The wooden components of the Horten Ho 229 V3 are of three main types. The outer skin of the aircraft is formed from plywood of varying shapes and thicknesses. The plywood skin is reinforced in some areas with laminated lumber supports. These supports also serve as a means to affix the plywood skin to the aircraft's metal structural framework. Lumber is also used in the construction of the canopy rear section and at various other places on the airplane including in the tail and air brake assembly. The small blocks of wood that serve as 'spacers' at points of contact between the plywood skin and the aircraft's metal frame represent a third wood type. These blocks of wood are made from layers of thinner veneers stacked and adhered under pressure to form spacers of the required thickness.
An investigation was undertaken to determine the wood species available to the German military at the time that the Horten construction was underway. Wood identification specialists Terry Connors, at the University of Kentucky, and Larry Osbourn, at West Virginia University, suggested that WWII aircraft plywood was traditionally of Birch, Beech, and in some cases included Balsa as a core ply. African gaboon wood (Aucoumea klaineana) was also suggested as a possible plywood raw material. Spruce was considered by the conservation team to be the wood most commonly used to make the structural members of wooden aircraft, an opinion echoed by wood identification specialists. However, caution was advised with regard to the consideration of commonly used materials as many of the resources available to the German military were depleted toward the end of WWII.
United States and British Joint Intelligence documents produced at the end of the war provide details of the locations and capabilities of Germany's plywood production facilities at that time.
Lacey, P.M.C., Rutherford, H.C., Pollard, G.J.T., Austin, J.B. 1945. Investigation of Targets connected with the German Plywood, Improved Wood, Shuttle Block, and Joinery Industries, British intelligence Objectives Sub-Committee Final Report No. 348
"Visit to:- I. Bruning & Sohn, 28, Goseburge Str, Luncburg ... Activity:- Plywood and blockboard manufacture ... Veneer and plywood production were as follows:- The beech logs, delivered by rail, are steamed with exhaust from the powerhouse turbines before cross cutting by chain saw and barking by hand, and then peeled in a large variety of peelers ... The plywood presses are heated by superheated hot water ("Caliqua" system) and were being used for making Tego-bonded plywood ... During the war beech aircraft plywood was made, from 5 plies of 0.2 mm veneer, Tego bonded; this was not sanded after manufacture ... "
Knight, R.A.G., Little, G.E., Leigh, J.H. 1945. Visits to Targets Connected with the German Plywood Industry, British Intelligence Objectives Sub-Committee Group 2, Final Report No. 4
"Visit to:- Prof. Dr. -Ing. Edgar Morathâ€¦ Activity:- Scientific worker in wood technology with particular interests in plywood ... Dr. Morath is a widely known wood technologist, who has specialised in plywood and glue ... For aircraft plywood, European birch was originally specified, but for reasons of supply, beech was soon introduced."
Joint intelligence reports document Beech as the wood most commonly used in the production of German plywood and indicate that plywood veneers were predominantly produced by peeling the Beech logs (as opposed to sawing). This Google map was produced from information available in post WWII intelligence reports and notes the locations of plywood production factories, plywood adhesive production plants, and factories involved in the production of plywood industrial machinery. The concentration of German plywood production facilities in the west and northwest of Germany suggest a Western European and Scandinavian supply of raw materials.
A small cross section of plywood was removed from the PL Jet Cutaway Front panel. The wood in this panel is in good condition having been shielded by a metal engine cowling and is representative of much of the wood used across the plywood skin. The sample was examined using a Hirox Digital Microscope and the Photomicrograph below was taken.
This individual veneer width and number of plys used to make the individual plywood layers in the cross section above is consistent with the historical research. From the above-mentioned Report No. 348:
"During the war beech aircraft plywood was made, from 5 plies of 0.2 mm veneer." (Lacey et all 1945)
Small variations in the veneer thickness are likely due to manufacturing irregularities and variations in moisture content at the time of pressing.
Identification of the wood species used to produce the plywood proved more complex than originally anticipated. Microscopic identification of wood draws on comparisons of features visible in the transverse, radial, and tangential planes of the unknown sample with those of known reference samples. In some cases, patterns of diagnostic features reveal themselves across an entire growth ring (in the transverse plane) or as a distribution across a known surface area.
With thin veneers, as in the sample cross section shown above, the available surface area of the transverse and radial planes for analysis is quite small. Furthermore, the penetration of resinous adhesive into the wood layer and the pressure involved in the plywood production process can distort some of the microscopic features useful for species identification.
The species identification for the plywood sample above started with the premise that the veneers had most likely been produced from common Birch (Betula pendula) or European Beech (Fagus sylvatica). The sample shown in the photomicrograph above was processed to produce thin tangential sections of the individual veneers. This involved slicing thin sections through the 0.2 mm thin veneer ends visible in the image above and mounting the sections on a microscope slide.
One of the main differences between these two species of wood is visible in the tangential plane. Ray cells are involved in the lateral transport of materials within the woody stem, and variations in the number of cells comprising the width of the ray can aid species diagnosis. In Birch (Betula pendula), the ray structures do not exceed four cells in width whereas in European Beech, the ray structure can measure up to 20 cells wide. Learn more about wood anatomy of central European species.
Spacer Blocks and Cockpit Wood
Microscopic analysis was carried out on small samples of wood taken from the instrument panel and from the small wooden frame/skin interface spacer blocks. These analyses confirm that Beech veneers were used to produce the wooden components of the cockpit instrument panel and the wooden spacer blocks throughout the jet.
A small sample of lumber was removed from the PL Belly panel of the aircraft. The sample was examined using a Hirox digital microscope, and the transverse photo-micrographs below were produced.