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
Overview
- 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
References
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.
Back to Contents
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.
Back to Contents
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.
Back to Contents
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.
Back to Contents
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.
Back to Contents
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
counterrotating 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.
Lateral control was aided by cascades of wings located in the
slipstream of the rotors. 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, Raul Pescara, an Argentinan
living and working in Spain and France, was building and attempting
to fly a coaxial helicopter with biplane-type rotors. On his first
machines, each rotor had four sets of biplane blades that were
mounted rigidly to the rotor shaft, and this was later increased to a
remarkable five sets per rotor, giving a total of 20 lifting
surfaces. As described by Boulet (1984), Pescara's work focused on
the need for complete control of the machine, which was achieved
through cyclic-pitch changes that could be obtained by warping the
blades periodically as they rotated. This was one of the first
successful applications of cyclic pitch. Yaw was controlled by
differential collective pitch. Early versions of Pescara's machine
were underpowered, which may not be surprising considering the high
drag of the bracing wires of his rotor, and the aircraft did not fly.
With a later version of his helicopter using a more powerful engine,
some successful flights were accomplished, albeit under limited
control. However, most flights resulted in damage or serious crashes
followed by long periods of rebuilding. By 1925, Raul Pescara had
abandoned his helicopter projects.
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.
In the late 1920s, the Austrian engineer
Raoul Hafner designed and built a single-seat helicopter called the
R-2 Revoplane -- see Everett-Heath (1986) and Fey (1987). The flights
were mostly unsuccessful despite some brief tethered flights of up to
a minute. His early machines used a single-rotor configuration with a
pair of wings located in the rotor downwash to provide an antitorque
moment. For rotor control, Hafner's machine in notable in that it
used a swashplate for blade pitch, which was a very early application
of the final mechanism that was to become the standard means of
providing pitch control on the modern helicopter. Hafner later
emigrated to England, where he and Juan de la Cierva independently
continued work on blade articulation and rotor control for autogiros,
and later, for helicopters.
Back to Contents
Not Quite a
Helicopter
The Spanish
engineer Juan de la
Cierva had built and flown another type of
rotating-wing aircraft as early as 1923 -- see Juan de la Cierva
(1926, 1930). This aircraft looked a lot like a hybrid between a
fixed-wing airplane and a helicopter, with a set of conventional
wings and a tail but with a rotor mounted on a vertical shaft above
the fuselage. However, unlike a helicopter, this rotor was not
powered directly and was completely free to turn on the shaft. It was
found that when the rotor disk was inclined backward at a small angle
of attack and as the machine was pulled forward by a propeller, the
rotor was turned by the action of the airflow on the blades. This
aerodynamic phenomenon, called "autorotation," had been
understood by Crocco and Yur'ev before 1910, but the idea of pulling
the rotor horizontally through the air to generate lift was clearly
that of de la Cierva. Juan de la Cierva called his rotating-wing
aircraft an "Autogiro." The name Autogiro was registered by
de la Cierva as a proprietary name, but when spelled with a small "a"
it is used as a generic name for this class of rotorcraft. Today, we
might call this type of aircraft an "autogyro" or a
"gyroplane."
De la Cierva's C-1 Autogiro of 1923 was a
coaxial design, the airframe being from a converted wartime
fixed-wing aircraft. The problem of asymmetric lift (decribed
earlier) was well known to de la Cierva, and his first idea of using
a counter rotating coaxial design was that the lower rotor would
counteract the asymmetry of lift produced on the upper rotor, thereby
balancing the net rolling moment on the aircraft. However, the
aerodynamic interference produced between the rotors resulted in
different rotor speeds, spoiling the required aerodynamic roll
balance, and the aircraft crashed. Undeterred, De la Cierva conducted
basic wind tunnel experiments on model rotors at Quatro Vientos
airport near Madrid, and was one of the first to establish a
scientific understanding of their aerodynamic behavior.
De la Cierva built two more full-scale machines with single
rotors before he achieved final success in January 1923 with the
C-4. Based on his tests with small models, this fourth
machine incorporated blades with mechanical "flapping" hinges at the
root, which de a Cierva used as a means of equalizing the lift on the
two sides of the rotor in forward flight -- see de la Cierva &
Rose (1931). This novel solution to the problem of asymmetric
aerodynamic forces allowed the blades to flap up or down about these
hinges, responding to the changing airloads during each blade
revolution. Although the principle of flapping propeller blades had
actually been suggested by Charles Renard in 1904 and
the idea of flapping hinges was also patented by Louis Breguet
in 1908, Juan de la Cierva is credited with the first successful
practical application to an rotating-wing aircraft.
In all of de la Cierva's early Autogiros, the
engine drove only the propeller. Starting the rotor required a team
of helpers to pull a rope wound around the rotor shaft.
Alternatively, taxiing around on the ground could get the rotor
spinning. Thereafter, the pilot opened the throttle and the thrusting
propeller pulled the machine forward until it quickly lifted off into
flight. The freely spinning rotor, or "windmills" as de la Cierva
called them, turned relatively slowly in flight. His blades were of a
distinctive planform, with a constant chord outer portion and a well
rounded tip, tapering inboard to a spar. Metal ties with turnbuckles
located near the mid-span linked each of the blades together.
With the success of his Auotgiros, in 1925 Juan
de la Cierva was invited to Great Britain by the Weir Company.
His C-6 Autogiro was demonstrated at the RAE, and these flights
stimulated early theoretical work on rotating-wing aerodynamics at
the RAE, mainly by Glauert and Lock. Early
theoretical developments were also conducted at NACA by Munk. De la
Cierva himself was to write two books for the fledgling rotorcraft
industry, albeit formally unpublished, called "Engineering Theory of the
Autogiro" and "Theory of Stresses in Autogiro Rotor
Blades." In later models of his Autogiro,
de la Cierva added a lag hinge to the blades, which alleviated
stresses caused by in-plane Coriolis forces and completed the
development of the articulated rotor hub for autogiros. A control
stick was also connected to the rotor hub, which allowed the rotor
disk to be tilted for control purposes. De la Cierva called this the
"orientable direct rotor control." While this allowed the ailerons to
be dispensed with, the rudder and elevator on the machine were
retained. The Cierva Autogiro Company went on to build many more
versions of the Autogiro, including the highly successful
C-19, the development of which is described by de la Cierva
(1935).
Although de la Cierva's autogiro was still not a
direct-lift machine and could not hover, it required only minimal
forward airspeed to maintain flight. Juan de la Cierva proved that
his autogiros were very safe and essentially stall-proof, and because
of their low speed handling capability, they could be landed in
confined areas. Takeoffs required a short runway, but this problem
was solved with the advent of the so-called "jump" take-off technique.
In the jump take-off technique, the blades are set to flat pitch and
the rotor rpm is increased above the normal flight rpm using the
engine. This is followed by the rapid application of collective blade
pitch, while simultaneously declutching the rotor and thereby
avoiding any torque reaction on the fuselage. This technique lifts
the aircraft rapidly off the ground, powered only by the stored
kinetic energy in the rotor system. As forward speed builds, the
rotor settles into its normal autorotative state -- see also Prewitt
(1938). To achieve jump takeoffs, in 1935 Cierva introduced a pitch
change mechanism into the rotor design in later versions of the
C-30 model.
About the same time, Raul Hafner introduced the
"spider" cyclic-pitch control system to autogiros; see Fay (1987).
This provided a means of increasing collective pitch and also tilting
the rotor disk without tilting the rotor shaft with a control stick
as in de la Cierva's direct control system. Hafner used this design
in his third autogiro, the 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. Picairn 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.
Back to Contents
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 codesigner. 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 Oemichen, 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 under-powered, and it had to have a
hydrogen balloon attached to provide additional lift and stability.
Nevertheless, Oemichen went on to design a pure helicopter that was
flown between 1923 and 1924. By 1924, Oemichen 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 Oemichen (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,
counterrotating 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 Bleeker 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
Bleeker 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, Bleeker'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. Bleeker'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.
Back to Contents
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 possibilities 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.
Back to Contents
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 a
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.
Back to Contents
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
amatuer 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.
Back to Contents
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
remaunfacturing 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 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.
Back to Contents
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,
counterrotating 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.
Back to Contents
References
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See also Scientific
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Back to Contents
Text on all pages ©
J. G. Leishman 2000, with extracts from the
author's book "Principles
of Helicopter Aerodynamics"
© Cambridge University Press
2000.