Most people think of plywood in terms of its use in construction or furniture making, but it has played an important role in aviation for as long as aircraft have been around. Birch plywood was one of the first engineered composites to appear in aircraft structures. Perhaps surprisingly, birch plywood, especially thin plywood, is still used in aircraft today, like Colin Cheese and his friends proved by restoring old DH-88 Comet racer using 1.5 – 3.0mm birch plywood from Koskisen.
Early aircraft structures were almost exclusively wood with metal fittings and usually covered in fabric. Weight reduction is a key objective of an aircraft designer and wood is a surprisingly efficient material when used correctly. The most commonly used wood is Sitka Spruce sourced from the NE coast of Canada where the cold winters encourage slow growth and a compact grain structure. Wood grown in warmer climates is less dense and hence weaker. Tests have shown that Sitka Spruce is the most efficient timber in strength/weight, but others such as Douglas Fir come close and are accepted alternatives, albeit at a weight penalty.
Where does birch plywood come in? Ignoring things like gussets where members are joined, the main use of birch plywood is in shear webs and to form the aircraft surfaces. Fabric as a covering works well for slow, light aircraft, but the need for speed demanded cleaner, more aerodynamic shapes with stiff surfaces. Birch plywood with its bi-directional construction makes excellent surfaces.
In 1934 Geoffrey de Havilland was worried that an acknowledged aviation lead in Britain could slip away unless prompt action was taken. The England to Australia “MacRobertson” air race focused international attention and a wide range of impressive entrants was likely to be forthcoming.
He decided to design and produce a highly specialised long-range twin engine racing aircraft that would incorporate a number of innovations with few compromises for comfort or simplicity. His team needed a method of construction that was suitable for fast prototyping and of a known quality, so they turned to what they knew best, spruce and birch plywood.
The resulting design, the DH-88 Comet was a world-beater and one of the aircraft, G-ACSS ‘Grosvenor House’ (pictured above) won the race in the face of stiff competition.
That aircraft is still flying today as part of the Shuttleworth Collection and may be seen at Old Warden, their airfield in Bedfordshire in the UK.
Another of the original three aircraft entered in the race still survives and is under restoration at Derby Airfield in the UK. This aircraft was flown by a married couple; the Mollisons, but more people will know the famous female pilot under her maiden name of Amy Johnson. Unfortunately, after leading in the early stages of the race the Mollisons got lost in India and when they finally found an airfield, they took on poor quality fuel that damaged their engines and put them out of the race.
However, after being abandoned for years, their aircraft is now being carefully restored by a group of dedicated volunteers overseen by aircraft engineers and they plan to see it flying as soon as practicably possible. The following photos show the extensive use of birch plywood.
“The fuselage (see header photo) is skinned in 1.5 – 3.0mm plywood, the control surfaces, fin and rudder are all similarly skinned in 1.5mm plywood. The wing outer ends are plywood but this is replaced by much thicker spruce planking towards the centre section.”, explains Colin Cheese, one of the volunteers.
“These pictures illustrate just how much we depend on having a supply of the highest quality plywood. Close inspection of the parts will reveal that most of the plywood used in the restoration has those important stamps showing that it was made by Koskisen.”
The first picture shows the uncovered wing positioned upside down. The metal frame is the undercarriage/engine frame. The deep members running diagonally from bottom left to top right are the wings spars which have internal spruce top and bottom booms and are faced with plywood in varying thicknesses tapering from about 10mm thick the centre to 1.5mm at the tips. The brown stains are glue. Restores use a resorcinol glue called Aerodux which is very dependable.
“Thank you, Koskisen, for making and supplying the top-quality plywood needed. Without it, projects like this would be unviable. “, said Colin Cheese.
For more information about the project go to their website.
Koskisen thin plywood, also known as aircraft plywood or aviation plywood is known for its elasticity modulus, bending strength and for the fact that the material won’t chip and break when worked. Therefore, it is a perfect material for aircraft building, not forget its sustainability and naturalness compared for example to plastic.