Aircraft Construction Details

An aircraft is build up from a number of major components: fuselage, wings, empennage, undercarriage and one or more engines. Each of these parts consists of hundreds sometimes thousands of parts and all flying in very close formation. All these components need to be designed and constructed in such a way that they are able to withstand all these loads an aircraft encounters during its lifetime of flying. Plus a healthy safety margin in the construction of 50%.

The problem aircraft designers face is to keep the weight as low as possible but at the same time it must be strong enough to withstand all expected and unexpected loads as well.

Aircraft loadings

Aircraft are designed to be able to take a number of loads from standstill to very high speeds at low level and high altitudes with ambient temperatures of +30°C to -56°C these loads take the form of manoeuvring, gusts from turbulence and last but not least pilot induced control surface loads.

Control surface loads
Deflection of control surfaces induce a load on the wings and tailplanes of the aircraft. More so when the pilot must compensate for turbulence or is manoeuvring the aircraft around.

Manoeuvring loads
When in straight and level flight the lift generated of an aircraft is equal to its weight. But when the aircraft is turning or otherwise manoeuvring the lift increases, sometimes more than twice as high. Especially in 60° banked level turns were generated lift must be twice the weight for the aircraft not to loose altitude.

Gust loads
Air turbulence causes changes in direction and speed (velocity) and these will create gust loads on the aircraft, within a short duration of time. These rapid loads can exceed the maximum load factor an aircraft is certified for and cause buckled skin or worse.

Landing loads
Sometimes a landing is not as smooth as we would want too, intentionally or not. But the aircraft needs to take care of that too without falling apart.

All these various loads have their effect on the aircraft. They will try to bend, stretch (tensile), twist (torsional) or shear parts or even the whole aircraft. The construction must handle all these loads safely during the lifetime of the structure.

Fuselage Construction

The fuselage of an aircraft can be constructed in basically three different ways: truss, monocoque and stressed skin. The truss is a steel tube box like the construction of a crane. A good flying example is the Piper Super Cub or the engine mount of any aircraft. The strenght of the truss comes from the diagonal bracing and the truss takes all the loading in shear, bending and twisting motion.

The monocoque or single shell, is a design where all the loads are taken by the skin and there is no internal framework to assist. The strenght comes purely from its rounded form, like the shell of an egg. A good example of a monocoque airplane is the Lancair and other composite type airplanes. The skin must be fairly thick to take all loading encountered in flight and on the ground.

A combination of the two mentioned forms above is the stressed skin or semi-monocoque style. The structure consists of a series of bulkheads, longerons and skins and sometimes with stringers to stiffen the skin. Most aluminum type aircraft are of stressed skin construction. For example: Piper, Cessna, Murphy and Tecnam.

Wing construction

The wing of an aircraft must be able to withstand the bending and twisting loads it encounters during flight caused by lift, weight and inertia. There is an extra challenge for the designer as wing are usually long, not very thick and can contain fuel and sometimes the undercarriage too. Most wings are straight, on jets usually swept and can have a eliptical planform (Spitfire) as well. Wings are most of the time constructed as a stressed skin. But the truss type, with tubes and fabric, can be found too.

There are three types of wing categories: Biplane, where there are two wings above each other on struts and cross bracing wires or rods for rigidity. A very strong construction.
Braced monoplane, where the wings are supported by struts to the fuselage. Seen on many Cessna and other high wing types. Some low wing types have struts running to the top of the fuselage, not very common.
The last one is the cantilever wing, it has no struts or any outside bracing and is common on many airliner aircraft and on the strutless Cessna's. Most low wing aircraft types are cantilever.

Wing internals
The wing itself is constructed around a main spar, like a H-beam, running the length of the wing to wich the nose and main ribs are attached. The end of the main ribs are mounted to the rear spar. In some wing constructions the nose ribs are also attached to a nose spar, Murphy Aircraft uses this throughout the wings, elevator and vertical fin. The main and rear spar form a torsion box and sometimes contain the fuel tanks. Other aircraft use the nose rib area for fuel tanks (VANs does this).

The ribs form the wing and determine the aerodynamic shape and thus its performance. Made from aluminum alloy and take the load of the skins to the spar. The skins are attached by solid rivets or avex rivets to the spar and ribs. The skins are reinforced with stringers, they run length wise and stabilize the skins to protect against buckling when compressed. Not all aircraft have these stringers, for example: Tecnam doesn't use them but Murphy does.

The wing flaps and ailerons are attached to the rear spar and they are usually made of a spar, ribs and a formed skin around it.

The connection from wing to the fuselage must be quite strong as all loading runs through it. Cantilever and braced monoplanes high wing aircraft have some structural members in the roof to which the wings are attached. Low wing aircraft have a H-beam section across the floor so that the fuselage sits on the right and left wing structure.