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The airframe of a fixed-wing aircraft consists of the following five major units:
Fuselage
Wings
Stabilizers
Flight controls surfaces
Landing gear
A rotary-wing aircraft consists of the following four major units:
Fuselage
Landing gear
Main rotor assembly
Tail rotor assembly
The primary factors to consider in aircraft structures are strength, weight, and reliability. These factors determine the requirements to be met by any material used to construct or repair the aircraft. Airframes must be strong and light in weight. An aircraft built so heavy that it couldn’t support more than a few hundred pounds of additional weight would be useless. All materials used to construct an aircraft must be reliable. Reliability minimizes the possibility of dangerous and unexpected failures.
Many forces and structural stresses act on an aircraft when it is flying and when it is static. When it is static, the force of gravity produces weight, which is supported by the landing gear. The landing gear absorbs the forces imposed on the aircraft by takeoffs and landings.
During flight, any maneuver that causes acceleration or deceleration increases the forces and stresses on the wings and fuselage. Stresses on the wings, fuselage, and landing gear of aircraft are in tension, compression, shear, bending, and torsion. These stresses are absorbed by each component of the wing structure and transmitted to the fuselage structure. The empennage (tail section) absorbs the same stresses and transmits them to the fuselage. Stresses are analyzed and considered when an aircraft is designed.
Members of an aircraft are subjected to following stresses:
Tension
Compression
Shear
Bending
Varying stresses
All structural members of an aircraft are subject to one or more stresses. Sometimes a structural member has alternate stresses; for example, it is under compression one instant and under tension the next. The strength of aircraft materials must be great enough to withstand maximum force of varying stresses.
Fig: Engine torque creates tension stresses in aircraft fuselages.
Fig: Bending action occurring during carrier loading
AIRCRAFT CONSTRUCTION MATERIALS
An aircraft must be constructed of materials that are both light and strong. Early aircraft were made of wood. Lightweight metal alloys with strength greater than wood were developed and used on later aircraft. Materials currently used in aircraft construction are classified as either metallic materials or non-metallic materials.
METALLIC MATERIALS
The most common metals used in aircraft construction are aluminium, magnesium, titanium, steel, and their alloys.
Aluminium
Aluminium alloys are widely used in modern aircraft construction. Aluminium alloys are valuable because they have a high strength-to-weight ratio. Aluminium alloys are corrosion resistant and comparatively easy to fabricate. The outstanding characteristic of aluminium is its lightweight.
Magnesium
Magnesium is the world’s lightest structural metal. It is a silvery-white material that weighs two-thirds as much as aluminium. Magnesium is used to make helicopters. Magnesium’s low resistance to corrosion has limited its use in conventional aircraft.
Titanium
Titanium is a lightweight, strong, corrosion resistant metal. Recent developments make titanium ideal for applications where aluminium alloys are too weak and stainless steel is too heavy. Additionally, titanium is unaffected by long exposure to seawater and marine atmosphere.
Alloys
An alloy is composed of two or more metals. The metal present in the alloy in the largest amount is called the base metal. All other metals added to the base metal are called alloying elements. Adding the alloying elements may result in a change in the properties of the base metal. For example, pure aluminium is relatively soft and weak. However, adding small amounts or copper, manganese, and magnesium will increase aluminium’s strength many times. Heat treatment can increase or decrease an alloy’s strength and hardness. Alloys are important to the aircraft industry. They provide materials with properties that pure metals do not possess.
Steel Alloys
Alloy steels used in aircraft construction have great strength, more so than other fields of engineering would require. These materials must withstand the forces that occur on today’s modern aircraft. These steels contain small percentages of carbon, nickel, chromium, vanadium, and molybdenum. High-tensile steels will stand stress of 50 to 150 tons per square inch without failing. Such steels are made into tubes, rods, and wires. Another type of steel used extensively is stainless steel. Stainless steel resists corrosion and is particularly valuable for use in or near water.
NON-METALLIC MATERIALS
In addition to metals, various types of plastic materials are found in aircraft construction. Some of these plastics include transparent plastic, reinforced plastic, composite, and carbon-fiber materials.
Transparent Plastic
Transparent plastic is used in canopies, windshields, and other transparent enclosures.
Reinforced Plastic
Reinforced plastic is used in the construction of radomes, wingtips, stabilizer tips, antenna covers, and flight controls. Reinforced plastic has a high strength-to-weight ratio and is resistant to mildew and rot.
Composite and Carbon Fiber Materials
High-performance aircraft require an extra high strength-to-weight ratio material. Fabrication of composite materials satisfies this special requirement. Composite materials are constructed by using several layers of bonding materials (graphite epoxy or boron epoxy). These materials are mechanically fastened to conventional substructures. Another type of composite construction consists of thin graphite epoxy skins bonded to an aluminium honeycomb core. Carbon fiber is extremely strong, thin fiber made by heating synthetic fibers, such as rayon, until charred, and then layering in cross sections.
FIXED-WING AIRCRAFT
Fig: Principle Structural units of a fixed wing aircraft
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