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Aerodynamic Forces Before we dive into how wings keep airplanes up in the air, it's important that we take a look at four basic aerodynamic forces: lift, weight, thrust and drag. Straight and Level Flight In order for an airplane to fly straight and level, the following relationships must be true: * Thrust = Drag * Lift = Weight If, for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down. If the thrust is increased so that it is greater than the drag, the plane will speed up. Similarly, if the amount of lift drops below the weight of the airplane, the plane will descend. Drag Drag is an aerodynamic force that resists the motion of an object moving through a fluid (air and water are both fluids). If you stick your hand out of a car window while moving, you will experience a very simple demonstration of this effect. The amount of drag that your hand creates depends on a few factors, such as the size of your hand, the speed of the car and the density of the air. If you were to slow down, you would notice that the drag on your hand would decrease. If you've ever wondered why, after takeoff, a passenger jet always retracts its landing gear (wheels) into the body of the airplane, the answer (as you may have already guessed) is to reduce drag. Just like the downhill skier, the pilot wants to make the aircraft as small as possible to reduce drag. The amount of drag produced by the landing gear of a jet is so great that, at cruising speeds, the gear would be ripped right off of the plane. Weight and Lift 747-400 Facts * Length: 232 feet (~ 71 meters) * Height: 63 feet (~ 19 meters) * Wingspan: 211 feet (~ 64 meters) * Wing area: 5,650 square feet (~ 525 square meters) * Max. takeoff weight: 870,000 pounds (~ 394,625 kilograms) * Max. landing weight: 630,000 pounds (~ 285,763 kilograms) (explains why planes may need to dump fuel for emergency landings) * Engines: four turbofan engines, 57,000 pounds of thrust each * Fuel capacity: up to 57,000 gallons (~ 215,768 liters) * Max. range: 7,200 nautical miles * Cruising speed: 490 knots * Takeoff distance: 10,500 feet (~ 3,200 meters) Weight This one is the easiest. Every object on earth has weight (including air). A 747 can weigh up to 870,000 pounds (that's 435 tons!) and still manage to get off the runway. (See the table below for more 747 specs.) Lift Lift is the aerodynamic force that holds an airplane in the air, and is probably the trickiest of the four aerodynamic forces to explain without using a lot of math. On airplanes, most of the lift required to keep the plane aloft is created by the wings (although some is created by other parts of the structure). A principal concept in aerodynamics is the idea that air is a fluid. Let's investigate that concept more closely. A Few Words About Fluid As we mentioned, a principal concept in aerodynamics is the idea that air is a fluid. Like all gases, air flows and behaves in a similar manner to water and other liquids. Even though air, water and pancake syrup may seem like very different substances, they all conform to the same set of mathematical relationships. In fact, basic aerodynamic tests are sometimes performed underwater. Another important concept is the fact that lift can exist only in the presence of a moving fluid. This is also true for drag. It doesn't matter if the object is stationary and the fluid is moving, or if the fluid is still and the object is moving through it. What really matters is the relative difference in speeds between the object and the fluid. Consequently, neither lift nor drag can be created in space (where there is no fluid). This explains why spacecraft don't have wings unless the spaceship spends at least some of its time in air. The space shuttle is a good example of a spacecraft that spends most of its time in space, where there is no air that can be used to create lift. However, when the shuttle re-enters the earth's atmosphere, its stubby wings produce enough lift to allow the shuttle to glide to a graceful landing. Popular (and Imperfect) Explanations of Lift Creation If you read any college-level aerodynamics textbook, you will find plenty of mathematical methods for calculating lift. Unfortunately, none of these explanations are particularly satisfying unless you have a Ph.D. in mathematics. There are many simplified explanations of lift that appear on the Internet and in some textbooks. Two of the most popular explanations today are the Longer Path explanation (also known as the Bernoulli or equal transit time explanation) and the Newtonian explanation (also known as the momentum transfer or air deflection explanation). While many versions of these explanations are fundamentally flawed, they can still contribute to an intuitive understanding of how lift is created. The Longer Path Explanation What is it? The Longer Path explanation holds that the top surface of a wing is more curved than the bottom surface. Air particles that approach the leading edge of the wing must travel either over or under the wing. Let's assume that two nearby particles split up at the leading edge, and then come back together at the trailing edge of the wing. Since the particle traveling over the top goes a longer distance in the same amount of time, it must be traveling faster. Bernoulli's equation, a fundamental of fluid dynamics, states that as the speed of a fluid flow increases, its pressure decreases. The Longer Path explanation deduces that this faster moving air develops a lower pressure on the top surface, while the slower moving air maintains a higher pressure on the bottom surface. This pressure difference essentially "sucks" the wing upward (or pushes the wing upward, depending on your point of view). How Lift is Created Pressure Variations Caused By Turning a Moving Fluid Lift is a force on a wing (or any other solid object) immersed in a moving fluid, and it acts perpendicular to the flow of the fluid. (Drag is the same thing, but acts parallel to the direction of the fluid flow). The net force is created by pressure differences brought about by variations in speed of the air at all points around the wing. These velocity variations are caused by the disruption and turning of the air flowing past the wing. The measured pressure distribution on a typical wing looks like the following diagram: A. Air approaching the top surface of the wing is compressed into the air above it as it moves upward. Then, as the top surface curves downward and away from the airstream, a low-pressure area is developed and the air above is pulled downward toward the back of the wing. B. Air approaching the bottom surface of the wing is slowed, compressed and redirected in a downward path. As the air nears the rear of the wing, its speed and pressure gradually match that of the air coming over the top. The overall pressure effects encountered on the bottom of the wing are generally less pronounced than those on the top of the wing. C. Lift component D. Net force E. Drag component When you sum up all the pressures acting on the wing (all the way around), you end up with a net force on the wing. A portion of this lift goes into lifting the wing (lift component), and the rest goes into slowing the wing down (drag component). As the amount of airflow turned by a given wing is increased, the speed and pressure differences between the top and bottom surfaces become more pronounced, and this increases the lift. There are many ways to increase the lift of a wing, such as increasing the angle of attack or increasing the speed of the airflow. Here is the standard equation for calculating lift using a lift coefficient: L = lift Cl = lift coefficient (rho) = air density V = air velocity A = wing area As an example, let's calculate the lift of an airplane with a wingspan of 40 feet and a chord length of 4 feet (wing area = 160 sq. ft.), moving at a speed of 100 mph (161 kph) at sea level (that's 147 feet, or 45 meters, per second!). Let's assume that the wing has a constant cross-section using an NACA 1408 airfoil shape, and that the plane is flying so that the angle of attack of the wing is 4 degrees. We know that: * A = 160 square feet * (rho) = 0.0023769 slugs / cubic foot (at sea level on a standard day) * V = 147 feet per second * Cl = 0.55 (lift coefficient for NACA 1408 airfoil at 4 degrees AOA) So let's calculate the lift: * Lift = 0.55 x .5 x .0023769 x 147 x 147 x 160 * Lift = 2,260 lbs Calculating Lift Using Computer Simulations Data was collected, engineers have used this information to calculate the lift (and other aerodynamic forces) produced by wings and other objects in fluid flows. Software packages, such as FLUENT, have been developed to create simulated fluid flows in which solid objects can be virtually immersed. The applications of this type of software range from simulating the air flowing over a wing, to mapping the airflow through a computer case to ensure that there is enough cool air passing over the CPU to prevent the computer from overheating. Interesting Things about Wings There are several interesting facts about wings that are useful in developing a more detailed understanding of how they work. Wing shape, the angle of attack, flaps, slats, rotating surfaces and blown surfaces are all important elements to consider. Let's start with wing shape. Wing Shape The "standard" airfoil shape that we examined above is not the only shape for a wing. For example, both stunt planes (the kind that fly upside down for extended periods of time at air shows) and supersonic aircraft have wing profiles that are somewhat different than you would expect: The upper airfoil is typical for a stunt plane, and the lower airfoil is typical for supersonic fighters. Note that both are symmetric on the top and bottom. Stunt planes and supersonic jets get their lift totally from the angle of attack of the wing. Angle of Attack The angle of attack is the angle that the wing presents to oncoming air, and it controls the thickness of the slice of air the wing is cutting off. Because it controls the slice, the angle of attack also controls the amount of lift that the wing generates (although it is not the only factor). Flaps In general, the wings on most planes are designed to provide an appropriate amount of lift (along with minimal drag) while the plane is operating in its cruising mode (about 560 miles per hour, or 901 km per hour, for the Boeing 747-400). However, when these airplanes are taking off or landing, their speeds can be reduced to less than 200 miles per hour (322 kph). This dramatic change in the wing's working conditions means that a different airfoil shape would probably better serve the aircraft. To accommodate both flight regimes (fast and high as well as slow and low), airplane wings have moveable sections called flaps. During takeoff and landing, the flaps are extended rearward and downward from the trailing edge of the wings. This effectively alters the shape of the wing, allowing the wing to turn more air, and thus create more lift. The downside of this alteration is that the drag on the wings also increases, so the flaps are put away for the rest of the flight. Slats Slats perform the same function as flaps (that is, they temporarily alter the shape of the wing to increase lift), but they are attached to the front of the wing instead of the rear. They are also deployed on takeoff and landing. Rotating Surfaces Given what we know so far about wings and lift, it seems logical that a simple cylinder would not produce any lift when immersed in a moving fluid (imagine a plane with wings shaped like cardboard paper-towel tubes). In a simplified world, the air would just flow around the cylinder evenly on both sides, and keep right on going. In reality, the downstream air would be a little turbulent and chaotic, but there still would be no lift created. However, if we were to begin rotating the cylinder, as in the figure below, the surface of the cylinder would actually drag the surrounding layer of air around with it. The net result would be a pressure difference between the top and bottom surfaces, which deflects the airflow downward. Newton's Third Law states that if the air is being redirected downward, the cylinder must be deflected upward (sounds like lift to me!). This is an example of the Magnus Effect (also known as the Robbins Effect), which holds true for rotating spheres as well as cylinders (see any similarities to curveballs here?) Blown Surfaces Let's take our cylindrical wing from the above examples and find another way to create lift with it. If you've ever held the back of your hand vertically under the faucet, you may have noticed that the water did not simply run down to the bottom of your hand and then drip off. Instead, the water actually runs back up and around the side of your hand (for a few millimeters) before falling into the sink. This is known as the Coanda Effect (after Henri Coanda), which states that a fluid will tend to follow the contour of a curved surface that it contacts. In our cylinder example, if air is forced out of a long slot just behind the top of the cylinder, it will wrap around the backside and pull some surrounding air with it. This is a very similar situation to the Magnus Effect, except that the cylinder doesn't have to spin. The Coanda Effect is used in specialized applications to increase the amount of additional lift provided by the flaps. Instead of just altering the shape of the wing, compressed air can be forced through long slots on the top of the wing or the flaps to produce extra lift. The Propeller Probably the most important parts of an airplane, after the wing, are the propeller and engine. The propeller (or, on jet aircraft, the jets) provides the thrust that moves the plane forward. (Check out How Gas Turbine Engines Work to learn about jet engines.) A propeller is really just a special, spinning wing. If you looked at the cross section of a propeller, you'd find that a propeller has an airfoil shape and an angle of attack. Just by looking at the propeller pictured above, you can see that the angle of attack changes along the length of the propeller -- the angle is greater toward the center because the speed of the propeller through the air is slower close to the hub. Many larger propeller aircraft have more elaborate three-blade or four-blade props with adjustable pitch mechanisms. These mechanisms let the pilot adjust the propeller's angle of attack depending on air speed and altitude. Horizontal and Vertical Stabilizers The tail of the airplane has two small wings, called the horizontal and vertical stabilizers, that the pilot uses to control the direction of the plane. Both are symmetrical airfoils, and both have large flaps on them that the pilot controls with the control stick to change their lift characteristics. Controlling the Direction The Main Wing and Flaps The plane's main wing is 40 feet (~ 12 m) long from end to end, and about 4 feet (~ 1.2 m) wide. On the inner portion of the wing, there are flaps used during takeoff, landing and other low-speed situations. On the outer ends, there are ailerons used to turn the plane and keep it level. Main wing Your browser does not support JavaScript or it is disabled. Flaps The flaps are actuated by electric motors in the wing. Also enclosed in the wings are two fuel tanks, each of which holds about 20 gallons of gas. Airplane Sensors From this description you can see that a plane has four different moveable control surfaces, as shown here: The plane also has two different sensors mounted on the wing: The L-shaped tube is called a pitot tube. Air that rams into this tube during flight creates pressure, and that pressure moves the needle on the air-speed indicator in the cockpit. The small opening on the right is a whistle that sounds as the wing nears a stall. The larger opening visible near the cockpit is used for ventilation. |