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Before we dive into how wings keep airplanes up in the air, it is important that we take a quick 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. By increasing the lift, the pilot can make the airplane climb.
Thrust
Thrust is an aerodynamic force that must be created by an airplane in order to overcome the drag (notice that thrust and drag act in opposite directions in the figure above). Airplanes create thrust using propellers, jet engines or rockets. In the figure above, the thrust is being created with a propeller, which acts like a very powerful version of a household fan, pulling air past the blades.
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.
We see another example of drag reduction when we watch downhill skiers in the Olympics. You'll notice that, whenever they get the chance, they will squeeze down into a tight crouch. By making themselves "smaller," they decrease the drag they create, which allows them to move faster down the hill.
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
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).
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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.
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
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. These methods and others are discussed in more detail later in this article.
It is important to realize that, unlike in the two popular explanations described earlier, lift depends on significant contributions from both the top and bottom wing surfaces. While neither of these explanations is perfect, they both hold some nuggets of validity. Other explanations hold that the unequal pressure distributions cause the flow deflection, and still others state that the exact opposite is true. In either case, it is clear that this is not a subject that can be explained easily using simplified theories.
Likewise, predicting the amount of lift created by wings has been an equally challenging task for engineers and designers in the past. In fact, for years, we have relied heavily on experimental data collected 70 to 80 years ago to aid in our initial designs of wings.
Calculating Lift Based on Experimental Test Results
In 1915, the U.S. Congress created the National Advisory Committee on Aeronautics (NACA -- a precursor of NASA). During the 1920s and 1930s, NACA conducted extensive wind tunnel tests on hundreds of airfoil shapes (wing cross-sectional shapes). The data collected allows engineers to predictably calculate the amount of lift and drag that airfoils can develop in various flight conditions.
The lift coefficient of an airfoil is a number that relates its lift-producing capability to air speed, air density, wing area and angle of attack -- the angle at which the airfoil is oriented with respect to the oncoming air flow (we'll discuss
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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)
Air Force One
(Redirected from VC-25A)
Air Force One is the callsign of any United States Air Force aircraft carrying the President of the United States of America. The Air Force operates two VC-25A aircraft, tail numbers 28000 and SAM 29000, for this primary purpose. They are custom-configured versions of the civilian Boeing 747-2G4B. Before 28000 and 29000 entered service in 1990, two Boeing 707-320B-type aircraft, tail numbers 26000 and 27000, had been operated as Air Force One starting in 1958.
Air Force One
Larger imageThe custom modifications include interior reconfiguration for presidential duties: sleeping quarters, office areas, two kitchens, a medical operating table and pharmacy, communications systems, telephones and television sets, even workout rooms. There is also space for the president's family, staff and news media. The plane can also be operated as a millitary command center in the event of an incident such as a nuclear attack. Operational modifications include in-flight refueling capability and anti-aircraft missile countermeasures.
Air Force One flights are handled as military operations with all flights managed by the Presidential Airlift Group of the Air Mobility Command's 89th Airlift Wing at Andrews AFB in Maryland. The President often flies a US Marine Corps helicopter, callsign Marine One, between the Andrews AFB and the White House. Similarly, Army aircraft carrying the President bear the callsign Army One, and Navy aircraft are called Navy One. A civilian plane carrying the President gets the callsign Executive One, and a plane carrying a member of the first family will be called Executive One Foxtrot.
The callsigns were established for security purposes during the Dwight D. Eisenhower administration, after a commercial flight with the same callsign as a flight the President was on coincidentally entered the same airspace.
Both planes are only designated Air Force One while the President is onboard. In 1974, when Richard M. Nixon resigned the presidency and departed from Andrews AFB on Air Force One, it was arranged that the plane's callsign would switch from Air Force One to its SAM designation the moment Gerald Ford took the oath of office.
From its inception Air Force One has become a symbol of Presidential power and prestige, carrying the president on several diplomatic missions. It has also played a role in history. On November 22, 1963 SAM 26000 carried President John F. Kennedy to Dallas, Texas where he was assassinated. It was on the plane that Vice President Lyndon B. Johnson took the oath of office, and the plane carried Kennedy's remains back to Washington. SAM 26000 also carried president Nixon on his historic trip to mainland China
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