AIRCRAFT WOOD STRUCTURE FINAL.ppt

AIRCRAFT WOOD STRUCTURE, FABRIC

COVRING, & FINISHING MATERIAL

AFC-2

INSTRUCTIONAL TIME

• THEORY …………………………….…..….………………14HRS

– Aircraft Wood Structure ……................................…7hrs

– Aircraft Fabric

Covering……………………………………………..4hrs

– Aircraft Finishing Material ………………….………..

…………..3hrs

• PRACTICAL ………………..………………………………14HRS

• TOTAL ……………………………………….……………..28HRS

COURSE OBJECTIVE

• By the end of this course you will be able to understand

– Application of wood for aircraft structure

– Advantage & disadvantage/limitation of wooden aircraft

structure

– Classification and Types of woods and their attributes

– Different types of acceptable & unacceptable wood

structural defects and their repair method

– Types of glues used for wooden construction

– Precaution to be followed during gluing process

CONTENT

• Introduction

• Aircraft Wood classification and their types

• Different terminologies on structure of wood

• A/c wood structure

• Requirements of a/c wood & evaluating the quality of wood

for aircraft use

• Glues and Gluing

• Construction & Repair of Wood Structure

• Protection and Inspection of Wooden Structure

REFERENCES

• Aircraft Maintenance Technician Series

– Volume 1 – Structures

• Chapter 3 – Non-Metallic Structure

• AC65-9A – Airframe Handbook

– Chapter 5 – Aircraft Structural Repair/Wood Structures

AIRCRAFT WOOD STRUCTURE

INTRODUCTION

AIRCRAFT WOOD STRUCTURES CONTD…

• Introduction

– Wood been early aircraft construction material

– Desirable characteristics

• Light weight

• Strong

• Long life when properly preserved

– Amateur-built aircraft are often made of wood

• Less expensive than metal

• Less special tooling required

AIRCRAFT WOOD STRUCTURES CONTD…• Introduction contd…

– Amateur-built aircraft

• Home built by individuals as hobby than by factories as commercial product

• Subject to FAA regulation operating under 14 CFR (Code of Federal Regulation)

Part 135, but not required to meet stringent requirements

CFR:- is the codification of the general and permanent rules in USA and is

divided in to 50 titles. Title 14 deals with aeronautics & space (also known as

the FAR).

•


• Introduction Contd…

– Completely phased out by commercial manufacturers

• Difficulty for automated high volume production

• High cost of labor required

– Solution

• All metal aircraft

– Current trend

• Composite structures

AIRCRAFT WOOD STRUCTURE CONT…

• Many of lighter aircraft that were produced in 1930s and 40s

have made use of wood for structural component such as:

– Wing spars

– Ribs

– Control surface

– Fuselage

RECALL: Major a/c parts & structural (load-carrying)

members of each part.

AIRCRAFT WOOD STRUCTURE CONT….

AIRCRAFT WOOD STRUCTURE CONT….

• Introduction Cont.…

– Since 1950 – use of Mg, plastic, wood & fabric

disappeared from aircraft construction

– Since 1950 – use of Al decreased from 80% to 15%

– Current Trend – reinforced plastic and advanced

composites

– Aerospace industry requires:

– High strength to weight ratio

What do we mean by composite material?

CLASSIFICATION OF WOOD


• Aircraft Wood

– Based on their cellular structure woods are generally

classified into two (There are two principal types of woods)

• Hard Wood

• Soft Wood

- The distinction between the two is not based on the

hardness of the wood, but the cellular structure of the

wood.

- The difference in physical properties of various species of

wood are due to the cell size and wall thickness


• Hard Wood

– Come from deciduous broadleaf trees (has broad leaves and

classified as deciduous – shedding leaves)

• Have mixture of large cells causing visible pores in the

wood and

• Usually but not always heavier & denser than soft

wood

– E.g. Mahogany

– Deciduous : - A type of tree that shed or drop its foliage/leaf

at the end of growing season

EXAMPLE OF HARD WOOD

– Mahogany

– Birch

Has no smooth,

even

(uniform)

appearance

when cut in

cross section.


• Soft Wood

– Comes form evergreen trees that bear cones (fir-cone)

and has needles rather than leaves called firs.

• Characterized by fiber-like cells

– E.g. Sitka-Spruce

SOFT WOOD

HENCE SOFT WOODS:

• Have needle like or spine like leaves

• Ever greens or conifers

• Generally have high strength to weight ratio

• Used as a solid wood for spars and cap strip

• Used as a veneer for plywood cores

• Is composed primarily of fibrous cells

• Has a smooth, even (uniform) appearance when cut

in cross section.

EXAMPLE OF SOFT WOOD

• Spruce

• Fir

• White Ash/pine

There is also a third

type of tree known as

Monocotyledons

• This group includes

the palm and bamboo

trees

• They have little or

no structural value


• There are many different grades and species of woods

• Properties of aircraft structural wood


• Spruce

– Soft evergreen tree with needle like leaves & cones

• Used in making papers, medicine, A/C construction


• Douglas Fir/Noble Fir

– Important and valuable timber tree (firewood tree)

– Historically used for telephone poles & railway

attachment.

CLASSIFICATION SUMMARY

• The two general groups of trees are:

1. Conifers – softwood, needle-leaf, evergreen

2. Hardwood – deciduous, broadleaf, non-coniferous

• Conifers is the most common name applied to the first group

• The other names are not all applicable b/c some of the woods of

this group are not soft, some do not have narrow leaves as

indicated by needle-leaf, and others are not evergreen

• Coniferous trees wood is comparatively light in weight, is

easy to work, and is obtainable in large, straight pieces

CLASSIFICATION SUMMARY CONT…

• Hard wood

• Neither this name nor the others applied to this group, are

wholly be true

• Some of the woods of this group are soft

• Others are not deciduous but retain their leaves

• Birch, mahogany, maple, oak, poplar, and walnut belong in this

classification

• Hard wood are relatively heavy in weight,

• Difficult to work because of their complicated cell

structure and

• Obtainable only in relatively small lengths

WOOD TYPES FOR A/C USE

WOOD STRUCTURES

• Wood in a/c construction has been largely superseded by

aluminum alloys and steel.

• It is still used extensively. Wood propellers are still in common

use . Wood is also used with good effect for interior cabin

paneling and flooring

• ADVANTAGES

• Wood construction has very definite weight advantages when

parts are lightly loaded as in gliders or light airplanes

• Wood has the advantage of large bulk for a given weight,

combined with relatively great strength

• The ease of working is also important when only small quantities

of planes being built

ADVANTAGES CONT….

• Wood has excellent elastic properties which permit stressing

almost to the breaking point without excessive permanent

deformation

• It also has the ability to resist a greater load for a short period of

time than it is capable of carrying for a long period

• DISADVANTAGE

• The nonhomogeneity of wood is its greatest disadvantage

• The properties of wood vary even for two pieces taken from the

same tree

• The properties of a pieces of wood are also dependent upon the

moisture content

• Moreover the direction of the grain is of prime importance to their

physical properties


• Types of Wood

– Based on their construction, woods employed for a/c

use are classified as:

• Solid Wood

• Laminated wood

• Plywood

The above listed types of wood are the three forms

of wood commonly used for a/c use


• Solid Wood

– Often used for aircraft wing spars

– Disadvantage

• Difficult to get single piece large enough for spar

construction that meets all the specification

– Solution

• Less expensive laminated/plywood construction


• Plywood

• Plywood is a material made by gluing a number of plies of

thin wood together (sheets of wood glued together)

• Each sheet of thin wood is known as veneer

• Veneer over 1/10 inch thick is seldom used in the

manufacture of ply wood

• The grains of adjacent layers of veneer run at right angles

to each other, which makes plywood equally strong in

two directions

• E.g. surface plies of Mahogany or Birch & Spruce with

poplar or basswood core provide strong glue bond

PLYWOOD CONT…• Made from imported African mahogany or American birch

veneer laminated to cores of poplar or basswood with

waterproof glue

• Composed of uneven number of layers (plies) of wood

veneer bonded with special glues

• The out side layers are called face and inner layer are

called core. The rest are called cross bands.

– Plywood up to 3/16” normally has three plies

– Plywood of ¼” and higher has 5 plies

• Not as strong as solid wood or laminated wood

PLYWOOD CONT…

• Plywood is used in the construction of box spars for wings, webs

of ribs, wing and fuselage covering, specially for the leading edge

of the wing, as well as for flooring and interior cabin paneling

• Ply wood most commonly used for aircraft are made from:

•Mahogany

– It has a reddish brown appearance

•Birch

– It has light yellow or cream color

Note :- Mahogany offer better gluing than birch because of its

porosity.

•Thermosetting glue is used for aircraft plywood construction

PLY WOOD

PLYWOOD CONT….

• Except for 2 ply plywood, all plywood is manufactured with

an odd number of plies to obtain symmetry

• The center ply or plies are usually made of a soft wood

and are considerably thicker than the two face plies made

of a hardwood

• A hard wood is used for the face plies to resist abrasion, to

furnish a better contact for washers and fittings, and to

take a better finish

• A plywood with a metal sheet cemented to one face to take

excessive wear is often used for flooring

PLY WOODS

ADVANTAGE OF PLY WOODS OVER SOLID WOOD

•Highly resistant to cracking

•Not likely to warp (twist, bend, distort)

•Strength is equal in any direction

•Change in dimension with changes in moisture content is

negligible


• Laminated Wood

• Strips of wood glued together with grain of all strips run

parallel or run in the same direction

• Made up of several wood layers glued together

• Layers of solid wood bonded together with an adhesive

– Spars made of laminated wood are acceptable

• If the same high quality wood material is used

– E.g. laminated wood of strips of Sitka Spruce

– Lamination of Birch is used in making wooden propeller

• Lamination enhances uniformity & strength

LAMINATED WOOD

• Used to form a specific shape or size

• Used for:

– Components that require curved shape like

• Wing tips

• Fuselage formers

– Components which are difficult to manufacture from single

piece

• Can be used in place of any solid wood provided it meets

quality specifications

• Created from wood that may not have met solid wood

specifications but has had all defects removed

• Subject to the same specifications as solid wood

LAMINATED WOOD

• Prepare the laminates

• Apply glue on the mating surface

• Clamp the laminates together

• Wait until the glue sets

DIFFERENT TERMINOLOGIES AND

STRUCTURE OF WOOD

TERMINOLOGY OF WOOD

TERMINOLOGY OF WOOD

• Bark is the external cover

• Sapwood is part of the

tree which is alive or

partially alive and still

carries sap/liquid

• Heartwood is part of the

tree which is dead and

does not carry any sap. It

supports the treeAnnual ring

TERMINOLOGY OF WOOD

ANNUAL RINGS concentric layers of wood that can be seen at

the end of a tree trunk/stem that has been cut perpendicular

to its length.

(Concentric rings called annual rings of alternately light and

dark – represents growth)

SPRINGWOOD formed in the first season of the year, has large

cells. Light rings – Spring wood – representing fast growth.

SUMMERWOOD formed in the later growing season, denser

and stronger. Dark rings – Summer wood - Represent slow

growth

SHAKE a separation between the annual ring layers

TERMINOLOGY OF WOOD

Annual rings

TERMINOLOGY OF WOOD

GRAIN indicates the direction of the wood fibers relative

to the axis of the tree or longitudinal edge of a piece of cut

lumber (wood, timber, board, plank).

COMPRESSION WOOD a wood in which the fibers have

been damaged by compression load

KNOT a portion of a branch or a limb/member of a tree which

has been incorporated in to the body of the tree (is a

deformation of grains caused by the growth of a branch)

PIN KNOT a knot resulting from growth of a twig/branch

CHECK a radial crack that runs across the grain line

TERMINOLOGY OF WOOD

TERMINOLOGY OF WOOD

• DECAY disintegration of actual wood substance by the

action of fungi

• SPLIT lengthwise separation of fibers

• SPIKE KNOT a knot that runs through the depth of a beam

perpendicular to the annual ring

• COMPRESSION FAILURE a structural failure in wood caused

by the application of too great compressive load

• KNOT is a deformation of grains and structure in a tree

trunk/stem caused by the growth of a limb/branch

A/C WOOD STRUCTURE

STRUCTURE OF WOOD

• A tree trunk/stem is composed of four distinct parts:

1. A soft central core called the pith

2. concentric rings immediately surrounding the pith called

heart wood, which in turn surrounded by

3. Sapwood followed by

4. The bark

TERMINOLOGY OF WOOD

STRUCTURE OF WOOD CONT…

• The pith, or medulla - as it is technically called, is evident

in the sections of young trees for which it serves as a food

storage place

• In mature trees the pith is nothing but a point or a small

cavity

• Heartwood or duramen - is a modified sapwood

• Each year as a new annular ring is added to the sapwood

the heart wood also increases


• Heartwood - is heavier, tougher, and darker than sapwood

• In a living tree the heart wood is subject to attack by fungi,

but after cutting it is more resistant to insect attack, decay,

stain, or mold than sapwood.

• Sapwood or alburnum - is the younger, lighter, colored,

more porous wood located just under the bark of the tree


• The cells of sapwood are alive and serve for the storage

and transformation of food

• Sapwood is more pliable/flexible than heartwood and is

preferable when severe bending must be done

• Sapwood is as strong as heartwood except in the case of

very old trees in which the sapwood is inferior

• Bark is the husk or outer cover that protects the tree

• Bark does not serve any useful structural purpose


• Wood is composed of a great number of minute structural

units or cells

• These cells vary considerably in size and shape within a

piece of wood and b/n species

• The thickness of the cell walls and their arrangement

together with associated materials such as water,

determine the physical properties of the wood

• Due to its cellular structure, wood has good bending

strength and stiffness for a given weight

• But it has low hardness


• The difference in physical properties of various species of

wood are due to the cell size and wall thickness

• For any particular wood the strength is proportional to its

specific gravity

• All trees except monocotyledons, grow annually by the

addition of a concentric layer of wood around the outside

surface of the sapwood

• An examination of any tree trunk or log will show these

concentric layers, which are called annual rings


• GRAIN:- The grain of wood is determined by the direction of the

fibers

• It always runs along the length of a piece of lumber but it is not

always straight

• The strength of a piece of wood without reasonably

straight grain is greatly reduced

• A/C wood specifications require that the grain shall not deviate

more than 1 inch in 15 inches from a line parallel to the edge of

the lumber

• Spiral grain is a defect often found in lumber/wood

• Spiral grain occurs when the fibers take a spiral course in the tree

trunk/stem as if the tree had been twisted

GRAIN CONT….

• Diagonal grain is produced when the direction of sawing is not

parallel to the bark

• Interlocked grain occurs when adjacent layers of wood are

spirally inclined in opposite direction. This condition is found

mostly in hardwood trees

• Wavy and curly grain are the result of the wood fibers in a

tree following a contorted/distorted course

• The grain is always distorted when knots or wounds are

grown over

• These type of irregular grain weaken the wood and cause

irregular shrinkage and rough surfacing when machined

STRENGTH OF WOOD

• The strength of wood depends upon a great many factors:

• The absence or limitation of defects is a primary

consideration

• The density of the wood as indicated by its specific gravity

is a very definite indication of its quality and strength

• Its moisture content affect its strength probably more

than any other one item

• The rate of growth of the tree as shown by the number

of annual rings per inch

WOOD STRENGTH CONT…

• The strength of wood varies with density

• Wood must be kiln dried to satisfy aircraft quality

– As moisture decreases, strength increases

– Kiln drying ensures that wood has an acceptable moisture

content

• Moisture can cause wood to swell and crack.

• It also can allow fungus to develop and cause decay.

• In some instances the strength of a piece of wood is dependent

upon the locality in which it was grown

• Great care must be taken in selecting a piece of lumber for a/c

use when its strength is all important

REQUIRMENTS OF A/C WOOD

&

EVALUATING THE QUALITY OF

WOOD FOR AIRCRAFT USE


• Requirement of Aircraft Wood

– Sitka spruce is reference wood for aircraft structure

– Typical requirement for aircraft spruce & approved

substitute are found in FAA advisory circular 43.13-1B

• Acceptable Methods, Techniques, and Practices –

Aircraft inspection and repair


• Requirement of Aircraft Wood cont.…

– Typical requirements in evaluating wood for a/c use:

• Moisture (water) content – between 8 and 12%

• Minimum annual rings per inch – 6

• Maximum slope of the grain (grain deviation or slope)

– 1inch in 15inches

• Its strength

• Type of cut - Cuts with edge grain

DETERMINING QUALITY CONT…

• The slope of grain shall not be steeper than 1 inch in 15 inches

• Wood must be sawn vertical grain

– Flat grain is more susceptible to warping/distortion than

vertical grain

• No fewer than 6 annular rings per inch

– Assures that there is adequate strength due to spring and

summer wood growth

– Too much of springwood or summerwood will make the

wood unusable

REQUIREMENT OF AIRCRAFT WOOD CONTD…

• Evaluating Wood for Aircraft Structure

– End section evaluation

• Concentric rings called annual rings of alternately

light and dark – represents growth

• Light rings – Spring wood – representing fast growth

• Dark rings – Summer wood

– Represent slow growth

– Denser and heavier


• Evaluating Wood for Aircraft Structure contd…

– Wood fibers swell as they absorb moisture and shrink as they

lose it

– Changes are greater along annular rings than across

– Cutting direction determines the final strength & characteristic of

the wood

– Planks cut tangent to annular ring (Flat grain)

• Distort or warp as moisture content

changes

– Planks cut across annular rings (Vertical grain)

• Change dimension very little as moisture content changes

SAWING/CUTTING WOOD

• In sawing logs into planks, the wood can be sawed in either

of two ways:

1. Radial sawing - along any of the radii of the annual rings

which will expose the radial or vertical grain surface or

2. Tangential sawing - tangent to the annual rings.

Tangentially cut lumber is commonly called plain-sawed or flat

grain surface. Plain sawing (unacceptable for aircraft)

• A modification called quarter sawing is actually used

•Quarter sawed lumber shrinks and swells less than

tangential lumber, and develops fewer flaws in seasoning

•Quarter sawing also wastes considerable material and is

therefore more expensive than tangential sawing


• Aircraft Wood Cutting contd…

– Best cut is in such a way that most of the grain lie at 45

to 90 degree with the face of the board

• Also called quarter sawing

– A log is quarter sawn to produce maximum number of

planks with vertical ‘C’ grain

– Two methods are shown below

TYPE OF CUT

• The way a board is cut has an effect on the strength and

shrinkage characteristics of the wood

• Aircraft solid wood should be cut so that the annual rings are

parallel to the edge of the board

AIRCRAFT STRUCTURAL WOOD DEFECTS

• Aircraft Structural Wood defects - permitted

– Cross grain – Spiral/diagonal grain or combination

• If not exceed 1 inch in 15 inches

• Checked using free flowing ink

– Wavy, curly and interlocked grain

• Slope not exceeding 1:15

– Mineral Streak/line or stripe

• If careful inspection fails to reveal any decay (not allowable

if decay is present)


• Aircraft Structural Wood defects – permitted cont.…

– Knots – Sound hard knot up to 3/8” maximum if

• Not in the edges or flanges

• Not causing grain divergence more than 1:15

• No knot or other defect with in 20 inches

– Closer for smaller knots

– Knots greater than ¼” must be used with caution


• Aircraft Structural Wood defects – permitted cont.…

– Pin knot / clusters

• If small and produce small effect on grain direction

– Pitch pocket in the center portion of the beam if

• Minimum 14” apart when in the same growth ring

• Do not exceed 11/2” length, 1/8”width & 1/8” depth

• Not located on edges or flanges


• Aircraft Structural Wood defects - Not Permitted

– Decay

• Start as stains or discolorations and end up being Rot

/disintegration

– Spike knot/check

• Running completely thru depth of beam

• Perpendicular to annual rings

• Appear frequently in quarter sawed lumber


• Aircraft Structural Wood defects - not permitted

contd…

– Checks, Shakes and Splits

• Checks – longitudinal cracking across annual rings

• Shakes – longitudinal cracking between annual rings

• Splits – longitudinal cracking induced by stress


• Defects not permitted cont.…

– Compression Wood

• Very detrimental to strength & difficult to recognize

• Characterized by high specific gravity

• Has appearance of excessive growth of summer

wood

• Show little contrast in color between & summer and

spring wood


• Defects not permitted contd…

– Compression Failure

• Caused by natural overstressing forces during growth

• Also by rough handling of logs and planks

• Characterized by buckling of fibers appearing as

streaks at right angle to the grain

WATER CONTENT OF WOOD

• Free water

– Water that flows up and down the tree carrying nutrients

• Cell water

– Trapped within the walls of the wood cells and is part

of the tree

KILN DRYING

• It is a process of drying wood in an oven controlling the

temperature and the humidity

• Aircraft woods moisture content

– Free water - none

– Cell water - between 8 and 12 percent .

• Too low cell water causes

– Poor glue penetration

• Too much cell water causes

– Weak wood structure

– Unnecessary weight

– Easy attack by fungus

GRAIN ARRANGEMENT

- The grain lines should be

parallel to the edge of the

board but a deviation or

slope of 1:15 is allowed.

- The grain lines shall not

deviate more than 1 inch

in 15 inches from a line

parallel to the edge of the

lumber

NOTE

• It has been determined through research that Sitka spruce is

generally the best wood for aircraft structure due to its

– Strength

– Lightness

– Stiffness per unit weight

– Toughness when compared to other species

• Other substitutes

– Douglass fir

– Noble fir

– Western hemlock

– White cedar

GLUES AND GLUING


• Glue and Gluing

– Glues - chemicals with the ability to bond wooden aircraft

structures together

– Glues are used for joining wood in aircraft construction and

repair

– Wood aircraft depend entirely on glued joints for their

strength

– Glued joints should carry full strength across the joint under

all stress conditions

– Specification & gluing procedure – AC43.13-1B, Ch1

GLUES AND GLUING

• In wooden a/c construction glue plays a very prominent

part

• Many wood joints depend wholly upon the joining power

of glue for their strength

• It must retain its strength under adverse conditions

- As when wet

- Hot, or

- Attacked by fungus and

• It must not deteriorate rapidly with age.


• Types of Glue

– FAA approved for use on certificated aircrafts

• Plastic resin (Urea resin - all purpose glue for a/c work)

• Resorcinol

– Epoxies – need specific approval by FAA inspector before use on

certificated aircraft

– Vegetable and liquid glues such as fish glue are not satisfactory for a/c

work

TYPES GLUES CONT…

CASEIN GLUES

• Casein glue was the all purpose glue in a/c construction

prior to the development of urea resin glue

• Casein is obtained from curdled/thickened milk and is

combined with other materials to form a glue (it is usually

sold in powdered form)

- Manufactured from milk product

- Highly water resistant

- Attacked by microorganisms

TYPES GLUES CONT…

SYNTHETIC GLUES

– Are not attacked by microorganisms

1. Resorcinol formaldehyde

2. Urea formaldehyde

3. Phenol formaldehyde

4. Epoxy formaldehyde

– Component of carbon, hydrogen and oxygen

ASSEMBLY METHODS OF WOOD PART WITH GLUE

• OPEN ASSEMBLY

- Glue is applied to both surface to be joined

- Surfaces are exposed to free circulation of air

for a specified length of time

- This reduce the time required for the glue to set

up

• CLOSED ASSEMBLY :- The pieces are laid together

as soon as the glue is spread. 86

GLUES AND GLUING PROCEDURE

• A part is regarded as satisfactorily glued

– If there is continuous contact of glue film and wood, not

broken by foreign particles or air bubble

– If the strength of the joint is equal to the strength of the

wood

• Advantages of adhesive bonding(gluing)

1. Are economical

2. Distribute the stress over bonding surface

3. Resist moisture and corrosion

REQUIREMENTS FOR STRONG GLUE BOND

• The most important requirements for strong and durable

structural bonds are:

1. Selecting good quality aircraft standard glue

2. Preparation of the wood surface before applying the

adhesive

3. Performing a good bonding technique consistent with

the manufacturer’s instructions

SURFACE PREPARATION FOR GLUING

• To assure a sound glue bond the wood must be:

1. Free of any surface contaminant

2. Have proper moisture content which is between 8–12 %.

3. Clear of loosened or chapped grains

4. Free of machine marks

5. Free of any paints, oils ,waxes…

6. Should be smooth and true

GLUING WOOD

• Glues are supplied in liquid or powder form

• Powder glues must be mixed with water and catalyst

• Wood to be glued must be seasoned to the proper moisture

content.

• Thin pieces of wood, such as laminations, should have a

lower moisture content (5 – 10%) than thicker pieces (8 –

12%) in order to compensate for the relatively greater

amount of moisture they absorb from the glue.

• Wood should be machined after seasoning when it is to be

glued

• The surface must be smooth and square


• Gluing

– Good glued joint should be stronger than the wood

• i.e. when glue joint is broken, fibers should tear

before glue separation

– Surface should be perfectly flat and smooth

– Apply glue according to mfg instruction

– Clamp parts together to appropriate amount of pressure


• Proper Gluing Procedure

– Mix glue according to mfg instruction

– Apply one coat of glue to one or each surface

– Force it into the wood with putty knife

– Apply second coat and assemble parts

– Clamping pressure for soft woods – 125 to 150 psi

– Clamping pressure for hard woods – 150 to 200 psi

– Ensure both piece at same level of moisture content by

• Keeping both piece in same room at least overnight.

GLUING PRESSURE

• The function of pressure on a glue joint:

– To squeeze the glue in to a thin continuous film between

the wood layer

– To force air from the joint

– To bring the wood surface in to intimate contact with the

glue

– More important is :

• Both piece be at same level of moisture content

PRESSRE APPLICATION

GLUING PRESSURE

• The method of applying pressure depends on the size, shape, and

contour of the surface

• Prepare the laminates

• Apply glue on the mating surface

• Clamp the laminates together

• Wait until the glue sets

AIRCRAFT WOOD WORKING TOOLS:

(HAND & POWER TOOLS)

96

AIRCRAFT WOOD WORKING TOOLS

- woodworking is the forming or shaping of wood to

create, restore or repair useful object

- The quality of wood structure repair depends on: selection of the proper tools and the skill of the repair

man

HAND TOOLS

– Many of the repairs

of aircraft wood structure

are still made with hand tools.97

CUTTING SAWS

SAWS :- There are many varieties of manually operated saws designed

for all kinds of cutting

Wood saws are generally made with one of the two type of teeth: - Teeth

for Crosscut saws and Teeth for Ripsaws

SAWS CONT….

CROSS CUT SAWS :- are designed for cutting across the grain

of the wood.

The teeth of the crosscut are filed at an angle to produce sharp

and knifelike edge and are alternately bent out wards

One tooth bent to the left and one to the right

The bending of the teeth is done to provide set for clearance

This set causes the saw to make a cut, a little wider than the

saw blade so there will be adequate clearance for free

movement

CROSSCUT SAW & RIP SAW

100

SAWS CONT…. RIPSAW:- The teeth of ripsaw are filed to form small chisels

Designed to cut parallel to the grain of the wood

The ends of the teeth are almost straight across

The chips are carried out of the cut by the teeth.

Unlike cross cutting, which shears the wood fibers, a ripsaw

works more like a chisel, lifting off small splinters of wood

NOTE: if a ripsaw is used for

crosscutting, a very rough

cut will result . If a crosscut

saw is used for ripping the

teeth will load up and cause

the saw to cut very slowly

BRACE AND AUGER BIT

Brace:- is a tool in which the auger bit is mounted to provide for

hand driving of the bit

The shell of the brace is a threaded cylinder or sleeve, placed

over the jaws into which the bit is inserted

The shell & jaws together form the chuck

102

BRACE AND AUGER BIT CONT…

Wood bit or Auger bit:- is used for boring holes in wood.

The most critical part of the bit are:- feed screw, cutting edge and spurs

(cutting parts).

When the cutting parts are kept sharp & even in dimension, the bit will

cut most effectively

103

CLAMP

C-clamp :- is used to hold two or more pieces of materials together

temporarily while glue or other adhesive is setting and also used to hold a

workpiece aganist the bench when performing different activities

Parallel clamp :- is used to hold a spliced spar in position and under

pressure while the glue is drying

104

NOTES

1. When boring with an auger bit:

Clamp a flat piece of scarp wood tightly against the back side of

the wood

Bore through from one side until the feed screw breaks through

the other side. Then back the bit out and bore through from the

other side

2. The pressure of the C- clamp must be evenly distributed over an

area of the C- clamp jaws.

3. The pressure should never be released once it has been applied to

prevent air from entering the joint and reducing the effectiveness

the bond105

HAMMERS

Nail hammer also called

carpenters hammer used to

drive nails or to remove nails

Magnetic tack hammer:-

Is particularly useful to the

aircraft maintenance

technician because it is light

and it will hold steel tacks or

small nails on its magnetic

face.

106

PLANE

One of the most essential

tools for the woodworker

Used to smooth the

surface of wood by

shaving.

There are three common

types of bench plane which

are categorized by their

length

block plane 7 in.

smooth plane 7-9 in.

Jack plane 11-15 in.107

APPLICATION OF PRESSURE WHEN SMOOTING BY USING PLANES

108

POWER TOOLS FOR WOOD WORKING-

109

• The following power tools are some of the many which

have great importance

1. Circular saw :- is employed principally for:

- Ripping:- cutting parallel to the grain of wood; Beveling;

Crosscutting

2. Bench saw:- is provided with an adjustable fence & a

protractor to provide for a wide variety of sizes and angles

- The fence (rip fence) is designed to the board being ripped

- The work moves against (in a direction opposite) the

rotation of the saw

CIRCULAR SAW

110

The saw blade for a

circular saw is a disk of

steel with teeth cut on

the rim.

The teeth of a ripsaw

have the appearance

of sharply angled

chisels

The teeth of a crosscut

saw are ground with a

bevel on opposite sides

of the alternate teeth

POWER TOOLS CONT…

3. Band saw:- is used primarily for cutting curved outlines and

is better swited for cutting very small pieces than is the

bench saw (has smaller teeth – thinner & narrower)

4. Jig saw: - is used primarily for very fine and intricate work on

comparatively small parts

- It can also be used for cutting very small curves and

irregular outlines in sheets of wood

5. Disk sander:- is a valuable tool for producing fine wood

work

- Is generally used to smooth rough cut surfaces & to aid in

shaping small parts.

CONSTRUCTION AND REPAIR OF

WOOD STRUCTURES

BENDING AND FORMING WOOD

• It is necessary to achieve the desired shape of components

while maintaining the structural strength

• Wood structures made by bending are stronger than those

made by forming

• Whether the curved wooden parts are steamed and bent or

laminated and bent without steaming, the grain follows the

curvature of the a/c part

• The degree of being shaped depends on: - Size of the piece

– Type of wood and - Technique used in bending the wood

(hand bending for slight curvature & forming die and

holding strap for pronounced curvature)

FORMING DIE AND HOLDING STRAP

BENT DONE BY USING A FORMING DIE AND HOLDING STRAP

(Bending stock around forming die)

The wood to be bent is

fitted snugly/tightly b/n

the bulkheads and

then bent over the

forming die

The wood having been

bent, should remain in

the forms until it has

cooled and dried

enough to keep its

shape

WOODS FOR BENDING

• Commonly used woods for bending are:

- Sprus, Ash & Oak

• Airframe component made of bent solid wood

- Wing tip bow

- Rib cap strip

- Fuselage stringers

• Solid wood is bent only for a very large radius

• Curved laminated structure are used for:

• Tip bows

• Former

• Bulkheads

NOMENCLATURE FOR WOODEN WING

WOODEN FUSELAGE

WOOD STRUCTURE PROBABLE DFECTS

FOUND DURING INSPECTION

• DRY ROT AND DECAY

– Caused by fungus in damp or wet wood

• SEPARATED GLUE JOINT

– Improperly glued

• CRACKS

– Caused by shrinkage of the wood or stress applied to it

• SURFACE CRUSHING

– Caused when the wood is struck by hard object

WOOD STRUCTURE DEFECTS CONT….

• COMPRESSION FAILURE

– Caused by a compression force acting essentially

parallel to the grain of the wood

• STAINING

– Caused by moisture indicating that a glue joint has failed

or that the protecting coating is deteriorating.

• CORROSION

– Caused due to the presence of moisture in attachment

points of hard wares

INSPECTION METHODS

• Visual inspection

– Likely locations are any possible points for moisture

entry

– Overall check for swelling or warping

– Check finish for any cracking or defects that may expose

wood to outside environment

• Tapping/beating

– Suspected areas will sound hollow or be soft

• Probing/questioning

– If soft or mushy/sappy then the wood is rotted.

INSPECTION METHODS

•Odor

•Musty or moldy smells when removing access panels

indicates moisture or fungal growth

•Moisture meters

– Any reading over 20% indicates probable fungal growth

•Destructive testing of sample bonded joints

– Testing sample bonds after making repairs is highly

recommended to test adhesive strength

WING SPAR CONSTRCTIOn

1. SOLID SPAR

• Solid rectangular spars

• Externally or internally routed/channeled or directed spar

• Laminated solid spars

WING SPAR CONSTRUCTION

2. BUILT UP SPARS• Combination of solid wood and play wood

– I- beam – C- beam– Box beam

RIBS CONSTRUCTION

• Two types of construction

– Three piece construction

• Nose, center and trailing edge sections are built separately

and fastened with glue and nails.

– Single piece construction

• Ribs are constructed in one unit and slipped over the spars

• Wood rib is usually assembled in a rib jig – to facilitate production

and ensure accuracy without making repeated measurements.

• During rib assembly, cap strips are inserted between the blocks

to hold them in proper position for attachment of the vertical

and diagonal members and the plywood gussets.

• Gussets are attached to the cap strips, vertical and diagonal

members with nails and glue.

RIB JIG AND COMPLETED RIBA rib jig is made by drawing a pattern of the rib on a

smooth flat plank and then nailing small blocks of wood to the

plank so that they outline the rib pattern

REPAIR OF DAMAGED PLYWOOD

• Follow the manufacturer instruction

• Major Types of wood structure repairs

– Surface patches

– Splayed patches

– Flush (Scarf) patches

– Plug patches

SURFACE PATCHES

• Applied to outer surface of a plywood skin

• Applied where damage occurs between or along framing

members

• It is trimmed to triangular or rectangular shape with corners

rounded

• Doublers at the bottom and fabric at the top are applied

• The angles of the triangle/rectangle should be rounded with a

radius of at least five times the thickness of the skin

• The leading edge of the surface patch should be beveled with an

angle of at least 4:1

• Surface patches are not allowed in the range 10% of the cord

length from the leading edge

SPLAYED PATCH• Is a patch fitted into a plywood to provide a flush surface.

• The slope of the edges/ mating surfaces is cut at a 5 : 1 angle

• Used where the largest dimension of the hole to be repaired

is not more than 15T & thickness is not more than 1/10 (0.1)

inch. No reinforcement behind the splayed patch

• Two concentric circles are drawn around the damaged area

with the difference b/n the radii of the circles 5T.

• The inner circle is cut out and a patch is cut and tapered to fit

the hole

• After a patch is in place, a pressure plate cut to the exact size

of a patch is centered over the patch, with waxed paper b/n

the two

SCARF/FLUSH PATCHES• May be employed , where ever the damaged plywood skin has

a curvature with the radius of more than 100 times the

thickness of the skin.

• Similar to splayed patch, but the edges are beveled to a

slope of 12:1 instead of 5:1 for splayed patch

• In all type of large scarf patches, it is necessary to support

the glued joints with backing blocks/strips which are again

supported by means of saddle gusset at the end of framing

members ( i.e if the backing strips join the framing members).

• The thickness of the backing (reinforcement) is 3T, with a

minimum thickness of ¼ inch

PLUG PATCH• The edge is cut at right angle/ perpendicular to the surface of the

skin

• Because the plug patch is only a skin repair, use it only for

damage that does not involve the supporting structure under the

skin. Doubler (reinforcement ) required

• The skin is cut out to round or oval hole and the patch is cut to

the exact size of a hole, and when installed, the edge of the patch

forms a butt joint with the edge of the hole.

• Approved circular (round) patches have been designed for holes of

circles with diameter not more than 6 inch.

– 6 inches for large patches ; - 4 inches for small patches

– The steps to be followed during installation are given in

OJT w/s

PLUG PATCH CONT..Doubler is cut so that its outside radius is 5⁄8-inch greater than the

hole to be patched and the inside radius is 5⁄8-inch less

FINISHING REPAIRED WOOD SURFACE

• To prevent absorption of moisture, oil or other

contaminant

• To preserve the wood quality

• It is done on both the interior and exterior surfaces

• On interior surface finishing:

– Make sure to remove all loose items

– Apply two coats of spar varnish

– Areas to be glued should not be covered

FINISHING REPAIRED WOOD SURFACE

• Exterior surface finishing

– Clean properly

– Small depressions should be filled with plastic

wood

– Apply two coats of spar varnish or wood sealer

– After drying remove roughs

– Apply as many coats(skin) as possible

CARE OF AIRCRAFT WITH WOOD STRUCTURES

The key to success in the maintenances of the wood structure are:

• Selection of best wood

• Proper finishing of the wood to prevent the absorption of moisture

• Moisture is the deadliest enemy of wood

– Facilitates fungus growth

– Causes warp and crack

– Reduces structural strength

– Causes corrosion

IMPORTANT FACTOR IN PREVENTING MOISTURE (CONTROL OF MOISTURE)

• Finish with an effective water resistance coating like spar

varnish

• Provide a drain hole

EFFECT OF TEMPRATURE ON WOODEN STRUCTURE

• Causes stresses and dimensional changes that can lead to

cracks

• Loosens fittings

• Evaporation of plasticizer in coatings which leads to

brittleness and cracking.

OPERATION AND HANDLING

• Airplane having wood structure can be damaged by

improper operation and handling

– Pilot must not exceed flight limits of the air plane

– Care in landing

– Careful taxing

– Use of approved walkways on the a/c

– Lift and push on solid structure

ASSINMENTS

1. Discuss about Protection (care of a/c with wood structure)

and Inspection of wooden aircraft structure

2. Discuss and characterize the different types of glues

commonly used in a/c assembly operations which possess

the necessary properties to a satisfactory extent

3. Explain in detail about the classification and types of wood

which are employed for construction of a/c structure

THE END

THANK YOU

Documents

AIRCRAFT WOOD STRUCTURE FINAL.ppt