Aviation Week At 100: How The Helicopter Has Developed
Getting from idea to reality took far longer for the helicopter than for the fixed-wing aircraft
Efforts to develop what became the helicopter began long before those that led to the airplane and continued well after the Wright brothers achieved powered, controlled and sustained flight of a fixed-wing aircraft. It would take another three decades to arrive at a practical helicopter, but once developed, the idea was quickly embraced, entering military and commercial service within 10 years.
The helicopter, with its ability to take off and land in small spaces, and hover and maneuver at low speed, has become indispensable. But its pioneers might be disappointed with how narrowly the helicopter is used because of speed and range limitations and safety and noise concerns. Much research has been expended over decades on these issues, but true progress has been recent.
VERTICAL PIONEERING
Europe was the hotbed of rotorcraft invention through the first decades of the 20th century. France in 1907 saw not one but two landmark flights: In September, the Gyroplane No. I, a quadrotor built by brothers Jacques and Louis Breguet, lifted its pilot 2 ft. into the air for a minute. But it was unstable and uncontrollable and steadied by four men on the ground, so it is considered the first manned but tethered flight. What is considered the first manned free flight, albeit brief, came in November, when Paul Cornu’s tandem-rotor helicopter lifted its pilot to 1 ft. for 20 sec.
Pioneering flights followed thick and fast, not all of them in Europe. In the U.S., Russian refugee George de Bothezat built an experimental quadrotor for the Army. The H1 first flew in December 1922, and went on to carry passengers, but it was complex and hard to control and was scrapped. Also in 1922 Emile Berliner, a German immigrant and inventor of the gramophone record, modified a Nieuport biplane with twin wing-mounted rotors that could tilt slightly to control yaw. This aircraft could hover and fly forward, but had poor roll control and performance.
But the breakthroughs continued in Europe. Working in France, Argentine marquis Raul Pateras Pescara developed a coaxial-rotor helicopter, its twin rotors each having four biplane blades with wing warping for cyclic control. The Model 2 hovered in 1922, and the improved Model 3 set a distance record of 738 meters (2,400 ft.) in 1924. Frenchman Etienne Oehmichen’s helicopter No. 2 had four rotors for lift, six propellers for stability and control and two for propulsion. First flown in 1922, this aircraft completed the first 1-km (0.6-mi.) closed-circuit helicopter flight in 1924.
COMPETING CONFIGURATIONS
Twenty years after the Wright brothers had first flown their Flyer II in a complete circle, demonstrating their mastery of stability and control, the helicopter was still far from a practical machine. World War I had accelerated development of the airplane; by 1924, airmail and airline services were established and the surviving Douglas World Cruisers had circumnavigated the globe.
Helicopter pioneers were still trying to find a successful configuration with sufficient lift for vertical flight on the power available, with the stability and control needed for prolonged flight. The Breguet Gyroplane had four rotors, each with four biplane blades, Cornu used tandem rotors, Berliner transverse, and Pescara coaxial biplane rotors.
In 1925, Dutchman Albert Gillis Von Baumhauer designed a helicopter with a single main rotor, two slender aerodynamically effective blades, collective and cyclic control via a swashplate and an anti-torque tail rotor powered by its own engine. It flew numerous times, but was destroyed in 1930 when a blade separated due to fatigue failure of a hinge bolt. Lacking funds, it was not rebuilt.
Italian aeronautical engineer Corradino D’Ascanio in 1930 built the D’AT3, a coaxial-rotor helicopter that used three small propellers for roll, pitch and yaw control. Rotor control was by servo tabs on the blades-—a technique used later by Charles Kaman. In the lean years after World War II, D’Ascanio went on to design the Vespa scooter for Piaggio before designing helicopters for Agusta.
Another early attempt that approached practicality was the 1-EA built by the Soviet Union’s TsAGI aerohydrodynamic institute. Flown in 1932, this had a single four-blade main rotor and pairs of small propellers on both nose and tail for anti-torque control. The 1-EA had two rotary engines and good performance, setting an unofficial altitude record in 1932, but poor control.
Soviet-Georgia-born engineer Nicolas Florine, working in Belgium, designed the first successful tandem-rotor helicopter—a configuration later perfected by Frank Piasecki. In Florine’s design, both rotors turned in the same direction, and torque reaction was countered by tilting the rotor disks in opposite directions. Florine’s second helicopter flew in 1933 and set modest flight endurance and altitude records.
AUTOGYRO’S BRIEF REIGN
By the mid-1930s, the practical helicopter was tantalizingly close. But another type of rotorcraft had already in been in service for almost a decade—the autogyro. This used air flowing through an unpowered rotor to turn the blades and generate lift in forward flight—a phenomenon called autorotation that had first been demonstrated by Pescara.
The autogyro could take off and land in a short distance, but not hover. Nevertheless its development pioneered many of the breakthroughs that ultimately led to the practical helicopter. The autogyro was invented by Spanish engineer Juan de la Cierva, whose fourth design, the C.4, was the first to fly, in 1923. The improved C.6, based on an Avro 504K fuselage, followed in 1924, and its success led to formation of Cierva Autogiro Co. in the U.K. to develop and produce the machines.
His breakthrough was development of the flapping blade hinge. This overcame a fundamental problem with rotors in forward flight: The blade on the advancing side sees rotor speed plus airspeed and generates more lift, while the blade on the retreating sides sees rotor speed minus airspeed and generates less lift.
In early designs, this dissymmetry of lift caused an unbalanced rolling moment as soon as the rotorcraft started flying forward. Cierva added a hinge at the rotor hub that allowed the advancing blade to flap upward, reducing its angle of attack and lift, while the retreating blade flapped downward, increasing its lift. The flapping hinge balanced the rotor, and was a key to enabling practical helicopters.
In 1929, Harold Pitcairn acquired the rights to Cierva’s designs and the Pitcairn-Cierva Autogiro Co.—later the Autogiro Co. of America—began developing and producing aircraft in the U.S. Kellett Autogiro Corp. acquired a license from Pitcairn and autogyro production accelerated, laying the foundation for today’s rotorcraft industry. Cierva was killed in an airliner crash in 1936, the year the first practical helicopter flew, but roughly 500 autogyros were produced through 1945, when the helicopter’s arrival essentially ended the autogyro’s reign.
A NEW BREED
Although the ability to spin up the rotor on the ground to enable a “jump” takeoff was added to later autogyros, notably Austrian Raoul Hafner’s AR.III, their inability to hover doomed them once a workable helicopter emerged. And by the mid-1930s most of the pieces were in place. Cierva had developed the articulated rotor. Hafner, in the AR.III flown in 1935, introduced the modern swashplate—a device that converted control inputs in a fixed frame into blade pitch commands in a rotating frame.
The coaxial-rotor Gyroplane Laboratoire designed by Louis Breguet and flown in June 1935 was perhaps first, but was quickly overshadowed when German engineer Heinrich Focke’s F.61 made its first flight in June 1936. With its stability, control and performance, the transverse twin-rotor F.61 is considered the first functional helicopter. The aircraft made its first autorotation landing in 1937, a capability Focke—and all subsequent designers—saw as a crucial safety feature of the helicopter.
Focke-Achgelis & Co. went on to produce the larger transverse-rotor Fa.223 Drache, but credit for the first series-production helicopter was to go to Germany’s Anton Flettner in 1942 with the Fl.282 Kolibri. This had an entirely different configuration, dubbed the synchropter, with intermeshing rotors, first flown in 1939 with the Fl.265 and later used by both Kellett and Kaman.
While Germany had taken a lead, the progenitor of the modern helicopter is recognized as being Igor Sikorsky’s VS-300. In the early 1900s, Sikorsky had experimented unsuccessfully with a coaxial-rotor helicopter but abandoned this to design large fixed-wing aircraft, first in Russia then in the U.S. after emigrating in 1919. The VS-300 first flew untethered in May 1940, but in December 1941 took to the air in its final configuration: single main rotor with cyclic control and a single tail rotor. The modern helicopter was born.
Developments came thick and fast. The VS-300 led quickly to the Sikorsky R-4, the first helicopter produced in significant numbers (131 were built) and the first to enter service with the U.S. military, in 1942. In 1943, Arthur Young flew a two-bladed rotor with a stabilizer bar that acted as a mechanical gyroscope, partially controlling the rotor and stabilizing the helicopter. This led to the Bell 47, which in 1946 became the first helicopter to receive civil certification.
BIRTH OF AN INDUSTRY
The second helicopter to fly in the U.S. was Piasecki’s single-main-rotor PV-2, in 1943, but he achieved fame developing the tandem-rotor transport helicopter, beginning with the HRP Rescuer first flown in 1945 and leading to the Boeing CH-47 Chinook, still in production 55 years later. Kaman flew his intermeshing-rotor K-125, ancestor of today’s K-MAX, in 1947. Stanley Hiller developed a control system that made his helicopters easy to fly, leading in 1948 to the UH-12.
The industry spread rapidly. In the Soviet Union, Nikolai Kamov’s first helicopter, the Ka-8, first flew in 1947, introducing the design bureau’s signature coaxial-rotor configuration. But the first helicopter to enter series production there was Mikhail Mil’s single-main-rotor Mi-1, first flown in 1948. Today the two design bureaux are part ofRussian Helicopters.
In postwar Europe, French state aircraft manufacturers began helicopter development, SNCASO flying the SO1100 Ariel I in 1949, its rotor powered by compressed-air tipjets. This led to France’s first production helicopter, the army’s SO1121 Djinn. SNCASE, meanwhile, flew the SE3110 in 1950, leading to the Alouette family. The companies were merged to form first Sud Aviation, then Aerospatiale, which joined with Germany’s MBB in 1992 to create Eurocopter, now Airbus Helicopters.
In the U.K., Cierva and Hafner started a postwar rotorcraft industry that grew to encompass Bristol, Fairey, Saunders-Roe and Westland, but which was consolidated in 1960 around Westland, by then building Sikorsky helicopters under license. In 2000, Westland merged with Italy’s Agusta, which had entered the industry in 1952 by building Bell helicopters under license. They formed AgustaWestland, now Leonardo-Finmeccanica’s helicopters division.
BIGGER, BETTER . . .
Helicopters developed rapidly in capability after World War II, helped by the turbine engine with its higher power-to-weight ratio than the piston engine. The first turbine-powered helicopter to fly, in 1951, was Kaman’s XHTK-1, with a Boeing 502 turboshaft. A modified HTK-1 became the first twin-turbine helicopter in 1954. (The HTK-1 was also the first to fly on electric power and unmanned.)
But the first turbine-powered helicopter to enter production was Sud Aviation’s SE3130 Alouette II, powered by a Turbomeca Artouste. First flown in 1955, the helicopter quickly smashed the altitude record and accomplished two noted high-altitude rescues while still in development. The first turbine helicopter to enter production in the U.S. was the Bell 204, better known as the UH-1 “Huey.”
As performance improved, so did size. First flown in 1961, the tandem-rotor CH-47A Chinook had a maximum takeoff weight of 33,000 lb. and a payload of 10,000 lb. Today’s CH-47F weighs 50,000 lb. with a 24,000-lb. payload. Sikorsky’s single-rotor CH-53A weighed in at 35,000 lb. but in 1968 set a record by lifting off at 51,900 lb., including a 28,500-lb. payload, a record—for a non-Soviet helicopter, that is.
The largest helicopter ever built, Mil’s transverse-rotor V-12, flew in 1968. With a maximum takeoff weight of 231,484 lb. and payload of 88,000 lb., the four-turbine helicopter had the same cargo-compartment dimensions as the Antonov An-22 airlifter so that it could rapidly deploy strategic ballistic missiles. The V-12 was never produced, so the modest-by-comparison Mil Mi-26 became the largest helicopter to enter series manufacture. First flown in 1977, the Mi-26 weighs 123,450 lb. with a 44,000-lb. payload.
. . . BUT NOT FASTER
While most aspects of performance have improved dramatically over the 75 years since Sikorsky ushered in the modern helicopter, one has not changed much: speed. Fast for a conventional helicopter is 160 kt. Because of the retreating-blade stalling and the advancing-blade going supersonic, rotors do not like flying edgewise at higher speeds.
But there have been repeated attempts in that time to make helicopters faster, and some of them are making a comeback. One of these is to compound the helicopter by adding a wing for lift and propulsion for thrust. This offloads the rotor of the need to produce both lift and thrust to go faster, and delays retreating-blade stall to higher speeds.
The first attempts involved brute force. The Bell 533 High Performance Helicopter was an early Huey prototype fitted with a wing and two jet engines. Ultimately propelled by a pair of 3,300-lb.-thrust Pratt & Whitney J60s mounted on the tips of a stub wing, the 533 reached 274.6 kt. in 1962. Built to test a rigid, hingeless rotor designed for high speed, Lockheed’s XH-51 reached 263 kt. the same year, fitted with a wing and single 2,500-lb.-thrust J60.
The XH-51 led to the U.S. Army’s Lockheed AH-56 Cheyenne attack helicopter, which first flew in 1967 and reached 212 kt. on a wing and the power of a single 3,925-shpGeneral Electric T64 turboshaft driving both the rigid rotor and a pusher propeller on the tail. The Cheyenne was canceled and replaced by the conventional, 150-kt.-plus Hughes (later McDonnell Douglas and now Boeing) AH-64 Apache.
TRYING AGAIN
Airbus resurrected the compound helicopter when it flew its experimental X3 to 263 kt. in 2013. A refined version is to fly in 2019 under the European Clean Sky research program’s LifeRCraft technology demonstration. Airbus’s hybrid-helicopter interpretation of the compound has twin turboshafts driving both the rotor and, via shafts running through the wing, variable-pitch propellers at the tips. LifeRCraft is expected to lead to a production high-speed helicopter in the mid-2020s.
Two other avenues of high-speed rotorcraft research have borne fruit. One traces back to the Sikorsky XH-59A Advancing Blade Concept. This reached 263 kt. in 1973. It was fast, but noisy, shaky and complex to fly, needing two pilots to control four engines: two turboshafts driving the coaxial rigid rotors and two turbojets for propulsion.
In 2008, Sikorsky revisited the concept, applying the latest in blade and airframe structures, and flight and vibration control to cure the XH-51’s vices and unlock its potential to combine high speed with the helicopter’s low-speed agility. The small X2 technology demonstrator reached 260 kt. in 2010. Sikorsky is now flight testing the 240-kt. S-97 Raider light tactical helicopter and, with Boeing, building the 250-kt. SB-1 Defiant medium-lift demonstrator for the U.S. Army, to fly in 2017.
The other avenue was the tiltrotor. First to fly was the Transcendental 1-G, in 1954, but it never made a complete transition to wingborne flight. Instead, the Bell XV-3, flown in 1955, made the first conversions between helicopter and airplane mode. This led to the highly successful Bell XV-15, first flown in 1977, which reached a speed of 300 kt. and paved the way for the 270-kt. Bell Boeing V-22 Osprey, which in 2007 became the first tiltrotor to enter service.
Development of a civil tiltrotor was begun by Bell in 1998 and ultimately taken over by partner Agusta in 2011. After development delays and difficulties, the nine-passenger, 275-kt. AW609 is now planned to be certificated in early 2018. Bell, meanwhile, is building its next tiltrotor, the 280-kt. V-280 Valor medium-lift demonstrator for the U.S. Army, which is planned to fly in 2017.
The service has not decided whether to buy a high-speed rotorcraft-—compound, tiltrotor or some other configuration—but for the first time since Sikorsky flew the VS-300 there is a real prospect that the helicopter’s gains in performance and sophistication over the past 75 years will at last be joined by a long-sought increase in speed and range.
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