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#338289 - 08/21/07 07:46 PM The case of the plane and conveyor belt  
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LanierBoater Offline
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"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"

Lots of "experts" on both side of the fence. What say you?


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#338293 - 08/21/07 07:52 PM Re: The case of the plane and conveyor belt [Re: LanierBoater]  
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Jack T Offline
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Unless the propeller creates enough air movement to give the wings lift, the plane will not take off--assuming your standard fixed wing aircraft.


Have a great day of boating
2012 Monterey 224 FS,
300 HP Volvo Penta with catalytic converters

#338294 - 08/21/07 07:54 PM Re: The case of the plane and conveyor belt [Re: LanierBoater]  
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markas Offline
Captain
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Western PA
Nope. Plane needs airspeed relative to the atmosphere. The conveyor will cause the plane to stay in the same place relative to the surrounding air, therefore the windspeed over the wings will be zero, therefore no lift.


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#338295 - 08/21/07 07:59 PM Re: The case of the plane and conveyor belt [Re: LanierBoater]  
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FatDog Offline
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the plane won't lift off

if all it took was for the plane's wheels to be moving fast, then since the earth rotates at 700,000 mph all planes should immediatly be able to just instantaneously fly,
it also takes a difference in the earth's speed and the plane's speed .. some air flowing neath the wings


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pugs, boxer, bulldog

"Let us cross over the river and sit under the shade of trees." ...... General Thomas 'Stonewall' Jackson
#338296 - 08/21/07 08:01 PM Re: The case of the plane and conveyor belt [Re: markas]  
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BillyB Offline
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+3 no, there is no air passing over the wing to create lift.


I'm just happy to be here!



#338302 - 08/21/07 08:14 PM Re: The case of the plane and conveyor belt [Re: BillyB]  
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deepv Offline
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No. Didn't we have this discussion last year about the kite tube thing?


72% of fatal boat accidents are caused by
boaters that haven't taken a safe boating course.

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#338305 - 08/21/07 08:23 PM Re: The case of the plane and conveyor belt [Re: deepv]  
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Hockey Family Offline
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No Take Off!

Likewise, if there is enough wind speed coming from the back of the plane, No Take Off!

That's why our local airport reverses direction of the take offs when the winds are-a-blow'n off shore.


HF
2006 Monterey 214FS
Volvo/Penta 5.0 GXi SX
#338307 - 08/21/07 08:32 PM Re: The case of the plane and conveyor belt [Re: Hockey Family]  
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LanierBoater Offline
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From "Straightdope.com" :

"First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off."


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"Politicians never accuse you of 'greed' for wanting other people's money --- only for wanting to keep your own money."
#338310 - 08/21/07 08:38 PM Re: The case of the plane and conveyor belt [Re: LanierBoater]  
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LanierBoater Offline
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Georgia
Also see:

http://www.newton.dep.anl.gov/askasci/phy05/phy05023.htm

I told you there were different opinions.....


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"Politicians never accuse you of 'greed' for wanting other people's money --- only for wanting to keep your own money."
#338328 - 08/21/07 09:30 PM Re: The case of the plane and conveyor belt [Re: LanierBoater]  
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Al Offline
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Al  Offline
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This is a joke, right?

Lift is generated across a wing by air flowing across it. The upper part of the wing is curved in respect to the lower part.

This means the airflow must be going faster across the upper part if it is going to meet up with the air going across the lower part. Since the air is going faster across the upper part, the air pressure is lower, and the wing is sucked upwards.

So the ability for a wing to create lift is airflow across the wing, and it really doesn't matter how that air flow is created, as long as it is sufficient to create enough lift.

You could fly an aircraft in a hurricane coming at you if you could maintain control.

But generally engines are not powerful enough themselves to generate enough lift, or generate enough air movement across the entire wing area, so it is a requirement for the aircraft to have forward movement in respect to the immediate air surrounding it to create sufficient lift.

Sure, there are exceptions such as an F-15 where the engines have more thrust than the aircraft weight (but you would have to launch it rocket-style if you didn't want to use the wings for lift).

This is especially true of engines mounted below the wings, such as a 747 or Cessna 172. There is little or no airflow from the engines above the wing, only below the wing. Therefore, the engines actually create negative lift. The job of the engines is to get the aircraft moving in respect to the surrounding air, and its the surrounding air that creates lift, not the engines.

If the "reverse conveyor" logic were true, then if the conveyor was not running at all, but stationary, it seems the system would be even more efficient, so the pilot should be able to put the brakes on, fire up the engine, generate sufficient lift from just the engines, then vertically lift off the ground.

As well, if the conveyor theory were true, you should be able to throttle the engines so they are in equilibrium with the amount of lift needed to fly the airplane without causing forward movement, and park the aircraft in the sky.

And how could you even land the aircraft unless you turned the engines off?

But I could be wrong.

But I'll caveat that you could probably get a RC model aircraft to lift off from a dead stop, but this is because the engine is much more powerful than in real life, and the engine itself is pulling the aircraft into the air.

------------------------------------------------------------------

Uh oh, this reminds me of a story.

When I was a dumb teenager, me and a buddy of mine were really into gas powered model airplanes. It was in the old days, before computers or video games, we used to have fun with gas airplanes.

We would build several of them, then on fly day, fly them all day long. Eventually, we would usually bust them up, but that did not mean the end of fly day.

If any of you are familiar with Cox 049 engines, some had a gas tank mounted in the back of the engine. We actually would grasp on the gas tanks with the palm of our hand, and actually START THE ENGINE, then vertically throw them up into the sky!

They would go up maybe 100ft, but the engine would be unbalanced, and at some point, the engine would quit and fall to the ground.

As long as it didn't land on something hard, we simply cleaned the dirt off the engines with a bit of gas, fire them up, and repeat.

We had larger engines as well, but would not dare try anything like that on a .40 engine.

(You cannot make this stuff up)





President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
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#338332 - 08/21/07 09:39 PM Re: The case of the plane and conveyor belt [Re: Al]  
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D-Rod Offline
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If I see someone from the aerospace engineering dept tomorrow, I will ask. \:\)


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#338335 - 08/21/07 09:54 PM Re: The case of the plane and conveyor belt [Re: Al]  
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Jack T Offline
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Jack T  Offline
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"Unless the propeller creates enough air movement to give the wings lift, the plane will not take off--assuming your standard fixed wing aircraft."

Isn't what Al said what I said? \:b
And everyone around here says I use too many words.


Have a great day of boating
2012 Monterey 224 FS,
300 HP Volvo Penta with catalytic converters

#338339 - 08/21/07 10:04 PM Re: The case of the plane and conveyor belt [Re: Jack T]  
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Al Offline
Nautical Alchemist
Al  Offline
Nautical Alchemist
Grand Poobah

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Vagabond Wanderer from Mich.
Yep, but we just said the same thing differently.

Besides, I probably took a couple of hours to say what I said, and maybe you snook in ahead of me....

(but I thought of the negative lift thing with a below-wing engine)


President and CEO - Napmoor and Doolittle.


2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.6L Diesel

previous boats:
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180
#338347 - 08/21/07 10:31 PM Re: The case of the plane and conveyor belt [Re: Al]  
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Jack T Offline
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Jack T  Offline
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Southern California
\:b Al once again.


Have a great day of boating
2012 Monterey 224 FS,
300 HP Volvo Penta with catalytic converters

#338350 - 08/21/07 10:59 PM Re: The case of the plane and conveyor belt [Re: Jack T]  
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Scott L Offline
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Scott L  Offline
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NJ
You would think if would work there would not be much of a need for Aircraft Carriers.... - just big floating conveyor belts

Last edited by Scott L; 08/21/07 10:59 PM. Reason: sp

2003 Chaparral 200 SSi - Volvo 5.7 Gi-Sx - " Ready or Knot "

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#338351 - 08/21/07 11:16 PM Re: The case of the plane and conveyor belt [Re: Scott L]  
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Lambert Laker Offline
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You guys are too smart for me – I thought the entire thing was about the V-22 Osprey...


LL

"Common Sense is not common to everyone"
#338409 - 08/22/07 08:18 AM Re: The case of the plane and conveyor belt [Re: Lambert Laker]  
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Bowline Offline
Boatless in LV
Bowline  Offline
Boatless in LV
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Yeah, but the Osprey is, essentially, a helicopter on take-off.


2001 Cobalt 206, 280hp V/P DP - SOLD
2003 Toyota 4-Runner Ltd
#338424 - 08/22/07 08:28 AM Re: The case of the plane and conveyor belt [Re: Bowline]  
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WaterMutt Offline
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Massachusetts
The whole premis is a bit skewed. How can a conveyer moving prevent an air craft from moving? The air around the air craft is its medium for thrust, not the ground. Like the explanation says, this isn't a car. The Conveyer has no influence on the air around the plane. Therefore, it will take off. The wheels will be spinning like mad, but the plane will move forward. Now, if this were a wind tunnel....


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#338436 - 08/22/07 09:12 AM Re: The case of the plane and conveyor belt [Re: WaterMutt]  
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athiker Offline
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athiker  Offline
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What WaterMutt said...absolutely. Like a boat prop in the water an airplane prop bites into the air to create movement through the air. The conveyor belt has no or minimal effect on the surrounding air so the plane takes off. The wheels may need good bearings for the extra-fast spinning for a bit until take-off is achieved. The extra friction might make take-off take a little more time than usual.


2000 Cobalt 206 5.7L EFI Alpha 1
#338453 - 08/22/07 11:02 AM Re: The case of the plane and conveyor belt [Re: athiker]  
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Rocnat4 Offline
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It depends. What airline carrier are we talking about?

No matter, their sure to blame the delay on "weather in Chicago".

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