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Aircraft Requirements and Safety

The design process of an aircraft starts with specification of the requirements. An aircraft design is always a compromise. The first and most important requirement of an aircraft part is that it fulfils its function in all circumstances, particularly in critical situations.
The strength of a structure is a measure of the risks taken – the acceptance that the structure will fail in extreme conditions. Society sets standards for such risks. We accept that all structures fail in certain conditions. When calculating the loads, we name the force which will just make the structure fail, the ultimate load. Structural failures often occur due to a very large series of normal repetitive loads that cause fracturing of the material: metal-fatigue. It is very important to know the rate of crack-growth and the residual strength (the strength in the presence of cracks) of a structure. A number of European countries have formulated a set of Joint Airworthiness Requirements, the J.A.R, which are based on the American Federal Airworthiness Requirements, or F.A.R. The
airworthiness standards define primary structures, those that would endanger the aircraft upon failure, secondary structures, those that do not cause immediate danger upon failure, and non load-bearing structures, which do not carry loads. There are multiple ways of considering part safety. The fail-safe principle accepts that there is a chance that part of the structure fails. However, there should be no chance of the whole structure failing. In the safe-life philosophy, the chance of the structure failing within its prescribed lifetime should be zero. If this were to happen, then the chance of the whole structure failing is substantial. The stiffness of a structure is a measure of its resistance to a change in shape when subjected to forces. The stiffness of a complete structure is always a combination f its material properties and its geometry. Aircraft wings and tail-sections can be subjected to three types of forces, namely aerodynamic forces, elastic forces and mass forces. These forces can work together in such an unfortunate way that they induce a type of vibration known as flutter. Flutter only occurs above a certain speed, which we call the critical speed. Flutter is caused by two coordinated types of vibration that amplify each other’s effect. Air-transport safety is the responsibility of the manufacturer, the user and the government. As part of this responsibility, the government exercises control over the airfield through the State Air-Transport Service (In the Netherlands this is the Luchtvaart Autoriteit). The Luchtvaart Autoriteit is responsible
for monitoring design, manufacturing, use and maintenance of aircraft, education, training and testing of personnel, and operational guidelines, accident investigation, traffic management and traffic regulations.


Aircraft Engines

Aircraft power plants fall into five main types.

  • Ramjet engines, for very high speed aircrafts.
  • Turbo jet engines, for high speed aircrafts.
  • Turbo-fan engines, for Mach 0.3 to Mach 2.
  • Turbo-prop engines, for relatively low speeds.
  • Piston engines, for simple low speed aircrafts.

Each variantis most suited to a particular aircraft flight speed. The operating efficiency,  loosely defined as power absorbed divided by the rate of fuel burn, is maximized when the velocity of the air expelled from the jet, fan or propeller is close to the speed of the aircraft.
In turbo-fan engines, some of the exhaust gases are made up of air that has by-passed the engine core or gas generator, and only passed through a fan. They are therefore called by-pass engines. The higher the by-pass ratio, the larger the engine’s diameter.
Engines can be positioned in many ways. Most transport aircraft have externally mounted engines, leaving the fuselage interior volume clear for payload. Engines can then be rear-mounted, wing-mounted, or a combination of them. Both have advantages and disadvantages. Twin- and four-engined turbo-prop aircrafts will almost inevitable require the engines to be wing-mounted. In combat aircrafts the fuselage is not required to carry an internal payload, so it’s an ideal location for the engines.
Twin- or multi-engined propeller-driven aircrafts must have their engines spaced out along the wing to provide clearance between the propeller tips and the fuselage. The closer the engines are to the fuselage, the more noise is generated inside the fuselage, and the further away they are, the more the aircraft yaws if an engine fails. Wherever the engines are located, they must be supplied with fuel. In twin- or multi-engine installations it is a requirement that the fuel supply can be maintained if any component fails. In case an aircraft should land due to some kind of emergency early on the flight, the aircraft overall weight may be too high for a safe landing. Therefore, the pilot has the possibility to dump fuel. Propellers fitted to many aircrafts often have the capability of varying their pitch. If the pitch is controlled automatically, an engine can be operated at constant speed. If an engine fails, the propeller will windmill. This causes extra drag, and may further damage the engine, so the propeller is feathered: the blade pitch is changed until the blades sit approximately in line with the air stream.

Aircraft Wing Contents

Providing lift is the main function of the wings of an aircraft. The wings consist of two essential parts. The internal wing structure, consisting of spars, ribs and stringers, and the external wing, which is the skin.
Ribs give the shape to the wing section, support the skin (prevent buckling) and act to prevent the fuel surging around as the aircraft manoeuvres. They serve as attachment points for the control surfaces, flaps, undercarriage and engines. The ribs need to support the wing-panels, achieve the desired aerodynamic shape and keep it, provide points for conducting large forces, add strength, prevent buckling, and separate the individual fuel tanks within the wing. There are many kinds of ribs. Form ribs consist of a sheet of metal, bent into shape. Plate-type ribs consist of sheet-metal, which has upturned edges and weight-saving holes cut out into it. These ribs are used in conditions of light to medium loading. Truss ribs consist of profiles that are joined together. These ribs may be suitable for a wide range of load-types. Closed ribs are constructed from profiles and sheet-metal, and are suitable for closing off sections of the wing. This rib is also suitable for a variety
of loading conditions. Forged ribs are manufactured using heavy press-machinery, and are used for sections where very high loads apply. Milled ribs are solid structures, manufactured by milling away excess material from a solid block of metal, and are also used where very high loads apply. The stringers on the skin panels run in the length of the wing, and so usually need to bridge the ribs. There are several methods for dealing with this problem. The stringers and ribs can both be uninterrupted. The stringers now run over the rib, leaving a gape between rib and skin. Rib and skin are indirectly connected, resulting in a bad shear load transfer between rib and skin. The stringers can be interrupted at the rib. Interrupting the stringer in this way certainly weakens the structure, and therefore extra strengthening material, called a doubler, is usually added. Naturally, the stringers can also interrupt the rib. The stringers now run through holes cut into the rib, which also causes inevitable weakening of the structure.
The ribs also need to be supported, which is done by the spars. These are simple beams that usually have a cross-section similar to an I-beam. The spars are the most heavily loaded parts of an aircraft. They carry much more force at its root, than at the tip. Since wings will bend upwards, spars usually carry shear forces and bending moments.
Aerodynamic forces not only bend the wing, they also twist it. To prevent this, the introduction of a second spar seems logical. Torsion now induces bending of the two spars, which is termed differential bending. Modern commercial aircrafts often use two-spar wings where the spars are joined by a strengthened section of skin, forming the so-called torsion-box structure. The skin in the torsion-box structure serves both as a spar-cap (to resist bending), as part of the torsion box (to resist torsion) and to transmit aerodynamic forces.

The Fuselage

The fuselage should carry the payload, and is the main body to which all parts are connected. It must be able to resist bending moments (caused by weight and lift from the tail), torsional loads (caused by fin and rudder) and cabin pressurization. The structural strength and stiffness of the fuselage must be high enough to withstand these loads At the same time, the structural weight must be kept to a minimum. In transport aircraft, the majority of the fuselage is cylindrical or near-cylindrical, with tapered nose and tail sections. The semi-monocoque construction, which is virtually standard in all modern aircraft, consists of a stressed skin with added stringers to prevent buckling, attached to hoop-shaped frames. The fuselage also has members perpendicular to the skin, that support it and help keep its shape. These supports are called frames if they are open or ring-shaped, or bulkheads if they are closed. Disturbances in the perfect cylindrical shell, such as doors and windows, are called cutouts. They are usually unsuitable to carry many of the loads that are present on the surrounding structure. The direct load paths are interrupted and as a result the structure around the cut-out must be reinforced to maintain the required strength. A typical freighter aircraft will have a much larger door than a passenger aircraft. It is therefore necessary for them to transmit some of the loads from the frames and stringers. Where doors are smaller, the surrounding structure is reinforced to transmit the loads around the door.
In aircraft with pressurized fuselages, the fuselage volume both above and below the floor is pressurized, so no pressurization loads exist on the floor. If the fuselage is suddenly de-pressurized, the floor will be loaded because of the pressure difference. The load will persist until the pressure in the plane has equalized, usually via floor-level side wall vents. Sometimes different parts of the fuselage have different radii. This is termed a double-bubble fuselage. Pressurization can lead to tension or compression of the floor-supports, depending on the design. Frames give the fuselage its cross-sectional shape and prevent it from buckling, when it is subjected to bending loads. Stringers give a large increase in the stiffness of the skin under torsion and bending loads, with minimal increase in weight. Frames and stringers make up the basic skeleton of the fuselage.
Pressure bulkheads close the pressure cabin at both ends of the fuselage, and thus carry the loads imposed by pressurization. They may take the form of flat discs or curved bowls.
Fatigue is a phenomenon caused by repetitive loads on a structure. It depends on the magnitude and frequency of these loads in combination with the applied materials and structural shape. Fatigue-critical areas are at the fuselage upper part and at the joints of the fuselage frames to the wing spars.


Airfoil can be definead as a shape of wing, as seen in cross-section. In order
to describe an airfoil, we must define the following terms.

  • The mean camber line is a line drawn midway between the upper and
    lower surfaces.
  • The leading and trailing edge are the most forward an rearward of the
    mean camber line.
  • The chord line is a line connecing leading an trailing edge.
  • The chord length is the distance from the leading to the trailing edge, measured along the chord line.
  • The camber is the maximum distance between mean camber line and chord line.
  • The thickness is the distance between the upper and lower surfaces.