Flight, October 1927

THE SIKORSKY S.36.B AMPHIBIAN
An Interesting Flying-Boat of Unusual Design

   WE have, on previous occasions, referred to the work of Igor Sikorsky, the Russian aircraft designer - who produced the first successful "giant" multi-engined aeroplane some twelve years or so ago in Russia - now settled in America, where his firm, the Sikorsky Manufacturing Co., of New York, has produced several different types of aircraft. This week we are able to give some particulars of one of his recent productions, the S.36B amphibian flying-boat, which is, perhaps, of more than usual interest.
   One point of interest attached to this Sikorsky flying-boat is the fact that it is on one of these machines that Mrs. Grayson is making her attempt to cross the Atlantic from America to Denmark. Technically, also, it possesses several noteworthy features.
   The Sikorsky S.36 is a sesquiplan flying-boat of the short-hull variety, the tail surfaces being carried, not on the hull, but by an outrigger from the main plane, braced from the stern of the hull by two struts. The tail surfaces are thus entirely independent of the boat, and being placed high, are well protected from damage when alighting or taking off, besides being located in the line of thrust.
   Attached to the hull is a very short lower plane, used mainly for structural purposes, at the ends of which are mounted wing floats. High up above the hull is the large main plane, supported by a form of Warren bracing. In this way the structural advantages of the biplane are obtained with, to a very large extent, the aerodynamical efficiency of the unobstructed monoplane wing; for it is possible with this arrangement to employ a high aspect wing of medium thickness. The engines, two J.5C. Wright "Whirlwinds," are located slightly below and in front of the upper plane.
   There are three models of the S.36, one with open cockpits and one with an enclosed cabin, each seating a pilot and seven passengers. The third model has a front cockpit seating two side-by-side, and immediately behind is a cargo compartment of 175 cub. ft. All three models are again divided into two types, a service type and a long-distance type, the latter having a larger wing area, a different installation of tanks, etc.
   The first, open cockpit, type is, we believe, a straightforward flying-boat, while the others are equipped with amphibian gear, which can easily be dismounted, if desired. One of the service open cockpit type was recently supplied to the Andean National Corp., Ltd. - a Canadian concern operating an oil pipe line in Colombia, S. America. This line runs along the Magdalena River, where no landing fields are available, so the amphibian gear was not required. The machine used by Mrs. Grayson and Wilmur Stulz in their Atlantic attempt is an enclosed cabin amphibian.
   Metal construction is used very largely on the S.36, the wings and tail structure being entirely of metal, fabric covered, while the hull is of hard wood with sheet duralumin covering.
   The wing and tail trusses are of simple design, with external wire bracing replaced by streamline steel struts wherever possible, thus reducing vibration to a minimum and also facilitating assembly. The wing construction is also of simple yet original design. The internal structure is of riveted and bolted duralumin - no welding being employed. It is remarkably light and lends itself readily to the fabrication of most of the members on a production basis.
   The spars consist of two main members of T-section one above the other - the lower one being inverted with the stem of the T upwards. These T-members are a special Sikorsky development, having small "bulbs" at the ends of the flanges (the head of the T) placed on the inside (toward the stem). By having these small "bulbs" tangent to the ends of the T-head on the side towards the stem a small flange results on that side increasing the moment of inertia about the vertical axis yet still leaving the top and bottom surfaces of the spar flanges flat and unobstructed for riveting or reinforcing plates. The stems of the upper and lower T-section members are joined by diagonal members riveted in place by a single rivet at each end, making the spar into a Warren truss. All rivets are duralumin, and chrome nickel bolts are used on the major assemblies.
   Drag bracing of the conventional type is employed, consisting of tie rods and compression ribs bolted to the spars, and of the same type of construction as the spars except that the flanges are perforated to reduce the weight. The ribs a production job - consist of cap strips and diagonal bracing members of duralumin channels riveted together to form a conventional type rib truss. The flanges of the channels of the diagonals are flattened at the ends to facilitate riveting. Where the diagonal bracing meets the cap strips, a simple duralumin fitting - interchangeable at all points - is used; two rivets hold this fitting to the cap strip, while a third rivet is used to fasten diagonal braces to each fitting. All parts in the wing structure that do not come in direct contact with fabric covering are finished with a coat of varnish, while those coming in contact with fabric are protected by a primer.
   The outrigger carrying the tail is built up like a fuselage, of four longerons and cross bracing, of duralumin and of usual Sikorsky construction. It is attached at the mid point of the upper wing where two diagonal struts support it from the sides of the hull, while two struts also extend from the stern of the hull to a point about two-thirds from its forward end.
   Both elevators and ailerons are of the mitred type, operated by cables carried up from the cockpit to the wing and passing through the latter (to the ailerons) or along the bottom of the outrigger to the tail. The rudders are of the Sikorsky compensating type, that is they are cambered so that with one engine stopped the camber of each rudder - one in the slipstream and one out of it - tends to counteract the off-set thrust of the running engine, thus giving complete control of the machine. It may be mentioned here that this arrangement has proved very satisfactory in practice, and the S. 36, during its tests, maintained an altitude of 1,500 ft., with a load of 2,500 lbs., on only one engine and was entirely under control.
   In the cabin type, the hull is 26 ft. long, 4 ft. 3 in. wide and 5 ft. 2 in. high. It has a 12° Vee bottom with a step slightly to the rear of the mid-point. As previously stated, it is built up of a hardwood frame to which are screwed varnished duralumin struts. Duralumin wood-screws are used, and at the chine and wherever the covering is reinforced, duralumin rivets are employed. Steel tubular members across the hull are bolted in place and gusset plates are of duralumin, protected from corrosion by a special primer; the wood structure is finished with a coat of varnish.
   The frames comprising the internal hull structure are of both solid wood and built-up wood sections, reinforced with sheet duralumin, depending upon the stress at that point. The hull is divided into numerous water-tight compartments by struts of duralumin, which are so arranged that by removing a few bolts one can easily crawl inside the hull to inspect or repair the structure. An opening is provided at the bow of the hull through which the pilot can drop anchor or effect a mooring when the boat is afloat.
   The cockpit, with side by side dual control, is in the front part of the cabin, just forward and below the leading edge of the top plane where excellent visibility in all directions is obtained. Behind the cockpit are two seats, each comfortably accommodating three passengers; and behind these is a compartment for luggage. The cabin is equipped with an efficient ventilation system and is electrically lighted. In the open type, wind shields are provided for each cockpit. In the service type the petrol tanks are located in the top plane centre section, while on the long-distance types there are additional tanks behind each engine and in the nose of the hull.
   The principal characteristics of the Sikorsky S. 36 are :-
   Service Type. Long-Distance.
   Span (top) 62 ft. 0 ins. 72 ft. 0 ins.
   ,, (bottom) 18 ft. 1 in. 24 ft. 0 ins.
   Overall length 34 ft. 0 ins. 34 ft. 0 ins.
   Height on wheels 12 ft. 0 ins. 12 ft. 0 ins.
   Wing area 585 sq. ft. 668 sq. ft.
   Weight empty 3,950 lbs. 4,400 lbs.
   Useful load 2,050 lbs. 3,000 lbs.
   Weight laden 6,000 lbs. 7,400 lbs.
   Loading per sq. ft. 0-25 lbs. 11-1 lbs.
   ,, h.p. 13 lbs. 16-1 lbs.
   Speed range 49-120 m.p.h. 52-118 m.p.h
   Cruising speed 100 m.p.h. 100 m.p.h.
   Climb (ground level) 600 ft./min. 400 ft./min.
   Ceiling 15,000 ft. 14,000 ft.