6.3.2 Wooden Structures Flashcards
What two classifications do wood fall into?
Softwood and hardwood
List 2 of each classification?
(Softwood /Hardwood)
Softwood - Sitka spruce and Ash
Hardwood - Mahogany, Walnut, Birch and Balsa
What would you be looking for when deciding on a wood type - properties?
High Strength to weight ratio
Have a high modulus of elasticity ( Greatest in the direction of the grain)
Able to withstand bending and compression loads
Which wood is normally used for ply?
Aircraft Plywood has traditionally been made from Birch, Mahogany spruce and gaboon ebony
Describe the word “seasoned”
After a tree is cut down, the wood is dried out or ‘Seasoned’ allowing the wood to skrink
The moisture content is reduced to 15% by weight
Which direction would you describe the grain to run in?
Where does the most shrinkage take place in timber?
Maximum shrinkage takes place along the annual growth rings and so the timber is usually rift sawn or quater sawn
Tangential shrinkage is the most
Longitudeinal shrinkage is the least
What way is wood normally cut to reduce this effect?
(shrinkage effect)
Rift sawing (Radial sawing) is a technique of cutting boards from logs radially so the annual rings are nearly 90 degrees to the faces. When rift-sawn, each piece is cut along a radius of the original log, so that the saw cuts at right angles to the tree’s growth rings.
Quarter Sawn - is defined as boards made by sawing a log into quarters and then sawing out boards in parallel cuts with varying angles of the sides to the growth rings up to 30, 45 or 60 degrees from the annual rings.
Wood has a high elasticity in what direction?
The greatest modulus of elacticity is in the direction of the grain
Name another type of sawing timber?
Tangential Surface is when the wood is cut vertically but the cut does not pass through the centre of the wood.
Basically it is cutting a vertical slice off the wood
Describe a “shake” that is said to run with the rings and one that is with the heart?
Ring Shake - This appears as a separation of the annual growth rings and is caused by frost, perhaps after heavy rain.
Heart Shake - These are cracks that radiate from the centre of the tree. After the timber is converted they may appear as cracks in the direction of the rain in the member. The defect is cut out of the timber
When might a compression shake occur?
This is a partial fracture of the timber which may have occurred when the timber was cut down.
What is the difference between a live knot and a dead knot in wood?
Dead knots were the roots of branches which were cut off when the tree was living and so stopped growing with the tree. Timber with dead knots in it is not used on aircraft.
A live knot is the root of a branch that was living when the tree was cut down and so, it is part of the srrounding timber.
Can you describe how a compression failure might happen and how it would look (grain)?
A compression failure occurs 90 degrees to the grain.
(right angle to the grain)
A large sudden or shock load, such as a heavy landing, may cause a compression failure in wooden wing spars or fuselage members.
How are ply layers laid up (number of and direction)?
What is a scarf repair?
