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H-19 Chickasaw / HO4S / HRS

The year 1949 was to become a banner year for Sikorsky helicopters. In early 1949, Sikorsky Aircraft was at a crossroad with its single main rotor helicopters. The company's four-place S-51 (naval HO3S-1) was the most widely used, but competitions for larger helicopters were being won by tandem-rotor designs that didn't require balancing fore and aft loading under a single rotor. Along with general improvements, such as all-metal, untapered rotor blades, Sikorsky engineers explored rotor hub design and other improvements, such as hydraulically powered flight controls. They also investigated other rotor configurations.

The benefit of direct military and commercial use of its two- and four-place helos led Sikorsky to initiate a larger follow-on design, carrying 10 passengers or equivalent cargo and a crew of two. The Company had developed a follow-on to the S-51, the S-55, by rearranging and reconfiguring existing components. The Sikorsky commercial Model S-55 helicopter was built by Sikorsky without the aid of government funds and first flown in November 1949. It had been designed originally to compete against Piasecki's PD-22 (H-21) for use as the Air Force's Arctic Rescue helicopter.

This re-arrangement included placing of the engine forward of the center section and relocating the cockpit above it. This allowed for a roomy passenger area. Placing the engine in the nose of the HRS-1, where it was easily accessible, was ingenious. It not only had tremendous advantages in servicing the aircraft, but it completely eliminated the critical center of gravity problems previously encountered by placing the payload directly below the rotor hub. A drive shaft transmitted engine power to the three-bladed main rotor through the center rear section of the cockpit. The cabin, which had foldable seats for 10 passengers, was situated directly beneath the main rotor.

With the payload cabin directly under the rotor and transmission, fuel tanks under the cabin, the hub and flight control improvements and the engine moved to the fuselage nose, the new design attracted a great deal of operator interest since the principal drawbacks of the single-rotor design were largely overcome.

Compactness of the new design was aided by tilting the rearward-mounted engine, with an upward-slanted extension shaft running between the raised pilot's cabin and the payload cabin to the transmission directly above. Two clamshell doors, opening to provide engine access, formed a bulbous nose with air intakes to the engine-driven cooling fan extending around the upper fuselage ahead of the wind-shield, and an exit opening under the nose. The result looked something like a box with four wheels underneath and a tail boom extending aft from the top.

Chopping off the tail boom with a blade was an all-too-frequent occurrence. Most often this type of accident happens as the helicopter makes a hard vertical touchdown or a run-on landing following autorotation. There were two contributing factors in these accidents. The blades keep on coming down even after the fuselage has stopped, and the sudden nosedown motion following the contact of aft-mounted wheels or the back of skids makes the pilot naturally want to pull the stick back to counteract it. Designing the landing gear to have a long energy-absorbing stroke may alleviate these landing problems. To counteract these tail-boom strikes, designers have sometimes been forced to redesign the tail boom. An example of such a redesign was the Sikorsky S-55 (H-19), which originally had the tail boom coming straight out of the main fuselage. After a series of accidents, it was redesigned with the tail boom angled down 3 degreese.

The subsequent impact of the S-55 from both commercial and military perspectives was unforseen at the time of its initial introduction. This unique design marked the dawn of a new era for a single-main-rotor lifting helicopter. Here was a helicopter that was reliable, versatile and adaptable to diverse requirements. This resulted in a requirement for a total of 1,700 units of which 1,281 were produced in the United States.

The Sikorsky model S-55, initially procured in November 1949, was the world's first transport helicopter. During their early years, the naval designation of the H-19 (redesignated UH-19 in 1962) was HO4S and the US marines variants of the were the HRS-1, HRS-2 and HRS-3 before the normalization to the H-19 family. The 12-place craft, with a crew of two, served as a utility, troop carrier, and rescue helicopter with winch. Model numbers ranged from A thru D. In the MedEvac role it could carry six litters and one medical attendant. For the first time casualties could be carried and attended to under cover, within the aircraft.

H-19 Chickasaw

The Air Force ordered the first five S-55s to be built as YH-19s for evaluation. Rolled out and first flown in November 1949 when jet fighters, bombers and high-speed research aircraft were getting all the aviation interest, the clamshell nose doors did attract some attention. Flight test changes provided a faired-in triangular section aft of the cabin and a small inverted V tail below the tail rotor, smoothing out the boxy appearance.

A mock-up of this reconfigured model was proposed to the United States Air Force (USAF) in Dayton Ohio and they were most enthusiastic. An arrangement was made to provide, in place of the last model of the S-51 series, half the contracted number of S-55's for the same price. In due course the S-55 model was evaluated as a litter carrier for evacuation of wounded US Army soldiers. These trials were completed in 1949-50 and the helicopters were used extensively during the Korean War.

The H-19, successor to the H-5, was a larger and more powerful aircraft than its predecessor. The H-19 also had a longer range, four-wheeled landing gear, better controls, and easier handling. A unique feature of this helicopter was that 17 percent of its total airframe weight consisted of magnesium sheet and castings, the greatest percentage of any operational aircraft then in service.

The development of the H-19 as a replacement for the H-5 expanded the helicopter's promise by providing a more capable and longer-range platform, dramatically demonstrated by the transatlantic crossing of two H-19s in July 1952.

The Sikorsky H-19 Chickasaw was sent to Korea for service tests in March 1951. Its capabilities surpassed the H-5. In March 1951, a YH-19 and its crew arrived in Korea for an air evacuation service test. For five weeks, they participated in a variety of tests to evaluate its ability in tactical control missions, downed aircrew rescue, medical evacuation, cargo support of Army front lines, and clandestine operations. The helicopter also helped to recover vital components of a MiG-15 that had crashed behind enemy lines. The final conclusions were positive. The H-19 was an improvement over the H-5 in rescue and evacuation because of its greater load capability, longer range, and its increased flexibility in rugged terrain and poor weather conditions. However, the H-19's slow speed and excessive vibration and buffeting at low altitudes resulted in the conclusion that it was not suitable for tactical control missions.

Two companies of H-19 Chickasaw helicopters proved the value of helicopter transport by moving cargo and personnel during the final months of the Korean war and then by participating in prisoner exchanges and other functions after the cessation of hostilities. Routine support engineering for the H-19 involved minor problems with windshield wipers, defrosters, brake and rudder controls, and radio equipment. Other work on the H-19 in 1952 included a new adhesive used in the fabrication of blades and changes in the tail cone due to marginal clearance between the assembly and the main rotor blades during landing. The H-19 performed satisfactorily in Korea, prompting both the Air Force and Navy to consider further development of the basic design.

In addition, the British firm, WESTLAND AIRCRAFT/HELICOPTER Ltd, built 364 of these venerable aircraft under license from SIKORSKY and named them the WS-55 WHIRLWIND. Also the Japanese conglomerate, MITSUBISHI INDUSTRIES built 44 and the French firm SNCA du Sud-Est a further quantity. The French called the S-55 the "Joyeux Elephant". At least 40 countries used military variants of the S-55 and it was equally ubiquitous in both military and civil roles.

The UH-19 had a three-bladed main rotor and a metal two-bladed tail rotor. Models A thru C were powered by a single Pratt & Whitney R-1340-57 550 hp engine. Model D was powered by a Curtis-Wright R-1300-3D 700hp piston engine. The Chickasaw had a speed of 113 mph (98 knots). The UH-19D had a cruising speed of 75 mph (65 knots).

HRS US Marine Corps

Beginning in early 1950, S-55 events moved rapidly with expanded antisubmarine warfare (ASW) and Marine combat troop transport applications. The transport version of the H04S-1 was redesignated as the HRS-1. Its design features included one Pratt and Whitney R-1340-57 600-horsepower engine installed behind clam-shell doors in front of the helicopter.

The HRS's empty weight was 4,462 pounds with its gross weight originally predicted and listed as 8,070 pounds. The maximum forward airspeed was 90 knots. Other features included all metal main and tail rotor blades, instruments suitable for night VFR flight, and an external cargo sling and hook situated underneath the fuselage. The aircraft stood 14 feet high, had a rotor blade diameter of 53 feet, and measured approximately 42 feet long with its blades folded.

It was built upon similar, but enlarged, mechanical components of the H03S-1. In appearance it was entirely different although it retained the typical Sikorsky single main rotor design.

Before any were delivered, North Korea invaded South Korea in June 1950, initiating the Korean War and a big buildup for US military assistance. In August transports were ordered as HRS-1s. Since Sikorsky had not received the contract for the Arctic rescue model, the company could commence production immediately on a first-come, first-serve basis with delivery of the first aircraft in six months. Piasecki, on the other hand, also had the capability of building an assault version of its H-21, the PD-22C, although delivery could not be made until approximately September 1951, a difference of eight months. Time was of the essence to the Marine Corps, however, and the most readily available model was chosen, the HRS-1.

HO4S-1 deliveries were completed in early 1951, as HRS-1s began to reach the Marines-first with Marine Helicopter Squadron 1 and then the first squadron to deploy to Korea, Marine Helicopter Transport Squadron (HMR) 161. A total of 59 Marine HRS-1s were completed in late 1951. All versions could have a hoist installed above the single sliding cabin door on the starboard side for rescue and other lift.

When HMR-161 entered the Korean conflict in August 1951, the Marines demonstrated the HRS's effectiveness in combat operations. The use of sling loads for cargo and outsized combat equipment proved to be a particular asset. Sixty-eight upgraded Marine HRS-2s were delivered through 1952.

While the HRS-3s were late for Korean combat, they served many roles in post-armistice Korean operations. Subsequent Marine peacetime operations included staging helicopter assault exercises from carriers, leading to dedicated helicopter assault carriers. The HRS-3s played a major role in these operations, and when the redesignated escort carrier Thetis Bay (CVE 90) was recommissioned as CVHA (assault helicopter aircraft carrier) 1 in 1956, the HRSs fulfilled their last major role as the ship's first helicopter complement.

Difficulties in helicopter carrier operations, such as manual blade folding, led to phasing out the HRSs starting in 1957. By the time the various converted and newly built assault helicopter carriers went to sea with LPH (amphibious assault ship) designations, the complement was mostly later HUSs.

HO4S US Navy

The H04S-1 was the Navy's designation for the S-55. In May 1950, after the Air Force had chosen the H-21, the Navy purchased 10 S-55s for use in an evaluation project to determine its value as an interim ASW helicopter. Requiring enough aircraft for a high-priority operational investigation of helicopter ASW using dipping sonar, 10 were ordered as HO4S-1s. The HO4S-3s that could carry and drop one torpedo were assigned to early Navy ASW helicopter antisubmarine and helicopter utility squadrons. Power limitations of the 600 hp Pratt & Whitney R-1340 Wasp engine were evident in all uses. Engine replacement by a new Wright 800 hp R-1300 Cyclone 7 resulted in the -3s, and 144 were built for the Navy, Coast Guard and Marine Corps in 1953 and 1954. Some -2s were similarly converted to -3s.

HSS-1s replaced HO4S-3s on CVSs (ASW support aircraft carriers). Three HO4S-3s were among Air Development Squadron 6's aircraft in Antarctica for Operation Deep Freeze, 1955-1956.

Both HRS-3s and HO4S-3s continued in utility and air station rescue roles into the 1960s; HO4S-3s were also Naval Air Training Command helicopter pilot trainers. To comply with common military designations in 1962, HRSs still in service became CH-19Es; HO4S-3s UH-19Fs; and Coast Guard HO3S-3Gs HH-19Gs-joining their Air Force and Army counterparts under the original Air Force H-19 designation series.

UH-19Fs were phased out of training in 1963, and CH-19Es completed rescue and utility duties in 1969. The "Horses" (the unofficial Navy nickname) became memories in Naval Aviation, but other military and civilian versions, some converted with turbine engines, would continue lifting for many years.

HO4S-1G and 2G - Coast Guard

Beginning in 1951, the Coast Guard acquired a number of the HO4S-1G and 2G models for use in search and rescue operations, building upon the experiences gained through the use of the earlier Sikorsky helicopters. Seven HO4S-1s for the Coast Guard were completed in late 1951. Coast Guard H04S-1s were modified as -2Gs. The 1G and 2G models were powered by a 550 horsepower Pratt & Whitney R-1340. The service later acquired 23 of the 3G models which were powered by a 700 horsepower Wright R-1300. All were fitted with a rescue hoist.

Late in 1951, the U.S. Coast Guard acquired a new type helicopter, the Sikorsky HO4S-2G, for search and rescue duty. Seven of the planes are to be put in operation in various Coast Guard Districts throughout the country. Cruising speed is 80 knots and top speed, 115 knots, ceiling 16,000 feet and range, 400 miles. Equipped for instrument and night flying, the helicopter included such equipment as a hydraulic hoist to pick up personnel or equipment up to 400 pounds in weight. Plans were also drawn up for the installation of a collapsible rubber float attached to the landing gear, which can be inflated automatically by the pilot. The helicopters were painted bright yellow, and are the largest helicopters operating in the Coast Guard.

The Coast Guard continued to develop and refine their search and rescue techniques and helicopter-borne equipment, including rescue hoists, slings, and baskets, with the HO4S. They also conducted some rather unique experiments with them. In an experiment known as Project Tugbird, the Coast Guard tested the practicality of towing vessels using a helicopter. The tests, which began at AIRSTA St. Petersburg in August of 1957, were carried out through January of the following year. HO4S helicopters based at AIRSTA St. Petersburg practiced towing a number of vessels, the largest being the 794-ton buoy tender Juniper. The tests were successful and on 1 July 1958 the Coast Guard directed that every airstation have at least one HO4S helicopter "permanently equipped with necessary apparatus ready for towing operations." The towing line was a steel cable, one-quarter of an inch in thickness, and had a pulling strength of 6,900 pounds. The cable was attached to the helicopter on a padeye installed on the bottom rear section of the fuselage. The helicopters were capable of towing a vessel of up to 800 tons for a total of 1 1/2 hours, which was the duration of their fuel supply, for a distance of up to 20 miles.

HUS-1Gs replaced the Coast Guard's HO4S-3Gs. HH-19Gs left Coast Guard rescue in 1966.

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Page last modified: 12-02-2014 19:05:11 ZULU