Category Archives: Rotorcraft

Dorand Gyroplane G.20 (G.II)

By William Pearce

Since the early 1900s, Frenchman Louis Bréguet was interested in rotorcraft. But, the technical challenges of controlling such machines and the lack of suitable power plants led Bréguet to pursue the development of aircraft instead. In the late 1920s, Bréguet’s interest returned to rotorcraft, and he created the Syndicat d’Etudes de Gyroplane (Syndicate for Gyroplane Studies) in 1931 with René Dorand as its Technical Director. The syndicate produced a successful experimental helicopter known as the Bréguet-Dorand Gyroplane Laboratoire, which first flew on 26 June 1935. The Gyroplane Laboratoire used two sets of two-blade, coaxial, contra-rotating rotors. No tail rotor was used, as the contra-rotating rotors cancelled out the torque reaction of the blades. The helicopter set a number of speed and distance records.

Dorand Gyroplane G20

A drawing of the Dorand Gyroplane G.20 in what appears to be its final form. The drawing illustrates one of the two inverted Renault inline-six engines and the two-person cockpit.

In 1938, Dorand amicably parted with Bréguet and established the Société Française du Gyroplane (French Gyroplane Company), abbreviated SFG or just Gyroplane for short. The French Navy (Marine Nationale) commissioned the SFG to design a combat helicopter for costal defense and anti-submarine warfare. Dorand designed the new machine, and its layout was similar to that of the Gyroplane Laboratoire. The new helicopter was designated the Gyroplane G.20, but it is also known as the Dorand G.20 or the Dorand G.II.

The G.20 had a cigar-shaped fuselage of all metal construction. A butterfly tail was attached to the extreme end of the fuselage, and the tail’s control surfaces were fabric-covered. The streamlined nose of the G.20 was covered with plexiglass panels. The pilot sat in the nose of the helicopter with either one or two crewmen behind.

Dorand Gyroplane G20 org drw 2

An early drawing of the Dorand G.20 showing what appears to be a three-man crew. Note the turret on the rotor mast and the side-mounted machine gun (pointed toward the rear).

At the center of the helicopter were two three-blade, coaxial, contra-rotating rotors. The upper rotor had a diameter of 50 ft 6 in (15.4 m) while the lower rotor’s diameter was around 6 ft 7 in (2.0 m) smaller, at 43 ft 11 in (13.4 m). The magnesium blades were made of two parts: a box forming the leading edge and a separate trailing edge. As with the Gyroplane Laboratoire, articulation of the blades allowed for both cyclic and collective pitch control, and no tail rotor was used.

The rotor blades were powered by two Renault 6Q-04 engines. The 6Q was an air-cooled, inverted, inline, six-cylinder engine with a 4.72 in (120 mm) bore and a 5.51 in (140 mm) stroke. The engine’s total displacement was 580 cu in (9.5 L). The 6Q-04 was supercharged and produced 240 hp (179 kW) at 2,500 rpm up to 13,123 ft (4,000 m). A special gearbox transferred power from the engines to the rotors. If one of the engines were to fail, that engine would be automatically disconnected, and the remaining engine would power both sets of rotors.

Dorand Gyroplane G20 org drw

This top view drawing of the G.20 clearly shows the side-mounted machine gun and engine placement. The outline of bombs can be seen under the rotor mast.

The G.20 was supported by two main wheels and a tail wheel. The main and tail wheels all retracted backward into the fuselage and were fully enclosed by gear doors. The space in the fuselage between the main gear and below the rotors was for either bombs or a depth charge. In addition, Dorand’s original design included a machine gun mounted on the helicopter’s side and a turret mounted on top of the rotor mast—with the guns operated by separate crewmen. However, all armament was omitted from the G.20, and most sources list a two-person crew consisting of a pilot and copilot. The G.20’s role was defined as observation, liaison, and mail-carrying.

The G.20’s fuselage had a length of 36 ft 4 in (11.08 m) and a height of 10 ft 3 in (3.13 m). The helicopter’s empty weight was 3,086 lb (1,400 kg); normal operating weight was 5,512 lb (2,500 kg), and maximum weight was 6,614 lb (3,000 kg). The G.20’s hover ceiling was 9,843 ft (3,000 m), and it had a maximum ceiling of 16,404 ft (5,000 m). The helicopter’s range was 497 mi (800 km). Its cruise speed was 103 mph (165 km/h), and its maximum speed was 155 mph (250 km/h).

Dorand Gyroplane G20 complete

The completed Dorand G.20 after World War II. With the machine guns no longer part of the design, nothing is left to interrupt the helicopter’s sleek lines. Note the long gear door.

Construction of the G.20 started in Guethary in south-western France, near Spain. When the German Army invaded France in 1940, the helicopter was moved to Chambéry in south-eastern France, near Italy, and construction resumed. By this time, Marcel Vuillerme had taken over the project from Dorand. As the Germans pushed into southern France, the G.20 was discovered. The Germans showed little interest in the helicopter and allowed its construction to continue, albeit slowly.

The G.20 was completed after World War II and underwent ground tests. It was the French officials who now showed little interest in the project, and funding was not forthcoming. Its estimated performance was optimistic, and while its streamlined appearance and retractable gear appeared futuristic, in many ways the G.20 was obsolete after war-time helicopter developments made in the United States and Germany. Further development and testing of the G.20 was abandoned, and the helicopter never flew. However, the SFG continued to develop helicopters for a time. The SFG worked with Bréguet to construct a four-passenger helicopter, the G.11E, which first flew in 1949. The G.111 was a follow-on project that first flew in 1951. The SFG went out of business in 1952.

Breguet G11E

The G.11E was designed by SFG after the G.20. It was built by Bréguet and powered by a 9-cylinder Potez 9E radial engine. It first flew in 1949 and had a layout similar to the G.20.

Sources:
Les Avions Breguet 1940/1971 by Jean Cuny and Pierre Leyvastre (1973)
http://en.wikipedia.org/wiki/Breguet-Dorand_Gyroplane_Laboratoire
http://en.wikipedia.org/wiki/Breguet_G.11E

Brennan Helicopter

By William Pearce

Born in 1852, British inventor Louis Philip Brennan had contemplated the concept of a flying machine (helicopter) as early as 1884. However, the art and science of aeronautics was very much in its infancy, and there was much to be learned. Even if the knowledge had existed, there were no suitable power plants light enough and powerful enough to propel man aloft in any machine, much less in a helicopter.

Brennan Helicopter early

The Brennan Helicopter with its inventor Louis Brennan standing before it and engineer/pilot Robert Graham in the pilot’s seat. This early tethered test photo shows the machine before its fuselage was completed.

In the mid-1910s, technology began to catch up with Brennan’s idea for a helicopter, and the British Ministry of Munitions backed Brennan’s experimental helicopter project on 29 June 1916. Under strict secrecy, design work progressed slowly, and it was not until 1919 that construction actually began at the Royal Aircraft Establishment at Farnborough. The helicopter project was encouraged and supported by Winston Churchill, who transitioned from Minister of Munitions to Minister of Air at that time. Engineer Robert Graham joined Brennan’s team in 1920.

The helicopter was constructed with a small fuselage (or pilot’s car) supported on the ground by four outrigger legs. The fuselage was equipped with a small rudder, and the fuel tank was under the pilot’s seat. Above the fuselage were the rotor blades and engine attached to a pyramid-shaped, steel supporting frame. The two wide chord rotor blades themselves were not powered; their rotation was achieved by a small four-blade propeller positioned at the tip of each rotor blade. This configuration provided virtually torqueless rotation of the rotor blades. The blades had a steel and wood frame and were covered in doped fabric. Each rotor blade was supported by a hollow tube (spar) through which the drive shaft ran to the tip propeller. The pilot could articulate the hollow tubes to change the incidence of the rotor blades. Each blade also had an aileron to further enhance the helicopter’s control.

Brennan Helicopter Patent

A drawing from Brennan’s “Improvements relating to Aerial Navigation” patent submitted in 1916. Note the small propellers (marked “n”) that were to be used for control. These propellers and their complicated drive systems were omitted from the actual helicopter.

The controls were pneumatic, and the base cross tubes of the pyramid structure were enlarged to act as air reservoirs. Pressurized air was provided by an engine-driven compressor. The entire pyramid structure rotated when the helicopter was under power. The structure was attached to the fuselage via a universal joint. This joint was hollow, and flight controls and fuel lines passed through it.

The tip propellers were geared to the engine via shafts and right angle gear boxes. Originally, a 150 hp (112 kW) Bentley BR 1 rotary engine was used, but a 230 hp (172 kW) Bentley BR 2 rotary engine was substituted because more power was needed. The nine-cylinder BR 2 engine had a 5.5 in (140 mm) bore and a 7.1 in (180 mm) stroke, resulting in a total displacement of 1,522 cu in (24.9 L). The rotary engine was positioned horizontally and revolved the same direction as the rotor blades (but faster). The helicopter blades had a 61 ft (18.6 m) diameter and a 6 ft (1.8 m) cord and rotated between 50 and 60 rpm. The total blade area was 240 sq ft (22.3 sq m). Brennan’s helicopter weighed 2,765 lb (1,254 kg).

Brennan Helicopter Tethered

The Brennan Helicopter tethered in a hangar at Farnborough. Brennan is standing next to the machine with Graham occupying the cockpit. The fuselage has been completed and its rudder is visible. Note the cross braces of the pyramidal structure that acted as air reservoirs and the aileron on the rotor. The axillary rotors have not been added.

An attempt was made to begin Engine-powered spin tests of the rotor blades on 4 November 1921. The helicopter did not have its own starter at the time. A Hucks starter was used, engaging one of the tip propellers. The rotor blades were braced to prevent their movement. However, once the engine was started, the brace moved and was hit by a propeller. All testing stopped until the damage could be repaired.

By 7 December, Brennan had designed and fitted his helicopter with a mechanical starter. The helicopter was rigged for tethered tests inside a balloon hangar at Farnborough. Brennan decided that since Graham had complete knowledge of the machine, he should be the one to fly it. The first tethered flight occurred on 22 December. Although the Air Ministry assigned pilots Paul “George” Bulman and Cecil Bouchier to the project, Graham made about 90% of the helicopter flights.

Brennan Helicopter

Brennan’s helicopter ready for outside tests at Farnborough. One of the square-shaped auxiliary rotors can be seen in the middle of the photo.

The tethered tests showed the helicopter to be underpowered, and it was at this time that the BR 2 engine was installed. With this engine, the helicopter could lift the pilot, four men, and an hour’s worth of fuel. However, the 60 ft (18.2 m) tall hangar provided a limited test environment, and the helicopter’s ascents did not top 20 ft (6.1 m). Stability tests, while still in the hangar, showed that directional control was not sufficient, and use of the ailerons hindered the helicopter’s control compared to blade articulation alone. A solution to the control deficiencies was to add additional rotor blades. Ideally, two full blades would have been added, but for simplicity, cost, weight, and speed of conversion, two small auxiliary blades were added. The auxiliary blades increased the lifting surface area by 60 sq ft (5.6 sq m), bringing the total to 300 sq ft (27.9 sq m).

On 16 May 1924, the helicopter was tested out of the hangar for the first time. Initially, the helicopter was tethered, but free-flight tests commenced once confidence in the helicopter was gained. Since space was limited, the helicopter’s height was kept under 10 ft (3 m). A number of flights were made, some of which were flown by Brennan. Complete control was still a concern, and Brennan had designed a gyro to automate minor control adjustments and stabilize the helicopter. However, the Air Ministry wanted proof that unassisted control was achievable before any type of automatic control was added. It was estimated the helicopter could achieve a forward speed of 20 mph (32 km/h) and an altitude of 600 ft (183 m).

Brennan Helicopter flight

The helicopter in low flight and perhaps lacking a little control. For all the flights, it was never flown higher than 10 ft (3 m).

On 2 October 1925, the helicopter was being demonstrated before a number of officials when partial control was lost. Close to landing, the pyramid structure of the helicopter tilted and the blades struck the ground. While the blades were damaged, the propellers and gear boxes at their tips were destroyed. Although a definitive cause was not found, it was believed that contaminants had been introduced into the air system and caused a valve to malfunction.

At this time, some in the Air Ministry felt there was no future for the helicopter and were not interested in continuing with Brennan’s experiment. Also, Juan de la Cierva’s Autogiro was showing some promise without all the complications of the helicopter. Subsequently, the decision was made to terminate the Brennan Helicopter project and investigate the Autogiro. Brennan was notified of the decision on 29 January 1926. He felt that cancelling the work on his helicopter was a mistake and that the Autogiro would never do everything a helicopter could. Brennan predicted helicopters would eventually provide unparalleled service to the world.

Even though most components were undamaged, Brennan’s helicopter was never repaired. Louis Brennan died in 1932, about five years before other pioneers began to demonstrate the unique abilities of the helicopter. The Brennan Helicopter made over 70 free flights, and it was the first helicopter to fly in the United Kingdom.

Brennan Helicopter flight front

A good photo of the Brennan Helicopter in steady flight. Note that the aileron is at a slightly different angle than that of the rotor blade.

Sources:
“Brennan—his helicopter and other inventions” by Robert Graham The Aeronautical Journal (February 1973)
Louis Brennan Inventor Extraordinaire by Norman Tomlinson (1980)
W O Bentley Rotary Aero Engines by Dr. Tom Dine (2014)
“Improvements relating to Aerial Navigation” G.B. patent 281,735 by Louis Brennan (granted 10 December 1918)

Petróczy-Kármán-Žurovec PKZ 2 Helicopter

By William Pearce

In 1916, Major Stephan Petróczy von Petrócz of the Austro-Hungarian Army envisioned replacing hydrogen-filled observation balloons with tethered helicopters. These helicopters would have been used as static observation platforms. Compared to hydrogen balloons, the helicopters’ were much less likely to catch fire, presented a smaller target for the enemy, increased operational readiness, required fewer ground and support crew, and eliminated the need for hydrogen generating equipment.

Follow-on to the PKZ 1, the Petróczy-Kármán-Žurovec PKZ 2 is shown here with the observation basket attached above the rotors. This image was taken after the PKZ 2 was modified in May 1918 and the 120 hp (89 kW) La Rhône engines are installed.

To achieve his goal, Petróczy, along with Oberleutnant Dr. Theodor von Kármán and Ingenieurleutnant Wilhelm Žurovec, conceived the Schraubenfesselflieger (S.F.F) mit Elektromotor (captive helicopter with electric motor). This machine is now commonly refered to as the Petróczy-Kármán-Žurovec 1 (PKZ 1) helicopter. Built in 1917 and primarily designed by von Kármán and Žurovec, the PKZ 1 consisted of a rectangular frame with an observation basket in the middle. On each side of the basket were two lift rotors. All four rotors were powered by a single 190 hp (142 kW) Austro-Daimler electric motor.

The PKZ 1 was flight tested and was able to lift three men to a tethered height of 20 in (50 cm).  However, the electric motor generated 50 hp (37 kW) less than anticipated, and on the fourth flight, the straining motor gave out. Because of the scarcity of high-grade electrical copper and quality insulation, Daimler was not able to repair the motor. In addition, the PKZ 2, which was originally known as the S.F.F. mit Benzinmotor (captive helicopter with petrol engine), was nearing completion. No further work was done on the PKZ 1.

PKZ 2 rotary engine arrangement with the 100 hp (75 kW) Gnomes installed.

The PKZ 2 helicopter (for which he received German patent 347,578) was designed solely by Wilhelm Žurovec. The PKZ 2 was privately funded by the Hungarian Bank and the iron foundry / steel fabrication firm of Dr. Lipták & Co AG, who built the machine. The PKZ 2 utilized two coaxial contra-rotating sets of two-blade propellers to cancel out torque and provide lift. The rotors, made of high-quality mahogany, were 19 ft 8 in (6.0 m) in diameter and were rotated at 600 rpm by three 100 hp (75 kW) Gnome rotary engines. A removable observation basket sat atop the rotors.

The craft had three outrigger legs; each supported one engine. All engines were connected to the rotors via a common gear box. The PKZ 2 was supported by a central air cushion and three additional air cushions; one on each outrigger leg. These air cushions were filled by an air pump driven from the rotor drive. Attached to each outrigger was a tethering cable that was connected to the ground and controlled by an electric winch. With one hour of fuel, The PKZ 2 weighed 2,645 lb (1,200 kg).

PKZ 2 shown just off the ground and without the observation basket on April 5, 1918, powered by the 100 hp (75 kW) Gnome engines.

Tethered and unmanned, the PKZ 2 was test flown on April 2, 1918. After several flights, including one that lasted about an hour, tests were suspended on April 5th because of insufficient power from the Gnome engines. The engines were replaced by 120 hp (89 kW) La Rhône (captured and rebuilt) engines and, with a few additional modifications, tethered and unmanned flight tests resumed on May 17th. With the new engines and calm winds, an altitude of 165 ft (50 m) was achieved, and the PKZ 2 could lift 330–440lb (150–200 kg). The craft would lose lift at higher altitudes, but the PKZ 2 was kept under control as long as tension remained on the tethering cables.

PKZ 2 in a tethered high hover with power provided by the 120 hp (89 kW) La Rhône engines on June 10, 1918.

On June 10, 1918 the PKZ 2 was demonstrated for high ranking members of the military. A flight was made with the observation basket in place, but the engines were not running well and the craft became unstable. The basket was removed and another flight attempted. The wind had picked up, and as the PKZ 2 hovered at 40 ft (12 m) tethered to the ground, the craft began to rock. The overheating engines lost power, and the tether winch crew could no longer maintain control. The PKZ 2 crashed from a height of 6.5 ft (2.0 m), severely damaging the airframe and completely destroying the rotors.

Realizing the technical problems could not be overcome quickly, the government cancelled the project on June 21, 1918. However, Žurovec pressed on and began to design an individual cylinder water jacket to water-cool the rotary engines. The craft was being rebuilt to resume flight tests in November 1918 when the end of the war and revolution caused all development to cease. The PKZ 2 made over 15 tests flights, but it is doubtful any were manned.

Remains of the PKZ 2 after it crashed on June 10, 1918.

Sources:
Austro-Hungarian Army Aircraft of World War One by Grosz, Haddow, and Schiemer (2002)
Recent Developments in European Helicopters NACA Technical Note No. 47 by von Kármán (1921) pdf
http://forumeerstewereldoorlog.nl/wiki/index.php/PKZ_2_Schraubenflieger
http://www.valka.cz/clanek_13499.html
Comment by Kees Kort

Papin-Rouilly Gyroptere (Gyropter)

By William Pearce

The Papin-Rouilly Gyroptere as depicted on the cover of the September 1922 edition of Popular Science.

The Gyroptere was designed in France from 1911-1914 by Alphonse Papin and Didier Rouilly. Their monocopter was based on the sycamore seed; a single blade extends from the seed to spin the seed and slow its decent as it falls. Though unsuccessful, the machine was the first air-jet helicopter. Papin and Rouilly obtained French patents 440,593 and 440,594 for their invention and later obtained U.S. patent 1,133,660 in 1915 (filed in 1912).

Construction of Papin and Rouilly’s Gyroptere began in February 1914 and was completed in June of the same year. The prototype was named Chrysalis (Chrysalide). Constructed of molded wood, the Gyroptere was well built with compound curves and a smooth sweep of its single, long, airfoil-shaped blade. The fabric-covered blade was hollow and approximately 19.5 ft long (5.9 m) and 4.4 ft wide (1.33 m), giving it an area of 130 sq ft (12 sq m). The blade was counterbalanced by an 80 hp (60 kW), nine-cylinder Le Rhone rotary engine. The pilot occupied a nacelle between the blade and engine. The bottom of the nacelle included a structure to support the machine while it was on the ground or act as a float when on water.

The Le Rhone engine was started with a pulley system. The engine, turning at 1,200 rpm, drove a fan that produced an output of just over 250 cu ft (7 cu m) of air per second. The air, along with the engine’s hot exhaust for thermal expansion, was directed through the length of the blade and exited the blade’s tip through a nozzle on the trailing edge at 330 ft/s (100 m/s). This jet of air would turn the blade, and the gyroscopic force of the motor would lift the blade into a positive angle of attack. The nacelle that carried the pilot was centered on the axis of rotation. The nacelle was mounted on ball-bearings and was centered against four horizontal rollers. The entire machine weighed 1,100 lb (500 kg), which was 220 lb (100 kg) more than originally planned.

This image offers a good view of the Gyroptere. The blade does not have its covering, the float and directional control tube can clearly be seen in the center nacelle, and the Le Rhone engine in its fan housing is on the right.

The pilot controlled the Gyroptere through the use of two foot pedals: one pedal opened a valve to admit air to the blade; and the second pedal allowed air into an L-shaped tube above the craft that served as a rudder for directional control. The L-shaped tube was directed by the pilot; its discharge provided forward thrust, steering, and stabilized the center drum to prevent it from spinning with the blade. A switch in the nacelle allowed the pilot to engaged or disengaged the engine.

This view highlights the air-jet nozzle on the trailing edge of the blade, which can be seen on the left.

The outbreak of World War I delayed testing until 31 March 1915. During tests on Lake Cercey (Reservoir de Cercey), near Pouilly-en-Auxois, France, the craft achieved a rotor speed of only 47 rpm, well below the 60 rpm calculated as necessary for liftoff. Even so, the machine was wildly out of balance, and the blade repeatedly contacted the water, damaging itself and shaking up the pilot. In addition, the Le Rhone engine used was not powerful enough; the Gyroptere had been designed to use a 100 hp (75 kW) engine which could not be obtained.

A military commission observing the test determined that such a machine could not aid the war effort and halted further evaluation. The Gyroptere remained at Lake Cercey until it was sold for scrap in 1919.

The completed Gyroptere awaiting tests on Lake Cercey on 31 March 1915.

Sources:
French Aircraft of the First World War by Davilla and Soltan (1997)
Helicopter, U.S. Patent 1,133,660 by Papin and Rouilly (1915) pdf
http://fr.wikipedia.org/wiki/Gyropt%C3%A8re
http://en.wikipedia.org/wiki/Monocopter
http://flyingmachines.ru/Site2/Crafts/Craft29386.htm
Will This ‘Whirling Leaf’ Flying Machine Solve Greatest Problem in Aviation?Popular Science September 1922.

Sikorsky S-56 (CH-37 Mojave/Deuce)

By William Pearce

At the time of its introduction, the Sikorsky S-56 was the largest and fastest military helicopter in the Western world. In 1956, the helicopter set two height-with-payload world records and one world speed record. The S-56 was also Sikorsky’s first multi-engined helicopter and remains the largest piston-engined helicopter ever built.

CH-37B Mojave in flight. The screen on the side of the engine nacelle to maximize engine cooling is visible.

The S-56 was powered by two Pratt & Whitney R-2800 air-cooled radial engines with a take-off rating of 2,100 hp (1,566 kW) and a cruise rating of 1,900 hp (1,417 kW). R-2800-50 engines were initially used, but late production aircraft used R-2800-54 engines, which had upgraded magnetos and additional mounting studs. The engines were connected to the main rotor’s transmission via a hydro-mechanical clutch. This clutch allowed the engines to run independently from the main rotor for starting or in the event of an engine failure. Power for the tail rotor was taken off the main transmission.

Unlike most heavy lift helicopters, the engines were not located in the upper section of the fuselage. Instead, each engine was housed in a nacelle fixed to the end of a short shoulder-mounted wing on each side of the helicopter. The engine nacelles also accommodated the machine’s fully retractable, twin-wheeled main landing gear.

The S-56’s engine arrangement allowed for an unobstructed cargo bay of nearly 1,500 cubic feet (24.5 cu m)—large enough to carry three Jeeps, 24 stretchers, or up to 26 fully-equipped troops. The cargo bay measured 30 ft 4 in (9.2 m) long, 7 ft 8 in (2.3 m) wide and 6 ft 8 in (2.0 m) high. The S-56’s nose section was equipped with large clam-shell doors which allowed vehicles to be driven straight into the cargo area. The cockpit was placed above and slightly aft of the doors to ensure good visibility. The single main rotor was five-bladed and designed to sustain the aircraft in flight with one blade shot away in combat. For storage, the helicopter’s main rotor blades folded back along the fuselage and the tail rotor mast folded forward.

Head on shot of the CH-37 with the clam-shell doors open and distinctive engine pods visible.

Sikorsky originally developed the Model S-56 in response to a 1950 Marine Corps requirement for a heavy-lift assault transport able to carry 26 fully equipped troops. In 1951 the Navy ordered four XHR2S-1 prototypes for USMC evaluation, and the first of these made its maiden flight on December 18, 1953. In 1954 the Army borrowed one of these pre-production machines. Re-designated YH-37 Mojave, it was subjected to rigorous evaluations before it was returned to the Marines.

Based on the helicopter’s excellent showing during the Marine and Army evaluations, Sikorsky was awarded a contract for 55 HR2S-1 for the Marines, who nicknamed the helicopter Deuce, and 94 H-37A Mojaves for the Army. The H-37A was delivered to the Army during the summer of 1956, at about the same time the HR2S-1 naval variant was entering regular Marine squadron service. The Marines set three world records for helicopters in November 1956 when a HR2S-1 carried an 11,050 lb (5,012 kg) payload to an altitude of 12,000 ft (3,658 m), carried 13,250 lb (6,010 kg) to over 7,000 ft (2,134 m), and set a three kilometer speed record of 162.743 mph (261.910 km/h). All aircraft were delivered to their respective services by May 1960.

In 1961 Sikorsky began converting the Army’s H-37As to B model standards. The upgrade included the installation of automatic flight stabilization systems that gave the H-37B the ability to load and unload while hovering, crash-resistant fuel cells, and modified nose doors. All but four A models were eventually converted. In 1962 military aircraft designations in the United States were unified, and the H-37A were re-designated CH-37A; the modified B machines became CH-37B, and the Marine’s HR2S-1 became CH-37C.

Three CH-37C Deuces on a carrier deck. The hole on the front of the engine nacelles is the air intake for the R-2800 engine.

The S-56 was developed just prior to the widespread adoption of the turbine engine as a helicopter powerplant. As a result, the type was forced to rely on larger, heavier, and less powerful piston engines. Even so, the S-56 ultimately proved to be a more than capable heavy-lifter. In June 1963, four CH-37B Mojaves were temporarily deployed to Vietnam to recover downed U.S. aircraft. By the following December, the Mojaves had recovered an estimated $7.5 million worth of equipment. Most of the recoveries were sling-lifted out of enemy-held territory that was virtually inaccessible by any other means.

In September 1965 a Marine detachment of eight Deuces was sent to Vietnam for general transport duties in support of Marine Air Group 16 at Marble Mountain. Although this detachment had only ten pilots for eight aircraft, they flew about 5,400 hours in 1,500 missions, hauled more than 12.5 million pounds (5.67 million kg) of cargo, and transported some 31,000 passengers without an air accident.

The Mojave did not see more extensive service in Vietnam primarily because of its replacement by the turbine-powered Sikorsky CH-54 Tarhe (S-64 Skycrane), a machine that weighed slightly less than the CH-37 but could carry nearly four times as many troops or five times as much cargo. In addition, the R-2800 engine was very loud, created a lot of vibration, and required much more maintenance than a turbine engine. The last CH-37 was withdrawn from Army service in the late 1960s. The Marine’s Deuces were replaced by CH-53 Sea Stallions in 1967.

When built, no helicopter could surpass the CH-37’s lift capacity.

Sources:
U.S. Army Aircraft Since 1947 by Stephen Harding (1997)
R-2800: Pratt & Whitney’s Dependable Masterpiece by Graham White (2001)
http://en.wikipedia.org/wiki/Sikorsky_CH-37_Mojave
http://www.aviastar.org/helicopters_eng/sik_s-56.php
http://www.popasmoke.com/notam2/showthread.php?1489-CH-37C-Deuce-A-History