6.3.2- Wooden Structures

Question 1

What structure do fabric covered aircraft have?

Answer 1

Truss type.

Question 2

What are the main lengthwise of a wing called and in wooden structures what are they made of.

Answer 2

Spars

Solid or laminated sitka spruce.

Question 3

What are spars separated by?

Answer 3

Compression members (joints). They can be made of steel or alloy tubing.

Question 4

Why are compression ribs sometimes used?

Answer 4

Specially strengthened for compressive loads.

Question 5

What are the trusses held together with?

Answer 5

Steel wires that cross the bays formed by the compression struts.

Question 6

What is the definition of drag wires?

Answer 6

Extending from front to rear spar they opposing drag forces.

Question 7

A wire that attaches to the rear spar inboard and front spar outboard are called?

Answer 7

Anti-Drag Wires, oppose forces that move the wing forward.

Question 8

What is the box spar and adaptation of?

Answer 8

Truss type.

Question 9

What does a box structure built between the spars do?

Answer 9

Carries all the bend and torsional loads.

Question 10

Why would you apply varnish to the wood surface?

Answer 10

After gluing its applied to exposed surfaces to protect the timber and its joints.

Question 11

What can cause timber shrinkage?

Answer 11

Large cyclic changes of temperature and humidity.

Question 12

Does glue age?

Answer 12

Yes.

Question 13

What happens if ply structures are exposed to inclement weather and neglected of care?

Answer 13

The wood will crack and allow ingress of moisture.

Question 14

When inspecting the aircraft what condition should the aircraft be in?

Answer 14

Housed in a dry, well ventilated hangar and all inspection panels open or removed.

Question 15

How long should timber be stacked before shipment?

Answer 15

Approx. 60 days.

Question 16

How long is timber seasoned for in air drying sheds?

Answer 16

1-3 years. If needed sooner it can be artificially seasoned in a kiln.

Question 17

Once timber is seasoned properly what is the next step?

Answer 17

Take sample cuts to be tested for suitability, approx. 6 inches should be removed from the ends of the timber before the samples.

Question 18

How do you determine moisture content of timber?

Answer 18

Weigh it, then dry it in a oven at 100°c until two successive weighing’s have the same result.

Question 19

What’s a quicker way to determine the moisture content instead of the weighing method?

Answer 19

To use a moisture meter, it should be checked for accuracy periodically. The guide for moisture content is 15% +/-2.

Question 20

What’s a practical way of measuring the density of the timber?

Answer 20

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).

Question 21

How can you test the brittleness?

Answer 21

Using an Izod impact tester or drop weight machine.

Question 22

How is the splitting test carried out?

Answer 22

With a blunt chisel.

Question 23

What happens to timber with a rate of growth less than 6 rings per 25mm?

Answer 23

Its rejected.

Question 24

How long should the test samples of timber be kept?

Answer 24

No less than two years.

Question 25

What is rift sawing?

Answer 25

Cutting along the radius of the annular ring. Very similar to near-quarter sawing.

Question 26

What is Tangential Sawing?

Answer 26

Cutting at a tangent to the annular rings, it produces what is known as the flower face.

Question 27

How is it decided which sawing method to use on timber?

Answer 27

Direction of the annular rings at the end section.

Question 28

What’s the main advantage of rift sawing?

Answer 28

It shrinks in one direction and doesn’t warp much.

Question 29

What is the most economical way to saw wood?

Answer 29

Tangential or flat sawing.

Question 30

Name some of the defects seen in timber? (9 examples)

Answer 30

Dote Disease
Decay
Incorrect Grain Inclination
Heart Shake
Ring Shake
Compression Shake
Knots
Pitch holes
Blue Stain

Question 31

Describe Dote Disease.

Answer 31

Worst of all defects. Occurs at base of tree. Identified through brown-yellow patches. Its contagious and infected wood should be burnt.

Question 32

What is the limit of incorrect grain inclination?

Answer 32

Spruce Grade A- 1 in 15

Spruce Grade B- 1 in 12

Question 33

What is heart shake and how can it be solved?

Answer 33

A defect along the sap duct longitudinally and visible on the tangential surface. It can be cut out.

Question 34

What is ring shake and what causes it?

Answer 34

Causes parting of annular rings, usually caused by frost, it can be cut out.

Question 35

Why are compression shakes so dangerous?

Answer 35

They form a partial fracture of the timber and future loads may be fatal.

Question 36

What’s the limit on Knots?

Answer 36

No more than a quarter of an inch but this is not a hard and fast rule.

Question 37

What is blue stain?

Answer 37

Only occurs in sapwood which should not be used in aircraft.

Question 38

What two woods are good for spruce repairs as they are stronger and have good characteristics?

Answer 38

Douglas fir
Western Hemlock
White cedar.

Question 39

What is BS 1204

Answer 39

Gives terminology for wood resin and adhesives.