How Airplanes Fly!
 It's natural to see a bird in the sky. But sometimes it's hard to believe that a huge chunk of metal like the Air Bus can float through the air! It all boils down to basic airplane aerodynamics. Newton's Three Laws of MotionSir Isaac Newton (1642-1727) was by far one of the greatest scientists in the history of the world. He was long gone before the invention of the airplane. That doesn't mean that he didn't play a key role in inventing the airplane! The three simple laws that he discovered control the motion of every object in the universe. In order to understand how airplanes fly we have to start with Newton's three laws of motion. First Law: Second Law: Third Law: Forces Acting on an AirplaneIn order to understand how airplanes fly we need to look at all of the forces that are acting it. According to Newton's first law if these forces aren't balanced the airplane will accelerate in the direction of the higher force. A force is basically a push or pull in a specific direction. Pressure acting on an object can be thought of as a bunch of small forces acting over certain area. Force = pressure x area There are four forces that are acting on an airplane at any given time. They are lift, weight, drag, and thrust. Refer to the diagram below as we learn about each force. 
LiftThe factors that determine lift are the shape of the airfoil, the surface area of the wing, the angle of attack, and the speed of the airplane. Airfoil Have you ever been cruising down the expressway with the windows down and all the sudden a loose plastic bag or piece of paper gets sucked out of the window? Since the air moving past the window is moving faster relative to the air inside the car, its pressure is much lower than the pressure inside the car. For this reason the bag is pulled to the low pressure and out the window it goes! What’s this have to do with how airplanes fly? Take a look at the cross section view of this airplane wing below. This cross section is referred to as an airfoil. When the air hits the leading edge (front of the wing), some of the air goes over the top of the wing and some of the air goes under the wing. 
The shortest distance between any two points is a straight line. You already know that! Take a close looks at the airfoil. You can see that the air going under the wing travels a shorter distance before it gets to the trailing edge (the back of the wing). The air going over the wing travels a longer distance before it makes it to the trailing edge. Both the air going over the wing and the air going under the wing go from the leading edge to the trailing edge in the exact same amount of time. The only way this is possible is if the air going over the wing is going faster than the air going under the wing. When this happens the air pressure on top of the wing becomes less than the air pressure on the bottom of the wing. Since force = pressure x area we have more force pushing up on the wing than we do force pushing down on the wing. As a result, we have lift! As you can see, the airplane wing design is critical for obtaining the desirable flight characteristics. The faster the air is moving the greater the pressure difference will be and the more lift the wing will produce. This is why the speed of the aircraft is critical. Are you wondering how airplanes fly upside down if this theory describes how airplanes fly? The shape of a wing's airfoil is a key factor that determines how much lift a wing will produce. However, the shape of the airfoil is only one factor that determines lift. Angle of Attack When the front of the wing is raised up it deflects the air passing by downward. According to Newton's third law the air must in turn be pushing the wing up. This is lift!
Take a look at the graph above. As the angle of attack increases the lift increases. But when the angle of attack gets too large the airplane will stall. A stall means that the airplane is no longer flying, it is falling! This can be bad in certain situations! WeightWeight is a force that pulls the airplane straight down. The weight is actually distributed throughout the entire airplane. However, the weight can be thought of as acting on the center of gravity. The center of gravity is the point at which the airplane balances both laterally and longitudinally. When the airplane is in flight it actually rotates about its center of gravity.DragWhen the airplane is flying it encounters air resistance. This air resistance is called drag. Drag can be thought of as a force pulling the airplane's tail backwards. The shape of the airplane, how fast the plane is moving, and the surface texture all contribute to drag.ThrustTo get off the ground the airplane must be moving fast enough for the air moving over the wings to produce lift. The force to move the airplane forward is called thrust. Thrust is needed to get the airplane airborne. Think of thrust as a force pulling the airplane towards its flight direction.Thrust is provided by the propulsion system. This could be a single engine propeller, multiple engine propeller, a turbine(jet) engine, or multiple turbine engines. ConclusionScroll up and take another look at Newton's first law. Since we are learning how airplanes fly we will focus on the second part of this law. Pertaining to an airplane, this means that the speed and direction of an airplane will not change unless the forces acting on it are unbalanced.If the drag outweighs the thrust then the forces acting on the airplane are unbalanced. The airplane will begin to slow down. Slowing down is simply negative acceleration. When the thrust is greater than the drag, the airplane will begin to accelerate. When the airplane accelerates to a sufficient speed, the airfoil and the angle of attack of the wing will produce more lift than the airplane weighs. When the force of lift is greater than the force of weight the airplane begins to fly! Now you know how airplanes fly!
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