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#338572 - 08/22/07 04:55 PM Re: The case of the plane and conveyor belt [Re: Al]  
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I'm with Al

The argument is that the forward speed of the aircraft is matched by the conveyor, but that the seemingly free wheeling wheels would take the abuse and turn twice as fast (aircraft forward motion + conveyor rearward motion), which would still allow the aircraft to move forward.

What's not taken into consideration is the friction of the wheel contact to the conveyor.
The aircraft would still need to generate enough thrust to overcome the conveyor speed plus the forward speed needed for lift.

An earlier post said something about being on a treadmill on skates and pulling on a rope attached to a wall. The problem with this analigy is that the wall is stationary. If you hold the rope, you will not move on the tread mill and yes, you can pull yourself forward. But if the rope you were holding onto were not attached to something stationary (like air is not), you would move backwards.

You would need to pull aggresively at the rope (or actually move your A@# and skate) to supply enough forward movement to overcome the treadmill.

Like the wheels, the aircraft engines would need to supply enough thrust to overcome forward inertia plus the ever-increasing rearward speed of the conveyor.

So let's say 2x.

If the plane needs 100mph forward motion for lift, and the conveyor is matching 100mph rearward motion, my guess is the engines need to provide 200mph or thrust and would be over rev'd to the point of throwing a rod thru the cockpit killing the pilot and thus not acheiving lift.


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#338573 - 08/22/07 04:57 PM Re: The case of the plane and conveyor belt [Re: deepv]  
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Last edited by deepv; 08/22/07 05:00 PM.

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#338575 - 08/22/07 04:58 PM Re: The case of the plane and conveyor belt [Re: deepv]  
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Let's go back a minute to the original question. The conveyer tracks the speed of the plane and equals that of the plane, but in opposite direction. Speed of the plane relative to ground, or relative to conveyer, or even relative to how fast it should be going, hmmmmmm.......

Anyway, deepv, I would agree that the prop can not create enough air flow for lift, but I don't agree the plane would be sitting still. I think athiker and I are on the same page here, there isn't enough drag from the conveyer to keep the plane sitting still. The sea plane is a good analogy.


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#338579 - 08/22/07 05:02 PM Re: The case of the plane and conveyor belt [Re: WaterMutt]  
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deepv 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?"

The original problem statement requires that it be still.


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#338581 - 08/22/07 05:03 PM Re: The case of the plane and conveyor belt [Re: WaterMutt]  
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Al Offline
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If the conveyor tracks the speed of the plane in reverse, isn't that by definition mean the plane isn't going forward.

I think that the original scenario may not be adequately explained.


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#338583 - 08/22/07 05:11 PM Re: The case of the plane and conveyor belt [Re: Scott L]  
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Al Offline
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 Originally Posted By: Scott L
 Originally Posted By: athiker
Again I agree w/ WaterMutt. Yeah, the engine doesn't provide lift, but the the engine turns the propeller, which pulls the airplane through the air, like a prop pushes a boat through water, the subsequent air movement over the shape of the wings lifts the plane into the air.


Yes but a boat facing upriver in order to stay still must prouduce enough forward thrust to negate the riverflow. 5mph riverflow against 5 mph of forward thrust and the the boat stays still relative to the banks - How fast is the boat going ? Relative to what ? 0 relative to the river bank, 5 mph realitive to the water going behind it.


Maybe 5mph relative to the water going behind it, but zero speed relative to the banks, and zero speed relative to airflow.


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previous boats:
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#338586 - 08/22/07 05:25 PM Re: The case of the plane and conveyor belt [Re: Al]  
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Last edited by deepv; 08/22/07 05:25 PM. Reason: spelling.

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boaters that haven't taken a safe boating course.

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#338590 - 08/22/07 05:27 PM Re: The case of the plane and conveyor belt [Re: deepv]  
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Al Offline
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Would that be with a big "G" or small "g".

No worries on 550 pages, we'll parlay this topic into something else as equally superficial by then.


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

previous boats:
1995 Carver 325
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#338592 - 08/22/07 05:38 PM Re: The case of the plane and conveyor belt [Re: Al]  
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Like I said initially, the plane will not fly. QED.


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boaters that haven't taken a safe boating course.

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#338595 - 08/22/07 05:43 PM Re: The case of the plane and conveyor belt [Re: deepv]  
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Al Offline
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And I don't think you can totally discount friction. Due to gravity, it can be significant. Ever push a caboose?


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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
#338598 - 08/22/07 05:58 PM Re: The case of the plane and conveyor belt [Re: Al]  
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Agree, friction is the key...I just don't have the ability to gauge how big a factor it is sitting here on my high-friction bum. \:\)

LanierBoater wrote:
 Quote:
"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?"


Just to revisit the original question. Within the question it is stated that the plane moves. So this much we know.

There is an assumption that the movement is relative to the earth's surface correct? I mean that would be the reference for a spot on the conveyor belt, yes?...the conveyor belt's speed. So that would be the reference for the plane as well.

So the question states the plane moves in one direction, let's say 100 mph, which means the conveyor belt is moving in the other direction at 100 mph. So whatever speed the plane needs to move it can, there is no limitation in the question, it's just that the coveyor belt moves the same speed the other direction. Who cares tho what the conveyor belt is doing, we just care that the plane is moving at take-off speed.



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#338599 - 08/22/07 05:59 PM Re: The case of the plane and conveyor belt [Re: deepv]  
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I studied Mechanical and Aerospace engineering in college. This is not an engineering problem. This is just common sense physics. An airplane flies because the flow of air across the wings creates an imbalanced flow between the top of the wing and the bottom of the wing.To create enough lift to overcome the weight of the aircraft, there is a demand for X amount of wing area at Y speed. This is not an all or nothing factor, as even at low air speeds, some lift is produced, just not enough to overcome gravity.

If a plane has a tailwind, it will get to its destination quicker and with less fuel usage, but if the relative airspeed is lower than the minimum requirement for that aircraft, the plane will not stay in the air. This is why airspeed is a relative factor.

The wheels on an airplane are only a indulgence that recognizes that planes do not stay airborne continuously. They support the airframe when the wings do not.

Answer these questions:

1. If the engine dies while the treadmill is going 100 mph, does the plane stay still?

2. If the plane is turned around on the treadmill and it is cranked up to 100 mph, will the plane fly without the engine?

3. If you take the wheels of the aircraft to where it is setting on the treadmill, and you have rockets/jets that will generate enough thrust to equal the movement of the treadmill, what will happen?


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#338610 - 08/22/07 07:22 PM Re: The case of the plane and conveyor belt [Re: seadog]  
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Im suprised KCook and Seabuddy have not chimed in. I have always been impressed with their fluid dynamics expertise.


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#338617 - 08/22/07 07:43 PM Re: The case of the plane and conveyor belt [Re: LanierBoater]  
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Not rightly sure what yer dad burned plane will do but I'm bettin' I can sale a bunch of tickets to watch it do this little stunt to my NASCAR buddies....
....can you make it do a left turn every now and then?.....



By the way, I don't think it will fly....


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#338622 - 08/22/07 07:59 PM Re: The case of the plane and conveyor belt [Re: Parrott_head]  
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I must admit, this kind of thing is way over my head. I'm just a dumb hick farm boy from the middle of Illinoise. But will someone please explain to me how this silly conveyor belt is actually going to stop the plane from moving?


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#338631 - 08/22/07 08:23 PM Re: The case of the plane and conveyor belt [Re: 2Suns]  
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 Quote:
But will someone please explain to me how this silly conveyor belt is actually going to stop the plane from moving?



It won't. The plane takes off. IMO, the wheels/tires and magic conveyor belt have nothing to do with an airplane's ability to create positive thrust.

Last edited by LanierBoater; 08/22/07 08:24 PM.

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#338635 - 08/22/07 08:34 PM Re: The case of the plane and conveyor belt [Re: LanierBoater]  
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WaterMutt, ATHiker and seadog have it right. As long as I got something out of all that $$$ I spend on a couple of engineering degrees, they do... ;\)

The wheels will be spinning over twice as fast as if the plane was taking off from the ground because the conveyer belt is trying to catch up to the plane speed, but it never will unless the brakes are on. I think if you compare the drag from the wheel bearings to the output of the engines, you will see that the plane will move forward almost as fast as without the conveyer belt, right up to the point the bearings melt or the plane leaves the ground!

Like WaterMutt first said, the engines are pushing against the air, not the ground, so the conveyer belt is almost irrelevant, except to the wheels' rotational acceleration. It must be a pretty long conveyer belt...



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#338636 - 08/22/07 08:37 PM Re: The case of the plane and conveyor belt [Re: Al]  
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It would if the wheels where somehow connected to the engine/thrust.

I agree, you can change one word and have a totally different scenerio.


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#338640 - 08/22/07 08:45 PM Re: The case of the plane and conveyor belt [Re: Al]  
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 Originally Posted By: Al
And I don't think you can totally discount friction. Due to gravity, it can be significant. Ever push a caboose?


I think the bigger issue pushing a caboose is inertia. Ever try to stop a caboose?



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#338650 - 08/22/07 09:18 PM Re: The case of the plane and conveyor belt [Re: WayWeGo]  
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If a speedboat were placed into a huge lap pool (narrow swimming pool that produces a current), and the water movement is automatically adjusted to match the boat's speed, but in the opposite direction, will the boat fly?


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