Post by Diarist on Sept 17, 2016 19:19:59 GMT 1
Geoffrey met me at the railway station and handed me a document to read. It was an introduction to building a wooden bomber. Fascinating reading.
Fuselage
The oval-section fuselage is a frameless monocoque shell built in two halves being formed to shape by band clamps over a mahogany or concrete mould, each holding one half of the fuselage, split vertically. The shell halves are made of sheets of Ecuadorean balsawood sandwiched between sheets of Canadian birch, but in areas needing extra strength, (such as along cut-outs), stronger woods replace the balsa filler; the overall thickness of the birch and balsa sandwich skin is only 7⁄16 inch (11 mm). This sandwich skin is so stiff that no internal reinforcement is necessary from the wing's rear spar to the tail bearing bulkhead. The join along the vertical centre line greatly aids construction as it allows technicians easy access to the fuselage interior.
While the glue in the plywood skin dried, carpenters cut a sawtooth joint into the edges of the fuselage shells, other workers install the controls and cabling on the inside wall. When the glue is completely dried, the two halves are glued and screwed together. The fuselage is strengthened internally by seven bulkheads made up of two plywood skins separated by spruce blocks, which formed the basis on each half for the outer shell. Each bulkhead is a repeat of the spruce design for the fuselage halves; a balsa sheet sandwich between two plywood sheets/skins. Bulkhead number seven carries the fittings and loads for the tailplane and rudder.
The glue is Casein-based although "Aerolite", a synthetic urea-formaldehyde, which is more durable is being tested. Many other types of screws and flanges (made of various woods) also hold the structure together. The fuselage construction joints are made from balsa wood and plywood strips with the spruce multi-ply being connected by a balsa V joint, along with the interior frame. The spruce is reinforced by plywood strips at the point where the two halves join to form the V-joint. Located on top of the joint the plywood forms the outer skin.
During the joining of the two halves ("boxing up"), two laminated wooden clamps are used in the after portion of the fuselage to act as support. A covering of doped Madapolam (a fine plain woven cotton) fabric is stretched tightly over the shell and a coat of silver dope is applied, after which the exterior camouflage is applied. The fuselage has a large ventral section cut-out (braced during construction) that allows the fuselage to be lowered onto the wing centre-section. After the wing is secured lower panels are replaced and the bomb bay or armament doors fitted.
Wing
The all-wood wing is built as a one-piece structure and is not divided into separate construction sections. It is made up of two main spars, spruce and plywood compression ribs, stringers, and a plywood covering. The outer plywood skin is covered and doped like the fuselage. The wing is installed into the roots by means of four large attachment points. The engine radiators are fitted in the inner wing, just outboard of the fuselage on either side. These give less drag. The radiators themselves are split into three sections: an oil cooler section outboard, the middle section forming the coolant radiator and the inboard section serving the cabin heater.
The wing contains metal framed and skinned ailerons, but the flaps are made of wood and are hydraulically controlled. The nacelles are mostly wood, although, for strength, the engine mounts are all metal as are the undercarriage parts. Engine mounts of welded steel tube are added, along with simple landing gear oleos filled with rubber blocks. Wood is used to carry only in-plane loads, with metal fittings used for all triaxially loaded components such as landing gear, engine mounts, control surface mounting brackets, and the wing-to-fuselage junction. The outer leading wing edge has to be brought 22 inches (56 cm) further forward to accommodate this design. The main tail unit is all wood built. The control surfaces, the rudder and elevator, are aluminium framed and fabric covered. The total weight of metal castings and forgings used in the aircraft is only 280 lb (130 kg).
At the assembly site I was led through the process step by step. A truly ingenious construction. We then had a small evening meal before he drove a couple of us to Saffron Walden.
( Source of the construction information: Wikipedia - en.wikipedia.org/wiki/De_Havilland_Mosquito#Fuselage )