RE: Seriously now, can it take off or not? What do you think?  
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All Forums >> RC Airplanes >> Aerodynamics >> RE: Seriously now, can it take off or not? What do you think?
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[Poll]

Seriously now, can it take off or not? What do you think?
Yes, it will take off.
  69% (61)
No, it won't take off.
  30% (27)


Total Votes : 88
(last vote on : 8/21/2007 11:55:01 PM)
(Poll will run till: -- )
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RE: Seriously now, can it take off or not? What do yo... - 12/8/2005 11:30:30 AM   
CHassan



Posts: 1230
Joined: 8/26/2002
From: Beavercreek, OH, USA
Status: offline 		Boast all you want but don't be so sure of yourself. I'll bet you 10,000 USD that you are wrong. I'll even go so far as saying the plane will take off in 1/2 the normal distance!!
Any takers???

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(in reply to mr_matt)
       Post #: 26

RE: Seriously now, can it take off or not? What do yo... - 12/8/2005 5:25:56 PM   
mr_matt



Posts: 6635
Joined: 12/7/2001
From: Oak Park, CA,
Status: offline
quote:

ORIGINAL: CHassan

Boast all you want but don't be so sure of yourself. I'll bet you 10,000 USD that you are wrong. I'll even go so far as saying the plane will take off in 1/2 the normal distance!!
Any takers???



Depends on who you will take as a definitive judge....Mythbusters maybe??

http://community.discovery.com/groupee/forums/a/tpc/f/9701967776/m/7451937218/p/1


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Matt
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(in reply to CHassan)
       Post #: 27

RE: Seriously now, can it take off or not? What do yo... - 12/8/2005 6:22:24 PM   
Scar



Posts: 2276
Joined: 10/1/2002
From: Peoria Hts, IL, USA
Status: offline
quote:

ORIGINAL: mr_matt

There is a 67 page thread on this on rcgroups it is running about 54% "no" (the correct answer, BTW)

I'll challenge that "correct answer".

F=Ma
a=(thrust/wt) - (wheel axle friction)

axle friction is negligible, thrust accelerates the plane through the air, slowed only by axle friction (as is normal under takeoff conditions, regardless of wheel speed) until airspeed reaches takeoff. This thing would take off with any engine/plane combination that will lift off a grass runway. Heck, make it a paved runway.

What makes you think a moving runway would make any difference to the airplane?

Or has this just been a troll?
Dave Olson

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(in reply to mr_matt)
       Post #: 28

RE: Seriously now, can it take off or not? What do yo... - 12/8/2005 6:47:28 PM   
dfalcon


 

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Joined: 10/26/2004
From: Bozeman, MT, USA
Status: offline
quote:

...even if it means the wheels get dragged at some point. a;; that stuff about friction etc just complicates matters needlessly.


that stuff about friction is a necessary part of the problem.

quote:

I'll challenge that "correct answer".

F=Ma
a=(thrust/wt) - (wheel axle friction)

axle friction is negligible,


unless you've included it as a form of friction (which it is at a molecular level) you've forgotten the rolling resistance from tire deformation. rolling resistance is substantial and measurable. think of your car for instance, (and no, i'm not stating that the plane is applying power to the treadmill through it's wheels) if the pressure in your tires is 5 psi low, your tires must deform more than normal and fuel economy can be cut by 10%.

also, someone earlier mentioned the original problem, "the treadmill exactly matches the airplanes speed." after trying it's hardest to take off for a while, the bearings in the axles have potential to reach extremely high temperatures. i know bearings are there to reduce friction, but they can not eliminate all friction. mechanical energy lost due to the minute amount of friction in the greased bearings is dissipated as heat energy, which can build up over time and (hey, here's a new problem) cause the viscosity of the grease in the bearings to break down, rendering it useless. now we have axle friction that is not negligible.

oops, i forgot to start this post with, "now i don't mean to get off on a rant here, but...."

the plane will never take off if the treadmill performs as originally stated in the problem.

(in reply to Scar)
       Post #: 29

RE: Seriously now, can it take off or not? What do yo... - 12/8/2005 6:52:31 PM   
dfalcon


 

Posts: 214
Joined: 10/26/2004
From: Bozeman, MT, USA
Status: offline 		i mentioned bearings and axle grease and stuff assuming the problem was aimed at a full-scale aircraft. are we talking full-scale or a model?

(in reply to dfalcon)
       Post #: 30

RE: Seriously now, can it take off or not? What do y... - 12/8/2005 7:17:40 PM   
PowerPlay


 

Posts: 181
Joined: 9/6/2002
From: Glendora, CA, USA
Status: offline 		All right, for all you that think it won't take off

What if you replace the wheels on the plane with wheels that are twice the diameter. Now the belt is matching the speed of the plane but the wheels are only spinning half as fast, Will the belt now have to turn twice as fast as the airplanes speed in order to prevent it from taking off?

(in reply to dfalcon)
       Post #: 31

RE: Seriously now, can it take off or not? What do y... - 12/8/2005 7:23:30 PM   
dfalcon


 

Posts: 214
Joined: 10/26/2004
From: Bozeman, MT, USA
Status: offline 		yep

< Message edited by roguespear -- 12/8/2005 7:24:41 PM >

(in reply to PowerPlay)
       Post #: 32

RE: Seriously now, can it take off or not? What do y... - 12/8/2005 7:28:36 PM   
dfalcon


 

Posts: 214
Joined: 10/26/2004
From: Bozeman, MT, USA
Status: offline 		and the bearings will last almost twice as long before they seize from the heat build-up.

(in reply to dfalcon)
       Post #: 33

RE: Seriously now, can it take off or not? What do y... - 12/9/2005 2:42:30 AM   
Craig-RCU



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Joined: 1/10/2002
From: minneapolis, MN, USA
Status: online 		----------------------------------------------------------------------------------------------------------------------------------------------------------
From the Mythbusters link above:

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 opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

----------------------------------------------------------------------------------------------------------------------------------------------------------
From the original RCU thread:

Imagine an airplane is on the beginning of a massive conveyor belt, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off?


The disagreement between the 'will fly' and the 'won't fly' groups stems from a different interpretation of the question. The RCU wording of the question, I think, adds to the confusion while the Mythbuster wording is clearer. Some interpret the 'wheel speed' in the RCU version to mean the reading of a speedometer attached to the wheel. Thus, the only way for the speedometer reading and the conveyor belt to exactly match the belt speed at all times is for the plane not to be able to move forward because if the plane moves forward the belt speed will be less than the speedometer reading. This however, requires that the belt provides a force equal and opposite to the propeller thrust through the wheels. The wheels are connected to the plane by bearings with small friction forces at normal operating rpm. So in order for the belt to exert this force on the plane, it (the belt) would have to be moving at very, very high speed at which other normally small and negligible forces begin to have a significant effect. A belt moving at high speeds (probably 1000's of miles per hour) would create a strong enough wind for the plane to lift off. A belt moving that fast would also create mach effect pressure waves that could fling the plane into the air not to mention the belt itself would also probably deform and fling the plane into the air. So, you see all sorts of problems begin to arise with this interpretation.

The correct and meaningful interpretation of the problem is one that doesn't push the laws of physics to the limit and where negligible forces at the outset remain negligible throughout the experiment. This means interpreting the 'wheel speed' to be the same as the 'plane speed' which is backed up by the Mythbusters wording in which only 'plane speed' is mentioned. Plane speed must be measured in relation to ground speed not the belt or we're back to the same 'pushing the laws of physics' situation stated in the previous paragraph. So, now we have a situation in which the rolling and bearing friction remain small enough to completely ignore them. The belt has no influence on the acceleration of the plane because it exerts zero force on the plane opposing engine thrust. With no opposing forces acting on the plane it is free to accelerate to flying speed. If the plane's takeoff speed is 60kts the belt will have a speed of 60kts in the opposite direction and the speedometer attached to the plane's wheel would read 120kts at liftoff.

I hope this clears things up for those who don't understand the 'will fly' side.

< Message edited by Craig-RCU -- 12/9/2005 3:11:21 AM >

(in reply to dfalcon)
       Post #: 34

RE: Seriously now, can it take off or not? What do y... - 12/9/2005 3:26:21 AM   
mr_matt



Posts: 6635
Joined: 12/7/2001
From: Oak Park, CA,
Status: offline 		I think you are my new hero, Craig. I just noticed the 2 versions of the question now. Your explanation is spot on (although many of the "wheel speed" guys can still cannot agree that the very fast conveyor can hold the plane back through wheel friction)

I am going to quote you elsewhere!

Thanks,



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       Post #: 35

RE: Seriously now, can it take off or not? What do y... - 12/9/2005 3:42:06 AM   
Phlip



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From: Auburn, MA, USA
Status: offline
quote:

I think you are my new hero, Craig. I just noticed the 2 versions of the question now. Your explanation is spot on (although many of the "wheel speed" guys can still cannot agree that the very fast conveyor can hold the plane back through wheel friction)


YESSS!!!! THANK YOU!!!!

Although I did post something similar in the other thread, I don't think anybody caught my drift.


http://www.rcuniverse.com/forum/fb.asp?m=3623172

And, the argument keeps getting derailed by those that picture the plane needing to push against the belt somehow to attain speed. Or possibly the belt moving the air at the same speed as well.

THANKS AGAIN CRAIG!


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(in reply to Craig-RCU)
       Post #: 36

RE: Seriously now, can it take off or not? What do y... - 12/9/2005 4:32:47 AM   
iron eagel



Posts: 1267
Joined: 7/15/2004
From: Middleboro, MA, USA
Status: offline 		I would advise not standing in front of the belt and plane while this was being tried!
LOL

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(in reply to Phlip)
       Post #: 37

RE: Seriously now, can it take off or not? What do yo... - 12/9/2005 11:37:00 AM   
CHassan



Posts: 1230
Joined: 8/26/2002
From: Beavercreek, OH, USA
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quote:

ORIGINAL: mr_matt


quote:

ORIGINAL: CHassan

Boast all you want but don't be so sure of yourself. I'll bet you 10,000 USD that you are wrong. I'll even go so far as saying the plane will take off in 1/2 the normal distance!!
Any takers???



Depends on who you will take as a definitive judge....Mythbusters maybe??

http://community.discovery.com/groupee/forums/a/tpc/f/9701967776/m/7451937218/p/1




Only if they can build a belt to 100% match the speed of the plane(wheels or what ever methode you are measuring) instantly. Otherwise the there are flaws in the testing method!



_____________________________

The knack of flying is learning how to throw yourself at the ground and miss.
--Hitchhikers Guide to the Galaxy--

(in reply to mr_matt)
       Post #: 38

RE: Seriously now, can it take off or not? What do yo... - 12/9/2005 2:13:17 PM