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Aircraft Structures and Systems
R Wilkinson

CHAPTER 1 – AIRFRAME DESIGN FEATURES

Objectives: introduce the main airframe components, describe their main design features, something about the forces acting upon them and the effects of their weight and shape.

INTRODUCTION

Most people who fly, perhaps when going on holiday, look at an aircraft and see just that – a complete aircraft – without really considering the huge complexity of parts that lie under the skin. Perhaps they don’t want to think about it, preferring the bliss of ignorance at a stressful time. However, others realise that an aircraft is an amazingly complex machine. Just how complex it is can only really be grasped when an understanding of some of the principles has been achieved. Of course, the aircraft is not made so complex just for the sake of it – every component is there for a specific purpose.

The high cost of an aircraft can only be justified if it can provide a return on the money invested. For commercial aircraft this seems quite straightforward – buy the cheapest aircraft that can carry the required number of passengers or amount of freight. But the initial cost of buying the aircraft is usually only a small part of the total cost of ownership throughout its entire operating life. It is often worth paying extra for an aircraft that will be cheaper to operate. When depreciation, operating life and reliability or down time (the time during which the aircraft is unavailable because it is awaiting repair) are taken into account, this can be a difficult decision to make. For military aircraft, the questions are different, and decisions can be more or less straightforward than for a commercial one, especially when political aspects need to be considered.

Whatever the purpose of the aircraft, it must be strong enough and stiff enough, and able to sustain a long life in service, often thirty years or more. It must also be constructed so that if any part fails, as some are bound to do, the failure does not cause the loss of the aircraft, and possibly many lives. There are numerous ways of achieving this, but these are the subject of later chapters. It is important to say, however, that aircraft are among the safest means of transport. This is due in no small measure to the rigorous and detailed requirements applied by aircraft designers and operators, safeguarded by airworthiness authorities world-wide.

THE STRUCTURE

Airframe Components. Almost any airframe may be split into four main components:

• the mainplane or wing
• 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. The main features of each major component are defined in the next few paragraphs, together with a brief description of the loads they will see during operation. 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.

The wing. 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.

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 supporting 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 (8 000 feet) for transport aircraft, and up to 7600 metres (25 000 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, as defined below. 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.

WEIGHT

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. 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.

AERODYNAMIC FORCES – LIFT AND DRAG

When air flows over aerodynamic surfaces, its pressure will change as its speed changes. This change in pressure, acting over a large area, will produce large forces. Depending on the direction in which these forces act, they will behave as lift or drag forces. Lift is defined as a force at right angles to the direction of flight (i.e. up or down as experienced on the aircraft, with up positive) and drag as acting along the direction of flight (i.e. fore and aft, with aft positive). If the force is neither exactly at right angles nor exactly along the direction of flight, it will have components of lift and drag. As already explained, the lift produced by a large aircraft can be several hundred tonnes, with drag forces typically one tenth of the lift. It is impossible to generate lift without also producing drag, but careful design can ensure that all forms of drag are minimised.

The operator of an aircraft wishes the aircraft to fly at the highest economical speed, to use it to its maximum potential. Commercial operators wish to deliver the maximum cargo or passenger load in the shortest time, and the armed forces gain a great advantage from very fast combat aircraft. The problem they all face is that higher speeds mean higher drag loads. These in turn need more powerful engines, and so increased fuel consumption. The loads increase as the square of the airspeed. Inevitably, there is an optimum speed at which to operate any aircraft, and this speed will normally be chosen as the cruise speed for that particular aircraft. To get the best possible performance and best fuel economy an aircraft must be shaped to minimise drag. The external shape has a great influence on the design of the underlying structure. Conversely, it is not possible to design an aircraft to give the minimum drag without taking into account structural factors.

Fortunately, this relationship between load and speed also applies to lift. However, the structure must be able to cope with the higher loads generated. Most of the loads that generate the stresses on the airframe structure come about from the effects of aerodynamic pressures on the airframe external surfaces, rather than other sources. These pressures will vary over a wide range, depending on whether the aircraft is cruising, diving, climbing or in turbulent air, and also of course on its speed. Thin wings normally give low drag but may be prone to flutter (a damaging oscillation caused by an interaction between aerodynamic and structural effects); thicker wings are stiffer, and can carry more fuel. Inevitably, the final design is a compromise. The most successful aircraft are those in which the best compromises are found.

INERTIA FORCES

Inertia forces resulting from manoeuvres may be considerable. If an aircraft is turning, each part of the aircraft will resist any change to its motion by exerting an inertia force on its attachments. The attachments will then exert an equal and opposite force, according to Newton’s Laws of Motion. If the aircraft is turning at 4 ‘g’, i.e. four times the acceleration due to gravity, the component will appear to weigh four times its static or normal weight. This ‘weight’ must be resisted by the mountings of the component. Thus an engine weighing 15 000 N will generate a load of 60 000 N in a 4 ‘g’ manoeuvre.

THRUST

The final type of load to consider is the thrust generated by the engine or engines. These loads are transmitted through the engine mounts into the surrounding structure. In constant-speed flight, the thrust will equal the total drag of the aircraft, but usually acts along a different line – the engine centre-line. Pylon-mounted engines are common on large commercial aircraft, and the thrust acting along a line below the wing produces torsional loads on the wing. It is possible to use some loading actions in opposition to other loads on the structure, thereby reducing the overall strength requirements of the structure and saving weight. This is one of many techniques used by designers to make their aircraft more efficient.

Low- and high-drag arrangements. Note how streamlined the SR-71 is in comparison with the Fairey Swordfish, showing the importance of low drag for high-speed flight. Streamlining an aircraft usually carries a penalty in terms of higher weight or reduced volume, but is essential in this case. Photographs: Alistair Copeland.

CONCLUSIONS

An airframe must be capable of satisfying many requirements. It must cope with the aerodynamic forces produced at the speeds at which the aircraft is to fly, to resist the inertia forces created by the manoeuvres of the aircraft, and of course to carry the payload that it was designed to transport. The aircraft must also achieve the other performance aspects for which it was designed, not least to fly at the speeds and with the fuel economy required to provide an effective means of transportation. It must have the lowest-possible structural weight, but have a stiff and strong structure with a long life and a high degree of safety. To achieve all of this, designers must resolve many problems. They must have a thorough understanding of the loads on an aircraft structure, and how they are best supported. They must balance the diverse requirements of the operators, the airworthiness authorities, the manufacturing organisation and all of the other specialists, so that the aircraft is safe, economical and effective to operate and to maintain. We can look at the examples in later chapters to see how light yet strong and stiff structures are built.

© Copyright MechAero and R Wilkinson 2001

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