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Aviation History
1960
1960 - 1232.PDF
172 FLIGHT, 5 August 19,o I Rotor Blade De-icing SPRAYMAT TRIALS ON WESSEX THE Napier Spraymat de-icing system has been extended forapplication to helicopter rotor blades, and three prototypesets of heating mats are now being manufactured, on a production basis, for trials in Canada next winter. The systemwill be tested on the Westland Wessex, Spraymat de-icing being incorporated in an all-weather version of this anti-submarinehelicopter entering production next year. Unlike the majority of Spraymat de-icers produced for fixed-wing applications, the helicopter heating mats are prefabricated, for bonding to the components by the aircraft manufacturers. Thiswill avoid the transport of vulnerable rotor blades from the makers to Napier's Spraymat shops at Luton and back again. Actual Wessex rotor spars are being used for the moulds onwhich the 24ft-long prototype mats are being made. These follow standard Spraymat practice in containing Kumanal heat-sprayedcopper-manganese heating elements between two glasscloth laminates. Six such elements, with electrical power suppliedat the root, run spanwise, and are connected by a copper contact at the tip. The circuit is completed by a low-resistance purecopper foil bus-bar running spanwise along the upper surface of the heating mat. The whole laminate is symmetrically disposed around the leadingedge, extending on both surfaces to 30 per cent chord, but the heating elements are asymmetrically disposed, their coverage Preparing the connections to the ends of the supply bus-bars of a rotor-blade heater mat General view of Napier's heater-mat balancing rigs. The large rig on the left is for spanwise and balancing; the other is used for balancing in the chordwise plane extending to eight per cent chord on the top surface and 27 percent on the lower surface, where the ice catchment area is greater due to positive angles of incidence in flight. The heating mats'for the tail-rotor blades are essentially similar, but each comprises only one element along the extreme leading edge. Heating is required to a greater degree over the inboard sectionsof the blade than toward the tip, where both centrifugal force and kinetic heating aid ice-shedding. This is achieved by loweringthe specific resistance of the heating elements in four stages, from 30W/sq in at the root to 20W/sq in at the tip, simply by increasingthe thickness of the sprayed element. To speed ice-shedding— and so reduce the possibility of asymmetrical shedding, whichleads to rotor imbalance—very high power-loadings are applied. For speed of surface heating the insulation gaps between elementshave been reduced to only 0.030in. The heating mats, and blades, are protected from hail and tainerosion, and from stone damage, by bonding over the outboard 10ft of the leading edge a 2in electriform micrograin nickel sheath.Inboard, where blade flexure is greater, and the erosion problem less critical, the mat is protected by the application of NapierStoneguard, a synthetic resin heavily loaded with stainless-steel particles, and further coated with Erocoat anti-erosion paint. Thenickel sheath is used along the whole length of the tail rotor blades. Napier work to very close weight and balance tolerances, sothat any blade assembly will be interchangeable with all others Applying Stoneguard anti-damage coating. The blade is being longitudinally vibrated and gently warmed by an air blower of its type. Two complex balance-rigs have been built for chord-wise and spanwise balance, and heating mats are at present tested for balance at all stages in their manufacture. If the balance is outit can usually be made good in subsequent stages. The primary method of correction is to spray metallic "false elements" in thatpart of the top surface laminate behind the heating elements. When in full production, Napier expect that three weight-and-balance checks during manufacture will suffice. Present tolerance1 on main rotor blades allow ±2oz in the 140oz weight of a mainblade mat, +0.5in in spanwise movement of the e.g. and ±0.05in in the chordwise position. Tail-rotor mats weigh 9oz and +sozis allowed in the weight and ±gin in spanwise e.g. position. To avoid asymmetrical ice-shedding, matching heating elementson opposite blades are cycled together, so there are twelve cyclic areas for the four main-rotor blades. Two phases of a three-phaseRotax alternator, driven from the main gearbox, provide the power for main-rotor heating, while the third phase supplies thepower for the tail rotor mats. An 81b "cyclic interrupter," developed for Napier by Austinlite Ltd and housed within thefuselage, switches the current, which passes through an 1 lib cyclic distributor mounted on the rotor head. The distributorcontains a rotary solenoid which is activated when current is pass- ing to one pair of elements and winds a spring device which movesthe contacts to connect the subsequent pair. The total power reauirement for the system is about 18kVA. The tail rotor is cycled four times for every full cycle of themain rotor, for, although the aerodynamic effects of ice on the tail blades are negligible, asymmetrical self-shedding leads to verypronounced vibrations. Cycling and cycle timings have not been finalized but will be determined during the winter tests in Canada.Napier claim that the Spraymat de-icing system can be adapted, to any type of helicopter, and one projected application is on theRotodyne, where the 25O°C gases ducted through the rotor blades in the helicopter regime will introduce new problems.
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