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David L
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 04, 2007 4:27 am

Aaargh... "I am just going outside and may be some time..."

Quoting Iwok (Reply 648):
not to nit pick, but the loss of grip could affect the aircraft's ability to show a negative acceleration i.e. braking, which is exactly what would be needed in the absense infinite thrust from the engines.

Um, what "infinite thrust"? My comments were relevant to the cross-runway conveyor belt (Problem B, Option 1), introduced here:

Quoting XT6Wagon (Reply 623):

Keep up.  biggrin 
 
superstring
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RE: If A Plane Took Off A Conveyor Belt...

Thu Aug 09, 2007 5:38 am

Yes, Yes, Yes... I am No. 651. And still counting...
 
Ralgha
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RE: If A Plane Took Off A Conveyor Belt...

Thu Aug 09, 2007 6:18 am

Good Lord are you people seriously still arguing about this? Every possible argument for why the airplane will not take off has been shredded by either myself or SlamClick way back up in this thread.

THE AIRPLANE WILL TAKE OFF
09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
 
superstring
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RE: If A Plane Took Off A Conveyor Belt...

Thu Aug 09, 2007 6:26 am

Well we could construct a situation where the aircraft wont fly. Simply use some tons of glue and put the aircraft into the glue on the conveyor belt, it will definitely not fly...

To be obvious the basic question is not wrong. The aim of this forum should be to answer any question (well of course as long as the quesiton is not insulting anybody). It is also clear that the question is very theroretical as nowhere (with the notable exception that anyone builds a special conveyor belt for a model plane) such a "anti aircraft lifting device" exists, and therefore it is quite simple to construct the strangest situations...

But it is always interesting when a threat boosts past 500...
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Thu Aug 09, 2007 7:28 pm

Quoting Ralgha (Reply 652):
Every possible argument for why the airplane will not take off has been shredded by either myself or SlamClick way back up in this thread.

For one option of one of the three different scenarios, yes, but what about the later ones? Incidentally, while SlamClick's contributions were immense as ever, there were many other valuable contributions, in my opinion.
 
zak
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RE: If A Plane Took Off A Conveyor Belt...

Thu Aug 09, 2007 10:59 pm

hilarious thread, shows the amount of hot air between the ears present, sadly.
of course it will fly, slamclick > tecops
10=2
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Thu Aug 09, 2007 11:20 pm

Quoting Zak (Reply 655):
hilarious thread, shows the amount of hot air between the ears present, sadly.
of course it will fly,

In which scenario? One of the two original scenarios, the "hampster wheel" scenario or the cross-runway conveyor belt scenario?
 
deltaamtrak058
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 12, 2007 12:25 pm

Lift is generated by the wings, the engine just pulls it forward. Thus I agree in that the engines will generate thrust, however if the wheels are "fixed" ie. f net=0 then the airplane will never leave the ground simply because the wings are not moving through the air, the "engines are".

Edit: The faster the engines "pull" the a/c through the air the faster the wheels spin. And if the belt moves at the "same" speed but in the opposite direction then fnet is again zero because thrust = motion and motion is always being counteracted (in this example)

2nd Edit: IF...the belt spins at the same speeds as the wheels then yes, it will take off because for the plane to remain stationary the wheels would be spinning 2x the speed. But in this example f net is .5x greater because the velocities are matched.
Its confusing, But the belt is actually moving at .5 times the speed of the wheels, thus matching the relative velocities.

-Deltaamtrak058

[Edited 2007-08-12 05:31:30]

[Edited 2007-08-12 05:46:37]

[Edited 2007-08-12 05:50:44]
The engine is the heart of an aeroplane, but the pilot is its soul. -Sir Walter Raleigh
 
XT6Wagon
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 12, 2007 1:22 pm

Quoting Deltaamtrak058 (Reply 657):
Lift is generated by the wings, the engine just pulls it forward. Thus I agree in that the engines will generate thrust, however if the wheels are "fixed" ie. f net=0 then the airplane will never leave the ground simply because the wings are not moving through the air, the "engines are".

Edit: The faster the engines "pull" the a/c through the air the faster the wheels spin. And if the belt moves at the "same" speed but in the opposite direction then fnet is again zero because thrust = motion and motion is always being counteracted (in this example)

2nd Edit: IF...the belt spins at the same speeds as the wheels then yes, it will take off because for the plane to remain stationary the wheels would be spinning 2x the speed. But in this example f net is .5x greater because the velocities are matched.
Its confusing, But the belt is actually moving at .5 times the speed of the wheels, thus matching the relative velocities.

-Deltaamtrak058

If you secure the airplane to the ground such that the ground speed can't exceed 0mph, you can still take off. This is why I suggested the windtunel approch as the aircraft's engines will accelerate the air, creating an airpseed with 0 ground speed.
 
vikkyvik
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 12, 2007 2:13 pm

Disclaimer: I haven't read all the replies since probably reply 150 or so....

Quoting XT6Wagon (Reply 658):
If you secure the airplane to the ground such that the ground speed can't exceed 0mph, you can still take off. This is why I suggested the windtunel approch as the aircraft's engines will accelerate the air, creating an airpseed with 0 ground speed.

How do you figure? Even with the most powerful jet engines out there right now, the airspeed over and under the wings will not be anywhere near enough to enable the airplane to fly...

I'm pretty sure that windtunnels have fans that cover the entire cross-sectional area of the wind tunnel - the air has nowhere to go but around the tunnel. Wouldn't quite be the same with a couple jet engines inside the tunnel, I wouldn't think.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
zak
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 12, 2007 11:45 pm

Quoting XT6Wagon (Reply 658):
as the aircraft's engines will accelerate the air, creating an airpseed with 0 ground speed.

come on, you cant be serious.
10=2
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 13, 2007 6:19 am

Okay it is time for me to settle this.

The plane would take off but you would not be able to guess where it was going to end up.

Proof: They put your luggage on a conveyor belt and IT takes off, but may end up anywhere in the world.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 13, 2007 7:00 am

Quoting SlamClick (Reply 661):
The plane would take off but you would not be able to guess where it was going to end up.

Proof: They put your luggage on a conveyor belt and IT takes off, but may end up anywhere in the world.

I think you're on to something there. If you tag the luggage to go to the airport it's already at and put it into the hold, the aircraft would have to take-off in order to transport it to "the wrong place".
 
MidEx216
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RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 13, 2007 9:15 am

To address the subject of the belt moving at the exact same speed as the wheels, this is much like the idea of an un-movable object meeting and unstoppable force. It just plain doesn't work.

The engines are going to push the plane forward, and as the wheels begin to turn, they will be matched by the belt, but the engines will continue moving the aircraft forwards. Essentially, if the belt is spinning the same speed as the wheels, but the aircraft is not going anywhere, the belt IS spinning the wheels. Both speeds could be reduced to zero and it would have no effect on the aircraft's speed. The belt and the wheels are equal, but the engines are the tiebreaker, completely overruling that whole 'same speed' idea.
 
XT6Wagon
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RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 13, 2007 3:05 pm

Quoting Vikkyvik (Reply 659):
How do you figure? Even with the most powerful jet engines out there right now, the airspeed over and under the wings will not be anywhere near enough to enable the airplane to fly...



Quoting Zak (Reply 660):
come on, you cant be serious.

Very serious. A jet engine moves alot of air. Given a long tunnel of minimum area the jet engines are plenty capible of generating a hell of a huge movement of air. The problem is that It takes alot of time, and takeoff thrust is time limited.

We don't need full windtunnel speeds, we only need enough to get the plane airborne.... and in this rather theoretical case, you can use a wieght for the aircraft that is empty, with minimial fuel at the takeoff point..
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 13, 2007 8:02 pm

Quoting MidEx216 (Reply 663):
To address the subject of the belt moving at the exact same speed as the wheels, this is much like the idea of an un-movable object meeting and unstoppable force. It just plain doesn't work.

That would Option 3 above. I think we've all agreed that just can't happen.
 
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Jetlagged
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RE: If A Plane Took Off A Conveyor Belt...

Tue Aug 14, 2007 1:30 am

Quoting David L (Reply 665):
I think we've all agreed that just can't happen

Dangerous ground. The one thing that we're all agreed on is that we aren't all agreed on anything.

I think this thread is like Halley's Comet, it re-appears in regular cycles (sadly not 76 years in this case).
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
FredT
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RE: If A Plane Took Off A Conveyor Belt...

Tue Aug 14, 2007 2:14 am

Quoting XT6Wagon (Reply 664):
Very serious. A jet engine moves alot of air. Given a long tunnel of minimum area the jet engines are plenty capible of generating a hell of a huge movement of air. The problem is that It takes alot of time, and takeoff thrust is time limited.

A tunnel will not do the trick.

There's a reason that most wind tunnels are closed loops.

Think about it.

Quoting XT6Wagon (Reply 664):

We don't need full windtunnel speeds, we only need enough to get the plane airborne.... and in this rather theoretical case, you can use a wieght for the aircraft that is empty, with minimial fuel at the takeoff point..

And the take off speed will still be quite significant, as far as wind tunnels go.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Tue Aug 14, 2007 3:54 am

Quoting FredT (Reply 667):
A tunnel will not do the trick.

There's a reason that most wind tunnels are closed loops.

Think about it.

I didn't really want to go there but, since you mention it...

It seems to me that, although you could obviously get the engines to generate an airflow through the tunnel, at some point equilibrium would be reached, when the drag of the tunnel and aircraft would balance the accelerating force of the engines. It's only my hunch that it would occur at too low a speed to produce lift - I really don't feel like actually working it out.  Smile

Quoting Jetlagged (Reply 666):
Quoting David L (Reply 665):
I think we've all agreed that just can't happen

Dangerous ground. The one thing that we're all agreed on is that we aren't all agreed on anything.

I think we were all OK with it once we'd separated the physical (Options 1 and 2) from the philosophical (Option 3).  Smile
 
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Jetlagged
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RE: If A Plane Took Off A Conveyor Belt...

Tue Aug 14, 2007 4:36 am

Quoting David L (Reply 668):
I think we were all OK with it once we'd separated the physical (Options 1 and 2) from the philosophical (Option 3).

I'm not disagreeing, I just thought it was tempting fate to say so though.  Wink
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
aauzou
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RE: If A Plane Took Off A Conveyor Belt...

Thu Aug 23, 2007 4:02 pm

The plane will indeed lift into the air, that is if the indicated air speed is equal to that which it would usually have at Vr, however the engines must not be turning in this scenario or otherwise it would not be the conveyor belt helping the aircraft to take off, and because of this simple little fact it would not be too long before the plane lost its speed and came crashing back down not that it would get very high anyways.

The closest to this happening in real life I suppose is an aircraft taking off from an aircraft carrier, being catapolted out by the cable to assist in building IAS.

Regards
760611
 
SilverComet
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RE: If A Plane Took Off A Conveyor Belt...

Fri Aug 24, 2007 1:23 am

Quoting Aauzou (Reply 670):

Umm I **think** you misunderstood the question. The conveyor belt runs the opposite way to that which you are probably assuming. Read the 650 or so posts before yours and I'm pretty sure whatever you have say about it has already been covered Big grin.

This thread has evolved to a stage which one of my former instructors would eloquently describe as 'enculer les mouches'  Wink.
 
BAE146QT
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RE: If A Plane Took Off A Conveyor Belt...

Fri Aug 24, 2007 3:40 am

Quoting SilverComet (Reply 671):
Read the 650 or so posts before yours

Why don't you just tell him to take some Ketamine and curl up in the corner? Yeesh.  Wink
Todos mis dominós son totalmente pegajosos
 
SilverComet
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RE: If A Plane Took Off A Conveyor Belt...

Fri Aug 24, 2007 5:38 am

Quoting BAe146QT (Reply 672):

Heh my personal record on this thread is like 250 posts or so in a day. Yes, there was a time when I was interested to know if the plane would finally be allowed to take off, and why. Now I just sit back with the popcorn and watch the circus.
 
newagebird
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RE: If A Plane Took Off A Conveyor Belt...

Fri Aug 24, 2007 11:37 pm

longest thread ever!

it wont take off, aircraft produces thrust but isnt going anywhere, it has to be in motion to develop lift. This is assuming the belt is going in the opposite direction to the wheels.

It will take off if the belt direction is reversed though (same direction as forward motion), as lift can be produced by the air flowing over the wings.

l = Cl 1/2 rho V^2S

rgds newagebird

[Edited 2007-08-24 16:45:28]
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 12:08 am

Remarkable.

People still jump on here believing that they could possibly offer an opinion or "facts" that have not already been considered, disputed, chewed up and digested five hundred posts back.

Not that it will make the slightest difference but here are a few things to keep in mind:

1. The thread title askes "If a Plane Too Off ..." Now it does not matter what words come next. If a plane "took off" then the plane TOOK OFF and the answer is YES.

2. The question in the o/p is different from the thread title.

3. The conditions in the original question are magic. Get that? MAGIC! So answer any old thing you want. It just flat does not matter.

4. On the off chance that you are still seriously considering the basic question AND you are willing to permit some magic and disallow other magic I don't think anyone here can shed any real light on the questions of your private universe. All answers you get will be rooted in their private universe.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
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Jetlagged
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 2:38 am

Quoting Newagebird (Reply 674):
it wont take off, aircraft produces thrust but isnt going anywhere, it has to be in motion to develop lift. This is assuming the belt is going in the opposite direction to the wheels.

It will take off if the belt direction is reversed though (same direction as forward motion), as lift can be produced by the air flowing over the wings.

Oh, well, that settles it then.....  sarcastic 

I suppose 673 replies are hard to wade through, but I'm amazed there are still posts like this. Even the no-fly group accept that the belt can't possibly stop the plane in real life, only in fantasy land.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 6:00 am

Quoting SlamClick (Reply 675):
2. The question in the o/p is different from the thread title.

It sure is.  Smile

Quoting Newagebird (Reply 674):
it wont take off, aircraft produces thrust but isnt going anywhere, it has to be in motion to develop lift. This is assuming the belt is going in the opposite direction to the wheels.

Which option are you addressing? If it's the one I think you're addressing, I think you have 600+ posts to review - hint: the wheels don't drive the aircraft (Copyright, SlamClick Consultancy, 2003).  Smile

Quoting SlamClick (Reply 675):
People still jump on here believing that they could possibly offer an opinion or "facts" that have not already been considered, disputed, chewed up and digested five hundred posts back.

Yup. Even when someone presents a later variation on the theme, some can't help reading the thread title and responding to the the new question as if it were the one from nearly 700 posts back.

Quoting Jetlagged (Reply 676):
I suppose 673 replies are hard to wade through, but I'm amazed there are still posts like this. Even the no-fly group accept that the belt can't possibly stop the plane in real life, only in fantasy land.

 checkmark  An additional gripe I have is that some feel the need to waltz in at various intervals and demand the thread be locked. If no-one's interested, it'll die naturally. Fortunately, most of the evidence for that has "disappeared".

It was a good thread... most of the time... for those who actually bothered to read it before posting...
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 6:30 am

Quoting David L (Reply 677):
If no-one's interested, it'll die naturally.

Exactly. I see this thread as rather like the "Police Squad" or "Airplane!" movies, or even like demolition derby. Totally stupid and you are free to walk away any time you wish. When these things stop entertaining us or satisfying some need we have for stupid then they wither and die.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
AirframeAS
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 6:53 am

It wont fly.

I'm surprised that this thread is still going after all this time! Wow! Impressed!  wave 
A Safe Flight Begins With Quality Maintenance On The Ground.
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 7:03 am

Quoting AirframeAS (Reply 679):
It wont fly.

Which one?  duck 
 
newagebird
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 9:49 am

Remarkable

the same blokes arguing about this for months, clearly im not in the loop. Well enjoy yourselves
gday all
newagebird
 
Fly2HMO
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 10:01 am

So this beast of a thread is dangerously close to braking 700 replies, yet the correct answer was given in reply 2.

Only in tech-ops Big grin
 
zak
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 10:12 am

Quoting FLY2HMO (Reply 682):
So this beast of a thread is dangerously close to braking 700 replies, yet the correct answer was given in reply 2.

Only in tech-ops Big grin

its not about facts and figures, its about that fetish  Wink this thread here kind of proves the proliferation of the "magic fantasy wonderland" plane fetish  airplane   alert   hissyfit   white 
10=2
 
Stealthz
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 3:35 pm

Quoting FLY2HMO (Reply 682):
So this beast of a thread is dangerously close to braking 700 replies, yet the correct answer was given in reply 2.

Gonna bust thru 700 and then some if you consider reply 2 as the correct answer.

Cheers
If your camera sends text messages, that could explain why your photos are rubbish!....well that might have changed!!!
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Sat Aug 25, 2007 8:47 pm

And still some haven't noticed there were at least 6 different scenarios discussed, only one of which involved "magic". Now that's remarkable.  Smile
 
AirframeAS
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 6:10 am

Quoting David L (Reply 680):
Which one?  duck 

See reply #2.
A Safe Flight Begins With Quality Maintenance On The Ground.
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 6:55 am

Quoting StealthZ (Reply 684):
if you consider reply 2 as the correct answer.

Without explaining what force is holding the plane back.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 6:58 am

Quoting David L (Reply 685):
there were at least 6 different scenarios discussed, only one of which involved "magic".

Actually every solution permitted infinity to be used, as if it was a real option.
Magic enough for you?
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
AirframeAS
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 2:56 pm

Quoting SlamClick (Reply 687):
Without explaining what force is holding the plane back.

Maybe the conveyor belt is off. Maybe its on. But still....the relative wind over the wing is still needed to provide lift. If the belt was on, and the wheels are moving, the plane does not move. Of the conveyor belt is off, the plane can roll off of it and still take off, hence it can then fly.

Fair enough??
A Safe Flight Begins With Quality Maintenance On The Ground.
 
Stealthz
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 7:07 pm

Quoting AirframeAS (Reply 689):
the plane does not move.

You have not explained why the plane is not moving, have you forgotten the only role the wheels play on an aircraft is to stop the gear struts scraping on the ground.
They play no part in the propulsion of the aircraft.

All this has been explained before but some just don't get it.
If your camera sends text messages, that could explain why your photos are rubbish!....well that might have changed!!!
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 10:28 pm

Quoting AirframeAS (Reply 686):
See reply #2.

That deals with one interpretation of one of the options. For other interpretations and scenarios, see replies 3 to 600+

Quoting SlamClick (Reply 688):
Actually every solution permitted infinity to be used, as if it was a real option.

I was referring to these options:

Quoting David L (Reply 523):
1. Belt opposes aircraft's ground-speed, not its "belt-speed" ("classic" scenario)
2. Belt stops tyres rotating ("as stated" version 1),
3. Magic force stops the aircraft from gaining air/groundspeed ("as stated" version 2)
4. The question was never asked in the first place and what are you talking about?

Results:

1. Aircraft takes off at normal air/groundspeed but wheels rotate twice as fast as normal,
2. Aircraft takes off at normal air/groundspeed but wheels not rotating,
3. Twighlight zone with magic force - belt changes colour, slips in and out of focus, we all feel one with the universe. "But what happens to the plane?"... "I don't care, look at all the colours!"...
4. With hindsight, I'd like to believe this was the case.

- OK, Option 4 doesn't really count.  

Then we had the cross-runway belt and the hamster wheel.  

[Edited 2007-08-26 15:30:34]
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 11:27 pm

Quoting AirframeAS (Reply 689):
relative wind over the wing is still needed to provide lift.

We agree on this.

It is this:

Quoting AirframeAS (Reply 689):
If the belt was on, and the wheels are moving, the plane does not move.

Explain this.

It doesn't matter a damn if there is an airplane or a skateboard attached to those wheels.
The belt could move a gazillion miles an hour and have no effect whatever on the AXLES.

All the belt can do is spin the freewheeling wheels.

The axles will still move forward because of thrust. The axles will move forward in response to jet thrust.
When they do they will take the airplane (and its wings) forward with them.

In the real world the only thing the conveyor can do is impart a linear speed through the contact patch of the tire. This is seen as rotation of the wheel. The AXLE is not turning.

So if you disagree tell us all how you can hold the axle back just by retarding the forward motion of the BOTTOM OF THE WHEEL which is free to rotate in either direction or even stand still. I mean walk out of the hangar and watch an airplane take off. At the moment of liftoff the relative motion between the main tires and the concrete of the runway is ZERO. The tire is stationary relative to the concrete even at 150 knots. Otherwise it would be skidding!

What force is acting on the AXLES which are the part that is actually bolted to the rest of the plane?
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 11:45 pm

Disagree with you on this one point:

Quoting David L (Reply 691):
1. Belt opposes aircraft's ground-speed, not its "belt-speed" ("classic" scenario)



Quoting David L (Reply 691):
Results:

1. Aircraft takes off at normal air/groundspeed but wheels rotate twice as fast as normal,

I was thinking along this line too then I realized something. Without getting into equations that never "prove" anything unless one is already predisposed to believe, let me 'splain.

Normally a plane begins a takeoff roll. The rotational speed of the wheel is nil. It starts moving, the RPM of the wheel is whatever is required to keep the contact patch on the bottom NOT MOVING with respect to the concrete immediately beneath it. It is being rolled by that concrete. At the far end of the takeoff roll the RPM of the wheel is whatever is needed to keep that contact patch not moving with respect to the ground.

On the backwards-moving conveyor it would start out that way but another factor gets introduced immediately.

Through some magic machinery as soon as the axle begins to move forward the wheel begins to rotate in order to keep the contact patch neither skidding nor slipping. The ROTATION of the wheel is sensed and a rearward belt motion is introduced to MATCH the expected wheel circumference linear speed. Now the wheel RPM must match that required for the actual motion forward PLUS the rearward motion of the footprint of the wheel. This increases the wheel RPM a certain amount.

This repeats - over and over in infinitely small units of time. The result is that by the time the AXLE has moved an infinitessimal distance forward, the belt speed has increased on a rising curve to the point that the wheel has reached infinite RPM.

Not double, infinite.

That is why I say the question involves magic.

There are really only only two indisputable truths.

1. Jet thrust is completely independant of belt motion.

2. Unless there is some solid linkage between the belt and the AXLE the belt can have no effect on the airplane's ability to move forward. (Except in the real world where the tires will explode.)
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
David L
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 11:57 pm

Quoting SlamClick (Reply 692):
The belt could move a gazillion miles an hour and have no effect whatever on the AXLES.

Before someone else chips in without reading the rest of the thread, it's probably worth a reminder that we agreed that failure of the bearings and/or tyres due to over-speeding were to be ignored since they were clearly not intended to be relevant to the will-fly/won't-fly outcome.

Quoting AirframeAS (Reply 689):
If the belt was on, and the wheels are moving, the plane does not move.

This is why "See Reply #2" is not a useful response. See the replies to Reply 2.  Smile

If you're saying the aircraft will not move because the belt is opposing its motion, I think you're misunderstanding Option 1 (Classic Scenario), as SlamClick suggests. If you're saying the aircraft will not move because the wording of the thread starter "dictates" that it can't move, then you're arguing Option 3 (Here Be Magic). There is no Single Answer to what's been discussed over 600+ posts.
 
bond007
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RE: If A Plane Took Off A Conveyor Belt...

Sun Aug 26, 2007 11:58 pm

Quoting SlamClick (Reply 693):
There are really only only two indisputable truths.

1. Jet thrust is completely independant of belt motion.

2. Unless there is some solid linkage between the belt and the AXLE the belt can have no effect on the airplane's ability to move forward. (Except in the real world where the tires will explode.)

Correct. The airplane WILL move forward. It really is that simple.

You can all discuss whether the belt can match the speed of the wheels, etc. etc. ... but the airplane will move forward.


Jimbo
I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!
 
nema
Posts: 483
Joined: Fri Feb 24, 2006 3:18 am

RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 27, 2007 12:32 am

Just done a quick calculation.

With the amount of rubber needed to construct a conveyor belt as wide and as long as the average runway you would need about 1.2 million big macs to feed the workers.


Well, only as daft as the original post
There isnt really a dark side to the moon, as a matter of fact its all dark!
 
tdscanuck
Posts: 8572
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RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 27, 2007 12:52 am

Quoting SlamClick (Reply 692):
Quoting AirframeAS (Reply 689):
If the belt was on, and the wheels are moving, the plane does not move.

Explain this.

The problem is all in the way the original question was formulated (yes, I have read all of the posts down to here...I'm bored at work).

From the OP:

Quoting Sovietjet (Thread starter):
The conveyer belt is designed to exactly match the speed of the wheels at any given time

We don't know how the conveyor does this, but it's a condition of the problem so we have to accept it. The axle's forward speed relative to the air (assuming no skidding) is the wheel speed minus the conveyor speed. That's just kinematics. As stated by the problem constraints, the wheel speed is always equal to the conveyor speed. Therefore the axle speed must be zero.

If the axle speed is non-zero, then the original constraints of the problem are violated and you're talking about a different situation.

The problem is that any solution to the problem constraints requires some kind of magic: frictionless wheels, infinite speeds, etc. The real answer is that the problem doesn't have any physical meaning because you can't actually build the system that's required by the constraints.

If you allow skidding, a bunch of other possibilities open up.

Tom.
 
SlamClick
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RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 27, 2007 4:09 am

Quoting Tdscanuck (Reply 697):
If the axle speed is non-zero, then the original constraints of the problem are violated and you're talking about a different situation.

In other words - MAGIC.

Simplify the question: "If a plane can be kept from moving will it take off?

It is just silly.

Move ONE STEP toward the real world, however, and you can see there is no way for a conveyor to do anything but spin the wheels. It cannot affect the axles. There IS NO FORCE being transmitted from conveyor to axle. The one and only force the conveyor can exert (no matter how magic it is) on the airplane at all is to SPIN THE WHEELS. That does nothing to the axles, landing gear, gear box, wings, fuselage, engines or forward biffy.

Next question: Can we keep a helicopter from taking off by holding onto its shadow?
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
David L
Posts: 8551
Joined: Tue May 18, 1999 2:26 am

RE: If A Plane Took Off A Conveyor Belt...

Mon Aug 27, 2007 4:59 am

Quoting SlamClick (Reply 693):
I was thinking along this line too then I realized something. Without getting into equations that never "prove" anything unless one is already predisposed to believe, let me 'splain.

OK, I'm going to have to think about that. Thanks ever so much.  Sad

In the meantime...

Option 1 was supposed to be the classic scenario, where the belt speed is equal and opposite to the aircraft's speed over the earth's surface. It could involve the belt operator simply viewing a remote readout of the aircraft's speed and turning a dial to set the belt speed accordingly. The speed doesn't have to match exactly at every moment - just enough for some to fall for the old "belt speed has to be subtracted from aircraft speed" trap.

Similarly, Option 2 doesn't have to be exact - only close enough for some to fall for the old "wheel rotation speed determines whether or not the aircraft will fly" trap.  Smile

Quoting Tdscanuck (Reply 697):
We don't know how the conveyor does this, but it's a condition of the problem so we have to accept it.

Hence the reason we split it into three options, 1 and 2 for those who wanted to discuss related, but more "realistic", issues and 3 for those who wanted to discuss the philosophical conundrum of the actual wording of the thread starter. I'm not suggesting you didn't know that but, clearly, many didn't.  Smile

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