- My Forums
- Tiger Rant
- LSU Recruiting
- SEC Rant
- Saints Talk
- Pelicans Talk
- More Sports Board
- Fantasy Sports
- Golf Board
- Soccer Board
- O-T Lounge
- Tech Board
- Home/Garden Board
- Outdoor Board
- Health/Fitness Board
- Movie/TV Board
- Book Board
- Music Board
- Political Talk
- Money Talk
- Fark Board
- Gaming Board
- Travel Board
- Food/Drink Board
- Ticket Exchange
- TD Help Board
Customize My Forums- View All Forums
- Show Left Links
- Topic Sort Options
- Trending Topics
- Recent Topics
- Active Topics
Started By
Message
re: Myth Busters/Can a plane take off on a conveyor belt
Posted on 10/24/07 at 7:02 pm to ags01
Posted on 10/24/07 at 7:02 pm to ags01
Has anybody ever swam against a current? This is the same thing. As "takeoff power" is increased the plane will begin to move forward then the conveyor can be runup to counteract the plane's relative motion. This will actually slow the plane down until it comes to a stop. With no "relative wind" flowing over the wings there is no lift so the plane will not fly.
Has anybody ever freakin run on a treadmill? It doesn't matter how hard you run you don't feel any air in your face unless it's from the damn fan or AC. I'm surprised nobody thought of this.
BTW, I'm a pilot so don't question me.
Has anybody ever freakin run on a treadmill? It doesn't matter how hard you run you don't feel any air in your face unless it's from the damn fan or AC. I'm surprised nobody thought of this.
BTW, I'm a pilot so don't question me.
Posted on 10/24/07 at 7:07 pm to ags01
quote:
Is it possible to catch up with someone in freefall if that person jumps out a plane before you do?
Yes.
The first person falls until he hits the ground. When you hit the ground, you will have caught up with him.
Posted on 10/24/07 at 7:08 pm to Putty
quote:Not just um, yes. But HELL YES.
um, yes?
if the plane's at takeoff speed, it's at take off speed...it has enough forward speed and lift to take off.
the fact that the conveyor belt is "matching" the plane's ground speed doesn't mean that the plane is traveling slower, it just means that the wheels are spinning faster.
You can analyze this at several levels, and you'll have a plane taking off.
First, you can analyze it definitionally like Putty did. If the plane's at takeoff speed, it's at take off speed. Up, up, and awsy.
Second, you can analyze by forces. The plane's engines are exerting a force on the air behind the engines, NOT the earth or the conveyor belt. The conveyor is not going to move the air behind the plane backwards. Thus, the engines push off the air and moves forward. There is nothing to eliminate the force of the plane's engines.
Ah-ha you say, what about the conveyor belt? Unlike a car in gear, there is no direct linkage between the speed of the conveyor belt and the speed of the plane (with the slight exception of the friction of the bearings). The wheels are spinning free. The only linkage would be the external friction of the wheels with the conveyor belt and the internal friction of the wheel, i.e. the bearings.
The wheels have negligible external friction with the conveyor belt because they are wheels; they roll. The internal friction of the whee;s will not be enough to link the conveyor belt's speed to the plane's speed.
It's kinda like pulling the table cloth out from the dishes. There is not sufficient linkage between the cloth and dishes to overcome the inertia of the dishes. The cloth moves back, but the dishes stay in place. It wouldn't change if you put wheels on the bottom of the dishes.
And with a plane, unlike the dishes, you have more than just inertia, you have the thrust of the engines pushing on the air behind the engines. Again, the conveyor belt is not moving the air backwards. Thus, the conveyor belt cannot negate the force of the engines pushing the plane forward against that air.
If the bearings were completely locked up, it gets a little more complicated. However, it's still like the dishes to some extent. At some point even locked up wheels will start to skid across the conveyor belt. And you don't need the plane to do 100+ knots. The engines will be pushing air back at 100 knots against air that's already piling up at 100 knots.
Posted on 10/24/07 at 7:16 pm to LSUBoo
I think the only confusion here is what they mean by the use of the treadmill.
If the plane is sitting on a treadmill that is stationary, relative to the ground and the surrounding air, and the "treads" of the treadmill are spinning to match the spead of the turning wheels...
The plane's jet engines will push the plane off of the treadmill.
If the treadmill in question is as long as a runway, the plane will be propelled by its jet engines forward along the length of the treatmill until it reaches an "air speed velocity" sufficient to create the lift on the wings - it will fly. The motion of the wheels is immaterial. They are "free spinning". In fact the speed of the wheels will be equal to the speed of the treadmill, PLUS the forward speed of the plane, no matter how fast the treadmill is spinning. If the speed of the treadmill were lightspeed, the speed of the wheels would be lightspeed in the other direction, PLUS the forward speed of the plane.
(faster than the speed of light? - theoretically, yes.)
If the plane is somehow secured to the treadmill, like a jogger is secured to a treadmill by holding on to a handle, the wheels will be travelling at the SAME speed as the treads (although, since there is no force being exerted by the wheels, they will be stationary if the treadmill is simply designed to match the speed of the wheels), the plane will remain stationary and will not fly. There will be a huge force being exerted on the "handle" of that treadmill!
The bottom line, the plane will fly if it moves relative to the surrounding air. It won't fly if there is no air moving over the wings.
The wheels are immaterial (besides creating some negligible friction)
If the plane is sitting on a treadmill that is stationary, relative to the ground and the surrounding air, and the "treads" of the treadmill are spinning to match the spead of the turning wheels...
The plane's jet engines will push the plane off of the treadmill.
If the treadmill in question is as long as a runway, the plane will be propelled by its jet engines forward along the length of the treatmill until it reaches an "air speed velocity" sufficient to create the lift on the wings - it will fly. The motion of the wheels is immaterial. They are "free spinning". In fact the speed of the wheels will be equal to the speed of the treadmill, PLUS the forward speed of the plane, no matter how fast the treadmill is spinning. If the speed of the treadmill were lightspeed, the speed of the wheels would be lightspeed in the other direction, PLUS the forward speed of the plane.
(faster than the speed of light? - theoretically, yes.)
If the plane is somehow secured to the treadmill, like a jogger is secured to a treadmill by holding on to a handle, the wheels will be travelling at the SAME speed as the treads (although, since there is no force being exerted by the wheels, they will be stationary if the treadmill is simply designed to match the speed of the wheels), the plane will remain stationary and will not fly. There will be a huge force being exerted on the "handle" of that treadmill!
The bottom line, the plane will fly if it moves relative to the surrounding air. It won't fly if there is no air moving over the wings.
The wheels are immaterial (besides creating some negligible friction)
Posted on 10/24/07 at 7:20 pm to just me
quote:
The engines will be pushing air back at 100 knots against air that's already piling up at 100 knots.
But you haven't addressed the lift issue.
Posted on 10/24/07 at 7:22 pm to General
quote:No, to be the same thig, the air would have to be moving backwards. Then, the plane would not take off because the thrust from the engines would have nothing to push backwards against. But your analogy is useful. Imagine swimming in a river and the river bed is moving backwards, but the water in the river is not moving backwards. You still move forward because when you are swimming, you are pushing forward against the water, not the riverbed. Even if you drag you foot along the riverbed, you will still move forward as long as there is no linkage between the riverbed and you, i.e. there is little friction between your dragging foot and the riverbed.
Has anybody ever swam against a current? This is the same thing.
The real key is to remember two things. First, the plane's thrust is against the air behind it, not the conveyor belt. Second, the wheels are free spinning and can spin at the combined rate of the conveyor belt and the speed of the plane.
Posted on 10/24/07 at 7:33 pm to just me
quote:
First, the plane's thrust is against the air behind it, not the conveyor belt.
Popular misconception. A jet engine does not cause propulsion by pushing against the air behind it. A jet engine theoretically works in space but there is no air in space to be accelerated to cause the propulsion. In simpler terms: Air is not being pushed on, it is being speeded up. This change in speed is effectively a change in the air's momentum. The change in the air's momentum is balanced equally by a change in the plane's momentum in the oposite direction (Newton's Third Law). It's not a very easy concept to explain without higher mathematics.
quote:
Second, the wheels are free spinning and can spin at the combined rate of the conveyor belt and the speed of the plane.
The plane will lift if the wheels are going the speed of the treadmill PLUS take-off spead.
This post was edited on 10/24/07 at 7:40 pm
Posted on 10/24/07 at 7:36 pm to just me
Your looking to deep into this. In case you didn't know the preferred method to take off is INTO the wind. This slows the plane's "ground speed" but provides a better relative wind and a shorter takeoff roll. The engine can produce all the thrust it wants but if the conveyor is sped up to counteract the forward motion of the plane then there will be no forward motion and therefore no air flowing over the wings. This is assuming of course that the wind is calm.
Do an experiment: take a wind speed detector and a john boat. go to the mississippi river and launch said john boat. turn into the current (ie. conveyor) open that mug up to full power take your wind reading when you reach top forward speed (if you do). Then take said john boat to a calm lake and do the same thing. Think about it.
Do an experiment: take a wind speed detector and a john boat. go to the mississippi river and launch said john boat. turn into the current (ie. conveyor) open that mug up to full power take your wind reading when you reach top forward speed (if you do). Then take said john boat to a calm lake and do the same thing. Think about it.
Posted on 10/24/07 at 7:39 pm to just me
Naw dawg, it ain't happnin:
See those blue arrows? Those don't exist in our scenario of a plane essentially sitting "motionless" on a treadmill. The engines can haul as much arse as they want, but when you put the flaps/ailerons down they're hitting still air - not air going 100mph (or whatever take-off speed is). The end.
Boom I got your girlfriend
See those blue arrows? Those don't exist in our scenario of a plane essentially sitting "motionless" on a treadmill. The engines can haul as much arse as they want, but when you put the flaps/ailerons down they're hitting still air - not air going 100mph (or whatever take-off speed is). The end.
Boom I got your girlfriend
This post was edited on 10/24/07 at 7:40 pm
Posted on 10/24/07 at 7:41 pm to TigersRuleTheEarth
quote:Okay, let's let look at the relative speed of the plane. The plane will be going almost 100 knots relative to the earth. It won't be going exactly 100 knots because there will be some small amount of friction in the wheels to link the speed of the conveyor belt (in the opposite direction) to the plane. That friction is a force that is working opposite to the force exerted by the plane's engines. But it's negligible.
But you haven't addressed the lift issue.
Thus, the plane is still will travel at ALMOST 100 knots relative to the speed of the earth. That ALMOST 100 knots is also relative to the speed of the air. (The conveyor belt is not moving the air.) Thus, lift.
Now, if you start the conveyor belt at 100 knots before you start the engines. The plane is going -100 knots (vectorally) relative to the earth. The plane is also going -100 knots relative to the air. (The plane could take off backwards because it's going 100 knots relative to the air, but planes don't fly well backwards.) When you start the engines, the thrust of the engines starts pushing against air that is already piling up at 100 knots. You double the thrust.
But you're still not flying yet. However, the thrust of the engines reduces the negative speed of the plane and eventually starts the speed going positive (vectorally) relative to the earth. (-100 knots, -75 knots, -50 knots, -25 knots, 0 knots, 25 knots, 50 knots, 75 knots, ALMOST 100 knots relative to the earth.) Thus, you get lift.
Posted on 10/24/07 at 7:47 pm to Putty
quote:
um, yes?
if the plane's at takeoff speed, it's at take off speed...it has enough forward speed and lift to take off.
the fact that the conveyor belt is "matching" the plane's ground speed doesn't mean that the plane is traveling slower, it just means that the wheels are spinning faster.
Um no - it's about the wind over the wings and in the hypo the plane would not be moving hence no wind.
Posted on 10/24/07 at 7:48 pm to just me
Justme, your almost there. We are asumming that the conveyor can match whatever the plane does thus the relative motion will be zero knots. it doesn't matter what the engines are doing if the conveyor can match it.
Posted on 10/24/07 at 7:50 pm to just me
quote:
But you're still not flying yet. However, the thrust of the engines reduces the negative speed of the plane and eventually starts the speed going positive (vectorally) relative to the earth. (-100 knots, -75 knots, -50 knots, -25 knots, 0 knots, 25 knots, 50 knots, 75 knots, ALMOST 100 knots relative to the earth.) Thus, you get lift.
I see what you are saying. We are looking at the problem at two different ways. In your senario the plane will lift. The question is worded in a way that can be seen in two different ways.
Bottom line: If they are talking about getting the plane up to takeoff spead relative to the surrounding air, then lift. If they are talking about getting the plane up to takeoff spead relative to the treadmill, then no lift. **yes I am captian obvious**
Should be an interesting experiment to see. I wonder if they could run a few full scale tests?
This post was edited on 10/24/07 at 7:55 pm
Posted on 10/24/07 at 7:54 pm to TigersRuleTheEarth
Why is everyone assuming a jet? The hypo is "plane" and the guy participating in the experiment is in an "ultra-light flying machine".
The deal is that you have to have the proper windspeed over the wings on a conveyor belt going the same speed as the plane you get zero forward motion so no wind to lift the plane.
The deal is that you have to have the proper windspeed over the wings on a conveyor belt going the same speed as the plane you get zero forward motion so no wind to lift the plane.
Posted on 10/24/07 at 7:56 pm to Jimbeaux
quote:
"air speed velocity"
unless you're just quoting Monty Python, that's redundant
Posted on 10/24/07 at 7:57 pm to General
Im with just me on this.
Because the plane doesn't use the friction between the ground and its wheels to propel itself (like a car), the conveyor is immaterial.
The engines will propel the plane forward regardless of how fast the wheels are turning.
Because the plane doesn't use the friction between the ground and its wheels to propel itself (like a car), the conveyor is immaterial.
The engines will propel the plane forward regardless of how fast the wheels are turning.
Posted on 10/24/07 at 8:01 pm to General
quote:Wrong analogy. Your john boat is not like a plane; it's like a car. The john boat engine is pushing against the oppositely moving current, not the stationary air. There is almost complete linkage between the speed of the current and the speed of the john boat.
Do an experiment: take a wind speed detector and a john boat. go to the mississippi river and launch said john boat. turn into the current (ie. conveyor) open that mug up to full power take your wind reading when you reach top forward speed (if you do). Then take said john boat to a calm lake and do the same thing. Think about it.
The plane's engine is pushing against the stationary air, not the conveyor belt. There is almost no linkage, except for friction between the conveyor belt's speed and the speed of the plane.
The important thing in all of these analogies is what the the thing is pushing against. If you push against something that is moving in the opposite direction, the force is negated. You go nowhere relative to a fixed point on earth.
If you push against something that is stationary, and the only thing to negate that force is negligible friction, then you are moving relative to a fixed point on earth.
John boat in current pushes against something going in opposite direction. No movement relative to fixed point on earth.
Swimmer in stream pushes against something going in opposite direction. No movement relative to fixed point on earth.
Car on a conveyor belt pushes against something going in opposite direction. No movement relative to fixed point on earth.
John boat on calm lake pushes against water = something stationary. John boat moves relative to fixed point on earth.
Plane on conveyor belt pushes against air = something stationary. Plane moves relative to fixed point on earth.
Here's one that ought to really explain it. Car on the ground in a wind tunnel. The air is going 100 kots. The car's engine starts. The wheels start pushing against the ground = something stationary. The air only exerts negligible friction, a force in the opposite direction. The car moves relative to a fixed point on earth. The friction of the air cannot negate the thrust of the tires pushing against the ground.
This post was edited on 10/24/07 at 8:17 pm
Posted on 10/24/07 at 8:01 pm to JollyGreenGiant
well you would be wrong
Posted on 10/24/07 at 8:03 pm to cwill
quote:
The deal is that you have to have the proper windspeed over the wings on a conveyor belt going the same speed as the plane you get zero forward motion so no wind to lift the plane.
First part of you sentence is correct, but the second is not taking into consideration the fact that the wheels are free rolling. A car gets its propulsion from pushing against the ground while a plane (if jet propelled) gets its propulsion from the air itself - the wheel speed has nothing to do with the propulsion of the plane. The jet will just have to overcome the friction in the bearings and get enough power to get the plane up to take off.
If I understand the question correctly now, then JustMe is correct.
Posted on 10/24/07 at 8:03 pm to GaryMyMan
quote:Where is the thrust (air) from the engines going?
The end.
Boom I got your girlfriend
You never had her.
Page 2 of 29
Back to top
Follow TigerDroppings for LSU Football News