A means of air transportation that is propelled by an internal combustion, turboprop, or jet engine.
Airplanes fly with the help of the laws of physics and engineering. They come in all shapes and sizes and serve different purposes. Some aircraft are used for training; others are used for transporting goods and freight. Military aircraft are used in waging warfare. Passenger airliners are used for the daily transportation of travelers.
Although airplanes have different designs and functions, all airplanes share common traits. The fuselage, or body of the aircraft, carries people, cargo, and baggage. Attached to the fuselage are the wings, which provide the lift to carry the aircraft and its payload. To balance the airplane in flight, the tail, or empennage, is very important. The landing gear allows the airplane to operate on the earth’s surface. The flight controls are used to maneuver the aircraft in flight. Flaps provide additional lift and drag for takeoffs and landings.
The primary job of the fuselage is to provide space for the flight crew and passengers. The attachment of the wings and other load-bearing structures is also an important function of the fuselage.
Depending on the size and function of the aircraft, the fuselage provides a safe haven for those inside the craft. For large airplanes that fly at high altitudes, these compartments are pressurized and air-conditioned. In smaller general aviation airplanes, the cockpits can be drafty, noisy, and either cold or hot, depending on the time of the year.
In airliners, seats are arranged to allow the greatest number of paying passengers to ride inside the fuselage. In older airliners that have been converted to cargo carriers, the fuselage is a cavernous hold without seats in the cabin.
Wings are as varied as other parts of the airplane. They come in different shapes and sizes, depending on the aircraft’s speed and weight requirements. A slower airplane may have a rectangular wing or a tapered wing. A rectangular wing is one in which the chord line, or cross section, of the wing, remains constant from the root of the wing near the body of the aircraft to the wingtip. A tapered wing is one that becomes narrower toward the tip. High-speed aircraft, such as jet transports, airliners, or fighter aircraft, have swept-wing designs. The purpose of the swept wing is to allow the airplane to fly at higher airspeeds.
The size of an airplane’s wing in relation to the airplane’s size is important. The larger the airplane, the bigger the wing must be to support it in flight. Many factors determine how the wings work in lifting an airplane.
The first factor is that of the wingspan. This is the distance from one wingtip to the other. Small general aviation airplanes typically have wingspans from 35 to 40 feet. Larger airplanes, such as the Boeing 747, have wingspans that easily exceed 100 feet.
The second factor is that of chord. The wing chord is the distance as measured from the leading edge of the wing, or front, to the trailing edge. In a rectangular wing, the chord is constant and, as such, is a constant-chord wing. On tapered, elliptical, or other odd-shaped wings, the chord is not constant. On these wings, the average chord, or mean aerodynamic chord (MAC), is required in equations dealing with the wing.
One important equation in aircraft wing design involves the load the wing will bear while in flight. Wing loading directly relates to the size, or the wing area, of the airplane wing. The first mathematical step in determining wing loading is to determine the wing area by multiplying the wingspan by the chord, or MAC.
After the wing area has been determined, the wing loading can be determined, using the weight of the airplane. The gross weight, or GW, is the operational weight of the airplane. To determine the wing loading of a particular aircraft, the weight of the airplane is divided by wing area. For the lightest of civilian airplanes, wing loading may reach values as low as 6 pounds per square foot, whereas a tactical jet bomber will have a wing loading of more than 375 pounds per square foot.
As the wing flies through the air, it does so at a particular angle. This angle, measured by the relationship between the relative wind and the chord line of the wing, is directly related to the speed of the aircraft. An airplane flying at high airspeeds will have a small angle of attack, whereas one flying slowly will have a large angle of attack.
The lift equation aptly expresses the relationship between the speed, angle of attack, and weight of the aircraft. An airplane’s lift must equal its weight in order for the airplane to remain in flight. Pilots are unable to change either the density of the air or the area of the wing. However, they are in control of the other two variables, the airplane’s speed and angle of attack.
Because lift must always equal weight in level flight, if the airplane slows down, the angle of attack must increase. Accordingly, an increase in speed will require a decrease in the angle of attack.
The empennage is the tail structure of the aircraft, which includes the vertical stabilizer and rudder, along with the horizontal stabilizers and elevator. These essential components provide stability for the airplane in flight.
The vertical stabilizer stands straight up, like a fin, from the aft portion of the airplane’s fuselage. It is important to the stability of the aircraft in that it helps the airplane track a straight path. The larger the vertical fin is in area, the more stable the aircraft is around the vertical axis.
Attached to the trailing edge of the vertical stabilizer is the rudder. By way of the pedals at the pilot’s feet, the rudder controls movement about the vertical axis of the airplane.
Acting in concert with the vertical stabilizer are the horizontal stabilizers. Located on each side of the fuselage and near the vertical stabilizer, they provide longitudinal stability to the airplane about the craft’s lateral axis. The combination of the horizontal stabilizers and elevators resembles the main wing in shape. However, the function of the horizontal stabilizers and elevators is totally different from that of the wing. Whereas the wing lifts up in force, the horizontal stabilizer provides a downward force that provides longitudinal stability to the airplane.
In order to move around on the earth’s surface, all aircraft have landing gear. The most common arrangement of the landing gear is the tricycle landing gear, in which the aircraft has two main wheels that extend from either the wing or the fuselage and a third wheel that extends from the nose of the aircraft. The brakes are located on the main wheels, or mains, whereas the steering is the function of the nose gear. Depending on the size and model of the aircraft, nose-gear steering maneuvers the airplane on the ground. The nose wheel can be freewheeling, with the maneuvering done by differential braking. Aircraft steering can be actuated by rods, cables, or hydraulics systems.
Another arrangement of the landing gear is the conventional landing gear, typically seen on older aircraft. In the conventional arrangement, there are two main wheels in the front of the fuselage, with a smaller tailwheel located on the aft end. Conventional gear was the norm in the early period of aviation, but it fell out of fashion in the 1950’s and 1960’s, because the tricycle landing gear is inherently more stable.
Another type of arrangement, found on the B-52, is the bicycle landing gear, which has two sets of main landing gear centered on the fuselage, one behind the other along the centerline of the fuselage. Because there are no supporting landing gear outside the body of the craft, devices known as outriggers keep the wingtips from striking the ground.
The flight control system controls the aircraft in flight and comprises the devices that command movement of the aircraft around all three axes: longitudinal, lateral, and vertical.
The elevator controls the airplane’s longitudinal movement about its lateral axis. In other words, it causes the airplane’s nose to go up or down. In this manner, combined with the power output of the engine, the elevator adjusts the wing’s angle of attack. This adjustment will have a direct influence on the airspeed of the craft.
Ailerons are small airfoils located on the outer portion of the wing. When one deflects up, the other deflects down. The down aileron increases the lift on that wing while the opposite aileron spoils the lift on the opposite wing. This starts a rolling movement about the longitudinal axis.
Finally, the rudder controls the airplane about the vertical axis. Actuated by the pedals at the pilot’s feet, the trailing edge of the vertical stabilizer moves the airplane’s nose either left or right, depending on which pedal is depressed.
Airplanes have flaps for both takeoffs and landings. Located on the inboard portion of the wing at the rear, flaps change the shape of the wing in a way that creates both lift and drag. The first half of travel, after takeoff, creates more lift than drag, whereas the last half of travel, before landing, creates more drag without a noticeable increase in lift. With the flaps partially extended for takeoff, the wing will generate more lift at lower airspeeds, allowing for shorter and safer takeoffs. On the other end of the spectrum, an aircraft approaching a landing with full flaps extended is generating more drag. This will allow the pilot to fly a steeper approach, land more slowly, and stop in a shorter distance.
There are four types of flaps: plain flaps, split flaps, slotted flaps, and Fowler flaps. Each has its own characteristics, with the first three found typically on general aviation airplanes. The fourth type, the Fowler flap, is typically found on larger air transports. The Fowler flap system is heavier and more complex than the other three types of flap, necessitating a larger aircraft.
The internal combustion engine powers many of today’s light airplanes. The most popular arrangement of the engine is in the horizontally opposed configuration. The engine is air-cooled and typically arranged in a flat four- or six-cylinder configuration, allowing the best cooling for all of the cylinders.
Unlike the aircraft engines built before World War II, the modern aircraft engine is highly engineered and very reliable. Although modern engines may still fail, the likelihood of complete power loss is minimal.
Most aircraft engines are four-stroke engines, which means each cylinder has an intake stroke, a compression stroke, a power stroke, and, finally, an exhaust stroke. The amount of power the engine puts out depends on the engine’s size. Essentially, an engine’s power increases with its size.
Aircraft engines come in all sizes, from one of the smallest, the 65-horsepower Continental A-65, to the 350-horsepower TSIO-540. As horsepower requirements reach higher than 350, many aircraft manufacturers opt to equip their high-end models with turboprop engines.
The advantages of a turboprop engine over an internal combustion engine are increased power output, smoother operation, and the ability to operate at higher altitudes. At higher altitudes, a pilot can take advantage of winds that are more favorable and realize better specific fuel consumption. Typical cruise speeds for airplanes equipped with turboprops are in the 230-knot to 350-knot range. For faster cruise speeds, a jet engine is required.
Jet engines are very simple devices. The thrust of a jet engine is determined in pounds of force rather than in horsepower, as are reciprocating engines and turboprop power plants.
The heart of the jet engine is the compressor and turbine. Linked together by a common shaft, the turbine and compressor spin at rates as high as 20,000 revolutions per minute. As the fuel and air mixture burns in the combustion chamber, the exhaust gases escape through the turbine, spinning it at high speeds. The turbine, by way of the common shaft, spins the compressor, ingesting more air into the engine. The potential power available from a jet engine is phenomenal.
More phenomenal than jet engines are future engine possibilities. Presently under development, the Stirling engine may be the most significant innovation for aviation in the near future. The Stirling engine, an external combustion engine originally invented in 1896, is on the verge of becoming the power plant of choice not only for airplanes but also for cars, boats, and many other applications.
There are as many airplanes as there are reasons for their existence. Small, privately owned aircraft such as Cessna, Beechcraft, and Piper aircraft are used for transportation and recreation. Most privately owned airplanes are single-engine, one- or two-seaters that have a range of about 400 miles and a speed of 100 miles per hour. Higher-end privately owned airplanes are turboprops and jets that are rather expensive to acquire and maintain. Powered by two engines, these more complex airplanes require more training and certification for their operation than do smaller craft. The turboprops are capable of 230- to 275-miles-per-hour cruise speeds, whereas some privately owned jet aircraft can reach a speed of 600 miles per hour.
The cost of the privately owned aircraft varies. In 2001, small two-seaters in flying condition could be purchased by bargain hunters for less than $12,000. Such airplanes are rudimentary but capable of flight and cost effective for flight training.
At the same time, the cost of a typical four-seater family airplane began at $25,000 for an older, used craft and could reach as high as $150,000 for a new model. Still more sophisticated models could cost as much as $500,000. The smaller light twin-engine craft cost $50,000 to $75,000, on the low end of the market for used craft. On the high side, a newer aircraft cost as much as $750,000.
In 2001, the cost of turboprop aircraft and small corporate jets started at well over the $1 million mark. Depending on the make and model of the corporate jet, the cost can reach as high as $40 or $50 million. Typically used in commercial endeavors, these aircraft are a strain for one or two individual owners to manage financially.
The airliner, the type of airplane with which most passengers are familiar, flies at high speeds and altitudes. Smaller commuter airliners carry an average of fifty passengers and a crew of five or six, including the two pilots. As the airline industry moves into the twenty-first century, there is a desire to move away from the turboprop aircraft of the 1980’s and 1990’s, as passengers prefer the smoother, higher, and seemingly safer ride of jet aircraft.
The final category of aircraft is military aircraft. The armed forces use different types of airplanes for different jobs. The task of protecting the nation from intruders falls to fighter planes, jets that can fly at almost twice the speed of sound. Fighters carry one or two crew members, and their mission is to stop any unannounced intruder into national airspace. The military branches also operate airline-type aircraft to move personnel and cargo throughout the world.
Bergman, Jules. Anyone Can Fly. Garden City, N.Y.: Doubleday, 1977. An outstanding and easy-to-understand explanation of aviation written for the beginner. Langewiesche, Wolfgang. Stick and Rudder: An Explanation of the Art of Flying. New York: McGraw-Hill, 1972. Hailed as the most important book on aviation, this classic text explains basic principles of flight in a simple manner. Stinton, Darrol. The Design of the Aeroplane: Which Describes Common-sense Mechanics of Design as They Affect the Flying Qualities of Aeroplanes Needing Only One Pilot. New York: Van Nostrand Reinhold, 1983. Outstanding text relating aircraft design to flying qualities; written in a technical format, with a great deal of mathematical explanation. Van Sickle, Neil D. Van Sickle’s Modern Airmanship. New York: McGraw-Hill, 1999. A technical work about flying and the aviation industry that extensively covers all aspects of the business.