Aircraft/Structures

Subject classification: this is an engineering resource.

Subject classification: this is a physics resource.

General

Most of the aircraft operating today are made with lightweight but strong aluminum, with most newer aircraft being made of advanced composites. Note the Cirrus SR-22 and Boeing Dreamliner 787 as examples of the newest modern aircraft developed at this time.

It has only been very recently that mainstream automobiles are starting to switch to lighter materials like aluminum.

There are several categories of aircraft:

• Fixed-wing Airplane
• Rotorcraft
• Glider
• Lighter-than-air vehicles

The aircraft is made up of an airframe, specifically fuselage, booms, nacelles, cowlings, fairings, airfoil surfaces, and landing gear. There are also accessories and controls that go with these structures. A notable fact is that the rotors of a helicopter serve the same purpose as the wings of a plane, so are considered part of the helicopter's airframe, unlike the propeller of an airframe.

Fixed Wing

The fixed wing airplane is made up of five principal components:

• Fuselage
• Wings
• Stabilizers
• Flight Control Surfaces
• Landing Gear

Rotorcraft

Helicopter airframes consist of

• Fuselage
• Main rotor and driving gearbox
• Tail Rotor
• Landing Gear or Skids

Structural Members

Structural member of the aircraft fuselage include

• Stringers
• Longerons
• Ribs
• Bulkheads
• Frames and Formers

And the main structural member of the wing is the wing spar.

Skin

The skin of the aircraft is contemporarily made of aluminum or composites, but in the past has been made of impregnated fabric or plywood, when an appreciable amount of the aircraft stress is carried strictly by the frame.

Major Stresses

There are five major stresses or loads on the aircraft are

• Tension
• Compression
• Torsion
• Shear
• Bending

Tension

Is the stress that tends to pull something apart. i.e. tension on the length of the fuselage from the engine pulling the fuselage against the air resistance of drag.

Compression

Is the stress that resists a crushing force

Torsion

Is the stress that produces twisting. The rotary force of the engine tries to twist the engine in one direction, but this force is resisted by the supporting components it is connected to, most notably the engine mounts.

Shear

Is the stress that resists the force tending to cause one layer of a material to slide over an adjacent layer. Two plates riveted together in tension subject the rivets to a shearing force. Screws bolts and rivets are often subject to a shearing force.

Bending

Is a combination of compression and tension. When a rod is bent, the outside of the bend is stretched, and the inside of the rod is compressed.

Aircraft Structural Components

The major components of the Aircraft Structures can be broadly combined into:

• The Fuselage
• The Wings
• The Empennage

Fuselage

The fuselage is the body of the Airplane that carries the Crew and the Payload, the payload being passengers, cargo, fuel and weapons. It is subjected to very high Bending Loads from its own and the Payload's Weight.

Older planes with stretched impregnated fabric or plywood skin were made with Truss type, a rigid framework of members made of beams, struts, and bars to resist deformation by applied loads.

Fuselage structures can be of monocoque construction, or of semi-monocoque construction

Monocoque

Full monocoque fuselages uses formers, frame assemblies and bulkheads to give shape to the fuselage. The skin of the aircraft carries most of the structural load of the aircraft.

Semi-monocoque fuselage

To make up for the shortcomings of monocoque fuselage design, semi-monocoque construction was developed. Like monocoque fuselages, it is made of frame assemblies, bulkheads and formers, but the skin is stiffened using longitudinal members called longerons that carry bending loads, and stringers that are also longitudinal members, used to give shape and provide attachment of the skin. Stringers and longerons together prevent tension and compression from bending the fuselage.

Pressurization

A further stress that the fuselage and the aircraft skin must absorb is pressurization of aircraft that fly at higher altitudes. Pressurization causes significant repeated stress from the cycling from unpressurized to pressurized and unpressurized again. A result is that aircraft that regularly fly at altitude, like commercial airlines, must be periodically be completely inspected so that any damage that does occur can be fixed before further damage occurs.

Wings

The Wings provide the Lift to an Aircraft. The Wings are subjected to High Stress due to Aerodynamic Forces as well as the Weight of the Engines and reaction loads from Landing Gears. The tailplane of the wing is the main contributor to Directional controls. Ailerons, Rudder and Elevators enable the maneuverability and provide stability, while in flight. The Wing Flaps provide the extra Lift during Take Off and Landing.

Empennage

The term "Empennage" refers to the tail section or tail assembly of the airplane/aircraft that consists of the Horizontal Stabilizers and the Elevators, Vertical Stabilizer and a Rudder that provides sufficient stability for the aircraft along the lateral and the vertical axis. The Rudder provides Directional Stability, whereas the Elevators provide Longitudinal Stability.