Aircraft Construction Details
Airframe Construction, II
An aircraft is build up from a number of major components: fuselage, wings, empennage, undercarriage and one or more piston or turbine engines. Each of these components consists of hundreds sometimes thousands of individual parts and all are 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 possible loads an aircraft may encounter during its lifetime of flying. 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 too and with a healthy safety margin of at least 50%.
This page takes a look into aircraft fuselage and wing construction methods.
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 strength 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 strength 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.
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 elliptical 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 in smaller ultralight types.
Wing Construction Details
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 type 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 as the Cardinal and P210. Most low wing aircraft types are cantilever.
The wing itself is constructed around a main spar, like a H-beam, running the length of the wing to which 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, either in a wet wing form or with composite tanks or a fuel bladder. Other aircraft are known to use the nose rib area (ahead of the spar) as a fuel tank (VANs does this).
No Spare Ribs
The ribs form the wing and determine the aerodynamic shape and thus its performance in terms of lift and drag. Made from aluminum alloy and they take the load of the skins to the spar.
The skins are attached by solid or avex rivets to the spar and ribs. The skins are reinforced with stringers, they run length wise (from root to the tip) and stabilize the skins to protect against buckling when the wing is under compression loads. Not all aircraft have these stringers, for example: Tecnam doesn't use them but Murphy does and their wing structure is really strong.
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 usually have a H-beam section across the floor so that the fuselage sits on the right and left wing structure.