mark manda
Well Known Member
Anybody get into this debate within the last few weeks? My trumpet forum is having a real field day with this brain teaser. see below........
Conveyor-belt runway
trumpetherald.com Forum Index -> The Lounge
Location: Atlanta, GA
Posted: 13 Dec 2005 04:52 Post subject: Conveyor-belt runway
--------------------------------------------------------------------------------
OK! I've got another debate for you wanna-be engineers! Any other Boortz fans (or haters) will recognize this from last week. (Actually, the debate spilled over to this week, even!)
Sorry about the wording as I find a bit ambiguous but I'm going to faithfully reproduce the "riddle" as I found it:
Quote:
On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?
Enjoy!
Oh, let me be the first to say it will fly! (I think)
Absolutely it will fly.
The apparant ground speed (with respect to the moving runway) will be twice the forward speed (with respect to the still air) but it isn't ground speed that makes lift.
The plane will take off at exactly the same distance from the start of the roll that it would if the ground were at rest.
.
.
The key to this riddle is to realize that a planes forward motion isn't created by the wheels driving the plane forward (If it was, then in this instance the plane would stand still), but instead comes from a source that pushes against the air and thus can't be countered by the motion of the ground.
_________________
The only problem with trouble-shooting is that sometimes trouble shoots back.
Location: Clinton, MS
Posted: 13 Dec 2005 05:59 Post subject:
-
If the aircraft is sitting on a runway that is propelling it backwards at the same speed as the aircraft is going forward, how would it generate the necessary airspeed to provide lift from the wings? Interesting problem. Maybe I should have paid a little more attention in high school and college physics.
Location: Spokane/Everett WA
Posted: 13 Dec 2005 06:11 Post subject:
--------------------------------------------------------------------------------
No, the plane will not fly.... the lift is created by the air moving around the plane. The thrust from the engines just keeps the plane moving through the air fast enough for that air to hold the weight. If the plane is creating the thrust but the ground is moving opposite so that the plane isn't moving, the air is not moving to lift the plane. It's the same as if you put a plane's wheels on rollers. all that happens is the engines run and the wheels and rollers spin, there will be no gravity defying act.
Think about running on a treadmill. When you run outside, the forward movement causes wind resistance against you. If you were to harness that force and were shaped like a wing, you'd theoretically fly. But, as you run on a treadmill, you don't feel any wind resistance because you are not moving.
In the riddle, the conveyor belt does not move the air, it only moves along with the plane.
Location: Atlanta, GA
Posted: 13 Dec 2005 06:14 Post subject:
--------------------------------------------------------------------------------
Heh heh heh. ...and the fun begins! After the_lip's post, (nice job, btw), I thought it was going to be this thread that wasn't going to get off of the ground. But now we're gettin' somewhere!
_________________
John Ford
Conveyor-belt runway
trumpetherald.com Forum Index -> The Lounge
Location: Atlanta, GA
Posted: 13 Dec 2005 04:52 Post subject: Conveyor-belt runway
--------------------------------------------------------------------------------
OK! I've got another debate for you wanna-be engineers! Any other Boortz fans (or haters) will recognize this from last week. (Actually, the debate spilled over to this week, even!)
Sorry about the wording as I find a bit ambiguous but I'm going to faithfully reproduce the "riddle" as I found it:
Quote:
On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?
Enjoy!
Oh, let me be the first to say it will fly! (I think)
Absolutely it will fly.
The apparant ground speed (with respect to the moving runway) will be twice the forward speed (with respect to the still air) but it isn't ground speed that makes lift.
The plane will take off at exactly the same distance from the start of the roll that it would if the ground were at rest.
.
.
The key to this riddle is to realize that a planes forward motion isn't created by the wheels driving the plane forward (If it was, then in this instance the plane would stand still), but instead comes from a source that pushes against the air and thus can't be countered by the motion of the ground.
_________________
The only problem with trouble-shooting is that sometimes trouble shoots back.
Location: Clinton, MS
Posted: 13 Dec 2005 05:59 Post subject:
-
If the aircraft is sitting on a runway that is propelling it backwards at the same speed as the aircraft is going forward, how would it generate the necessary airspeed to provide lift from the wings? Interesting problem. Maybe I should have paid a little more attention in high school and college physics.
Location: Spokane/Everett WA
Posted: 13 Dec 2005 06:11 Post subject:
--------------------------------------------------------------------------------
No, the plane will not fly.... the lift is created by the air moving around the plane. The thrust from the engines just keeps the plane moving through the air fast enough for that air to hold the weight. If the plane is creating the thrust but the ground is moving opposite so that the plane isn't moving, the air is not moving to lift the plane. It's the same as if you put a plane's wheels on rollers. all that happens is the engines run and the wheels and rollers spin, there will be no gravity defying act.
Think about running on a treadmill. When you run outside, the forward movement causes wind resistance against you. If you were to harness that force and were shaped like a wing, you'd theoretically fly. But, as you run on a treadmill, you don't feel any wind resistance because you are not moving.
In the riddle, the conveyor belt does not move the air, it only moves along with the plane.
Location: Atlanta, GA
Posted: 13 Dec 2005 06:14 Post subject:
--------------------------------------------------------------------------------
Heh heh heh. ...and the fun begins! After the_lip's post, (nice job, btw), I thought it was going to be this thread that wasn't going to get off of the ground. But now we're gettin' somewhere!
_________________
John Ford