On the pros and cons of using plywood vs. composite skins here's a couple of interesting posts. The first is an excerpt by Ron Scott in a piece in the Winter 2000 SAA magazine. ( www.sportaviation.org ) Of course prices have gone up since the article was written.
ALTERNATE MATERIALS Ron Scott, SAA #67 "Have you priced plywood lately? A well known aircraft supply house lists a 4'x8'x1/16 " of 45° mahogany plywood at $161.00 plus freight. This would be the variety one would skin a Tailwind wing with. Then the plywood is normally covered with polyester fabric to pre-vent cracking of the final paint job on the wing...a further cost. Fiberglass sheet might be an alternate material for this application. Looking at today's prices for glass cloth (10 oz.) and vinyl ester resins, the cost of a 4'x8x(approximately)1/16" sheet would be somewhere in the neighborhood of $40.00. This assumes a 2 ply lay-up on a smooth waxed table. Once set up, one could lay-up a 4'x8' skin in about 15 minutes. After the skin has cured, it is bonded to the wing structure using an epoxy adhesive. All bonding areas must be cleaned and scuffed before bonding. Ol' Ironsides is now 31 years old and still going strong using the above process on the whole airframe in a skin stressed application. Since no additional fabric is required on top of the plywood, the glass skins are just sanded and painted...another cost reduction. Unless you are blessed with a mattress full of greenbacks, you may want to check further into alternate materials and "beat the system" on the ever increasing high cost of construction materials. If you would like further information on fiberglass covering and fiberglass landing gear fabrication, see November, 1971, January, 1972 and February, 1972 in Sport Aviation." A different viewpoint (in part) is put forward by Bob Waldmiller concerning wing skins. He is building a highly modified aircraft called "Excalibur" based on the Corby Starlet for One Design aerobatic competition. This is a short excerpt from a very interesting article. Some of his ideas could be used by KR builders. The extended article and pictures are at www.eaa1000.av.org . Click on "Aircraft our members are building" and navigate down to Excalibur. The heaviest thing about a plywood covered wing is the skin. Unfortunately, even though a wing skin is lightly stressed, the skin must be stiff enough to resist buckling. I needed the plywood skin to carry wing torsional loads so going back to a fabric covered wing was completely out of the question. How about high tech composites? Most people don't realize just how heavy a high tech material such as carbon fiber/epoxy really is; it's roughly three times heavier than birch plywood. So why don't I just make the carbon wing skins 1/3 the thickness of the plywood skins? Well, when you do that, the carbon skins are only 25% as stiff as plywood for buckling resistance. The bottom line is a solid carbon fiber skin, designed to the same buckling criteria as a solid wood skin, will be heavier. So next time someone tells you that they wouldn't consider flying a wooden airplane, you can tell them that wood is, pound for pound, more efficient than carbon fiber when used for wing skins...and it's cheaper! The wing spar is a totally different animal, however. A wood wing spar is quite heavy when compared to a carbon fiber equivalent. I did the math and there was no way to justify a wood spar in Excalibur. Carbon fiber has material properties (strength and modulus of elasticity) that are an order of magnitude higher than Douglas Fir and even though the density of carbon fiber is 3 times higher, you only need 1/10th the amount of material. Unlike wing skins, a spar doesn't generally have limits placed on it for buckling problems. Its shape takes care of that. The result was I could build a carbon fiber spar that would carry my 750 pound airplane to ±10 g's and it would weigh only 19 pounds! That's probably half the weight of the Douglas Fir equivalent! All in all, the lighter but stronger spar made up for some of the weight of the plywood wing skins. Thomas Dalby Boulder City, NV KR-1.5
