AIRCRAFT FUSELAGE STRUCTURE

FUSELAGE STRUCTURE

Fuselage is the body of the aircraft. It is the main structure of the aircraft. Fuselage provides:

a)                 Space for cargo, passengers, accessories/equipments

b)                 Routes for controls

c)                  Installation of power plants

d)                 Attachment of wings, tail unit, Landing Gears

Fuselage structural members are:

a)     Longerons

b)     Stringers

c)      Bulkheads

d)     Formers

These members carry loads and so they are stressed members. Sometimes, skin of the aircraft may be designed to carry load and then the aircraft skin is termed as stressed-skin.

Fuselage Construction: There are two basic concepts of fuselage construction:

a)                 Truss construction

b)                 Stressed skin construction

A.      Truss Type Fuselage Construction:

Early evolution of fuselage construction is the TRUSS. A truss is an assembly of bars, rods, tubes, wires etc forming a rigid framework. Primary strength members of a truss fuselage construction are four longerons running lengthwise. They are the principal longitudinal members which are braced at intervals by various methods like:

a)                 Wire bracing

b)                 N bracing

c)                  Warren bracing

Earlier truss used wooden members as longerons, vertical and lateral supports, and wires as diagonal bracing. Later on, entire truss was constructed with steel thin wall tubing welded together. Some of them used aluminum alloy tubing riveted or bolted together.

Pratt Truss (Figure 2.3) and Warren Truss (Figure 2.4) are two variations of truss constructions.

After built up, the truss is covered with:

a)                 Fabric (usually)

b)                 Plywood

c)                  Fiberglass or

d)                 Metal sheet

In truss fuselage, skin covering is only an enclosure and the truss is the load bearing structure.

Truss construction fuselage is used in un-pressurized aircraft only.

B.      Stressed Skin Fuselage Construction:

Truss type fuselage construction carried the total load on its rigid TRUSS framework. The  ‘skin’ or the cover is only for giving the aerodynamic surface.

The next logical step in aircraft structural development came with the discovery of a form of construction in which loads are carried in the outside skin. Hence, ‘stressed skin’ construction came into the fuselage.

There are two types of stressed skin construction:

a)                 Monocoque

b)                 semi-monocoque

Figure 2.3: Pratt truss 

Figure 2.4: Warren truss

1.   Monocoque Fuselage Construction (Figure 2.5)

‘Mono’ means single and ‘coque’ means shell. Natural monocoque structures are:

a)                 Egg shell

b)                 Crab shell

c)                  Legs of lobster

Example of an artificially made monocoque structure is a beverage can made of aluminium.

The fragile shell of an egg can support an almost unbelievable amount of load when it is applied in the proper direction as long as the shell is not cracked. Thin aluminium can of coke will withstand a great amount of force applied to its end when it is free of dents.

 

Figure 2.5:  Monocoque Fuselage Construction 

Monocoque construction for an aircraft fuselage is the simulation to the natural monocoque. It has a skin built with a very clean & smooth surface and with aerodynamically efficient shape. It is sufficiently thick and rigid requiring no skeleton or TRUSS beneath it. Thus, construction is a single shell that is a monocoque fuselage. This type of construction involves the construction of a metal tube or a cone without internal structural members. Some monocoque fuselage is constructed by riveting two pre-formed halves together.

In some cases, it is necessary to have rings or formers beneath the skin but those are necessary only to give shape.  Rings/formers are not connected by longerons, so they do not carry load.

As long as there is structural integrity, monocoque construction is capable of taking designed amount of load. But, if there is a slight damage, it collapses very easily like an eggshell that crushes with a very little effort if it has a little crack. Besides this disadvantage, monocoque fuselage involves unfavorable strength/weight ratio.