Sikorsky S-51 - History and technical description

The S-51 was broadly similar in overall appearance to its predecessor, although it nevertheless had a number of distinctive features. The cabin, first of all, had been completely redesigned to accommodate three passengers instead of one in addition to the pilot. The tricycle landing gear was also modified, with the nose wheel repositioned further forward.
The S-51 prototype, registered NX92800, made its maiden flight on February 16, 1946. After the Bell 47, it was the second helicopter to be certified by the Civil Aeronautics Administration (CAA). It received its provisional type certificate (Provisional Type Certificate H-2) on March 26, 1946, while the definitive type certificate was issued on April 17, 1947.
The first aircraft intended for commercial use was delivered to Helicopter Air Transport (HAT) on July 29, 1946 at the Sikorsky plant in Bridgeport. HAT, founded in Philadelphia in November 1945, operated three S-51s. The aircraft were based at Central Airport in Camden, New Jersey. From August 1946 onwards, they were used for a variety of services, including the transportation of passengers, freight and mail, aerial photography, liaison flights, aerial observation, and various experimental activities such as beach patrols and sea rescue.
The price of a Sikorsky S-51 was approximately $48,500, nearly twice that of the Bell 47B, which sold for $25,000, a considerable sum at the time. Operating costs were also high. For a civilian operator, therefore, keeping the helicopter in service represented a substantial financial commitment.

The aircraft underwent a wide range of tests, including airmail flights, aerial crop-spraying trials, and experiments carried out in cooperation with the Dutch Navy.

Production under licence

The British company Westland Aircraft of Yeovil, which had already experimented with the construction of several autogiro models in the 1930s, signed an agreement with Sikorsky on January 10, 1947 to manufacture the S-51 under licence.
On April 17, 1947, the S-51 received British certification. The first licence-built example (G-AKTW, WA/H/1), designated Westland-Sikorsky WS-51 Mk. 1A Dragonfly, made its maiden flight at Yeovil on October 7, 1948, with test pilot Alan Bristow at the controls.
The helicopter was not, however, an exact copy of the American model. Westland Aircraft replaced the original 335/450 kW/hp Pratt & Whitney Wasp Junior engine with a 374/500 kW/hp Alvis Leonides, while introducing a number of minor modifications to the original design.
In the United Kingdom, British European Airways (BEA) – Helicopter Experimental Unit acquired three Sikorsky S-51s, along with two Bell 47Bs, in 1947 to assess the feasibility of commercial helicopter services. In 1948, the unit conducted trials of scheduled airmail services using the S-51s; BEA subsequently inaugurated the first scheduled passenger helicopter service in June 1950.

Among the early European operators was Pest Control Ltd, which used S-51 and WS-51 helicopters for aerial crop-spraying operations.

In an effort to increase the passenger-carrying capacity of the WS-51 Mk. 1A, the British company developed the Westland Widgeon, which made its maiden flight on August 23, 1955. The helicopter could carry four passengers in addition to the pilot, but achieved only limited commercial success and was built in just 15 examples, three of which were originally WS-51s.
With regard to production figures, the archives consulted indicate that Sikorsky built 294 aircraft of the R-5/S-51 series, while Westland produced 149 WS-51s. Civilian operators used approximately fifty of these aircraft.
Series production in the United States ceased during 1951.
At the end of the Korean War (1950–1953), numerous helicopters of the R-5 series, which had received various military designations while in service with the U.S. armed forces, became surplus military aircraft. Some of these were subsequently offered for sale. Sikorsky Aircraft took advantage of their availability by purchasing a number of examples and subjecting them to a complete overhaul. The company also sold conversion kits and provided instructions for converting military helicopters to meet CAA (FAA) certification requirements.
The helicopters refurbished in this way were subsequently marketed to civilian operators under the S-51 designation, in an attempt to give a new lease of life to an aircraft originally developed for military purposes.
A few examples did find civilian employment, but the rapid evolution of helicopter technology, coupled probably with the dwindling availability of spare parts and specialized personnel, brought the S-51's commercial career to an end during the 1960s.

Final considerations

Like other contemporary helicopters, such as the Bristol 171 Sycamore and the Mil Mi-1 Hare, the Sikorsky S-51 was one of the aircraft that contributed significantly to transforming the helicopter from a mere technological curiosity into a practical means of transport for civilian use.
At a time when rotary-wing aviation was still in its infancy, the S-51 provided an important testing ground for exploring the potential of the helicopter as a working aircraft, and for developing operating procedures and methods of use that would become commonplace only in the years that followed.
Various difficulties nevertheless contributed to limiting the S-51's spread in the civilian market and, more generally, made it difficult for companies operating helicopters commercially to remain viable. It is also against this background that the brief existence of companies such as HAT and Greyhound Air Lines can be understood.
In the civilian sector, operating a helicopter could be economically viable only with some form of government support, or through profits from other activities carried out by the operator that could be used to offset the high operating costs.
Immediately after the war, the armed forces were among the few—or rather, the only organizations—that could readily afford the costs associated with operating helicopters.

The testimony of “Tug” Gustafson

The renowned French test pilot Jean Boulet (1920–2011), author of L’Histoire de l’hélicoptère racontée par ses pionniers, 1907–1956, interviewed many helicopter pioneers and recorded numerous first-hand accounts. Among them was E. E. “Tug” Gustafson (1915–1994), former marketing director of Sikorsky Aircraft’s division at United Technologies, who went on to become a true legend in the field of vertical flight.
Known for his pioneering efforts to promote the use of helicopters in the oil, forestry, construction, agricultural and mineral exploration industries, as well as for heavy-lift operations and military applications, Gustafson left behind many recollections of the early days of rotary-wing aviation.
Gustafson recalled that, at the end of the Second World War, orders from the U.S. armed forces declined sharply. Sikorsky then decided to explore the possibility of entering the civilian market. In this context, Gustafson was sent to Washington to determine the concrete needs of various government agencies, beginning with the Department of Agriculture and the Forest Service, and to assess whether new opportunities existed for the civilian use of helicopters.
“Everywhere I went to talk about this wonderful machine, I found very few people who really knew what it was. They had heard about it, they knew that it could hover and fly at low speeds, but that was about the extent of their knowledge. They would tell me that, if we produced it, they would buy 500 for forestry, 200 for agriculture, and so on. When I returned, I announced that we would sell thousands of civilian helicopters. And that is how Sikorsky decided to launch the development of the S-51.
We had planned to produce 60 aircraft in the first year, at a price of $48,500, and we genuinely believed that we would sell them like hot cakes, because everywhere we went people told us: ‘We will buy them.’ In the first year, however, instead of selling the 60 aircraft on which we had based our selling price, we sold only 11; the following year, 3.
The reason for these poor sales was that we had a helicopter capable of hovering and doing everything a hummingbird could do, but we still did not know how to make practical use of it. At first, we believed that 95% of our problems would be technical in nature and that, once those problems had been solved, selling helicopters would be easy. During the first two years, however, we realized that the real problem was the lack of a market. The first thing to do, therefore, was to go out into the field, discover potential applications for helicopters, and develop them”.
The first experiences of commercial—and military—operations inevitably highlighted both the machine’s potential and its limitations. Performance, range and payload capacity were still limited, while operational use was hampered by a lack of experience. Pilots often found themselves operating in an environment where established procedures were still largely absent, having to acquire in the field skills and knowledge that could not be drawn from any long-standing aviation tradition.

The value of the Sikorsky S-51 should therefore be considered not only in terms of its technical characteristics, but also in relation to the role it played in the development of the industry as a whole. The knowledge gained from its operation helped establish procedures and design criteria that would later be adopted and refined in subsequent generations of helicopters. In this respect, the S-51 was above all a pioneer: it was not yet capable of delivering the performance and reliability that the market would demand in later years, but it made a tangible contribution to creating the technical and operational foundations that would allow the helicopter to establish itself as a tool of commercial aviation.

Technical description

The Sikorsky S-51 is a four-seat helicopter of conventional configuration, belonging to the first generation of helicopters. The fuselage, constructed from welded steel tubing with a metal skin, consists of three main sections: the cabin, the central section housing the engine and main transmission, and the tail boom.
The occupants access the cabin through two sliding doors, one on each side. Because the cabin floor is more than one metre (3 ft) above the ground, the helicopter is fitted with a footstep to facilitate boarding.
The pilot sits centrally at the front and enjoys good visual in all directions. The three passengers sit behind the pilot on a single bench seat, which can be removed when necessary, side by side. The cabin is fitted with large windows and has an interior height of 136 cm (53 inches), a maximum width of 142 cm (56 inches), and an overall length of 254 cm (100 inches).
The helicopter could be equipped with dual controls, but in this configuration the passenger bench had to be removed to make room for the instructor’s seat.
The instrument panel is located directly in front of the pilot.

Originally, the conventional flight controls were not fitted with hydraulic boost, but then it was offered as an optional kit, a feature greatly appreciated by pilots. The S-51 is equipped with three separate hydraulic systems: one for the main rotor dampers, one for the rotor brake, and one for the wheel brakes.
The central section of the fuselage houses the engine and its accessories, together with the main transmission. The S-51 is powered by a nine-cylinder Pratt & Whitney R-985-B4 Wasp Junior radial engine, air-cooled and displacing 16,14 litres. Horizontally mounted, the engine develops a maximum take-off power at sea level of 335 kW (450 hp) at 2,300 rpm (37.0 inHg).
The engine compartment is isolated at the front and rear by metal firewalls.

The transmission drives the three-bladed articulated main rotor. The original main rotor blades were constructed with metal spars, plywood ribs and structural components, with the entire blade covered in two layers of treated fabric. The airfoil section was the NACA 0012. In later production models (H-5H), all-metal blades were introduced; these were also available as a retrofit kit for earlier models.
The new blades were well received by pilots, as they significantly improved the helicopter’s flying qualities.
The helicopter is equipped with two fuel tanks (91-octane fuel). One is located forward of the engine, while the other is positioned behind it. The tanks are separated by firewalls and have a combined capacity of 100 USG (379 litres). The oil tank, with a capacity of 7.92 US gal (30 litres), is located behind the rear fuel tank.
The electrical system comprises a 24 V, 17 Ah battery installed in the tail boom and a 28.5 V, 50 Ah generator. The external power connection is located on the left-hand side of the tail boom. On the same side is the access door to the baggage compartment, which can accommodate a load of up to 145 lbs (65.7 kg), subject to centre-of-gravity limitations.
The monocoque tail boom supports the three-bladed anti-torque tail rotor at its rear. The original tail rotor blades were made of laminated spruce and birch plywood, with a steel leading-edge strip. Some operators replaced this rotor with a new two-bladed, all-metal unit identical to that fitted to the Sikorsky S-55/H-19. The tail rotor was generally not fitted with a protective guard or bumper.
The tricycle landing gear, with a track of 3.66 m (12 ft), consists of two main wheels and a steerable nose wheel capable of rotating through 360 degrees. The helicopter is not fitted with either horizontal or vertical stabilizers.

Performance

Under standard atmospheric conditions, at the maximum authorized take-off weight of 5,300 lb (2,404 kg), the helicopter had a maximum cruising speed at sea level of approximately 73 knots (136 km/h). The maximum authorized speed (Vne) at sea level was 90 knots (165 km/h). Above 3,800 ft (1,158 m), Vne had to be reduced by 3 knots (5 km/h) for every additional 1,000 ft (305 m) of altitude.
At a weight of 5,300 lb (2,404 kg), the initial rate of climb was 1,000 ft/min (305 m/min). Under the same conditions, the helicopter could hover in ground effect at approximately 5,500 ft (1,675 m), and out of ground effect at approximately 3,500 ft (1,066 m). The maximum hover ceiling in forward flight was approximately 12,300 ft (3,750 m).
With an average fuel consumption of 28–30 US gal/h (106–114 litres/h), the S-51 had a range of approximately 212 nautical miles (392 km), corresponding to about 3 hours 20 minutes of flight time.

Dimensions, weights and useful load

The Sikorsky S-51 had a main rotor diameter of 14,63 m (48 ft), with a rotor disc area of 168 m² (1,808 sq ft), while the tail rotor had a diameter of 2,56 m (8 ft 5 in). The fuselage was 12,52 m (41 ft 1 in) long, while the overall height measured to the rotor mast was 3,94 m (12 ft 11 in).
The aircraft had an empty weight of approximately 1’723 kg (3,798 lb), while the maximum take-off weight was 2’404 kg (5,300 lb), initially limited to 2’263 kg (4,989 lb).
Allowing for a standard pilot weight of 77 kg (170 lb) and one-hour reserve of fuel and oil weighing approximately 90 kg (198 lb), the aircraft had a useful load of approximately 516 kg (1,138 lb), or three passengers with their baggage.

According to the flight manual, rotor speed had to be maintained between 168 and 245 rpm. Engine speed in flight had to remain between 2,050 and 2,300 rpm. Cylinder head temperature had to be maintained between 212 and 500 °F (100 and 260 °C).

Optional Equipment

The Sikorsky S-51 could be fitted with a range of optional equipment, including cabin heating and ventilation, rescue equipment, landing lights, interior lighting, external protective equipment, a crop-spraying kit, external stretchers for carrying two casualties, an external hoist, a cargo hook for sling-load operations, and a cabin kit for carrying mail.

Civilian operations

The Sikorsky S-51 was used primarily for passenger and mail transportation, aerial crop spraying, and flight training. It was also frequently employed experimentally to assess the practical capabilities of the helicopter, for example as a search-and-rescue aircraft, aerial crane, observation platform—including observation of vessels and convoys at sea—, and for aerial photography and filming, aerial advertising, and other applications.

The Sikorsky S-51 in Switzerland

During the summer 1948, Air Import of Lucerne informed the Swiss Federal Air Office (Federal Office for Civil Aviation, FOCA) of its intention to present the Sikorsky S-51 in Switzerland. Both civil and military authorities showed considerable interest in the programme organized by the newly established Lucerne-based company, which had planned flight demonstrations in various cities and locations across Switzerland.
On September 22, British pilot Alan Bristow landed at Kloten Airport at the controls of Sikorsky S-51 G-AJHW (c/n 5117, built in 1946), the demonstrator aircraft operated by Westland. The following day, he experimentally carried postal bags on two flights between the Zürcher Sihlpost, departing from the barracks parade ground, and Kloten and Dübendorf airports. The remarkable event was commemorated with a special postmark.

In the afternoon, the aircraft proceeded to Meiringen military airfield. The following day, the S-51 carried out a series of demonstrations on behalf of the hydroelectric power companies. During the flights, Bristow transported equipment in a sling suspended from the center cargo hook, and hoisted people and various items aboard using the winch. Reconnaissance flights were also conducted over power lines. The helicopter then proceeded with a first series of mountain landing trials.

On September 25, the British pilot arrived in Lausanne, where he carried out further demonstrations after landing in the grounds of the Comptoir Suisse.
On September 26, he presented the helicopter at an airshow in Porrentruy and then returned to Meiringen.
The following day, he resumed evaluation flights on behalf of the Swiss Army, with several representatives of the Federal Office for Civil Aviation (FOCA) and the Swiss Alpine Club also taking part. During the morning, the helicopter made several landings in the area of the Krautmättli alpine pasture (1,800 m/5,900 ft) and Lütschentalalp (2,050 m/6,730 ft), with a passenger on board. Later, with Bristow as the sole occupant, the helicopter landed in the vicinity of the Ebenfluh-Axalp shooting range (2,300 m/7,550 ft).

After returning to Meiringen, Alan Bristow once again carried out postal transport flights to Bern.
On September 28, hovering above the roof of Bern's main post office, the helicopter collected mailbags. It could not land because the roof was too fragile to support the aircraft's weight. The following day, similar flights were carried out to and from Biel/Bienne.
The trials concluded on September 30, when the helicopter left Switzerland.

The demonstrations attracted considerable interest, and received extensive coverage from journalists and reporters. These experimental flights were primarily intended to showcase the helicopter’s potential as a means of transport, as well as for search-and-rescue and reconnaissance operations.
Air Import, which had been appointed the official representative for the sale of the aircraft in Switzerland, probably decided against purchasing one for financial reasons. The following year, the company opted instead for the smaller—and above all much less expensive—Hiller 360. 
At the time, the Sikorsky S-51 cost the astronomical sum of CHF 360,000, while an hour of flight time was charged to the customer at approximately CHF 800.

The “Spraycopter” arrives in Valais

At the end of April 1950, the British company Pest Control, which carried out aerial crop-spraying operations using WS-51 helicopters, arrived in Valais to undertake a campaign against an infestation of cockchafers, which were causing serious damage to agriculture. For this purpose, the company used G-ALEI (c/n WA/H/4, built in 1949).
On May 1, the helicopter was damaged during a test flight while landing, making it necessary to replace the tailboom. The tailboom belonging to helicopter G-ALEG (WA/H/2, built in 1949) was shipped from Britain and installed on May 4.
That same day, the aircraft began its pest-control flights, piloted in rotation by Jimmy Harper, Lincoln Lord and Peter Moore. Unfortunately, in the early afternoon, Moore struck a telephone cable and crashed west of the village of Aproz, sustaining serious injuries.
In the accident, the first involving a helicopter in Switzerland, the aircraft was almost completely destroyed.

The Sikorsky S-51 G-AJHW made another appearance in Switzerland in May 1954, on the occasion of the Mustermesse Basel (MUBA). Chartered from Flugzeughandels of Zurich, it was used experimentally by the police as an observation platform.
At the time, the aircraft was also being used for advertising purposes and carried the slogan “Persil bleibt Persil”, meaning “Persil remains Persil”, the famous advertising slogan of the detergent manufacturer Henkel.
On this occasion, the aircraft was flown by John Crewdson. This was most probably the aircraft’s final appearance in Switzerland.

Did you know…

HAT operated a flight school headed by Major Frank T. Cashman, who also served as the company’s chief pilot.
To enroll in the Sikorsky S-51 course, candidates had to have accumulated at least 500 hours of flight time on fixed-wing aircraft and hold a commercial pilot rating.
For the Bell 47B course, candidates were required to have at least 200 hours of fixed-wing flight time and hold either a commercial or private pilot certificate.
The S-51 course cost $2,000 for 20 hours of flight training, while the Bell 47B course cost $1,500 for 25 hours. A combined course consisting of 20 hours on the S-51 and five hours on the Bell 47B cost $2,300.
Thanks to HAT’s flight school, a number of pilots from the United States, Canada and Europe obtained their helicopter pilot licences.

In western Pennsylvania, a HAT S-51 was experimentally used to prevent frost damage. By flying at low altitude over an orchard, the helicopter was able to raise the temperature by several degrees, preventing the formation of ice that could otherwise have damaged the crop. This unusual application was requested by the Fruit Growers Exchange Organization.

In 1948, during his campaign for the United States Senate, Texas Senate candidate Lyndon Johnson, who would later become President of the United States, chartered a Sikorsky S-51 that took him on a tour of 18 towns and cities.