FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1962
1962 - 0696.PDF
694 FLIGHT International, 3 May 1962 BRISTOL 188... determined by the pilot and ejection seat and by the retracted main wheels, and length adequate to accommodate the fuel, systems and payload. When the design was first laid out it was considered important to obtain good cruising performance at low supersonic speeds, and this led Bristol to design for minimum drag at Ml.4. Accordingly, the fuselage is gently waisted in plan and elevation, and the nacelles are tapered to reduce wave drag within the fields of flow interference. As noted in the next section, this accentuated the difficulty of manufacturing the aircraft. Materials The Bristol 188 is the first British aeroplane, and one of the first in the world, designed to fly so fast that kinetic heating prohibits the use of traditional materials. The problem has been met in more severe form with the X-15; as described in this journal on May 8,1959, the North American designers had to build that aeroplane out of high-nickel alloys capable of retaining reason able strength and stiffness at over 600°C. The Bristol 188 is not expected to exceed 300°C, but is designed to soak at this temperature for appreciable periods. Moreover, it must be able to withstand considerable manoeuvre loads in relatively dense air in either the fully heated or a transient condition. Evaluation of possible materials began by plotting specific strength and modulus against temperature. These curves suggest that commercially-pure beryllium and the Al/Mn titanium alloy are outstanding; but the former is even today out of the question as a major structural material, and seven years ago the titanium alloy was neither a known quantity nor commercially available. Eventually it was decided to make the major part of the airframe from two grades of stainless steel, which promised to be more read ily available in usable form and to pave the way for structures potentially capable of use at temperatures higher than those which the 188 is designed to reach. Today several types of stainless steel are commercially available significantly better than anything on the market six years ago. When the 188 was in the preliminary-design stage the only materials which appeared to meet all requirements were a titanium-stabilized 18-8 austenitic steel and a more specialized 12%-Cr steel used in hot parts of gas turbines. The former was in production by diverse means, and specimens exhibited relatively wide variation in proper ties and unacceptable tolerance on dimensions. It was appreciated that in a steel aeroplane it is especially important to maintain a tight tolerance on material properties and gauges in order to hold down structure weight. Appreciating the magnitude of the problems facing them, Bristol Aircraft joined forces with Firth-Vickers and associated companies in order to achieve better and more uniform properties, in sheets of reasonably large size having a higher standard of surface finish and flatness and closer tolerance on thickness than had previously been Tensile strength divided by density plotted against temperature for a variety of possible airframe materials: A, c.p. beryllium; 6, titanium alloy 4%AI 4%Mn; C, titanium alloy 5%AI 2.5%Sn; D, l2%Cr complex stainless steel; £, 18-8 Cr-Ni hard-rolled stainless steel; F, 18-8 soft stainless; G, c.p. titanium; H, Mg-Zr-Th alloy; J, heat-resistant alumin ium alloy; K, high room-temperature strength aluminium alloy O IOO 200 JOO 400 SOO 600 700 TEMPERATURE (°C ) This is one of the earliest three-view drawings to be proposed by Bristol Aircraft to meet the ER.I34 specification. The overall similarity to the final aircraft is remarkable demanded. An extensive programme of research was also instituted to evolve optimum methods of fabrication and jointing, using the best possible heat-treatment to achieve the desired finished properties. FV.448, the 12%-Cr steel, is characteristically the material with a higher modulus, but strength-levels had to be chosen to match the manufacturing processes and heat-treatments. It was impossible to carry out extensive forming or stretch-flattening, and the only way to produce large sheets thicker than 0.2in in the high-heat- treated condition with the desired degree of flatness was by "block- tempering" and hammer-flattening, followed by milling, taper-milling and taper-grinding to close tolerances for such parts as the torsion boxes of wing and tailplane. Thinner sheets, around 10G, were manufactured by hot and cold rolling, annealing, stretch-levelling and hardening in a vertical furnace before tempering. They were then finish-ground at Bristol to uniform thickness matched to the flying control surfaces. In the case of the FDP 18-8 steel the im mediate task was to tighten up the tolerances on strength and dimensions, and material of consistently high strength was finally achieved by Firth-Vickers with a continuous strip-rolling process. One of the minor complications was to settle which Firth-Vickers strength would yield the particular (and different) heat-treated strength required for each aircraft part. Having ensured a supply of raw material, means had then to be found of turning it into an airframe. As outlined in an earlier article (Flight, August 19, 1960) experiments showed that fully hardened and tempered FV.448 was an excellent material for rivets, which could be set relatively easily by squeeze (up to &in-diameter) or percussion (up to ^in) methods. The limiting factor was usually that poor accessibility to the joint concerned placed a restriction on the load that could be applied. For joints in areas so restricted that solid rivets could not be used, recourse was had to FV.448 Jo-bolts and Hi-Shear tubular rivets, the latter with FI(G) chrome steel collars. Special bolts and screws required throughout the structure were produced by Unbrako in FV.448 and u.h.t. steel, intimate co-operation being maintained at all stages with the materials supplier, specialist wire-drawers and fasteners manu facturers. But what really made the 188 possible was the technique, evolved by Bristol Aircraft, known as "puddle-welding." This is a controlled local fusion of the material to be joined beneath the arc struck from an electrode surrounded by an inert argon atmosphere. Its gradual evolution had an appreciable effect on both the materials chosen and the design of the aircraft, and by the time airframe manufacture began it was an approved process for components between 0.3in and 0.012in thickness. In the heavier sections shear strengths of about 6,0001b per weld have been attained; but each batch of austenitic sheet had to be checked to ensure consistent weld be haviour, and skill was necessary both in design and manufacture to avoid an accumulation of minute shrinkages and distortions. Puddle-welding was finally used as the principal means of jointing throughout the 188 structure. Bristol Aircraft subcontracted the work of manufacturing several major portions of the 188 to Sir W. G. Armstrong Whitwonh Aircraft, who had conducted their own research into this field. The Coventry firm were responsible for the complete tailplane, fin, rudder, outer wings, ailerons and cockpit canopy. One of their important contributions was a method of isolating argon-arc welds by Lassovic rubberized insulation tape, the precise thickness of
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
