Aircraft Structures

Marwa Grimes

Airframe components

Almost any airframe may be split into four main components:

• the mainplane or wings

• the fuselage or body

• the tail unit (or foreplanes, for a canard-type aircraft)

• mountings for all other systems (undercarriage, engines, etc.)

Each main component is designed to perform a specific task, so that the complete airframe can carry out the job for which it was designed in a safe and efficient way.

Airframe structures and design

All aircraft are made up of a great many individual parts, and each part has its own specific job to do. But even if it were possible to build an aircraft in one single piece, this would not be the best option. Some parts will become damaged, wear out or crack during service, and provision must be made for their repair or replacement. If a part begins to crack, it is imperative that the structure does not fail completely before it is found during maintenance inspections, or the safe operation of the aircraft may be jeopardised. This is the basis of our industry.

The aircraft wings

The wing must generate lift from the airflow over it to support the aircraft in flight. The amount of lift required depends on how the aircraft is flying or manoeuvring. For straight and level flight, the total lift produced must be equal to the weight of the aircraft. To take off and climb, the required lift must be developed at a low airspeed. If the aircraft is to fly in very tight turns, the wing must produce lift equal to perhaps eight times the aircraft weight. For landing, the slowest possible forward speed is required, and enough lift must be produced to support the aircraft at these low speeds. For take-off and landing, lift-augmenting devices are normally added to make this possible – flaps, leading-edge slats, etc. The wing needs to be stiff and strong to resist high lift forces, and the drag forces associated with them.

So it could be argued that the wing is the most essential component of an airframe. In fact, aircraft have been designed which consist only of a wing. More commonly, an arrangement that moves some way towards this ideal can be seen in aircraft like the Boeing B-2, F-117 and delta aircraft like Concorde.

In most large aircraft, the wing carries all or most of the fuel, and also supports the main undercarriage; in military aircraft it often carries a substantial part of weapon loads and other external stores. All of these will impart loads onto the wing structure. This is why the UK contribution to Airbus is a critical one.

The fuselage.

The fuselage serves a number of functions:

It forms the body of the aircraft, housing the crew, passengers or cargo (the payload), and many of the aircraft systems – hydraulic, pneumatic and electrical circuits, electronics.

It forms the main structural link between the wing and tail or foreplanes, holding them at the correct positions and angles to the airflow to allow the aircraft to fly as it was designed to do. The forces transmitted from these components, particularly the wing and tail, generate a variety of types of load on the fuselage. It must be capable of resisting these loads throughout the required life of the aircraft.

Engines may be installed inside or attached to the fuselage, and the forces generated can be very high.

Because of the altitude at which they fly, most modern aircraft have some form of environmental control system (temperature and pressurisation) in the fuselage. The inside of the fuselage is pressurised to emulate a lower altitude than outside, of around 2400 metres (8000 feet) for transport aircraft, and up to 7600 metres (25000 feet) for military aircraft (with crew oxygen), and temperatures are maintained within comfortable limits. These pressure loads generate tensile forces along and around the fuselage, as with the material in an inflated balloon.

These many loading actions can all exist at once, and may vary cyclically throughout the life of the airframe. The fuselage needs to be strong and stiff enough to maintain its integrity for the whole of its design life.

The fuselage is often blended into the wing to reduce drag. In some aircraft it is difficult to see where the fuselage ends and the wing begins.

The tail unit

The tail unit usually consists of a vertical fin with a movable rudder and a horizontal tailplane with movable elevators or an all-moving horizontal tailplane. There is, however, another form of control surface that is finding increasing popularity in fighter aircraft, and even some sport and executive aircraft. In this layout, the horizontal tail surface is replaced or supplemented by moving control surfaces at the nose of the aircraft. These surfaces are called foreplanes, and this layout is known as the canard layout, from the French word for duck, which these aircraft resemble.

Whichever layout is used, these surfaces provide stability and control in pitch and yaw. If an aircraft is stable, any deviation from the path selected will be corrected automatically, because aerodynamic effects generate a restoring effect to bring the aircraft back to its original attitude. Stability can be provided artificially, but initially it will be considered to be achieved by having a tail unit, with a fixed fin and tailplane, and movable control surfaces attached to them. It is an advantage if the tail is as far from the centre of gravity as possible to provide a large lever – it can then be small and light, with low drag. For this reason it is placed at the rear of the fuselage

Forces created by the tail act up and down (by the tailplane), and left and right (by the fin). All of these forces, plus the associated bending and torsion loads, must be resisted and absorbed by the fuselage.

Aerospace composites and the weight of aircraft composite structures.

It is good engineering practice for the design of all parts to be as efficient and economical as possible, keeping weight and cost low. Of course, the requirements of low weight and low cost often conflict. In aircraft low weight and high strength are especially important, and great efforts are made at the design stage to achieve this. The maximum weight of an aircraft is set by its design, and any extra weight taken up by the structure is not available for payload or fuel, reducing its operating efficiency. This is made worse by the weight spiral effect, where an increase in weight in one area means that other areas need to be strengthened to take the extra loads induced. This increases their weight, and may mean more powerful engines or bigger wings are required to maintain the required performance. In this way, an aircraft may become larger or less efficient purely as a result of poor weight control during design.

There are many ways of saving weight, but one of the most common ones is to use improved materials like advanced aerospace composites. Often these may be more expensive, but the extra cost may be justified by the improved performance and reduced operating costs. At the design stage, such questions are the subject of extensive trade-off studies.

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