A History of Helicopter Flight
J. Gordon Leishman Professor of Aerospace Engineering, University of Maryland, College Park.
"The idea of a vehicle that could lift itself vertically from the ground and hover motionless in the air was probably born at the same time that man first dreamed of flying."
Igor Ivanovitch Sikorsky
Contents
- The Dream of True Flight
- Key Problems in Attaining Vertical Flight
- Early Thinking
- Engines: A Key Enabling Technology
- The First Hoppers
- Not Quite a Helicopter
- Success at Last
- More Successes - First Production Machines
- Don't Forget Autogiros
- Maturing Technology
- Compounds, Tilt-Wings and Tilt-Rotors
Text on all pages © J. G. Leishman 2000,
with extracts from the author's book "Principles of Helicopter
Aerodynamics" © Cambridge University Press 2000.
Overview
During the past sixty years since their first successful flights,
helicopters have matured from unstable, vibrating contraptions that could
barely lift the pilot off the ground, into sophisticated machines of
extraordinary flying capability. They are able to hover, fly forward, backward
and sideward, and perform other desirable maneuvers. Igor Sikorsky lived long
enough to have the satisfaction of seeing his vision of a flying machine
"that could lift itself vertically from the ground and hover motionless in
the air" come true in many more ways than he could have initially
imagined. At the beginning of the new Millennium, there were in excess of
40,000 helicopters flying worldwide. Its civilian roles encompass air
ambulance, sea and mountain rescue, crop dusting, fire fighting, police
surveillance, corporate services, and oil-rig servicing. Military roles of the
helicopter are extensive, including troop transport, mine-sweeping, battlefield
surveillance, assault and anti-tank missions. In various air-ground and air-sea
rescue operations, the helicopter has saved the lives of over a million people.
Over the last forty years, sustained scientific research and development in
many different aeronautical disciplines has allowed for large increases in
helicopter performance, lifting capability of the main rotor, high speed cruise
efficiencies, and mechanical reliability. Continuous aerodynamic improvements
to the efficiency of the rotor have allowed the helicopter to lift more than
its empty weight and to fly in level flight at speeds in excess of 200 kts (370
km/h; 229 mi/h). Since the 1980s, there has been an accelerating scientific
effort to understand and overcome some of the most difficult technical problems
associated with helicopter flight, particularly in regard to aerodynamic
limitations imposed by the main rotor. The improved design of the helicopter
and the increasing viability of other vertical lift aircraft such as the
tilt-rotor continue to advance as a result of the revolution in computer-aided
design and manufacturing and the advent of new lightweight composite materials.
The helicopter today is a safe, versatile, and reliable aircraft, that plays a
unique role in modern aviation provided by no other aircraft.
The Dream of True Flight
Compared to airplanes, the development of which can be clearly traced to Otto Lilienthal, Samuel Langley, and the first fully controlled flight of a piloted powered aircraft by Orville and Wilbur Wright in 1903, the origins of successful helicopter flight are considerably less clear. A pure helicopter can be defined as any flying machine using rotating wings (i.e., a rotor with blades that spin about a shaft) to provide lift, propulsion, and control forces that enable the aircraft to hover relative to the ground without forward flight speed to generate these forces. In addition, to be practical, the machine must also be able to fly forward, climb, cruise at speed, and then descend and come back into a hover for landing. This is the dream of true flight, a feat only achieved in nature by the hummingbird or dragonfly. Nature has inspired humankind for literally hundreds of years before the vertical flight machine we now know as a helicopter became a practical reality.
The price of an aircraft that could safely and efficiently perform these very demanding flight maneuvers under full control of a pilot is significant mechanical and aerodynamic complexity. While one can draw several parallels in the technical development of the helicopter when compared to fixed-wing aircraft, the longer and more tumultuous gestation of vertical flight aircraft is a result of the greater depth of knowledge required before all the various aerodynamic and mechanical problems could be understood and overcome. Besides the need to understand the basic aerodynamics of vertical flight and improve upon the aerodynamic efficiency of the helicopter, other technical barriers included the need to develop powerplants (engines) with high power-to-weight ratios, as well as high-strength, low-weight materials for the rotor and airframe. As these key technologies have matured during the last seventy years, the helicopter has grown from a vibrating rickety contraption that was barely able to lift its own weight into a modern and efficient aircraft of considerable engineering sophistication.
Key Problems in Attaining Vertical Flight
There are several authoritative sources that document the development of helicopters and other rotating-wing aircraft such as autogiros. These authors include Gregory (1944), Lambermont (1958), Gablehouse (1967), Gunston (1983), Apostolo (1984), Boulet (1984), Lopez & Boyne (1984), Taylor (1984), Everett-Heath (1986), Fay (1987) and Spenser (1999), amongst others. Boulet (1984) has a unique perspective, giving a first-hand account of the early helicopter developments though interviews with the pioneers, constructors, and pilots of the machines. A detailed history of very early, and perhaps even obscure, helicopter developments is given by Liberatore (1950, 1988, 1998). For original publications documenting the earliest technical developments of the autogiro and helicopter, see Warner (1920), von Kármán (1921), Balaban (1923), Moreno-Caracciolo (1923), Klemin (1925), Wimperis (1926) and Seiferth (1927).
As described by Liberatore (1998), the early work on the development of the helicopter can be placed into two categories: inventive and scientific. The former is one where intuition is used in lieu of formal technical training, whereas the latter is one where a trained, systematic approach is used. At the beginning of the twentieth century nearly all prior attempts at vertical flight can be considered as inventive, the inherent aerodynamic and mechanical complexities of building a vertical flight aircraft resisting many ambitious efforts. A contributing factor was the relatively few scientific investigations of flight or studies into the science of aerodynamics -- see Anderson (1997). The history of flight documents literally hundreds of failed helicopter inventions, which either had inadequate installed power or limited control capability, or more often than not, the machine just vibrated itself to pieces. Some of the better designed early machines made brief hops into the air, but control of the aircraft was limited. Yet, the quest for true mastery of the air continued to inspire many inventors and, in time, their work led to sustained technical efforts by trained engineers and, ultimately, to the successful development of the modern helicopter. The technical contributions of Juan de la Cierva, Louis Breguet, Heinrich Focke, Raoul Hafner, Igor Sikorsky and Arthur Young stand out, and their work was instrumental in the design of truly safe and practical helicopters.
Six fundamental technical problems can be
identified that limited early experiments with helicopters. These problems have
been described by Igor Sikorsky (1938) and other sources. In summary, these
problems were:
- Understanding
the basic aerodynamics of vertical flight: The theoretical power required to produce a
fixed amount of lift was an unknown quantity to the earliest
experimenters, who were guided more by intuition than by science. While
basic theories describing the operation of thrusting rotors had been
established by the end of the nineteenth century by William Rankine
(1855), W. Froude (1878) and R. E. Froude (1889), the first significant
application of aerodynamic theory to helicopter rotors came about in the
early 1920s.
- The
lack of a suitable powerplant (engine): This was a problem that was not to be overcome
until the beginning of the twentieth century by the development of
internal combustion (gasoline) powered engines. Yet, it was not until the
mid-1920s that engines with sufficient power and with the high power to
weight ratios suitable for vertical flight became more widely available.
- Minimizing
structural weight and engine weight: Early power
plants were made of cast iron and were relatively heavy. Aluminum, a
common material used on modern aircraft, was not available commercially
until about 1890, but even then was inordinately expensive. Aluminum was
not widely used in aeronautical applications until 1920.
- Counteracting
rotor torque reaction: The
idea of a tail rotor to counter torque reaction and provide directional
control was not used on most early designs. Most early machines were built
with either coaxial or laterally side-by-side rotor configurations. Yet,
building and controlling two rotors was even more difficult that for one
rotor. Igor Sikorsky was the first to successfully use the tail rotor in
the single rotor helicopter configuration we know today.
- Providing
stability and properly controlling the machine: A primary concern was to devise a means of
defeating the unequal lift produced on the blades advancing into and
retreating from the relative wind when in forward flight. These were
problems that were only to be fully overcome with the use of blade
articulation in the form of flapping and lead/lag hinges, ideas that were
pioneered by Cierva, Breguet, and others, and with the development of
blade cyclic pitch control.
- Conquering the problem of high vibrations: Vibration was a source of many mechanical failures of the rotor and airframe, and reflected an insufficient understanding of the dynamic and aerodynamic behavior of rotating-wings.
While all of the factors listed above contributed in some way to the lack of initial progress in achieving successful vertical flight, the development of a practical helicopter had to wait until engine technology could be refined to the point that lightweight engines with considerable power could be built. By 1920, gasoline powered piston engines with higher power-to-weight ratios were more widely available, and the control problems of achieving successful vertical flight were at the forefront. This era is marked by the development of a vast number of prototype helicopters throughout the world. Most of these machines made short hops into the air or flying slowly in ground effect. Many of the early designs were built in Great Britain, France, Germany, Italy, and the United States, who led the field in several technical areas. However, with all the various incremental improvements that had been made to the basic helicopter concept during the pre-World War 2 years, it was not until the late interwar period that significant technical advances were made, and more practical helicopter designs that could lift both a pilot and a substantial payload began to appear.
Early Thinking
The ideas of vertical flight aircraft can be traced back to early Chinese tops, a toy first used about 400 BC. Everett-Heath (1986) and Liberatore (1998) give a detailed history of such devices. The earliest versions of the Chinese top consisted of feathers at the end of a stick, which was rapidly spun between the hands to generate lift and then released into free flight. These toys were probably inspired by observations of the seeds of trees such as the sycamore, whose whirling, autorotating seeds can be seen to carry on the breeze. More than 2,000 years later, about 1754, Mikhail Lomonosov of Russia had developed a small coaxial rotor modeled after the Chinese top but powered by a wound-up spring device. The device flew freely and climbed to a good altitude.
In
1783, the French naturalist Launoy, with the
assistance of Bienvenu, his mechanic, used a
coaxial version of the Chinese top in a model consisting of a counterrotating
set of turkey feathers. This relatively large device was powered by a string wound
around the rotor shaft and tensioned by a crossbow. When the tension was
released, the blades whirled and the device climbed high into the air. Launoy
& Bienvenu's invention created quite a stir in scientific circles. Inspired
by the early success with these and other such whirling tops, the French
mathematician A. J. P. Paucton published in
1786 one of the first scientific papers on the problem of rotating wings
entitled "Theorie de la vis D'Archimede."
Amongst
his many elaborate drawings, the Renaissance visionary Leonardo
da Vinci shows what is a basic human-carrying helicopterlike machine.
His sketch of the "aerial-screw" or
"air gyroscope" device is dated to
1483 but it was first published nearly three centuries later. (Da Vinci's
original drawing is MS 2173 of Manuscript (codex) B, folio 83 verso, in the
collection of the Biblotheque L'Institute de France, Paris.) Da Vinci's idea
was an obvious elaboration of an Archimedes water-screw, but with keen insight
to the problem of flight. His proposed device comprised a helical surface
formed out of iron wire, with linen surfaces made "airtight with
starch." Da Vinci describes that the machine should be "rotated with
speed that said screw bores through the air and climbs high." He obviously
realized that the density of air was much less than that of water, so da Vinci
describes how the device needed to be relatively large to accomplish this feat
-- the number "8" in his backward mirror image script and to the left
of the sketch indicates that the size of the rotor is eight braccia. (A braccia
is an old Florentine unit of measure approximately equal to one arm's length,
which translates into a rotor of roughly 20-feet in diameter.) Da Vinci clearly
did not build his machine, except perhaps for some small models, but his idea
was clearly far ahead of its time. See Hart (1961) or Giacomelli (1930) for
further reading on da Vinci's aeronautical inventions. Although da Vinci worked
on various concepts of engines, turbines, and gears, his sketches did not seem
to unite the ideas of his aerial-screw machine to an engine. Nor did da Vinci
seem to appreciate the concept of torque-reaction -- a well-known problem to
all rotary-wing engineers where a torque applied to the rotor shaft will result
in a reaction torque tending to rotate the platform from which the torque is
applied.
Sir George Cayley is famous for his work on the basic principles of
flight, which dates from the 1790s -- see Pritchard (1961). As a young boy,
Cayley had been fascinated by the Chinese top, and by the end of the eighteenth
century had constructed several successful vertical-flight models with rotors
made of sheets of tin and driven by wound-up clock springs. As a young man, his
fascination with flight led him to design and construct a whirling-arm device
in 1804, which was probably one of the first scientific attempts to study the
aerodynamic forces produced by lifting wings. Cayley (1809-10) published a
three-part paper that was to lay down the foundations of modern aerodynamics --
see Anderson (1997). In a later paper, published in 1843, Cayley gives details
of a relatively large vertical flight aircraft design that he called an "Aerial Carriage." The machine had two
pairs of lateral side-by-side rotors to provide lift, and were pushed forward
by propellers. His idea seemed to be that the disks flattened down in forward
flight, becoming circular wings. However, Cayley's device only remained an idea
because the only powerplants available at the time were steam engines, and
these were much too heavy to allow for successful powered flight.
The lack of a suitable powerplant continued to stifle aeronautical progress, both for fixed and rotating wing applications, but the use of miniature lightweight steam engines met with some success. In the 1840s, another Englishman, Horatio Phillips, constructed a steam-driven vertical flight machine where steam generated by a miniature boiler was ejected out of the blade tips. Although impractical to build at full-scale, Phillips's machine was significant in that it marked the first time that a model helicopter had flown under the power of an engine rather than stored energy devices such as wound-up springs.
In the early 1860s, Ponton
d'Amecourt of France flew a number of small steam-powered helicopter
models. He called his machines helicopteres,
which is a word derived from the Greek adjective "elikoeioas" meaning
spiral or winding, and the noun "pteron" meaning feather or wing --
see Wolf (1974) and Liberatore (1998). However, the novelist Jules Verne was still impressed with d'Amecourt's
attempts, and in 1886 he wrote "The Clipper of the Clouds" where the
hero of the novel cruised around the skies in a giant helicopterlike machine
that was lifted by thirty-seven small coaxial rotors and pulled through the air
by two propellers.
Other notable vertical flight models that were
constructed at about this time include the coaxial design of Bright in 1861 and the twin-rotor steam-driven
model of Dieuaide in 1877. Wilheim von Achenbach of Germany built a single
rotor model in 1874, and he was probably the first to use the idea of a
sideward thrusting tail rotor to counteract the torque reaction from the main
rotor. Later, Achenbach conducted experiments with propellers, the results of
which were published by NACA -- see Achenbach (1923). About 1869 a Russian
helicopter concept was developed by Lodygin,
using a rotor for lift and a propeller for propulsion and control. Around 1878,
Enrico Forlanini of Italy also built another
type of flying steam-driven helicopter model. This model had dual
counterrotating rotors, and it is recorded that it flew freely at heights of
over forty feet for as much as twenty seconds.
In the 1880s, the well-known scientist and inventor Thomas Alva Edison experimented with small helicopter models in the United States. He tested several rotor configurations driven by a gun cotton engine, which was an early form of internal combustion engine. A series of explosions deterred further efforts with these engines. Later, Edison used an electric motor for power, and he was one of the first to realize from his experiments the need for a large diameter rotor with low blade area to give good hovering efficiency. Unlike other inventors and experimenters of the times, Edison's more scientific approach to the vertical flight problem proved that both high aerodynamic efficiency of the rotor and high power from an engine were required if successful flight was to be achieved. In 1910, Edison patented a rather cumbersome looking full-scale helicopter concept with boxkite-like blades, but there is no record that it was ever constructed. Edison, however, was to remain a staunch supporter of helicopter concepts for the rest of his life.
Engines: A Key Enabling Technology
The development of the engine (powerplant) is fundamental to any form of flight. While airplanes could fly with engines of relatively lower power, the success of the helicopter had to wait until aircraft engine technology could be refined to the point that much more powerful and lightweight engines could be built. A look at the historical record shows that the need for engines of sufficient power-to-weight ratio was really a key enabling technology for the success of the helicopter.
To the early pioneers, the power required for successful vertical flight was an unknown quantity and an understanding of the problem proceeded mostly on a trial and error basis. The early rotor systems had extremely poor aerodynamic performance, with efficiencies (figures of merit) of no more than 50%. This is reflected in the engines used in some of the helicopter concepts designed in the early 1900s, which were significantly overpowered and overweight. Prior to 1870, the steam engine was the only powerplant available for use in most mechanical devices. The steam engine is an external combustion engine and, relatively speaking, it is quite a primitive form of powerplant. It requires a separate boiler, combustor, recirculating pump, condenser, power producing piston and cylinder and well as a fuel and an ample supply of water. All of these components would make it very difficult to raise the power to weight/ratio of a steam engine to a level suitable for aeronautical use. Nonetheless, until the internal combustion engine was developed, the performance of steam engines was to be steadily improved upon, being brought to a high level of practicality by the innovations of James Watt.
The state-of-the-art of aeronautical steam engine technology in the mid-nineteenth century is reflected in the works of British engineers Stringfellow and Hensen and also the American, Charles Manly. The Hensen steam engine weighed about 16lb (7.26kg) andproduced about 1hp (0.746kW), giving a power-to-weight ratio of about 0.06, which as about three times that of a traditional steam plant of the era. Fueled by methyl alcohol, this was also a more practical fuel for use in aeronautical applications. However, to save weight the engine lacked a condenser and so ran on a fixed supply of water. With a representative steam consumption of30lb/hp/hr (18.25kg/kW/hr) this was too high for aircraft use. A steam engine of this type was also used by Erico Forlanini of Italy in about 1878 for his experiments with coaxial helicopter rotor models.
In the United States, Charles Manly built a relatively sophisticated five cylinder steam engine for use on Langley's Aerodrome. The cylinders were arranged radially around the crankcase, a form of construction that was later to become a basis for the popular air-cooled radial reciprocating internal combustion aircraft engine. Manly's engine produced about 52hp (36.76kW) and weighed about 151lb (68.5kg), giving a power-to-weight ratio of 0.34hp/lb (0.56kW/kg). The Australian, Lawrence Hargreve, worked on many different engine concepts, including those powered by steam and gasoline. Hargreve was probably the first to devise the concept of a rotary engine, where the cylinders rotated about a fixed crankshaft, another popular design that was later to be used on many different types of aircraft including helicopters
The internal-combustion engine came about in the mid-20th century and was a result of the scientific contributions from many individuals. Realizing the limitations of the steam engine, there was gradual accumulation of knowledge in thermodynamics, mechanics, materials and liquid fuels science. One of the earliest studies of the thermodynamic principles was by Sadi Carnot in 1824 in his famous paper "Reflections on the Motive Power of Heat." In 1862, Alphose Beau de Rochas published the first theory describing the 4-stroke cycle. In 1876, Nikolaus Otto was to use Rochas's theory to design an engine that was to form the basis for the modern gasoline powered reciprocating engine. The development of the internal combustion engine eliminated many parts, simplified the overall powerplant system and for the first time enabled the construction of a compact powerplant of high power/weight ratio.
The earliest gasoline powered aircraft engines were of the air-cooled rotary type. The popular French "Gnome" and "Le Rhone" rotary engines had power-to-weight ratios of 0.35hp/lb (0.576kW/kg) and were probably the most advanced lightweight engines of their time. This type engine was used by many helicopter pioneers of the era, including Igor Sikorsk in his test rig of 1910. The rotary engine suffered from inherent disadvantages, but compared to other types of engines that were available at the time, they were smooth running and sufficiently lightweight to be suitable for aircraft use. The technology to enable vertical flight was now finally at hand.
The First Hoppers
In 1907, about four
years after the Wright brothers' first successful powered flights in fixed-wing
airplanes at Kitty Hawk in the United States, a French bicycle make named Paul Cornu constructed a vertical flight machine
that was reported to have carried a human off the ground for the first time.
Boulet (1984) gives a good account of the work. The airframe was very simple,
with a rotor at each end. Power was supplied to the rotors by a gasoline motor
and belt transmission. Each rotor had two relatively large but low aspect ratio
blades set at the periphery of a large spoked wheel. The rotors rotated in
opposite directions to cancel torque reaction. A primitive means of control was
achieved by placing auxiliary wings in the slipstream below the rotors. The
machine was reported to have made several tethered flights of a few seconds at
low altitude, but this has never been satisfactorily verified. Certainly, the
24-hp engine used in the machine was hardly powerful enough to have sustained
hovering flight out of ground effect.
In 1904
French scientist and academician Charles Richet
built a small, unpiloted helicopter. While the machine was unsuccessful, one of
Richet's students was the future famous aviation pioneer, Louis Breguet. During the latter part of 1906, the
brothers Louis and Jacques Breguet had begun
to conduct helicopter experiments of their own under the guidance of Professor
Richet. The Breguet Brothers were of an affluent famous clock making family,
and were subsequently to become pioneers in French aviation. Louis Breguet made
meticulous tests of airfoil shapes, paralleling those of the Wright Brothers
[see Anderson (1997)], and without a doubt understood the essential aerodynamic
theory of the helicopter. In 1907, the Breguet Brothers built their first
helicopter. Their ungainly quad-rotor Gyroplane No.~1 consisted of four long girders
made of steel tubes and arranged in the form of a horizontal cross. A rotor
consisting of four biplane blades was placed at each of the four corners of the
cross, giving a total of 32 separate lifting surfaces. The pilot sat in the
center of the cross next to a 40-hp engine. The machine is reported to have
carried a pilot off the ground, albeit briefly. Photographs show the assistance
of several men stabilizing and perhaps even lifting the machine. Clearly, the
machine never flew completely freely because, like the Cornu machine, it lacked
stability and a proper means of control. However, the Breguet machine was more
sophisticated and probably closer to achieving proper vertical flight than the
machine built about the same time by Paul Cornu.
In the early 1900s, Igor
Ivanovitch Sikorsky and Boris Yur'ev
independently began to design and build vertical-lift machines in Czarist
Russia. By 1909, inspired by the work of Cornu and other French aviators,
Sikorsky had built a nonpiloted coaxial helicopter prototype. This machine did
not fly because of vibration problems and the lack of a powerful enough engine.
Sikorsky (1938) stated that he had to await "better engines, lighter
materials, and experienced mechanics." His first design, the S-1, was unable to lift its own weight, and the
second machine, the S-2, only made short
(nonpiloted) hops even with a more powerful engine. Discouraged, Sikorsky
abandoned the helicopter idea and devoted his skills to fixed-wing
(conventional airplane) designs at which he was very successful. Although he
never gave up his vision of the helicopter, it was not until the 1930s after he
had emigrated to the United States that he again pursued his ideas of vertical
flight. Good accounts of the life and work of Igor Sikorsky are documented by
Bartlett (1947), Delear (1969), Sikorsky (1964, 1971), Sikorsky & Andrews
(1984), Finne (1987), and Cochrane et al. (1989).
Unbeknown to Igor Sikorsky, Boris
Yur'ev had also tried to build a helicopter in Russia around 1912. This
machine had a very modern looking single rotor and tail rotor configuration.
The large diameter, high aspect ratio blades suggested some knowledge that this
was the configuration for high aerodynamic efficiency. Yet, like Sikorsky's S-1
and S-2, Yur'ev's aircraft lacked a powerful enough engine. Good accounts of
Yur'ev's machine are given by Gablehouse (1967) and Liberatore (1998). The
machine never flew properly, being plagued with mechnical failures. Yet,
besides being one of the first to use a tail rotor design, Yur'ev was another
one of several firsts to propose the concepts of cyclic pitch for rotor
control. In this vein, another early cyclic pitch design was patented by Gaetano A. Crocco of Italy in 1906. Crocco, who
pioneered the ideas of hydrofoil boats, recognized that if a helicopter was to
work properly when in forward flight, a means of changing the pitch on the
blades would be needed to account for the dissymmetry in the aerodynamic loads
between the side of the rotor advancing into the relative wind and the side
retreating away from the wind. As mentioned earlier, the concept of cyclic
pitch was one key to attaining full control of the helicopter.
There is also evidence of the construction of a primitive coaxial helicopter by Professor Zhukovskii (Joukowski) and his students at Moscow University in 1910 -- see Gablehouse (1967). Joukowski is well known for his theoretical contributions to aerodynamics, and besides other contributions to the field he published several papers on the subject of rotating wings and helicopters -- see Margoulis (1922) and Tokaty (1971). While prior to 1900 Rankine and Froude had already established the general theory of propellers and rotors using momentum theory, there were a number of rapid developments in the basic aerodynamic theory. For example, the Frenchman Drzewiecky had developed a hybrid momentum/blade element concept about 1900. In 1909 Drzewiecky published a book entitled "Des Helices Aeriennes Theorie Generale des Propulseurs." In 1904, Joukowski published a paper entitled "On the Useful Load Lifted by a Helicopter." In 1906, Joukowski's well-known work "About Connected Vortices" was published. A year later, a paper entitled "A Multi-Bladed Propeller-Screw" appeared. In 1909 Joukowski began to investigate the theory of the effects of forward flight speed on a rotor. This paper was entitled "Experiments on the Theoretical Determination of the Effect of the Airflow on the Surface of a Propeller." Here, like Crocco, Joukowski proved that because of the non-axisymmetric distribution of velocity and airloads, asymmetric forces and moments must act on a propeller when in edgewise (forward) flight. Of course, this implied that to control a helicopter such that it could fly forward would be a difficult task indeed. Although this problem was later to be solved with the invention of cyclic blade pitch and blade flapping hinges, Joukowski offered no method of solution in his 1909 paper.
About 1914, the Danish aviation pioneer Jen C. Ellehammer designed a coaxial rotor
helicopter. Boulet (1984) gives a good description of the machine. The rotor
blades themselves were very short; six of these were attached to the periphery
of each of two large circular aluminum rings of about 20-ft in diameter, with
the wings extending out about another 5-feet. The lower disk was covered with
fabric and was intended to serve as a parachute in the event the blades or the
engine failed. A cyclic pitch mechanism was used to change the pitch of the
rotating wings and to effect control, this being another one of many early
applications of the cyclic pitch concept. The pilot was supported in a seat
that could be moved forward and sideways below the rotor, allowing for
additional kinesthetic control. The aircraft made many short hops into the air
but never made a properly controlled free flight. It was finally destroyed in a
crash in 1916.
An Austrian, Stephan Petroczy, with the assistance of the
well-known aerodynamicist Theodore von Kármán,
built and flew a coaxial rotor helicopter during 1917-20. Interesting design
features of this machine included a pilot/observer position above the rotors,
inflated bags for landing gear, and a quick-opening parachute. The machine was
powered by three rotary engines. While the machine never really flew freely, it
accomplished numerous limited tethered vertical flights restrained by cables.
The work is summarized in a report by von Kármán (1921) and published by the
NACA. It is significant that von Kármán also gives results of laboratory tests
on the "rotors," which were really oversize propellers. With the work
of William F. Durand [see Warner (1920) and
the analysis of the measurements by Max Munk
(1923)] these were some of the first laboratory experiments to study rotor
performance and the power required for vertical flight.
In the United States, Emile
and Henry Berliner (a father and son) were interested in vertical flight
aircraft. As early as 1909, they had designed and built a helicopter based on
pioneering forward flight experiments with a wheeled test rig. They were one of
the first to observe the fact that the rotor power required for hovering flight
was substantially greater than for flight at low forward speeds. In 1918 the
Berliners patented a single-rotor helicopter design, but there is no record
that this machine was built. Instead, by about 1919, Henry Berliner had built a
counter-rotating coaxial rotor machine, which made brief uncontrolled hops into
the air and reached a height of about four feet. By the early 1920s at the
College Park airport, which is close to the University of Maryland, the
Berliners were flying an aircraft with side-by-side rotors. The rotors were
oversized wooden propellers, but with special airfoil profiles and twist distributions.
Differential longitudinal tilt of the rotor shafts provided directional
control. Cascades of wings located in the slipstream of the rotors aided
lateral control. All variants used a conventional elevator and rudder assembly
at the tail, with a small vertically thrusting auxiliary rotor on the rear of
the fuselage. This machine made only short hops into the air, and because the
true vertical flight capability was limited, the Berliners abandoned the pure
helicopter in favor of another hybrid machine they called a
"helicoplane." This still used the rotors for vertical lift but
incorporated a set of triplane wings and a larger oversized rudder. The
Berliner's final hybrid machine of 1924 was a biplane wing configuration with
side-by-side rotors. However, the Berliner's early flights with the coaxial
rotor and side-by-side rotor machines are credited as some of the first
rudimentary piloted helicopter developments in the United States. See also
Berliner (1908, 1915). The Berliner's subsequently went on to form the Erco
Company or Riverdale, Maryland, which became a well-known manufacturer of light
planes and propellers.
In Britain during the late 1910's and early 1920's, Louis Brennan worked on a helicopter concept with
an unusually large single two-bladed rotor. Fay (1987) gives a good account of
Brennan's work. Brennan, who was an inventor of some notoriety, had a different
approach to solving the problem of torque reaction by powering the single rotor
with propellers mounted on the blades themselves. Control was achieved by the
use of servo-flaps or "ailerons" inboard of the propellers. The
machine was powered by 230-hp Bentley rotary. While Brennan's work was
initially carried out with considerable secrecy, in 1921 the machine was moved to
the Royal Aircraft Establishment (RAE) at Farnborough. In 1922, the machine
flew successfully inside a balloon shed. Further flights outdoors were
undertaken through 1925, where the machine made flights at low altitude. The
machine crashed on its seventh flight, and official interest in the Brennan
machine quickly faded because of increasing interest in the autogiro.
During
the early 1920s, Raoul Pateras-Pescara worked on
helicopter development in Spain and France. Pescara is mostly known in the
helicopter field for his technical contributions to methods of achieving
effective flight control. He was one of the first to implement cyclic and
collective blade pitch control into a prototype helicopter. Pescara was able to
achieve roll, pitch, and yaw control over his helicopters solely through blade
pitch variations in his rotor systems. Additionally, Pescara was one of the
first to recognize the phenomenon of autorotation, and how a pilot should
control an unpowered helicopter in the event of engine failure. Adopting a
coaxial rotor system with biplane blades for his four helicopter variants,
Pescara achieved collective and cyclic pitch angle variations through warping
displacements of the torsionally compliant blades. Despite several serious
crashes, and a mechanically complex and less than effective control system,
Pescara’s machines demonstrated modest levels of performance. The most notable
flight was a FAI straight-line record flight of 2,415 feet (736 meters), which
was set in France during April of 1924. Pescara also dabbled in hybrid forms of
aircraft, coupling vertical takeoff capabilities using a vertically thrusting
rotor with a fixed wing to provide lift during forward flight. Despite his
technical innovations, and over 30 patents relating to rotating wing
technology, Pescara eventually abandoned his helicopter experiments in the
mid-1920s. Although he never was able to refine his helicopters to a point of
true practical success, Pescara was arguably the first pioneer to build a
prototype helicopter that addressed all the aspects of lift, propulsion,
control, and stability, both in powered flight and in autorotation. Pescara continued his
attempts at Saint Raphael (France) in 1926 with the 3F model of his helicopter.
Pescara later settled with his brother in Barcelona (Spain) where he developed
the 4S version of his helicopter, which the hands of the engineer, Mr. Pouit,
and pilot, Lieutenant Vaissseau Barrera, demonstrated some successful flights
in hover and in forward flight at low altitude.
Between 1924 and 1930, a Dutchman named A. G. Von Baumhauer designed and built one of the
first single-rotor helicopters with a sideward thrusting tail rotor to
counteract the torque reaction from the main rotor. Boulet (1984) gives a good
description of Von Baumhauer's machine. The fuselage consisted essentially of a
tubular truss, with an engine mounted on one end. The other end carried a
smaller engine turning a conventional propeller to provide a thrust force,
which with the long moment arm, counter the main rotor torque reaction. The
main rotor had two blades, which were restrained by cables so that the blades
flapped about a hinge like a seesaw or teeter board. Control was achieved by a
swashplate and cyclic-pitch mechanism, which was another very early application
of this mechanism. Unfortunately, the main and tail rotors were in no way
connected, and this caused considerable difficulties in achieving proper
directional control. Nevertheless, the machine was reported to have made
numerous short, semi-controlled flights.
AR-3, which flew in
1935. With its jump take-off capability, the autogiro was to closely rival the
helicopter in performance capability. Several other British companies including
Weir, Avro,
Parnall, de
Havilland, and Westland went on to
build variants of the de la Cierva Autogiro designs. The first Weir designs
were developments of de la Cierva's models and used the orientable direct rotor
control system. The Weir W-1 through W-4 models were all autogiros and were some of the
first machines to use a clutch to help bring up the rotor rpm prior to takeoff.
The de Havilland and Westland companies built a few larger prototype autogiros.
The Westland C-29 was a five-seat cabin
autogiro built in 1934. The aircraft was never flown because it exhibited
serious ground resonance problems, and the project was canceled with the
untimely death of Juan de la Cierva in 1936. However, de la Cierva's work was
carried on by designers from Weir, and another Westland designed autogiro
called the CL-20 was flown just before World
War 2; see Monday (1982).
In the United States, the Kellett and Pictairn companies entered into licensing agreements with de la Cierva, resulting in the first flight of an autogiro in the USA in 1928. Pitcairn went on to design and patent many improvements into the de la Cierva rotor system [see Smith (1985)], but it became clear that it was a true helicopter with power delivered to the rotor shaft that was required. The autogiro was extensively tested in the United States by the NACA. Gustafson (1971) gives an authoritative account of the early NACA technical work on autogiros and helicopters. In addition, the entire first issue of the Journal of the American Helicopter Society, published in January 1956, was devoted to the early autogiro and helicopter developments in the United States. In Russia, the TsAGI built autogiros derived from the de la Cierva designs. Kuznetsov and Mil built the 2-EA, which was derived from the Cierva C-19 -- see Everett-Heath (1988). Later developments of this design led to the first Russian helicopters built with the assistance of Vittorio Isacco, who had led basic helicopter developments in Italy during the 1920s.
First Successes with Helicopters
In 1922, a Russian émigré to the United States by the name of Georges de Bothezat built one of the largest
helicopters of the time under contract to the US Army. De Bothezat had been a
student of Professor Zhukowski in Russia and had written one of the first
technical manuscripts on rotating-wing aerodynamics -- see de Bothezat (1919).
De Bothezat's machine was a quadrotor with a rotor located at each end of a
truss structure of intersecting beams, placed in the shape of a cross. Ivan Jerome was the co-designer. Each rotor had
six wide chord blades. Control of the machine was achieved by collective,
differential collective and cyclic blade pitch variations, and the blade pitch
design likely derived directly from those of Yur'ev. A set of four smaller
rotors served to help control the machine. In 1922, the ungainly Jerome-de Bothezat quad-rotor or "Flying
Octopus" flew successfully many times, albeit at low altitudes and slow
forward speeds. However, because of insufficient performance, high financial
costs, and the increasing military interest in autogiros at the time, the
project was canceled. Surprisingly, it was to be fifteen or more years before a
pure helicopter was again to fly again in the United States and better de Bothezat's
accomplishments.
In 1920, Etienne hemichen,
an employee of the French Peugeot car company, built a quad-rotor machine in a
similar style to that of de Bothezat, but with eight additional rotors for
control and propulsion. His machine typified the cumbrous mechanical complexity
of the various helicopters of that time. His initial design was underpowered,
and it had to have a hydrogen balloon attached to provide additional lift and
stability. Nevertheless, Oehmichen went on to design a pure helicopter that was
flown between 1923 and 1924. By 1924, Oehmichen was making reasonable flights
and his machine proved that a vertical flight machine could be stable and
somewhat maneuverable, although cumbersome. In May 1924 he was awarded a prize
by the FAI for demonstrating the first helicopter to fly a standard closed 1 km
circuit, which took 7 minutes 40 seconds at an average speed of only 7.8 km/hr
(4.9 mph). The machine, however, was impractical for any realistic use. See
also NACA (1921) and Oehmichen (1923).
In 1930, Corradino
d'Ascanio of Italy built a relatively successful coaxial helicopter,
which flew under good control. His relatively large machine had two,
two-bladed, counter-rotating rotors. Following the work of de la Cierva, the
blades had hinges that allowed for flapping and a feathering capability to
change blade pitch. Control was achieved by using auxiliary wings or servo-tabs
on the trailing edges of the blades, a concept that was later adopted by
others, including Bleecker and Kaman in the United States. D'Ascanio designed
these servo-tabs so that they could be deflected cyclically by a system of
cables and pulleys, thereby cyclically changing the lift on the blade as it
swept around the rotor disk. For vertical flight, the tabs on all the blades
moved collectively to increase the rotor thrust. Three small propellers mounted
to the airframe were used for additional pitch, roll, and yaw control. This
machine held modest FAI speed and altitude records for the time, including
altitude (57 ft, 17.4 m), duration (8 minutes 45 seconds) and distance flown
(3,589 ft, 1,078 m).
In
1930, Maitland leecker of the United States
followed Brennan's approach to the torque reaction problem by using a single
rotor and delivering power to propellers that were mounted on each rotor blade.
Power was supplied through a system of chains and gears from an engine mounted
at the center of the machine. Like d'Ascanio's machine, Bleecker's helicopter
was controlled by auxiliary aerodynamic surfaces he called "stabovators"
that were fastened to the trailing edges of each of the four blades. Both
collective and cyclic pitch capability were incorporated into the design.
Bleecker's machine accomplished numerous precarious hovers in ground effect. It
was not as successful as d'Ascanio's machine and high vibration levels and
control problems caused the project to be abandoned during 1933. Liberatore
(1998) gives one of the best accounts of the project.
In Belgium during 1929-30, the Russian born engineer Nicolas Florine built one of the first successful
tandem rotor helicopters. The rotors turned in the same direction but were
tilted in opposite directions to cancel torque reaction. Boulet (1984)
describes the various mechanical aspects of the machine. Florine's first aircraft
was destroyed in 1930, but he had a second design flying successfully by 1933,
which made a flight of over 9 minutes to an altitude of 15-feet. This exceeded
d'Ascanio's modest flight duration record of the time. Yet, Florine's designs
suffered many setbacks, and work was discontinued into the pre-World War 2
years. His machines were ultimately destroyed during the war.
Success at Last
During the period
1930--1936, the famous French aviation pioneers Louis
Breguet and Rene Dorand made particularly
notable advances in the development of a practical helicopter. Their machine of
1935 was relatively large for the era, with a coaxial rotor configuration.
Boulet (1984) and Kretz (1987) give an excellent account of the work. Each
rotor had two modern looking tapered blades that were mounted to the hub with
flap and lag hinges. The blades were controlled in cyclic pitch using a
swashplate design. Yaw control was achieved by differential torque on one rotor
with respect to the other rotor. Horizontal and vertical tails were used for
increased stability. For its time, the aircraft had held several FAI records,
including a duration flight of 62 minutes and distance flown of 44 km (27 mi).
Further work on the Breguet-Dorand machine
was stopped prior to the outbreak of World War 2.
While helicopters were becoming more and more successful, the safety of the machine was still an issue. They were difficult to fly, and the possibility of loss of power was always present. All aircraft must possess safe flight characteristics after a loss of power, the helicopter being no exception. While a fixed-wing aircraft can glide, the helicopter can take advantage of autorotation with the rotor unpowered as a means of maintaining rotor rpm, lift, and control in the event of engine failure. In this mode, the helicopter behaves very much like an autogiro so that the relative wind comes upward through the rotor disk. However, with the higher disk loadings (the thrust carried per unit disk area) found on helicopters, to get the rotor to autorotate the helicopter must descend at a relatively high rate. The pilot, in effect, gives up altitude (potential energy) at a controlled rate for kinetic energy to drive the rotor and with care, can autorotate the aircraft safely onto the ground. The ability to "autorotate" can be viewed as one distinguishing feature of a safe and successful helicopter.
Heinrich Focke of the
Focke-Wulf Company began his work on rotating-wing aircraft as early as 1933.
He acquired a license to build de la Cierva's autogyros, and successfully
manufactured the C-19 and the C-30 models. From the experience he gained by
working on these machines and after many wind tunnel tests with small models,
Focke began developing the FW-61 helicopter
in 1934, named after his current company, Focke-Wulf. Later, in early 1936,
Focke and Gert Achgelis finally built and
demonstrated a successful side-by-side, two-rotor machine, called the Fa-61. The details of this machine are described by
Focke (1938, 1965) and Boulet (1984). This machine was constructed from the
fuselage of a small biplane trainer with rotor components provided by the Weir-Cierva Company. The rotors were mounted on
outriggers and were inclined slightly inward to provide lateral stability. The
blades were tapered in planform and were attached to the rotor hub by both
flapping and lagging hinges. Longitudinal control was achieved by tilting the
rotors forward and aft by means of a swashplate mechanism, while yaw control was
gained by tilting the rotors differentially. The rotors had no variable
collective pitch, instead using a slow and clumsy system of changing rotor
speed to change the rotor thrust. A vertical rudder and horizontal tail
provided for additional directional stability. The cut-down propeller on the
front of the machine served only to cool the radial engine.
The Fa-61 machine is significant in that it was the first helicopter to show fully controlled flight and also to demonstrate successful autorotations. To this end, provision was made in the design for a fixed low collective pitch setting to keep the rotor from stalling during the descent. It also set records at the time for duration, climb to altitude (3,427 m, 11,243 ft), forward speed (122 km/h, 76 mph), and distance flown in a straight line (233 km, 143 miles). The machine gained a certain amount of notoriety prior to the outbreak of World War 2 when the famous German test pilot Flugkapitan Hanna Reitsch flew it inside Berlin's Deutschlandhalle sports arena. The Fa-61 aircraft was used as a basis to develop the first German production helicopter, the Fa-266 (Fa-233E), which first flew in 1940. This was a fairly large aircraft, with two three-bladed rotors, and could carry up to four crew. Yet, the machine saw limited production during the Second World War. Boulet (1984) gives a good account of the later helicopter work of Focke. After the War, some of the German machines were used as a basis to develop helicopters in Russia [see Everett-Heath (1988)] and France.
With the assistance of Juan de la Cierva, the Weir
Company had formed an aircraft department in Scotland in 1932. The W-5 was the Weir Company's first true helicopter
design. Initially, the W-5 was a coaxial design, but concerns about stability
and control as well as the success of the Fa-61 led to the redevelopment as a
lateral side-by-side configuration, which flew successfully in June 1938.
Control was achieved with cyclic pitch but there was no collective pitch;
vertical control was obtained by altering the rotor speed, a cumbersome feature
used also on the Fa-61. The W-5 reached speeds of 70 mph in forward flight. The
Weir W-5 (and later the W-6) and the Fa-61 were technically ahead of
Sikorsky's VS-300 in terms of flight capability, but the VS-300 was ultimately
to set the new standard for helicopter design. The Weir W-6, which first flew in 1939, was a much larger version of the
W-5 but still used the lateral side-by-side rotor configuration. Further work
on the Weir designs was suspended at the outbreak of World War 2.
During
the period 1938-43, Antoine Flettner, also
of Germany, developed several helicopter designs. Flettner's success came with
using a side-by-side intermeshing rotor configuration, which became known as a
synchcropter. This rotor idea was first
patented to Bourcart in 1903 and by Mees in 1910, and the synchropter configuration
was pursued by other developers in other countries. In the synchropter design,
the rotor shafts are close together but arranged so that they are at a significant
outward angle with the overlapping rotors turning in opposite directions. A
gearing system ensures the exact phasing of the rotors. In 1939, Flettner's Fl-265 synchropter flew successfully and was the
first helicopter to demonstrate transition into autorotation and then back
again into powered flight. Flettner built several other machines, including the
Fl-282 Hummingbird. With the Focke Fa-266 (Fa-233E), the Fl-282
was one of the first helicopters to enter into production. However,
production was limited because of World War 2. After the war, in the United
States, the Kellett Aircraft Company (which,
as mentioned earlier, also built autogiros as a licensee to Pitcairn) adopted
Flettner's synchropter configuration but used three-bladed instead of two-bladed
intermeshing rotors. The aircraft flew very successfully, but it never went
into production. The synchropter concept was also adopted by Charles Kaman, who's company Kaman Aircraft Corp. was later to put the type
into successful production.
As
described earlier, Igor Sikorsky had
experimented in Czarist Russia with primitive vertical lift aircraft as early
as 1907 -- see Sikorsky (1938) and Finne (1987). After Sikorsky had emigrated
to the United States, he went on to design and build giant flying boats. In
1935, Sikorsky was issued a patent, which showed a relatively modern looking
single rotor/tail rotor helicopter design with flapping hinges and a form of
cyclic pitch control. Although Sikorsky encountered many technical challenges,
he tackled them systematically and carefully. To the workers at the Sikorsky
plant in Connecticut, the machine was known as "Igor's nightmare" and
reflected the mechanical complexity of his early prototypes. Sikorsky's first
helicopter, the VS-300, was flying by May 1940.
A good summary of the technical design is given by Sikorsky (1941, 1942, 1943).
His first machine had one main rotor and three auxiliary tail rotors, with
longitudinal and lateral control being obtained by means of pitch variations on
the two vertically thrusting horizontal tail rotors. Powered only with a 75 hp
engine, the machine could hover, fly sidewards and backwards, and perform many
other maneuvers. Yet it could not easily fly forward, exhibiting a sudden
nose-up pitching characteristic at low forward speeds. This phenomenon was to
be traced to the downwash of the main rotor wake, which as airspeed built, blew
back onto the two vertically thrusting tail rotors and destroyed their lift.
The main lifting rotor of the VS-300 was used in the later VS-300A with a more powerful 90 hp engine, but
only the vertical (sideward thrusting) tail rotor was retained out of the
original three auxiliary rotors. In this configuration, longitudinal and
lateral control was achieved by tilting the main rotor by means of cyclic-pitch
inputs; the single tail rotor was used for antitorque and directional control
purposes. This configuration was to become the standard for most modern
helicopters.
More Successes - First Production Machines
Before long, Sikorsky had refined
his first machines and by 1941 he had already started production of the R-4. In 1943 Sikorsky developed the R-5, which, although still only a two-seater
helicopter, was much larger, more powerful, and more capable than the R-4,
which became used extensively for pilot training. The R-5 was produced in
substantial numbers, and while it had a limited payload and forward speed
capability, several hundred of them saw military service in the Pacific during
World War 2. Find out more about the history of Sikorsky Aircraft by checking
out the Sikorsky Archives or the Sikorsky Timeline at the
Helicopter History Site. In 1946 Westland Helicopters in Great
Britain obtained a license to build models of the Sikorsky machines. Westland
already had a history as a successful fixed-wing manufacturer. Their first
machine was designated as the WS-51 after
the S-51, which was a development of the R-5
and the first commercial helicopter designed by Sikorsky. This post-War period
was the start of a long relationship between the two companies. After
significantly reengineering the Sikorsky machine to meet British airworthiness
standards, Westland called the aircraft the Dragonfly.
The Westland Widgeon later followed, and
this was a very modern looking and powerful version of the Dragonfly with a
larger passenger cabin. Find out more about the history of Westland Aircraft by
checking out the Westland
Timeline at the Helicopter History Site.
During 1944, the Cierva-Weir
Company, prompted by the initial success of Sikorsky's R-4 and R-5, proposed a
rather large single-rotor machine called the W-9.
This machine was rather unique in its use of jet thrust to counteract rotor
torque reaction -- see Everett-Heath (1986). However, because the rotor lacked
any collective pitch control, rotor thrust was controlled by changing rotor
speed as in the pre-war Weir W-5/6 models. The W-9 crashed during a test flight
in 1946, and the project was subsequently abandoned. Subsequently, the Weir and
Cierva companies went on to design the W-11
Air-Horse, which was an unorthodox three-rotor helicopter of considerable
lifting capability. Mainly designed for crop dusting, the machine crashed
during a test flight and any further work was terminated. The final helicopter
of the Cierva--Weir line was the diminutive W-14
Skeeter used by the British armed forces. This was a two-seater training
helicopter designed in 1948, but it saw only a limited production run through
1960.
Several
other young helicopter design pioneers were working in the United States during
the 1940s. These included Arthur Young, Frank
Piasecki, Stanley Hiller, and Charles Kaman. In the late 1930s, Arthur Young began a series of experiments with
model helicopters that were ultimately to lead to the design of the renowned Bell-47 helicopter. After much research Young
invented a teetering rotor with a stabilizer bar; see Young (1948, 1979). The
bar had bob weights attached to each end and was directly linked to the rotor
blades through the pitch control linkages. The idea was that if the rotor was
disturbed in pitch or roll, the gyroscopic inertia of the bar could be used to
introduce cyclic pitch into the main rotor system, increasing the effective
damping to disturbances and giving stability to the entire rotor system -- see
also Kelly (1954). Young received financial support from Lawrence Bell of the Bell Aircraft
Corporation and their first prototype, the Bell-30,
was built in 1942. This two-place machine had a single main teetering rotor
with Young's stabilizer bar. The first untethered flights of the Bell-30 took
place in 1943, and the machine was soon flying at speeds in excess of 70 mph.
The
Bell-30 formed the technical foundation for the famous Bell
Model 47, which became the world's first commercially certified helicopter.
During its nearly thirty-year manufacturing period over 5,000 were produced in
the United States alone, and at least another 1,000 were license built in more
than twenty other countries. Tipton (1989), Brown (1995) and Spenser (1999)
give a good historical overview of the enormously successful Bell helicopters.
Schneider (1995) gives a brief biography of Arthur Young and his novel
teetering rotor design. To find out more about Arthur Young go to his website. To find
out more about the history of Bell Helicopter, check out the Bell Timeline at the
Helicopter History Site.
In 1943, Frank Piasecki designed and flew a tiny
single-seater helicopter that was called the PV-2.
This was the second successful prototype helicopter to fly in the United
States, the first being Sikorsky's VS-300. Piasecki's company went on to
develop the overlapping tandem rotor configuration, a concept patented by Gish
Javanovitch and demonstrated with a flying prototype as early as 1944. Piasecki
immediately turned to larger helicopters, and in 1945 the Piasecki Helicopter Corporation built a tandem
rotor helicopter called the PV-3 Dogship.
Further details are given by The Piasecki Corporation (1967) and Spenser
(1999). This aircraft was popularly called the "Flying Banana"
because of its long, distinctive, curved fuselage shape. Despite its nickname,
however, the aircraft was very successful and larger and more powerful versions
of the tandem rotor design quickly followed, including the H-16 and H-21
"Workhorse" model of 1952. To find out more about the history of the
Piasecki, Vertol and Boeing Helicopters, check out the Boeing Timeline at the Helicopter History
Site.
Despite the success of the tandem rotor design, the
only other company in the United States to build a tandem helicopter was Bell
Helicopter who manufactured the XSL-1 during
the 1950s. The British company, Bristol Helicopters,
had designed and built a tandem helicopter during the late 1940s under the
leadership of the helicopter pioneer Raoul Hafner -- see Hobbs (1984) and
Everett-Heath (1986). The Bristol Type-173
had a long, slim fuselage with two three-bladed rotors at each end, similar to
the Piasecki machines. The Bristol Type-192 Belvedere was an improved tandem rotor design,
which followed in 1958 with more powerful engines. While it saw service with
the British forces, it was not as successful as the American machines. The
other Bristol helicopter design of note was the single rotor Type 171 Sycamore, which had a well-streamlined
fuselage and quite good performance -- see Hafner (1949). However, the Bristol
Company found it difficult to compete with helicopters being produced by
Sikorsky, Bell and Westland, and limited numbers of their machines were
produced.
In the United
States, Charles Kaman adopted Antoine
Flettner's synchropter rotor design. One of Kaman's innovations was the use of
torsionally compliant solid spar spruce rotor blades with servo-flaps. The
servo-flaps were mounted at the three-quarter-rotor radius, some distance
behind the elastic axis of the blade -- a system first used by d'Ascanio. When
these flaps were deflected cyclically, the aerodynamic moments caused the
blades to twist, changing their angle of attack and thus introducing a cyclic
rotor control capability. The first Kaman helicopter, the K-125A, flew in 1947. An improved version, the K-225, became the first helicopter to fly powered
by a gas turbine engine. A family of larger Kaman machines, known as the H-43 Husky and its derivatives, were produced
through 1964. While Kaman reverted to conventional single-rotor helicopter
designs in the later 1950s, the servo-flap concept continued to be a trademark
of the Kaman helicopters. The H-2 Seasprite
first flew in 1959 and has been produced in considerable numbers. Kaman has recently
returned to the synchropter concept with the design of the K-Max, which first flew in 1991. See Kaman Aircraft Corp. for further details of
their helicopter lineage. To find out more about the history of Kaman Aircraft,
check out the Kaman Timeline
at the Helicopter History Site.
Stanley Hiller is another
well-known pioneer who contributed to the development of the modern helicopter
[see Straubel (1964) and Spenser (1992, 1999)]. Hiller built several helicopter
prototypes, including the coaxial XH-44,
which flew successfully in 1944. Although Hiller pursued various other coaxial
and tip-jet driven rotor machines, his later helicopters used a conventional
main rotor and tail rotor configuration. His main breakthrough was the "Rotormatic" main rotor design, where
the cyclic pitch controls were connected to a set of small auxiliary blades set
at ninety degrees to the main rotor blades. These auxiliary blades provided
damping in pitch and roll helping to augment the hovering stability of the
machine. While today this can be done electronically by an automatic flight
control system, the "Hiller paddle"
concept continues to be used for flying scale model helicopters. It is
significant to note that both Hiller and Young designed in stability-producing
mechanisms for their helicopters from the outset, whereas the Sikorsky machines
had none and so they had a reputation for being harder to fly. While the Hiller
machines are probably less well known than those of Sikorsky or Bell, the
Hiller company went on to build many thousands of helicopters, including the Model 360 and later the UH-12A
and H-23. To find out more about the history
of Hiller helicopters, check out the Hiller Timeline at the
Helicopter History Site.
Don't Forget Autogiros
It is significant to note that while helicopters were becoming more and
more successful throughout the 1950's, the development of the autogiro
continued in Europe and the United States. Considerable development work was undertaken
by the Pitcairn Company [see Pitcairn (1930)
and Smith (1985)] and the Kellett Aircraft Company
in the United States. Harold Pitcairn
patented many basic concepts in rotor design, many of which were licensed and
used by other helicopter manufacturers. The Pitcairn and Kellett autogiros or
"Mailwings" flew on part of Eastern Airlines' network delivering mail
from the top of the United States Post Office in Philadelphia to the nearby
Camden Airport in New Jersey.
In
Great Britain during the 1940s and 50s, the autogiro concept was pursued to
some significant end by the Fairey Aviation Company.
The Fairey Girodyne compound aircraft used a
propeller set on the end of a stub wing to provide both propulsion and
antitorque; see Everett-Heath (1986). The Fairey Company went on to develop the
Jet Girodyne in which the rotor system was
driven by tip jets. This ultimately led to the Rotodyne,
which was the world's biggest giroplane with a cabin big enough for forty
passengers -- see Hislop (1958). The aircraft set a world speed record for a
convertiplane in 1959 before the project was canceled, for reasons that were
perceived by many as political rather than technical.
During the 1960s small single- and two-seat autogiro designs were developed in the United States by Umbaugh and McCulloch for the private market. Similar single- and two-seat autogiros were later built in Britain by the Wallis Company. Today, there is a strong interest in autogiros by the amateur aircraft builder, mainly because of its mechanical simplicity, light weight, and forgiving flight characteristics. There is only a limited interest in autogyros for other commercial uses, mainly because it cannot compete with either a fixed-wing aircraft or the helicopter. However, the autogiro concept continues to be pursued by a number of enthusiasts, including the Groen Brothers Company in the United States.
Maturing Technology
The early 1950s saw helicopters quickly maturing into safe, successful,
and highly viable aircraft that were easier to fly and more comfortable for
crew and passengers alike. This era is marked by significant mass production of
helicopters by various manufacturers in the United States and also in Europe.
The Sikorsky S-55 and S-58 models made great advances in helicopter
design. These aircraft had a large cabin under the rotor, and to give a wide
allowable center of gravity position, the engine was placed in the nose. Westland also maintained their relationship with
Sikorsky and built versions called the S-55
Whirlwind and S-58 Wessex.
The
1960s saw the development of the Sikorsky S-61 Sea
King, the heavy-lift S-64 Sky Crane,
and the larger five- and seven-bladed CH-53
models. Later, the S-70 (UH-60) Blackhawk
was to become the mainstay of the Sikorsky Company, and the machine is expected
to remain in production well into the twenty-first century. The civilian S-76 has been successful in its role as an executive
transport and air ambulance, amongst other roles. In the 1970s, Sikorsky and
Boeing teamed to build the military RAH-66
Comanche, which will be a scout/attack helicopter for the new millennium. The
latest Sikorsky machine, the civilian medium lift S-92
Helibus, flew for the first time in 1998. For more information, check
out the Sikorsky Aircraft home page, or
the Sikorsky Timeline
at the Helicopter History Site.
The
success with the Model-47 led Bell Helicopter to develop the UH-1 Huey, which were delivered starting in 1959.
The Bell 212 was a two-engine development of
the UH-1D, and proved to be a successful
military and civilian machine. The Huey-Cobra
also grew out of the UH-1 series, retaining the same rotor components, but
having a more streamlined fuselage with the crew seated in tandem. The type is
still in production in 1999 as the AH-1W
Super-Cobra, which uses an advanced composite four-bladed rotor. The Bell 412 is basically a 212 model, but with a
four-bladed composite rotor replacing the two-bladed teetering rotor. Bell also
conquered the civilian market with its 206
Jet-Ranger and variants, which first flew in 1966 and has become one of
the most widely used helicopters. The OH-58
military version was sold in considerable numbers and with sustained
improvements over the years, with the OH-58D
having an advanced four-bladed rotor with mast-mounted sight. One of most
recent civilian models is the Bell 427,
which is an eight-place light twin. See also Bell Helicopter Textron and the Bell Timeline at the
Helicopter History Site.
Piasecki's
corporation became The Vertol Company in
1956, which went on to develop the civilian Vertol
107 and two highly successful military tandem rotor models, the CH-46 and CH-47.
The company finally became Boeing Helicopters.
An overview of the Boeing-Vertol machines
produced up to the mid-1970s is given by Grina (1975). In the late 1980s, the
Boeing Company produced a demonstrator of an advanced technology tandem rotor
helicopter called the Model 360, which was
made almost entirely of composite materials. Production and remanufacturing of
the Boeing CH-47 continues today, and in
1998 Boeing announced the launch of the CH-47F
and the CH-47SD "Super-D" Chinook.
See also The Boeing Company and the Boeing Timeline at the
Helicopter History Site.
Hughes built the military TH-55
and later the Hughes-500 series, which has
seen extensive civilian use in various models. However, the AH-64 Apache, which was designed in 1976, proved
to be the biggest success story for the Hughes company. The AH-64D Longbow model is still in production over
twenty years later. It is also produced under license in the UK by
GKN-Westland, where it is called the WAH-64. McDonnell Douglas have also
produced a line of light commercial helicopters including the MD 500 and 600
series, and most recently it has marketed the MD-900 Explorer. This aircraft
uses a new bearingless rotor design and the "No
Tail Rotor" (NOTAR) circulation control antitorque concept. To find
out more about the history of Hughes and McDonnell-Douglas helicopters, check
out the McDonnell-Douglas
Timeline at the Helicopter History Site.
Although the bulk of helicopters produced are for the military, several manufacturers produce training helicopters or helicopters aimed at the general aviation market, including Robinson, Schweizer, and Enstrom. In the United States, Robinson produces the R-22 two-seat and R-44 four-seat helicopters. Both are powered by piston engines. Schweizer produces an updated version of the two-seat Hughes 300 for the training market, and a larger derivative, designated as the Model-330, has a gas turbine.
The
European manufactures Aerospatiale, Agusta, MBB,
and Westland have produced many successful
helicopter designs since the 1960s. Augusta and Westland have also
license-produced helicopters designed in the United States, such as those of
Sikorsky and Bell. The Aerospatiale (formally Sud-Aviation) Alouette was
one of the most successful European helicopters, and in 1955 it was one of the
first machines to be powered by a gas turbine. The Aerospatiale Super Frelon was a large transport machine, first
flown in 1962. In the early 1970s the Aerospatiale/Westland SA330 Puma became Europe's best selling transport
helicopter. The Aerospatiale/Westland Gazelle
was a successful successor to the Alouette, first flown in 1967, and it
introduced the fenestron tail rotor. The fenestron
is a ducted tail rotor design, fully integrated into the fuselage and vertical
fin. The Dauphin, first flown in 1972, used
an improved fenestron tail rotor and a composite main rotor hub. Messerschmitt-Bolkow-Blohm (MBB) introduced the BO105 in 1967 with a hingeless titanium rotor,
with the larger and more capable BK117
machine first flying in 1979. In the 1990s, Aerospatiale and MBB joined
resources to form Eurocopter, which produces
a large number of civilian and military helicopter models -- see Eurocopter. To find out more about the
history of Aerospatiale, check out the Aerospatiale Timeline
at the Helicopter History Site.
In 1952, Agusta purchased a license to build the Bell Model-47, and through 1965 it built several variants of the Bell machine to their own specifications. Agusta also began to design their own machines, with the large three-engined A-101 flying in 1964, but it never went into production. The Agusta A-109 was one of the most aerodynamically attractive helicopters. First flown in 1971, this high-speed transport and multirole helicopter has been very successful and is used in both civilian and military roles. The A-129 Mangusta, first flown in 1983, is a militarized version of the A109 with a different fuselage. To find out more about the history of Agusta, check out the Agusta Timeline at the Helicopter History Site.
Westland Helicopters (now GKN-Westland) has been a key player
in British aviation since the 1930s -- see Mondey (1982). The earliest
helicopters built by Westland were under license from Sikorsky, but these were
significantly modified to meet British airworthiness standards. During
1959--60, Westland took over the operation of the Bristol,
Saunders-Roe, and Fairey
companies. Saunders-Roe (SARO) had previously taken over the Cierva Company in
1951.The Westland/SARO/Cierva Skeeter was a
small two-seat trainer, which led to the bigger and relatively successful Wasp in 1962. The Westland Wessex was a development of the Sikorsky S-58, which was built
in many configurations through 1970. The Sea King
and Commando were derived from the S-61,
which were steadily improved upon since the first models flew in the late
1960s. The latest versions of the Sea King sold through 1990 have used
composite rotor blades and various airframe improvements. Westland designed its
own line of helicopters, starting with the military Lynx,
which first flew in 1971. The Westland WG-30
was a larger multirole transport version of the Lynx. Although this aircraft
saw some civilian use, production was limited. New versions of the Lynx (Super Lynx) are fitted with the Westland/RAE
British Experimental Rotor Program (BERP) blade, which has improved airfoil
sections and special tip shape. A Lynx with the BERP rotor currently holds the
absolute straight-line speed record for a single-rotor helicopter at some
250-kts (400-km/hr; 287-mi/h). The BERP blade design is also used on the
Westland-Agusta built EH Industries EH-101,
which is a medium-lift helicopter that entered production in 1996 in both
civilian and military variants. Westland also have a license agreement with
Sikorsky to build the WS-70 Blackhawk. See
also GKN-Westland for more information
on the current lineage, and also the Westland Timeline at the
Helicopter History Site.
Significant
numbers of helicopters have also been built in the former Soviet Union. In the
1930s, the TsAGI Technical Institute in
Moscow built a series of autogiros based on the de la Cierva designs.
Everett-Heath (1988) gives a good account of the early work. Later, work with
the Focke-Achgelis Company of Germany resulted in a number of prototype
helicopter designs with a lateral side-by-side rotor configuration. The Mil, Kamov, and
Yak companies all went on to build
successful helicopter lines. An overview of the early Russian machines is given
by Free (1970). Mikhail Mil adopted the
single main rotor tail rotor configuration, with the Mi-1
flying in 1950. The Mi-2 was a turbine-powered
version. The more efficient Mi-3 and larger Mi-4 machines quickly followed. The Mi-4 looked
very much like the S-55, but it was much bigger and more capable. The Mi-2 was
also built in significant numbers in Poland, with the Mi-4 being produced in
China. The Mi-6 of 1957 was one of the
largest helicopters ever built, with a rotor diameter of 35-m (115-ft) and a
gross weight of over 42,500-kg (93,700-lb). This was followed by the smaller Mi-8 (similar to the Mi-4), which went into
civilian service. The Mi-10 of 1961 was a
flying crane development of the Mi-6, with a tall, wide, quadricycle landing
gear. However, the credit for the world's largest and heaviest helicopter goes
to the Russian Mil Mi-12. This aircraft had
a side-by-side rotor configuration, with the span of the aircraft from rotor
tip to rotor tip exceeding that of the wing span of the Boeing 747. Power was
provided by four gas turbines, installed as pairs at the end of each wing
pylon. The Mi-24 assault/transport
helicopter was designed in 1972, and it has been produced in large numbers. The
Mi-26 entered service in 1982 and is the
largest helicopter currently flying. The Mi-28
is an attack helicopter, similar in configuration to the AH-64 Apache. The
latest Mil design, the Mi-38, is planned as
a successor to the Mi-8/17 and is similar in size and weight to the EH-101. To
find out more about the history of Mil, check out the Mil Timeline at the Helicopter
History Site.
The Kamov Company built a series of very successful
light and medium weight coaxial rotor helicopter designs, including the Ka-15 and Ka-18
in 1956 and the Ka-20 in 1961. Kamov was the
only company to ever put the coaxial helicopter design into mass production.
The Ka-25 and most of the later models were
all gas turbine powered. The Ka-27 and the
civilian model Ka-32 have been in production
since 1972. One of the most recent Kamov designs is the Ka-50, which is a lightweight attack helicopter of considerable
performance. One exception to the Kamov coaxial line was the Ka-22 convertiplane of 1961. Another new prototype
design is the Kamov Ka-62, which is a
conventional light utility helicopter design incorporating a fenestron. Alexander Yakolev built many successful fixed-wing
designs, but with the assistance of Mil designed the large tandem Yak-24 helicopter in the early 1950s. This
helicopter was produced from about 1952 to 1959, but it was not very
successful. Further information on Russian helicopter developments is given by Anoschenko
(1968) and Everett-Heath (1988). To find out more about the history of Kamov,
check out the Kamov Timeline
at the Helicopter History Site.
Compounds, Tilt-Wings and Tilt-Rotors
The conventional
helicopter is limited in forward flight performance by the aerodynamic lift and
propulsion limitations of the main rotor. These rotor limits arise because of
compressibility effects on the advancing blade, as well as stall on the
retreating blade. In addition, the high parasitic drag of the rotor hub and
other airframe components leads to a relatively poor overall lift-to-drag ratio
of the helicopter. This generally limits performance of conventional
helicopters to level-flight cruise speeds in the range of 150 kts (278 km/h;
172 mi/h), with dash speeds up to 200 kts (370 km/h; 230 mi/h). Although
somewhat higher flight speeds are possible with compound
designs, which use auxiliary propulsion
devices and wings to offload the rotor. The
idea is to enhance the basic performance metrics of the helicopter, such as
lift-to-drag ratio, propulsive efficiency, and maneuverability. The general
benefits are an expansion of the flight envelope compared to conventional
helicopter, but this is always at the expense of much higher power required and
fuel burn than would be necessary with a fixed-wing aircraft of the same
gross-weight and cruise speed. Furthermore, compound designs suffer from an
increase in empty weight and loss of payload capability, download penalties in
hover, and reduced vertical rate of climb. The Lockheed Cheyenne is an example
of a helicopter that used both lift compounding
and propulsion compounding. While
technically successful, it did not enter into production. While there are no
compound helicopter designs in current production (although many prototypes
have been built), except from a few Russian designs such as the Mi-6 that have
an element of lift compounding. One of the first experimental compound
helicopter designs was the McDonnell XV-1. This was a pressure jet driven
rotor, with a wing and a pusher propeller. After a vertical take-off, the power
was shifted from supplying the tip jets to driving the propeller, and the rotor
continued to turn in autorotation. In 1954, the aircraft was flown at speeds
approaching 200 mph. The Sikorsky NH-3A was based on the S-61, and used a wing
mounted with two turbo-jets for auxiliary propulsion. It achieved speeds of up
to 230 kts. The Bell UH-1 compound also had a wing and two turbo-jets, and
reached a speed of 275 kts in level flight. Boeing-Vertol flew the tandem Model
347 with relatively large wings. The ideas of compounding have recently
received renewed attention by some helicopter manufacturers. It remains to be
seen, however, if the compound helicopter design will re-emerge as a viable
design concept for the 21st century.
The
need for a machine that could combine the benefits of vertical takeoff and
landing (VTOL) capability with the high speed cruise of a fixed-wing aircraft
has also led to the evolution of tilt-wing
and tilt-rotor concepts. A history of the
many VTOL designs, including tilt-wings and tilt-rotors, is given by Hirschberg
(1997). However, this potential capability comes at an even greater price than
for a conventional helicopter, including increased mechanical complexity,
increased weight, and the susceptibility for the rotors and wing to exhibit
various aeroelastic problems. The tilt-wing
is basically a convertiplane concept, but it never became a viable
rotating-wing concept to replace or surpass the helicopter. The idea is that
the wing can be tilted from its normal flying position with the propellers
providing forward thrust, to a vertical position with the propellers providing
vertical lift. Several companies seriously considered the tilt-wing concept in
the 1950s, with Boeing, Hiller, Vought-Hiller-Ryan, and Canadair all producing
flying prototype aircraft. The Boeing-Vertol VZ-2 first
flew in 1957 and went on to make many successful conversions from hover into
forward flight. However, flow separation produced by the wing during the
conversion flight regime resulted in some difficult piloting, and these issues
were never satisfactorily resolved.
The Hiller X-18 was a large tilt-wing aircraft
compared to the VZ-2. The aircraft used two large diameter, counter-rotating
propellers (from the earlier Ryan Pogo concept) -- see Straubel (1964). The
aircraft underwent flight-testing in 1960, but the program was canceled in 1961
after the aircraft suffered a loss of control. In the 1980s, the Ishida Co. developed the TW-68
tilt-wing aircraft as a private venture, but the company went into bankruptcy
before the aircraft could be completed.
The tilt-rotor aircraft takes off and lands
vertically with the rotors pointed vertically upward like a helicopter. For
forward flight, the wing tip-mounted rotors are progressively tilted to convert
the aircraft into something that looks like a fixed-wing turboprop airplane. In
this mode, the tilt-rotor is able to achieve considerably higher flight speeds
(about 300-kts; 555-km/h; 344-mi/h) than would be possible with a helicopter.
Therefore, the tilt-rotor combines some attributes of the conventional
helicopter with those of a fixed-wing aircraft. Because the rotors of a
tilt-rotor are not large, the hovering efficiency of the tilt-rotor is not as
high as that of a helicopter. In the design of the Bell-Boeing V-22 Osprey, the rotor diameter was also limited by the
need to operate and hanger the aircraft on board an aircraft carrier. The
tilt-rotor concept was first demonstrated in a joint project between the Transcendental Aircraft Corporation and Bell in 1954.The first aircraft, the Model 1-G, had two three-bladed fully articulated
rotors. Various technical problems were encountered, especially in the
conversion from helicopter mode to fixed-wing flight. Bell later led the
development of the XV-3 in 1951, which had
two fully articulated 3-bladed rotors. The XV-3 was damaged in an accident in
1956 after an aeroelastic problem with the rotor. The second XV-3 used a
two-bladed teetering rotor system, and the aircraft was successfully flown in 1958.
However, several aeromechanical problems were again encountered, including
pylon whirl flutter.
By
the late 1960s, Bell had developed the Model 266
tilt-rotor and later the Model 300. Various
wind-tunnel tests of scaled models led to an improved understanding of the
rotor and wing aeroelastic issues involved with tilt-rotors, especially during
the conversion mode, and Bell continued to develop the Model
301. This aircraft later became the XV-15,
which fully demonstrated the viability of the tilt-rotor concept; but the
aircraft was never designed for production. However, in 1983 the much larger V-22 Osprey tilt-rotor program was begun. This
joint Bell/Boeing project has resulted in several test and preproduction
aircraft, and in 1997 the decision was made to put the aircraft into production
for the United States Navy and Marines. In 1997, Bell announced the development
of the Model 609 civilian tiltrotor, which
will be capable of transporting 9 passengers at 275-kts (509-km/h; 315-mi/h)
over 750-nm (1,390-km; 860-mi) sectors. See Bell Helicopter Textron for further
information.
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Text on all pages © J. G. Leishman 2000,
with extracts from the author's book "Principles of Helicopter
Aerodynamics" © Cambridge University Press 2000.