Originally posted by aginis i'm confused the first half of your post says that the plane will take off. In the second half you give the reason why a propeller isn't enough to create air movement under the wing and so create lift.
Now for my 2 cents. propellers/jets/other forms of propulsion create
ACCELERATION. Assuming that the conveyor belt is moving at a constant rate eventually ...[text shortened]... ment breaks out. I place the blame squarely at the feet of the original author of the problem.
The plane will take off exactly as if it were on a solid runway.
The propeller does not cause air to flow over and under the wings. At least not directly.
The propeller PULLS the aircraft through the air and creates THRUST in a forward direction. As the craft moves through the air, air is caused to pass the wing, and due to the shape of the wing lift is developed, which opposes the force of gravity. Once enough thrust and lift are developed, the plane will overcome gravity and fly. So actually the wing is passing through the air rather than the air passing the wing. Hope that makes sense.
A screw or propeller on a ship or boat does the same thing basically, which is to push the vessel through the water. If it were mounted on the front of the boat, it would pull it through the water the same as an aircraft propeller pulls the airplane through the air.
Originally posted by mwmiller The plane will take off exactly as if it were on a solid runway.
The propeller does not cause air to flow over and under the wings. At least not directly.
The propeller PULLS the aircraft through the air and creates THRUST in a forward direction. As the craft moves through the air, air is caused to pass the wing, and due to the shape of the wing lif ...[text shortened]... pull it through the water the same as an aircraft propeller pulls the airplane through the air.
Do not confuse THRUST and LIFT here, they are two completely diffeent forces.
THRUST is created by the propeller, creating forward momentum.
LIFT is created by the movement of air around the wings. The airfoil shape of the wing causes air to move faster over the wing, causing a lower pressure on top of the wing in relation to below the wing. Angle of attack determines how much lift is created for a given air speed.
There seems to be a misconception here that wind from the propeller causes lift on the wings. This is not correct. Although there is some propeller wash over the wings and elevator, it is not enough to create enough lift to fly.
Originally posted by mlprior Do not confuse THRUST and LIFT here, they are two completely diffeent forces.
THRUST is created by the propeller, creating forward momentum.
LIFT is created by the movement of air around the wings. The airfoil shape of the wing causes air to move faster over the wing, causing a lower pressure on top of the wing in relation to below the wing. Angle of ...[text shortened]... propeller wash over the wings and elevator, it is not enough to create enough lift to fly.
🙂
Sounds exactly like what I said. I'm certainly not confusing the two.
Originally posted by aginis i'm confused the first half of your post says that the plane will take off. In the second half you give the reason why a propeller isn't enough to create air movement under the wing and so create lift.
Now for my 2 cents. propellers/jets/other forms of propulsion create
ACCELERATION. Assuming that the conveyor belt is moving at a constant rate eventually ...[text shortened]... ment breaks out. I place the blame squarely at the feet of the original author of the problem.
My point was that the airstream the propeller pulls over the wings gives an inconsequential amount of lift.
The only job a propeller has is to pull the plane forward, as has been phrased, rephrased and paraphrased many times in this and other threads.
Propellers pull plane forward
Forward motion of plane makes air move over wing
Air moving over wing gives lift
Combination of forward motion and lift = flying
From above "If on the other hand the conveyor accelerates at the same rate as the plane then the plane will not move and its velocity will remain zero thus no lift is created."
This is totally wrong and betrays a fundamental misunderstanding of why flying works.
OK I have read all three posts and have swung with whom I agree. The reason for my confusion is in understanding the original question and the concepts of flying.
My initial understanding of the question was that if you were to place a flag beside the aeroplane and the "treadmill" runway was turning and the engines work just enough to keep the flag in the same place relative to the plane is there at any time a speed at which the treadmill will travel which would cause the plane to take off - the answer to this is no because of lack of air pressure change over wings - air stationary relative to flag and plane.
Now what I understand the question was asking did not include the flag because of how planes fly - if the belt is travelling at 100mph and the propulsion of the plane is enough to move the plane at 100mph in normal circumstances, after the initial friction the plane will move forward along the runway.
The plane is not stationary relative to the ground and therefore can take off when changes of air presure over wings allow.
I have no doubt this has been put far more eloquently however after initially believing the answer to be No I firmly believe the answer to be yes - and for my realistion I have Marsan to thank - good anology.
the propellor is creating thrust, but not lift. Air flow over the wings creates lift.
If you tied a plan to a pole with a rope, took the engines to full blast, and then let go of the rope, the plane wouldn't instantly take off, it would slowly gain speed moving forward and then take off.
Originally posted by Mat Kelley OK I have read all three posts and have swung with whom I agree. The reason for my confusion is in understanding the original question and the concepts of flying.
My initial understanding of the question was that if you were to place a flag beside the aeroplane and the "treadmill" runway was turning and the engines work just enough to keep the flag in ...[text shortened]... ly believe the answer to be yes - and for my realistion I have Marsan to thank - good anology.
No, the plane is stationary relative to the ground and surrounding air - it's only moving relative to the belt.
Originally posted by sugiezd No, the plane is stationary relative to the ground and surrounding air - it's only moving relative to the belt.
Thus no air-speed and no take off.
No matter how many times you say it you are still wrong. Here's a pretty picture.
http://img153.imageshack.us/img153/4073/plane01bfc6.jpg
The black force arrows are present in a normal takeoff. The red is when we have the belt in motion.
So how does the velocity of the belt give a force on the plane? Well it gives a force on the portion of the plane touching the belt only. That's the wheels. The wheels on a plane are free turning. They are not driving and they can spin in both directions. Therefore the plane and the belt are what is called decoupled. They would be completely decoupled if the wheels were frictionless. As the wheels have a small amount of friction they are partly decoupled. This means that a small force will act on the plane itself. This will be equal to a small percentage of the force that the belt can exert. This small force is easily overcome by the Thrust of the plane which means that the plane will move forwards through the air and thereby create lift which will eventually exceed weight and the plane will takeoff.
Originally posted by XanthosNZ No matter how many times you say it you are still wrong. Here's a pretty picture.
http://img153.imageshack.us/img153/4073/plane01bfc6.jpg
The black force arrows are present in a normal takeoff. The red is when we have the belt in motion.
So how does the velocity of the belt give a force on the plane? Well it gives a force on the portion of the plane ...[text shortened]... he air and thereby create lift which will eventually exceed weight and the plane will takeoff.
Love the picture.
Aren't you forgetting the plane's moment of inertia?
Originally posted by XanthosNZ So how does the velocity of the belt give a force on the plane? Well it gives a force on the portion of the plane he air and thereby create lift which will eventually exceed weight and the plane will takeoff.
I thought the original question stated the plane was not moving forward relative to the ground.
Ie if you had a flag on the runway, the plane remained at the same spot as the flag.
Aren't you forgetting the plane's moment of inertia?
Moment of inertia refers to object which are being made to rotate you are using this term out of context.
Yes i know the wheels rotate but that is completely irrelevant, they are their own little closed system, a horizontal force cannot be transmitted from the conveyor belt to the plane via the wheels because they spin freely. They decouple the plane from the conveyor belt (as has been said many many many times)
You can prove this for yourself.
Take a small toy car and push it along a plank of wood. easy.
Next push the car with one hand and simulate the action of the conveyor belt with the plank of wood, don't you see the movement of the wood does not and cannot affect the motion of the car? It is exactly the same as with the plane.
Originally posted by uzless I thought the original question stated the plane was not moving forward relative to the ground.
Ie if you had a flag on the runway, the plane remained at the same spot as the flag.
The original question stated that the plane started stationary relative to the air. This quickly changes thanks to forward thrust pushing the plane forward relative to the air.
Why the plane takes off in laymans (my) terms
30 Oct '06 23:01
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