Flight, April 1937
SALUTE to the ENSIGN
A Complete Survey of the New Armstrong Whitworth Monoplane, of which Twelve have been Ordered by Imperial Airways
CHARACTERISTICALLY effusive, recent publicity releases from America announcing orders for fleets of large new transport aircraft herald what they are pleased to call the ''four-engine era.''
The authors, in their overwhelming enthusiasm for the highly promising designs concerned, seem conveniently to have forgotten the superb record of service compiled by the Hannibal, Atalanta, Scipio, Scylla and Diana classes of Imperial Airways on the European and Empire routes since 1930, so we hereby offer for their consideration the fact that by the time American air lines have four-engined landplanes in everyday service Imperial Airways will be quietly celebrating at least eight years of experience with aircraft of this calibre.
Familiar with the largest practical transport machines in the world, Imperials did not hesitate, as their pioneer four-engined types passed into the twilight of their days, to order from Sir W. G. Armstrong Whitworth Aircraft, Ltd., of Coventry (whose Argosies and Atalantas with their Siddeley Jaguar and Serval engines had given such exemplary service), a dozen monoplanes powered with four Tigers apiece.
Allotted the Armstrong Whitworth works designation "A.W.27" and bearing the contractor's class name Ensign, the new machines exhibit great originality, as might well have been expected in view of the surprises sprung upon the aeronautical world by Mr. Lloyd and his colleagues in the Atalanta.
The entire batch of Ensigns is to be assembled at Hamble (leaving the Coventry works free for the production of Whitley bombers, which, incidentally, bear a number of structural similarities to the "27"), where four fuselages are now on the stocks, the first awaiting its wing. No effort is being spared by Mr. A. E. Arch and his staff, and in a few weeks the world will be shown the first completed machine of the series.
Structural Features
An outline of the structural features of the A.W.27 may prove acceptable before setting out to describe the equipment, power plants, accommodation, etc.
Essentially the machine is an all-metal, high-wing monoplane with an undercarriage the halves of which retract into the inner pair of the four wing-mounted engine nacelles.
The wing structure was dealt with in some detail in Flight of January 7 this year. Tapering in plan form and thickness, the wing has comparatively square-cut tips. It is built round a single patented rectangular box-spar with top and bottom sides of corrugated light-alloy sheet, the box being stabilised by tubes arranged in diamond formations. Metal covering is used for the leading edge, but aft of the spar, where the structure is of rolled light-alloy sections, fabric is employed. Automatic servo flaps are inset into the trailing edge of each of the mass-balanced Frise ailerons, that on the port side being under the control of the pilot for lateral trimming purposes. Split trailing-edge flaps, actuated by four Lockheed hydraulic jacks, extend between the ailerons and fuselage.
The high-wing arrangement offers not only aerodynamic advantages, but is a godsend to passengers, who might otherwise have to endeavour to crane their necks to view scenery past the trailing or leading edge of a low wing. Typically practical, Imperial Airways claim that the high wing also has the advantage of sheltering passengers entering or leaving the machine from sun or rain.
Fuselage Features
An oval monocoque structure a hundred and ten feet long, the fuselage is more or less conventional in conception, incorporating transverse frames, longitudinal stringers and sheet-metal covering. A section is suitably recessed to receive the wing and in this region embodies a massive structure to take the flying stresses. A substantial system of tubular girders is built into the lower portion of the fuselage beneath the floor.
The student of modern design will doubtless be struck by the fact that, as on the Empire flying boats, a single fin-and-rudder assembly is employed, whereas the general tendency abroad is to duplicate this structure. The length of the fuselage and ample fin and rudder area should result in good directional qualities.
Structurally the tail unit is of the monoplane type, the tailplane being built up in a similar manner to the wing. Fore-and-aft trim is maintained by tabs in the elevator, which, incidentally, is not aerodynamic ally balanced, but has a spring-loaded balancing device incorporated in the internal controls. A servo flap, which is also arranged to function as a trimmer, extends over the entire trailing edge of the rudder. Like the fin, this member is fabric-covered.
Since each of the two Ensign types has been designed to the order of a single operating company, it might be expected that the accommodation and interior equipment would bear some resemblance to those of the Short Empire flying boat. This resemblance is, however, superficial, for the machines have been developed for somewhat different purposes.
As in the case of the boat, the big Armstrong Whitworth has its passenger quarters arranged so that either chairs or bunks may be fitted, and it is interesting to learn that the change-over in the long-distance or Empire class can be made in a matter of a quarter of an hour. Twenty-seven seats are removed and are replaced by twenty bunks, with partitions, curtains, and all the necessary equipment for sleeper comfort.
On through services, when a machine is flying by day and night, the change-over would probably be made while the passengers were eating their dinner at the aerodrome hotel or rest-house. The bunks are arranged in tiers of two; those using the lower ones will have the normal cabin windows at their level, while those in the upper berths have small auxiliary windows which normally serve to increase the amount of light in the upper parts of the exceptionally deep cabins.
An interesting aspect of the method of making the day-to-night change-over is the fact that the seats and backs of the daytime chairs are transferred to the bunks and form part of the mattresses; this somewhat peculiar transfer is necessary because these parts of the chair are designed to act as the life-savers which are a legal necessity for any sea-crossing service. The chairs are entirely similar to those developed for the Empire beats, which are the result of some two years of test and experiment and are made at Imperial Airways' own furniture factory at Wandsworth.
There are three passenger compartments in the Ensign, and each of these has its own emergency exit in the form of a sliding panel in the roof. The drawing on the two previous pages clearly shows the general layout. The first compartment lies directly behind the control cabin while the other two are towards the rear of the fuselage. All of them, of course, are well away from the plane of the airscrews and from the vicinity of the main spar, which carries the power units. Furthermore, the corrugations in the bulkheads and so forth should prevent any sign of drumming. The cabins should, consequently, be very quiet even without the addition of the soundproofing equipment which one might expect to see.
Behind the forward passenger compartment is a large freight hold (which can be loaded from the outside through two wide doors), a pantry, a mail department and a lavatory. Beside these compartments, on the port side, is a feature which appears to be very popular with passengers on the Empire boats - the promenade deck. Both here and in the front compartment smoking will be permitted. In the upper part of the fuselage, above the promenade deck area, is a space for normal luggage. The main entrance for the passengers is on the promenade deck and there is another at the rear of the cabin - where, also, there is a lavatory and a second freight compartment with a sliding door. The interior of the tail may be reached, for purposes of inspection, through the rearmost bulkhead.
Apart from the normal fresh-air ventilators, which can be adjusted by each passenger, an exhaust-heated ventilation system is under the control of the steward. Tins system is described elsewhere.
In the European class of A.W.27 forty-two passengers are carried. Obviously it would not be possible to accommodate them all in the three main compartments, and in the case of this type the forward freight compartment becomes a "cardroom," where four passengers are comfortably ensconced. In addition, there are twelve seats in the forward cabin and twelve and fourteen, respectively, in the two rear compartments.
The cabin windows are of Perspex made up by the Triplex Company, the pilots' windows are of Splintex safety glass, and various inspection panels are covered with Rhodoid.
Generally speaking, the instrument layout is similar to that in the Short boats - not unnaturally, since this layout was the subject of a very great deal of thought and because the boat pilots may occasionally be expected to take over these landplanes at short notice.
In the centre of the dashboard is the Sperry automatic pilot panel, while above this are the engine switches and a master switch controlling the undercarriage and flap indicators; until this master switch is moved to the "on" position the engine switches themselves are inoperative and the captain, therefore, cannot possibly take off without first checking the positions of the flaps and undercarriage. Above the screen, in the centre, are the four engine starter and two windscreen wiper switches.
On the left of the Sperry panel are all the essential navigational instruments, including the usual Sperry horizon and gyro, a sensitive altimeter, a rate-of-climb indicator, and so forth. Beside the captain, on his left, are levers controlling the oil relief valve (permitting a heavy circulation of oil during the warming-up process) and the slow-running cut-outs for the four Tiger engines. Farther aft on the same side are the fuel tap controls.
The central throttle bank column carries, in addition to and alongside the four throttle levers, the v.p. airscrew control on the left, and the mixture control on the right. Ahead of the gates, on either side of the column, there are two other levers; that on the left controls the valves for the undercarriage retraction pumps, and that on the right controls the flap-operation valves. The pilot has merely to move one or other of the levers into its new position and the engine-driven pumps do the rest.
On the face of this central column is a small lever which can be moved up, down, or sideways. This little affair controls the entire Dunlop pneumatic braking system. Movement in the fore-and-aft plane applies or releases the brakes, while sideways movement brings the differential operation into play for manoeuvring on the ground. On the left of the column and at the level of the brake control lever is the usual Sperry pilot cut-out lever.
A useful feature which is not often to be seen on British transport machines is a direct-reading drift-sight, which is arranged on the floor of the cabin between the two pilots. Each pilot, of course, has his own compass.
The Ensign type can not only be trimmed in the fore and aft and directional planes, but has tabs on each of the ailerons, so that any slight lateral trim differences may be corrected while in flight. This aileron trimming wheel is mounted on the roof above the chief pilot's seat, while the elevator and rudder trimming wheels lie, one behind the other, on the right-hand side of his seat, so that they may be reached by either pilot. An interesting comfort detail concerns the first officer's chair, which not only slides backwards and forwards, but also swivels in order to simplify movements in and about the cabin. Entrance is normally effected through a trap-door in the floor behind the "bridge," though there is, of course, a door leading to the front compartment. An emergency exit is provided.
Behind the pilot's "platform” is the radio operator, who sits facing starboard, on which side are his Marconi transmitters and receivers. In the case of the European type these will have the normal medium-wave tuning range, while the Empire class will, in addition, have short-wave equipment.
Additionally, D/F. equipment is installed; this may be used either for obtaining direct bearings or, with the loop in a fixed position, for "homing." An indicator lies in front of the captain, immediately below his more important navigational instruments. The loop is not of the retractable type.
On the port side of the operator's compartment is a small petrol motor, similar to that used in the Empire boat equipment, which is permanently coupled to a generator. Thus, in case of a long wait on the ground with, perhaps, the radio and lighting in action, the main battery system may be replenished without the necessity of running one or other of the engines, which, through their generators, normally keep the battery up to scratch. A Rotax switchboard, covering the domestic electrical equipment of the machine, is in charge of the steward. The following Rotax equipment is specified: Two 24-volt, 1,000-watt engine-driven generators with two voltage control boxes; cabin lamps; roof lamps; navigation lights; switches and fuse boxes.
Power is supplied by four Siddeley Tiger IX geared and moderately supercharged radials, which are carried on welded tubular structures forward of the leading edge, these being provided with Armstrong Siddeley patented flexible rubber mountings.
The Siddeley Tiger IX is a recently introduced engine developed from the Tiger VI, as used in large quantities by the Royal Air Force. The engines for the Ensigns will be standard units except for certain drives introduced specially for service with Imperial Airways.
Rated at 790 h.p. at 2,375 r.p.m. at 6,500 feet, the Tiger will deliver 810 h.p. maximum at 2,450 r.p.m. at 6,400 feet. With the three-bladed De Havilland two-position variable pitch airscrews set a t fine pitch, 880 h.p. is available for take-off at 2,375 r.p.m., the automatic boost control then being "over-ridden."
"Two-Row" Engines
The engine has fourteen cylinders, arranged in two staggered rows, the capacity being 1,996 cubic inches, and measures 50.8 inches in diameter.
The engines are enclosed in standard Siddeley long-chord cowling rings, which have inner and outer sections for the purposes, respectively, of directing the air flow round the cylinders and giving an aerodynamically efficient exterior.
Petrol and oil tanks with respective total capacities of 670 gall, and 40 gall, are mounted in the leading edge.
A boiler on the exhaust pipe of one of the inboard engines supplies steam to a heater box. Cold air is blown through this box, warmed, and delivered to the cabin, the supply being regulated by the steward. A system of branch pipes supplies cold air.
The A.W.27 has the largest retractable undercarriage developed to date, the entire mechanism for raising and lowering being of Lockheed production. Each of the two independent units is located under each of the engine nacelles, which are of considerably larger dimensions than their outboard neighbours. The Dunlop wheels stand over 6ft. 3m. high and are about 27 inches wide. The retracting movement is rearward and upward into the fairings behind the main spar, a portion of each wheel being left protruding below each nacelle when the gear is completely raised. Doors are provided to preserve a smooth contour when the undercarriage is up, these being interconnected with the retracting mechanism. A special valve allows the fluid controlling these doors to be jettisoned in an emergency without the loss of fluid in the other systems.
Lockheeds were faced with some pretty problems in developing gear to operate the flaps, undercarriage and wheel doors. In all, there are sixteen jacks - four for retracting and lowering the undercarriage, four for the flaps, four for the wheel doors and four to lock the undercarriage in the raised position. The fluid system has three engine-driven pumps. The pilot is provided with the new Lockheed selector valves which return the operating levers to neutral when an operation is completed.
The undercarriage jacks are 3 1/2 inches in diameter, and should raise the wheels in 1 1/3 minutes and lower them in 50 seconds.
Over 200 m.p.h.
Complete estimated performance figures for the A.W.27s are not available, but the top speed should exceed 200 m.p.h., and the economical cruising speed is expected to be about 160 m.p.h., although, undoubtedly, the machines could be continuously operated quite safely at a higher speed. Fully laden they will weigh rather more than 20 tons and should be able to maintain height at 12,500ft., using only three of their four Tigers and at 4,000ft. on two engines.
The normal range will be 500 miles against a 40 m p.h. head wind, but with extra tankage and reduced payload this can be increased to 1,000 miles under the same conditions.
Among the firms which are stated by the makers to have supplied components and accessories for the Ensigns are the following :-
Accles and Pollock, Ltd., Aircraft Materials, Ltd., Albion Drop Forgings, Ltd., Aluminium Corporation, Ltd., Arens Controls, Ltd., Automotive Products, Ltd.
Benton and Stone, Ltd., J. Biresford, Ltd., T. M. Birkett, Ltd., Birmabright, Ltd., Birmingham Aluminium Castings, Ltd., P. H. Bonnella, Ltd., James Booth and Co., Ltd., Bowden Engineering Co., Ltd., British Aluminium Co., Ltd., British Celanese, Ltd. (Celastoid), British Marine Aircraft, Ltd., Brooke Tool Mfg. Co., Ltd., E. G. Brown and Co., Ltd., Brown Bros. (Aircraft), Ltd.
Cork Mfg. Co., Ltd., De Bergue Patents, Ltd., De Havilland Aircraft Co., Ltd., Dunlop Rubber Co., Ltd., Fairey Aviation Co., Ltd., Firth-Vickers, Ltd., General Aircraft, Ltd., High-Duty Alloys, Ltd., Hoffman Mfg. Co., Ltd., Lightalloys, Ltd., Magnesium Castings and Products, Ltd., Manganese Bronze and Brass Co., Ltd., Masson, Seeley and Co., Ltd., May and Baker, Ltd. (Rhodoid).
Northern Aluminium Co., Ltd., Parnall Aircraft, Ltd., Ransome and Maries, Ltd., Reynolds Tube Co., Ltd., Rotax, Ltd., Rotherhams, Ltd., Rubery Owen and Co., Ltd.
Geo. Salter and Co., Ltd., Short Bros., Ltd., Simmonds Aerocessories, Ltd., Skefko Ball Bearing Co., Ltd., Smith's Aircraft Instruments, Ltd., Sperry Gyroscope Co., Ltd., Splintex Safety Glass Co., Ltd., Stirling Metals. Ltd.. Superflexit, Ltd., Tecalernit, Ltd., Geo. Tucker Eyelet Co., Ltd , Viekers Aviation, Ltd., Vokes, Ltd.
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Flight, October 1938
British Commercial Aircraft
ARMSTRONG WHITWORTH
LAST week the first example of the Imperial Airways’ fourteen Armstrong Whitworth Ensign class transports went into preliminary service on the Paris run. Actually this first machine is one of those intended for the overland service to India, and when experience has been gained and others of the class have been delivered it will go on to this route.
The A.W.27, as this machine is officially designated, is the largest commercial landplane to be put into regular service. It is, moreover, one of the very few high-wing monoplanes among transport types, imperial Airways having decided on this layout in order to give the passengers a range of view which is not obtainable in low-wing machines. The Ensigns, which are each powered with four 880 h.p. Siddeley Tiger IX engines, are being laid out in two versions. One, for European operations, will carry forty passengers, and the other, for the Empire routes, will carry twenty-seven by day or twenty by night. In the empire version there are three main passenger compartments, one immediately aft of the control cabin, and two others aft of the monumental centre section. As in the case of the Empire boats, a promenade deck has been arranged below the wing between the forward and the centre cabins.
The wing of the Ensign is built round a single rectangular box spar in which the top and bottom sides are of corrugated sheet, and the entire structure is stabilised by means of tubes arranged in diamond formation. Metal covering is used for the leading edge, but aft of the spar, where the wing is built up of light alloy sections, fabric covering is used. Split trailing-edge flaps, actuated by four hydraulic jacks, extend between the ailerons and the fuselage. The latter is a monocoque structure, the length of which - 110ft. - enters the realm of marine engineering. It is more or less conventional, involving the use of transverse frames, longitudinal stringers, and a metal covering. The top of the fuselage is recessed to take the wing spar, and the area concerned is, of course, substantially strengthened to take the flying stresses.
The Ensign is somewhat unusual, according to modern practice, since the fin and rudder assembly is not duplicated. Apparently the length of the fuselage and the ample area of this assembly give good directional stability. Most interesting of all, perhaps, is the engineering feat involved in designing and manufacturing an undercarriage retracting gear to suit a high-wing machine of such dimensions; the hydraulic jacks in the undercarriage are 7ft. long in their extended position, yet the raising operation takes only 1 1/4 minutes.
Ensign Data.- Span, 123ft.; length, 110ft.; height, 23ft.; wing area, 2.443 sq. ft.; all-up weight, 48,500 lb.; weight empty, 33,000 lb.; payload, 9,500 lb.; wing loading, 20 Ib./sq. ft.; power loading, 13.8 lb./h.p.; span loading, 3.2 Ib./sq. ft.; maximum speed (sea-level), 182 m.p.h.; maximum speed (7,000ft.), 205 m.p.h.; cruising speed (7,000ft.) on two-thirds power, 170 m.p.h.; stalling speed, 66 m.p.h.; initial rate of climb, 700 ft./min.; rate of climb at 10,000ft., 500 ft./min.; service ceiling, 16.500ft.; range at cruising speed, 850 miles; price, 752,000.
Makers:- Sir W. G. Armstrong Whitworth Aircraft. Ltd.. Coventry (Coventry 60071).
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