Key Technical Problems in Attaining Vertical Flight

There are many authoritative sources that record the development of helicopters and other rotating-wing aircraft such as autogiros. These 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), among others. Boulet (1984) takes a unique approach in that he-gives a first-hand account of the early helicopter developments though interviews with the pioneers, constructors, and pilots of the machines. A remarkably detailed history of early helicopter developments is given by Liberatore (1950, 1988, 1998). For original publications documenting early technical developments of the autogiro and helicopter, see Warner (1920), von Karman (1921), Balaban (1923), Moreno-Caracciolo (1923), Klemin (1925), Wimperis (1926), and Seiferth (1927).

As Liberatore (1998) described, the early work on the development of the helicopter can be placed into two distinct categories: inventive and scientific. The former is one where intu­ition is used in lieu of formal technical training, and the latter is one where a trained, system­atic approach is used. Prior to the nineteenth century there were few scientific investigations of flight or the science of aerodynamics. The inherent mechanical and aerodynamic com­plexities in building a practical helicopter that had adequate power and control and did not vibrate itself to pieces, resisted many ambitious efforts. The history of flight documents literally hundreds of failed helicopter projects, which at most made only brief uncontrolled hops into the air. Clearly, some designs provided a contribution to new knowledge that ultimately led to the successful development of the modem helicopter. Yet, it was not until the more scientific contributions of engineers such as Juan de la Cierva, Henrich Focke, Raoul Hafner, Harold Pitcairn, Igor Sikorsky, Arthur Young, and others did the design of a truly safe and practical helicopter become a reality.

Seven fundamental technical problems can be identified that limited early experiments with helicopters. These problems are described by Sikorsky (1938 and various editions) and in many other sources. In summary, these problems were: [2] [3]

combustion engines with sufficient power-to-weight ratios suitable for use on a helicopter did not occur until the 1920s.

3. Keeping structural weight and engine weight down so the machine could lift a pilot and a payload. Early power plants were made of cast iron and were heavy. Aluminum was not available commercially until about 1890 and was inordinately expensive, it not being used as a construction material for airframes and aircraft engines until about 1915.

4. Counteracting rotor-torque reaction. The relatively simple idea of a tail rotor to counter torque reaction was not used on most early helicopter designs, these machines were either coaxial or side-by-side rotor configurations. Yet, building and controlling two or more primary lifting rotors was even more difficult than controlling one rotor, a fact that seemed to evade many inventors and constructors.

5. Providing stability and properly controlling the machine, including 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, ideas that were pioneered by Cierva, Breguet, and others, and with the development of practical forms of cyclic blade pitch control by Hafner and others.

6. Conquering the problem of vibrations. This was a source of many mechanical failures of the rotor and airframe because of an insufficient understanding of the dynamic and aerodynamic behavior of rotating wings. It was to be many years before such problems could be reduced to the thresholds where the helicopter was to become as reliable as a fixed-wing aircraft.

7. The capability to recover safely to the ground in the event of engine failure (i. e., a “gliding55 or autorotational requirement – see page 28). It is fair to say that this capability is critical to the success of any practical helicopter or other type of rotorcraft because it would simply not be accepted otherwise.

The relatively high weight of the structure, engine, and transmission was mainly respon­sible for the painfully slow initial development of the helicopter. In particular, the success of the helicopter had to wait until aircraft engine technology could be refined to the point that lightweight engines with considerable power could be built. By 1920, gasoline-powered reciprocating engines with higher power-to-weight ratios were more widely available and the anti-torque and control problems of achieving successful vertical flight were at the fore­front. This resulted in the development of a vast number of prototype helicopters. Many of the early designs were built in Britain, France, Germany, and Italy, which 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-WW2 years, it was not until the late interwar period that significant technical advances were made and more practical

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