Malaysian Institute of Aviation TechnologyAircraft Structure

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Airframe Structure General Concepts

Prepared by: Syed Idrus Syed OmarConstruction Specfications and documentation

Specifications

The relevant construction specifications must not be considered and complied with during construction only but also during the design phase. Figure 1 shows the main relevant construction specifications according to FAR (Federal Aviation Regulations) and JAR (Joint Aviation Requirements).

These construction specifications are important for

Possible damage assessmentSpecial installationsPrototypes.

Specifications to be met can be found in the relevant FAA and JAA regulations.Refer to Figure 1.

Figure 1:

Documentation

In addition to the construction specifications, a complete set of documentation must be available. This includes

All documents from the project up to the delivery (configuration management) All papers issued after delivery.

When an aircraft is purchased it is to be ensured that a maintenance manual is handed over to the operator. This maintenance manual must include the essential information for correct maintenance to maintain the airworthiness.

Truss Fuselage Construction

By definition, a truss is a form of construction in which a number of members are joined to form a rigid structure

Pratt Truss Construction

Used by many early aircrafts in which wooden longerons served as the main lengthwise structural members and were held at proper distance apart by wood struts. Each bay or space between the struts was crossed by 2 piano wire stays whose tension was adjusted by brass turnbuckles. (See figure 2)The basic characteristic of this construction is that its struts carry only compressive loads, while the stays carry only the tensile loads.

Figure 2: Pratt Truss Construction

Warren Truss Construction

When technology progressed to the extent that fuselages could be built of welded steel tubing, the Warren truss construction became popular. In this type of truss type, the longerons are separated by diagonal members that can carry both compressive and tensile loads. (See figure 3)

Figure 3: Warren Truss Construction

The smooth aerodynamic shape required by an aircraft fuselage is provided for those using both Pratt and Warren trusses by the addition of a non-load-carrying superstructure, and the entire fuselage is covered with cloth fabric.

Stressed-Skin Fuselage Construction

There are three types of stressed-skin fuselage construction, namely the natural stressed-skin, the monocoque structure and the semi-monocoque structure.

Natural Stressed-Skin Structure

This skin does not require the angular shape that is necessary for a truss, but can be built with a very clean, smooth, and aerodynamically efficient shape. All of the loads are carried by the outside skin.

One of the best examples of a natural stressed--skin structure is the chicken egg. The fragile shell of an egg can support an almost unbelievable load, when it is applied in the proper direction, as long as the shell is not cracked.

The main limitation of a natural stressed--skin structure is that it cannot tolerate any dents or deformation in its surface. This characteristic can be demonstrated with a thin aluminum beverage can. When the can is free of dents, it will withstand a great amount of force applied to

its ends, but if we put only a slight dent in its side, it can be crushed very easily from top or bottom.

Monocoque Structure

In this type of structure, the upper and lower skins are made of thin sheet aluminium alloy that have been formed into compound curved shapes with a drop hammer or a hydropress. The edges of both of these skins are bent to form a lip which gives the skin rigidity. These skins are riveted to bulkheads/frames that have been pressed from thin sheet aluminium in a hydropress. The sides of the fuselage between the top and bottom skins are made of flat sheet aluminium, riveted to the skins and to the bulkheads/frames. (See figure 4)

This type of construction is economical and has sufficient strength for these relatively low-- stress areas. It is very important that all repairs to monocoque structure restore the original shape, rigidity and strength to any area that has been damaged

.Semi-Monocoque Structure

Figure 4: Monocoque Construction

This type of structures provides more strength by providing a substructure that consists of frames and stringers and the skin is riveted to it. The frames and bulkheads are made of sheet metal that has been formed in a press and the stringers are made of extruded aluminium alloy.

The bulkheads can also act as compartment walls. The stringers can have a bulb on one of their sides to provide added strength needed to oppose bending loads. The longerons are also made of extruded aluminium alloy, but are heavier than the stringers and assist the skin to carry more of the structural loads in the fuselage. (See figure 5)

Wing

Figure 5: Semi Monocoque Construction

The wings of an aircraft have a special aerodynamically efficient shape called an airfoil section that allows them to force down the maximum amount of air as they move through it. The wing provides for lift generation and control (aileron) as well as for fuel storage and possibly for the accommodation of engine(s) and landing gear. The wings are designated left and right corresponding to left and right side of the pilot when seated in the flight deck or cockpit. Theparticular wing design for any given aircraft depends on a number of factors, such as size, weight, use of the aircraft desired speed in flight and at landing, and desired rate of climb.

Full Cantilever Wing Design

The wings are built so that no external bracing is needed. They are supported internally by structural members assisted by the skin of the aircraft. (See figure 6)

Semi-Cantilever Wing Design

Aircraft wings that use external struts or wires to assist in supporting the wing and carrying the aerodynamic and landing loads. Wing support cables and struts are generally made from steel.

Figure 6: Full Cantilever Wing

Many struts and their attach fittings have fairings to reduce drag. Short, nearly vertical supports called jury struts are found on struts that attach to the wings a great distance from the fuselage. This serves to subdue strut movement and oscillation caused by the air flowing around the strut in flight. (See figure 7)

Figure 7: Semi- Cantilever Wing

Wing Configuration

Airplanes with a single set of wings are referred to as monoplanes while those with two sets are called biplanes. (See figure 8)

MONOPLANEBIPLANE

Figure 8: Wing Configuration

The monoplanes are further divided In 3 categories in relation to the rooting or attachment of the wings to the fuselage.

Low wing configuration. The wing is rooted at the bottom part of the aircraft fuselage, usually just below the pilots seat in GA aircraft. Good for touring and passenger aircraft, low drag, easier to land due to higher ground effect but need fuel pumps. (See figure 9)

Figure 9: Low wing attachment

Mid wing configuration. The rooting takes place approximately half way up the fuselage.Nice very strong wing and fuselage binding for aerobatics. (See figure 10)

Figure10: Mid wing attachment

High wing configuration. Here the wings are rooted on top of the fuselage. Better ground visibility, gravity fuel feed, but high drag. (See figure 11)

Empennage

Figure 11: High wing attachment

The empennage is also known as the tail section and consists of the rear fuselage section, the vertical and horizontal stabilizers and movable control surfaces. The empennage with vertical tail surfaces is joined to the rest of the fuselage in the main assembly jig. The stabilizers help to stabilize the aircraft and the moveable control surface help to direct an aircraft during flight. The movable control surfaces are usually a rudder located at the aft edge of the vertical stabilizer and an elevator located at the aft edge the horizontal stabilizer. (See figure 12)

Figure 12: Empennage or Tail Section