Helocopter

Introduction

aircraft with one or more power-driven horizontal propellers or rotors that enable it to take off and land vertically, to move in any direction, or to remain stationary in the air. Other vertical-flight craft include autogiros, convertiplanes, and V/STOL aircraft of a number of configurations.

The idea of taking off vertically, making the transition to horizontal flight to the destination, and landing vertically has been for centuries the dream of inventors. It is the most logical form of flight, dispensing as it does with large landing fields located far from city centres and the inevitable intervening modes of travel—automobile, subway, bus—that flight in conventional aircraft usually requires. But vertical flight is also the most demanding challenge in flying, requiring more sophistication in structure, power, and control than conventional fixed-wing aircraft. These difficulties, solved over time by determined engineers and inventors, made the progress of vertical flight seem slow compared to that of conventional flight, for the first useful helicopters did not appear until the early 1940s.

 

History

One important characteristic of the history of vertical flight is the pervasive human interest in the subject; inventors in many countries took up the challenge over the years, achieving varying degrees of success. The history of vertical flight began at least as early as about AD 400; there are historical references to a Chinese kite that used a rotary wing as a source of lift. Toys using the principle of the helicopter—a rotary blade turned by the pull of a string—were known during the Middle Ages. During the latter part of the 15th century, Leonardo da Vinci made drawings of a helicopter that used a spiral airscrew to obtain lift. A toy helicopter, using rotors made out of the feathers of birds, was presented to the French Academy of Science in 1784 by two artisans, Launoy and Bienvenu; this toy forecast a more successful model created in 1870 by Alphonse Pénaud in France.
The first scientific exposition of the principles that ultimately led to the successful helicopter came in 1843 from Sir George Cayley, who is also regarded by many as the father of fixed-wing flight. From that point on, a veritable gene pool of helicopter ideas was spawned by numerous inventors, almost entirely in model or sketch form. Many were technical dead ends, but others contributed a portion of the ultimate solution. In 1907 there were two significant steps forward. On September 29, the Breguet brothers, Louis and Jacques, under the guidance of the physiologist and aviation pioneer Charles Richet made a short flight in their Gyroplane No. 1, powered by a 45-horsepower engine. The Gyroplane had a spiderweb-like frame and four sets of rotors. The piloted aircraft lifted from the ground to a height of about two feet, but it was tethered and not under any control. Breguet went on to become a famous name in French aviation, and in time Louis returned to successful work in helicopters. Later, in November, their countryman Paul Cornu, who was a bicycle maker like the Wright brothers, attained a free flight of about 20 seconds' duration, reaching a height of one foot in a twin-rotor craft powered by a 24-horsepower engine. Another man who, like the Breguets, would flirt with the helicopter, go on to make his name with fixed-wing aircraft, and then later return to the challenge of vertical flight, was Igor Sikorsky, who made some unsuccessful experiments at about the same time.
The next 25 years were characterized by two main trends in vertical flight. One was the wide spread of minor successes with helicopters; the second was the appearance and apparent success of the autogiro (also spelled autogyro).
The helicopter saw incremental success in many countries, and the following short review will highlight only those whose contributions were ultimately found in successfully developed helicopters. In 1912 the Danish inventor Jacob Ellehammer made short hops in a helicopter that featured contrarotating rotors and cyclic pitch control, the latter an important insight into the problem of control. On Dec. 18, 1922, a complex helicopter designed by George de Bothezat for the U.S. Army Air Force lifted off the ground for slightly less than two minutes, under minimum control. In France on May 4, 1924, Étienne Oehmichen established a distance record for helicopters by flying a circle of a kilometre's length.
In Spain in the previous year, on Jan. 9, 1923, Juan de la Cierva made the first successful flight of an autogiro. An autogiro operates on a different principle than a helicopter. Its rotor is not powered but obtains lift by its mechanical rotation as the autogiro moves forward through the air. It has the advantage of a relatively short takeoff and a near vertical descent, and the subsequent success of Cierva's autogiros and those of his competitors seemed to cast a pall on the future of helicopter development. Autogiros were rapidly improved and were manufactured in several countries, seeming to fill such a useful niche that they temporarily overshadowed the helicopter. Ironically, however, the technology of the rotor head and rotor blade developed for the autogiro contributed importantly to the development of the successful helicopter, which in time made the autogiro obsolete.
In 1936 Germany stepped to the forefront of helicopter development with the Focke Achgelis Fa 61, which had two three-bladed rotors mounted on outriggers and powered by a 160-horsepower radial engine. The Fa 61 had controllable cyclic pitch and set numerous records, including, in 1938, an altitude flight of 11,243 feet and a cross-country flight of 143 miles. In 1938 the German aviator Hanna Reitsch became the world's first female helicopter pilot by flying the Fa 61 inside the Deutschland-Halle in Berlin. It was both a technical and a propaganda triumph. Germany continued its helicopter development during World War II and was the first to place a helicopter, the Flettner Kolibri, into mass production.
In the United States, after many successes with commercial flying boats, Igor Sikorsky turned his attention to helicopters once again, and after a long period of development he made a successful series of test flights of his VS-300 in 1939–41. Essentially a test aircraft designed for easy and rapid modification, the VS-300 was small (weighing 1,092 pounds) and was powered by a 65-horsepower Lycoming engine. Yet it possessed the features that characterize most modern helicopters: a single main three-bladed rotor, with collective pitch, and a tail rotor. As successful as the VS-300 was, however, it also clearly showed the difficulties that all subsequent helicopters would experience in the development process. For many years, compared with conventional aircraft, helicopters were underpowered, difficult to control, and subject to much higher dynamic stresses that caused material and equipment failures. Yet the VS-300 led to a long line of Sikorsky helicopters, and it influenced their development in a number of countries, including France, England, Germany, and Japan.
After World War II the commercial use of helicopters developed rapidly in many roles, including fire fighting, police work, agricultural crop spraying, mosquito control, medical evacuation, and carrying mail and passengers.
The expanding market brought additional competitors into the field, each with different approaches to the problem of vertical flight. The Bell Aircraft Corporation, under the leadership of Arthur Young, began its long, distinguished history of vertical-flight aircraft with a series of prototypes that led to the Bell Model 47, one of the most significant helicopters of all time, incorporating an articulated, gyro-stabilized, two-blade rotor. Frank Piasecki created the Piasecki Helicopter Corporation; its designs featured a tandem-rotor concept. The use of twin tandem rotors enabled helicopters to grow to almost twice their previous size without the difficulty of creating very large rotor blades. In addition, the placement of the twin rotors provided a large centre of gravity range. The competition was international, with rapid progress made in the Soviet Union, the United Kingdom, France, Italy, and elsewhere.
To an even greater extent than fixed-wing aircraft, the development of the helicopter had been limited by engine power. Reciprocating engines were heavy, noisy, and less efficient at high altitude. The first application of jet-engine technology to the helicopter was accomplished in 1951 by the Kaman Aircraft Corporation's HTK-1, which had Kaman's patented aerodynamic servo-controlled rotors in the “synchropter” configuration (i.e., side-by-side rotors with intermeshing paths of blade travel).
In conventional aircraft the power of the jet engine was used primarily for increased speed. In the helicopter the thrust of the jet turbine had to be captured by a gearbox that would turn the rotor. The jet engine had many advantages for the helicopter—it was smaller, weighed less than a piston engine of comparable power, had far less vibration, and used less expensive fuel. The French SNCA-S.E. 3130 Alouette II made its first flight on March 12, 1955, powered by a Turbomeca Artouste II turbine engine. It rapidly became one of the most influential helicopters in the world and started a trend toward jet-powered helicopters everywhere.
There are now a vast number of helicopter types available on the market, ranging from small two-person private helicopters through large passenger-carrying types to work vehicles capable of carrying huge loads to remote places. All of them respond to the basic principles of flight, but, because of the unique nature of the helicopter's rotor and control systems, the techniques for flying them differ. There are other types of vertical-lift aircraft, whose controls and techniques are often a blend of the conventional aircraft and the helicopter. They form a small part of the total picture of flight but are of growing importance.