Autogiros and Gyroplanes[44]

The outstanding inspiration of the Autogiro’s story was the recognition that there might be more than one type of flying machine, soundly mechanical but basically independent of conventional theory and practice.

Juan de la Cierva (1931)

12.1 Introduction

An autogiro or gyroplane has a rotor that can turn freely on a vertical shaft. However, unlike a helicopter, its rotor is not powered directly. Instead, the rotor disk inclines backward at a positive angle of attack, and, as the machine moves forward in level flight powered by a propeller, the resultant aerodynamic forces on the rotor blades causes the necessary torque to spin the rotor and produce lift. The natural self-rotation of the rotor is called autorotation, a phenomenon introduced in Chapter 2. The autorotating rotor provides the lift for the machine (perhaps also control), with forward propulsion being provided by a conventional tractor or pusher propeller arrangement — see Fig. 12.1. Compare this to the helicopter where the rotor provides both lift and propulsion, as well as control. If the machine also has a fixed wing, or has the capability to hover (using tip jets), but is still flown as an autogiro in forward flight, then the aircraft may be referred to as a gyroplane or perhaps even as a convertiplane or gyrodyne. The names autogiro, autogyro, giroplane, and gyroplane are, however, often used synonymously.

In 1923, the autogiro was the very first type of rotating-wing aircraft to fly successfully and thus to demonstrate a useful and practical role in aviation, pre-dating the first successful flights of helicopters by about fifteen years. The autogiro was also the first powered, heavier – than-air aircraft to fly successfully other than a conventional airplane. Conceived by Juan de la Cierva about 1920 [see Cierva & Rose (1931), Gablehouse (1967), C. A. Cierva (1998), and Chamov (2003)] over thirty different autogiro designs were produced in just fifteen years of development (1923-1938). Cierva proved that his Autogiros1 were very safe and because of their low flight speed capability they could be landed in confined areas. Takeoffs required a short runway to buildup airspeed, but this was rectified later with the advent of a prerotator for the rotor and the development of the “jump” and “towering” takeoff techniques. This gave the autogiro a capability that was comparable to that of the future helicopter in terms of overall performance. However, the autogiro often seems to be a half- forgotten machine that occupies a lower place in the list of aviation firsts. It has perhaps been viewed by many only as an interim step toward the helicopter, and a temporary makeshift

Figure 12.1 The autogiro rotor (a) provides lift to sustain flight, with forward propulsion being provided by a conventional propeller, compared to the helicopter (b), where the rotor provides both lift and propulsion.

at that. As discussed in Chapter 1, however, the autogiro played a fundamental role in the technological development of all types of modem rotating-wing aircraft. There are also several interesting technical aspects of autorotative flight that make this aircraft unique when compared to a helicopter. It is for this reason that a separate chapter on autogiros is an appropriate part of this book.

Autorotation can been seen naturally in the flight of a variety of seeds, such as maple or sycamore – see Azuma & Yasuda (1989) for an aerodynamic discussion. However, the somewhat curious aerodynamic phenomenon of “autorotating bodies” had been observed in variety of scientific experiments by the beginning of the twentieth-century, which probably were inspired by earlier theoretical work by the Scottish physicist James Maxwell – see Tokaty (1971). The Italian Gaetano Crocco and the Russian Boris Yuriev had examined the principle of autorotation on spinning rotors around 1910. Munk (1925) conducted experiments with “helicopter propellers,” where the phenomenon of autorotation was again demonstrated.

However, the Russian scientist Yuriev and his students probably made the most significant studies, showing that under some conditions of steeply descending and horizontal flight with the rotor at a positive angle of attack, a lifting rotor would actually turn of its own accord. Yuriev called this phenomenon “rotor gliding,” and he apparently realized, even then, that the ability of the rotor to self-rotate might be used to bring a helicopter safely to the ground in the event of an engine failure. The ability to autorotate in an emergency condition (such as after a power or tail rotor failure) is a fundamental safety of flight capability that must be designed into all helicopters (see Chapters 2 and 5).

Chapter 1 described the numerous types of primitive human-carrying helicopters pro­posed and built from 1900 to 1920. Yet nobody had previously considered the idea that a successful rotating-wing aircraft could be built such that the rotor was unpowered and always operated in the autorotative state during its normal flight. In the spring of 1920 Juan de la Cierva of Spain built small, unpowered free-flying models of a rotating wing

aircraft, with the rotor free to spin on its vertical shaft. The model had a rotor with five blades, with horizontal and vertical tails to give it stability. Cierva launched the model and the rotor spun freely of its own accord as the model glided softly to the ground. Cierva had rediscovered the principle of autorotation, which at first he was to call “autogiration.” These first experiments with models were to pave the way for the design of a completely new type of rotating-wing aircraft that Cierva was to ultimately call an Autogiro.

Juan de la Cierva had become interested in aviation as early as 1908 when the Wright Brothers demonstrated their Flyer machine in Europe. Cierva was subsequently to build the first Spanish airplane in 1912. His third airplane of 1919 was a large three-engined bomber. While the aircraft flew well, the test pilot became over ambitious and the machine stalled and crashed. This tragedy motivated Cierva to think of a way of improving the flight safety of an aircraft when it operated at low airspeeds and, in particular, when it was flying close to the ground. Cierva set out to design a safe flying machine that ensured “stability, uplift, and control should remain independent from forward speed” and suggested further that it should be one that could be flown by a pilot with average skill [see Cierva & Rose (1931)]. Cierva went on to point out that “the wings of such an aircraft should be moving in relation to the fuselage. The only mechanism able to satisfy this requirement is a circular motion and, moreover, in order to give adequate security to the aforementioned requirement it must be independent of the engine. It was thus necessary that these rotary wings were free-spinning and unpowered.”

Thus was borne the vision of an autogiro. Juan de la Cierva was not the first to suggest or observe the phenomenon of autorotation, but he was certainly the first to understand better the aerodynamic principles and to put the phenomenon toward serving a useful purpose. He was to make some of the first theoretical studies on rotors and conducted a series of wind tunnel tests [see Cierva & Rose (1931)] as he writes: “with valuable results, among them the determination of the fact that the rotor would continue to turn at every possible angle of flight – a point that was somewhat disputed by critics of my earlier experiments.”