Wood structures

Question 1

What species of wood is used as a standard for strength properties of other species of wood used in aircraft structures?

Answer 1

Spruce wood

Question 2

Is northern white pine an acceptable substitute for spruce?

Answer 2

Yes, but only if the pine is a larger size to make up for its lower strength.

Question 3

Are knots allowed in aircraft wood?

Answer 3

Yes if within limitations.

Question 4

What type of wood quality must be used in the repair of aircraft structures?

Answer 4

The wood must be of aircraft quality and equivalent to its original strength or greater.

Question 5

What is the difference between laminated wood and plywood?

Answer 5

Laminated wood has the grain of all layers approximately parallel. The grain of plywood
runs 90° with the adjacent ply.

Question 6

What are two types of glue used for aircraft wood structure repair?

Answer 6

casein and synthetic resin glue.

Question 7

What method is used to prepare plywood for bending?

Answer 7

The wood is soaked in hot water, and then place in a form until completely dry.

Question 8

What are the results of using insufficient gluing pressure when making a wood joint?

Answer 8

thick glue lines, and a weak joint.

Question 9

What is the recommended clamping pressure for a glued joint?

Answer 9

125 to 150 psi.

Question 10

How long should a wood glue joint be kept under pressure before removing the clamps?

Answer 10

The pressure time will vary depending on the temperature of the curing room. The curing
temperatures are from 70°F and up. Higher temperatures will cause the glue to set faster. To be
certain(sure) that a glue joint is satisfactory, it is recommended that it remain under pressure at about 85°F for a period of at least seven hours. Full joint strength will develop only after conditioning for at least two days.

Question 11

What are some of the various methods used to apply pressure to joints in aircraft gluing operations?

Answer 11

Brads, nails, screws, clamps, or hydraulic and electric power presses.

Question 12

Why does the strength of a beveled scarf joint depend on the accuracy of the beveled cut?

Answer 12

The effective glue area will be reduced if both beveled cuts are not the same.

Question 13

Why is sanding unacceptable when preparing a joint for splicing?

Answer 13

Because the dust created from sanding will clog the pores of the wood, preventing proper
penetration of glue.

Question 14

What type of patch is preferred for repair of stressed plywood skin and why?

Answer 14

A properly prepared and inserted scarf patch is the best repair for plywood, and is preferred for
most skin repairs. It differs from a splayed patch in that the edges are scared to a 12 to 1 slope
instead of a 5 to 1 slope(higher ratio of slope), and it uses reinforcements under the patch where the glue joints occur.

Question 15

when are mineral streaks acceptable in aircraft wood?

Answer 15

When inspection fails to reveal any decay.

Question 16

How do you inspect for decay in wood?

Answer 16

Visual (dark discoloration or gray stains). Odor (musty or moldy). Tapping (sharp solid report). Probe
(sharp metal tool). Prying (bonded joints).

Question 17

How can the moisture content of wood be determined?

Answer 17

One way is to use a moisture meter. However, the most accurate method is to take a sample of the
wood, weigh it, dry it in an oven at a temperature of 100’C to 105°C, reweigh it, and then compute
the moisture content by using the following formula
(W1-W2)/W2 X 100 = Percent moisture content.
W1 = the weight of the wood sample before drying.
W2 = the weight of the wood sample after drying.

Question 18

What area of the wood spar may not be spliced?

Answer 18

under wing attachment fittings, landing gear fittings, engine mount fittings, or
lift strut fittings.

Question 19

What type of joint is generally used in splicing structural members in aircraft?

Answer 19

A scarf joint.

Question 20

What is compression wood and why is it not acceptable for aircraft structures?

Answer 20

Compression wood is wood that is characterized by high specific gravity and has the appearance of
an excessive growth of summer wood (hard, non-porous). This defect is difficult to recognize and is
very detrimental to strength. Reject all material containing compression wood.

Question 21

What are three forms of wood?

Answer 21

Solid, Ply, and Laminate.

Question 22

What are two forms of wood that are acceptable for the repair of wooden aircraft skin?

Answer 22

The plywood and laminated woods are preferred for this application.

Question 23

What would be the greatest amount of splices placed into one spar?

Answer 23

Two.

Question 24

What is the largest hole diameter that can be repaired in a plywood wing spar when using a fabric patch?

Answer 24

1 inch.

Question 25

How would you inspect wood for rot?

Answer 25

Inspect for a dark discoloration of the wood surface or gray stains along the grain.

Question 26

Are a hard knots acceptable?

Answer 26

Yes, within specified limitations.

Question 27

Why are light steel bushings sometimes used in wooden structures?

Answer 27

To prevent crushing the wood when the bolts are tightened.

Question 28

Why does the strength of a beveled scarf joint depend on the accuracy of the beveled cut?

Answer 28

The effective glue area will be reduced if both beveled cuts are not the same.

Question 29

What is the minimum temperature for curing wood joints with resin glue?

Answer 29

70 F.

Question 30

Why is it necessary to examine all stains and discoloration carefully?

Answer 30

To determine whether or not they are harmless, or preliminary, or advanced decay.

Question 31

What methods are used to apply pressure to wood glue joints?

Answer 31

brads, nails, screws, clamps, or the use of electric and
hydraulic power presses

Question 32

Always splice and reinforce plywood webs with what type of wood?

Answer 32

The same type of plywood as the original.

Question 33

How is aircraft plywood constructed?

Answer 33

It is usually made of an odd number of thin plies, or veneers, with the grain of each layer placed 90° with the adjacent ply or plies.

Question 34

What effects do room temperature have on gluing wood structures using resin glue?

Answer 34

Curing time as well as joint strength.

Question 35

What type of glues are used in aircraft wood structure repair?

Answer 35

Casein glue, plastic resin glue, resorcinol glue, and epoxy adhesives.

Question 36

Why is compression wood not acceptable?

Answer 36

It is very detrimental (harmful) to strength.

Question 37

What method will prevent a patch and a plywood pressure plate from adhering together due to extruding glue from the patch?

Answer 37

The use of waxed paper between the patch and pressure plate.