Wood structures Flashcards
What species of wood is used as a standard for strength properties of other species of wood used in aircraft structures?
Spruce wood
Is northern white pine an acceptable substitute for spruce?
Yes, if the pine is larger in size to make up for its lower strength.
Are knots allowed in aircraft wood?
Yes if within limitations.
What type of wood quality must be used in the repair of aircraft structures?
The wood must be of aircraft quality and equivalent to its original strength or greater.
What is the difference between laminated wood and plywood?
Laminated wood has the grain of all layers approximately parallel. The grain of plywood
runs 90° with the adjacent ply.
What are two types of glue used for aircraft wood structure repair?
casein and synthetic resin glue.
What method is used to prepare plywood for bending?
The wood is soaked in hot water, and then place in a form until completely dry.
What are the results of using insufficient gluing pressure when making a wood joint?
thick glue lines, and a weak joint.
What is the recommended clamping pressure for a glued joint?
125 to 150 psi.
How long should a wood glue joint be kept under pressure before removing the clamps?
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.
What are some of the various methods used to apply pressure to joints in aircraft gluing operations?
Brads, nails, screws, clamps, or hydraulic and electric power presses.
Why does the strength of a beveled scarf joint depend on the accuracy of the beveled cut?
The effective glue area will be reduced if both beveled cuts are not the same.
Why is sanding unacceptable when preparing a joint for splicing?
Because the dust created from sanding will clog the pores of the wood, preventing proper
penetration of glue.
What type of patch is preferred for repair of stressed plywood skin and why?
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.
when are mineral streaks acceptable in aircraft wood?
When inspection fails to reveal any decay.
How do you inspect for decay in wood?
Visual (dark discoloration or gray stains). Odor (musty or moldy). Tapping (sharp solid report). Probe
(sharp metal tool). Prying (bonded joints).
How can the moisture content of wood be determined?
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.
What area of the wood spar may not be spliced?
under wing attachment fittings, landing gear fittings, engine mount fittings, or
lift strut fittings.
What type of joint is generally used in splicing structural members in aircraft?
A scarf joint.
What is compression wood and why is it not acceptable for aircraft structures?
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.
What are three forms of wood?
Solid, Ply, and Laminate.
What are two forms of wood that are acceptable for the repair of wooden aircraft skin?
The plywood and laminated woods are preferred for this application.
What would be the greatest amount of splices placed into one spar?
Two.
What is the largest hole diameter that can be repaired in a plywood wing spar when using a fabric patch?
1 inch.
How would you inspect wood for rot?
Inspect for a dark discoloration of the wood surface or gray stains along the grain.
Are a hard knots acceptable?
Yes, within specified limitations.
Why are light steel bushings sometimes used in wooden structures?
To prevent crushing the wood when the bolts are tightened.
Why does the strength of a beveled scarf joint depend on the accuracy of the beveled cut?
The effective glue area will be reduced if both beveled cuts are not the same.
What is the minimum temperature for curing wood joints with resin glue?
70 F.
Why is it necessary to examine all stains and discoloration carefully?
To determine whether or not they are harmless, or preliminary, or advanced decay.
What methods are used to apply pressure to wood glue joints?
brads, nails, screws, clamps, or the use of electric and
hydraulic power presses
Always splice and reinforce plywood webs with what type of wood?
The same type of plywood as the original.
How is aircraft plywood constructed?
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.
What effects do room temperature have on gluing wood structures using resin glue?
Curing time as well as joint strength.
What type of glues are used in aircraft wood structure repair?
Casein glue, plastic resin glue, resorcinol glue, and epoxy adhesives.
Why is compression wood not acceptable?
It is very detrimental (harmful) to strength.
What method will prevent a patch and a plywood pressure plate from adhering together due to extruding glue from the patch?
The use of waxed paper between the patch and pressure plate.
