Post on 05-Dec-2015
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 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).
•
AIRCRAFT WOOD STRUCTURES CONTD…
• 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….
• 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?
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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.
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• There are many different grades and species of woods
• Properties of aircraft structural wood
AIRCRAFT WOOD STRUCTURES CONTD…
• Spruce
– Soft evergreen tree with needle like leaves & cones
• Used in making papers, medicine, A/C construction
AIRCRAFT WOOD STRUCTURES CONTD…
• 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 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
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
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
• 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
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
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
STRUCTURE OF WOOD CONT…
• 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
STRUCTURE OF WOOD CONT…
• 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
STRUCTURE OF WOOD CONT…
• 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
STRUCTURE OF WOOD CONT…
• 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
STRUCTURE OF WOOD CONT…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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 STRUCTURES CONTD…
• 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 WOOD STRUCTURES CONTD…
• 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 WOOD STRUCTURES CONTD…
• 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 WOOD STRUCTURES CONTD…
• 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 WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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.
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
AIRCRAFT WOOD STRUCTURES CONTD…
• 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
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
- 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
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
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.
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)
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
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