Wood Structures Flashcards
What are the main lengthwise members in a wing called?
Spars
What are spars made from?
Sitka spruce
What separates the spars?
Compression joints (struts)
What are the compression joints usually made out of?
Steel or heavy-wall aluminium
Why have compressive ribs strengthened?
To take on the compressive loads
What are the wires that cross bay formed by the compression struts called and what do they extend from and to?
Drag wires
Extend from the front spar inboard to rear spar outboard
What are the wires that attach from the rear spar inboard to front spar outboard called?
Anti-drag wires
What is the use of the anti-drag wires?
They oppose any force that tend to move the tip of the wing forward
What do the drag wires do?
Oppose the force that tends to drag against the wing and pull it backwards
What are box spars used for?
Placed between spars to stiffen the spars so it can withstand all of the bending and torsional loads on the wings during flight
Besides work as an adhesive, what must glue also do to the wood it is applied on?
Must also protect the wood
What are some main factors that may cause deterioration to glued structures?
Chemical reactions such as moisture, ageing, extreme temperature
Mechanical forces due to timber shrinkage
Development if microbiological growths
Oil percolating from Engine installation
Fuel contamination due to leakage /spills from tank bays
Spillage of water drainage holes
What will natural causes will cause deterioration to fabrics?
Inclement weather and strong sunlight as it allows moisture in which will destroy weather proofing qualities
What should you be aware of when surveying a structure?
Inspections of panels and covers removed in a dry, well ventilated hangar
Smell of fungus and dampness
Look for warping and misalignment
What is warping?
The bending or twisting of a material caused by heat or dampness
What must be done when checking the glue line condition on a structure?
All protective coatings of paint should be removed by careful scraping
Use of a magnifying glass, torch and a stipulated feeler gauge to probe the joint
How does glue become defective?
If penetration of the joint is possible
What is a good indication of possible moisture ingress or decay on screws?
Discolouration on brushings
Corrosion on screws
What are some examples of miscellaneous defects on glued structures?
Shrinkage Elongated bolt hole Bruising and crushing of structural members Compression failure (shakes) Previous repairs Dry rot
What does shrinkage do?
induces stress on members and causes looseness of attachment
Can cause damage to wood fibres at the edges of the fittings or around the bolt holes
How can shrinkage be detected?
Removing any paint or varnish and attempting to insert a thin feeler gauge between the timber and the fitting
How should inspection of elongation bolt holes take place?
Bolts should be removed
Check for looseness
What causes bruising and crushing of structural members?
Over tightening bolts
How are compression failure defects made?
When there is a rupture of wood fibres
How can you test for compression failures?
Shining a light parallel to the grain
Why should attention be paid to the previous repairs if inspecting for defects?
Must know whether previous repairs were done correctly
If not, more damage may have been made
What do you look for when inspecting for dry rot?
When the wood structure begins to crumble away
How can you detect joint failure?
Breaking away of the joint and it leaves splinters
If there is joint failure but no splintering, what is likely to be the cause?
Glue failure
How could load cause joint failure?
Loads can be transmitted to the other end - secondary damage
How is timber ‘seasoned’?
After cutting, before shipment timber is stacked for approximately 60 days
Why is timber stacked for 60 days before shipment?
To allow moisture to evaporate prior to shipment
How is timber usually seasoned after shipment?
In an air-drying shed for 1 to 3 years or longer
What happens when timbers moisture content is the same as the surrounding atmospheres?
Protective covering can be applied
What does seasoning do?
Reduces the moisture content of timber to the point where it is in equilibrium to the surrounding atmospheres and enables protective treatments to be applied more efficiently
After timber has been cut and seasoned, what must happen to determine its suitability for use on aircraft?
Samples should be cut and tested
How much should be cut off the end of a test sample of timber after seasoning and why?
Around 6 inches as it may be drier than the rest
What are some characteristics of timber that must be tested to determine if it is fit for aircraft use?
Moisture Density Brittleness Splitting Rate of growth Test recording
What is the ideal moisture content for timber and what is it measured with?
15% + or - 2
Measured with a moisture metre
What must be checked for density?
Density must be checked against the grade of wood being sampled
How would you carry out brittleness tests?
The izode impact testing or the drop weight testing
How would you check splitting for timber?
Use a blunt chisel to split the sample and it will reveal the grain direction
When should timber samples be rejected?
Timber with a growth rate of less than 6 rings per 25mm should be rejected
How long should the test recordings be kept?
No less than 2 years
What are the two types of sawing used for timber?
Rift sawing
Tangential sawing
Why are the rift and tangential sawing methods used?
They produce the best strength and durability
What is rift sawing?
A method of cutting timber along the radius of the annular rings
What is tangential sawing?
A method of cutting timber at a tangent to the annular rings
What does tangential sawing produce?
Flower face
What are the 3 main woods used in aircraft maintenance?
Spruce
Noble fir
Douglas fir
Which out of the three woods used in aircraft is the priority choice?
Spruce
What are the most common defects of timber?
Dote disease Decay or rot Incorrect grain inclination Heart shake Ring shake Compression shakes Knots Pitch holes Bluestain
What is dote disease?
The worst of all the defects
An infectious, inherent disease which only occurs in the base of the tree
What is the precaution you need to take when cutting a tree with dotes disease?
Cut 10ft or more above the ground/base
What is decay or rot?
Decay happens when the wood is exposed to excessive soaking or partial drying
What is the limit of grain inclination for soruce grade A and grade B?
Grade A is 1” in 15”
Grade B is 1” in 12”
When are mineral streaks acceptable?
When there is no sign of decay
What type of wood is blue stain found in?
Sap wood
Why are Synthetic Resin Adhesives extensively used in joints for wooden structures?
Avoids localising the stresses and strains in the structure
What standards myst the SRA comply with?
CAA
What does the strength of the joint depend on?
The effectiveness of the glued joint
What preparation must be made when using SRA on plywood?
Light sanding in the direction of the grain to remove loose fibres and all dust should be removed
What preparation must be made when using SRA on timber?
Checking of the blind joints -> chalking and previous glue must be removed before gluing
Check the moisture levels are within the limits
What should you do to prepare when working with adhesives on wooden structures?
Make sure all surfaces are clean and free from grease
Make sure all traces of adhesives are removed when working with wood that is being reused
What is an adhesives pot life affected by?
Temperature
What temperature must all wooden joint structures attain?
Room temperature
What must wooden joints keep clear of during curing?
Heating sources
What must you remember when applying adhesive?
Apply to both surfaces Completely cover the surfaces Should be tacky when pressure is applied Softer surfaces need a precoat Avoid high clamping pressure May require small vent holes on large areas to prevent air process on large glue joints Temperature affects curing times
How many days must you leave the joint until it is at full joint strength?
2 days
What are two different ways to test adhesive efficiency?
Test the samples
Wet tests
How would you test the samples?
Carry out the test on off cuts of the actual components to be used or use representative test pieces.
The glued sample must be put in a vice and the joint should be broken by leverage exerted on the overhanging member. The fracture faces should show an even distribution of at least 75% of fractured wood fibres
What are wet tests?
Carried out to test the adhesive efficiency
Joints should be allowed to condition over a 2-3 week period before testing
What are glued joints designed to do?
Provide maximum strength under shear loading
What is the cause if there is a a failure in glued joints without any broken fibres?
Caused by the mix or pot life
How should adhesives be stored?
Kept in controlled environments
Shelf life observed
