Plywood for R/C Airplanes – What Type Should I Use?
Recent discussions with friends and on message boards regarding the common types of plywood used to build R/C airplanes led me to think that a short treatment of the subject would be helpful.
There are three primary types of commercially available plywood common to R/C airplane construction, each with its own benefits and drawbacks. Those three are: aircraft grade birch, interior grade birch, and poplar plywood better known as "lite" plywood.
While the details of the design requirements for aircraft grade ply may be a riveting read, I think what most people want to know is how each type is different and where and how to work with them. For those of you so inclined, you may find the Germanischer Lloyd Rules for Surveying and Testing of Plywood for Aircraft and MIL-P-6070 both provided on the Aircraft Spruce website .
Aircraft Grade Birch Plywood
Pro: very high quality and strength
Con: weight and workability
Aircraft grade plywood is possibly the highest quality commercially available plywood. Aircraft grade plywood consists of birch plies, often Finnish Birch, that are adhered together with an exterior grade waterproof glue that is dark in color. All plies, including both face and interior sheets are free of voids and knots.
Due to its high quality and relatively large number of plies, aircraft grade ply is the strongest and densest of the plywood varieties described in this article. For this article, I weighed a sample set of aircraft ply sheets and found the density to vary between 40 and 50 lbs per cubic foot. Aircraft grade ply is best used in those areas of a design where the greatest stresses are expected, such as firewalls, wing mounts, and control horns.
The downside of such a durable material is in its workability. Aircraft grade ply can be very difficult to cut. If you intend to cut the material with a saw, chose a saw blade with a fine tooth and cut slowly to avoid chipping. It has been my experience that aircraft grade plywood up to 1/16” (2mm) in thickness can be cut very well with a laser, the thicker materials suffer from excessive edge burning due to the heat generated by the reaction of the exterior grade glue with the laser. The edges of parts cut by a laser that are to be bonded should be lightly sanded first to remove any residue, ensuring a strong glue joint.
Interior Grade Birch Plywood
Pro: good quality and strength
Con: weight
Interior grade birch plywood is often called “European,” “Baltic,” or “Russian” depending on its origin. Interior grade birch plywood differs from aircraft grade plywood in the number and quality of the plies, and the glue that is used. Interior grade plywood uses a light-colored glue that is not waterproof. The interior sheets and up to one face sheet may or may not have voids and knots depending on the grade of plywood.
Interior grade birch plywood is not available in the same ultra-thin dimensions of aircraft grade, with the thinnest common dimension being 1/8” (3mm). Interior grade plywood is slightly less dense than aircraft grade plywood, and is generally not as strong. Interior grade plywood is a good substitute for aircraft grade ply where high stress levels are expected, but caution should be exercised in selecting a high quality sheet, since voids can decrease the strength of the parts.
The workability of interior grade birch plywood, especially in terms of laser cutting is superior to aircraft grade plywood. When cut by a laser, the edges tend to be clean with a minimal amount of residue, and very little to no burning.
Poplar or “Lite” Plywood
Pro: weight and workability
Con: relatively low quality and strength
Lite plywood is exactly what it sounds like – lightweight. The poplar used in Lite plywood construction is substantially lighter than birch (almost 50% lighter). The sheets that make up Lite plywood are of much lower quality than birch plywood, and depending on the grade may contain filled cracks, knots, and voids. Interior grade glue is used to bond the plies together.
Lite ply is available in dimensions beginning at 1/8” (3mm). Lite ply is the lightest of the three common types of ply described in this article, with a density of roughly 25 lbs per cubic foot, according to my measurements. For comparison purposes, balsa varies from 4-12 lbs per cubic foot. Lite ply is also has the lowest strength of the plywood types described in this article. I have found Lite ply to be susceptible to changes in the moisture content of the air, which leads to warped sheets. Lite ply is best used in areas where low stress is expected, such as formers and bulkheads.
Lite ply is very easy to work with. It cuts easily by saw and by laser. Laser cut edges are very clean, and require very little to no preparation before assembly.
And the Winner Is…
You decide. It is a matter of design requirements and build technique. Because the laser is my tool of choice, I prefer to use aircraft grade plywood where thin strong material is called for (doublers and horns), interior grade where strength and thicker dimension are needed (firewall), and Lite ply where I want something stronger than balsa, but I still want to save on weight (formers and bulkheads).
I hope this discussion was useful, and helps you select the right materials for your next build project.
Tags: aircraft grade, build, laser, plywood, structure