Aircraft Wood Information

8/2/2019 Aircraft Wood Information

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Aircraft wood information


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Aircraft wood

In order to really appreciate the use of timber orwood in aircraft construction, It is necessary tolearn something about wood growth and

structure.

There are two types of trees:

Conifer/evergreen Deciduous


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Both hardwood and softwood trees aresaid to be exogenous meaning their

growth progress outwards from the core or

heart by the development of additionalrings.


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If a fully developed tree would be sectioned, closerexamination would reveal obviously different types of

growth:Core/Pith-Soft center of the trunk (oldest part)

Annual rings-Principle portion of the tree

Bark-outer covering/has 2 layers labeled as-cambium


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The inner part of the woody layers or annual rings isknown as the heartwood usually dark in colour in a

healthy tree.

The outer part of the woody layers is unripe and istermed sapwood. It is porous and full of sap. The

moisture causes it to be lighter than the heartwood.Sapwood contains very little strength and possesses asugary substance that could hasten decay.


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Cutting and seasoning


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Trees are normally felled at a period whenthe sap is at rest. Any other time results inthe timber containing an excess of sapwhich is difficult to dry out withoutadversely affecting the durability of thewood.

To obtain best results , a tree should be

felled when mature. Immature trees andold trees produce inferior wood, which isneither strong nor durable.


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Cutting

A felled tree is termed as from, which maybe obtained timber in various usable sizes.The process of cutting up a log is known

as conversion. There are six standardterms of reference for timber that has beenconverted which are used to define certain

sizes. There are as follows


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CUT SIZES:

Plank: 2 to 4 thick x 11 and over inwidth. Deal: 2 to 4 thick x 9 to 10 inwidth. Batten: 2 to 4 thick x 5 to 8 inwidth. Scantling: 2 to 4 thick x 2 to 4/2in width. Board: under 2 thick x 4 andover in width. Strip: under 2 thick andunder 4 in width. Wood when cut, tends towarp and twist by virtue of shrinkage as itdries out. This distortion is due toshrinkage or contraction along the lines ofthe annual rings.


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Cutting

Timber for aircraft use is converted in sucha manner as to reduce the effect ofshrinkage and to eliminate as far as

possible warp in service.

Wood cut in this manner is known as rift-sawn which is easy to distinguish

because it is cut with the direction of theannual rings at right angles to its surfce.


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Cutting

Broadly speaking rift sawing is not themost economical method of convertingtimber because there is a high percentage

of wastage.

However all the wood prepared in thismanner is usable without fear of warping.


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Seasoning

Seasoning generally refers to the drying-out process that would eliminate sap frominside of the wood.

When seasoned, timber becomes moreharder, stiffer and has an increasedresistance to decay (the presence of sapinvites decay and disease.)

Furthermore, seasoned timber is unlikelyto warp or shrink in service.


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If all moisture would be removed from thewood, timber will become excessively hardand brittle. Therefore it is important to

allow a small percentage of moisture tokeep it supple and to an extent resilient.

The amount of seasoning is measured bythe quantity of moisture left in the woodafterwards and is expressed aspercentage moisture content of the timber.


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The optimum moisture content of aircrafttimbers is 15 percent.


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Several ways of seasoning timber:

One method is to stack the timber undercover in the open to allow free passage ofair through the pile.

The second method would be to float thelog before conversion in a stream of freshwater to allow the sap to be washed out as

the water flows through the pores of thetimber.


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The third and last method is the mostwidely used method for engineering

timber and is now used exclusively for

aircraft woods. This consists of drying thetimber by means of hot air in a kiln. Boardscan be seasoned by this manner in two

weeks.


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Kiln drying consists of stacking theconverted timber in a chamber in whichthe temperature is maintained between 80

to 220 degrees Fahrenheit according tothe species of the timber being processed.The heat is provided by use of steam

pipes.


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2 types of kilns:

Compartment type- this is where a chargeof timber is placed in a chamber andremains there until the seasoning is

completed.

Progressive kiln- The lumber is stacked ontrucks and moved slowly through the

whole length of the tunnel operation.


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If we are to use the first method, It wouldproduce well-seasoned wood yet it wouldtake one to ten years in order to complete

the process.

The second process is a time efficient one,although speedy in action, it is likely to

affect the durability of the timber.


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Diseases and Defects

As with all living things, trees are subjectto nature diseases and defects duringgrowth, which can have a very serious

effect on the strength of the resultingtimber.

Diseases are caused by the action offungi, A fungus is a form of vegetationwhich can only live by feeding on organicsubstance.


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Fungus consists of minute thread-like cellswhich can penetrate wood and absorb thesubstance of the cellular tissue as food.

This process breaks up the wholestructure of the timber and can beidentified by obvious mod on the surface afine, cobweb like mould, discoloration

patches and in advanced stages, therendering of the timber into a useless,spongy mass.


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Diseases and defects

Dry-rot:cannot occurin living wood butattacks wood

exposed to warm,moist conditionscombined with poorventilation. Sapwood

and unseasonedtimber are susceptibleto this.


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Wet-rot: may occur inboth living trees andconverted timber. It is

a decomposition ofthe woody fibersbrought about bywater entering

through the bark intothe circulation of thesap.


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Druxiness: is similarto wet rot but in thiscase the water

becomes stagnant inthe timber and setsup decomposition ofthe surrounding fiber

structure.


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Foxiness: affects the fibrous structure ofover mature trees. It is a form of rot, whichis discernible on converted timber by

reddish-brown stain.


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Rind galls: are lumpyswellings on thetrunks or treebranches of trees,they are caused bythe growth of newlayers over a woundmade either by insect

attacks or abranchbeing brokenoff.


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Rammy grain: is the term given to woodwith curly grain. This is caused by faultygrowth. Rammy grained wood is difficult to

work and is weak. Therefore it isunsuitable for aircraft structural purposes.


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Cross grain: is occasioned by a bend inthe tree or due to a branch causing graindivergence. It can also be induced by

faulty conversion. Spiral grain: Is similar to cross grain and is

caused by high winds twisting the trunk.This type of disease invites insect larvaethat are deposited in tiny cracks or poresthat starts insect infestations.


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Spiral grain

Cross grain


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Heart and starshakes: Are radialcracks extending from

the center outwardsdue to shrinkage ofthe annual rings.


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Cup shakes: are theresult of theseparation of thelayers forming the

annual rings and thusappear concentricwith these rings.Normally the result of

local drilling of thecambium layer duringgrowth by abrasion.


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Windshakes: are cracks produced by thebending of the tree in the wind and takethe form of extended ring separation to a

far greater extent than specified under cupshakes.


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Ring shakes: Aresplits, which runaround the

circumference andare produced by theaction of frost due tothe expansive force of

ice crystals in thecells.


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Splits or cracks: arelength wise separationsof the wood and arecaused by the tearing

apart of the wood cells.Normally they follow thedirection of the grain andare usually caused by

excessive stresses set inwood shrinkage due tochanges in moisturecontent.


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Knots: are caused bythe growth ofbranches. In growth ,

they may beprevented by theremoval of buds asthey form.


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Pitch pockets/resin ducts:are openings in the woodbetween annual ringsfilled with resin, since

they do not interfere toany great extent with thegrain direction, smallresin ducts may be

permissible as long asgrain divergence is notappreciable.


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Woods used in aircraftconstruction:

Sitka spruce: Softwood,which grows in Canadaand the US, Is inbrownish yellow color,

straight grained andsatiny in appearance. Itsqualities are: resistant tosplitting, bending and

stiffness, due to itsstrength weight ratio, it iswidely used for spars,longerons and so forth.


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Birch: is a hardwoodand grows mostly inEurope. It isyellow/brown-white,

strong and does notsplit easily. Replacesash for laminatedmembers such as

wingtip bows inaddition to being usedfor bearing block.


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Ash: hardwood,grows in England it isyellow-white/brown in

color an is closegrained. It is toughand has good shockresisting qualities.

Used for bearingblocks


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Douglas fir: Soft wood,grown in Canada and theUS, varies from reddishyellow to orange brown

and has prominentgrowth rings. Often usedin place of spruce for itsstrength and similar

although it is heavy.Straight grained althoughresinous.


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Mahogany: Hardwoodgrown in Honduras andcentral America. Reddishbrown to dull red withstraight close grain. Has

good bending strengthwith stiffness and alongthe grain, it will withstandcompressive loads whenproperly seasoned, it willnot warp or shrink andretains glue exceptionallywell. Used for bearingblocks, propellers.


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Balsa: although very softand low in strengthproperties, it is ahardwood, which grows in

central America. Lightesttimber, due to its porosityif badly stored wouldeasily rot. Mostly used in

Aircraft fairings, fillets andlight low density contourblocks.


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Pine: or Oregon pine,is very similar tospruce and fulfills

similar duties. It isslightly heavier thanspruce, but lighterthan Douglas fir.


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Gabon: is a tropical wood ofthe mahogany family it islight, open-grained andfibrous. It is used in themanufacturing of plywoodfor aircraft use in place ofbirch. The main advantageof gabon ply is that a thickerand more stable ply skinmay be used for the sameweight as a somewhatthinner skin of birch ply.Due to gabons porosity, it isnecessary to cover it withmadapolam fabric.

W d d i i f


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Noble fir: has satisfactorycharacteristics withregards to workability,warping, splitting and

glueing. May be used asa direct substitute forspruce in the same sizesprovided that shear

stresses do not becomecritical. It is somewhatsofter in texture thanspruce.

W d d i i f


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Western Hemlock:Has a less uniformtexture as compared

with spruce andrequires carefulselection. Otherwise itis satisfactory.

W d d i i f


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White pine: (port orford)can be used as a directsubstitute for spruce inthe same sizes orslightly less providedthat such reductionsare substantiated. Canonly be glued

satisfactorily withsynthetic resinadhesives.

W d d i i f


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Yellow poplar: hasexcellent workingqualities but cannotbe used as a direct

spruce substitutewithout allowance forits slightly reducedstrength properties. It

is somewhat low inshock resistingcapacity.

W d d i i ft


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Although many types of wood werenamed, there are only four that are widelyused, these are:

Sitka Spruce

Birch

Ash

Douglas Fir

H t h k th ifi it f


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How to check the specific gravity ofwood

The best and simple way of determiningthe specific gravity of wood is to obtain atest piece fro the batch it is required to

test, This test piece should be ten incheslong and fashioned accurately into tenequal divisions.

H t h k th ifi it f


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How to check the specific gravity ofwood

In example; mark off the units ofmeasurement, and clearly mark thesedivisions with a hard pencil line at right

angles to length. Obtain a transparent tubeor beaker into which a sample has to beplaced vertically and held vertically by thesides of the container with water and insert

the sample. The relation of the resultantwater line to the total length ca at once beread off as a percentage of the length.

How to check the specific


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How to check the specificgravity of wood

e.g.. If three and a quarter inches arebelow the water level, the specific gravityis 0.325.Provided that the container is held

vertical and the test piece does not jamthe sides, a sufficiently accurate readingcan be made. At all times, read off the

percentage of submersion(the specificgravity) immediately the test piece isinserted and before it has time to absorbwater

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Aircraft Wood Information