6.3.2 Wooden Structures Flashcards
What type of structure do fabric covered aircraft have
Truss type
What are the main lengthwise members in a wing called
Spars
What are wing spars usually made from
Sitka spruce
May be solid or laminated
What is the benefit of a laminated spar over a solid spar
A laminated spar is much cheaper and just as strong
What are the compression members which separate the spars made from
Steel tubing or heavy wall aluminium alloy tubing
What are sometimes used instead of compression members
Compression ribs but they must be strengthened to take the loads
What are drag wires
Wires that extend from the front spar inboard to the rear spar outboard oppose the forces that tend to drag against the wing and pull it backwards
What are anti drag wires
Wires that attach the rear spar inboard and and go to the front spar outboard are called anti drag wires
They oppose any force that tends to move the tip of the wing forward
What do most lightweight wing trusses consist of
- Spars
- Compression members
- Drag wires
- Anti drag wires
What type of loads does a built up box spar take
Torsional loads and bending loads
What part of a wooden aircraft take majority of the load in flight
Spars
What happens if a fabric covered ply structure is subject to excess bad weather and exposure to sunlight over a large amount of time with out maintenance
The surface finish will crack allowing moisture to penetrate to the wooden structure resulting in considerable deterioration through water soakage
What should be done before commencing a survey on a wood and fabric aircraft
Aircraft should be housed in a dry, well ventilated hangar. With all inspection covers, panels and hatches removed before survey.
What must be done before examining glued joints
The aircraft must be completely dried out
What should be the first check done when the inspection panels have been opened
A smell check to check for damp
How is NDT testing done on glued wooden joints
There is no Non destructive testing available for glued joints which makes it hard to accurately gauge the quality of the joint
How would inspection of a complete aircraft for glue or wood deterioration be carried out
- Checks on trouble spots of the structure
- Considerable dismantling may be required
- Must be done in accordance with maintenance manual
- If a repair that needs to be carried out doesn’t have an approved scheme, the manufacturer must be contacted or approved body
How would access holes be cut
Circular in shape and with a sharp trepanning tool to avoid jagged edges
What should be done prior to inspecting a glue line
- All protective paint coatings should be removed
Why must the wood be completely dry before checking a glue line
To avoid getting a false impression by wet swollen wood in the glue line
How would checking a glued joint take place
After the glued line is sufficiently dried the joint must be inspected with a thin feeler gauge and if penetration is possible the joint must be discarded
How is dry rot indicated
Small patches of crumbling wood
How is water penetration indicated
Gray streaks or stains running along the grain
If water penetration is suspected what as one main method which is carried out to confirm this
Removing a few screws in the area and inspect for corrosion
How would timber shrinkage be indicated
- Stress on glued joints
- Can cause looseness in metal fittings and bolts
- If timber shrinkage is suspected all paint and varnishes should be removed and a thin feeler gauge placed between the timber and bolt head
What would be the cause of elongated bolt holes
Where bolts secure fittings which take load carrying members or where bolts are subject to landing or shear loads, the bolt holes should be examined for elongation or surface crushing of the wood fibres
What would bruising or crushing of timber be caused by
Usually by over tightening bolts
How would you detect a compression failure (compression shakes)
By running a torch over the grain to check for ruptured fibres
This would be caused to excess bending during a heavy landing
What type of loads are glued joints designed to take with out the assistance of bolts or screws
Shear loads
What is done to timber immediately after it has been cut
Timber which has been cut from selected trees is stacked prior to shipment for approx 60 days. During this time the timber looses most of its moisture and is ready for shipping
Where is timber usually seasoned
In air drying sheds ranging from 1 to3 years
It can be artificially dried with a kiln
What is the purpose of seasoning
To reduce moisture content of timber to a point where it is in equilibrium with the surrounding atmosphere and enables protective treatments to be applied more affectively
What should be done to the timber after it has been seasoned to ensure it is suitable to be used on aircraft
Samples should be cut and tested
Before this is done approx 6 inches should be cut from the end and discarded as this could be drier than the rest
What are the two methods of measuring moisture
- Weighing and drying
- Moisture meter
What is the quicker and more effective method of measuring moisture
Moisture meter
How would density of seasoned timber be measured
Weighing and measuring the volume of a sample cut from the plank
How would brittleness be detected in wood
By using izod impact testing
How can the rate of growth be determined
The number of rings per 25mm (1 inch)
The timber rate of growth less than 6 rings per 25mm should be discarded
How long should test pieces be kept
For at least 2 years
What is rift sawing
The process of cutting timber along the radius of the annular rings is known as rift sawing. Rift sawing and near quarter sawing are much the same
What is tangential sawing
The process of cutting at a tangent to the annular rings is known as tangential sawing.
Produces what is known as a flower face
What is the main advantage of rift sawn timber
It shrinks chiefly in one direction only and does not warp very much
What is the most economical form of cutting timber
Tangential or flat sawing
What is the worst form of timber defect
Dote disease
What is dote disease
Worst of all defects does a lot of damage to the wood
Occurs at the base of the living tree only
Dote is contagious and should be burnt
How can dote be identified
Brownish yellow patches similar to thin mineral oil spots
What is decay or rot
Usually developed after the tree has been felled due to excessive soaking and partial drying
How can decay or rot be identified
Brown in colour and and appears as though it has been charred. The timber is rendered soft and dry and will flake off easily
What is incorrect grain inclination
The limit of grain inclination for spruce is 1 in 15 for grade A and 1 in 12 for grade B. Inclination should be checked to ensure the above limits are not exceeded
What is heart shake
Usually follows the course of the sap duct longitudinally and visible on the tangential surface
What is compression shake
Appears on the cross section and usually takes the form of a wavy thin line
What is blue stain
This defect only occurs in sapwood which should not be used in aircraft parts
Why are synthetic wooden adhesives used extensively in wooden structures
To avoid localised stresses and strains which may be set up by the use of mechanical methods of attachment
What forms do synthetic resin adhesives come in
Liquid or powder form
What type of synthetic resin adhesive has a longer shelf life
Powder resins
How are powder resins mixed
With water, in accordance with the manufacturers guidance
How are liquid resins available to use
They are ready for immediate use
What type of utensils should be used for mixing synthetic resin adhesives
Acid proof utensils and must be very clean
What should be done to pry wood structures before gluing
They must be sanded to remove surface glazing and loose fibres
What should be done to timber before gluing
They should be suitably roughened up to form a key for the adhesive
Some adhesives contain solvents which must be allowed to evaporate before the joint is made. Why is this
If this is not done, bubbles may be created and a weakness caused
When gluing large surfaces such as boards of ply, what must be done to prevent air being trapped between the two surfaces
The drawing usually specifies the drilling of small vent holes in regular intervals
What does the setting time of wood glue depend on
Temperature (Increase in temperature will result in a decrease in setting time)
How would heat be applied to a glued joint to allow it to set
Electrically or steam heated platten
How should timber be tested for its characteristics
The test samples should be cut from the timber used for the component
How can the glue of a glued joint be tested
Should be in a vice and broken by leverage exerted on it. The fractured glue faces should show at least 75% of wood fibres evenly distributed over the fractured glued surfaces
When should wet tests be carried out
Should only be carried out in joints which have been conditioned form2-3 weeks
What type of loading are glued joints designed to take
Glued joints are designed to provide their maximum strength under shear loading
What would be a tell tale sign of a glued joint failing in tension
Tension failures appear to strip the glue from one surface leaving bare wood (a fine layer of wood fibres on the glue surface)
What temperature should not be exceeded for the storage of adhesives
21°c
