AIRFRAME STRUCTURES
2 AIRFRAME STRUCTURES – GENERAL CONCEPTS
2.7 CONSTRUCTION METHODS
Page 2-20 Issue 1 – Module 11.02 04 Sept 2001
2.7 CONSTRUCTION METHODS
2.7.1 STRESSED SKIN FUSELAGE
As previously described, a variety of loads act on the airframe during flight. If a proportion of these loads can be carried by the skin covering, the underlying framework can be made lighter without loss of overall strength.
In early aircraft, all loads were taken by the framework and the covering of fabric, doped to pull it taught or of thin sheets of wood achieved streamlining, but contributed little or nothing to the strength of the airframe. As aircraft design evolved, the fabric and wood was replaced with aluminium alloy sheet. Because of its extra strength, a large part of the load can be borne by this skin, reducing the weight of underlying structure. This is called Stressed Skin construction and this method also provides a very smooth surface, because the skin is stiff enough not to be distorted by the airflow. With the advent of pressurised cabins the usefulness of a strong skin is evident when considering pressurisation loads.
A method of construction where the skin carries all the loads without supporting structure is called pure monocoque construction. A good example of a pure monocoque construction is a chicken’s egg, since it has no internal support, only the egg shell carries the load. In practice, this construction is difficult to achieve, as the skin would have to be so thick, that the extra weight penalty incurred, would severely impair the ability to fly. However, the principle is sometimes used in the construction of composite material external fuel tanks, mainly for military aircraft and even here some internal strengthening is necessary.
Monocoque Construction Figure 18
JAR 66 CATEGORY B1 MODULE 11.02
AIRFRAME STRUCTURES
engineering uk
In a stressed skin fuselage construction, about half the loads are carried by the skin and half by the supporting structure. This type of construction is called semi monocoque and its advantage is that the space within the structure is unobstructed and is used for passengers and freight.
Semi-Monocoque Construction Figure 19
2.6.1 FRAMES AND FORMERS
Frames and formers provide the basic fuselage shape, with the frames, being of more robust construction, providing strong points for attachment of other fittings such as the wings and tailplane.
2.6.2 BULKHEADS
Where extra support is required within a fuselage for mounting of components such as wings and landing gear, bulkheads are to transfer the loads to the fuselage structure without producing stress raising points.
Bulkheads can be either a complete or a partial circular frame, which usually reinforces a fuselage frame. Other examples are solid pressurisation bulkheads which are normally found at the front of the fuselage ahead of the flight deck and at the rear of the pressure cabin, or an engine firewall on the nacelles.
Page 2-22 Issue 1 – Module 11.02 04 Sept 2001
2.6.3 LONGERONS AND STRINGERS
Longerons are used in fuselage construction, where either an aperture such as a door or window requires greater support, or where a number of structural high load points such as floors, landing gear attachments, etc. need to be interconnected. They are usually of much heavier construction than stringers and can be solid extrusions or fabricated multiple part construction.
Stringers provide longitudinal shape and support to the fuselage skin. They are also the spanwise members of the mainplanes, vertical and horizontal stabilisers and flying control surfaces. Often stringers are attached to frames with fillets or gussets.
Longerons and Stringers Figure 20
JAR 66 CATEGORY B1 MODULE 11.02
AIRFRAME STRUCTURES
engineering uk
2.6.4 DOUBLERS AND REINFORCEMENT
Where the skin requires extra strengthening, at the junction of plates or around small apertures, a second layer of skin is attached over the original to reinforce it.
This extra plate is known as a doubler or a doubler plate.
Where loads are concentrated within the structure, it can be strengthened at these places by either making the material thicker, or by the addition of a number of layers of similar material. The actual amount of reinforcement being dictated by the amount of stress carried in each area.
Doubler Plate Figure 21 2.6.5 STRUTS AND TIES
Any structural item that is designed solely to take a compressive load is called a strut. Whereas an item that only takes a tensile load is called a tie. They can be found throughout a modern aircraft structure, although an ideal example would be a high performance biplane. In this type of aircraft often used for aerobatics, the struts which separate the pairs of wings, in compression and the interconnecting flying wires, in tension, take all the loads produced by the wing.
Struts and Wires Figure 22
Page 2-24 Issue 1 – Module 11.02 04 Sept 2001
2.6.6 BEAMS AND FLOOR STRUCTURES
Beams are often used laterally and longitudinally along the fuselage to support the flight deck and passenger cabin floors. Additionally they provide strong point attachments for the crew and passenger seats and as such, constitute primary structure. Modern cabin flooring is usually made up from a number of removable composite honeycomb core panels, examples of which are shown below, whereas the flight deck is often made from metal panels supported on beams.
Floor Structures Figure 23
2.6.7 METHODS OF SKINNING
Skins for light aircraft are usually simple, thin sheets of aluminium alloy, wrapped around and riveted to the internal structure.
Larger aircraft, developed since the 1950’s have their skins manufactured from heavier material with the additional use of even thicker sections in certain places where more strength is required.
As the aircraft designs became more complex, the excess weight of thicker skins in places where they are not necessarily required, became too big a penalty. To overcome this problem, the skins were rolled individually to produce a variety of differing thickness across each sheet, to cater for variations in stress.
JAR 66 CATEGORY B1 MODULE 11.02
AIRFRAME STRUCTURES
engineering uk
The latest methods are to machine or mill each skin panel individually from a solid billet, to include all stringers and risers and to provide a varying thickness all over the sheet. In this way, the skin panel is exactly the right thickness at each location, with no excess material and hence no extra weight. This method results in what is termed milled skin or machined skin. Milled wing skins give maximum strength and rigidity with minimum weight.
Panels containing areas of different thickness can also be produced from a chemical etching process where areas which have been treated, will be removed to about half their thickness by the chemical etch. The nature of the etching process ensures that no ‘stress raisers’ are introduced into the material. So called
‘waffle plates’ can be produced in this way and are shown in Fig 24.
Skinning Methods Figure 24
Page 2-26 Issue 1 – Module 11.02 04 Sept 2001
2.6.8 ANTI-CORROSIVE PROTECTION
Materials used in aircraft construction are selected primarily for their strength and tenacity. Unfortunately, many may readily suffer serious damage from corrosion unless effectively protected and the rate of corrosion attack can be extremely rapid in certain environments. One of the main considerations in the design of aircraft structure therefore, are measures for the control and prevention of corrosion.
During manufacture and assembly, a range of surface treatments are applied.
Materials are heat treated to refine grain structure, sacrificial coatings in the form of plating and cladding are employed, to retard the onset of corrosion. Epoxy primers, special paint finishes, wet-assembly techniques and the use of barrier sealants to prevent the ingress of dirt and moisture between component parts, all help to reduce the risk of corrosion. Additionally, drain holes, drainage paths and attention to good corrosion resistant design techniques for each component part, ensure that aircraft newly off the production line are protected as much as possible, before entering airline service.
Aircraft are required to operate in widely varying, often highly corrosive environments throughout the world and despite the high standard of protective treatments applied during manufacture, corrosion will still occur.
Corrosive attack may extend over an entire metal surface, may penetrate locally to form deep pits or may follow the grain boundaries within the metal. The weakening effect of corrosive attack may be aggravated by stresses in the metal and result in premature failure of the component. These stresses may be due to externally applied loads or may be internal stresses locked into the metal structure during manufacturing processes, despite the care taken to keep the risk to a minimum.
Whatever the cause and type of corrosive attack, unless preventative maintenance is carried out, damage may become so severe, it could present a serious hazard to the airworthiness of the aircraft. Rectification of advanced corrosion damage is time consuming and much of the corrosion during service can be prevented or contained by simple corrosion prevention measures
Corrosion seldom occurs on a clean dry aircraft especially if the protective coatings are completely in tact. Since aircraft have to operate outside throughout their lives, they are difficult to keep dry, but keeping the protective coatings free from scratches, dents and scores, ensuring drains which might allow water to accumulate are kept clear and keeping the aircraft clean and free of dirt are all within the scope of a good maintenance engineer.
In addition, the engineer should clear up spills from the galleys and toilets and remove deposits from engine exhausts as these are also very corrosive if left on the skin for too long.
JAR 66 CATEGORY B1 MODULE 11.02
AIRFRAME STRUCTURES
engineering uk
2.6.9 CONSTRUCTION METHODS – WING
The basic requirement for wing construction, particularly with cantilever types is for a spanwise member of great strength, usually in the form of a spar.
Conventionally, there are three general designs, monospar, two-spar or multispar.
Most modern commercial airliners, have a wing comprising top and bottom skins complete with spanwise stringers, front and rear spars and a set of wing ribs running chordwise across the wing between the spars. This forms a box-like shape which is very robust and the addition of nose ribs and trailing edge fittings produce the characteristic aerofoil shape.
Wing structures carry some of the heaviest loads found in aircraft structure.
Fittings and joints must be carefully proportioned so they can pick up loads in a gradual and progressive manner and redistribute them to other parts of the structure in a similar manner. Special attention must be paid to minimising stress concentrations, by avoiding too rapid a change in cross section and to provide ample material to handle any concentration in stress or shock loading that cannot be avoided, such as landing loads.
Typical Wing Construction Figure 25
Page 2-28 Issue 1 – Module 11.02 04 Sept 2001
2.6.10 CONSTRUCTION METHODS – EMPENNAGE
The vertical and horizontal stabilisers, elevators and rudder are constructed in a manner similar to the wings but on a smaller scale. The main structural members are the spars, with the stringers, ribs and stressed skin completing the basic design.
Typical Stabilizer Construction Figure 26
JAR 66 CATEGORY B1 MODULE 11.02
AIRFRAME STRUCTURES
engineering uk
2.6.11 CONSTRUCTION METHODS – ENGINE ATTACHMENTS
Engine mountings consist of the structure that transmits the thrust provided by either the propeller or turbojet, to the airframe. The mounts can be constructed from welded alloy steel tubing, formed sheet metal, forged alloy fittings or a combination of all three. Some typical examples are shown in Figures 27 to 29.
All engine mounts are required to absorb not only the forward thrust during normal flight, but the reduced force of reverse thrust and the vibrations produced by the particular engine/propeller combination..
Fabricated Piston Engine Mounting Figure 27
Tubular Turbopropeller Mounting Figure 28
Page 2-30 Issue 1 – Module 11.02 04 Sept 2001
Machined Turbojet Side Mounting Figure 29
JAR 66 CATEGORY B1 together. The most common method of attachment is by the use of rivets or more sophisticated types of rivets, known as fasteners. However, where high strength is required, nuts and bolts are used whilst other structural assembly is achieved by the use of adhesive bonding techniques.
Although aluminium alloy is the most common material for aircraft construction, more and more structural components and in some cases, complete aircraft, are being manufactured from composite materials like glass or carbon fibre.
Riveting is generally divided into two types: (1) solid shank rivets and (2) special fasteners. The special fastener category being sub-divided further into special and blind fasteners.
2.6.13 SOLID SHANK RIVETS
The vast majority of aircraft structure is held together with solid rivets. As will be explained later, many of the more modern designs use special fasteners and some bonded construction, but the majority are still solid rivets.
Head Shapes
In the past there have been a large number of rivet head shapes used in aircraft, but in recent years these have been reduced and standardised to four main types:
The Universal Head, sometimes known as AN70 or MS20470, is most popular and may be used to replace any protruding-head rivet. It is streamlined on top but thick enough to provide strength without protruding too much into the airflow.
A Round Head rivet, AN430, is used on internal structure where the thicker head is more suitable for automatic riveting equipment.
In internal locations where a flat head rivet can be driven more easily than either a round or universal head rivet, the AN442 Flat Head rivet may be used.
Where a smooth skin is important, flush rivets such as AN426 or MS20426, with a 100 countersink head are used. Additionally, rivets with a different countersink angle, such as 90 and 120 degrees can be found.
Rivet Head Types Figure 30
Page 2-32 Issue 1 – Module 11.02 04 Sept 2001
Types of Alloy used for Solid Shank Rivets
The identification marks on rivet heads serve two important functions. Firstly, the marks are used to identify the rivet alloy required for a special installation area and, secondly, the head markings are necessary when trying to identify which kind of rivets are being removed from an aircraft during disassembly or repair.
The alloy identifying marks are made on rivet heads at the time they are being stamped out during manufacture.
Generally, solid rivets are manufactured in five different materials:
Solid Rivet Identification Figure 31
For non-structural applications, rivets made from pure aluminium, sometimes known as 'A' rivets, may be used.
A very popular rivet is the 'AD' rivet, which has copper and magnesium added to the aluminium base metal. This rivet is heat treated during manufacture to make it strong, whilst still being soft enough to be formed easily.
When much more strength than the 'AD' rivets is required, there are two stronger rivets available. These are 'D' and 'DD' rivets but they must be heat treated to make them softer before they can be formed. The 'D' types are of 2017 alloy and the 'DD' types are manufactured from 2024 alloy. Both of these rivet types, after heat treatment, must be formed within a specific period of time (one hour for 'D' and ten minutes for 'DD' types) or they may be put into a refrigerator to maintain the softening effect. Once refrigerated they will remain useable for about 10 days.
JAR 66 CATEGORY B1
When riveting magnesium alloy sheets, there must be no copper in the rivet alloy, or dissimilar metal corrosion will set in. Therefore, a 'B' rivet, manufactured from 5056 alloy is used. This contains a large amount of magnesium with a little manganese and chromium but no copper.
Dimensions
Aircraft rivet dimensions are categorised by the diameter of the shank, ‘D’, and the length, ‘L’, measured from the end of the shank to the portion of the head that will be flush with the surface of the metal. This means that a countersink rivet is measured from the top of its head, whilst the remainder are measured from under the head.
Rivet Dimensioning Figure 32
Identification
The complete identification of a rivet includes its head style, its material, its diameter and its length. The identification code shows the diameter as a number of 1/32ths of an inch and the length as a number of 1/16ths of an inch.
For example, An MS20470AD4-4 has a universal head (MS20470), is made from alloy 2117 (AD), is 1/8" diameter (4 x 1/32”) and 1/4" long (4 x 1/16”).
2.6.14 SPECIAL AND BLIND FASTENERS.
When solid shank rivets become impractical to use, then special fasteners are used. These, you will remember, are of two types; special and blind fasteners.
The term ‘Special Fasteners’ refers first to their job requirement and second to the tooling needed for the installation. In certain locations, aircraft require strength that cannot be produced by a solid shank rivet, so a special high strength fastener is used. For example, if high shear strength is required, then special High Shear rivets are used. These are usually installed with special tools and will be discussed later in this chapter.
Page 2-34 Issue 1 – Module 11.02 04 Sept 2001
Blind Fasteners
There are several different types of blind fasteners which can be hollow or self-sealing. They include the following types, all of which can be installed from one side of the work.
Chobert
Avdel
Tucker/Pop
Cherry
Note: It is most important that the correct tools are always used with the types of rivets mentioned above.
Chobert Rivets
These are available with a snap (round) head or a countersink head and are closed by forcibly pulling a mandrel through the bore of the rivet. This closes the 'tail' and expands the rivet tightly into the hole. To seal Chobert rivets, a separate sealing pin is driven into the hollow bore of the rivet.
Chobert Rivet Figure 33
JAR 66 CATEGORY B1
Tucker/'Pop' rivets are manufactured with either domed or countersunk heads and are supplied on individual mandrels. The rivets can be either ‘break head’ or
‘break stem’ and when closed, can be sealed or open depending upon their application. Break head rivets are rarely used due to the 'foreign object' risk from the broken off heads lying within the internal aircraft structure.
Break stem rivets are be divided into two groups, short and long break mandrels.
Long break types leaves the stem in place, greatly increasing the shear strength of the rivet.
Tucker ‘Pop’ Rivet Figure 34
Cherry Rivets
These rivets, of American manufacture, are similar to Avdel rivets, except that the stem is positively locked in the rivet bore. During final forming, a locking collar is forced into a groove in the stem, preventing further movement. After the closing operation, the remainder of the stem is milled flush with the skin.
There are many different types of Cherry rivets, two of the most popular being the Cherry Lock and the Cherry Max. The Cherry Lock, however, requires a range of closing tools for different sized rivets, whilst the Cherry Max series can all be closed with a single tool.
Cherry Lock rivets are manufactured from 2017 or 5056 alloys, Monel metal or Stainless Steel, whereas Cherry Max are made from 5056 alloy, Monel or Inconel 750. They are all available with either universal or countersink heads and due to their positive locking method, can be installed in place of solid shank rivets.
Page 2-36 Issue 1 – Module 11.02 04 Sept 2001
Cherry Lock Rivet
Cherry Lock Rivet