Between the 1940s and the end of the 20^th century, the helicopter industry has benefited from technological innovations that have guided its evolution. Step by step, speed, payload, flight range and operating costs have been considerably improved. Here is an overview of the milestones that have marked these developments …
By François BLANC

Since the first helicopter to be certified by a civil aviation authority (the Bell 47B) was manufactured, many technological innovations have enabled this type of aircraft to improve its performance and respond to increasingly sophisticated demands. While the first official certification (March 1946) is considered as the starting point of the competition between helicopter manufacturers, it should be noted that the Bell 47 had already benefited from an important innovation that was previously implemented on both aircraft designed by Igor Sikorsky (the VS-300 prototype from 1939 and the serial produced R-4 military helicopter from 1941): the anti-torque tail rotor; technical solution necessary because of the mechanically driven single main rotor configuration.

Sikorsky, Piasecki, Focke, Flettner: the (re)inventors
 Starting from that moment, the rate of integration of innovative technological solutions, obtained from other industries or developed exclusively for helicopters, experienced a sort of golden age. Let’s take a closer look. The cyclic pitch control, for example, developed by Igor Sikorsky for its VS-300 prototype, can be regarded as a major invention, although other research engineers, between the two World Wars, had already developed similar systems. The first pontoons designed to give a helicopter amphibious capabilities were installed by Sikorsky on the same VS-300. The basic idea, since improved, is still used in the year 2011.
The result of the American engineer Frank Piasecki’s work, the first helicopter with two rotors in tandem deemed reliable, the PV-17 (HRP-2 for the military version), made its first flight on November 10, 1949. It had several descendants and is the ancestor of the current Boeing CH-47 Chinook. Before it, another helicopter with two main rotors (lateral and not longitudinal as on the PV-17) had almost been mass produced. This helicopter was called the FA-223 and was assembled in 1939 after promising trials conducted on the prototype Focke 61, also designed by German engineer Heinrich Focke (starting in 1934). The tandem dual rotor formula with wing-like pylons, had in its time, also given rise to an experimental aircraft in the United States: the Platt-LePage XR1A, which took its first flight on May 12, 1941 …
Helicopters with intermeshing main rotors, well known today because of the aircraft produced by the American manufacturer Kaman Aerospace Company, have a clearly identified ancestor that was mass produced during the Second World War: the Flettner 282, named after its inventor, German engineer Anton Flettner.
However, the post-war innovation that has taken the helicopter to its most dramatic phase of development was the integration of a gas turbine engine, replacing the piston engine.

Gas turbine engine and rigid rotors
In this domain history has mostly retained the name of Joseph Szydlowski, who founded the French company Turbomeca in 1938. Szydlowski, while conducting research in France managed by government manufacturers (notably including the Société Nationale des Constructions Aéronautiques du Sud-Est, or SNCASE, and the Société Nationale des Constructions Aéronautiques du Sud-Ouest, or SNCASO) supplied the first gas turbine engines designed for helicopters. After the prototype SO.1120 Ariel III, followed by the S0 1220 Djinn from SNCASO – helicopters with the main rotors being driven by compressed air jets at the end of each blade, exempting the aircraft from requiring an anti-torque tail rotor – the SE.3130 Alouette II, from SNCASE, was the world’s first mass produced turbine engine helicopter. Starting with this crucial technological advancement, the innovations that directly affected helicopters, improving engine efficiency as well as aerodynamics, strength and material weight, in general, have increased aircraft performance.

Research on the rotor, for example, led the U.S. firm Lockheed Aircraft Corporation, in 1959, to work on the rigid rotor concept. Thus, the prototype CL-475 was quickly replaced by a more refined aircraft, named the CL-595 by the manufacturer (XH-51 for the military version) which first flew on November 2, 1962. This helicopter was the first to perform barrel rolls and loops; maneuvers that were previously reserved for airplanes. During its trials, it also reached a speed of 320 km/h (172 kt).

Djinn – SO1220/1221 ©Eurocopter

Glass fiber, Fenestron and NOTAR
Also in the domain of rotors, SNCASE and the German company Messerschmitt-Bölkow-Blohm (MBB) started collaborating in 1966 on research that gave rise to the helicopter BO.105; a helicopter equipped with a titanium rotor head and four glass fiber

reinforced blades. Glass fiber was also used during the same period in the manufacturing of the three-bladed rotor for the prototype SA.340 Gazelle (first flight April 7, 1967); the first French helicopter to take advantage of this material. The second prototype, which took off for the first time on April 12, 1968, was also the world’s first to be equipped with a shrouded tail rotor with thirteen blades creating something completely new: the Fenestron. This type of helicopter was mass produced under the name SA.341 and SA.342.
In 1975, the American firm Hughes Aircraft launched the development of an anti-torque system that would not need a tail rotor: the NOTAR (for No Tail Rotor). Developed from an idea having already been experimented with at the end of the Second World War by the British company Cierva, then in Spain with Aerotechnica on the AC-13, the NOTAR was installed on the military helicopter Hughes 500 (OH-6A) in 1981. Acquired in 1984 by McDonnell Douglas, work on NOTAR technology continued and a better designed helicopter equipped with a NOTAR system flew in 1986. Today, the company MD Helicopters has three models equipped with this system: MD500N, MD600N and MD Explorer.
Increasingly, most of the helicopter manufacturers that drive global markets propose or have already integrated advanced materials. Over the last decade, their efforts have been focused on reducing vibrations, reducing fuel consumption, decreasing noise pollution and facilitating navigation for the crews. The current research is oriented towards reducing weight, saving fuel and lowering decibels while increasing reliability and efficiency. These qualities are expected to make real differences on the civil and military markets throughout the world.