Author Topic: AWOT is stupid today, lets see if this crowd is any smarter  (Read 4570 times)

0 Members and 1 Guest are viewing this topic.

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« on: November 23, 2005, 02:55:58 PM »
debate question of the day:

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?

lets see if you guys can get this right.
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

PasSoup

  • Lightly Modded
  • **
  • Posts: 379
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #1 on: November 23, 2005, 03:29:23 PM »
If I get the question correctly, the plane is not moving, therefore the would be no lift created by air over the wings.  So assuming this is not some stupid trick question, the answer would be no.  This is just like increasing the speed on a treadmill. You don't actually move.
Jonathon

1999 Passat 30V V6
2005 Jeep Liberty Sport 3.7L V6

Rocky Mountain Club B5

AWOT is stupid today, lets see if this crowd is any smarter
« Reply #1 on: November 23, 2005, 03:29:23 PM »

Eric18T

  • Heavily Modded
  • ****
  • Posts: 1,668
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #2 on: November 23, 2005, 03:29:52 PM »
sure, assuming the ammount of thrust created can overcome the gravitational forces.

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #3 on: November 23, 2005, 03:38:25 PM »
Quote from: "PasSoup"
If I get the question correctly, the plane is not moving, therefore the would be no lift created by air over the wings.  So assuming this is not some stupid trick question, the answer would be no.  This is just like increasing the speed on a treadmill. You don't actually move.


bzzzt.  you are as confused as the rest of AWOT is.   this isn't comparable to walking or driving on a treadmill.  in both of those the force to move you forward is transmitted to the ground through the wheels, (or legs).  Thus when walking or driving your thrust is offset by the reverse direction of the treadmill.  But with a plane the thrust is transmitted through the air not the treadmill, it won't matter what the relatively frictionless wheels are doing underneath the plane.  saying it won't take off would be like saying the plane would not be able to take off on a frictionless ice surface.
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #4 on: November 23, 2005, 03:40:11 PM »
Quote from: "Eric18T"
sure, assuming the ammount of thrust created can overcome the gravitational forces.


the thrust has to move the plane (which it will) enough to create airflow over the wings, which will create the lift to overcome gravity.
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

PasSoup

  • Lightly Modded
  • **
  • Posts: 379
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #5 on: November 23, 2005, 03:43:43 PM »
jackass
Jonathon

1999 Passat 30V V6
2005 Jeep Liberty Sport 3.7L V6

Eric18T

  • Heavily Modded
  • ****
  • Posts: 1,668
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #6 on: November 23, 2005, 03:48:46 PM »
so i was sorta right.

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #7 on: November 23, 2005, 03:55:09 PM »
Quote from: "PasSoup"
jackass


LOL.  sorry man.  I actually had the same initial reaction you did, then only after some reflection did I come up with the other answer. Most of the people posting on AWOT are comming up with the answer you did, but they won't let it go. Arguning and arguing that the plane will not take off.   it is kind of funny and kind of frustrating to watch at the same time.   8O
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

Lila

  • Lightly Modded
  • **
  • Posts: 246
    • http://
ok, I'll bite...
« Reply #8 on: November 23, 2005, 04:00:44 PM »
I'm probably a sucker for trying to answer this, but here goes:

Airplanes (normal, regular, fixed wing airplanes) use Bourneulis (can't spell) principle to achieve liftoff.  That is, the air flow around the wing is moving faster above the wing than below, and this differential in air speed created a difference in air pressure (less pressure above the wing) creates lift.

Ok, so if the airplane on the conveyer belt has a propeller or whatever creating air flow over the wing, the plane will lift off.  If the plane is a jet, and all of the air flow is behind the wing, it's doubtful that the plane will lift off.  

Bottom line:  the plane's movement relative to the earth (not moving in the given brain twister) is totally irrelevant.  But, the plane's wing location relative to the air flow around it is relevant --- if there's enough air flow, the plane will lift up.

Right?

hotani

  • Heavily Modded
  • ****
  • Posts: 1,954
    • G+
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #9 on: November 23, 2005, 04:12:20 PM »
ok, I was with pasSoup till I read the explanation. Yeah, if the plane was moving itself via wheels (hey, we're car people of course we just assume the wheels are moving it!), then it would remain stationary. But in reality the jet engines provide the inertia, only difference is that the wheels are spinning really frikkin fast now!

tricky tricky....

Which brings up a sub-question... if it really was a car instead (that moves itself with its wheels), would the belt ever move? The belt responds to forward movement of the vehicle, so in order for it to move at all, the car would have to move forward, but if the car were to move forward it wouldn't really move forward because the belt would instantly stop that forward movement thereby defeating the movement alltogether, so did the car ever move? Would the wheels ever turn?

Which is it young feller, you want I should freeze or get down on the floor? Coz if'n I freeze, I can't very well drop, and if'n I drop, I'ma gonna be in motion!
Quick! Everyone out of the universe!
New: 2006 A4 Avant
Old: 2001 A4 1.8TQM

gragravar

  • Supercar
  • *****
  • Posts: 4,315
Re: ok, I'll bite...
« Reply #10 on: November 23, 2005, 04:13:07 PM »
Quote from: "Lila"
I'm probably a sucker for trying to answer this, but here goes:

Airplanes (normal, regular, fixed wing airplanes) use Bourneulis (can't spell) principle to achieve liftoff.  That is, the air flow around the wing is moving faster above the wing than below, and this differential in air speed created a difference in air pressure (less pressure above the wing) creates lift.

Ok, so if the airplane on the conveyer belt has a propeller or whatever creating air flow over the wing, the plane will lift off.  If the plane is a jet, and all of the air flow is behind the wing, it's doubtful that the plane will lift off.  

Bottom line:  the plane's movement relative to the earth (not moving in the given brain twister) is totally irrelevant.  But, the plane's wing location relative to the air flow around it is relevant --- if there's enough air flow, the plane will lift up.

Right?


nope.  you are still assuming that the plane will not move because of the treadmill and is relying on the engines for the airflow over the wing.   that is where you are taking a  wrong turn.  the force generated by the rearward motion of the treadmill will have a negligible effect on the plane itself since the wheels are not driven.  the treadmill will merely cause wheel spin.  

The only horizontal force to oppose the thrust the plane makes on the air is the tiny amount of friction in the turning wheels. So the plane still accelerates forward. It accelerates until there is air flow over the wings at which point there is a vertical force of lift coming into play.
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #11 on: November 23, 2005, 04:15:59 PM »
Quote from: "hotani"
ok, I was with pasSoup till I read the explanation. Yeah, if the plane was moving itself via wheels (hey, we're car people of course we just assume the wheels are moving it!), then it would remain stationary. But in reality the jet engines provide the inertia, only difference is that the wheels are spinning really frikkin fast now!

tricky tricky....


yep!
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

Lila

  • Lightly Modded
  • **
  • Posts: 246
    • http://
additional note of explanation of my explanation...
« Reply #12 on: November 23, 2005, 04:17:58 PM »
Just to be clear here (I just re-read my post), if the propeller or jet engine is actually causing significant air flow across the wing, and that air flow is smooth and has a bunch of other "appropriate" conditions, the plane will lift off the conveyer belt.

But, many times an aircraft architect does not place the thrust from the engine near the wings (like on the elevator, away from the wings) in order to keep the turbulent air flow away from the wing.  The thrust from the engine in this case causes the wing (and the whole plane) to move forward relative to the air around it, and that air flow across the wing causes lift.  The idea here is to have smooth air flow across the wing.

Again, in short, as with most things in life, the correct answer "depends."

hotani

  • Heavily Modded
  • ****
  • Posts: 1,954
    • G+
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #13 on: November 23, 2005, 04:21:57 PM »
I actually went through all this once while waiting 7 hours one snowy evening in Detroit for a flight to Japan. I kept thinking about the icy runway and eventually realized that after a certain speed, it didn't really matter how slick it was because the wheels were just there for support at that point. Again, forward movement was provided by the jet engines, not the wheels. Which is probably why 747s don't need snow tires. :lol:
Quick! Everyone out of the universe!
New: 2006 A4 Avant
Old: 2001 A4 1.8TQM

PasSoup

  • Lightly Modded
  • **
  • Posts: 379
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #14 on: November 23, 2005, 04:25:28 PM »
The thrust is generated by the plane's engines, which while not in flight must overcome the friction and rolling resistence of the plane's wheels.

This question has to many unknowns to correctly answer, and make to many assumptions about what physical properties are present which are not.
Jonathon

1999 Passat 30V V6
2005 Jeep Liberty Sport 3.7L V6

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #15 on: November 23, 2005, 04:37:12 PM »
Quote from: "PasSoup"
The thrust is generated by the plane's engines, which while not in flight must overcome the friction and rolling resistence of the plane's wheels.

This question has to many unknowns to correctly answer, and make to many assumptions about what physical properties are present which are not.


you are right, but don't make it too hard.  yes, there is more thrust required for takeoff., but that additional thrust is equal to the force in the oposite direction which is the friction and rolling resistance of the wheels.   Assuming the wheels are functioning properly (nothing in the question to indicate otherwise) those forces should be small compared to the thrust forces of the plane's engines. (another assumption, I know, but I think a rational one)  the plane should still be able to move itself forward enough to generate lift and take off.
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #16 on: November 23, 2005, 04:43:54 PM »
one more analogy to try to help here,

lets put this back in car terms since we always seem to relate to that better. picture the car back on the dyno. we will all agree that if wheel speed equals roller speed, the car isn't going anywhere. now mount a winch on the front of the car and run the cable to the wall in front of the car. turn on the winch. even if the dyno spins faster, the car is going to move toward the wall.

the plane's thrust doesn't happen through the wheels, just like the winch line doesn't.  just like the car moves forward, so will the plane.
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

PasSoup

  • Lightly Modded
  • **
  • Posts: 379
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #17 on: November 23, 2005, 04:45:04 PM »
Ha, Ha I got this question to go two pages on PWOT.  They are still wrapped op on the tires turning, and the plane not moving.
Jonathon

1999 Passat 30V V6
2005 Jeep Liberty Sport 3.7L V6

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #18 on: November 23, 2005, 04:50:47 PM »
yep.  kind of funny to watch, huh?
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

hotani

  • Heavily Modded
  • ****
  • Posts: 1,954
    • G+
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #19 on: November 23, 2005, 04:55:49 PM »
Andrew, someone on the PW thread brought up the dyno thing saying the car wasn't moving and relating it to the plane. I responded similar to what you posted above (before reading that), but simply said "actually it's like dynoing the batmobile." :lol:
Quick! Everyone out of the universe!
New: 2006 A4 Avant
Old: 2001 A4 1.8TQM

PasSoup

  • Lightly Modded
  • **
  • Posts: 379
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #20 on: November 23, 2005, 05:02:42 PM »
A little free body diagram of the wheel.

Jonathon

1999 Passat 30V V6
2005 Jeep Liberty Sport 3.7L V6

SivNiz

  • Stock
  • *
  • Posts: 53
    • http://
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #21 on: November 23, 2005, 05:27:50 PM »
I love FBDs.  Where is it on passatworld(PW)?

EDIT: found it on PW, but I still love FBDs

hotani

  • Heavily Modded
  • ****
  • Posts: 1,954
    • G+
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #22 on: November 23, 2005, 05:31:09 PM »
Here is the original thread on AW - took me a LOOOONG time to find it actually! It was on page 10 or something, never realized how busy AWOT was. The PW thread is still going strong.
Quick! Everyone out of the universe!
New: 2006 A4 Avant
Old: 2001 A4 1.8TQM

SivNiz

  • Stock
  • *
  • Posts: 53
    • http://
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #23 on: November 23, 2005, 05:32:46 PM »
I still think the plane won't take off.  An airplane needs a pressure difference between the top part of the wing and the bottom part of the wing to lift-off from the ground.  No pressure difference can be generated if the plane is not moving.  If you look at a wing the top part is curved and the bottom part is relatively flat.  Air has to travel faster over the top half of the wing and slower on the bottom half.  Faster velocity = less pressure which will cause the plane to 'float' or lift off.

gragravar

  • Supercar
  • *****
  • Posts: 4,315
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #24 on: November 23, 2005, 05:36:49 PM »
Quote from: "SivNiz"
I still think the plane won't take off.  An airplane needs a pressure difference between the top part of the wing and the bottom part of the wing to lift-off from the ground.  No pressure difference can be generated if the plane is not moving.  If you look at a wing the top part is curved and the bottom part is relatively flat.  Air has to travel faster over the top half of the wing and slower on the bottom half.  Faster velocity = less pressure which will cause the plane to 'float' or lift off.


but why do you say the plane isn't moving?  it may not be moving relative to the conveyor which will just keep moving faster and faster, but it will be relative to the air, and to the stationary objects just off the conveyor.  go back to my example of the car on the dyno with the winch.  doesn't matter how ast the dyno spins, it won't keep the car still relative to the air, the wall, or anything else.
Any man who can drive safely while kissing a pretty girl is simply not giving the kiss the attention it deserves.  ~Albert Einstein

Lila

  • Lightly Modded
  • **
  • Posts: 246
    • http://
what is FBD ? n/m
« Reply #25 on: November 23, 2005, 05:37:08 PM »
what is FBD ?

SivNiz

  • Stock
  • *
  • Posts: 53
    • http://
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #26 on: November 23, 2005, 05:37:41 PM »
I have read some of the PW posts and is the plane moving with respec to the ground?

SivNiz

  • Stock
  • *
  • Posts: 53
    • http://
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #27 on: November 23, 2005, 05:40:56 PM »
Free Body Diagram (FBD).  I had a statics teacher who implanted this idea in my head.

Lila

  • Lightly Modded
  • **
  • Posts: 246
    • http://
airspeed relative to the wing is all that matters
« Reply #28 on: November 23, 2005, 05:45:13 PM »
Ok, if the plane moves forward, as I conceed it will move forward on this conveyer belt, there will be air flow across the wings, resulting in lift.

For the record (others have said this too), the plane's movement relative to the ground is irrelevant.  I have seen a glider land moving essentially perpendicular to the ground.  THis happened on a windy day.  The glider was able to slow its speed relative to the ground to zero, while maintaining lift based on wind speed.  Then the pilot made contact with the tarmack by moving down without moving forward (or backwards) relative to the ground.  It was a fun "trick" to observe.

SivNiz

  • Stock
  • *
  • Posts: 53
    • http://
AWOT is stupid today, lets see if this crowd is any smarter
« Reply #29 on: November 23, 2005, 05:45:40 PM »
"This conveyer has a control system that tracks the plane speed"

This to me sounds like the plane and the conveyer are moving at the same speed.  Assume the wheels are frictionless and massless.  The only force is the thrust.  So I guess it does take off.  Weird, I think I hurt my brain.

Good question!  :D


EDIT: there was no mention of wind so I'm assuming that there is none.