Lesson 6 : AIRCRAFT STRUCTURES AND TYPES OF CONSTRUCTION

 

AIRCRAFT STRUCTURES

AIRCRAFT STRUCTURES Types Construction


Start from Lesson 1: Principle of Airframe

Go to Lesson 2 : PRINCIPLES OF AERODYNAMICS

Go to Lesson 3 : AIRFOIL CHARACTERISTICS







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Click here for the previous lessons, to learn about: Principle of Airframe; Principles of Aerodynamics; Airfoil Characteristics; Primary Flight Control Surfaces; Description and Operation of Helicopter; Miscellaneous Components of an Aircraft…

An airplane has six main parts—fuselage, wings, stabilizer (or tail plane), rudder, one or more engines, and landing gear. 

The fuselage is the main body of the machine, customarily streamlined in form. It usually contains control equipment, and space for passengers and cargo. The wings are the main supporting surfaces. Modern airplanes are monoplanes (airplanes with one wing) and may be high-wing, mid-wing, or low-wing (relative to the bottom of the fuselage). At the trailing edge of the wings are auxiliary hinged surfaces known as ailerons that are used to gain longitudinal control and to turn the airplane.

The lift of an airplane, or the force that supports it in flight, is basically the result of air moving over the surfaces of the wings. The lift varies with velocity (airspeed); there is a minimum velocity at which flight can be maintained. This is known as the stall speed. Because speed is so important to maintain lift, objects such as fuel tanks and engines, that are carried outside the fuselage are enclosed in structures called nacelles, or pods, to reduce air drag (the retarding force of the air as the airplane moves through it).

Directional stability is provided by the tail fin, a fixed vertical airfoil at the rear of the plane. The stabilizer, or tail plane, is a fixed horizontal airfoil at the rear of the airplane used to suppress undesired pitching motions. To the rear of the stabilizer are usually hinged the elevators, movable auxiliary surfaces that are used to produce controlled pitching. The rudder, generally at the rear of the tail fin, is a movable auxiliary airfoil that gives the craft a yawing (turning about a vertical axis) movement in normal flight. The rear array of airfoils is called the empennage, or tail assembly. Some aircraft have additional flaps near the ailerons that can be lowered during takeoff and landing to augment lift at the cost of increased drag. On some airplanes hinged controls are replaced or assisted by spoilers, which are ridges that can be made to project from airfoils.

Airplane engines may be classified as driven by propeller, jet, turbojet, or rocket. Most engines originally were of the internal-combustion, piston-operated type, which may be air- or liquid-cooled. During and after World War II, duct-type and gas-turbine engines became increasingly important, and since then jet propulsion has become the main form of power in most commercial and military aircraft. The landing gear is the understructure that supports the weight of the craft when on the ground or on the water and that reduces the shock on landing.

The airframe components are constructed from a wide variety of materials and are joined by rivets, bolts, screw, and welding or adhesives. The aircraft components are composed of various parts called structural members. Aircraft structural members are designed to carry a load or to resist stress. A single member of the structure may be subjected to a combination of stresses. In most cases the structural members are designed to carry end loads rather than side loads; that is, to be subjected to tension or compression rather than bending.

Strength may be the principal requirement in certain structures, while others need entirely different qualities. For example, cowling, fairing, and similar parts usually are not required to carry the stresses imposed by flight or landing loads. However, these parts must have such properties as neat appearance and streamlined shapes. Every square inch of a wing and fuselage, every rib, spar and even each metal fitting must be constructed in relation to the physical characteristics of the metal from which it is made. Every part of the aircraft must be planned to carry the load imposed upon it. Although planning the design is not the function of the mechanic, it is important to understand and appreciate the stresses involved in order to avoid changes in the original design through improper repairs.

 

AIRCRAFT FUSELAGE.

See Figure 1-31. The word fuselage originated during the early developments in French aviation and is still used to designate the body portion of an aircraft. The original purpose of the fuselage was to support the empennage for directional control and stability. Later, the fuselage was used as a primary structure to which all other components, such as engines, landing gear, and wings were attached. This is still the primary function of the fuselage.

 The second purpose of the fuselage is to carry the crew and cargo. The type of configuration is often determined by the basic mission of the aircraft. The three types of fuselage construction are the truss, monocoque, and the semi-monocoque.

Major Structural Components of Aircraft


Figure 1-31, Major Structural Components of Cargo Type Aircraft

Construction style:

AIRCRAFT TRUSS.

See Figure 1-32, in architecture and engineering, a supporting structure or framework composed of beams, girders, or rods commonly of steel or wood lying in a single plane. A truss usually takes the form of a triangle or combination of triangles, since this design ensures the greatest rigidity. Trusses are used for large spans and heavy loads, especially in bridges and roofs. Their open construction is lighter than, yet just as strong as, a beam with a solid web between upper and lower lines. The members are known as tie-beams, posts, rafters, and struts; the distance over which the truss extends is called the span. The upper and lower lines or beams are connected by web members.


Fuselage Construction


Figure 1-32, Fuselage Construction

AIRCRAFT MONOCOQUE CONSTRUCTION.

 The monocoque type of fuselage construction is like a shell in which the skin of the fuselage carries the primary stresses. Rings and bulkheads are used to shape the fuselage. Since no longitudinal bracing members are present, the skin must have sufficient thickness and strength to keep the fuselage rigid. Thus, weight becomes a problem. Most early aircraft featured this type of construction.

SEMI-MONOCOQUE CONSTRUCTION.

The semi-monocoque type of construction uses aluminum alloy and is used in nearly all present-day aircraft. The fuselage members are arranged so that each member carries part of the load.

Fuselage Structural Components

AIRCRAFT LONGERONS.

AIRCRAFT LONGERONS.


The longerons are the longest and strongest structural members of the fuselage and are used primarily to obtain strength. They run from the nose of the fuselage to the empennage. In the total construction the longerons hold the rings, formers, and bulkheads together. The rings, formers, and bulkheads hold the stringers.

AIRCRAFT STRINGERS.

Stringers are small, lightweight members running lengthwise from the nose of the fuselage to the empennage. Stringers furnish some strength but are mainly used for attachment of the skin. The skin of the aircraft is fastened to the stringers by rivets, screws, and other fasteners.

AIRCRAFT BULKHEADS.

AIRCRAFT BULKHEADS


Bulkheads give cross-sectional strength to the fuselage and divide the aircraft into compartments bulkheads are the strongest support members, set at a 90° angle to the longitudinal axis. Bulkheads that separate pressurized compartments have doors with seals to prevent pressure loss. Bulkheads also have openings for conduits, hoses, lines, and control cables to pass through.

AIRCRAFT EMPENNAGE.

The empennage is the stabilizing group of surfaces located at the aft section of the fuselage, and is designed to balance and stabilize the aircraft during flight. The empennage has two main parts, the horizontal stabilizer and the vertical stabilizer.

 AIRCRAFT Horizontal stabilizer.

 The horizontal stabilizer is the horizontal tail surface which keeps the aircraft stable about the lateral axis. Attached by hinges to its trailing edge are the elevators, which control the up and down movement, or pitch, of the aircraft. On some aircraft, the entire surface moves. This type of unit is called a stabilator and is commonly found on fighter aircraft.

 

Empennage

Figure 1-33, Empennage

AIRCRAFT Vertical Stabilizer.

The vertical stabilizer, figure 1-33, gives the aircraft positive stability about the vertical axis. The rudder is the hinged flight control surface attached to the trailing edge of the vertical stabilizer. It controls the side-to-side movement, or yaw, of the aircraft (fishtailing effect).

 

Tags:

Aircraft Construction, TRUSS, Fuselage Construction, MONOCOQUE CONSTRUCTION, LONGERONS, STRINGERS, BULKHEADS, EMPENNAGE, Horizontal stabilizer, Vertical Stabilizer