Leonardo da Vinci

The History of Helicopters

1. Historical Background
2. Stepping Stones to the Helicopter
3. The Helicopter is Born
4. The Helicopter Comes of Age



At the height of its popularity, Juan de la Cierva's autogyro intrigued, and amused many with its odd up and down flying antics. But there were others who were moved to more serious thoughts. These were the helicopter experimenters. Now that they had been shown one way to build a practical rotary-wing airplane, they were more convinced than ever that the aerial machine they were seeking—a craft with far greater flying
ability—was possible. Instead of discouraging helicopter inventors with its near-vertical flying abilities, the autogyro spurred them on as never before. Both in Europe and America concealed in remote corners of flying fields and hangar sheds, they quietly tinkered with loving care over their mechanical brainchildren. The dream of their predecessors of a hundred years and more which they were carrying on was soon to come true. Their helicopters were as varied in style as the clouds in the sky, and all flew with differing degrees of success. They all contributed some little share toward bringing the great work to a victorious climax.

Louis Damblanc, for example, built what is considered the first twin-engine helicopter. But unfortunately for this French engineer, his double-rotor aircraft crashed and was wrecked during its test flight.

In Holland a quietly working aviation expert, Dr. A. G. von Baumhauer, built a machine with a single large lifting rotor and a smaller tail rotor. Although the idea for this type of helicopter had already been thought of many years previously by Louis Breguet, Dr. Baumhauer's was the first airplane of that kind to be built. Appearing in 1925, it demonstrated a fair amount of flying ability before a crash also ended its career. On one occasion the inventor managed to have it rise several feet from the ground and to hover for as long as five minutes. Many years later this style of helicopter design was to be revived and was destined to make aeronautical history.

But of all the unique helicopter designs to appear during the late twenties and early thirties, those of Isacco and Maitland B. Bleeker were perhaps the most unusual. Isacco had long been associated with Pescara while the latter conducted his helicopter experiments. It was through this contact that Isacco had his interest in helicopters awakened, as well as the beginning of his own ideas for their construction. He carried out much of his development work in England. The helicopter design which Isacco produced called for each of the rotor blades to have its own small motor and propeller. This eliminated the usual single large-power source for spinning the blades. With its own propelling units and controls, each of the rotating wings was in a sense converted into an individual airplane. Another and larger motor was also in the front of the airplane, but this was mainly for pulling the craft through the air.

Isacco called his invention a "helicogyro" rather than a helicopter. The rotor blades were only expected to give "lifting" assistance at take-off. They were not designed to raise the machine vertically to any great height. The motor in the nose of the plane pulled it over the ground after a short run, and this created the major lift to make it airborne.

The first Isacco helicopter to feature his unique rotor arrangement was a small single-seater affair. It had a two-bladed rotor, each blade forty-one feet in diameter and each having attached a miniature low-powered engine. The machine encouraged the inventor so much during its test flights (because it actually left the ground and showed a fair amount of stability and control) that he decided to build a second one in 1929. This helicogyro, a two-seater, had the same rotor arrangement but was larger in structure. The blades were forty-nine feet in diameter. This machine was sponsored by the British Air Ministry. To the disappointment of its inventor, the craft showed very little improvement over the first model. Isacco's difficulty was not in the theory behind his rotor design—in this he was far in advance of his day—but rather in his not being able to obtain more powerful miniature motors to spin the blades. Only after many long years and the advent of the jet-propulsion era were his helicopter ideas revived and made use of in a workable rotary-wing aircraft.

In the United States, meanwhile, Maitland B. Bleeker had very much the same thoughts concerning helicopter design. He also equipped the rotor blades of his experimental airplane with individual propellers almost seven feet in diameter, but here the similarity with Isacco's ended. Bleeker did not power them with their own motors; instead he used a complicated mechanical system of gears and shafts extending from a single powerful motor to the propellers. This engine was directly back of the pilot's compartment and below the huge rotor which spun about on its vertical shaft. This unit was made of four blades a little more than forty-seven feet in diameter. Small wings called "stabovators" were attached to these blades which helped to control them better as they rotated. The craft was equipped with a rudder for directional control, while horizontal flight was obtained by tilting the complete rotor unit forward.

The American engineer first conceived his helicopter design while still a student at the University of Michigan. After graduation, Maitland worked at the National Advisory Committee for Aeronautics, where he carried out some preliminary experiments with models of his flying windmill. In 1926 he captured the interest of the Curtis Aeroplane and Motor Company, who agreed to back the development of the helicopter.

Maitland's helicopter was built at an airport in Valley Stream, Long Island, New York, and completed by 1930. The first attempts at flying the craft were made in a hangar at this same field. These were brief and not very encouraging. Poor lifting qualities, difficulty in maintaining proper balance when aloft, and tremendous vibration were convincing factors, among others, that the machine had a long way to go before reaching, if ever, a successful flying stage. In fact, Bleeker and the Curtiss-Wright Company became so discouraged by the mountain of aerodynamic and mechanical problems which the craft's unhappy flying ability presented that they abandoned the project after four years of effort.

A more encouraging development took place when Oscar von Asboth came out with the fourth of his helicopter models in 1928. This aviation expert was head of the Austro-Hungarian State Propeller Factory during World War I and gave a good deal of valuable assistance to the Karman-Petroczy helicopter project. In later years he emigrated to England, where he found better opportunity for carrying out his own helicopter experiments.

He designed and built four helicopters, all of which were in some way fairly successful in flight. His fourth, however, was by far the best. It had two small wooden twin-bladed rotors about fourteen feet in diameter placed one above the other. A series of six vertical movable wing surfaces placed beneath the rotors and acted upon by the down sweep of air from the propellers helped control the craft while in flight. The airplane had no landing gear like other airplanes. Instead four legs extended downward ending in hollow cuplike units. Into these the inventor strapped four inflated soccer balls so the craft would make a soft landing. Crude though it was in appearance, Asboth's helicopter brought him considerable fame in aviation circles with its unusual flying ability.

With rotors flailing away at a great rate, this craft could leap skyward with surprising speed. At one time it was said to have reached a height of almost a hundred feet. According to the test pilot who put the machine through its trial paces, it could be held under excellent control at all times and was quite stable. One of the helicopter's greatest flying traits was its ability to hover. It could do this indefinitely and at any height within the machine's limits. The pilot said that during the course of this maneuver it was even possible to take one's hands off the controls.

But the helicopter that really made rotary-wing progress was the machine designed and built by Corridino D'Ascanio, an Italian aviation engineer, in 1930. This helicopter was the first of its breed to have its flying achievements officially recognized as true aeronautical records. The Federation Aeronautique Internationale, one of the aviation world's oldest groups for recording aerial accomplishments, felt that D'Ascanio's helicopter had brought the flying qualities of these aircraft to a point of efficiency where they could take their position beside conventional aircraft in the matter of breaking and holding records.

His rotary-wing airplane, however, was no world beater when compared to what these machines can do today. But at that time it appeared to be the last word both in what these craft should look like and in their accomplishments. D'Ascanio's helicopter was of the type known as the coaxial—that is, one rotor is placed above the other on the same shaft. These rotors had two blades each and spun in opposite directions. This was for the purpose of eliminating an opposite spinning tendency of the fuselage called "torque." The blades were thirty-nine feet in diameter and could flap or move in an up or down direction during their circular motion, a principle long previously developed by Cierva for his autogyros. D'Ascanio added another feature to the rotor blades in the form of smaller blades attached to the rear edge of the larger, which were expected to make them more efficient for lifting and control of the flying machine.

In addition to the large rotors, the inventor also equipped his craft with three others. One was attached vertically on the right side of the fuselage to help fly it in a right or left direction. The others were horizontally mounted—like the big lift rotors— one at the left side of the machine to help keep it in lateral balance and the other on the tail to prevent it from swaying in an up and down motion at the fore and aft ends. The entire machine, constructed in the form of a cross, weighed a little more than 1,600 pounds, and it was powered by a ninety-five horsepower engine.

The flying achievements of D'Ascanio's helicopter that brought it international recognition were an ability to rise straight up off the ground for a distance of sixty feet in 1:40 seconds; to fly in a direct line for a length of more than three thousand feet while remaining at a height of about twenty-one feet; climb straight up twenty feet above the ground, hover for one and a half minutes, and then land.

Further, the helicopter established an endurance record by staying aloft for almost nine minutes. It also managed to fly around a closed course of one mile under complete control of the pilot, Major Marinello Nelli. This meant taking off, flying the course, and returning to the starting point. In addition, the Italian engineer's airplane was said to be quite stable while aloft and have very little vibration. Despite its shortcomings in speed and climbing ability, D'Ascanio's helicopter came very close to filling the exact requirements for a successful helicopter.

Meanwhile a companion helicopter to Corridino's was exhibited at the Antwerp Exposition, Belguim, in 1930. This windmill flying machine was the work of Nicolas Florine, and American aviation experts who saw it were impressed with its construction and operation. The Belgian engineer built his rotating-wing airplane a bit differently from what had been the general practice up to this time. Instead of the coaxial arrangement with the rotors, he placed his on the fore and aft ends of the machine. Although the blades of the rotors spun in the same direction, torque was not created. The inventor had devised a mechanical arrangement whereby the whirling propellers could be tilted to eliminate this troublesome problem. Rather than the conventional wheels for a landing gear, Flor-ine's helicopter came down on four vertical legs. Even though this engineer's flying machine showed great promise, it never quite lived up to it. Certainly it could not equal the performance of the rotary-wing plane which Louis Breguet displayed in 1936. The latter, by this time, was once again deeply involved in helicopter experiments.

Breguet had returned to his beloved helicopters during the early 1930's but decided to combine his aeronautical talents this time with those of a clever young engineer, Rene Dorand. The machine resulting from their joint efforts, since known as the Breguet-Dorand helicopter, was a remarkable flyer. It was undoubtedly the most advanced yet to be achieved by helicopter experimenters.

The airplane was of the coaxial type so highly favored by Breguet, having rotors fifty-two and a half feet in diameter. It featured improved rotor systems based on the principles first worked out by the pioneer French aeronaut during the early 1900's. In the course of a series of test flights in 1936, it more than proved the hopes of its builders as a competent flyer. At one time the test pilot was able to make it soar to a height of five hundred feet and to keep it in the air for an hour and two minutes. A severe test occurred when the machine was flown over a restricted closed course of forty-four kilometers at a rate of ninety-nine kilometers an hour. The most important feature of this demonstration was that the pilot could maintain complete control of the craft throughout the flight. It also took off from and landed at the same spot.

Louis Breguet became so enthusiastic with results of this helicopter's flying ability that he, along with some others, organized a company to manufacture these windmill airplanes. World War II broke out, however, to interfere with their plans. This was a keen shock to the pioneer airman because in addition to the original design which he desired to build, he had still newer ones of helicopters with improved flying performance. One of these was a larger machine with two engines. To be known as the G-20, this rotary-wing aircraft was intended for military purposes. For the still more distant future, Louis Breguet envisioned helicopters that some day would be capable of flying across the Atlantic Ocean.

The aviation world during the late thirties had hardly stopped buzzing with excitement over Breguet's contribution to rotary-wing flight when news of a still more amazing helicopter seeped out of Germany. The world in 1936 was far more interested in political developments of a sinister nature taking place in that country than in the queer flying antics of a new type airplane. Nevertheless the aircraft managed to achieve some measure of attention, especially in aviation circles, and it has since been recognized as the world's first practical helicopter. Dr. Heinrich Focke, one of Germany's leading aviation experts, was mainly responsible for bringing this machine into existence.

Dr. Focke was at one time a partner in the Focke-Wulf Company, which turned out some very efficient fighter planes. But they also built autogyros through a license agreement with the Cierva Autogyro Company. While engaged with this particular activity of the company, Dr. Focke became deeply interested in rotary-wing aircraft. The autogyro aroused his admiration and was the spark that set his mind to thinking about the helicopter. He well knew that Cierva's invention, while not the complete answer for a wholly successful whirling-wing airplane, was certainly a long stride towards that objective. The more Dr. Focke worked with autogyros, the more he thought about helicopters. Finally, in 1932, after studying most of the available technical literature on previous helicopter experiments, he began the engineering work on his own rotating-wing airplane.

For several years the German aviation engineer busied himself with theoretical studies, especially relating to the aerodynamics of the lifting propellers and control of the machine. This eventually led him to build a small scale model of a helicopter design, powered with a tiny gasoline engine. The performance of this midget aerial vehicle was so successful that it encouraged Dr. Focke to go ahead and build a full-size, man-carrying airplane.

The Focke-Achgelis helicopter, known also as the FA-61, made its appearance on June 26, 1936, when test pilot Rohlfs made the first free flight in the machine. It did not differ too much from other more normal-looking airplanes of that day. The aircraft's only really odd features were an absence of the usual wings and in their place outriggers equipped with large three-bladed rotors. The latter were connected by a system of gears and shafts to the engine located in the nose of the plane. The two rotors spun in opposite directions. Weighing slightly more than one ton, the helicopter was powered by a 160 horsepower engine. A small propeller was hooked up to this for cooling purposes.

The flying windmill quickly proved to its builders that they had created a super-performer. It easily carried out the basic maneuvers which all helicopter experimenters had long known and dreamed of achieving. These were vertical, forward, backward, sidewards, and hovering flight, all under full control of the pilot. Once it was thoroughly tested and its abilities more than amply demonstrated, Dr. Focke and his associates decided to send the airplane out for a real test. In June, 1937, with one of Germany's most popular woman flyers, Fraulein Hanna Reitsch, at the controls, the helicopter made a cross-country flight from Bremen to Berlin at an average speed of sixty-eight miles per hour. This was a major accomplishment for an up and down flying airplane, but proved to be only the prelude to a whole series of outstanding nights. Not long afterwards the FA-61 established a number of helicopter records which were recognized by the Federation Aeronautique Internationale.

Among these were an altitude mark of eight thousand feet; endurance flight of one hour, twenty minutes and fifty seconds; a high speed record of seventy-six miles per hour; controlled flight over a closed course of fifty miles, and finally a record cross-country distance journey of 143 miles. At first there were skeptics when news of Dr. Focke's wonderful flying machine was announced to the world. Too often in the past had helicopter inventors claimed extraordinary feats for their creations which, when put to the test, failed to measure up to the claims. However, it gradually dawned on those in the aviation world that here was a helicopter that was different. Not only were its unusual flying abilities recognized by a responsible aviation organization, but they were also displayed before the fascinated eyes of the public.

In February, 1938, with the same skillful woman pilot, Hanna Reitsch, at the controls, Dr. Focke had his uncanny mechanical whirly bird perform inside the giant sports arena, Deutschlandhalle, in Berlin. Here, where the slightest error in judgment or mechanical failure could have caused a disaster, the pilot flew her aerial steed under perfect control. First she took it off the floor straight up, hovered a short distance from the floor and then flew sidewards the whole length of the hall before the astonished expressions of the spectators. Here was proof positive that the helicopter had at long last arrived.

By one of those strange coincidences, of which the history of inventions has so many, the first beginnings of another practical helicopter were taking place in the United States at this same time. Igor I. Sikorsky, one of the world's most famous designers and builders of aircraft, was also solving the problems of a successful rotary-wing aircraft. When finished, his helicopter used a completely different means of operation from Dr. Focke's and also proved itself an excellent flyer. In reality, Sikorsky's helicopter had its beginning many years previously, during the early days of the 1900's, when the inventor lived in the city of his birth, Kiev, Russia.

Igor first saw the light of day in that city in 1889, born into a family rich with the atmosphere of culture and learning. His father was a professor of psychology at the local university and was also famous throughout Russia for his writings on the subject of mental diseases. Igor's mother had studied for several years at a medical college.

When a young boy, Igor had the seeds of a love for technical and scientific subjects sown within him by his father. There was a wonderful relationship between father and son, and the elder Sikorsky often delighted in telling young Igor the wonders of the world of science. A versatile man of learning, he would explain in simple language many of the mysteries of astronomy, electricity, and physics. These fascinating talks were usually given outdoors, on the professor's vacation, when he and Igor would go to the Tyrol region of Germany and tramp through the forests.

But the spark of Igor's aeronautical genius was kindled by his mother. She was a student of the life of Leonardo da Vinci and often told her attentive son little anecdotes about that great man's life. Many of these were of Leonardo's attempts to build a flying machine. Igor liked these stories best and would ask his mother to tell them over and over again. In the later years of his life, long after he had become world famous for his aeronautical contributions, he told in his biography how important these stories were in starting him on his career. Although he confessed that many of the tales of da Vinci's life vanished from his memory, this was not the case when it came to the aerial accomplishments of that great man's career. Igor cherished these and locked them in a special vault of his eager, youthful mind.

It amuses him now to tell of another experience of his youth, when he used to approach his elders and ask questions about the possibilities of mechanical flight. They would look at him with a sort of tender expression and try to explain to him that flying of that kind was a virtual impossibility. Politely but firmly the young Igor would disagree with them, and there followed a long, sometimes tedious, discussion as to why a mechanically-powered aircraft could not fly. These were frequent episodes in the Sikorsky home.

Igor's interest in scientific and technical subjects was wisely encouraged by his parents during his boyhood days. They did this not only with his more formal school studies, but with his hobbies as well. Astronomy fascinated him and still occupies a favorite place among his relaxing activities. Electricity also captured his interest, and he spent many hours tinkering with homemade batteries and even an electric motor, which he considered a pinnacle of success. But the hobby which occupied the majority of his leisure hours was the building of model airplanes, helicopters especially. Scores of these tiny aerial vehicles were created by his nimble fingers and quick mind. The most successful of these he made when he was twelve years old. Powered with a rubber-band motor, this helicopter made a glorious trip to the ceiling of his room. Igor was slowly becoming ensnared in the web of aeronautics.

By the time he reached young manhood, Igor was thoroughly convinced that aeronautics was the kind of engineering he would rather pursue than any other. His father, with all the logic at his command, sought to persuade his son to follow a more practical field of technical work. But all the elder Sikorsky's talking was in vain, and reluctantly he let his son have his way. Igor's imagination at that time was completely captivated by newspaper accounts of the dirigible flights of Count Zeppelin and the first description he had read of the historic heavier-than-air flight of the Wright brothers. The Wright brothers' achievement, especially, was the last convincing proof he needed, if any, that flying machines were practical. The young man began giving serious thought to the construction of his own airplane.

It was in the summer of 1908, while on a vacation with his father in Germany, that Igor began laying plans for his flying machine. Since he was still under the spell of the aerial exploits of Leonardo da Vinci, it appeared quite logical that this craft should be in the form of a helicopter. First he had to have information about lifting screws or propellers, so he promptly went to work building a small test apparatus having a four-foot rotor. He fashioned this in the room of the hotel at which he and his father were staying, cluttering the place with tools and materials. With this device he expected to check his figures and other data for a design of a lifting propeller—calculate, if he could, the lifting power of this whirling fan.

Initial results were discouraging. Igor found that a propeller far too big to be spun by the weak motors of his day would be needed to raise a machine of even small size off the ground. But the young engineer was not discouraged. When he returned to his home in Kiev, he decided to build a more substantial test rig and hoped that his findings would be more promising.

By this time Igor had completely convinced his family that aeronautics was more than just a hobby with him. It was now his life's work. They no longer tried to discourage him in the pursuit of his aerial studies, but instead- even gave him encouragement. This went to the extent of temporarily permitting him to give up his formal technical studies so that he could concentrate on the building of his first helicopter. Despite the not-too-promising data he had obtained from his test devices, Igor had decided in December of 1908 to build a full-size helicopter.

There was little or no material and equipment in Russia during this era which Igor could obtain to build his airplane. It was necessary for him to go to Paris, then the world's center of aviation activities. His sister Olga was particularly helpful to him then by giving her brother the money with which he could buy an engine and other necessary mechanical parts. He left home in January of 1909. Igor Sikorsky had planned that his journey was to be a quick one, that he would buy an engine and other essential items for his helicopter, and then hurry home. As often happens with well-laid plans, his underwent a radical change when he arrived in the beautiful French capital.

His first sight of an airplane completely captivated the young engineer. He mingled with the pilots, mechanics, and engineers at the flying fields and overheard snatches of technical information which made him eager for more. After a day or two of this experience, he decided it would be to his advantage to stay longer among these aeronautical surroundings to pick up all the knowledge he could. In his autobiography Sikorsky writes with particular fondness of these "good old days" when the fragile, often graceful aerial vehicles before his eyes were the most wonderful objects in all the world. Even though removed to another age and a more distant world in later years, he could still picture in his mind's eye these colorful "good old days."

Day after day Igor spent long hours at the Paris air fields. He made the acquaintance of a number of daring airmen. They told him many things about airplanes that he had never known before, and Igor absorbed it all with the feeling that he was getting something of infinite value. One of these men whom he got to know very well was a Captain Ferber, a famous flyer of that pioneer era. Discussing his aeronautical plans with Ferber one day, Igor told him that he intended to build a helicopter. The Frenchman was quite frank in giving his opinion about this. To attempt to build a rotating-wing airplane would be a waste of time and money. The fixed-wing airplane, Captain Ferber felt, was destined to be the successful aerial vehicle.

Despite this dash of cold water on his aeronautical experiment by a seasoned aeronaut Igor still remained determined to carry out his plans. He had long ago shown that he had a will of his own. Even though he made repeated attempts to persuade the young engineer to give up the helicopter and concentrate on the more conventional aircraft, the French flyer was a great help to Igor. He was more than patient in answering the dozens of questions put to him, and generous in sharing his aviation knowledge. He strongly advised the young Russian to go to one of the aviation schools at the fields, where still greater information could be had. Sikorsky thought the suggestion a good one and attended a class for several weeks.

After a few months of this activity—talking, reading, and almost eating and breathing aviation matters, Igor felt that he had acquired about all the knowledge there was on the subject at the moment. He was much better equipped with technical information about airplanes than when he had arrived, and after purchasing a small aircraft engine and other materials for his helicopter, the young engineer returned home in May, 1909.

Among familiar home surroundings, Igor was bursting with ambition to begin his first aeronautical project, helicopter number one. This machine was extremely simple in structure. The body was nothing more than a box-like rectangular frame made of wood. The small Anzani engine was placed on one side of the frame while the seat for the operator was on the other. In this way the young inventor sought to keep his craft in balance.

Rising upward from this skeleton fuselage was a vertical shaft, on the upper portion of which was fastened a double set of two-bladed lifting rotors, one above the other. The top one was fifteen feet in diameter and the lower sixteen and a half feet. They turned at very high speed in opposite directions. The angle at which these blades struck the air could be changed by a rather complicated system of piano wire and turnbuckles. Tightening or loosening the turnbuckle changed the angles of the blades as desired. These rotors were hooked up to the engine by means of pulleys and a belt.

By July, Igor completed his helicopter and was eager to put it through its first test flights. But as often happens with newly developed mechanical apparatus, many adjustments had to be made before the long anticipated moment of possible flight could be realized. At first the belt gave the aeronaut trouble. It slipped around the pulleys and failed to turn the rotors properly. When he eliminated that, another rose to plague him. This time when Igor turned the engine on full power, the helicopter shook with a tremendous vibration. This problem took a little more time to fix, but after a good deal of head scratching, he managed to reduce it considerably. At last he was ready to fly his aerial vehicle.

Igor was quite cautious when he began testing the lifting ability of the helicopter. During this procedure he would stand outside the machine but close enough to be able to reach the throttle of the engine. Then, ever so slowly, he would apply more and more power, noting all the time with intense interest the behavior of the machine. It was during the course of one of these trials that the youthful inventor experienced both happiness and fright over a near disaster. He had been standing in his usual place near the engine operating the throttle when suddenly one side of the helicopter rose off the ground. Quickly Igor jumped on the frame to bring it back into balance. He also reduced the speed of the engine. In this way he prevented the craft from turning over and damaging itself. Once the danger had passed, a surge of encouragement swept through him because this was the first time his helicopter had shown any desire to leave the ground. While standing on its frame, the youthful aeronaut could feel some strange force resisting his efforts to push the vehicle downward.

Igor also had his first helicopter ride on this machine, but one which he was not especially happy about. As the rotors swished through the air above the pilot, the body of the machine would also revolve. This was the familiar "torque" problem which has always caused helicopter designers a mild form of headache. Try as he would, Igor could not eliminate it completely from his first aircraft. In his efforts to do so, he used to stand on the frame and have a sort of merry-go-round ride as the fuselage slowly turned around the shaft. However, while on these rides the inventor could also sense the machine trying mightily to raise itself off the ground. Whenever he experienced this feeling it balanced somewhat his unhappiness over the "torque" situation.

Throughout the summer of 1909 Igor worked patiently on his first aerial creation. Bolts were tightened here and there; old parts were removed and new ones put in their places; the engine was coaxed to run more smoothly and produce a little more power and, finally, the rotors were twisted this way and that in the hope they would lift the craft off the ground a little higher. However, by October the young inventor realized that no matter what he did the helicopter would never fly as he had dreamed it might. He came to the reluctant conclusion that much more technical information was needed about helicopters before he could hope for a greater measure of success. This was especially true concerning rotor blades and the need for a more powerful engine.

Igor did not become discouraged over the failure of his first helicopter. Rather he redoubled his efforts to build such an aircraft by engaging in a new series of test experiments to gain more accurate information about rotor blades. For this he went back to his miniature toy models and also other more substantial full-sized devices. Igor certainly did not consider his time wasted with helicopter number one. He had obtained experience in putting an airplane together that could not be had in any other way. And in the course of dismantling it, he began laying plans for a second machine, which included a second visit to Paris to get a more powerful engine and other improved equipment.

The young engineer returned from his visit to the French capital with two engines this time, plus new ideas for designing rotor blades. One of the engines which he had was of 25 horsepower, and he planned to put this in his helicopter number two. During the winter of 1910 this machine began to take form and when it was completed, Igor felt pleased with its appearance. To his eyes it looked like "a huge butterfly." He hoped it would have even half the flying qualities of that colorful insect.

Unfortunately for Igor, this helicopter performed little better than the first. It just managed to raise itself off the ground for a distance of several inches, but that was all. Try as he would, Igor could not improve the machine's flying ability, and gradually his interest in rotating-wing airplanes began to cool. He realized that far more aeronautical knowledge was needed before he could successfully cope with the peculiar technical problems connected with helicopters. As a start toward that goal, he decided to follow the advice of his friend Captain Ferber and concentrate on conventional airplanes. At least there was sufficient information available to build and fly one of these aerial vehicles.

Not long after he abandoned helicopter number two, Igor began building a fixed-wing flying machine, and with it achieved his first success as an aeronautical designer and builder. From this stepping stone he went on to become an even more skillful and famous aviation engineer. During the First World War he created Russia's most efficient multi-engined bombing planes. After the Communist upheaval in his native land forced him to flee to the United States, he continued to build some of the world's finest aircraft. His specialty was designing airplanes of two or more motors that could fly from both sea and land. Many of these played historic roles in helping to establish the far-flung air routes that spider-web the globe today. All brought their maker great fame throughout the world as an aviation expert of the very first rank.

Despite the fame which his fixed-wing airplanes brought, Igor never lost interest in helicopters. After the failures of his Kiev experiments, he filed a reminder in a remote nook of his mind to return some day to the building of a rotary-wing airplane and vowed that success would then be his. The first faint murmurs of this long held promise stirred the engineer in 1929 when he patented a novel idea for powering rotor blades by means of jet action. From that moment on helicopters once again occupied an ever larger share of his engineering thoughts. He began filling pages of his notebooks with mathematical calculations and other test data along with the first preliminary sketches of his third helicopter. By 1938 he was completely involved with his beloved rotating-wing airplanes and almost ready to start building. The first successful helicopter to be designed and constructed in the United States, the VS-300, soon appeared.

In the process of achieving his engineering goal, a good deal of individual testing of some of the main portions of the helicopter had first to be undertaken. For example, the lifting power of the rotor was thoroughly investigated. So, too, was the system which Sikorsky had designed for moving the blades of the rotors for control purposes. When all the data obtained from these tests met the inventor's approval, work on the actual helicopter was begun in the spring of 1939. By the fall of that year the craft was rolled out of the shop and prepared for flight. Soon Sikorsky would know whether his newer helicopter theories were an improvement over those he had originated in his youth.

The VS-300 did not fail its inventor. Even though it was far from the finished craft later refinements were to make it, the helicopter proved with the very first flight that Igor had solved a perplexing riddle. It could rise straight up off the ground, hover, and even fly backwards. During these pioneering hops, the craft was never permitted to fly more than a few feet off the ground, since its builder was still not too certain about complete control of the machine by the rotors. But all these "bugs" were eventually cleaned up. Sometimes in the refining process the machine was tethered to the ground while the pilot sat at the controls to familiarize himself with their actions. It was usually given sufficient slack to permit a foot or two of daylight between the wheels and the ground. At the very beginning of these trial flights, Igor did most of the piloting. In fact, he subsequently became the first licensed helicopter pilot in the United States.

Physically, Sikorsky's third helicopter did not present much of an appearance. It had a tubular framework body along the lines of a small conventional airplane. This structure was completely uncovered. It had no wings, and at the tail end were two smaller rotors fixed to short outriggers extending from the sides of the body. At the very end of the tail was still a third rotor which spun around in a vertical path. The pilot sat at the front of the machine, without any protection around him. This amused Igor and reminded him of the old flying days when pilots were exposed in the same way to the wind and other unpleasant weather conditions.

A short distance back of the pilot's seat was the engine that furnished the power for the large lifting rotor and the smaller auxiliary propellers at the rear. The main rotor was made up of three blades and was twenty-eight feet in diameter. During the later years of its existence, after some major mechanical refinements had been made, the body of the VS-300 was covered and, in general it was made to look more attractive.

This helicopter was equipped with the same controls found on a normal airplane, a control stick, and foot pedals. There was in addition, however, a brand-new device called a main pitch control lever. By properly manipulating all these, the pilot could make his machine rise vertically from the ground, stop and hover, fly forwards, backwards, or sidewards or in any other direction that he might dream up.

The main lift rotor governed the helicopter's up and down flying ability, hovering, and level flight. When the VS-300 first came out, its directional path of flying was controlled by the three small rotors at the tail. Later, this number was reduced to one, and this single rotor took over the task plus acting as an anti-torque force.

Igor loved to fly his helicopter or, as he sometimes called this airplane in moments of excitement, "heelikopeter." Throughout his thirty years in aviation, he observed, he had never flown in an airplane that had given him as much pleasure as the helicopter. The ability of this unique aerial machine to fly straight up and to hang motionless in the sky provided a thrill unlike any produced by the conventional airplane.

By the summer of 1940, Igor and his co-workers were ready to put this "dream" airplane through a short series of modest record-breaking flights. They had about completed all the changes in the craft which they felt necessary to make it perform the way it should. With the happy inventor at the controls, the VS-300 quickly showed that it was capable of staying aloft for a period of at least fifteen minutes under complete control from take-off to landing. It was the first American helicopter able to do this. Sikorsky then tested the craft in forward speed, reaching a mark of forty-five miles per hour, while in sideward and backward flight he managed to skim along at twenty miles per hour. Climbing ability was also tested when the pilot brought it to a cautious height of a little more than a hundred feet.

The following year, again with Sikorsky at the controls, the VS-300 established a series of far more substantial records. Among these, a new international endurance mark for helicopters was probably the most important. This achievement occurred on May 6, when Sikorsky patiently kept his pet airplane aloft for one hour, thirty-two minutes, and twenty-six seconds. This beat the mark held by Focke's helicopter by twelve minutes. Only the fact that the engine began to give him trouble prevented Igor from extending that record still further. A slightly different kind of record, but none the less historically important had been established a little more than a month previously, on April 17, 1941. The landing wheels of the VS-300 were replaced with rubber floats, and the world's first helicopter water take-offs and landings were accomplished. Throughout these maneuvers, Sikorsky had the machine under absolute command.

Probably the outstanding flying trait of the VS-300 was its precise, sensitive response to the pilot's command. This never failed to give Igor a deep feeling of satisfaction toward his windmill airplane, and he always delighted in showing off its uncanny aerial ability before the open-mouthed expressions of amazement of onlookers. These sky-flying circus acts were a common occurrence in the vicinity of the factory in which the craft was born. Often Igor would take off on what was normally a routine test flight. Then, with the serious part of the work over, he would scoot away from the test area, sometimes flying sideways, sometimes backwards, sometimes rising straight up into the sky like an elevator. Suddenly, to the surprise of those on the ground, the machine would drop down to earth and disappear behind some obstruction on the field. Time and again Igor would land his helicopter in areas too small and beyond the reach of other aerial vehicles.

On one occasion he used the roof of a small shed at the edge of the flying field. He outdid this stunt soon afterwards, however, by bringing his helicopter down gently on some packing cases! If this wasn't enough to cause spectators to scratch their heads in wonderment, Sikorsky had still another trick up his sleeve. He would bring his flying egg-beater very gently to within a few feet of the earth and hang there almost motionless. Then his companion beside him, who was a necessary part of the demonstration, would lean over the side of the machine and take hold of a suitcase handed to him by another person on the ground. With what appeared to be some magic touches on certain levers, Sikorsky would then rise skyward and fly away.

By the fall of 1943 the VS-300 had fulfilled its mission of helping to prove that the helicopter as a practical flying machine had finally arrived. It had buzzed around the Connecticut countryside for four years, performing with incredible perfection the aerial maneuvers peculiar to a workable helicopter. The airplane represented a deep personal triumph for Igor Sikorsky, a symbol of unflagging determination and skill of a great aeronautical engineer.

On October 7, under the guidance of its inventor, who perhaps felt a mixture of wistfulness and pride, the VS-300 dropped out of the sky for the last time and softly settled to earth. In colorful ceremonies on the grounds of the Edison Institute Museum at Dearborn, Michigan, Sikorsky's third and successful helicopter was enshrined with other historic aircraft which, in their own way, had also added new and exciting chapters to aviation history.

Now that Dr. Heinrich Focke and Igor Sikorsky had shown the world how to design and build practical helicopters, the questions might well be asked, "What is a helicopter?" and "How does it fly?" If the first question were to be put to six helicopter engineers, in all likelihood six different but correct answers would be given. The reason for this is the varied nature of the unique flying machine, simple, yet complex. Thus, in attempting to explain it or its operation, one tends either to become limited to brief but precise aeronautical principles or go into a lengthy description for fear of omitting essentials. Perhaps as good a description as any might be to call it an airplane with wings that rotate around a vertical shaft, sweeping great currents of air downward. The opposite action of these descending air currents causes pressure on the underside of the blades and supports the machine in the air. It will be seen from this that the rotor containing the wings is truly the heart of the helicopter.

The rotor assembly is made up of a number of mechanical parts together with movable wings, or blades, as they are more popularly called. The whole unit spins about a vertical shaft powered either by a piston engine to which it is connected by gears and shafts or, as in some modern experimental types, jet engines fixed to the blade tips. This arrangement marks the major difference between a helicopter and an autogyro. The helicopter rotor is spun by a motor while that on an autogyro is rotated solely by air currents flowing over the blades. As the rotor on a helicopter whirls faster and faster, it permits the airplane to fly up and down vertically, in any horizontal direction, including backwards, and to hang motionless in the air. Thus, man has achieved the abilities of nature's own helicopter, the hummingbird. This tiny winged creature, by an unbelievable rapid motion of its wings—a back and forth rate of fifty times a second—can also hang suspended in midair and fly backwards.

If one were to saw a rotor blade in half, its general outlines would follow quite closely those of the larger fixed wings of conventional airplanes. This shape of the wing, called an airfoil, is quite special in character. As the airfoil moves through the air, the latter acts upon it in such a way that a force called "lift" is created. This lift is what enables an airplane to fly.

Helicopter rotating wings obtain their lift by spinning rapidly in a circular path. This characteristic has also identified them as rotary-wing aircraft. As these blades spin around at great speed, they push part of the air downward in torrents with a simultaneous action of upward pressure of air against their undersides. Another swift stream of air, in far smaller quantities, flows over their top surfaces. The pressure of this air is so light as to cause a near vacuum. It is this combination of effects—pressure underneath the wing, vacuum-like void on top—that creates lift and makes the helicopter airborne. This vital force takes place without any relationship to the helicopter's forward speed. In this respect the blades differ from wings on a conventional airplane, which are only effective as long as they are being drawn through the air by the craft's forward velocity.

The ability of a helicopter pilot to vary this lift force through a wide range is one of the most important means for controlling the craft. He can go from a "no lift" condition, while parked on the ground during the warm-up period, to the greatest maximum force for upward, forward, and hovering flight.

The helicopter generally has four main controls for flying. Builders of these aircraft have made the controls resemble those of ordinary airplanes as closely as possible both in appearance and operation to reduce confusion. These are the cyclic pitch rod—similar to the control stick of other aircraft—rudder pedals, throttle, and the only stranger in the group, the collective pitch lever. The last one is also known by a variety of other names, such as pitch control lever and altitude control lever, but the first one seems to be the most popular. It performs a most important function for helicopter flight.

By using the cyclic stick control, the pilot can change the angle of any of the individual blades on the rotor as they sweep through the air. The angle of a blade on one side of the cycle or disc may be increased while that of the blade directly opposite is decreased. This action tilts the complete rotor and causes the helicopter to fly in the direction of the tilt. When the stick is pushed forward, the angle of the blades is changed to a position that tips the entire rotor head forward, enabling the helicopter to fly in that direction. Movement of the stick to the right or left tilts the rotor in either of these directions, and the aircraft flies accordingly. The cyclic pitch control is similar to the aileron action of conventional airplanes. Ailerons are small movable panels on the rear edge of the fixed wings and tilt in opposite up and down directions to one another. They help the pilot bank his airplane in a right or left direction as he-turns the craft in those paths of flight.

When the cyclic stick is pulled all the way back, the helicopter is brought to a momentary halt, and after the blades receive the proper angle adjustments, the rotor tilts to the rear and the machine flies backwards.

During the rotor-tilting operation, it is only the rotor that moves and not the vertical shaft to which it is attached. The shaft remains fixed. The rotor is connected to the shaft by a very complicated mechanical arrangement and in such a way as to permit a certain limited range of movement or tilt.

There is one other important function of cyclic pitch control. It helps to keep the helicopter in perfect lateral and horizontal balance. In forward flight, for example, the speed of the air over the blade advancing to the front of the machine is greater than that over the blade retreating to the rear. Ordinarily this creates an unbalanced lifting action with the greater force toward the front. If this is not corrected, as Juan Cierva painfully discovered, the craft would tip over. So by a combination of the cyclic control to change the blade angle and also by an automatic flapping action of the whirling wings, the lifting forces are equalized throughout the circular path of the spinning rotor. The ability of the rotor blades to flap, that is, climb upward as they spin to the fore and drop as they go aft, equalizing lift, was Juan Cierva's contribution via his autogyro to successful helicopter flight.

The rudder pedals permit the pilot to turn the body of the machine in the direction he wishes to fly. On a conventional airplane they operate the tail rudder. On certain types of helicopters they control the pitch angle of the blades of a smaller rotor fixed to the stern of the machine. By increasing or decreasing the angle at which these blades strike the air, the pilot can make the tail end of the helicopter swing to the right or left. This small tail rotor, commonly called an anti-torque rotor, has two main jobs to perform. One of these is steering as described, and the other to provide an anti-torque pressure action. On helicopters that have a single large lifting rotor an odd physical action takes place. While the rotor is spinning in one direction, the body of the machine desires to turn in an opposite path. This is known as torque action. To overcome this problem builders have placed a small rotor on the tail which turns in a vertical circle. During normal or forward flight the pitch on these smaller blades is just enough so that the pressure produced by the rotor against the tail equalizes that of the torque force. As a result, the fuselage remains in a straight position to the line of flight.

If the pilot alters the pitch of these blades to a greater angle, the pressure created will be greater than that of torque and push the tail end through that resisting force. However, if the pitch is decreased so the blades are no longer getting a good "bite" out of the air, the torque force will be greater and the tail end will be pushed through the rotor pressure. By proper manipulation of this tail rotor, therefore, the aft end can be made to swing to the right or left. The vertical spinning propeller is powered by the same motor that turns the large lift rotor and is connected to it by a long shaft and gears.

The fourth control is one that is unique with the helicopter and will never be seen on a conventional airplane. This is the collective pitch lever, which is normally located at the lower left side of the pilot's seat. It operates in an up and down manner and in close harmony with the throttle. As a matter of fact, the throttle is a part of this same lever. It is formed in the shape of a bicycle grip and is located at the end of the control rod. The collective pitch lever also changes the angles at which the main rotor blades cut through the air. However, unlike the cyclic control, it changes all the blades to the same angle at the same time, hence its name collective pitch lever. Its purpose is to vary the lift force of the helicopter while it is flying in a vertical direction and for hovering. As the pilot pulls upward on this lever, the blades of the rotor are increased to a very large angle and grip the air firmly. The lifting force becomes very strong, and the helicopter rises from the ground. As long as the lever is held upward, the machine will rise to any desired height within the craft's limits. When the desired altitude is reached, the pilot stops the lever action which in turn halts the angle changing movement of the blades.

To bring the helicopter back to earth, the collective pitch lever is pushed downward towards the floor. This reduces the angle at which the blades strike the air, cutting down the lifting force and causing the flying machine to descend. By its proper manipulation, the helicopter pilot can fly his aircraft within inches of the ground, to a height of several thousand feet, or he can hang motionless in the sky between these two extremes.

At the same moment that the pilot is moving this collective control lever he must also operate the throttle. As the angle of the blade is increased, more power is required to push the rotor around its circular journey. When the angle is decreased, less power is needed. Improper throttle action during both these maneuvers could result in serious consequences. The two controls, therefore, are almost as one in their importance and operation.

Flying the helicopter is not a simple process. The pilot is an extremely busy man, with both his hands and feet manipulating controls to keep the craft airborne. Not only is a detailed knowledge of the workings of these parts essential, but a fine coordination of their movements is of almost equal importance. Even experienced airmen of conventional airplanes find it necessary, in learning to fly a helicopter, to go through a rigorous training course.

There are a number of odd flying habits of rotary-wing aircraft that a pilot must became thoroughly familiar with. Two of the most important perhaps, are "ground cushion" and "auto-rotation."

Ground cushion is a mass pile-up of air between the helicopter and the ground, created by downward currents of air from a large rotor. It only occurs when the machine is within inches or a few feet of the landing surface. Actually, it is nothing more than a billowy, invisible cushion of air having great buoyancy. While the helicopter is operating within the limits of this cushion, its airborne qualities are greatly improved. Indeed, the power of ground cushion is such that it can actually govern limited up and down movements of the helicopter without the pilot touching the controls. While in the grip of this concentrated air mass, the rotating-wing aircraft will rise to the topmost bounds of the cushion, stop momentarily, then settle gently to within inches of the ground, at which point it will rise again. This gentle, elevator-like action will repeat itself over and over. A pilot who understands this characteristic of helicopter flight can make skillful use of it in handling his craft. This is especially so if for example, he is required to fly a heavy load into an area where a landing is impossible. With the aid of ground cushion, he will be able to hover within inches of the ground if necessary until the cargo is unloaded.

The second of the "different" flying habits of helicopters is "auto-rotation." This is strictly for emergency use. What it means is that the rotor blades will continue to spin and provide lift even though no longer powered by the engine. In a sense the helicopter has now become transformed into an autogyro because the blades are turning by the action of air over their surfaces. The normal downward flowing currents of air while the craft is flying as a helicopter are reversed to upward streams of air with Cierva's autogyro. Although the pilot will be descending at a sharp angle and at a speed greater than he would desire, the use of auto-rotation nevertheless will give him some assurance of completing the landing in one piece.

Designers and builders of helicopters have devised a variety of different styles for the control of their flying egg-beaters. The one just described—single large overhead rotor and small tail rotor as perfected by Sikorsky—appears to be the most popular at the moment. Bell Aircraft Corporation and the Hiller Helicopter Company are among those in the United States who are building excellent models of similar design.

A second type of helicopter has a single large rotor at the front and another at the back. This is known as a tandem arrangement. It overcomes the torque problem by the rotors spinning in opposite directions. For turning this type of helicopter in a right or left path of travel, one rotor is held in a vertical position while the other is tilted in the direction to be flown. The Piasecki Helicopter Corporation pioneered this form in the United States and built some of the earliest of the larger helicopter models. Their HRP-1 was the first of the transport helicopters to be put in use by the military services of the United States.

A third variation of helicopter styles is that of the pioneer Focke model. This machine, it will be recalled, featured the rotors at the ends of booms extending from either side of the aircraft's body. As with the tandem type, this arrangement also eliminates the small tail rotor. Torque is balanced out by a counter-rotation of the twin-rotors.

In the United States, Platt-LePage carried out partially successful flights with a helicopter of this kind which they were building for the Air Forces. A later version was attempted by the McDonnell Aircraft Corporation when they produced an experimental model for the Navy. Except for the rotors placed at the tips of conventional-looking wings, this craft might easily have been mistaken for an ordinary airplane.

Probably the most interesting of the rotor arrangements is that which has two sets of large lifting rotors intermeshing with one another. Among helicopter technical groups this type of rotary-wing aircraft is called a syncropter. The blades spin around their shafts in the same manner as those on the egg-beater in your mother's kitchen. It is the real representative among helicopters when the latter are referred to as "flying eggbeaters." On this airplane the two rotor hubs are placed side by side rather close together. As the blades whirl about on their circular journey, they pass over and below one another. Their intermeshing method of rotating eliminates torque. Rudder control or changing the machine's direction of flight is brought about by increasing the pitch of the blades of one rotor while decreasing those on the other. This action turns the machine in the path of flight desired. Anton Flettner, a German aeronautical engineer, built and flew the first successful intermeshing type helicopter in 1939. His machine performed so well that the German government ordered large numbers of it during World War II.

Kellet Aircraft Corporation, pioneer autogyro builders, developed an experimental model of this kind for the United States Air Forces in 1948. The craft was designed to carry twelve people and was powered by two engines located on either side of the fuselage. A later and more successful model of this group has been designed and built by the Kaman Aircraft Corporation. This is the HTK.-1, which the United States Navy is using for training helicopter pilots, observation, and a variety of other tasks.

In point of years the coaxial type of rotor placement is undoubtedly the oldest. On helicopters of this design the lifting propellers are placed one above the other on a single vertical shaft. These blades counter-rotate in order to neutralize the problems of torque. By changing independently the angle of the blades, the pilot is able to steer the machine in any desired direction. A brilliant young aviation engineer, Stanley Hiller, Jr., designed and built the first coaxial helicopter in the United States. He flew the machine with some success in 1943, using a football field in Berkeley, California, as a take-off and landing area. At the moment the Gyrodyne Company of America is the only one in the United States exploiting this particular design. Their Model 2c, currently undergoing development, offers interesting possibilities for rotating aircraft of this type. Its builders claim a number of advantages over other helicopter types of a similar size. Chief among these is a more efficient use of the power used to spin the rotors.

Directly opposite to the coaxial in age is the sixth and newest group, which has its rotor blades spun by various types of jet power plants and rockets. With these propulsion units there is no torque problem to overcome because of their nature of operating. Controls for vertical, hovering, and other flight directions are very much like those for the single large rotor. However, for turning the body of the fuselage in a right or left flight path, they are equipped with a small rudder. This movable vertical panel normally extends from the rear of the pilot's compartment and does its job mainly because of the strong downward streams of air acting upon it from the overhead rotor.

A number of these experimental models are currently flying. In the United States, the McDonnell Aircraft Corporation has produced one of the oldest of these which they have called, "Little Henry." A small simple structure of tubular aluminum, a rudder, and two cylindrical fuel tanks, Little Henry has two rotor blades fixed with tiny ram-jet engines at their tips. It has undergone a great many test flights from which the company expects to gather valuable data for the construction of larger and more efficient models.

Even more miniature in size is the American Helicopter Company's XH-26. This one-man rotating-wing airplane is nothing more than a glass and metal enclosure just large enough for the pilot and is suspended beneath a two-bladed rotor. It is powered by two pulse-jet engines fixed to the tips of the rotor blades. These are the same type of reaction plants that powered the German Buzz Bomb of World War II. A boom extends from the rear of the tiny cabin, at the end of which are two vertical rudders. It has a top speed of eighty miles per hour and can fly without refueling for an hour and ahalf. This helicopter is being produced for the United States Army, which intends to use it for observation work and other front-line reconnaissance duties. In an emergency it can carry two litter patients strapped to the outside of the fuselage. This one-man whirly-bird weighs less than three hundred pounds and is collapsible, a feature which the Army desired so that the craft could be parachuted into combat areas.

One of the youngest and most talented of the helicopter designers in the United States, Stanley Hiller, Jr., has also entered the jet-helicopter field with an amazingly simple yet efficient model. This flying pin-wheel, called the Hiller Hornet, weighs less than four hundred pounds and is powered by two tiny ramjet engines fastened to the blade tips. It has a cabin somewhat similar to that of the XH-26, but only a single rudder extends to the rear. The lightweight engines are versatile, since they can operate on almost any kind of fuel—gasoline, kerosene, and fuel oil. The young designer hopes some day after the requirements for military defense are over with, to make the Hiller Hornet a must along with the family automobile.

Going from aerial mosquitoes to flying giants, the Howard Hughes Aircraft Company has constructed the world's largest helicopter with the jet-powered XH-17. This whirling-wing monster, whose blades are 136 feet in diameter, is powered in a different manner from the other craft just described. It has two large turbo-jet engines fixed to the sides of the fuselage, and the gas pressure produced by these units is piped up through the rotor shaft and out to the tips of the incredibly long blades. As the gas escapes from openings in the edges of these wings, it spins them around at great speed. An open frame-work tail boom extends to the rear, and this is equipped with a smaller anti-torque rotor and an equally small horizontal wing.

The XH-17 is being developed for the United States Air Force and is designed for the specific job of lifting weighty objects rather than personnel. More like a flying crane than a transport plane, the helicopter is expected to haul heavy equipment such as artillery, tanks, bridge sections, and trucks over areas that would be all but impassable for ordinary means of travel. For this specialized work, the giant helicopter has a landing gear with four very long legs permitting it to straddle its cargo, which is then made secure. At this writing the flying windmill is undergoing its first test flights.

Jet-powered helicopters have a number of advantages over those with conventional piston engines. For one thing they are much simpler in construction and therefore lighter in weight. The weight-saving feature alone is very important, since it means that they can carry more useful cargo. The fact that jet helicopters don't require a complicated mechanical arrangement of gears and shafts for connecting the rotors to the engine also makes them cheaper to build and maintain. However, this type of helicopter has one great disadvantage when compared to other forms and that is its short flying range because of the large fuel consumption of the jet engines. Their superior qualities, however, are potentially so much greater that almost every helicopter manufacturer both here and abroad has at least one model either in the design or test-flying stage. Continued technical advances, especially in regard to reaction power plants, are increasing the advantages of the jet helicopter all the time.

Present-day military and commercial helicopters, such as the Sikorsky S-55, Bell H-13D, and the Piasecki HUP-1, have a cruising speed ranging from eighty to ninety miles an hour. Their top speed is about a hundred miles an hour. They can soar to a maximum height of 16,000 feet. For normal flying, however, the helicopters usually stay anywhere from 2,000 to 5,000 feet. These flying pinwheels are able to travel an average distance of 150 miles with a full cargo varying between 1,000 and 2,000 pounds.

More advanced models now coming off the drafting tables of helicopter builders will have cruising speeds of more than 120 miles an hour. They will also be able to fly with a greater pay-load for a distance of three hundred miles with little difficulty.

From "Flying Windmills" by Frank Ross, 1953

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