How do planes fly?
 
 

The way an airplane flies may seem magical. How can something so big and so heavy fly. Several forces act on an airplane. A force is a push or a pull on something. You use force when you throw a ball, pull a wagon, or walk. When you walk, you push backward on the floor, and the resistance from the floor pushes you forward. Forces always work in pairs, on different bodies, in opposite directions, and at the same time.               Four main forces act on airplanes. Lift acts upward and allows the airplane to fly. The weight of the airplane acts downward. Thrust is the force that moves the airplane forward. The force that holds an airplane back is called drag. Lift is generated as a result of the special shape of an airplane's wing. The top of the wing is curved more than the bottom of the wing. The air that flows over the top of the wing moves faster than the air moves along the bottom. The air that is moving faster exerts less force than the slower moving air. Thus, the air pushes down on top of the wing less than the air pushes up on the bottom of the wing. The greater push upward is the lift of the wing. Lift holds an airplane up.    Lift acts in opposition to the weight of an airplane, which acts downward. Weight is the force generated by gravity. Weight always acts downward toward the Earth. For an airplane to fly, lift must equal weight. Weight holds an airplane down.  Thrust is the forward force that is produced by the pull of a propeller or the push jet exhaust. A propeller is a small rotating wing that is lifting forward. A jet engine forces exhaust gases out the back of the engine. These exhaust gases push the engine and thus, the airplane forward. Thrust makes an airplane go.  Drag is a force that slows an airplane down. You feel drag when you ride your bicycle. The air and the wind hold you back. Thrust equals drag for an airplane that is flying at a constant speed. Drag holds an airplane back.            So now we know how a plane flies.Right?. Then lets try to find an answer to this question. If the plane's flight is mainly due to the shape then how come planes can fly upside down?           Before trying to answer that.let's see 2 basic concepts 1.  Relative wind-  What relative wind means, is that the outcome is the same whether the airplane is moving through still air, or whether the air is moving over a stationary airplane. To understand this, think of a really windy day, when the winds get up to say twenty miles an hour. When the wind blows on you, you can feel it pushing you back. Now, if you're in a car, moving at twenty miles an hour, and stand up through the sunroof, you'll feel the same force pushing on you. So it did not matter whether you were moving, or whether the air was moving, as long as the wind was going past you at twenty miles an hour.  2. Concept of a vacuum- The technical definition of a vacuum is not a vacuum cleaner used to clean carpets, but rather an area where there is nothing. Vacuums are very hard to make. If you try to make one, air will fill it in right away. So basically, without special equipment, a vacuum will not exist.           Now think of an airplane wing. The simplest airplane wing that we could have is just a flat plate. This flat plate is analogous to the wall discussed above. If we could somehow create a pressure on the bottom of the wing that was greater than that on the top of the wing, the wing would be pushed up, and it would take the rest of the airplane with it. The pushing up of the wing is known as lift. Airplane wings do this by increasing the pressure on the bottom of the wing, and decreasing the pressure on the top of the wing. But how do they do this?             Airplane wings create lift by both increasing the pressure on the bottom of the wing, and decreasing the pressure on the top of the wing. The main way they accomplish this is by being mounted at some angle to the direction the airplane is moving. Now remember the concept of relative wind, if we imagine the airplane to be stationary, it is as if the wind is moving past the airplane at some angle. The diagram to the right shows what this will look like. The diagram is a cross section, where the thick black line is the wing, and the thin blue lines show the path of the air flowing over the wing. The air wants to keep flowing in a straight line, but the wing interferes with this. On the bottom side of the wing, the air is being pushed down. When the wing pushes it down, it pushes back up. This increases the pressure on the bottom side of the wing. Another way to envision this is to go back to our tennis ball analogy. If the tennis balls are the air, flying towards the wing, when they hit the bottom side of the wing, they will bounce off of it, increasing the pressure.  On the top of the wing, the air has to go down. This is due to the fact that was stated before, that a vacuum can't exist. If the air kept going straight, and nothing filled in between it and the wing, there would be a vacuum. Air would try to fill in this vacuum. The closest air to do it is the air flowing over the top of the wing, so it will just bend its path and flow right next to the wing. But, since it wants to be flowing straight over, instead of down and over, it will not be pushing as hard on the wing, so the pressure on the top of the wing will be less. So now the pressure beneath the wing has been increased, while the pressure above the wing has been decreased. The bottom of the wing pushes on the wing harder, so the wing is pushed up. This is the lift.  Concept of Stalling a Wing  In the above discussion, it was shown that increasing the angle of the wing would increase the lift it created. However, common sense tells us that this can't work forever. When the wing is straight up and down, we can see that it won't create any lift at all. The air would no longer be flowing over the wing. So the question is at what angle does the wing create the most lift, or at what angle does the air quit flowing over the wing? Well, the answer is a lot sooner than you might think. For a wing to work as discussed above, the air must bend its path to flow over the top of the wing. There comes a point when the angle is too great, and the air can't change its path enough to stick to the wing. It will keep on flowing straight, and other air will fill in the area right behind the wing. Remember that above it was said that it was the air flowing over the wing, but wanting to flow straight was what decreased the pressure on top of the wing. But now, the air on top of the wing is not trying to flow anywhere, so the pressure is greater. The pressure difference between the top of the wing and the bottom of the wing is much less than it was before, so the wing will create less lift. When the wing does this, it is known as stalling. The angle at which this occurs is about twenty degrees.   How Wing Shape Affects Lift  As was stated above, a wing quits producing lift at about twenty degrees. If you have designed an airplane, with flat plates for wings, and it needs more lift than what the wings can provide at twenty degrees, you must do something to change your wings. You could make them bigger, but that would take more material, which would increase the weight of your airplane, increase the drage of the airplane, and cost more. The other solution is to change the shape of your wing. This is what designers do. Wings for airplanes are usually shaped such that the top surface is curved, and the bottom surface is flat. But why does this cause an increase in lift?  An airplane wing can be thought of as the constriction in the tube. This is not exactly true, as the tube constricts the airflow on all sides, and an airplane wing only constricts the flow from one side, but it still does accelerate the airflow. The importance of this is that increasing the velocity of an airflow decreases the pressure. This is known as Bernoulli's Principle. This can be seen with a strip of paper. If you hold the paper up just below your mouth, then purse your lips and exhale, as was discussed above to increase the velocity of the air, you will see the paper lift up. This is because your blowing decreased the pressure above the paper, while the pressure below it stayed the same, causing the paper to move upwards. The curve on top of an airplane wing decreases the pressure, causing the wing to create lift even when the wing is not at an angle to the wind, and the pressure on the bottom of the wing is not increased. So, making an airplane wing curved is not what allows it to create lift, it just makes it better at creating lift.               For more information please go to these sites..... http://www.engr.umd.edu/~jeffl/aviation_theory.html http://observe.ivv.nasa.gov/nasa/exhibits/planes/planes_0.html http://k12unix.larc.nasa.gov/flyingstart/module1.html