quote:

Originally posted by lytwarrior:
Ok, after watching the video, and here isthe link;

http://www.youtube.com/watch?v=U9STo3fjfGg&NR=1

I saw what they did. Perhaps I am wrong, but how I interpreted this Myth would be that a plane, at normal take of throttle position, could take off. However, what seemed to have happened was that there was no record of the throttle position for take off while the "convayer" went up to take off speed and the plane was allowed to throttle up to take off speed.


For this to be absolutly confirmed, throttle position needs to be set and if it is to be proved that wheels and forces upon landing gear do not bother the plan in this respect, they need to run the convayer in reverse.

To start with, in a small aircraft, there is only one position for take off throttle... Full. Only when you get into lartger, multi engine aircraft do you set the throttles to a certain setting. Therefore, in regards to this myth, under the terms of the MB episode, it was full throttle or nothing...

Okay, I think that some here are confusing "inertia" with "drag".

The difference is whether the plane was already moving before the conveyor belt started to move. In other words, if the plane's take off speed is about 60+ miles per hour and it was setting on a conveyor belt already moving the opposite direction at 60+ miles per hour, then the conveyor belt might have an effect, though only temporarily. This is because the plane would have to overcome the built up inertia of it already moving the opposite direction at 60+ miles per hour. So the question becomes "Does the Cessna have enough power to overcome this inertia?" Depending on its top speed, then, eventually, yes, because once the plane starts to inch forward at all, it has countered the inertia to a zero point and, from then on, it will build speed and take off, regardless of what the conveyor belt is doing under its wheels. The only thing that will be different is the speed of rotation on the plane's wheels, which will simply be twice as fast as normal. But there will never be enough "drag" induced by the conveyor belt to keep the plane from taking off. Now, if the conveyor belt were moving the opposite direction faster than the plane's top speed and the conveyor belt were already in motion before the plane attempted take off, then that would be a different story.

Roger

Again, how is answering a different question in any way relevant?

The question says what happens... what *might* happen in other circumstances is irrelevant and just confuses those who can't see why the plane WILL take off for the tested question.

echo4charlie is absolutely correct. Until lift off speed is achieved, the plane is pulled forward by the propeller. The most the tread mill does is make the wheels turn faster. If this wasn't so, planes on skis or pontoons wouldn't get into the air at all. The correct analogy for a plane taking off is an air boat, not a car.

Roger"Again, how is answering a different question in any way relevant?

My point is that the initial question can be subject to a variety of influences as I outlined previously. Obviously, if the conveyor belt were moving in the opposite direction at a speed matching take off speed before the plane throttled forward, then the reverse air flow over the wings would need to be negated by forward motion of the plane before any kind of lift could be realized.

Don't worry. I know that a plane will take off on a moving conveyor belt. However, the myth simply asks if a plane on a conveyor belt can take off. The myth doesn't even state if the conveyor belt is moving! (though logically it would be) But because the myth was kind of vague in its presentation on many points, that is leading to a lot of confusion with different people arguing different points of view in this thread. That is why I tried to make the distinction between drag and inertia because I think there are a number of people in this discussion that are confusing one with the other. Hopefully (assuming they read this) they will step back and rethink their arguments as to why the plane would not take off.

Roger

quote:

The myth doesn't even state if the conveyor belt is moving!

No? Seems to say "the conveyor moves...":

quote:

"A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?"

Sorry. I'm not trying to be a butt. But most posters don't read the previous posts. They may read the last post, if they're feeling charitable, but mostly they just come in and say "it was tested wrong", because the plane moved and they were expecting the plane to remain stationary.

The fact remains that the setup, as described in the question, does not exist if the plane's "speed" is the wheel speed. And even if you approximated it by sitting a plane on a moving conveyor, the plane can still take off by the mere act of applying full throttle, like any other takeoff.

The question really does boil down to thrust and rolling resistance. The belt offers no more resistance than a normal runway would. If the plane's thrust will overcome the rolling resistance and move the plane on a normal runway, then it will also move it on a moving treadmill (up until something fails).

The biggest problem is that people can not see (or believe) that the "wheel speed" condition they were envisioning is invalid and impossible. And that the plane would *still* take off if you applied full throttle.

No? Seems to say "the conveyor moves...":

quote:
"A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?"

Ah! I did see the episode but did not remember them being specific about it. If that was how the myth was proposed then, yeah, that's all there is to it. As I said previously, I know that a plane will take off of a conveyor belt. Of course, people posting still may be confused about inertia vs drag but it really doesn't make any difference. The myth was tested as stated.

Of course, as I posted in related thread, the myth was pretty much busted before it was tried. Any plane facing east or west is already on a conveyor belt moving at about 1000 mph, which is the rotational speed of the earth. If a plane facing west can overcome a 1000 mph conveyor belt below its wheels, any additional movement of the conveyor belt is kinda moot.....

Roger

I think there is a simple way to visualize this.

Imagine a conveyor moving from front to back at .. say.. 100 mph. Then imagine that the plane is slowly lowered onto the conveyor from an over-head crane. As the wheels make contact with the conveyor they will begin to spin rapidly until they match the speed of the conveyor. At that point the plane could be held stationary (relative to someone beside the conveyor) with very little effort. A man holding a rope tied to the plane could very easily hold it in place, and even walk forward and pull the plane.
Now imagine the pilot starting the engine and applying a little power. If he applies the same amount of thrust as the man holding the rope the plane would remain in a stationary position (again relative to an observer beside the conveyor) even without the man holding the rope.

As the pilot applies additional power, the plane would move forward, accelerate and eventually take off. The wheels would be spinning a lot faster than normal but the plane would still move forward. Even if the conveyor accelerated the only effect that would have would be to make the wheels spin faster - the plane still moves forward.

Another way to visualize this is to imagine the conveyor is made of ice. Regardless of how fast the ice conveyor moves backwards the plane just sits there as the ice surface slides beneath the wheels. As the pilot applies power, the plane moves forward and eventually takes off. (we're ignoring the effect of friction in this example, as it's a small force easily overcome by a small amount of aircraft power).

The key point - which has been pointed out several times in this discussion - is that a plane's wheels are not powered, but spin freely. Forward motion is achieved not by the turning wheels (as in a car) but by the prop pulling the plane through the air. The conveyor therefore has a negligible effect on the forward motion of the plane.

quote:

Originally posted by MovieStuff:
No? Seems to say "the conveyor moves...":

quote:
"A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the plane take off?"

Ah! I did see the episode but did not remember them being specific about it. If that was how the myth was proposed then, yeah, that's all there is to it. As I said previously, I know that a plane will take off of a conveyor belt. Of course, people posting still may be confused about inertia vs drag but it really doesn't make any difference. The myth was tested as stated.

Of course, as I posted in related thread, the myth was pretty much busted before it was tried. Any plane facing east or west is already on a conveyor belt moving at about 1000 mph, which is the rotational speed of the earth. If a plane facing west can overcome a 1000 mph conveyor belt below its wheels, any additional movement of the conveyor belt is kinda moot.....

Roger

I don't think you know what inertia is! And so for you to be constantly wittering about other people not understanding the difference between inertia and drag is amusingly ironic.

Here's one way to look at the problem: What would happen if a plane were to LAND on a moving conveyor belt, without applying brakes? Would it come to an immediate stop or would it still require the normal stopping distance (relative to the still ground, not the conveyor belt surface). And when I say stop, I mean stops moving forward...

"I don't think you know what inertia is! And so for you to be constantly wittering about other people not understanding the difference between inertia and drag is amusingly ironic."

Actually, "irony" is someone that goes by the handle "rational" that has little knack for research. Look carefully and you'll see that I've made only two posts regarding inertia and a total of seven posts entirely on this forum. I'd hardly call that "constantly wittering about", especially since few could compare to the sheer volume of your own missives. Tell me, do you post from the toilet? Heaven forbid you should step away from your computer!

I would suggest that you re-read my posts for the sake of your continued education but, then again, someone that has amassed over 800 posts in as little as 9 months can't be bothered with reading when there is so much writing to be done! What a full, full life you must lead, "rational".

Alas, do not worry. As a mechanical/electrical engineer, I am very familiar with the effects of inertia, as referenced in my posts previously, as well as the effects of drag, as illustrated by your posts currently.

Enough?

I insulted no one and didn't come here to be insulted so drop the presumptive attitude or go play king of the hill with someone else.

Roger

As for the new imagined scenario where the plane starts off rolling backwards faster than the top speed its engines can give it… it’ll still take off! It will, of course, just need more runway to do so.

quote:

Originally posted by MovieStuff:
"I don't think you know what inertia is! And so for you to be constantly wittering about other people not understanding the difference between inertia and drag is amusingly ironic."

Actually, "irony" is someone that goes by the handle "rational" that has little knack for research. Look carefully and you'll see that I've made only two posts regarding inertia and a total of seven posts entirely on this forum. I'd hardly call that "constantly wittering about", especially since few could compare to the sheer volume of your own missives. Tell me, do you post from the toilet? Heaven forbid you should step away from your computer!

I would suggest that you re-read my posts for the sake of your continued education but, then again, someone that has amassed over 800 posts in as little as 9 months can't be bothered with reading when there is so much writing to be done! What a full, full life you must lead, "rational".

Alas, do not worry. As a mechanical/electrical engineer, I am very familiar with the effects of inertia, as referenced in my posts previously, as well as the effects of drag, as illustrated by your posts currently.

Enough?

I insulted no one and didn't come here to be insulted so drop the presumptive attitude or go play king of the hill with someone else.

Roger

I would be very interested in your analysis of what the "inertia" issues are and how people are getting them confused with "drag".

all that is needed are the speeds of the belt and plane. drag, friction, ect. is beyond the scope of the setup.

just to make it interesting the "belt moves" is an indicator that the belt actually moves or is moving and is not just part of a description of the direction the belt moves.

Now using ONLY the info provided there are different answers depending on how the speed of the plane is measured. This has all been explained before.

The belt isn't trying to match speeds it IS matching speeds, exactly.

When is your stunted brain going to absorb the fact that it is not mathematically possible for the belt to match the speed of the aeroplane if the speed of the aeroplane is measured relative to the belt?

-rational-, when will you realize that the plane's speed is not measured relative to the belt?

The measurement of speed in the plane is measured by the amount of throttle applied resulting in it's engines powering. The belt is measured in it's speed of rotation. That's how they're measured, and that's how tey pertain in the myth. The situation you speak of is a situation in which the plane is measured when it's sitting still on the treadmill, which is a point at which it's applying an amount of throttle (or none at all) to a point where it wouldn't even take off if it were on solid ground, which does not meet the concept of the myth.

Of you calculate a speed of the plane and use of the throttle that allows the plane to take off, then bring the treadmill's speed up to that same position as the MPH reading in the plane, and the plane were to sit on the treadmill with that throttle fixed to that point, the plane will always take off.

Your problem is how the speed is measured. Simply put, you don't measure the speed realative to the treadmill. It's as simple as that.

Planes take off due to lift. Lift is caused by airflow over the wings. Propeller speed is controlled by the throttle, independent of wheel speed (conveyor belt or not). Propellor is to pull the aircraft forward to create airflow over the wings. No matter how fast the engine / propellor the plane will not take off if the air speed over the wings does not meet the take-off criteria for the specific aircraft. So this will never work (aircraft standing still). If you want a plane to take off at a standstill, just put a big fan in front of the aircraft (much bigger than a propellor) that will cause airflow over the wings to meet the needed lift. Signed - aerospace engineer mike!

quote:

Originally posted by MMS19HOCKEY:
Signed - aerospace engineer mike!

THANK YOU!!

quote:

Originally posted by -rational-:
When is your stunted brain going to absorb the fact that it is not mathematically possible for the belt to match the speed of the aeroplane if the speed of the aeroplane is measured relative to the belt?

has been proven mathmatically possible depending on how both speeds are measured.

the same math applys to ALL objects on a treadmill/conveyor belt. The important FACT is that one speed is being said to have the same magnitude as another and the one doesn't care or know how the other is measured.

lets not forget that the Mythbusters frequently modify myths to get the results that desire for the sake of getting the desired results....and the bigger the explosion the better.

That point leads to some give and take in the interpretation of the myth given its arguably vague and poor wording.

Willgamer25 ,
you measure the speed of the plane in the manner you are given, whether it be in the original setup of in followup setups being explored. At the very least i;d say you have two options....normal for a plane or normal for an object on a conveyorbelt/treadmill. One leads to a very boring answer while the other doesn't.

quote:

Originally posted by -rational-:
When is your stunted brain going to absorb the fact that it is not mathematically possible for the belt to match the speed of the aeroplane if the speed of the aeroplane is measured relative to the belt?

Erm... How is it not mathematically possible? Solution is airplane's ground speed = 0. Maybe you wanted to say that it isn't mechanically possible? Even then I have to disagree.

It is a trivial matter to compute the acceleration of the belt required to keep the airplane still given a thrust setting. You just use the fact that wheels themselves have some inertia. Now, this will work until either:
1) Belt is mechanically incapable of delivering the acceleration at speed reached.
2) Friction in the wheel finally matches thrust.
3) Belt or wheels catastrophically fail.

First problem can be resolved by adding more power to the belt. Second is not very likely. So if you build it properly, plane will sit still, then the wheels will fail, and whole thing falls apart. Since there can be no takeoff after that, I think it would verify the myth in this particular setting.

quote:

Willgamer25:
The measurement of speed in the plane is measured by the amount of throttle applied resulting in it's engines powering.

No, it isn't. Myth in MBs interpretation requires speed to be measured relative to the ground. Airplane's speed indicator doesn't show airplane's speed at all. It shows speed of the air flow under the wing. Neither has anything to do with the throttle setting.