AIRCRAFT BONDED TYPE CONSTRUCTION

BONDED TYPE CONSTRUCTION

A bonded structure is a construction that is produced by chemical bonding of two or more layers of material with the application of a bonding agent (e.g thermosetting resin) between the layers.

Aircraft bonded structure is a form of LAMINATED CONSTRUCTION or a SANDWICH CONSTRUCTION.

A LAMINATED CONSTRUCTION is defined as a construction composed of laminations or layers of material firmly united by bonding.

A SANDWICH CONSTRUCTION is a laminated construction of three laminations: two facing sheets and a core sandwiched between the facing sheets.

Examples of bonded structure:

a)     A cricket bat or a wooden propeller: Laminated wood structure made of plank of    wood ( e.g birch)

b)     A honeycomb panel in a modern airplane: Sandwich structure with a cellular core (like honey cell) sandwiched between two facing sheets by bonding.

Varieties of bonded structures: There are wide variations of bonded structure, such as:

a)                 Laminated wood structure

b)                 Laminated metal structure or Metal bonded structure

c)                  Laminated fiberglass structure

d)                 Metal bonded honeycomb structure (sandwich construction)

e)                 Fiberglass honeycomb structure and so on.

Honeycomb sandwich structure in modern aircraft: Introduction of bonded structure, specially the sandwich construction (honeycomb)  in airframe design came as a major breakthrough in the search for a more efficient type of structure because bonded honeycomb structures are manufactured and used to perform their jobs in a manner different from the conventional structures.

Compared to the conventional structures, bonded structure has many excellent combinations of advantages like:

a)                 Much higher strength/weight ratio

b)                 Rigidity/Pliability as desired

c)                  Metallic or non-metallic or combination

d)                 Less or absolutely no corrosion problem

e)                 Better surface finish and aerodynamic smoothness

f)                   May be manufactured in a variety of shapes and sizes

Sandwiched constructed assemblies are used for such areas as bulkheads, control surfaces, fuselage panels, wing panels, and empennage skins, radomes or shear webs.

Figure 2.12 illustrates a section of bonded honeycomb. The honeycomb stands on end and separates facings, which are bonded to the core by means of an adhesive or resin. This type of construction as a superior strength/weight ratio over that of conventional structure. Also it is better able to withstand sonic vibration, as relatively low cost when compared with fastener cost and installation of conventional structures reduces the number of parts needed and greatly reduces sealing problems while increasing aerodynamic smoothness.

Special applications of metal bonded honeycomb may employ Stainless Steel, Titanium, Magnesium, and Plywood, Resin-impregnated paper, Glass, Nylon or Cotton cloth in various combinations.

Figure 2.12: Bonded honeycomb structure

AIRCRAFT WINDOWS & WIND SCREEN CONSTRUCTION

WINDOWS & WIND SCREEN CONSTRUCTION

Windows: The windows for the passenger compartment of a large airplane must be designed and installed so there is no possibility that they will blow out when the compartment is pressurized. They must be able to withstand the continuous and cyclic pressurization loading without undergoing a progressive loss of strength.

An understanding of the installation of cabin windows for pressurized airliners can be obtained from a study of Figure 2.9, which shows the details for the installation of a window in the Boeing 720 airliner. One passenger window consists of outer, center, and inner panes. The inner pane is nonstructural and is mounted in the cabin sidewalk lining. (It is not shown in Figure 2.9). The outer and center panes are each capable of taking the full cabin pressurization load. Fail-safe structure is ensured by the center pane, which can take shock loading subsequent to outer pane failure. All three panes are of acrylic plastic with the structural panes being stretched and formed to improve resistance to crazing and increase the strength.

Another example of the window installation for a jet airliner is shown in Figure 2.10. This window installation is utilized in the Douglas DC-10 airplane.

1. The passenger compartment window shown in Figure 2.10 consists of two acrylic panes, a silicone seal, eight clips, and a window ring pan. The inner pane is approximately 0.20 in (5mm)  thick, and the outer pane is approximately 0.40 in (10mm) thick. The two acrylic panes are installed in the seal and are separated by an air space. The outer pane takes the pressure load that exists when the compartment is pressurized. If the outer pane should fail, the inner pane is designed with the strength to withstand the pressure load, thus providing a fail-safe performance. A small hole at the top of the inner pane and a slit in the bottom of the seal permit conditioned air to circulate between the panes to prevent condensation.

Figure 2.9: Installation of window for a pressurized aircraft

Figure 2.10: Installation of  passenger compartment window 

Windscreen/windshield: Windscreen or windshield is the fixed windows in the flight compartment. Normally there are two windshields: captain’s windshield and the first officer’s windshield. Besides, the flight compartment has other windows: sliding clearview windows, and aft fixed windows.

The two windshields are to be installed from outside the airplane on either side of the flight compartment centerline and are heated for anti-icing and defogging.

Removal and installation procedures for the left and right windshield panels are normally identical.  The windshield panels are removed from outside the flight compartment after all electrical terminal blocks have been disconnected.

When a windshield panel is changed because of an overheat condition, the electrical system must be functionally checked. Figure 2.11 illustrates removal of a windshield.

Figure 2.11: Removal of windshield panel 

AIRCRAFT STRUCTURE IN THE EMPENNAGE

STRUCTURE IN THE EMPENNAGE

The stabilizers and the control surfaces of an airplane are constructed in a manner similar to the wings but wings but on a much smaller scale. They usually include one or more main longitudinal members (spare) and ribs attached the fuselage or may be a separate member which is both adjustable and removable.

The horizontal stabilizers often appear as the forward part of a wing, with the elevator serving as the rear part. Usually the airfoil section is that is, it has the same degree of chamber for both top and bottom.

STABILIZERS STRUCTURE

The internal structure consists of two main spars which extend the full length of the span. At the rear is an auxiliary spar to which four hinges are riveted to provide for installation of the elevators.

The principal structural members of the unit are rear spars and the ribs. The outside of the unit is covered with sheet-aluminum alloy, which adds considerably to the strength/the center section, which is within the fuselage. See Figure 2.9

 

Figure 2.9: Structures in empennage 

CONTROL SURFACE CONSTRUCTION

Control surfaces are:

a)                 Ailerons

b)                 Elevators

c)                  Rudders

d)                 Flaps & Slats

e)                 Spoilers

Construction structures of the control surfaces are basically same as the wings having attachment fittings to the main planes or tail planes or fins as applicable.

2.9   DOOR CONSTRUCTION

The doors for aircraft are usually constructed of the same materials used for the other major components.

Typically, the main framework of a door consists of:

a)                 A doorframe which is a strong and rigid sheet-metal structure

b)                 A sheet-metal outer skin which is riveted to the doorframe

The doors for a pressurized airliner must be much stronger and much more complex than the door for a light airplane. Typical of a door for the main cabin of a jet airliner is that the door consists of a strong framework of aluminum alloy to which is riveted a heavy outer skin formed to the contour of the fuselage. At the top and the bottom edges of the door are hinged gates that make it possible, in effect, to decrease the height of the door so it can be swung outward through the door opening.

The hinging and controlling mechanism of the door is rather complex in order to provide for the necessary maneuvering to move the door outside the airplane when loading and unloading passengers. For safety in a pressurized airplane, the door is designed to act as a plug fir the door opening and the pressure in the cabin seats the door firmly in place. To accomplish this, the door must be larger than its opening and must be inside the airplane with pressure pushing outward. This prevents the rapid decompression of the cabin that could occur if the door should be closed from the outside and the securing mechanism should become unlatched.

The doors and special exits for passenger carrying aircraft must conform to certain regulations designed to provide for the safety and well being of passengers. The FAA establishes these regulations, and they must be followed in the design and manufacture of all certificated aircraft for passengers.

The requirements for emergency exits for transport category airplanes are classified according to size and arrangement. The classifications are as follows:

Type 1 : A rectangular opening not less than 24 in wide by 48 in high with corner radii not less that one third the width of the exit. On each side of the fuselage must be located in the aft portion of passenger compartment unless the configuration of the airplane is such that some other location could afford a more effective means of passenger evacuation. All type-1 exits are floor level exits.

Type II :  A rectangular opening not less than 20 in (50.8 cm) wide by 44 in (112 cm) high with corner radii not greater than one-third the width of the exit. Unless type-I exits are required. one type-II exit on each side of the fuselage must be located in the aft portion of the passenger compartment except where the configuration of the airplane  is such that some other location would afford a more effective means of passenger evacuation. Type-II exit must be floor-level exits unless located over the wing, in which case they must have a step-up  inside the airplane of not more than 10 in (25.4 cm) and a step-down outside the airplane of not more than 17 in (42.18 cm).

Type-III : A rectangular opening not less than 20 in (50.8 cm) wide by 36 in (91.44 cm) high, with corner radii not greater than one-third the width of the exit, located over the wing a step-up inside the airplane of not more than 20 in (50.8 cm) and  a step-down outside the airplane of not more than 27 in (68.58 cm).

Type-IV : A rectangular opening not less than 19 in (48.26 cm) wide by 26 in (66.04 cm)high with corner radii not greater than one-third the width of the exit, located over the wing with a step up inside the airplane of not more than 29 in (72.66 cm) and a step-down outside the airplane of not more than 36 in (91.44 cm).

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.