Although airplanes are designed for a variety of purposes, most of them have the same major components. [Figure 2-4] The overall characteristics are largely determined by the original design objectives. Most airplane structures include a fuselage, wings, an empennage, landing gear, and a powerplant.
Figure 2-4. Airplane components.
Figure 3. Center of gravity (CG).
The fuselage is the central body of an airplane and is designed to accommodate the crew, passengers, and cargo. It also provides the structural connection for the wings and tail assembly. Older types of aircraft design utilized an open truss structure constructed of wood, steel, or aluminum tubing. [Figure 5] The most popular types of fuselage structures used in today’s aircraft are the monocoque (French for “single shell”) and semimonocoque. These structure types are discussed in more detail under aircraft construction later in the lesson.
Figure 5. Truss-type fuselage structure.
The wings are airfoils attached to each side of the fuselage and are the main lifting surfaces that support the airplane in flight. There are numerous wing designs, sizes, and shapes used by the various manufacturers. Each fulfills a certain need with respect to the expected performance for the particular airplane. How the wing produces lift isexplained in Aerodynamics of Flight.
Wings may be attached at the top, middle, or lower portion of the fuselage. These designs are referred to as high-, mid-, and low-wing, respectively. The number of wings can also vary. Airplanes with a single set of wings are referred to as monoplanes, while those with two sets are called biplanes. [Figure 6]
Figure 6. Monoplane (left) and biplane (right).
Many high-wing airplanes have external braces, or wing struts, which transmit the flight and landing loads through the struts to the main fuselage structure. Since the wing struts are usually attached approximately halfway out on the wing, this type of wing structure is called semi-cantilever. A few high-wing and most low-wing airplanes have a fullcantilever wing designed to carry the loads without external struts.
The principal structural parts of the wing are spars, ribs, and stringers. [Figure 7] These are reinforced by trusses, I-beams, tubing, or other devices, including the skin. The wing ribs determine the shape and thickness of the wing (airfoil). In most modern airplanes, the fuel tanks either are an integral part of the wing’s structure, or consist of fiexible containers mounted inside of the wing.
Figure 7. Wing components.
Attached to the rear or trailing edges of the wings are two types of control surfaces referred to as ailerons and fiaps. Ailerons extend from about the midpoint of each wing outward toward the tip, and move in opposite directions to create aerodynamic forces that cause the airplane to roll. Flaps extend outward from the fuselage to near themidpoint of each wing. The fiaps are normally fiush with the wing’s surface during cruising flight. When extended, the fiaps move simultaneously downward to increase the lifting force of the wing for takeoffs and landings. [Figure 8]
Figure 8. Types of fiaps.
With the Federal Aviation Administration’s (FAA) recent addition of the LSA category, various methods are employed to control flight and to produce lift. These methods are discussed in Chapter 4, Aerodynamics of Flight, which provides information on the effect controls have on lifting surfaces from traditional wings to wings that use both fiexing (due to billowing) and shifting (through the change of the aircraft’s CG). Handbooks specific to each category of LSA are available for the interested pilot. LSA illustrate various lifting surfaces and control methods. For example, the wing of the weight-shift control aircraft is highly swept, and the shifting of weight to provide controlled flight. [Figure 9]
Figure 9. Weight-shift control aircraft use the shifting of weight for control.
The empennage includes the entire tail group and consists of fixed surfaces such as the vertical stabilizer and the horizontal stabilizer. The movable surfaces include the rudder, the elevator, and one or more trim tabs. [Figure 10]
Figure 10. Empennage components.
The rudder is attached to the back of the vertical stabilizer. During flight, it is used to move the airplane’s nose left and right. The elevator, which is attached to the back of the horizontal stabilizer, is used to move the nose of the airplane up and down during flight. Trim tabs are small, movable portions of the trailing edge of the control surface. These movable trim tabs, which are controlled from the flight deck, reduce control pressures. Trim tabs may be installed on the ailerons, the rudder, and/or the elevator.
A second type of empennage design does not require an elevator. Instead, it incorporates a one-piece horizontal stabilizer that pivots from a central hinge point. This type of design is called a stabilator, and is moved using the control wheel, just as the elevator is moved. For example, when a pilot pulls back on the control wheel, the stabilator pivots so the trailing edge moves up. This increases the aerodynamic tail load and causes the nose of the airplane to move up. Stabilators have an antiservo tab extending acrosstheir trailing edge. [Figure 11]
Figure 11. Stabilator components.
The antiservo tab moves in the same direction as the trailing edge of the stabilator and helps make the stabilator less sensitive. The antiservo tab also functions as a trim tab to relieve control pressures and helps maintain the stabilator in the desired position.
The landing gear is the principal support of the airplane when parked, taxiing, taking off, or landing. The most common type of landing gear consists of wheels, but airplanes can also be equipped with fioats for water operations, or skis for landing on snow. [Figure 12]
Figure 12. Types of landing gear: floats (top), skis (middle), and wheels (bottom).
The landing gear consists of three wheels—two main wheels and a third wheel positioned either at the front or rear of the airplane. Landing gear with a rear mounted wheel is called conventional landing gear.
Airplanes with conventional landing gear are sometimes referred to as tailwheel airplanes. When the third wheel is located on the nose, it is called a nosewheel, and the design is referred to as a tricycle gear. A steerable nosewheel or tailwheel permits the airplane to be controlled throughout all operations while on the ground. Most aircraft are steered by moving the rudder pedals, whether nosewheel or tailwheel. Additionally, some aircraft are steered by differential braking.
The powerplant usually includes both the engine and the propeller. The primary function of the engine is to provide the power to turn the propeller. It also generates electrical power, provides a vacuum source for some flight instruments, and in most single-engine airplanes, provides a source of heat for the pilot and passengers. [Figure 13] The engine is covered by a cowling, or a nacelle, which are both types of covered housings. The purpose of the cowling or nacelle is to streamline the fiow of air around the engine and to help cool the engine by ducting air around the cylinders.
Figure 13. Engine compartment.
The propeller, mounted on the front of the engine, translates the rotating force of the engine into thrust, a forward acting force that helps move the airplane through the air. The propeller may also be mounted on the rear of the engine as in a pusher-type aircraft. A propeller is a rotating airfoil that produces thrust through aerodynamic action. A lowpressure area is formed at the back of the propeller’s airfoil, and high pressure is produced at the face of the propeller, similar to the way lift is generated by an airfoil used as a lifting surface or wing. This pressure differential pulls air through the propeller, which in turn pulls the airplane forward.
There are two significant factors involved in the design of a propeller which impact its effectiveness. The angle of a propeller blade, as measured against the hub of the propeller, keeps the angle of attack relatively constant along the span of the propeller blade, reducing or eliminating the possibility of a stall. The pitch is defined as the distance a propeller would travel in one revolution if it were turning in a solid. These two factors combine to allow a measurement of the propeller’s efficiency. Propellers are usually matched to a specific aircraft/powerplant combination to achieve the best efficiency at a particular power setting, and they pull or push depending on how the engine is mounted.
The subcomponents of an airplane include the airframe, electrical system, flight controls, and brakes.
The airframe is the basic structure of an aircraft and is designed to withstand all aerodynamic forces, as well as the stresses imposed by the weight of the fuel, crew, and payload.
The primary function of an aircraft electrical system is to generate, regulate, and distribute electrical power throughout the aircraft. There are several different power sources on aircraft to power the aircraft electrical systems. These power sources include: engine-driven alternating current (AC) generators, auxiliary power units (APUs), and external power. The aircraft’s electrical power system is used to operate the flight instruments, essential systems such as anti-icing, etc., and passenger services, such as cabin lighting.
The flight controls are the devices and systems which govern the attitude of an aircraft and, as a result, the flight path followed by the aircraft. In the case of many conventional airplanes, the primary flight controls utilize hinged, trailing- edge surfaces called elevators for pitch, ailerons for roll, and the rudder for yaw. These surfaces are operated by the pilot in the flight deck or by an automatic pilot.
Airplane brakes consist of multiple pads (called caliper pads) that are hydraulically squeezed toward each other with a rotating disk (called a rotor) between them. The pads place pressure on the rotor which is turning with the wheels. As a result of the increased friction on the rotor, the wheels inherently slow down and stop turning. The disks and brake pads are made either from steel, like those in a car, or from a carbon material that weighs less and can absorb more energy. Because airplane brakes are used principally during landings and must absorb enormous amounts of energy, their life is measured in landings rather than miles.