Flight, September 1926

THE SIKORSKY S.35
The Three-Engined ("Jupiter”) Transatlantic Biplane

   THE Sikorsky S.35 biplane - on which Capt. Rene Fonck, the famous French Ace, made the tragic attempt to fly across the Atlantic from New York to Paris - was, as its name implies, designed by M. Igor I. Sikorsky the Russian designer and pioneer of the Giant Multi-engined aeroplane. As we have previously mentioned in FLIGHT M. Sikorsky has now settled down in the United States where, under his direction, the Sikorsky Airplane Co. of Roosevelt Field, Long Island, has produced during the last year or so several very successful aeroplanes of various types.
   The S.35, which we are able to describe and illustrate this week, was primarily designed as a passenger and freight machine, but for the Atlantic flight certain modifications were made. To these we will refer as occasion arises, as otherwise the machine as originally designed and described herewith is the same as in its Atlantic form.
   Almost entirely of metal construction the S 35 is the largest of the modern Sikorsky machines and is similar in most respects to some of these previous models. It is a three-engined fuselage biplane with a large upper plane and a much smaller lower plane.
   Upper and lower planes have neither sweep-back nor stagger and are braced by two sets of M-struts at the fuselage, and by the wing engine struts, and further by a pair of interplane struts in each of the outer wing extensions. On the Trans-atlantic machine, however, an extra bay had been added on each side of the wing engines, in order to cope with the extra load of petrol that had to be carried for this flight; in this case, therefore, there are two pairs of interplane struts on each side.
   The wing-section employed is one specially developed by Sikorsky, and section and chord are the same throughout the entire span, except at the tips, when the wing is rounded oil and raked. Ailerons are fitted on the top planes only. The chord of the top planes being comparatively large - just over 7 ft. - and the wing section somewhat thick, a very deep main spar is obtained.
   The latter consists of a sheet duralumin web, braced top and bottom by duraluminum angles which are riveted on either side of the central web. The compression wing drag struts between the spars are square section tubes, of the same material, similar to those used in the construction of the fuselage, arranged in the form of an N, lying on its side, with the parallel legs connecting the tops of the spars and the diagonal member running from the bottom of the front spar to the top of the rear spar. Internal wire bracing of the usual type is employed throughout the entire structure.
   The ribs are built up of U-shaped duralumin sections, and are extremely strong and simple. Top and bottom cap strips of the ribs are approximately curved to the wing section, and then set in an erection jig when they are braced by a Warren truss system of "U" members. The ribs are then slid over the spar and riveted to stamped duraluminum pieces which rigidly secure the ribs and provide a sound fitting for the curved cap strip over the angular shape of the spar.
   Following the same construction as that of the main planes the horizontal tail surface - composed of two duralumin spars and U-shaped dural ribs, but with drag struts of angle section batterns instead of square tubes - is mounted on the top longerons of the fuselage. There are no vertical fins, but there are three sets of rudders - one centrally mounted on the fuselage sternpost, and a pair on each side on the tail plane with one rudder above and one below the latter. These rudders have the usual Sikorsky compensation arrangement, which consists of offsetting the rudders, which are of aerofoil section, so that if one of the side engines stops the slip stream of the other side engine affects the rudder in such a manner as to counteract the offset thrust, thus adding greatly to safety and ease of control.
   As the S. 35 was originally designed for passenger and freight carrying, the fuselage is of the cabin type, the cabin being exceptionally large and roomy - the actual dimensions being 4 ft. wide by 6 ft. high, by 15 ft. 6 in. long. The pilot's cockpit is located in the nose of the fuselage, which slopes sharply down to the central engine, so that, together with the fact that the lower wing is comparatively small, an excellent range of vision is obtained from all parts of the machine. A duralumin door located at the rear of the cabin provides easy access to the latter.
   The fuselage structure consists of a truss system of square duralumin tubes, about 1 1/2 in. gauge, the vertical struts being bolted to the longerons by a simple system of gusset plates which obviates the need of welding and enables parts to be changed when required. The cabin portion is free from cross tracing wires, but the rest of the fuselage is wire braced.
   The landing gear is of the divided axle type consisting of two separate units, made entirely of steel with a very wide track - 18 ft. 4 in. Each shock-absorbing unit consists of 24 independent rubber rings, readily interchangeable. It is possible to dismount each landing gear unit in a few minutes by removing only three bolts.
   For the Transatlantic flight provision had been made for an extra landing gear being fitted, directly under the fuselage in order to cope with the enormous load carried at the start. This extra landing gear could be dropped as soon as the machine got away. Naturally, a large amount of space is provided for petrol, necessary for a flight across the Atlantic. Petrol is carried in four tanks formed in the leading edge of the top plane, in tanks in the wing-engine nacelles, and tanks in the fuselage. The fuel is pumped up to the wing tanks, whence it flows by gravity to the engines. Several fuel pumps are provided and all the tanks can be interconnected or used separately, so that there is little danger of petrol failure during flight. The wing tanks hold sufficient petrol for four hours at full throttle, and for ordinary commercial work only these tanks are employed.
   Three Gnome-Rhone "Jupiter" (built in France under licence from the Bristol Co.) 400 h.p. air-cooled radial engines are fitted, one being mounted in the nose of the fuselage and the other two on either side in nacelles midway between top and bottom planes.
   The general characteristics of the standard type Sikorsky S.35 are :-
   Span 76 ft. 0 in.
   O.A. length 44 ft. 0 in.
   Height 15 ft. 18 in.
   Wing area 840 sq. ft.
   Weight, empty 7,200 lbs.
   Total weight 13,800 lbs.
   Weight per sq. ft. 16-1 lbs.
   Weight per h.p. 11-3 lbs.
   Speed (est.) range 59 - 160 m.p.h.
   Cruising speed 140 m.p.h.
   Speed (two engines) 115-135 m.p.h.
   Ceiling 15,000 ft.
   (two engines) 8,000 ft.
   Climb (ground) 1,100 ft. min.
   (two engines) 400 ft. min

   The Atlantic type S.35 had the following modifications :-
   Span -
   Top 101 ft. 0 in.
   Bottom 76 ft. 0 in.
   Wing area 1,095 sq. ft.
   Weight empty 8,000 lbs.
   Weight of equipment 490 lbs.
   Weight of fuel 15,200 lbs.
   Total weight 24,200 lbs.
   Weight per sq. ft. 21. 8 lbs.
   Weight per horse-power 19 lbs.