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Forces in Flight | |
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Forces in Flight The flight of an airplane, a bird, or any other object involves four forces that may be measured and compared: lift, drag, thrust, and weight. As can be seen in the figure below for straight and level flight, these four forces are distributed with the 1) lift force pointing upward; 2) weight pushing downward; 3) thrust pointing forward in the direction of flight; 4) and the drag force opposing the thrust. In order for the plane to fly, the lift force must be greater than or equal to the weight. The thrust force must be greater than or equal to the drag force. The terms and concepts that were defined earlier in this chapter can now be used to compute each of these forces.
Direction of Forces in Straight and Level Flight Weight The weight of the aircraft, as discussed earlier in the chapter, is a measure of a natural force that pulls the plane down towards the earth (gravity). Therefore, the direction assigned to the weight is downward. Lift The force that pushes an object up against the weight is lift. On an airplane or a bird, the lift is created by the movement of the air around the wings (the lift created by the body or tail is small). The figure below shows two streamlines about a typical airfoil (or wing); one travels over the top of the airfoil, the other moves underneath it.
If two particles were released from the same point at the same time, one on each streamline, they would start out moving together. As they approach the front of the airfoil, however, their velocity will start to change. Due to the shape of the airfoil, the air moves faster over the top of the airfoil than it does on the lower surface. The faster air leads to a lower pressure (from Bernoulli's Law) on the upper surface. A smaller force, on top, will be pointed downward, and a larger force (underneath) will be pointing upwards. When the two forces are combined, the net force is lift, which is directed upwards. The shape of the airfoil (wing) is a very important part of lift, and airplane designers design these shapes very carefully. Most airfoils today have camber, meaning they have curved upper surfaces and flatter lower surfaces. These airfoils generate lift even when the flow is horizontal (flat). The Wright brothers used symmetric airfoils to build the wings on their airplane. Since the upper and lower surfaces were the same, the particles on the streamlines above and below the symmetric airfoil move at the exact same velocity. The pressures on either surface (top or bottom) are exactly the same, so the net combined force on the airfoil is zero! No lift is generated by a symmetric airfoil in horizontal flow (flat wings moving straight ahead cannot fly). How, then, did the Wright brothers get their airplane off the ground? In order to generate lift with a symmetric airfoil, the airfoil must be turned (tilted) with respect to the flow, so that the upper surface is "lengthened" and the lower surface is "shortened". This "tilting against the airflow" is called angle of attack. It can be used for either cambered or symmetric wings. This is why an airplane rotates slightly at takeoff; the pilot is increasing the angle of attack to generate more lift. If the angle of attack is doubled, the lift doubles. There is a limit to how much lift can be generated, however. The angle of attack can be increased to a point where the net lift force drops drastically.
Airflow deflection is another way to explain lift. To understand the deflection of air by an airfoil let's apply Newton's Third Law of Motion. The airfoil deflects the air going over the upper surface downward as it leaves the trailing edge of the wing. According to Newton's Third Law, for every action there is an equal, but opposite reaction. Therefore, if the airfoil deflects the air down, the resulting opposite reaction is an upward push. Deflection is an important source of lift. Planes with flat wings, rather than cambered, or curved wings must tilt their wings to get deflection. Another way to increase lift on a wing is to extend the flaps downward. This again lengthens the upper surface and shortens the lower surface to generate more lift. The velocity of the freestream air (actually of the airplane) is the most important element in producing lift. If the velocity of the ariplane is increased, the lift will increase dramatically. If the velocity is doubled, the lift will be four times as large. The generation of lift can be found elsewhere. Race car designers use airfoil-like surfaces to generate negative lift, or downward-directed force. This force, combined with the weight of the race car, helps the driver maintain stability in the high-speed turns on the race track. Thrust Any force pushing an airplane (or bird) forward is called thrust. Thrust is generated by the engines of the airplane (or by the flapping of a bird's wings). The engines push fast moving air out behind the plane, by either propeller or jet. The fast moving air causes the plane to move forward. Drag The drag is the fourth of the major forces for flight. It is a resistance force. This force works to slow the forward motion of an object, including planes. There are four types of drag: friction drag, form drag, induced drag and wave drag. These drag types develop around the shape of the body, the smoothness of the surfaces, and the velocity of the plane. All four sum together for the total drag force. The drag forces are the opposite of thrust. If the thrust force is greater than the drag force, the plane goes forward, but if the drag force exceeds the thrust, the plane will slow down and stop. The friction drag is sometimes also called the skin friction drag. It is the friction force at the surfaces of the plane caused by the movement of air over the whole plane. If a person were to look at the furface of a wing, for example, he or she would see that all the sheets of metal join smoothly, and even the rivets are rounded over and are as flush with the surface as possible. This helps keep the friction drag at a minimum. The form drag, or pressure drag as it is sometimes called, is directly related to the shape of the body of the airplane. A smooth, streamlined shape will generate less form drag than a blunted or flat body. Any object that moves through a fluid (water/air) can get a decrease in form drag by streamlining. Automobiles are streamlined, which translates (allows) better gas mileage; there is less drag so less fuel is required to "push" the car forward. Buses, vans, and large trucks are less streamline, and this one reason why they use more fuel than smaller, streamlined cars (weight is another reason). Form drag is easy to demonstrate using a hand out the window of a moving car. If the hand is held flat, like a wing, it is a streamlined object. The person only feels a small tug or drag. If he or she turns the hand so that the palm is facing forward, the drag force is greatly increased, and the hand is pulled backwards! It is no longer streamlined. There are two additional drag forces, the induced drag and wave drag. Induced drag is sometimes called the drag due to lift. As the lift force is generated along a wing, a small amount of excess (lift) force can be generated in the opposite direction. This force acts like drag and slows the forward motion of the airplane. Aircraft designers try to design wings that lower induced drag. The last of the four types of drag is the wave drag. This generally only happens when the airplane is flying faster than the speed of sound. Wave drag is caused by the interactions of the shock waves over the surfaces and the pressure losses due to the shocks. Wave drag can also occur at transonic speeds, where the velocity of the air is already supersonic, locally. Since most commercial jets today fly at transonic speeds, wave drag is an important part of the total drag. Summary Every pilot knows and uses these four basic forces of flight. Aerobatic pilots are constantly balancing these forces to design amazing stunts to delight the crowds watching them. They will deliberately stall the wings of the airplane to cause the plane to lose lift and drop suddenly. They very carefully fly upside down, balancing the new lift force with the weight of the plane. They will point the airplane straught up into the air and fly staight up as far as they can, let the plane hang there for a second, then let it fall back down its original path. After a few heartbreaking seconds, the pilot will turn the airplane back so the nose points downward into the direction of the air flow to again regain level flight. These stunts are possible because the pilots carefully balance the forces of weight, lift, drag and thrust.
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