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Aviation History
1958
1958-1- - 0195.PDF
8 August 1958 199 A New General-purpose Helicopter Powered by a Free-turbine Engine SAUNDERS-ROE P.531 Flight trials of G-APNU are proceeding very satisfactorily, and several hours have been logged. Here the tailplone is not fitted. THE current year will be memorable as one in which theBritish aircraft industry produced a series of notable turbine-driven helicopters. Smallest of these, but probably the one which will be manufactured in the greatest quantity, is the P.531by Saunders-Roe, Ltd., whose helicopter division is located at Southampton Airport. The development of this attractivemachine has been outstandingly rapid; although detail design started as recently as January 1 of this year, the first prototype hasalready been flying three weeks. Saunders-Roe entered the rotary-wing field in 1951, when thecompany assumed responsibility for the further development of the Cierva Air Horse and Skeeter. The former was abandonedfor financial reasons, but the Skeeter is today in full production as a standard light helicopter. This is providing adequate currentbusiness; but, looking to the future, it had long been apparent to the company that there would be a demand for a new general-purpose helicopter with much better characteristics than any hitherto produced. The use of a turbine power unit would greatlyincrease the payload and performance of a helicopter of a given size, and at the same time the company appreciated the possibilityof evolving a machine with outstanding reliability and ease of servicing.In the summer of 1957 the Blackburn Turmo free-turbine engine was awarded a special^category type-test certificate, andthis powerplant seemed an obvious basis around which to plan the new helicopter. Saro have never had strong views on the questionof free-turbine versus single-shaft engines, but their inclination towards the former is being steadily strengthened by their experi-ence with an engine of this type. The simplest solution would have been to install a Turmo in a Skeeter and keep engineeringchanges to the minimum, but from the outset it was evident that it would be greatly preferable to start from scratch with a newdesign. At the same time, however, Saro were careful not only to adhere to proven principles but also to incorporate parts identicalwith, or based upon, the corresponding parts of the Skeeter wherever this could be done without compromising the newdesign. In practice this philosophy has been followed to a marked degree and has proven eminently successful. Serious design started last November. The production machine,designated P.531 Mk 1, will be a five-seater, scarcely larger than the Skeeter in overall dimensions and having rotors of the samediameter, but with tail-rotor blades of increased solidity and a main rotor widi four blades instead of diree. The transmissionand rotor system will be stressed for 650 s.h.p., and it is expected that the powerplant will be a fully rated Turmo of 425 s.h.p.Later a more powerful unit may be fitted, as noted at the end of this article. Meanwhile, construction has proceeded upon twoprototypes, virtually identical in appearance but employing a transmission initially limited to 300 s.h.p. and a correspondinglyderated Turmo. These two prototypes have enabled the difficult parts of thedesign to be investigated much more rapidly than would otherwise have been the case. Engine-handling, vibration, flying controlsand several other critical factors are all on the agenda confronting the first prototype G-APNU. It is appropriate now to describe theexisting two prototypes before once more picking up the thread of development and future sales. Structurally the P.531 is extraordinarily simple; and yet a verygreat deal of care has been taken to ensure the fitness of the airframe for its multitudinous tasks. A sketch on p. 201 clarifies thelocation of the main load-carrying members, and emphasizes the fact that almost all the visible parts of the forward fuselage aresecondary structure which can be cut away with impunity to provide access to the interior. Nearly the whole airframe is conventionally riveted from lightgauge aluminium-alloy sheet. The only significant departures are the employment of thin stainless-steel to form a box under thepowerplant, steel tubes to suspend the aircraft from the main rotor and the use of honeycomb in the ventral fin. The fuselage isdivided into a forward portion and a rear boom of circular section carrying the tailplane and tail rotor. The forward section is builtup on a pair of vertical webs placed side-by-side to form a box-like keel or spinal girder which at the rear is extended upwards tomeet the four tubes which suspend the helicopter from the main rotor. The basic structure is completed by the addition of lightbulkheads, a metal floor and the fire-resistant sheet-steel tray on which the engine is mounted. In a typical configuration twoindividual seats are attached to the forward part of the floor and a folding bench seat or stretcher racks can be pinned at the rear.Flying controls pass along the starboard side beneath the floor and up through a trunk on the starboard face of the upturned part ofthe right-hand main web, to emerge adjacent to the right-hand side of the rotor hub. The electrics similarly pass down the portside, both sides of the fuselage being liberally provided with access doors. The upturned parts of the main webs locate a pair of transversebulkheads running the full cross-section of the fuselage. Between these bulkheads is housed all the fuel, there being three flexiblecells by Fireproof Tanks, Ltd., joined to a common sump beneath the centre cell. Subdivision of the space into three portions isexpected to minimize surging during rapid rolling manoeuvres. Ventral doors provide access to the low-pressure warning trans-ducers and filters beneath the port cell and to the twin booster pumps under the central sump. Power is provided by a Blackburn-Turbomeca Turmo 600 free-turbine unit mounted completely externally on the deck at the rear of the forward fuselage. The entire installation is eminentlyaccessible and the decking around the engine has corrugated reinforcing beneath it to withstand the impact of heavy boots. (Continued on page 201)
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