6.3.2- Wooden Structures Flashcards
What structure do fabric covered aircraft have?
Truss type.
What are the main lengthwise of a wing called and in wooden structures what are they made of.
Spars
Solid or laminated sitka spruce.
What are spars separated by?
Compression members (joints). They can be made of steel or alloy tubing.
Why are compression ribs sometimes used?
Specially strengthened for compressive loads.
What are the trusses held together with?
Steel wires that cross the bays formed by the compression struts.
What is the definition of drag wires?
Extending from front to rear spar they opposing drag forces.
A wire that attaches to the rear spar inboard and front spar outboard are called?
Anti-Drag Wires, oppose forces that move the wing forward.
What is the box spar and adaptation of?
Truss type.
What does a box structure built between the spars do?
Carries all the bend and torsional loads.
Why would you apply varnish to the wood surface?
After gluing its applied to exposed surfaces to protect the timber and its joints.
What can cause timber shrinkage?
Large cyclic changes of temperature and humidity.
Does glue age?
Yes.
What happens if ply structures are exposed to inclement weather and neglected of care?
The wood will crack and allow ingress of moisture.
When inspecting the aircraft what condition should the aircraft be in?
Housed in a dry, well ventilated hangar and all inspection panels open or removed.
How long should timber be stacked before shipment?
Approx. 60 days.
How long is timber seasoned for in air drying sheds?
1-3 years. If needed sooner it can be artificially seasoned in a kiln.
Once timber is seasoned properly what is the next step?
Take sample cuts to be tested for suitability, approx. 6 inches should be removed from the ends of the timber before the samples.
How do you determine moisture content of timber?
Weigh it, then dry it in a oven at 100°c until two successive weighing’s have the same result.
What’s a quicker way to determine the moisture content instead of the weighing method?
To use a moisture meter, it should be checked for accuracy periodically. The guide for moisture content is 15% +/-2.
What’s a practical way of measuring the density of the timber?
Cut a sample 96mm long (3.8 inches), by 25mm (1 inch). For grade A spruce it should not weigh more than 10.89kg (24lbs).
How can you test the brittleness?
Using an Izod impact tester or drop weight machine.
How is the splitting test carried out?
With a blunt chisel.
What happens to timber with a rate of growth less than 6 rings per 25mm?
Its rejected.
How long should the test samples of timber be kept?
No less than two years.
What is rift sawing?
Cutting along the radius of the annular ring. Very similar to near-quarter sawing.
What is Tangential Sawing?
Cutting at a tangent to the annular rings, it produces what is known as the flower face.
How is it decided which sawing method to use on timber?
Direction of the annular rings at the end section.
What’s the main advantage of rift sawing?
It shrinks in one direction and doesn’t warp much.
What is the most economical way to saw wood?
Tangential or flat sawing.
Name some of the defects seen in timber? (9 examples)
Dote Disease Decay Incorrect Grain Inclination Heart Shake Ring Shake Compression Shake Knots Pitch holes Blue Stain
Describe Dote Disease.
Worst of all defects. Occurs at base of tree. Identified through brown-yellow patches. Its contagious and infected wood should be burnt.
What is the limit of incorrect grain inclination?
Spruce Grade A- 1 in 15
Spruce Grade B- 1 in 12
What is heart shake and how can it be solved?
A defect along the sap duct longitudinally and visible on the tangential surface. It can be cut out.
What is ring shake and what causes it?
Causes parting of annular rings, usually caused by frost, it can be cut out.
Why are compression shakes so dangerous?
They form a partial fracture of the timber and future loads may be fatal.
What’s the limit on Knots?
No more than a quarter of an inch but this is not a hard and fast rule.
What is blue stain?
Only occurs in sapwood which should not be used in aircraft.
What two woods are good for spruce repairs as they are stronger and have good characteristics?
Douglas fir
Western Hemlock
White cedar.
What is BS 1204
Gives terminology for wood resin and adhesives.
