The Brabazon II Rear Spar Frame: An Account of the Approach to the Problems that had to be Overcome when Designing Highly‐Stressed Components for the Largest British Aeroplane

THE rear spar frame, the first major member forward of frame ‘O’ forms, together with the front spar frame, the principal structural links between the inner wing and fuselage, bearing the loads imposed upon the inner wing and dispersing them into the fuselage. Constructionally, therefore, the frame is necessarily robust comprising a double diaphragm structure, spaced apart by heavy boom sections of light aluminium alloy situated around the inner and outer frame peripheries. The centre section of the frame is extensively cut away, leaving a diaphragm width of approximately 16 in. at the fuselage datum, tapering to 10 in. at the roof, and 4 in. at the bottom centreline of the aircraft. The lower portion of the frame, 24 in. below the datum is cranked at 3 1/2°, aligning with the wing incidence of the spars. The diaphragms front and rear are made in segments, and lap jointed; the outer edge periphery is flanged forming a skin attachment angle. For the front diaphragm construction, eight segments are utilized, those adjoining the roof and at the fuselage base are fabricated from 16 s.w.g. D.T.D. 546, while around the fuselage dytum 14 s.w.g. D.T.D. 546 is employed. Below the fuslage datum, a gap occurs in the frame segments, which is filled by the web of the rear spar after the frame and spar are brought together as an assembly. This web is 4 ft. deep and straddles the frame across the diameter, leaving a 2 ft. aperture between the top edge of the web and a floor support member. A similar aperture is left between the bottom edge of the web and the frame segments at the fuselage base.