see if there was any lift generated. This would say for certain if the plane, while moving on the treadmill at 25 mph when the treadmill is also going at 25 mph (no ambiguity of accelerations or skateboarder ropes that don’t exist), could take off.

Which, I assure you, it will not.

Like I said, I am a huge fan of the show, and I absolutely love watching. However, I cannot allow something as near and dear to my heart, and my livelihood, as a plane myth be classified so awfully wrong.

2LT "AFKeeker," USAF

quote:

AFKeeker wrote:
If a plane is “moving forward” at exactly 25 mph, while rolling on a conveyor belt that is moving exactly 25 mph the opposite direction, assuming no slippage, as well as no bounce and therefore constant tire contact, the plane WILL be stationary.

Is the plane moving forward at 25 MPH, or is it stationary? Make up your mind – it can’t be both.

quote:

Let me give YOU an example, so we don’t think about this totally in physics. Put a plane on a conveyor belt. Just let it sit there. Now turn on the belt. What does the plane do? It goes with the belt, just like anything else would. A small rock, a church, bread, even a plane will be pulled backwards. It’s only after you start the engines and begin thr•sting with them that you could start moving “forward” with relation to the belt, but even then, until you matched the speed of the belt, you would still be moving backwards in relation to the air in the room. And THAT is where your analogy fails. That rope you tied to your buddy is acting in lieu of 10 mph worth of thr•st, so of course, adding the one more mph worth of thr•st will move your forward, assuming he keeps the rope taught. Or if YOU add one more mph worth of thr•st, as long as he also moves forward with his 10mph equivalent of force to keep the belt from pulling you backwards, you’ll move forward. But remove his taught rope 10 mph worth of thr•st, and you’re back to 1 mph, and falling off backwards. In this case of the plane, you can put it on the treadmill, tie the rope, and sure, you can crank it up to 1000 mph and move forward with a taught rope + 1 mph worth of thr•st. But take off that rope, and you go flying backwards at 999 mph. But your analogy is entirely false with regards to the episode, as there were no ropes, taught or otherwise, attached to the plane in the experiment!

It’s funny how quick you tell us not to “think about this totally in physics”. It’s almost as if you subconsciously know how desperately wrong your physics are. Force and speed are not interchangeable as you seem to think they are.

If the plane’s wheel bearings were perfectly frictionless, the plane would actually remain stationary as you pull the belt backward. The reason a real plane would move backward along with the belt is because the small amount of friction in the wheels lets the belt apply some force to the plane.

quote:

were the plane to be on a moving belt, sitting still (much like Adam’s demonstration with the RC car), and therefore lacking any motion relative to the air, there is no air moving over the wings, and therefore no lift being generated. Liken this to the skateboarder. Adam’s car’s “rope” was his tires. Now, put the little plane in that same spot. If its engines are off, and there’s no rope, then it will crash to the back of the treadmill. It has to use its prop as its rope, much like the car uses the tires, in order to stay in place.

Wrong. The car’s tires apply a force against the belt. The plane’s prop / skateboarder’s rope work independently of the belt.

quote:

For the plane to sit still like the car did, the props have to drive air backwards at a rate that would push it forward at a rate equal to that of the treadmill.

Wrong. The props only have to drive air backwards with enough force to overcome the rolling resistance in the wheel bearings.

quote:

The problem is, your test with the truck/tarp didn’t create anywhere near the ideal situation. There is no way for the two vehicles to have matched each other pace for pace in acceleration. This is due partially to having two separate people controlling two separate vehicles. The solution to this problem would be to create a large scale treadmill, on your own if you must, to allow for actual acceleration control of both bodies. By syncing up the acceleration of the treadmill to the acceleration of the plane, you could truly say, FOR SURE, that both the plane and the conveyor belt were traveling at exactly the same speed. As the myth stands now, you can’t say that.

Now that’s just petty. The fact of the matter is, the conveyer belt cannot apply enough force to keep the plane stationary, so the myth is busted. Fine-tuning and matching the accelerations of the two bodies won’t do anything to change that.

quote:

If you want to have some numerical proof, try this: get a plane to taxi just below its takeoff speed. As it maintains this speed, get an rpm count of its tires. Then, when you put that plane on the belt, accelerate the belt until the tires match that rpm. THEN you could say that the plane is “traveling” at just below its takeoff speed

Only if you measure the speed relative to the belt. But since the variable you need to control is the speed of the belt, that’s invalid.

quote:

Were Jamie and the pilot matching each other for speed, there would have been no forward motion of the aircraft; but since the aircraft accelerated faster than the truck did, it did move forward, generated lift, and took off (that is NOT what the original myth was about).

Wrong. The plane didn’t move forward, etc. due to difference in acceleration, it did so because its free-spinning wheels prevented the truck from applying any significant force to oppose its forward motion.

quote:

2LT "AFKeeker," USAF

Are you really in the Air Force? That worries me. You desperately need a refresher in the basic physics of aircraft operation.

AFKeeker,

uh... the wheels of the plane are free wheeling. Like in your wheelbarrow or skates.

So the main effect of the conveyor belt moving opposite direction and speed of the plane is that the wheels of the plane will just turn faster.

The plane will take off.

Hope this helps

I agree with "the aviator"... his post is accurate and the Mythbusters got it WRONG! The problem with how they did it on the show is, the "conveyor belt" they used did not maintain a "stationary" position for the plane. THAT was what the myth was all about.. can a plane on a conveyor belt (which implies that the plane will be stationary while the belt moves) take off without rolling down a runway.. otherwise, what's the point of having it on a conveyor belt..

What I want to see is for the Boys to build a REAL conveyor belt for the plane to roll on and see if it will take off.. the plane's wheels should roll the conveyor belt freely or use a motor to roll the belt and let the wheels of the plane roll freely.. when they did it with the tarp being pulled by Jaimie's truck, all they did was to have the tarp moving under the plane, but they didn't effectuate the myth at all.

I CHALLENGE THE MYTHBUSTERS TO DO THIS MYTH RIGHT!!!!

Keith

attny36, you’re the one who’s wrong.

In the real world, a conveyor belt can’t ever hope to hold a plane back, because a plane is pushed forward by its propellers, not its wheels. All the conveyor belt will ever do is make the wheels spin faster while the plane moves at its normal speed.

And by the way, this thread isn’t even about the Plane on a Conveyor Belt. It’s about the Shaving Cream Car Prank. Way to go, champ.

Aww, how cute. The big PoaC thread had a baby .

Yeah, yeah, I didn't know about the whole wheel thing. Yes, I am in the Air Force, but the extent of my knowledge of planes comes from a single aeronautical engineering class I was forced to take. But this is more of a mechanical engineering situation than aeronautical, anyway; the mechanics of the wheel are the important part, whereas the movement forward and lift generation are after thoughts. I do believe the myth is well and truly busted, just not for the reasons they explained in the episode.

quote:

Originally posted by Markov Chain:
Aww, how cute. The big PoaC thread had a baby .

Hahahahahahahahaha.



AFKeeker, I am surprised to see you write some of the things you have written. And You're in the Chair Force?? I'm glad to see you're able to(kind of) admit that you are wrong.

Nice that people are not as rude as you are when you don't know what you're talking about.(see "cool")