RE: Can It Take Off??  
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RE: Can It Take Off?? - 12/6/2005 11:08:04 PM   
starwoes



Posts: 121
Joined: 10/6/2005
From: Maple Heights, OH, USA
Status: offline 		that's the most f-d up drawing of a fly i have ever seen LOL ...is it drunk or something???

just kidding anyway.....nice try


--- not too long ago, i read this article regarding the su-27 trying to set the fastest times to altitude records etc.
according to the sukhoi design bureau chief, they first tried to hold the specially "enhanced" su-27 on the runway by using its brakes while they gunned the jets, and kicked in the afterburner.

the big fighter practically hauled ass down the runway with breaks locked!!!! of course they had to abort because they'd have blown out the tires. now imagine if it had happened on a frozen lake. that jet would have taken off with no problems at all---locked tires and all. now what can a conveyor do to stop an aircraft from moving? rolling friction just cannot compare to static friction....and matching wheel speed is simply futile. you'd be better off using a huge blower ahead of the craft to keep it in place relative to the ground....it'll fly but hover above a spot if the speed of the fan air matches the aircarft propulsion speed.

and oh, according to the article, they eventually secured the aircraft to a military tank.....and let go at the right time.

< Message edited by starwoes -- 12/7/2005 1:45:41 AM >

(in reply to kriegsmacht)
       Post #: 101

RE: Can It Take Off?? - 12/6/2005 11:17:39 PM   
HighPlains


 

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From: Over da rainbow, KS, USA
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quote:

Which am I wrong about? The fly or the rocket?


Both.

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RE: Can It Take Off?? - 12/6/2005 11:52:36 PM   
Flyfalcons



Posts: 6100
Joined: 7/27/2002
From: Bonney Lake, WA, USA
Status: offline 		Your fly looks like it just let out a giant turd.

Even though the thrust of the rocket is being deflected, the deflector is still experiencing all of the thrust in a downward motion (assuming the rocket is vertical).

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Ryan Winslow

Cub flying - the new EXTREME SPORT!!! (evarrr)

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       Post #: 103

RE: Can It Take Off?? - 12/6/2005 11:52:53 PM   
kriegsmacht



Posts: 141
Joined: 12/8/2001
From: Castle, OK, USA
Status: offline 		Highplanes..

Care to go into more detail?

Could the fly fly?

Would the scale read zero?

Would the rocket move?

Starwoes...

haha he is a jolly little fly isnt he? He must be so wasted he only thinks he is flying because its actually impossible.

(in reply to HighPlains)
       Post #: 104

RE: Can It Take Off?? - 12/6/2005 11:56:31 PM   
Gremlin Castle


 

Posts: 1221
Joined: 2/9/2004
From: Arlington, TX, USA
Status: offline 		It looks like the physics majors have ignored this thread.
The airplane will take off nicely with the only loss of energy being what it takes to overcome the losses due to the excess wheel speed. To hold the plane stationary enough power has to be applied to equal the frictional losses in the bearings plus the rolling resistance of the wheels. Past that, any additional power will accelerate the plane forward down the moving conveyer belt with the wheels spinning at the rate that equals the conveyey belt speed plus the forward rate of the plane.
I sure would not want to be the first aviator to try a carrier landing on a high speed belt rather than an arresting system.
I also find that spraying the plane down with propwash just prior to taking off really helps with those downwind turns.

quote:

ORIGINAL: bentwings

Here is one to use up band width. The diesel truck guys made 302 posts in 5 days. only 2 personal flames recorded
can the experts beat this??? Let's have at it. Can the "wing guys" beat the "wheel guys"

*** Brain Teaser ***

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?




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RE: Can It Take Off?? - 12/7/2005 12:08:01 AM   
Void


 

Posts: 6
Joined: 11/10/2005
From: Denton, TX, USA
Status: offline 		Haha, there we have it! the plane will be flung into the air by the centripital bowing of the entire runway!!!!!

I am sorry Gremlin, but the question clearly states that "The conveyer belt is designed to exactly match the speed of the wheels at any given time". Which in my oppinion, blows the "they just spin faster" theory away. Even physics majors have to follow the rules of the experiment.

Also, kriegsmacht, Flyfalcons is correct about the deflector on your fly. It requires opposite force to deflect that air, so the scale will not read zero.

(in reply to Gremlin Castle)
       Post #: 106

RE: Can It Take Off?? - 12/7/2005 12:27:34 AM   
HighPlains


 

Posts: 2637
Joined: 3/2/2003
From: Over da rainbow, KS, USA
Status: offline 		When you draw all the vectors on your rocket, you will see that they all cancel.

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RE: Can It Take Off?? - 12/7/2005 1:19:54 AM   
Time Pilot



Posts: 1461
Joined: 10/25/2003
From: Vancouver, BC, CANADA
Status: online 		After reading a bit of this thread and having a wrong opinion in another, I put a pack in my Estarter and put it on the treadmill (I had to guide it a bit with my hand to keep in centered on the track).

At slow speeds, the plane is pushed back as the treadmill speeds up, but at a certain speed, the plane didn't care how fast the treadmill was going. At full speed (10mph on the treadmill), the Estarter would easily accelerate forward when full throttle was applied.

Maybe I should do this more often and let the wife think I really need to get out and fly more....

(in reply to HighPlains)
       Post #: 108

RE: Can It Take Off?? - 12/7/2005 1:39:16 AM   
Gremlin Castle


 

Posts: 1221
Joined: 2/9/2004
From: Arlington, TX, USA
Status: offline 		In that case it will take off even faster because it doesn't have to overcome either bearing friction or rolling resistance. The conveyor is adding additional energy into the takeoff effort by accelerating at a rate that keeps the wheel stationary while the plane is accelerating down the runway with the conveyor belt.
It makes very little difference in realitywhich way the belt is moving. As for the tires they will probably stand up because while the rpm increases the bearing load decreases in proportion to the developed lift.
quote:

ORIGINAL: Void

Haha, there we have it! the plane will be flung into the air by the centripital bowing of the entire runway!!!!!

I am sorry Gremlin, but the question clearly states that "The conveyer belt is designed to exactly match the speed of the wheels at any given time". Which in my oppinion, blows the "they just spin faster" theory away. Even physics majors have to follow the rules of the experiment.

Also, kriegsmacht, Flyfalcons is correct about the deflector on your fly. It requires opposite force to deflect that air, so the scale will not read zero.



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Up elevator is not always your friend.

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RE: Can It Take Off?? - 12/7/2005 1:50:10 AM   
Craig-RCU



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From: minneapolis, MN, USA
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quote:

ORIGINAL: Flyfalcons


quote:

ORIGINAL: Craig-RCU


quote:

ORIGINAL: Flyfalcons

The Avweb article is interesting but it only answers the question of if the belt will move relative to the airplane's speed, not the speed of the tires.

I don't understand how you are defining the speed of the tires. My definition of the speed of the tires is that since they are attached to the plane their speed is the same as the plane. What is your definition of tire speed that it can be different than the airplane's speed? If a plane is moving on the ground at 10mph, how fast are the tires moving by your definition?

If you are assuming that the speed of the tires means rotational speed, then you are confusing the original problem needlessly because the discussion here shows that the speed of the conveyor belt (no matter how it is measured or what it is matched to) is irrelevant to the airplane gaining airspeed enough to take off.




Yeah, rotational speed of the tires. If the belt were to somehow magically match the exact rotational speed of the tires, then the plane would not be allowed to move forward no matter what form of propulsion it has. However, we know that this is a physical impossibility which makes the wording of the question the trick. In reality, the belt would not be able to match the tire speed.


The wording of the question is not a trick if you define the speed of the tires as being the same as the speed of the airplane. There is nothing in the original question to force the definition of tire speed to mean rotaional speed. Instead, the wording of the question implies that the tire speed equals airplane speed because it states that it most definitly is possible for the conveyor belt to exactly match the speed of the tires. If you assume that tire speed means rotational speed then it is impossible for the belt speed to match the tire speed and the whole problem becomes nonsensical and meaningless with no prediction possible about what would happen in such an impossible situation.

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RE: Can It Take Off?? - 12/7/2005 2:33:03 AM   
Flyfalcons



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Joined: 7/27/2002
From: Bonney Lake, WA, USA
Status: offline 		Yeah, kind of like what I said in post #68. But the way the question is worded does lead to confusion about whether it's talking about the wheel rotation speed or speed of the whole plane (wheels included). I did do some thinking about if it were rotational speed earlier (sitting ready reserve will do that to a man) and concluded that as soon as power is applied to the engine, both the wheels and belt will instantaneously go to infinity. Since they are both inifinity, the plane can start moving forward and takeoff at say, 100 knots, because infinity +100 is still infinity.

(Trust me, I wish I had gotten called out instead of thinking about this crap).

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Ryan Winslow

Cub flying - the new EXTREME SPORT!!! (evarrr)

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RE: Can It Take Off?? - 12/7/2005 2:37:01 AM   
Phlip



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Joined: 11/17/2002
From: Auburn, MA, USA
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quote:

The fly (or RC helicopter, or whatever) must actually move air downward that is equal to its weight to stay aloft.


I don't think that's correct. The air moved downward may be a byproduct of the thrust that keeps a device aloft, but, in the case of a rocket or an untied balloon, the lift (or thrust) is created by the unbalanced vector. Remember this diagram from physics class? That's why rockets work in outer space. And a flying wing generates lift by the pressure differential from the top and bottom surfaces. All of the lift is not from air that is moved downward--Bernouli's principle does have something to do with it.

Phil

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(in reply to Flyfalcons)
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RE: Can It Take Off?? - 12/7/2005 5:36:27 AM   
Flyfalcons



Posts: 6100
Joined: 7/27/2002
From: Bonney Lake, WA, USA
Status: offline 		Let's forget acceleration and helium balloons for a second, and focus on a hovering object in equilibrium, such as a fly hovering. The fly must move air equal to its own weight downward to stay aloft. Ever watch a helicopter hover over tall grass? A 10 pound helicopter will push 10 pounds of air downward to hover, and in turn the grass is pushed down. Coincidentally, that's what makes grass cutting with an RC heli such a PITA, but that's another topic. Yes, this also applies to airplanes in steady state flight. That 100,000 pound airliner really is producing 100,000 pounds of lift to stay in the air (actually a little more, but we're keeping things basic here) and therefore must move 100,000 pounds of air to stay aloft. In the case of the fly hovering in an airplane, the 1 gram of air needed to stay in a hover is pushing against the floor of the plane, so no matter if the fly is sitting on the floor or hovering, the plane is going to weigh 1 gram more than if the fly wasn't bumming a ride. Bernoulli, Coanda, Updraft/Downdraft, etc, whatever theory you need to use, for an object to generate a lift vector it is going to need an equal and opposite reaction downward.

< Message edited by Flyfalcons -- 12/7/2005 5:39:19 AM >

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Ryan Winslow

Cub flying - the new EXTREME SPORT!!! (evarrr)

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RE: Can It Take Off?? - 12/7/2005 9:20:37 AM   
multiflyer


 

Posts: 251
Joined: 10/3/2005
From: simi valley, CA, USA
Status: offline 		BMatthews answered bentwings’ original question clearly, accurately, and completely, on the very second post. But I am only slightly surprised that wasn’t the end of it.

The length of this thread does demonstrate something useful to understand for anyone who is seriously interested in learning the finer points of how models operate. And that is that unfortunately a not so small amount of what is spoken or written by this hobby community can be simply wrong.

It’s just the nature of the beast. RC Modeling is a recreational pursuit of a fairly complex discipline. There is no “formal” minimum understanding required to participate (yet? there are pressures for regulation simmering out there). Consequently a significant part of the struggle to learn involves filtering out which advice is actually good.

Multiflyer

(in reply to Flyfalcons)
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RE: Can It Take Off?? - 12/7/2005 12:11:09 PM   
Phlip



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