Category Archives: Rotorcraft

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.

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
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.

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
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.

U.S. Army Aircraft Since 1947 by Stephen Harding (1997)
R-2800: Pratt & Whitney’s Dependable Masterpiece by Graham White (2001)