by Ron Alexander,VAA 27150
Upon examining the problems associated with selecting wood for use in your airplane, it is apparent that you must become familiar with how to properly inspect wood prior to installation. The previous articles provide an in-depth look at the types of wood you can use, the common defects found in these types of wood, and how to properly inspect the wood. Military Specification 6073 is presented and discussed as it relates to Sitka spruce. The articles suggest that you should purchase wood from a reputable kit manufacturer or aircraft supplier—one that further inspects lumber prior to shipment. Most of these companies complete a final inspection for obvious defects before shipping the order to you—the end user. Several of them will even do testing on samples of wood. Let’s take a look at aircraft plywood.
AIRCRAFT PLYWOOD
Confusion often results from the terms “veneer” and “plywood.” The term veneer is used to describe the relatively thin sheets of wood cut with special machinery from the surface of a log. The log typically revolves in a massive lathe and thin sheets are sliced or sawed from the log to form veneer. “Plywood” on the other hand, refers to the combination of several sheets of veneer that is glued together. These sheets are termed “plies”. You will usually hear plywood referenced according to a certain number of plies, each being one layer of veneer.
Plywood does have certain advantages over solid wood when used in aircraft construction. As compared with solid wood, one of the major advantages of plywood is the presence of more equal strength properties along the length and width of a specific panel. Plywood is also more resistant to checking and splitting and it has less change in dimension with corresponding changes in moisture content. These advantages are obtained by alternating the direction of grain in the plies of veneer.
Plywood used in aircraft construction must be manufactured according to certain specifications. The regular plywood you find at your local lumberyard should not be used in any structural component of your airplane. Why? Most common grade plywood is comprised of plies of veneer that probably have several defects. These defects result in “voids” or gaps between the sheets of core veneer. These voids cause a weak spot in the plywood itself. If moisture creeps into a void it can lead to rotting or delamination of the plywood. In addition, the glue used in common plywood may not have adequate strength or be waterproof.
Aircraft grade plywood, on the other hand, must be free of all voids and the veneers used must be free of most wood defects. The glue used must also meet certain specifications. Most manufacturers use phenol-phenolic glue applied in a hot press. The glue is both waterproof and fireproof. A military specification pertaining to the manufacture of aircraft plywood was developed by the government years ago. This specification is referred to as Mil-Spec 6070B. It outlines in detail the types of wood that can be used, adhesives to be used, thickness of veneer, defects not allowed in veneer, thickness tolerances, sample testing requirements, etc.
All plywood manufactured is done so in accordance with Mil-Spec 6070B. This plywood is measured in fractions of an inch and can legally be used in production aircraft. The veneer used is either mahogany or birch with a core material between the sheets of veneer. This core material is usually poplar. Birch, basswood, or maple is also acceptable as a core material. Each ply must be 90 degrees to the adjacent ply. The outside plies of material are called “faces” and the inner plies are termed “core and cross bands.” The basic construction of a 5-ply panel consists of a center ply made of core material with 2 inner plies whose grain will be oriented at a 90-degree angle to the 2 face plies.
Birch plywood is stronger than mahogany but is also heavier. Some builders prefer mahogany because it weighs about 10-15% less than birch. The appearance of mahogany is sometimes preferred over birch.
Plywood manufactured under Mil Spec 6070B must pass a boil test for 20 minutes. After the boil, the plywood is subjected to a peel test and the wood must fail before the glue line fails. This plywood is often referred to as fractional plywood and it is available in 4 ft x 8-ft panels. It is also sold both as 90-degree plywood and 45-degree plywood. The degree reference pertains to the outside or face veneer orientation. 45-degree plywood is more expensive and is mainly used where torsional stiffness is required.
Metric Plywood
Metric plywood is manufactured according to a European standard. It is termed GL-1 or GL-2, the GL meaning German Lloyd. GL-1 is a more strict specification than GL-2 that allows more defects. Metric plywood obviously is measured using the metric system with 1.5mm (approximately 1/16 inch) being a common thickness. It is only available in 4 ft x 4-ft panels. Metric plywood is a suitable alternative to fractional plywood for experimental aircraft. It is my understanding that most FAA inspectors will require fractional plywood to be used in production aircraft. This is because fractional plywood is manufactured according to a military specification. You can inquire further concerning this with your local FAA office. You will find GL-2 grade metric plywood readily available for purchase. It is acceptable to use GL-2 rather than GL-1 for experimental aircraft. Also, the glue used in manufacturing metric plywood is similar to that used in fractional.
Marine Plywood
Several aircraft builders use marine grade plywood in aircraft construction. It is available from aircraft supply companies. It is sold in two different grades A-A and A-B. The A-B grade means that the A side does not allow too many defects where the B side allows for more defects. Marine plywood is usually made of Douglas Fir. It is inexpensive but of poor quality. Care should be taken if you decide to use this type of plywood. I would not recommend using it for anything structural.
Use of Plywood
Plywood is used for a number of purposes in aircraft construction. Examples are gussets for wing ribs, reinforcement plates for wing spars, instrument panel construction, floorboards, skins for entire wing panels, leading edges of wings, etc.
Repairing & Storage of Plywood
Repairs on plywood surfaces are discussed in detail in FAA Advisory Circular 43-13. This publication discusses repairing ribs, patching plywood skin, bending plywood, etc. AC43-13 has several pages devoted to this subject and I would recommend referring to it prior to doing any wood repairs. Concerning the storage of plywood panels, you will want to ensure that you store this and any type of wood in a dry area. Remove it from the shipping crate and inspect it for moisture or damage. Pieces of plywood may be stacked on top of one another. Do not store plywood on concrete floors or in any area where moisture may present a problem. Store in a dry, well-ventilated area with all edges exposed to permit adequate ventilation.