Design and manufacture of the Concorde hinged nose fairing and retractable visor
A prominent feature of Concorde at take-off and landing is the pendulous nose of the fuselage drooped (at a faintly supercilious angle) to improve visibility from the flight-deck. This droop nose, or nez basculant, to give it the perhaps more euphonius French name, is the forward, unpressurised section of the Concorde nose fuselage. The nose fuselage proper is a pressurised shell, at the forward end of which is the flight deck, with the main windshield panels of laminated glass, electrically heated and toughened to withstand bird impact. The droop nose, which carries the retractable visor, is hinged to the forward end of the pressure-shell. The purpose of the hinged forward portion is to give the pilot, when near the ground, forward vision comparable with that in other subsonic aircraft. After take-off the entire nose-fairing forward of the main windshield, including the visor, is raised to give the aircraft a clean aerodynamic external form during subsonic and supersonic flight. In its raised position the visor also protects the main windshield panels from the effects of kinetic heating.
Delegated design and manufacture
The droop-nose component—that is, the variable-geometry nose fairing, with the retractable glazed visor—was designed and produced by Marshall of Cambridge (Engineering) Ltd on behalf of the Filton Division of the British Aircraft Corporation. It is the only task control delegation on the Concorde airframe structure outside the
BAC group in the United Kingdom. Marshalls, who own and operate Cambridge Airport, have an extensive background of experience in the aircraft and aerospace field.
At the time of manufacture they had a comprehensive design and production facility at Cambridge which included, on the production side, a programming department for numerically controlled machining and a machine shop equipped with NC machine tools.
Although small in dimensional terms, the droop-nose component presented considerable problems, both in design and manufacture. As produced for production Concordes, the component differed widely from that of the prototype aircraft: it is, in fact, of completely new design for the production Concordes. Differences included a new shape to give better visibility for the pilots at take-off and landing, and a visor that presented an acceptable aerodynamic form when in the raised position and also gave the pilots good external vision.
In cross-sectional form, the nose-fairing changes from a true ellipse at the forward-end bulkhead to a near-circle at the interface with the pressure-section of the nose fuselage. The consequently varying cross-sectional form along the length of the fairing, in addition to the closeness of the tolerances imposed on the external profile, demand accurate tooling and close control at all stages of manufacture. It must be remembered that the fairing and the visor move independently but in unison and, when in the flying position, the latter must be a glove fit to the aircraft to meet aerodynamic and noise requirements.
As with the rest of the Concorde airframe, the basic structural material is the heat- and creep-resistant aluminium alloy, Hiduminium RR58. From the forward-end bulkhead at fuselage station XA 10.85 the nose fairing extends to the droop hinge-point at station XA 196 and, overall, has a length of some 15ft 6in. Basically the structure is a traditional one of skin panels, stringers and frames. Construction is, however, mixed, in that with the formed sheet-metal members is embodied a considerable proportion of machined elements. Its strength derives from bulkheads at the forward and after ends, from twin keel-beams of box-section, port and starboard, and from top and bottom longerons, also port and starboard, in the upper part of the structure. Hinge-fittings for the drooping of the nose are embodied in the after ends of the keel beams and the droopmechanism operating jacks are mounted in fore-and-aft machined members behind the bulkhead at station XA 141. Anchorage points for the roller guide-carriages of the droop mechanism are provided in what are known as “banana” fittings which connect the end-fittings in the after ends of the top and bottom longerons. Side loads are resisted by links attached to the nose bulkhead and the nose fuselage.
On the port and starboard sides of the nose-fairing structure, a stainless-steel guide-rail is fitted between the frames at stations X 103-75 and XA 141 for the roller carriages of the visor-retraction mechanism. Between the forward bulkhead and the frame at station XA 70-25 a machined attachment fitting is introduced at the top of the fairing structure. At its forward end, this fitting carries integral lugs for the anchorage of the visor-retraction jack. At it’s after end are two pairs of integral lugs that serve as pivot points for the inverted A-frame, which is linked to the forward end of the visor, and through which the jack operates to retract and raise the visor. In order to maintain an efficient seal between the nose pressure-fuselage and the droop nose in all positions, a shutter fairing is attached to the top member of the fairing rear bulkhead.
The conical radome is mounted on the forward bulkhead of the nose fairing. For ground servicing it can be moved forward on two integral tracks which move on rollers in the nose-fairing structure. The radome shell has a light alloy mounting ring at its after end and carries the pitot/ static probe at its forward end. Structurally, the visor may be said to be built around a central fore-and-aft spine, which constitutes the central glazing-bar of the visor proper and is also the central member of the built-up decking-structure. There are six glazed panels in the visor proper, housed in a framework formed by intermediate glazing-bars parallel with the spine and by side frames, all of which are joined to the visor edge-members. To the lower part of the spine on each side are attached four ribs, which extend outwards to the visor edge-members and side-frames. Between adjacent pairs of ribs, fore-and aft intercostal members are introduced. The central (spine) and intermediate glazing-bars are supported by short vertical struts or props introduced between the deck-member of the spine, the top flanges of the ribs and the undersides of the glazing bars.
Tracking legs for the actuation of the visor take the form of triangular brackets attached to the outer ends of each third rib (from the front). These brackets are braced fore and aft by tubular struts giving a rigid, triangulated structure, which is additionally braced by a transverse tubular strut between the tracking legs. At the base of each tracking leg is a pivoted bracket carrying four rollers that engage the visor guide rail at each side of the nose fairing. The forward tubular bracing struts are connected to an actuating arm, or link, pivoted to the apex of the swinging A-frame and to the foremost decking rib of the visor structure.
Irrespective of the position of the nose fairing, the glazed visor can be retracted into the fairing cavity through the top skin-plating. At its forward end, the visor is carried by the actuating arm which is pivoted port and starboard to the foremast rib of the deck structure. At its lowest end, the arm is pivoted to the apex of the inverted A frame. This frame, pivoted to the nose-fairing structure at frame station X 70-25, is actuated in a fore and aft direction by a horizontally mounted hydraulic jack, pivotally anchored to the fairing structure immediately behind the forward bulkhead. At the after end, the visor is carried by the two roller carriages at the bottom of the tracking legs; the carriages run on the curved guide-rails at the sides of the fairing structure. When the actuating jack is retracted the A frame swings about its upper end, drawing the lower end of the visor actuating arm forward. This movement, combined with the guidance of the curved rails, draws the visor downward and forward on a path that takes it below the top panel of the nose fairing and down the slope of the main windshield of the flight deck. Drooping of the nose is effected by hingeing the entire nose-fairing, with the visor and its actuating mechanism, downward about an axis at the bottom of the forward part of the nose pressure-fuselage.
Movement is given by a pair of tandem jacks, working in parallel, each one of which has two cylinders with a common ram. The two jacks are mounted side by side, the lower cylinders in gimbal mountings in the base of the nose-fairing structure, the upper cylinders also in gimbal mountings in a fixed bracket on the forward pressure bulkhead of the nose fuselage. Extension of the jacks between these two points has the effect of moving the nose downward about the main hinge-point. The lower cylinders have a stroke equivalent to a droop angle of 5°. That of the upper cylinders is equivalent to a droop angle of 12]2°, giving the maximum droop of 1712° when the two cylinders are fully extended. In the first stage (5° droop) movement of the nose is effected by downward movement of the lower cylinders of the jacks. The remaining 121 2° of droop is given by a further downward movement of the rams from the upper cylinders. Roller carriages, anchored to the fittings at the rear ends of the top and bottom longerons of the nose fairing, track along guide rails, port and starboard, on the front of the nose pressure-fuselage and stabilise the droop movement of the nose. The four possible positions of the droop nose and visor are: Nose and visor up. Nose up, visor down. Nose at intermediate (5°) droop, visor down. Nose at maximum (1712°) droop, visor down. Two hydraulic systems, designated Green and Yellow, are provided for the actuation of the droop nose and visor. Normally, operation is by the Green system with the Yellow available for standby.
Operating time forin parallel, each one of which has two cylinders with a common ram. The two jacks are mounted side by side, the lower cylinders in gimbal mountings in the base of the nose-fairing structure, the upper cylinders also in gimbal mountings in a fixed bracket on the forward pressure bulkhead of the nose fuselage. Extension of the jacks between these two points has the effect of moving the nose downward about the main hinge-point. The lower cylinders have a stroke equivalent to a droop angle of 5°. That of the upper cylinders is equivalent to a droop angle of 12]2°, giving the maximum droop of 171 2° when the two cylinders are fully extended. In the first stage (5° droop) movement of the nose is effected by downward movement of the lower cylinders of the jacks. The remaining 1212° of droop is given by a further downward movement of the rams from the upper cylinders.
Roller carriages, anchored to the fittings at the rear ends of the top and bottom longerons of the nose fairing, track along guide rails, port and starboard, on the front of the nose pressure-fuselage and stabilise the droop movement of the nose. The four possible positions of the droop nose and visor are: Nose and visor up. Nose up, visor down. Nose at intermediate (5°) droop, visor down. Nose at maximum (1712°) droop, visor down. Two hydraulic systems, designated Green and Yellow, are provided for the actuation of the droop nose and visor. Normally, operation is by the Green system with the Yellow available for standby. Operating time for the visor is approximately 6sec. The nose can be fully drooped in 12sec and raised in less than 19sec. normally; the aircraft would be operated with the visor up during the greater part of the flight. For take-off, the nose is drooped 5° and the visor is lowered; both would be raised at the start of supersonic flight. During holding, both nose and visor remain up. During the final stages of approach and landing, the visor is lowered and the nose is fully drooped. Production As already mentioned the nose-fairing structure is basically traditional in type, consisting of skin panels, stringers and frames. It might, however, be described as traditional rendered in modern terms. Skin panels are contour-etched on both sides with, for example, rebates for lap-joints on the external surfaces and, on the inner surfaces, areas between stringer and stiffener lands for weight reduction.
This double-sided etching presented an interesting production problem as the etched areas on the opposite faces must be accurately related. The required result was achieved by locating the resist-area marking-out templates for both sides by tooling holes in integral lugs at the panel edges. These tooling holes were also used for panel location in the subsequent assembly stages and the lugs were removed when assembly was completed. Skin panels were stretched to form before being etched and the marking-out templates for the resist areas were also stretched to form on the same forming-tools. Both sides of the panel were etched simultaneously.
Many parts of both nose fairing and visor were machined units and for these, and indeed for a large amount of subcontract work done for the industry, Marshalls maintained a machine shop equipped with modern numerically controlled machine tools. The principal NC machines were two Marwin Max-E-Trace triple-head high-speed profiling machines, one with a 14ft and the other with a 28ft table; and a Marwin twin-spindle Max-E-Mill machine.
In general, NC procedure followed customary practice. On the two Max-E-Trace machines, Ferranti Mark NC control was used, but the Max-E-Mill had the Bunker-Ramo system. Programming languages used included Profile data (predominantly), Apt and Adapt. In-house programming was preferred because, despite its mathematical-factual basis, the process can become, to a degree, idiosyncratic and therefore more familiar in a particular environment.
Tape-proving, as always, presented its problems. A plastic material, Tancast, was largely used for the tape try-out stages. Commercial aluminium was also needed to some extent, although this was an expensive medium, approaching actual component billet cost. Flush-riveting was used for the nose-fairing skin panels, which were hot dimpled to receive solid rivets. What is known as the triple-action dimpling process was used. Triple-action dimpling was development by the American Zephyr Manufacturing Co of conventional Cain dimpling. It was claimed at the time that it produced sharper dimple definition on both the top and bottom of the sheet with better flushness of fitting by the rivet head in the result. As used by Marshalls the dimpling equipment was fitted to a CP-450EAf riveting machine.
For ease of manufacture, the complete nose fairing was divided into a number of subassemblies. They were built in separate fixtures and were later brought together in what may be termed a boxing-up fixture. The subassemblies were: bottom panel; port and starboard forward side-panels; port and starboard after side-panels; top panel; forward bulkhead; visor seal structure. Certain features were common to the nose fairing assembly fixtures. Steel tube of square sections was used as a structural material. Such tube has the advantage of being inherently rigid and can therefore be built into a self-supporting structure, free from additional bracing, that will give adequate support and maximum access to the component that is being built within it. Also, the flat faces of the square section afford attachment faces for pick-up and location fittings at any point where needed on the fixture. Assembly fixtures for panel subassemblies are of generally similar type: the panels were held vertically in a rectangular “picture-frame” of one or more bays, according to the length of panel. This frame was supported on triangulated standards at convenient assembly height. Such a structure has the advantages already noted of inherent rigidity and offers free access to both sides of the assembly. Fluorescent-tube lighting was arranged over all fixtures. Forward side panels, top panel and bottom panels were all assembled in fixtures of this type. Former-boards, or templates, of aluminium to prevent damage to the light-alloy skin panels, were mounted between the top and bottom longitudinal of the rectangular frame of the fixture. These templates served several purposes: they were slotted or cut out at appropriate points to serve as locations for the stringers; on some fixtures they were also station locations for the frame segments; and they were also a means of controlling the profile of the skin panels. Trimblocks were also mounted on the fixtures to give the cut-off points for stringer lengths. In these fixtures, the assembly sequence was first to locate stringers, longerons and other stiffeners in the template cut-outs. Frame segments were located against the templates or to brackets on the top and bottom fixture longitudinals. Location of the contour etched skins in the fixture was by tooling holes in integral lugs on the panel, as already mentioned. When in position on the fixture, the panels were held back against the templates, to control the skin profile, by steel straps anchored at top and bottom to the fixture longitudinals and covered with PVC to prevent damage to the skin. The skin panels were left full on all external edges to be trimmed to finished size at a later stage.
The skin panels were drilled-off from the flanges of the stringers and internal structure, which were drilled pilot size. The holes were then opened up to full rivet size by drilling back through the skin, after which the structure was dismantled, burrs were removed, and the skin panels and structure were hot-dimpled. For riveting-up, the structure is reassembled in the fixture.
Final assembly of the nose fairing was done in what may be termed a boxing-up fixture. It consisted of two vertical side frames with triangulated-standard bracing and bridging top members and a central structure upon which the fairing was supported. Provision was made for the location of the panel subassemblies and of all the key points of the structure. These points included the droop nose hinge-fittings and the top and bottom longeron end-fittings and also for the side-load link fittings. The forward (nose) bulkhead was located for assembly on a gate structure which was mounted across the forward ends of the two vertical side-frames and could be removed for withdrawal of the completed assembly from the fixture. Trim-line strip templates were provided for the skin edges of the panel subassemblies. Girder-section templates on the centre structure of the fixture controlled the build of the radial joint structure.
Assembly of the retractable visor was done in a single fixture and was a progressive sequence beginning with the location of the spine member. The deck structure, side frames and edge-member were built up, followed by the top and bottom decking and the glazing-bar structure. The structure was dismantled for treatment and application of interfaying material, and was then reassembled.