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|How Aircraft Fly|
How Aircraft Fly
The Bernoulli Principle
Airplanes fly when the movement of air across their wings creates an upward force on the wings (and thus the rest of the plane) that is greater than the force of gravity pulling the plane toward the earth.
The physics behind this phenomenon was first described by Daniel Bernoulli, an 18th century Swiss mathematician and scientist who studied the movement of fluids. Bernoulli discovered that the pressure exerted by a moving fluid is inversely proportional to the speed of the fluid. In other words, fluid pressure decreases as fluid speed increases, and vice versa.
The same principle applies to moving air. The faster that air moves through a space, the lower the air pressure; the slower it moves, the higher the pressure. Aircraft wings are designed to take advantage of that fact and create the lift force necessary to overcome the weight of the aircraft, and get airplanes off the ground. The undersides of wings are more or less flat, while their tops are curved. In addition, wings are slanted slightly downward from front to back, so air moving around a wing has a longer way to travel over the top than it does underneath. The air going over the top moves faster than the air going underneath, and the air pressure above the wing thus is lower than it is under the wing, where slower moving air molecules bunch together. The pressure differential creates lift, and the faster the wing moves through the air, the greater the lift becomes, eventually overcoming the force of gravity upon the aircraft.
The Phases of Flight
Push-Back and Taxi-Out
This first phase of flight, after all doors have been secured, involves the movement of the aircraft away from the terminal jetway and along taxiways to a runway. A motorized vehicle called a tug sometimes is used to push the aircraft back from its gate. At some airports, certain aircraft are permitted to power back. This means that following engine start at the gate, the thrust reversers are used to literally back the aircraft away from the gate. The aircraft then moves under its own power along the taxiways. Since aircraft are designed primarily for flight, and are not ground vehicles, they are taxied at very low speeds. Push-back occurs only when the pilot has clearance to do so from Air Traffic Control, which monitors all aircraft movements during taxi.
Takeoff and Climb
When ready for takeoff, and cleared by Air Traffic Control to proceed, the pilot or first officer of an aircraft releases the brakes and advances the throttle to increase engine power to accelerate down the runway. Once aligned on the runway, steering the aircraft is normally accomplished by using foot pedals that manipulate the nose wheel until the speed is sufficient enough that wind rushing by the rudder on the aircraft tail makes nose wheel steering unnecessary.
As the aircraft gains speed, air passes faster and faster over its wings and lift is created. Instruments onboard the aircraft display this airspeed, which equals not only the speed of the plane relative to the ground, but also the speed of any wind that may be blowing toward the aircraft (aircraft normally take off headed into the wind). When the airspeed reaches a certain predetermined point known as rotation speed, the pilot manipulates panels on the tail of the aircraft to rotate the nose of the plane upward. This creates even stronger lift and the plane leaves the ground.
Rotation speed, abbreviated VR, is one of three important airspeed settings calculated before every flight. The others are V1 - the speed beyond which a safe stop on a runway is no longer possible; and V2 - the minimum speed needed to keep a plane flying should an engine fail after the aircraft surpasses V1. Some of the factors affecting VR and V2 are the weight of the aircraft, the air temperature and the altitude of the airport. The heavier the aircraft, the more lift, and thus speed is needed to get it off the ground. Aircraft also need to go faster to fly on a hot day than on a cool day. Hot air is less dense than cool air and less density produces less lift for the same speed. Similarly, the higher the altitude, the less dense the air. Aircraft need more speed to leave the ground at a place like Denver than at a place like New York, with all other factors such as weight being equal. Some of these factors also are important in calculating V1, although the key factor is the length of the runway that is being used.
Most large jets leave the ground at about 160 miles per hour and initially climb at an angle in excess of 15 degrees. The angle of a plane's wings to the air flowing around them is extremely important to maintaining lift. If the so-called angle of attack is too severe, the flow of air around the wings becomes disrupted and the plane loses lift.
To make an aircraft more aerodynamically efficient, the wheels on which an aircraft rolls when it is on the ground are retracted into a cavity in the belly of the plane after it is airborne. There is less drag (wind resistance), and an aircraft can fly faster when its landing gear is retracted.
Once a plane is in the air, it continues to climb until it reaches its cruising altitude, which is determined by the pilot and must be approved by Air Traffic Control. At this point, power is reduced from the setting that was needed to climb, and the aircraft maintains a consistent, level altitude. To fly level, the weight of the aircraft and the lifting force generated by the wings are exactly equal.
There is no standard altitude for cruising. Generally, it is around 35,000 feet, but that can vary considerably depending on length of flight, weather conditions, air turbulence and the location of other planes in the sky. Cruising speeds are at a constant mach number, about 82 percent of the speed of sound. This translates to a groundspeed of about 550 miles per hour, although that too can vary considerably with headwinds, tailwinds and other factors.
During flight, pilots normally follow designated airways, or highways in the sky, that are marked on flight maps and are defined by their relationship to radio navigation beacons, whose signals are picked up by the aircraft. Some jets also have inertial navigation systems onboard to help pilots find their way. These computer-based systems calculate the plane's position from its point of departure, by closely tracking its heading, speed and other factors after it leaves the gate. Some aircraft also are capable of using signals from a constellation of satellites to pinpoint their position. This is known as the Global Positioning System. Commercial aircraft are increasingly using it. GPS enables aircraft to operate, with the permission of Air Traffic Control, to operate safely off predetermined airways. This capability makes for more efficient operations and adds capacity to the aviation system.
Pilots control and steer aircraft in flight by manipulating panels on the aircraft wings and tail. Those control surfaces are described in greater detail later in this chapter.
Descent and Landing
In this phase of a flight, the pilot gradually brings the aircraft back toward the ground, by reducing engine power and speed, and thus the force of the lift. The so-called final approach begins several miles from the airport. By this point, Air Traffic Control has put the aircraft in a sequence to land, carefully separating it from all other aircraft headed for, or leaving, the same airport. The landing gear is lowered, slowing the plane further. In addition, panels at the trailing edge of the aircraft's wings, known as flaps, are manipulated to increase drag and thus reduce speed and altitude. Other panels, known as elevators, and the rudder are used (as they are throughout the flight) to steer the plane and keep it on the localizer (heading) and glideslope (glidepath), the continuous radio signals the flight crew will follow to the end of the runway.
Airline aircraft generally are traveling at about 120 miles per hour relative to the ground when they touch down. The flight crew then slows the aircraft quickly with several actions: pulling back on the throttles, raising yet another set of panels on the top of the wings, called spoilers, that disrupt airflow and increase wind resistance, reversing the thrust of the engines, and, of course, applying the brakes.
Taxi-In and Parking
The final phase of a flight is a reverse of the first phase. The aircraft is driven at slow speed under its own power onto the taxiway and from there to a gate. Since most gates are equipped with moveable jetways, or covered ramps, aircraft generally are parked under their own power.
Major Parts of an Aircraft
This is the main body of an aircraft, exclusive of its tail assembly, wings and engines. The term derives from a French word, fusele, meaning tapered, because the fuselage is the shape of a long cylinder with tapered ends. It is made of aluminum sections that are riveted together, and inside are three primary sections: the cockpit, the cabin (which often is subdivided into two or three sections with different seating arrangements and different classes of service) and the cargo hold.
The cockpit is the most forward part of the fuselage and contains all the instruments needed to fly the plane. Sometimes referred to as the flight deck, the cockpit has seats for the pilot and co-pilot; a flight engineer, on some planes; and seats for one or two observers that could be from the airline itself, or from FAA. The cockpit is off limits to passengers during flight and to flight attendants during takeoffs and landings.
The cabin is the section of the fuselage behind (and below in the case of the double-deck Boeing 747) the cockpit, where an airline carries passengers, freight, or both, in the case of a combination carrier. A typical passenger cabin has galleys for food preparation; lavatories; one or more seating compartments, closets and overhead bins, for stowing baggage, coats, and other things carried onto the plane by passengers; and several doors to the outside, most of which are used only for emergency evacuations. The number of exits is determined by the number of seats. Small jets carry about 60 passengers, the larger ones like the Boeing 747 can carry more than 400.
This is the area of the fuselage below the passenger deck where cargo and baggage are carried. It is basically the lower half of the fuselage cylinder. It is pressurized, along with the rest of the fuselage, and has heating systems for areas designated for the carriage of live animals. Aircraft also have ventilation systems that force air into these areas. Access to the cargo holds is through doors in the belly of the aircraft. There is no access from the cabin area.
The wings are the airfoil that generates the lift necessary to get and keep, an aircraft off the ground. Like the fuselage to which they are attached, they are made of aluminum alloy panels riveted together. The point of attachment is the aircraft's center of gravity, or balance point.
Most jet aircraft have swept wings, meaning the wings are angled back toward the rear of the plane. Swept wings produce less lift than perpendicular wings, but they are more efficient at high speeds because they create less drag.
Wings are mostly hollow inside, with large compartments for fuel. On most of the aircraft in service today, the wings also support the engines, which are attached to pylons hung beneath the wings.
Wings are designed and constructed with meticulous attention to shape, contour, length, width and depth, and they are fitted with many different kinds of control surfaces, which are described below.
The empennage is the tail assembly of an aircraft, consisting of large fins that extend both vertically and horizontally from the rear of the fuselage. Their primary purpose is to help stabilize the aircraft, much like the keel of a boat. In addition, they also have control surfaces built into them that help the pilots steer the aircraft.
The control surfaces attached to an aircraft's wings and tail alter the equilibrium of straight and level flight when moved up and down or left and right. They are manipulated from controls in the cockpit. In some planes, hydraulic lines connect the cockpit controls with these various exterior panels. In others, the connection is electronic.
The rudder is a large panel attached to the trailing edge of a plane's vertical stabilizer in the rear of the plane. It is used to control yaw, which is the movement of the nose left or right. The rudder is used mostly during takeoffs and landings to keep the nose of an aircraft on the centerline of the runway. It is manipulated via foot pedals in the cockpit. Jet aircraft also have automatic yaw dampers that function at all times, to ensure a comfortable ride.
The elevators are panels attached to the trailing edge of an aircraft's two horizontal stabilizers, also part of the tail assembly, or empennage. The elevators control the pitch of an aircraft, which is the movement of the nose up or down. They are used during flight and are manipulated by pulling or pushing on the control wheel or side-stick controller in the cockpit.
The ailerons are panels built into the trailing edge of the wings. Like the elevators, they are used during flight to steer an aircraft and are manipulated by turning the control wheel or side-stick controller in the cockpit to the left or right. These steering motions deflect the ailerons up or down, which in turn affect the relative lift of the wings. An aileron deflected down increases the lift of the wing to which it is attached, while an aileron deflected up decreases the lift of its wing. Thus, if a pilot deflects downward the aileron on the left wing of the aircraft, and defects upward the aileron on the right wing, the aircraft will roll, or bank, to the right. Spoilers are panels built into the top surfaces of the wings and mostly are used during landings to spoil the lift of the wings and thus keep the aircraft firmly planted on the ground once it touches down. They also can be used during flight to expedite a descent.
The other major control surfaces are the flaps and slats, both designed primarily to increase the lift of the wings at the slow speeds used during takeoffs and landings. Flaps are mounted on the trailing edge of the wings, slats on the leading edge. When extended, they increase lift because they make the surface area of the wings larger and accentuate the curve of the wings. Flaps also are commonly deployed during final approach to increase lift, which provides control and stability at slower speeds. Flap and slat settings are controlled by the pilots, although automatic extension/retraction systems are sometimes provided to protect flight and structural integrity.
The landing gear is the undercarriage assembly that supports an aircraft when it is on the ground and consists of wheels, tires, brakes, shocks, axles and other support structures. Virtually all jet aircraft have a nose wheel with two tires, plus two or more main gear assemblies with as many as 16 tires. The landing gear is usually raised and lowered hydraulically and fits completely within the lower fuselage when retracted. Aircraft tires are filled with nitrogen rather than air because nitrogen does not expand or contract as much as air during extreme temperature changes, thus reducing the chances of a tire blowout.
The exact number of engines on an airplane is determined by the power and performance requirements of the aircraft. Most jet airplanes have two, three or four engines, depending on aircraft size. Some have the engines attached to the rear of the fuselage. Many have them mounted on pylons, hanging below the wings. Some have a combination of both, with an engine under each wing and one on top of the fuselage at the rear of the plane.
The power produced by the engines is controlled by the pilots, either directly or indirectly, through computerized controls. All large airliners are designed to fly safely on fewer than all engines. In other words, the remaining engine or engines have enough power to keep the aircraft airborne.
As mentioned above, some form of propulsion is required to move an aircraft through the air and generate sufficient lift for it to fly. The earliest forms of propulsion were simple gasoline engines that turned propellers. Most modern airliners are equipped with jet engines, which are more powerful and mechanically simpler and more reliable than piston engines. Jet engines first entered commercial service in the late 1950s and were in widespread use by the mid-1960s.
A jet engine takes in air at the front, compresses it into smaller and smaller spaces, by pulling it through a series of compressor blades. Then fuel is added to the hot, compressed air and ignites the mixture in a combustion chamber. This produces an explosion of extremely hot gases out the rear of the engine and creates a force known as thrust, which propels the engine (and thus the aircraft) forward. It is the same principle that propels a balloon forward when blown up with air and released. The air escaping from both a balloon and a jet engine creates a pressure differential between the front and back of the enclosed space that results in forward movement. Importantly, as the hot gases explode out the back of a jet, they turn a wheel known as a turbine. The turbine is connected by a center shaft to the compressor blades at the front of the engine and thus keeps the compressor spinning while the engine is on.
As with all combustion engines, power is increased by adding more fuel to the combustion chamber. Today's most powerful jet engines can produce more than 90,000 pounds of thrust. Expressed another way, each of these giant engines can lift 90,000 pounds straight up off the ground. Since aircraft rely on their wings for vertical lift and engines only for horizontal movement, these large engines can lift enormous amounts of weight off the ground and power aircraft at great speeds.
Types of Jets
There are three basic types of jet engines. Turbojets are engines that use exhaust thrust alone to propel an aircraft forward, as just described.
Turbofans, or fanjets, are an improved version of the turbojet. With a larger fan at the front, the turbofan pulls in more air. It also diverts some of the incoming air around the combustion chamber and later mixes it with the hot exhaust gases escaping out the back. This lowers the temperature and speed of the exhaust, increasing thrust at lower speeds and making the engine quieter.
The third type is the turboprop, or propjet. It uses a jet engine to turn a propeller. Thrust is generated by both the propeller and the exhaust gases of the jet itself. Turboprops are used on small, short-range aircraft such as those often operated by commuter and regional airlines. They are efficient in these types of operations, but less so at the high speeds and high altitudes flown by the large commercial jets.
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