FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1986
1986 - 3545.PDF
Tip features 1 Large swept tip 2 Leading edge notch 3 Highly swept extreme edge 4 Increased planform area 5 Balanced tip lift and mass BERP tip high-angle-of-attack behaviour Far left The world-speed-record- breaking Westland Lynx, with inset the so-called "Berp" tip, which permits both the fastest and slowest blade in the world. Its design makes possible a retreating-blade angle-of-attack of more than 20°, while the advancing tip is flying at MachO-97. Left It is the swept tip of extended chord (and therefore lower thickness/chord ratio) which permits the high Mach number on the advancing blade, while the vortex flow patterns induced by the sweep and the notch keep the retreating blade lifting at extreme angles of attack. This vortex flow is analogous to that of leading-edge root extensions on combat aircraft such as the F-16. Higher helicopter speed is a direct consequence of increasing the available speed differential between advancing and retreating blades and thrust balance for various areas of nozzle and, since the jet pipe was conical, small area increases could be made with a hacksaw! Use of such "linear" thrust (and even variable-area nozzles) is still the subject of study by Westland as a useful performance additive, because the more power that a main rotor has to deliver for forward flight, the more nose-down the helicopter's attitude becomes, and form drag increases. With reversers fitted, jet pipes could also be used to decelerate and manoeuvre helicopters of the future. An equally notable outcome of the record flight with the new blades was the confirmation that the reduced overall vibration levels shown in the earlier trials extended well beyond the 200kt barrier. In simple terms, the vibration stays at the 120kt level right through to 200kt. "The pilots lost vibration as a speed cue. In the run-ups they came to regard 180kt as a mild speed range." This was gratifying for Westland, which started with the aim of designing-out vibration at the rotor, instead of correcting points of peak fuselage vibration with anti-nodal mass. The associated design rules also ensured that the vibration data on the four blades were compatible with the five-blade rotor used for the W.30-300 variant, and with a 16,0001b aircraft. This speed capability and reduced vibration is the result of the unique tip design, whose shape would have been difficult, if not impossible, to make either in metal or without automated production. Early blades were composite reproductions of metal ones. Now Westland stresses that "automation is treated as a redesign process". Ease of manufacture and blade aeroelastic design factors are considered together. The mass distribution of composite blades for the Lynx and W.30 is substantially different from that of the metal ones, and this has involved considerable aeroelastic design work. One benefit of composite construction for the Lynx is that the acceptable speed for a 5,000hr life goes up from 120kt to over 145kt. Westland's first interest in composite construction was coincident with the major UK programme launched in the mid-1970s toward composite rotor blades. A highly cambered Sea King tail rotor blade was the first component which the company produced. A shortened version of this untwisted constant-thickness blade is now in production for the W.30 and for Sea King retrofit. The Westland composite blade manufacturing facility was opened in March 1985. It has a 40,000ft2 main layup area which can be extended to 60,000ft . "Layup" is hardly the right word in this context, as pre-preg composite material is used for the production of all dynamic components, with outside skins for blades being attached to the rest of the body by film adhesive. To avoid waste, stocks of pre-preg are kept refrigerated at -18°C in moisture- proof bagging and are labelled, palleted, and computer controlled so that stock is automatically taken out in strict date order. Stock is allowed to stabilise for 24hr upon withdrawal. Pre-preg is guaranteed in its quality for only 30 days at room temperature. An immense area, facing the curing facilities and set between the present and future assembly halls, is kept entirely clear for the transfer of moulds on air- cushion sleds. Such blade tools can be up to 40ft long, and can weigh from 2 to 5 tonnes, despite being made of aluminium rather than the more usual steel. They are opened and closed in the assembly area using 5-tonne full-span overhead cranes. The hand assembly area is based on wheel-mounted benches, which can be repositioned easily when a new product line is brought in. Air, vacuum, electrical power, and static earthing are all available from the floor. Initial spar wrapping and skin-to-spar mating is carried out here. Glass worktops are used for flatness, hardness, and cheapness. Cleanliness does not have to be to the level required for some electronics manufacture, but it is particularly important to keep hand grease off the exposed surface of the pre-preg once the protective paper backing is removed. All handling is done in nylon gloves, which are renewed daily or when the workroom is left for any time. Those preparing composites do not handle other components or material. Two shop employees have the sole task of restocking and re-equipping work stations at each shift—in consumables, equipment, and clothing. Items are tallied at transfer from stock, using portable bar code readers, having memories that can be dumped into a main stock-control computer. Blades are constructed in both unidirectional and +/-45° layers of E glass and XAS carbonfibres. Glass/carbon hybrid weaves are also used to give a blade with damage tolerance and less sudden failure under stress, but also a fair degree of torsional stiffness. Titanium and nickel shields give erosion protection to the leading edge. The trailing edge is formed about shaped and uncured Nomex honeycomb which is sized for controlled FLIGHT INTERNATIONAL, 27 December 1986 25
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
