Cierva C.1 Autogiro and Early Development

The loss of an airplane led Juan de la Cierva to seek a completely new way to fly. The main goal was to eliminate the danger of an aircraft losing speed. To do this, he decided to abandon the principle of lift generation by a fixed wing and began to search for a solution through other means. Initially, he looked into ornithopters and helicopters.

The principles of ornithopter and helicopter flight had been known since ancient times, but in practice, no one had managed to build a normally flying apparatus, although, of course, attempts were made. An ornithopter is a heavier-than-air flying machine that obtains lift through flapping wings. The idea of a helicopter, on the other hand, is based on obtaining lift through the rotation of a propeller on a vertical axis, driven by an engine.

Many developed this idea, but a practical machine was still far off. The first helicopter to lift off with a person aboard was built by the Bréguet brothers and Professor Richet on September 29, 1907, in Douai. The lift, however, was conditional, with four people holding poles to maintain stability. The first “free” lift was achieved by Paul Cornu on November 13 of the same year, rising several meters in his twin-rotor machine without any ground support.

After World War I, work in the field of helicopters was conducted in almost all industrialized countries. Names like Pescara (Spain), Botezat and Bleeker (USA), Baumhauer (Holland), d’Ascanio (Italy), Emichen and Bréguet (France), Focke and Flettner (Germany) can be mentioned. The results of early helicopter work in one country were generally little known in other countries, and often the development of new ideas in this field proceeded in parallel and independently.

In Russia, a number of interesting projects were also carried out. As early as 1907, military engineer K.A. Antonov began designing a helicopter, which was built three years later. Its two main rotors consisted of individual aluminum triangular plates (blades) that could pivot around their longitudinal axes. A small propeller was intended by the author to create horizontal thrust. The aircraft was powered by a 35 hp engine, but tests did not yield positive results.

The helicopter built in 1909 by I.I. Sikorsky, a student at the Kyiv Polytechnic Institute, had two coaxial two-bladed rotors driven by a 25 hp engine (which was heavily worn). By spring 1910, Sikorsky built a second helicopter. It had two three-bladed rotors and a new 25 hp engine. The aircraft weighed 180 kg. The designer did not complete his work, at that time focusing his efforts on creating airplanes.

In 1911, B.N. Yuryev, a student at the Moscow Technical School (later an academician), developed an original helicopter design. A 70 hp Gnome engine was placed in its central part. The machine had a main rotor and a tail rotor. To ensure helicopter controllability in flight, a swashplate mechanism was provided, which allowed changing the direction of the main rotor’s thrust, and consequently, the direction of flight.

In late 1911, Yuryev developed a second version (built only in early 1912)—a single-rotor helicopter with an 8-meter diameter two-bladed main rotor. The reactive torque of the main rotor was compensated by a tail rotor installed at the end of the tail truss. Both rotors were driven by a 32 hp engine through a transmission. In spring 1911, this helicopter was demonstrated at the Second International Aeronautical Exhibition in Moscow. The designer was awarded a gold medal for developing fundamentally new structural elements for the helicopter. For a number of reasons, this apparatus could not take to the air, but its scheme and swashplate give full right to consider it a progenitor of modern helicopters.

In the early 1920s, helicopters rose to a height of several meters and performed flights over a distance of several hundred meters (Emichen’s world record for straight-line helicopter flight on April 17, 1924, was 736 m). The reasons for such slow improvement in flight characteristics were strong vibrations, poor controllability and stability, and the impossibility of gliding upon engine shutdown. Designers persistently sought ways to solve these problems, working to eliminate these shortcomings, but no one could offer anything revolutionary or fundamentally new yet.

Alongside attempts to create a normally flying helicopter, research work was conducted, including in aerodynamic institutes, among which Nikolai Yegorovich Zhukovsky noted three of primary importance in 1910: the Roman, the TsAGI Institute in the USA, and the Aerodynamic Institute in Kuchino, established in 1904 and the first such institution in Russia. The creation of this institute and the initial period of its activity are associated with the name of N. E. Zhukovsky. The scientific results obtained at the Kuchino Institute and published in its “Bulletins” received positive evaluations not only in Russia but also abroad, thanks to their publication in French.

Cierva’s Breakthrough: The Autogiro Concept

Now we will make a small digression that may have a direct bearing on de la Cierva’s invention. D.P. Ryabushinsky, founder and director of the Kuchino Institute, writes in a brochure dedicated to the institute’s tenth anniversary: “The main results I obtained at the Aerodynamic Institute are as follows: I systematically investigated propellers and found the general laws of this remarkable mechanism’s functioning. In these studies, I did not limit myself to studying the field of propellers, but also captured the field of wind turbines and the field in which propellers throw fluid in the direction of motion – this allowed me to obtain a very complete picture of the phenomenon.”

“I expressed the obtained results using several simple formulas that are currently used in practice. I indicated the foundations of the vortex theory of the propeller and a graphical method in which the ratios of zero size are directly plotted along the coordinate axes. I also studied experimentally and theoretically propellers that operate stationary and with side wind; I found and explained several curious phenomena of great theoretical interest, such as the self-rotation of thin plates.”

We will be lenient with the author’s style and his personalization of all institute work. We only note that the results of the study of air propellers and the self-rotation of a symmetrical plate were published in 1909 in the “Bulletin” of the Kuchino Institute in French. Given the fact that La Cierva knew French well and at that time read everything about aviation that he could get his hands on, it is not so unlikely that he could have become familiar with the “Bulletin.” In this case, the seed could have fallen on long-prepared ground, the spark could have ignited the fire of thought.

After analyzing mankind’s accumulated experience in studying the problems of ornithopters and helicopters, La Cierva considered them quite complex in mechanical and aerodynamic terms. He concluded that the safety problem could be largely solved if a system of wings could be created that moved relative to the apparatus, being structurally connected to it, and furthermore, maintained this relative movement without the need to use an engine.

According to some of La Cierva’s biographers, and in particular José Varleta, the initial idea came from a children’s toy – the so-called “Chinese whirligig.” Observing the smooth descents of the “whirligig,” La Cierva came to the idea that there are regimes in which a propeller can self-rotate (autorotate). In practice, the problem was that the resultant lift force of each wing of the conceived system would simultaneously induce its movement relative to the apparatus.

La Cierva studied various options and settled on a system of wings that would rotate around an almost vertical axis. He called it a rotor. The rotor was intended to perform the same functions as a helicopter’s main rotor, but unlike the latter, the aircraft’s rotor was not engine-driven; it freely rotated under the action of the incoming airflow – it autorotated. To maintain autorotation, the incoming flow must approach the disc with some upward component, meaning the disc must always have a positive angle of attack relative to the incoming flow. For forward motion, the aircraft must have an engine with a pulling (or pushing) propeller, like a conventional airplane. In the incoming flow, the rotor is capable of maintaining autorotation at angles of attack up to 90°, i.e., during a vertical descent of the apparatus.

Helicopter designer Pescara had patented a self-rotating rotor several years earlier, but with a negative blade pitch, i.e., like a conventional windmill. The peculiarity of La Cierva’s discovery was that a rotor with a positive blade pitch of a few degrees, once spun up, continued to rotate in the incoming flow during the aircraft’s forward motion and created lift. The incoming flow induced the rotor to rotate, but unlike a windmill, the rotor rotated in a different direction. La Cierva discovered the autorotation effect when he was 25 years old. He immediately patented it and put in maximum effort to use this effect in practice.

La Cierva initially called the system of self-rotating wings “autogyroptero” (auto – self, gyro – rotation), then shortened this word to “autogyro” because it seemed to him “too Greek, too long, and too heavy.” The word “autogyro” came to us from French (autogire).

Cierva C.1 and Early Prototypes (C.2, C.3)

Having decided to apply the principle of autorotation to an aircraft, La Cierva, like all his predecessors who tried to create a helicopter, encountered problems of rotor asymmetry in horizontal flight. It is known that a blade moving forward into the incoming flow, at the same angle of attack, creates greater lift than one moving backward, which causes an overturning moment in the transverse plane towards the advancing blade. To avoid asymmetry, La Cierva adopted the then-common scheme of two coaxial rotors rotating in opposite directions.

On July 1, 1920, La Cierva filed an application for a patent titled “New Aviation Apparatus.” Patent No. 74322 was issued on August 27, 1920. It covered the basic principles of a freely rotating rotor for lift generation. While the draft for this patent contained the word “autogyro,” it was omitted in the final version. After several successful autogiro flights, La Cierva submitted a request on February 10, 1923, to register the word “autogyro”—”to distinguish aircraft with rotating wings.” Certificate No. 49038 was issued on November 7, 1923.

For the first autogiro, named C-1, the fuselage from an old “Deperdussin” monoplane was used, which La Cierva obtained from a civil pilot school in Getafe. A 60 hp Le Rhône rotary engine was installed on it. The wings were removed, and two four-bladed rotors were mounted above the fuselage, their blades having a “rigid” attachment in the hub. The diameter of the rotors was 6 m. The upper one was intended to rotate clockwise, when viewed from above, and the lower one in the opposite direction. A vertical plane, located above the rotors, which could pivot left and right and provide a moment relative to the longitudinal axis of symmetry during forward motion, was to perform the role of ailerons. The tail was left in its original form. The apparatus weighed 350 kg.

The C-1 was ready for testing in October 1920. The apparatus had such an unusual appearance that only a sky fanatic, eager to fly anything, or a relative saving family honor from ridicule, could undertake its testing. Captain Felipe Gómez-Acebo was both. He was Juan’s brother-in-law and a few days before the tests received the rank of initial training pilot. And subsequently, La Cierva – a fanatic of rotary-wing aviation – was lucky with such dedicated aviation people. The tests took place at the Madrid airfield of Getafe. The flashing blades of the spinning rotors created the impression of a street fight, for which the airfield wits nicknamed the apparatus “la bronca” (“the scuffle”).

After the initial spin-up of the rotor, Gómez-Acebo taxied across the field and began his takeoff run. Both rotors entered autorotation in the expected directions, but the lower one rotated much slower than the upper. The inventor had foreseen this effect, but did not assume that the speed of the lower rotor would be almost a third less than calculated. Thus, the total lift force on the left side of the rotors was greater than on the right and caused a tendency to tip over. Due to an engine breakdown, only one test was conducted, but it already provided an opportunity to draw at least two conclusions: 1) autorotation is a fact; 2) a new solution is required to compensate for the displacement of the aerodynamic resultant.

Of course, La Cierva could have achieved equal rotor speeds by using a gear drive, but he rejected this idea, believing that the system would be cumbersome. Furthermore, the cyclic change in aerodynamic forces applied to the highly extended upper rotor would, in La Cierva’s opinion, require significant structural reinforcement and consequently increase its weight. Thus, he concluded that it was more expedient to use a single rotor.

Using the experience gained, Juan de la Cierva, with his inherent strong consistency, set about solving the problem of creating a rotor with a centered aerodynamic resultant, which he called a “compensated” rotor. To obtain such a resultant, the blade moving backward with the flow had to operate at a greater angle of attack than the one moving forward. Since the lift force of each blade section is directly dependent on the square of the incoming flow velocity and the angle of attack, La Cierva believed that these two factors could have such a combination that would yield an aerodynamic resultant approximately centered with the rotor axis, i.e., the theory of the “compensated” rotor was based on precisely maintaining the calculated distribution of the angle of attack along the blade length.

To improve rotor efficiency, the inventor conducted thorough theoretical research to determine the blade shape, its profile, and the angles of attack of various sections, the combination of which would yield the desired result. Thus, a blade with a symmetrical profile and a strong negative twist appeared. The blade pitch could be controlled from the cockpit, i.e., to decrease the pitch of the blade moving against the incoming flow and increase it for the one moving with the flow. With such a blade (which, according to the inventor’s design, was to be warped in flight using cables), La Cierva expected to obtain a rotor whose aerodynamic resultant would be in the aircraft’s plane of symmetry for all operating angles of attack. Moreover, this system would allow the aircraft to be controlled in the transverse plane. The idea of warping blades or wings was not new. On the first monoplanes (e.g., Blériot), ailerons were absent, and lateral control was achieved by cable warping of the thin wing, which allowed changing the angles of attack. Thus, to decrease the angle of attack, the leading edge of the wingtip was lowered, and to increase it, the trailing edge was raised.

To build the autogiro, it was necessary to create a manufacturing base. To this end, on November 18, 1920, Juan de la Cierva, his brother Ricardo, and carpenter Pablo Díaz signed a contract, forming a community that owned a carpentry workshop for building “automobiles, airplanes, and spare parts.” Soon, the community began building the C-2.

On the advice of friends, La Cierva decided to publicize his invention. To do this, on March 5, 1921, he sent a message to the Royal Academy of Exact Sciences, Physics, and Natural Sciences, as well as to the Royal Aero Club, in which he generally described the main principles of the autogiro and the results of the C-1 test. At the same time, he reported that he was building a new apparatus with only one rotor. His message to the Royal Aero Club was published in the journal “Heraldo deportivo” on April 5, 1921, and in the official bulletin of the Royal Aero Club for the same month (Boletín oficial del estado del real aero club de España).

In March 1921, La Cierva built and tested a flying autogiro model. The model had a five-bladed rotor, and the pulling propeller was driven by a rubber motor. La Cierva invited representatives of the Royal Academy of Sciences and the technical commission of the Royal Aero Club to the demonstration of the autogiro model, among whom was the head of the aerodynamic laboratory at Cuatro Vientos, FAI sports commissioner Emilio Herrera; it was he who would later register the first achievements of the new aircraft. In the presence of distinguished guests, La Cierva wound the rubber band with the pulling propeller, spun the model’s rotor in the desired direction, and placed it on the ground. As soon as he released the propeller, the autogiro ran and took off, gained several meters in height, and then, as its speed decreased, began to descend slowly, demonstrating stable flight. It was evident that the loss of forward speed caused only an insignificant loss of altitude. The blades continued to rotate, and the model did not fall but descended smoothly.

The model covered a distance of several tens of meters, and its flight made a great impression on those present. Everyone agreed that the autogiro would enter the arena of practical use sooner than the helicopter. When Herrera was asked to comment on the flight, he replied that he previously thought someone would crash first in Pescara’s helicopter, but now he was sure that it would happen in an autogiro. In such an unusual form, one of Spain’s most authoritative aviation engineers expressed the idea that the autogiro would take its place in practical life sooner than the helicopter. In one thing Herrera was wrong: the autogiro proved to be such a reliable aircraft that during 10 years of mainly experimental flights, there was not a single fatal accident.

On March 28, 1921, La Cierva submitted an application for an addition to his patent No. 74322. This application covered three of his inventions: the “compensated” rotor; rotor spin-up at the start of autogiro movement by means of a cord wound on a drum on the rotor axis and attached at the other end to the ground; and lateral control by differential deflection of the elevator halves, which could simultaneously function as both an elevator and as surfaces for lateral control. Drawings of the C-2 in three projections were attached to the application. An additional certificate to patent No. 74322 was issued on April 20, 1921, as No. 77569.

Construction proceeded slowly. Financial constraints prevented the inventor from speeding it up. In addition, the supply of duralumin for the blade spars, ordered from France, was delayed. La Cierva, impatient to test his theory of the “compensated” rotor, decided to build another apparatus earlier, simpler and quicker to manufacture. This resulted in the C-3, which was to serve to test the idea of the “compensated” rotor and, in addition, an original lateral control scheme. In the chronological series of built and tested autogiros, the C-3 appeared second, and the C-2, which was laid down earlier but finished later, turned out to be the third. These autogiros are sometimes confused in literature.

The C-3 was fitted with a three-bladed rotor with a simple device for twisting the blades in flight. The blade had a symmetrical profile and was constructed like a conventional single-spar airplane wing. Unlike previous and all subsequent autogiros, the C-3 had wide blades, which facilitated their warping. The fuselage was borrowed from a “Sommer” monoplane. The engine was initially a 50 hp Gnome, and then it was replaced by an 80 hp Le Rhône. Trials began in June 1921. The test pilot was José Rodríguez, who had recently received his pilot’s license and was naturally eager to fly.

During the tests, as the autogiro accelerated for takeoff, the rotor sped up its rotation, but when the autogiro reached takeoff speed, the machine would jump and fall to the right. Every attempt to take off was accompanied by breakages, and tests proceeded intermittently. The last tests were carried out by Alejandro Gómez Spencer (who would later make the first flight in an autogiro) and yielded the same results: the apparatus, during its run, would fall to the right immediately after its wheels left the ground.

Despite the failures, La Cierva systematically continued his work. In total, he modified the C-3 nine times. Eventually, the designer concluded that the cause of rotor decompensation was the lack of blade rigidity necessary for lateral control. Due to the blades’ elasticity, aerodynamic forces created undesirable twisting. By late 1921, La Cierva was already captivated by a new idea, the implementation of which, he believed, would avoid the asymmetry of the resultant lift force. The C-2 was already almost ready, and the C-3 trials were stopped.

The C-2 rotor, with a diameter of 11.5 m, was five-bladed. Each blade had two spars, wooden ribs, and was covered with fabric. To maintain a constant angle of incidence, the blades were rigidly braced: La Cierva hoped that with their help, he would obtain a resultant lift force from the rotor passing through its axis. The C-2 fuselage (unlike the C-1 and C-3) was custom-made for this apparatus. The tail assembly had peculiarities – both halves of the elevator could operate simultaneously as an elevator or separately as surfaces for lateral control. This control, according to the inventor’s design, was intended to create a powerful rolling moment, so the fuselage truss was appropriately calculated and ensured the transmission of this moment. A 110 hp Le Rhône engine was installed on the C-2.

The C-2 was built in early 1922. Gómez Spencer again became the test pilot. Despite the new devices, in trials, the C-2 behaved exactly like its predecessor, the C-3. The rotor spun up during the apparatus’s runs, providing sufficient lift for the autogiro to make short hops, but as soon as the wheels lifted off the ground and the apparatus lost solid support, it invariably fell to the right, and the lateral control could not prevent this. Attempts to get the C-2 airborne ended in blade breakages and fuselage deformation due to the large moment applied by the lateral control. After each takeoff attempt, significant repairs were needed. The C-2 was rebuilt four times.

Having thoroughly analyzed all the experience gained from testing the first three autogiros, La Cierva concluded that with rigid blade attachment to the hub, it was impossible to obtain a centered aerodynamic resultant of the rotor’s lift, and he continued persistently to search for a solution to the problem, as he was absolutely confident in the correctness of the chosen path. After all, the autogiro principle had been demonstrated on a flying model. Eventually, La Cierva concluded that the difference between the tested aircraft and the model was that the model’s blades were more flexible. A flexible blade deforms more or less in accordance with the lift force and performs a cyclic flapping motion during rotor rotation. In forward motion, the blade moving forward against the incoming flow rises upward, and the one moving backward moves downward. Due to the flexibility of the blade, the overturning moment transmitted to the rotor axis becomes insignificant.

The Hinged Rotor and the Cierva C.4’s Success

So, the reason for the different behavior of the model and the real autogiro was determined. Now it was necessary to build a rotor in which the blades could perform flapping movements – and La Cierva decided to attach each blade to the hub by means of a hinge with a horizontal axis (for now only one hinge; in 1927 he would come to a second – with a vertical axis). About when exactly this thought came to him, La Cierva writes: “…in my notebook, in which I recorded calculations for several years, I found an entry that reads: January 2, 1922, the solution of hinging the blades first occurred to me.” Apparently, this solution was initially an alternative for the designer. Only after the C-2 trials did he finally become convinced of the necessity of using a hinged rotor.

On April 18, 1922, La Cierva filed a patent application for “Hinged connection ensuring flapping motion of blades in the vertical plane.” The patent “Improvement of aeroplanes with rotating wings” No. 81406 was issued on November 15 of the same year. Now, the hinged articulation allowed the blade during rotation to form a cone, determined by the equilibrium of aerodynamic and centrifugal forces. The flapping motion was aerodynamically damped. A blade that rose automatically decreased its angle of attack and, consequently, its lift, which caused it to reduce its flapping speed and then descend; and conversely, when the blade descended, the angle of attack increased.

On the first autogiros with horizontal hinges, La Cierva used rubber cords to suspend the blades, which kept them almost horizontal even with the rotor stopped. In motion, however, centrifugal force is approximately 10 times greater than lift, and suspension is not needed for the blades. The need for suspension arises in a state of rest or at low rotor speeds, when lift is still insufficient. The patent also contained another point: the blade spar near the root was bent downwards, and thus the hinges were below the blade’s center of gravity. The designer believed that during flight, the blades would occupy an almost horizontal position. This form of blade spar was used only on the C-4 (which will be discussed below).

Juan de la Cierva believed that with the introduction of the hinged rotor, he gained six advantages: a centered aerodynamic resultant; inherent stability, as the resultant lift is always applied at a point located above the center of gravity; the ability to use optimal profiles that cannot be applied to the “compensated” rotor; elimination of bending moments on the rotor axis; elimination of the gyroscopic effect; and lightness and simplicity of construction.

The first autogiro with hinged blades, the C-4, also had another innovation: the rotor axis could be tilted left and right at the pilot’s discretion. As we already know, for lateral control, the C-1 had a single aileron above the rotors. The C-3 had a device for overall simultaneous blade twisting, and the C-2 had a differential elevator. Now, lateral control was applied by means of the rotor. For the construction of the C-4, La Cierva used the C-3 fuselage but significantly modified it. He installed an 80 hp Le Rhône engine. The autogiro used a four-bladed rotor. The blades each had one tubular spar and wooden ribs.

Trials began in June 1922. The first taxiing and ground runs were performed by Gómez Spencer, but he was soon recalled to active military service, and José María Espinosa continued the tests. The tests again did not yield the expected result: the pilot could not tilt the rotor axis—the required effort exceeded human capabilities. It was necessary to return to an airplane control scheme. La Cierva understood that this was a step backward, but at that moment, the priority was to prove the effectiveness of the autogiro principle in practice, and only then systematically develop the most optimal control scheme. Rotor control was abolished, and the C-4 was fitted with a transverse bar with ailerons. By then, Gómez Spencer had returned and continued the trials.

In January 1923, the first successful flights finally took place. Literature on the history of the autogiro gives different dates for the first flights, and to give an accurate picture, we quote excerpts from La Cierva’s message to the Royal Academy, dated February 15, 1928: “This apparatus, tested in June 1922, demonstrated from the outset its inherent automatic centering and was already ready to fly, but a breakdown occurred, forcing a halt to trials, which then, due to the absence of a pilot and for other reasons, could not be resumed until January of the current year. Currently, the apparatus is equipped with two small ailerons, i.e., small wings for lateral control.”

“Piloted by Mr. Gómez Spencer, the autogiro first took off on the 10th. After compensating for the engine’s reactive torque, on the 17th, it performed several straight flights at a height of two meters, confirming all calculated data, with the exception of the landing, which it executed like an airplane. On the 20th, due to a defect in the engine control system that prevented the pilot from turning it off during landing, the apparatus ascended to a height of 8 m, which, with a loss of speed, would have threatened catastrophe, had it been an airplane. The pilot, recalling the apparatus’s theoretical property of being insensitive to speed loss and its ability to make a vertical landing, performed the necessary maneuver; the apparatus slowly descended and landed almost without speed, finally confirming all its theoretical properties.”

“On the 25th, official tests were conducted, and on the 31st, a flight lasting three and a half minutes was completed along a closed four-kilometer route at a height of over 25 m, as evidenced by a certificate, a copy of which I enclose.” Thus, on January 10, 1923, Gómez Spencer made the first takeoff in the C-4. The hop was brief. The autogiro banked left and fell. The cause was the reactive torque from the pulling propeller. This was eliminated by offsetting the rotor axis laterally. On January 17, Gómez Spencer performed the first straight flight at an altitude of 2 m, covering 183 m. So, the hinged rotor allowed the autogiro to fly.

On January 20, during a low-altitude flight, the engine suddenly stopped obeying and went to maximum RPM. The autogiro began climbing sharply. However, the pilot instinctively cut the ignition, pulled the stick back vigorously, and the autogiro landed with a small vertical speed. The commander of the Getafe airfield, Captain José González, described this flight: “During the test carried out by Lieutenant Alejandro Gómez Spencer on the flying machine, invented by road engineer Juan de la Cierva and named ‘autogyro’ by the inventor, due to a malfunction in the manual contact circuit, the apparatus suddenly rose approximately eight meters and at this altitude found itself without forward speed, i.e., in a situation analogous to a complete loss of speed for an airplane. However, during descent, the presence of lift was continuously observed thanks to the rapid rotation of the blade-wings, which led to a gentle, soft landing.”

Undoubtedly, González described this incident at La Cierva’s request. It was important for the inventor to have official confirmation of the autogiro’s main advantage over the airplane. After all, an airplane only survives with speed. Without speed, it cannot stay in the air. In the early days of aviation, airplane speeds were commensurate with air mass movements, and in some cases (excluding the pilot’s level of experience) conditions could arise where the airplane wing entered supercritical angles of attack, leading to a spin. Losing speed at high altitude is not so terrible. In a spin, the machine can be brought out of it and, gaining speed, return to normal flight. At low altitudes, however, there was almost no chance of saving the machine, and often lives. Therefore, La Cierva wanted to emphasize the advantages of his new apparatus, which had found itself in such a critical situation.

After these flights, the C-4 was transported from Getafe to Cuatro Vientos airfield, where the military aviation base was located. It was there, on January 31, 1923, that Gómez Spencer opened the era of normal rotary-wing aviation flights. FAI sports commissioner Herrera officially certified this flight: “At Cuatro Vientos airfield, on the afternoon of January 31st of this year, an ‘autogyro’ system apparatus, invented and built by Don Juan de la Cierva Codorniu, piloted by Lieutenant Alejandro Gómez Spencer, performed three flights, the last of which was completed along a closed route approximately 4 km long in 3 minutes 30 seconds, reaching an altitude of over 25 m above the ground.”

This historical flight practically proved the effectiveness of the autogiro principle. The C-4 had served its purpose, and for more complex experiments, another apparatus needed to be built. The autogiro’s successful flight resonated in the world press. The American magazine “Aviation” already on March 12 featured a photograph of the autogiro on its cover, and on April 9 published an article about it by Spanish aviation pioneer Heraclio Alfaro. The French magazine “L’Aéronautique” in its April issue published a short article by La Cierva, which provided the following characteristics of the C-4: rotor diameter – 8 m; total blade area – 10 m²; blade profile – “Eiffel-101”; engine – 80 hp Le Rhône; takeoff weight – 500 kg; specific blade loading – 50 kg/m²; maximum speed – 100 km/h. Of course, the maximum speed of the C-4 was not measured appropriately by anyone. This figure is, of course, an estimate and, most likely, quite optimistic.

In the aforementioned letter to the Royal Academy of Sciences dated February 15, 1923, La Cierva, after describing the C-4 flight on January 31, 1923, writes: “With this experiment, I conclude work with the current apparatus, not prepared for long-duration and long-range flights, and I will undertake the creation of another apparatus with the same characteristics but an improved design, which I hope to finish quickly. The purely experimental part is completed, but this does not negate possible subsequent improvements, especially in the area of useful efficiency.”

He concluded: “I have the honor to inform our Royal Academy of Sciences that the product of my research into the ‘autogyro’ system has resulted in a flying machine with the same characteristics as an airplane, with a relative superiority in stability and landing, ease of construction, and great simplicity of control. Simultaneously, the autogiro approaches the airplane in terms of useful efficiency and speed.” Thus, one historical stage was complete. The inventor began working on the further development of the idea.

After the successful flights of the C-4, La Cierva began building his fifth autogiro. Despite financial difficulties, he saw it through to completion. The C-5 was the last autogiro created by the inventor at his own expense (including modifications and major repairs, he built 30 autogiros at his own expense). The C-5’s rotor was three-bladed. In La Cierva’s opinion, a three-bladed rotor had less inter-blade interaction compared to four- and five-bladed rotors. The blades had a single spar of mixed construction, plywood skin, and metal leading-edge trim. The autogiro was powered by a 110 hp Le Rhône engine. Trials began in April 1923. Due to frequent breakdowns, work was intermittent. Eventually, the apparatus suffered a serious accident: during taxiing, a blade spar broke due to fatigue. The machine was not repaired, as the inventor’s resources were exhausted.

Technical Specifications

Modification	C.4
Main rotor diameter, m	12.70
Length, m	8.38
Engine type	1 Piston engine Le Rhone 9Ja
Power, hp	1 x 110
Maximum speed, km/h	72
Crew, crew members	1

Image and diagram gallery of the Cierva C.1 Autogiro and Early Development

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C.1(5)	C.1(5)
C.1(5)	C.1(5)
C.1(5)	C.1(5)
C.1(5)