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Airplane on a Treadmill

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Airplane on a Treadmill

Old 12-12-2007, 04:36 PM
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Tall Paul
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Default Airplane on a Treadmill

Mythbuster's is going to show what they came up with tonight.
Watch it, and be sure to sign on their site to tell them all the things they did wrong!
Old 12-12-2007, 04:57 PM
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meaden
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Default RE: Airplane on a Treadmill

Airplane on a Treadmill? Really, like it won't fly?

My amateur testing points to air speed and not ground speed that produces lift. I might be Wright...or wrong.
Old 12-12-2007, 05:07 PM
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camtheman000
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Default RE: Airplane on a Treadmill

how much lift would the prop wash cause?
Old 12-12-2007, 05:15 PM
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Default RE: Airplane on a Treadmill

Zip...prop wash disturbs the flow of air over the airframe. Talk to a pattern flyer...
Old 12-12-2007, 11:05 PM
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Tall Paul
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Default RE: Airplane on a Treadmill

The episode wasn't televised.
Bummer!
Old 12-12-2007, 11:49 PM
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Default RE: Airplane on a Treadmill


ORIGINAL: meaden

Zip...prop wash disturbs the flow of air over the airframe. Talk to a pattern flyer...
Actually if the plane is optimized to use the prop wash it can create a huge amount of lift in the wing.

The Ryan VZ-3ry is a perfect example.

http://www.centennialofflight.gov/es.../Aero34G12.htm
Old 12-13-2007, 11:28 AM
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Jim Thomerson
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Default RE: Airplane on a Treadmill

Launching control line sport and combat airplanes I jusrt turn them loose, a little nose up, and they climb right out. So there is adequate lift from the propwash and thrust.
Old 12-16-2007, 10:43 AM
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Default RE: Airplane on a Treadmill


ORIGINAL: BMatthews


ORIGINAL: meaden

Zip...prop wash disturbs the flow of air over the airframe. Talk to a pattern flyer...
Actually if the plane is optimized to use the prop wash it can create a huge amount of lift in the wing.

The Ryan VZ-3ry is a perfect example.

http://www.centennialofflight.gov/es.../Aero34G12.htm
http://www.custerchannelwing.com/04_facts.html

Another aircraft that used propwash to generate lift.



old git - - - - - aka John L.
Old 12-16-2007, 01:20 PM
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Tall Paul
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Default RE: Airplane on a Treadmill

http://www.youtube.com/watch?v=4owlyCOzDiE
The speed of the belt is irrelevant to the plane's motion.
It's that simple.
Old 12-17-2007, 01:58 PM
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Default RE: Airplane on a Treadmill

First, the wording of the problem is confusing. What does "matching the speed of the wheels" mean? Let me pose the question a little differently. Is there ANY way that the belt can move in order to prevent the plane from taking off? In other words, can we instruct the belt to move backwards in such a way as to keep the plane stationary with respect to the ground, and therefore the air?

In order to figure this out, you can to ask “how can the belt apply backward FORCE to the plane (through the wheel axles) that is equal or opposite to the thrust of the propeller against the air� But that turns out to be really, really hard, at least for me. Instead, I think it’s easier to think about whether there is a way that the belt can divert all of the POWER of the motor away from accelerating the plane.

First, the easy case. If the wheels have no friction and no mass, the belt is out of luck. There’s no way that it can apply any lateral force to the plane at all. So ALL of the motor power goes into accelerating the plane forward, and the plane takes off normally. In fact, better than normally, because there is no friction in the wheel bearings to worry about. This case is the mirror image of a man in slippery shoes on ice. His legs windmill, but he doesn’t move. Likewise, the belt moves under the plane, but the plane stays still by virtue of its inertia. With no force transfer between belt and plane, there can be no power transfer.

Now let’s add friction between the axle and the wheel (the wheel still has no mass). Remember that the motor normally has enough power to overcome the friction in the wheel bearing by a LONG way. That is, most of the power applied to the propeller goes into accelerating the plane, and a small amount goes into overcoming friction in the bearing (which heats the bearing). Let’s say that the amount of power stolen by friction near takeoff speed is about 1%, leaving 99% for acceleration. I actually think the number might be a little higher, but it doesn’t matter for our purposes.

To keep the plane still, the belt needs to increase the power dissipation in the bearings to be 100% of the motor’s power, so that no energy goes to acceleration. And in order to do that, the belt has to increase the wheel rotation speed by a factor of 100, because the power dissipated by friction is proportional to the relative speed of sliding parts. So with the throttle at max, the belt needs to go backwards at 100 times the normal takeoff speed of the plane. If it adjusts itself just right, the drag in the bearings will then exactly match the thrust. The wheels will be rolling at 100 times the takeoff speed, and the plane will be standing still because no energy remains to move it forward. Of course, with all the power going into the bearings, eventually the bearings will melt.

This same argument applies to rolling friction between tires and the belt. Now you are heating the tires and the belt as well as the bearing, and so the bearings will last a little longer.

Now let’s go back to assuming the wheels have no friction, but give them some mass. In this case, when the motor applies thrust, the power goes into two places: forward acceleration of the plane, and rotational acceleration of the wheels. That is, it takes power to increase the rotational speed of the wheels, in proportion to their moment of inertia.

So now what does the belt have to do in order to divert 100% of the motor power into rotating the wheels? If you apply constant power to a rotating mass, it will just spin faster and faster. So in order to keep constant power going to the wheels, the belt has to keep accelerating the wheel spin as long as the motor is on. The acceleration (I think) needs to be proportional to the square root of time, because rotational energy is proportional to the square of rotational velocity. In this case, however, the wheels will eventually fly apart at some large rotational velocity.

This last case is a little hard for me to grasp intuitively. With friction, you can see easily where the frictional force in the wheel will translate into a drag on the axles to counteract the thrust from the motor. Here it’s a little trickier. The plane is pulling forward on the axle. The belt is pulling backward on the bottom surface of the tire. The forces are hard to calculate, because they are applied to very different parts of the wheel, but in any case you can see that because the forces are applied obliquely to one another, the wheel will spin faster and faster and its kinetic energy will increase with time, just as we require.

Imagine that the belt can’t increase its speed forever. Let’s say it has a top speed of 1000 mph (and that the wheels can take these speeds). The belt keeps increasing its speed at first so that all the motor power goes into accelerating the wheel rotation. Then the belt maxes out. Can the plane now put power into accelerating itself instead of its wheels and begin to fly? I think so. There’s a huge amount of kinetic energy in the wheel rotation. The belt is whipping around. The wheels are spinning like crazy. But because there’s no friction, there’s no power required to maintain the wheel rotation. The airspeed of the plane is 0, and so much of its thrust will now go into moving the plane forward. But even in normal takeoff, SOME power goes into accelerating the wheel rotation. And the energy it takes to accelerate the wheel rotation from 1000 mph to 1010 mph is 20 times higher than the energy it takes to get them from 0 to 10 mph (because rotational energy is proportional to the square of velocity). So while the plane should eventually fly, the duration of its takeoff roll will be longer. And when it gets into the air, its wheels will keep rotating at 1010 mph. When you try to bank, there will be some wicked adverse yaw.

Finally, with both friction AND mass in the wheel, you’ll get some combination of the two effects, depending on bearing friction coefficients and wheel moments of inertia. Hard to tell whether the wheel will fly apart or melt first.

Addendum: There is a comment above from Aerospace Engineer (http://pogue.blogs.nytimes.com/2006/...ill-conundrum/) about the belt inducing laminar flow over the wings. If that happens, then the plane WILL fly. The only question is how fast that airflow will be at the point the wheels melt/explode.

Old 12-17-2007, 04:07 PM
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rmh
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Default RE: Airplane on a Treadmill

See post #9
To expound on that statement -Iwill add this :
unless the plane is attached to the belt -
it has no bearing on the matter.
No engineering is required
the question is one of logic .
But I do have one for you :
if a plane is too heavy, where should the CG be placed?
Old 12-17-2007, 06:05 PM
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Tall Paul
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Default RE: Airplane on a Treadmill

The question can and frequently is worded to make flight impossible.
The conditions for this impossibility are also impossible.
Therefore, no notice be paid.
The YouTube video more than adequately demonstrates the real world situation, not the one of the Zen koan mystical world, where otherwise normal people worry about the sound of one hand clapping.
Old 12-17-2007, 07:10 PM
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rmh
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Default RE: Airplane on a Treadmill

The EWE TUBE demo was nicely done
the thing that is strange to me is that the "problem to be disproved" -was a non sense issue and the result easily visualized
Old 12-17-2007, 08:49 PM
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Default RE: Airplane on a Treadmill


ORIGINAL: dick Hanson
the problem to be disproved was a nonsense issue and the result easily visualized
Unfortunately, it isn't easily visualized by everyone.
And that's the reason this crap goes on and on and on and on.
And the reason the guy produced the video, so they wouldn't have to understand, they'd just have to look.
And unfortunately.................
Old 12-18-2007, 12:55 AM
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Default RE: Airplane on a Treadmill


I agree/disagree with everyone.

Choose one!



old git - - - - aka John L.
Old 01-05-2008, 05:12 PM
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Default RE: Airplane on a Treadmill

Not THIS again!
Old 01-05-2008, 07:48 PM
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Default RE: Airplane on a Treadmill

If this runs for long I'm going to shut it down with a link to the 17 page thread that ran here a while back. It was all said over and over again there. No need to flog the decayed remains of this topic again....
Old 01-05-2008, 11:01 PM
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da Rock
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Default RE: Airplane on a Treadmill

Hey, it was on it's way to the cemetery. Nobody had posted since the middle of last month. So if nobody will dig the sucker back up again, it ought to blow away.

Truth is, this thread was supposed to be about the TV show, not a rehash of the stupid argument. So, any posts that are about something other than the TV show should be considered OFF TOPIC right.
Old 01-06-2008, 04:53 AM
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Default RE: Airplane on a Treadmill

Yep......
Old 01-08-2008, 07:03 PM
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Default RE: Airplane on a Treadmill

yeah, especially those which don't add anything to the thread! oh wait... dang.
Old 01-11-2008, 02:58 AM
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Default RE: Airplane on a Treadmill

Mythbusters is diong it on Jan 30

http://dsc.discovery.com/video/?play...eId=1344511100
Old 01-11-2008, 08:56 AM
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Default RE: Airplane on a Treadmill

I just looked at the video and true to the history of our modern media...............

They first sit a model airplane on a conveyor belt at a set speed. Blows the hypothesis right out the window with that.

And they also said they'd not seen anyone else actually put an airplane on a conveyor belt. Guess when they were reading all the contentious argument on the internet they didn't see the links to the video that showed a model airplane on a conveyor belt.

And if you look at the landing gear on their airplane, you'll instantly recognize that their test vehicle destroys the validity of the test. No way that crappy gear comes close to not biasing the results. With the wheels splayed and the tires dragging and not having any hope of freewheeling, the drag of that system also crops up in their test when it's not a part of the original joke.

First time I looked at the video it appeared to fade to black at that point so I clicked off. Well, I looked again and there is a 2nd half of the video where they have a tiny fullscale airplane at an airport sitting on a long tarp. They got a truck that is going to pull the tarp in one direction while the homebuilt goes in the other.

Jeez, I've got to admit that their credibility took a huge hit with that video. Their tests really have little real connection to the original problem. But if it attracts viewers what do they care.
Old 01-11-2008, 09:01 AM
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Default RE: Airplane on a Treadmill

ORIGINAL: bentwings

Mythbusters is diong it on Jan 30

http://dsc.discovery.com/video/?play...eId=1344511100

I can't believe they would waste perfectly good AIRtime on something like this. Everyone knows the plane can get off the ground, yet these threads continue and continue and continue and...

I sure hope they use a plane like and F-15 or something with greater than one to one thrust ratio!
Old 01-11-2008, 10:53 AM
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Default RE: Airplane on a Treadmill

Haha I agree with one exception...."everyone knows the plane will fly". From wasting time reading the posts on the new one and being part of the old one I think I started it, haha and wasting even more time following it on the truck forums where I started it, not everyone believes the plane will fly. Someone even went so far as to keep score. I think it was about 60-40 in favor on the truck forum. There were about 4500 posts as I recall.

It was supposed to be fun and I think it was. It's a good thing we use the "net" as in some cases name calling might have gone physical. It certainly turned over some interesting logic.

I would like to see the mythbuster attempt. It looks like they are going to use an ultra light of some kind. It looks like they are going to pull some kind of webbing with a truck for the runway. (they should be using a Dodge diesel instead of a Tundra haha)

bmathews...maybe it get put to bed once and for all. haha

The plane wil fly, it will fly, yes it will fly....everyone knows it. hahahhahah
Old 01-11-2008, 12:57 PM
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Tall Paul
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Default RE: Airplane on a Treadmill

"And if you look at the landing gear on their airplane, you'll instantly recognize that their test vehicle destroys the validity of the test. No way that crappy gear comes close to not biasing the results."
.
I mentioned the obvious lack of suitability of that PoC for the test, and the more obvious lack of ability of the 'busters themselves to fly r/c planes on the Mythbusters forum, and got my head chewed off.
With all the people in the SF area that -can- fly r/c, and all the much more suitable planes to use... I guess the "how hard can it be?" syndrome pokes its ugly head up way too frequently.
Proper testing demands all the variables be controlled, so that -only- the one being tested for can change. With unsuitable test gear and novice users, there's nothing but chaos to expect for results.
I'd also mentioned their poor procedures with their r/c full-scale cars, without any means of fail-safe.. the cars would frequently take off by themselves without being controllable. Now, they point out they have fail-safes incorporated in their cars.

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