The S-42: The Development and Characteristics of a Long-Range Flying Boat

A Speech Given To The Royal Aeronautical Society, London, On November 15, 1934 By Igor I. Sikorsky

Almost a decade ago, with the rapid world-wide development of air transportation, came the demand for new types of aircraft. It became evident that a plane capable of operating from land or water was the logical step needed to open up larger areas for air transportation. The proximity of water compared with the remoteness of available landing-fields to the centers of population was the deciding factor in amphibian development

While the advantages of the amphibian are obvious, the limitations and difficulties of creating an efficient plane of this type are also evident. Compared to the seaplane or landplane, the amphibian must take care of the extra weight and parasitic resistance, as well as solve various other problems connected with combining the characteristics of the land and seaplane in one aeroplane.

In order to overcome these difficulties and produce an amphibian that would compare favorably in performance with the best land or seaplane, it was necessary to make the utmost effort to refine all basic elements. In other words, it was necessary to create highly efficient power plant, wing and control surface units so that a wide margin of increased performance would be obtained that would compensate for the extra weight and resistance of the amphibian.

Evolved from painstaking study and research came the S-38- Its history is the conception of the Pan-American Airways, the air yacht of the private owner, an6 the pioneering craft of exploration into hitherto inaccessible regions of Central and South America and Africa.

Over a five-year period more than 100 of these aircraft were delivered and the majority of them are still in service’ As the characteristics of this aircraft became more and more evident, the hazy dream of large aeroplanes materialized into a definite plan. With the increasing business of the Pan-American Airways, their fleet Of S-38’s became inadequate. Upon their request, the big flying boat idea became a reality.

The three Clipper flying boats that now fly over the Southern Americas are the results. In our minds, at the time of designing, was a flying boat diverting in outline from the usual Sikorsky outrigger design, but it was felt that radical changes were not advisable, particularly because the large plane was to be an amphibian. Knowing what the S-38 could do, it was felt that a large size edition of this type would be the logical jump from an eight to a forty-passenger ship.

A considerable amount of work, research study and experimentation was spent in the development of the S-40. The theoretical work, design, stress analysis and research required the full time of the Sikorsky Organization for more than two years. The Sikorsky Company realized that upon the experience that this model would give, an even more efficient amphibian would be based.

Even then this new amphibian was gathering shape in the minds of the Sikorsky designers.From its first flight, the S-40 was a success. Capable of carrying forty passengers over a considerable range, the S-40’s dove-tailed with the need of larger equipment to meet an ever increasing passenger list. The engineers who built the S-40 watched the performance very closely. In November, I931, when the first ” Flying Clipper ” made its cruise from Miami to Colombia and the Panama Canal Zone and return.

The designer was on board and had several conferences with Colonel Charles A. Lindbergh, technical adviser to the Pan-American Airways and chief pilot in charge of this flight, wherein the basic ideas and main requirements for a new type
large flying boat were set down.

The engineers who built the S-40 watched the performance very closely. In November, I931, when the first ” Flying Clipper ” made its cruise from Miami to Colombia and the Panama Canal Zone and return, the designer was on board and had several conferences with Colonel Charles A. Lindbergh, technical adviser to the Pan-American Airways and chief pilot in charge of this flight, wherein the basic ideas and main requirements for a new type large flying boat were set down.

Immediately afterwards the engineers of the Sikorsky Company started actual research and development of a seaplane along these lines. The endeavor was to obtain a much higher all-round efficiency that would permit a much greater flying range and higher cruising speed, as well as to introduce various other improvements and refinements.
In the meantime, Pan-American Airways had placed in extensive service the second and third Clipper aircraft. The experience obtained from millions of miles of commercial flying gave Pan-American Airways excellent data for the development of a very complete set of detailed specifications for a plane that would cover the needs of further expansion and the possibilities of a commercial line across the Atlantic. Written into the specifications were requirements that were seemingly far in advance of the progress of aviation.

Gathering impetus from the reality of the S-42, power plant development advanced with great rapidity, so that when the question of engines and propellers became imminent, the Pratt and Whitney Aircraft Corporation was ready with its greatly improved Hornet model, and Hamilton Standard Propeller Company had to offer, what was considered by the Collier Trophy Committee an outstanding advance in the aviation world, a practical controllable pitch propeller.
Knowing the difficulties that are encountered in the endeavor to coincide the desired with the practical, close cooperation between the Pan-American Airways and the Sikorsky Company was necessary. Every detail from design to final fabrication was an approval based on careful criticism and study ; every phase that forms a part of the commercial airline was considered, including cost of operation, maintenance, durability, and passenger comfort.Design work again required over two years. Reports, records and tests, in seemingly endless procession, were culled of their best ideas and these were incorporated as far as was practical.

Stress men spent tireless hours figuring structural strength that would meet high self-imposed safety factors and still allow for simplified construction and low weight. Their complete analysis covers almost a thousand pages of closely typed matter with no consideration to the countless pages that contributed to the final figures.

Because of the stringent requirements of the contract calling for a useful load to gross weight ratio Of 47-53, a rigid weight control was set up. A threefold system was employed-one, an estimation from the design figures allocating to each unit a certain share of weight; another calling for calculation from the finished drawings and a comparison with the estimated; and the third, a program of actual weighing of parts. The value of this triple control is shown In the 48-52 ratio obtained.

No drawing was approved until it passed through the manufacturing department for a check against complicated design that would involve expensive fabrication or that would require undue maintenance when put into operation.

The S-42

In its very outline the S-42 represents simplicity. Diverting sharply from the past Sikorsky designs, external bracing’s have been reduced to a minimum. The tail, instead of being supported by outriggers, is attached directly to the hull.

The one-piece wing with tapering tips is attached to the hull by means of a superstructure. The necessary large external struts brace from the hull to the outer portion of the wing. These struts are the largest streamlined duralumin sections ever extruded. With a span of II4ft. 2in., the wing has an area of 1,330 square ft. Spars and compression members, of modified Warren Truss design, are constructed of extruded duralumin shapes. Stressed metal skin covers the major portion of the wing surface. Flush type rivets are used throughout the external surface.
Extending along the full straight portion of the rear spar is the hydraulically controlled flap. The flap is mechanically operated by means of a substantial hydraulic piston. The piston is actuated by an electrical pump that is controlled from the pilot’s compartment. For emergency use a manually operated pump is provided. The angular position of the flap can be altered in accordance with the attitude of flight, thus changing the performance of the whole wing. Ailerons of conventional design, tapering in conformity with the wing plan, are hinged to the rear spar outboard of the flaps. The power plant units, consisting of four Pratt and Whitney 700 h.p. geared Hornet engines, together with the necessary accessories, are attached to the front spar by means of welded steel tubular nacelles. Completing these units are the three-bladed variable pitch propellers, the largest of this type ever produced by the Hamilton Standard Propeller Company.

The full anti-drag rings and nacelle cowls merge into the wing at the front spar. Recessed into the leading edge are powerful landing lights. The lenses of these lights follow the curvature and form part of the leading edge. Eight sections of the leading edge, one on either side of each engine, fold down and form engine servicing platforms. Along the interior of the leading edge run the control cables, electrical conduits and other control and fuel units. All installations are made suitable for easy inspection and maintenance. Cradled between the spars and compression members are eight elliptical fuel tanks of a total capacity of 1,240 gallons and four similar shaped oil tanks of 74 gallons capacity. Holding these in place are metal straps which are covered with thick padding to insure vibration insulation.

Removable panels, above the fuel and oil tanks, and on the entire center of the lower surface afford access for inspection and servicing. The re-fueling system in the center portion of the wing is an interesting timesaving device. A single intake pipe directs fuel under pressure to any one or all tanks by means of a series of control valves. Equal in importance with the wing are the parts that make up the body group. The two-step, long stern type hull measures 67ft. 8in,. from bow to stern. Deep keel, transverse frames which are widely spaced in order to facilitate maintenance, and heavy stringers form the hull skeleton. Keel and frames are of plate girder type. Duralumin shapes and sheeting are used throughout.

The skin covering has the appearance of a smooth unbroken surface. This has been obtained by the use of a filler in the skin seams and the impressions of the slightly indented flush type rivets. Nine watertight doors separate the various compartments.

Marine equipment is located in the bow compartment. This compartment has been designed especially large to afford easy handling of a convenient anchor winch and to facilitate rapid mooring operations.

The pilot’s compartment following averages eight feet wide and seven feet from bulkhead to bulkhead.

An aisle 20 inches wide separates the pilot and copilot seats. These seats are extra roomy and are adjustable. They are designed to supply the maximum of comfort for long flights.

Both pilots have an unobstructed view of the complete flight instrument board. Special requirements as to making all instruments and parts readily removable for checking and servicing have been rigidly adhered to.

To the rear of the pilot’s seat is located the flight mechanic’s quarters. From his position the mechanic can readily attend to all the details under his control. Grouped on a separate panel directly in front of the mechanic’s seat. are all the instruments pertaining to power plant operation.

Opposite the mechanic’s quarters and in an uncrowded space is the radio receiving and sending station.

Surface controls are of the dual type featuring ease of operation. Controls are hooked up to the automatic pilot unit located beneath the floor. Dials and instruments for setting and regulating the automatic pilot are on the center of the flight instrument board within easy reach of either pilot.

Engine control units are centered overhead, comfortably reached by either pilot and the mechanic. It affords either unit control for all engines, or selective control for each engine.

Next to this unit are the engine fire extinguisher controls. A twist of a dial directs extinguishing gas to one or all engines.

Reached from the outside by a hatch large enough to accommodate large packages as well as permitting for passenger exit, and located between the pilot’s compartment and passenger cabins is the front baggage compartment. The allowable baggage space is 157 cubic feet. Here are also stored two life rafts and various tools. A strong box for valuables is located under the floor.

At the main passenger stairway in the rear are located additional baggage compartments with a total capacity of 95 cubic feet. Large packages find ready access through the main passenger entrance that measures 25 inches by 72 inches.

Between the front and rear baggage compartments are located the passenger cabins and the lavatories.

The four passenger cabins, measuring 76 inches by 110 inches each, seat eight passengers. More than sufficient space is allowed for wide aisles and comfortable leg room. The distance from the floor to the ceiling is well over six feet.

Tubular racks suspended from the bulkheads support the seats. This construction eliminates chair legs, thus permitting an unobstructed space beneath each seat for luggage.

Seats are adjustable to meet individual comfort. High head rests afford complete relaxation. Accessories, including- carpets, removable tables, magazine racks, and curtains, harmonize with the decorative scheme.

Sound and vibration have been the subjects of careful study. Thick pads of soundproofing material fill in the space behind the walls and ceiling. Because of their vibration, springs have been replaced in cushions by a special material that is proving more serviceable and comfortable. Windows are held in place by a clincher type rubber ring without the need of fasteners that would cause shattering. In any passenger cabin conversation may be carried on in a normal tone.

A concealed ventilating system supplies upwards of 30 cubic feet of air per passenger per minute. Auxiliary to this is an efficient exhaust system.

Installation fittings on walls, ceiling, floors, and seats are such that an entire passenger cabin can be stripped to its bare structure within 40 to 50 minutes and reinstalled in a similar period.

Each cabin is equipped with safety belts and lifebelts are distributed throughout the boat; in convenient locations are fire and emergency equipment. The two -life rafts in the front baggag6 compartment and the two located rearward of the main stairway are, suitable for, carrying a full capacity crew and passenger list.

The superstructure, already mentioned, also serves as a passage-way for items of controls that pass from hull to wing. Entrance is afforded to this structure from the inside of the cabin as well as from the outside. The internal space of this structure is such as to allow a man to work with comfort.

Every external part of the hull may be reached by a center ridge walk-way that extends from bow to stern. This walk-way, together with the wing walkways and the engine platforms, makes it possible to conduct almost any inspection or servicing operation, without the need of outside scaffolding. The economic feature of this is obvious.

Pontoons, hinged from the wing on two streamline struts and braced by cross wires, follow similar construction design as the hull.

Easy handling from sea to land, or vice versa, is offered by the beaching gear. This is a three-part unit consisting of two twin-wheels-on-a-strut carriages that attach to special fittings on the front of the hull and a single tail wheel that fits into a socket at the stern of the hull. All carriages may be turned on their axes so that motion is possible in any direction. Seven to ten minutes is required to attach or detach the entire gear.

Even more important than the structural characteristics of the S-42 is its performance. Reports on performances were handed in by three of the outstanding pilots in the aviation world-Captain Boris Sergievsky, holder of numerous world records and chief test pilot for the Sikorsky Aviation Corporation; Mr. Edwin Musick, chief pilot for the Pan-American Airways, and an airman of outstanding experience and ability; and the final stamp of approval by Colonel Charles A. Lindbergh. Thirty-two flights on an average of over two hours each filled in the five months’ period of performance testing.

The final accepted report shows the performance as:

• Gross weight ……………….38,000lbs.
• Full speed at 5,oooft. altitude …….188 m.p.h.
• Full speed at sea level …….180 M.P. 11.
• Speed at 75 per cent. b.h.p. at sea level …….160 m.p.h.
• Speed at 70 per cent. b.h.p. at 1 2, 000ft ..altitude …….170 m.p.h.
• Climb, initial, four engines …….1,000 f.p.m.
• Climb, initial, three engines ……..400 f.p.m.
• Ceiling, service, three engines …….7,500ft.
• Full speed at sea level, three engines ……..157 m.p.h.
• Speed at 75 per cent. b.h.p., sea level, three engines….I35 m.p.h.
• Stalling speed ………65 m.p.h.
• Time of take-off, dead cal………………………………..25-30 sec.
• Ceiling, service, four engines ………16,000 ft.
• Range at cruising speed, sea level ………1,200 miles
• Range with pay load of i,5oolbs. (cruising speed
• I45 m.p.h. at 6,oooft. altitude) ………3,000 miles
• During the test flights the following world records for Class C-2 seaplanes

were established :-

For a distance of 1,000 km. (621.4 mi-)-,

• Speed 253.7 km./hr (157-7 mi./hr.).
• Speed with 500 kg. (1,102.3lbs.) 253.7 km./hr (157.7 mi./hr.).
• Speed with 1,000 kg. (2,204.6lbs.) 253.7 km./hr (157.7 mi./hr.).
• Speed with 2,000 kg. (4,409.2lbs.) 253.7 km./hr (157.7 mi./hr.).

For a distance Of 2,000 km. (1,248.8 mi.).

• Speed ..253.4 km./hr. (157.5 mi./hr.).
• Speed with 500 kg. (1,1O2-3lbs.) 253.4 km./hr. (157.5 mi./hr.).
• Speed with 1,000 kg. (2,204.6lbs.) ..253.4 km-/hr. (157.5 mi./hr.).
• Speed with 2,000 kg. (4,409.2lbs.) ..253.4 km./hr. (157.5 mi./hr.).
• Greatest load to 2,000 M. (6,561.6ft.) 7,533 kg. (16,608lbs.).
• Altitude with 5,000 kg. (11,023lbs.) 6,203.6 M. (20,406.7ft.).

Operating Efficiency

In no way, however, has high performance reduced the practical purposes of this boat. the weight ratio of useful load to gross weight is very satisfactory. With a weight empty of 19,764lbs. and a licensed gross weight of 38,000lbs.. a useful load of 18,236lbs. is obtained. This ratio is then in the nature of 48:52, or the useful load equals 48 per cent. of the gross weight.

The weight allocations for a gross weight of 38,000lbs. and a range of 1,200 miles fit commercial requirements. The fuel and oil for this range weigh 7,995lbs. ; equipment weighs 2, 181lbs., and pay load equals 7,060lbs. Should consideration be given to using the S-42 for fast freight only, practically all the 2,181lbs. of equipment, plus the chairs, wall and ceiling trim and sound-proofing, can be dispensed with, giving a pay load of approximately 10,000lbs. over a range Of 1,200 miles. For a range Of 3,000 miles, cruising at I45 miles per hour and at 6,000 feet altitude, the allowable pay load will be 1,500lbs. Thus the S-42, keeping well within the safe limitations of structural strength, capable of maintaining a high ceiling with only three motors, and at the same time carrying a reasonable pay load, is entirely suitable for the establishment of trans-oceanic routes.

The following table indicates the benefits of the S-42 as compared to the S-40:-

S-40 S-42

Weight empty 2 1000lbs. 19,764lbs.

Gross weight 34,000lbs. 38,000lbs.

Equipment 1,000lbs. 2,181lbs.

Crew 1,000lbs. 1,000lbs.

Gas and oil (1,000 miles) 7,800lbs. 6,692lbs.

Pay load 3,200lbs. 8,363lbs.

Cruising speed (1,000ft.),m. p. h. 115 157

High speed, m.p.h. 137 182

Horse-power 2,300 2,800

Fuel consumption (cruising, per hr.) i4o gals. i62 gals.

Landing speed, m.p.h. 65 65

Range, miles 1,000 1,000

Time 8-7 hrs. 6-37 hrs.

Wing loading 19.5 28.58

Here is an indicated increase in the S-42 Of 5,163lbs. pay load over the S-40.

If equal pay loads are considered, that is, 7,500lbs., the range for the S-40 is 479 miles and in the S-42 is 1,130 miles, an increase of 651 miles.

The extensive test made with the S-42 made it possible to make some interesting comparative measurements. With the engines of the S-42 throttled down to 575 b.h.p. to conform with the b.h.p. of the S-40’s Hornet ” B ” engines, the speed obtained was 163 m.p.h. as against 137 m.p.h. obtained from the S-40.

The improvement in efficiency of the S-42 is better exemplified if a study is made of the cruising speed of the S-42 against the S-40, using equal horsepower in each case. Using 432 b.h.p. per motor, the S-40 cruises at 115 m.p.h., while with the same power and an increased gross weight of 4,000lbs. the S-42 cruises at 145 m.p.h., an increase of 30 m.p.h.

From an economic viewpoint, that is, comparing load carried against fuel consumed, an important deviation is found. Each plane using the same horsepower and the fuel consumption per hour of the S-42 being 144 gallons, and that of the S-40 140 gallons, the following ton-mile gallon is given:-

Payload in Tons

Miles per gallon (1,000 Miles) Ton mile per gallon.

S-40 0.82 3300/2000=1.65 1.35

S-42 1.0 8505/2000=4.25 4.25

In view of the fact that operating and maintenance costs are based on flying hours, consideration is here given to that item. Equal in size, the S-40 is again taken for our analysis. The unit of maintenance and operating costs per hour being considered equal, we find that for each flying hour the S-40 is credited with (1.65 tons x 115 mi.) 169.75 ton-miles, and the S-42 (4.25 tons x 145 mi.) 616.25 ton-miles.

It is quite clear that, in this consideration of pay load ton-miles that operation and maintenance cost vs. load carried would be decreased in the same proportion. In reality, the decrease will be substantially greater because of the refinement for fast and simplified inspection, servicing, and maintenance incorporated by the Sikorsky engineers in conjunction with the staff of the Pan American Airways.

General Considerations

Because of the general cleanliness of the S-42, due to the careful study of the aerodynamic interference of parts, the total parasite resistance at cruising speed of 160 m.p.h. is only 3,620lbs. At the time of designing, careful consideration was given to the cantilever wing, but it was felt that the increase in profile resistance, due to the greater thickness of the center section of the wing, would be greater than the drag of external struts. Research also revealed that the structural weight of the cantilever wing would be greater than the present wing and struts.

The table attached gives the curves of the original estimated performance compared with data measured during test flights. It should be noted that actual performance is in general accord with the figures estimated by the more liberal simplified method in which the profile drag of the wing and the resistance of engines are taken as having a constant value subject to the square of speed. It is believed that the slight excess of actual performance over the original estimate is due partly to greater scale speed effect other than the one that was considered, and partly due to the smooth surface created by flush riveting. It is evident that the ship with full load is able to maintain flight easily, using much less than half the power available. It is shown further that the best LID ratio of the whole aeroplane is above thirteen. The data on which the table is based were recorded during test flights with calibrated intake manifold pressure gauges mounted on all engines. The reading of these instruments, plus careful observation of the r.p.m., permitted the accurate establishment of the power used at various conditions of speed and load.

After a careful study of the conditions under which this ship would operate, it was decided to have a high wing loading. The ship was designed primarily for high cruising speed and operation over long trans-oceanic routes. A service of this character offers no intermediate landing possibilities, and in view of the distances and duration of flight, the ship must be able to withstand varying weather conditions. Good airworthiness in stormy weather was considered most essential. A simple aerodynamic study shows that the action of a squall or of a vertical air gust becomes more violent as the wing loading decreases. Therefore, a heavy wing loading of 28-5lbs., combined with ample h.p. per square foot of 2.1 b.h.p., was found desirable.

Flight tests confirmed this decision. The S-42 flies easily and smoothly in the roughest of weather. The comparatively small and rigid wing has the added advantage of safely weathering strong wind and heavy squalls while afloat, particularly with the flap in neutral position. Added approval of this consideration is given by nature. It is interesting to note that large birds that fly over the sea, having long distances to traverse before being able to alight in case of stormy weather, have a much heavier wing loading than birds of similar size that fly over land.

The disadvantages of heavy wing loading, namely, difficult take-off and fast landing, were avoided by the use of the specially designed flap. After a very careful wind tunnel study of several types of auxiliary surfaces, preference was given to the straight flap that now fills up the rear of the wing between the ailerons.

This flap produces an increase in lift of about 40 per cent. When placed at a 4o degree angle, the flap affords an actual stalling and landing speed of 65 m.p.h.

Test showed that the best position for take-off is with the flap set at + 10° to + 12°. Carrying a definite test load, the ship with the flap in neutral position took 12 seconds for take-off. Landing immediately, the flap was set at + 10° and the take-off time was reduced from 12 seconds to 7 seconds, indicating that the flap decreased take-off time by nearly 40 per cent.