AIRCRAFT WING CONSTRUCTION

WING CONSTRUCTION

The wings of an aircraft are surfaces, which are designed to produce lift when moved rapidly through the air. The particular design for any given aircraft depends on size, weight, use of the aircraft, desired speed in flight and at landing, and desired rate of climb. The wings of a fixed-wing aircraft are designated left and right, corresponding to the left and right sides of the operator when seated in the cockpit.

Wing design may use no external bracing (Full cantilever type) or may use external bracing like struts, wires etc to assist in supporting wing and carrying. aerodynamic and landing loads.

Structural Members in wing design: Structural members in the wing construction are mainly:

a)                 Spars

b)                 Stringers

c)                  Ribs

d)                 Formers

e)                 False Spar

Unpressurized aeroplanes normally uses fabric as upper and lower covering or ‘skin’ to cover wooden/metallic wing structure. Skin carries no load but gives aerodynamic shape only.

Pressurized aircraft normally uses metallic stressed skin covering forming part of the structure and carrying part of the wing loads.

shows a cross sectional view of an all metal full cantilever (no external bracing) wing section showing main structural members.

Figure 2.8 shows main parts of a wing internal structure. The internal structure is made up of spars and stringers running span wise, and ribs and formers running chord wise (leading edge to trailing edge).

Figure 2.7:  All metal full cantilever wing cross section

The spars are the principal structural members of the wing. They are like the Longerons of the fuselage structure. Wings are attached to the fuselage structure through front and rear spars.

The skin is attached to the internal members of the wing. The skin is attached to the internal members and may carry part of the wing stresses. During flight, applied loads, which are imposed on the wing structure, are primarily on the skin. From the skin they are transmitted to the ribs and formers, which in turn transmit the load to the front and rear spars. Spars transmit the wing loads to the fuselage structures.

Figure 2.8: Main parts of a wing internal structure.

Semi-Monocoque Fuselage Construction

Semi-Monocoque Fuselage Construction (Figure 2.6)

To overcome the problems encountered in the true monocoque construction, designers thought of a substructure to be placed beneath the stressed skin so that the sub-structure carries a large share of the total load. Hence, semi-monocoque construction came into practice. The substructure is made of vertical members (rings/formers, bulkheads) connected by horizontal members (longerons and stringers)

There is wooden semi-monocoque fuselage as well as all-metal semi-monocoque fuselage.

Today, majority of aircraft fuselage is all-metal semi-monocoque in design.

Figure 2.6 shows structural members/elements that are used in the internal framework to which a relatively thin skin is attached to produce semi-monocoque fuselage construction.

Figure 2.6: Semi-Monocoque Fuselage Construction 

2 Structural Members of Fuselage

Following are the names of the most important structural members used in the fuselage structure:

a)                 Formers/Rings/Frames : Vertical members to take load, give cross-sectional ‘form’

b)                 Bulkheads: Vertical members with no central apertures to isolate one section from another, taking end loads.

c)                  Longerons: Principal horizontal structural members.

d)                 Stringers: Lighter horizontal members to be used as fill-ins between longerons

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.

AIRCARFT STRUCTURE AND STRUCTURAL MEMBERS

AIRCARFT STRUCTURE AND STRUCTURAL MEMBERS

Structure: Structure is the skeleton or framework for giving ‘shape’ to a construction and for bearing ‘load‘ applied to the construction.

Examples:

a)                 Structure of a building

b)                 Skeleton of a human body

c)                  Framework of an airplane

Structural members: Structural members are the parts or elements of a structure. When a force is applied to a structure, we called that the structure is loaded. Structure may not deform under such load. Function of the structural members is to take such load and oppose deformation.  Different structural members will be described later on.

STRESSED AND NON-STRESSED PARTS

General: Structural members be designed to take load and prevent deformation or they may be designed mainly for the shape of the structure.

Stressed parts: Structural parts that are designed to take load are called stressed parts. Thus stressed parts serves to:

a)                Take load

b)                Prevent deformation

c)                 Keep shape

STRENGTH is the principal requirement and Strength/Weight ratio is the principal factor of choice of material of a stressed part.

Non-Stressed Parts: Structural parts that are designed to give neat appearance or streamlined shape to the structure are called non-stressed parts. Strength is not the principal requirement in choosing material non-stressed parts. Non-stressed parts are used in access doors, panels, fairings, cowlings etc.

STRUCTURAL DEFINITION

General: Airframe structures are loaded differently at different sections/positions. Structural definitions apply to the structures or members of the structures according to the amount of loading or stresses on them. Structures may be defined as follows.

Primary Structures: These are parts of the aircraft structure which are highly stressed and if damaged may cause failure of the aircraft and subsequent loss of life.

Examples: spars, longerons, engine, mountings, stressed skins etc.

Secondary Structures: These are also highly stressed structural parts but if damaged will not cause a failure of A/C or loss of life.

Examples: Flooring, auxiliary frames supporting equipment like oxygen bottles, radio etc.

Tertiary Structure: These are unimportant parts lightly stressed, but essential in the construction of the airframe, Examples: Fairing, wheel doors, minor component brackets.

AIRCRAFT STATION IDENTIFICATION SYSTEM

AIRCRAFT STATION IDENTIFICATION SYSTEM

There are various numbering systems in use to facilitate location of specific wing frames, fuselage bulkheads, or any other structural members on an aircraft. Most manufacturers use some system of station marking; for example, the nose of the air­craft may be designated zero station, and all other stations are located at measured distances in inches behind the zero station. Thus, when a blueprint reads "fuselage frame station 137," that particular frame station can be located 137 in. behind the nose of the aircraft. A typical station diagram is shown in Figure 1.5.

To locate structures to the right or left of the center line of an aircraft, many manufacturers con­sider the center line as a zero station for structural member location to its right or left. With such a system the stabilizer frames can be designated as being so many inches right or left of the aircraft center line.

Figure 1.5: Fuselage stations.

The applicable manufacturer's numbering system and abbreviated designations or symbols should al­ways be reviewed before attempting to locate a structural member. The following list includes loca­tion designations typical of those used by many manufacturers.

Fuselage stations: These stations (Fus. Sta. or F.S.) are numbered in inches from a reference or zero point known as the reference datum. The reference datum is an imaginary ver­tical plane at or near the nose of the aircraft from which all horizontal dis­tances are measured. The distance to a given point is measured in inches paral­lel to a center line extending through the aircraft from the nose through the center of the tail cone. Some manufacturers may call the fuselage station a body sta­tion, abbreviated B.S.

Buttock line or butt line (B.L.): BL is a width measurement left or right of, and parallel to, the vertical center line.

Water line (W.L.): WL is the measurement of height in inches perpendicular from a horizontal plane located a fixed number of inches below the bottom of the air­craft fuselage.

Aileron station (A.S.): AS is measured out­board from, and parallel to, the inboard edge of the aileron, perpendicular to the rear beam of the wing.

Flap station (F.S.): FS is measured perpen­dicular to the rear beam of the wing and parallel to, and outboard from, the in. board edge of the flap.

Nacelle station (N.C. or Nac. Sta.): NC is measured either forward of or behind the front spar of the wing and perpendic­ular to a designated water line.

In addition to the location stations listed above, other measurements are used, especially on large aircraft. Thus, there may be horizontal stabilizer stations (H.S.S.), vertical stabilizer stations (V.S.S.) or power plant stations (P.P.S.). In every case the manufacturer's terminology and station lo­cation system should be consulted before locating a point on a particular aircraft.

Airplane Zoning system

Airplane Zoning system

A. The "zoning" process employs a three-digit numbering system to identify the areas into which the airplane has been divided and subdivided. The first digit identifies the large areas of the airplane, called "major zones" (upper fuselage, wings, etc.). Each major zone is divided into "sub-major zones" (flight compartment, etc.), identified by the second digit. Sun major zones are broken down into "zones" (radome, etc.), identified by the third digit.

B. Zone numbers run preferentially from inboard to outboard, front to back, and bottom to top. Wherever applicable, one digit of the zone number will indicate left or right zones by using an odd number for the left side and an even number for the right side. Zones that straddle the centerline are assigned an odd or even zone number.

C. The zones will be defined, wherever possible, by actual physical boundaries, such as wing spars, major bulkheads, partitions, control surfaces, etc. Individual zone numbers will be assigned to major structural components, such as passenger and cargo doors, landing gears, elevators, flaps, ailerons, etc. The area enclosed by the wing-to-fuselage fillets will have individual fuselage zone numbers. The center wing area within the fuselage and areas between the wing and fuselage floor will have fuselage zone numbers.

D. Zone boundaries will enclose related structures, such as door jambs; that is, a jamb for a specific door will not be split by a zone boundary. A unit or component mounted on a zone boundary will take its zone number from the zone in which it is removed.

1.7.3 Major Zones: Major zones are as follows (Ref. Figure 1.2):

                                       

(i) 100 LOWER FUSELAGE:      From station 239 (including radome) to station 2007 aft pressure bulkhead, below fuselage floor, including wing-to-fuselage files and center wing.

(ii) 200 UPPER FUSELAGE:     From station 275 forward pressure bulkhead to station 2007 aft pressure bulkhead, above fuselage floor, including area above nose gear wheel well and area above center wing and main gear wheel well.

(iii) 300 EMPENNAGE:            Fuselage aft of station 2007, horizontal stabilizer, including center section, elevator, aft engine inlet duct, vertical stabilizer, and rudders.

(iv) 400 POWERPLANT AND PYLON:          Includes nacelle doors.

(v) 500 LEFT WING:                                    Includes control surfaces.

(vi) 600 RIGHT WING:                               Includes control surfaces.

(vii) 700 LANDING GEARS AND DOORS.

(viii) 800 DOORS:                                        Passenger and cargo.

Figure 1.2: Major zones of a typical aircraft

Submajor Zones: Major zones are subdivided into submajor zones. For example, some of the submajor zones of major zone 100 are as follows (Figure 1.3):

110    RADOME, AVIONICS COMPT, NOSE WHEEL WELL AND AIR-COND. COMPTS

120    FORWARD CARGO COMPT, AND COMPT, TUNNELS

130    CENTER ACCESSORY COMPT

140    CENTER WING, BELOW CENTER WING AND MAIN GEAR WHEEL WELLS/

150    LEFT AND RIGHT FUSELAGE TO WING FILLET.

Figure 1.3: Example of a sub major zone (110) 

Zones: Each sub-major zone is again divided into zones. For example, some of the zones of Submajor Zones 110, 120 and 130 are as follows (Figure 1.4):

111    RADOME COMPT

112    AVIONICS COMPT

121    FORWARD CARGO COMPARTMENT FORWARD SECTION LEFT TUNNEL

122    FORWARD CARGO COMPARTMENT FORWARD SECTION RIGHT TUNNEL

123    FORWARD CARGO COMPARTMENT FORWARD SECTION

131    CENTER ACCESSORY COMPT. LEFT

132    CENTER ACCESSORY COMPT. RIGHT SIDE

Figure 1.4: Example of zones (131, 132) 

AIRCRAFT ZONAL IDENTIFICATION SYSTEM

AIRCRAFT ZONAL IDENTIFICATION SYSTEM

The uniform method of dividing the airplane structure into various identifiable areas, called "zones," is developed to simplify the location of units/components/areas, the preparation of job instructions, and the identification of access doors and panels.