Attempts to solve with the helicopter the problems of vertical flight
A HORIZONTALLY ROTATING PROPELLER enables this aircraft to rise straight up from the ground. This machine, the invention of a German engineer named Zaschka, has been successfully tested at the Berlin Central Aerodrome. The power plant is a 300 horse-power engine.
AN aircraft which will rise vertically from the ground and sustain itself in flight without forward motion has been the dream of inventors since the time of Leonardo da Vinci. For the past thirty years the dream has been supported by the knowledge that it is possible to lift a flying machine into the air by means of airscrews rotating in a horizontal plane.
Yet the secret of vertical flight eluded inventors, for though they could, and did, build aircraft which made vertical power-driven ascents, they were faced with the still greater problems of controlling the machine in flight and of ensuring a safe descent in the event of engine failure.
Only recently have these two problems been solved. With their solution there has at last emerged the true helicopter— a heavier-than-air craft which, propelled by power-driven rotors, can rise vertically from the ground and hover in fully controlled flight.
In June 1937 the German Focke-Wulf helicopter Fw.61, designed by Professor Henrich Focke, set up the following world records: distance in a closed circuit 80-6 kilometres (50 miles), distance in a straight line 108-9 kilometres (67.7 miles), speed over 20 kilometres (12.1 miles) 76 miles an hour, duration 1 hour 20 minutes 50 seconds, and height 2,439 metres (8,000 feet). On June 20, 1938, the helicopter travelled 230 kilometres (143 miles) in a straight line, thus more than doubling its previous record in this category.
In appearance, the Focke-Wulf machine bears a closer resemblance to an ordinary aeroplane than to the conventional design of earlier helicopters, most of which have been dwarfed by one or more huge and centrally-mounted horizontal rotors. The Focke-Wulf has a normal aeroplane fuselage with a fin and rudder at the tail and a 160 horse-power Bramo Sh.l4a engine mounted in the nose. Instead of wings, two upwards-inclined steel-tube pylons project at right angles from either side of the fuselage and carry on their apexes two three-bladed rotors.
These rotors, which are about 23 feet in diameter, have double-jointed blades and their angle of attack varies with the speed of their rotation. They turn in opposite directions and are shaft-driven, through gearing, by the engine in the nose. The engine also is fitted with a small tractor airscrew, but it serves merely as a cooling fan and plays no part in the propulsion of the aircraft. The machine is supported on a normal wide-track divided undercarriage and a single third wheel is mounted under the nose of the aircraft.
The pilot is accommodated in an open cockpit in the fuselage and controls the machine in flight with the rudder and by tilting the rotor-heads for lateral and fore-and-aft control. In the event of engine failure, the power-driven rotors can quickly be disengaged from the driving mechanism to revolve freely in the relative wind, thus permitting a safe descent in the manner of an “Autogiro” aircraft.
Professor Focke’s first experiments were carried out in 1932 with a scale model which, fitted with a 0.7 horsepower engine, proved the soundness of his theories by reaching a height of 59 feet, the altitude record for full-sized helicopters at that time. The full-scale machine was then built and tethered to the ground by wire cables so that trial flights could be made to test the mechanism and controls without risk of serious accident.
The first free flight was made by Ewald Rohls on June 26, 1936. The machine reached a height of 1,300 feet, and made a perfect three-point landing. Exactly a year later, on June 25-26, 1937, the machine had reached such an advanced stage of development that the same pilot was able to obtain several world helicopter records.
Before Professor Focke’s entry into the field, the most promising helicopter had been one built, in 1935, to the joint designs of MM. Dorand and Louis Breguet. M. Breguet’s interest in the problem of vertical flight dates back to 1908 when, in collaboration with Charles Richet, he designed and built what appears to have been the first man-carrying helicopter to make a free and sustained flight.
This early Breguet machine was a tandem biplane structure with inclined lifting screws. It was powered with a 55 horse-power Renault engine which drove a three-bladed windmill-like propeller, consisting of aluminium wings mounted on steel blades. In this machine Breguet and a passenger are stated to have risen vertically and flown horizontally for a distance of 64 feet at a maximum height of 15 feet on July 22, 1908. A second machine, built soon afterwards and fitted with a 40 horsepower Antoinette engine, also rose successfully but was wrecked in landing.
For a long time afterwards Louis Breguet abandoned the helicopter in favour of fixed-wing aircraft, but a few years ago he made a new attempt to solve the problem that was still baffling inventors. This time he called his machine a “Gyroplane”, and quickly demonstrated its promise by winning large sums of prize money in a helicopter performance competition arranged by the French Government. In these trials, held in December 1935, the Breguet-Doratid’s achievements included a flight of 1,625 feet over a closed circuit at an average height of about 100 feet, and the attainment of a speed of 62 miles an hour over a straight course.
Large Power-Driven Rotors
The chief feature of the Breguet machine is its large power-driven rotor unit, consisting of a pair of two-bladed rotors, mounted on a pylon one above the other and turning in opposite directions. The diameter of each of the rotor blades is 52 feet, and they turn at the rate of approximately 132 revolutions a minute. The engine, a 350 horse-power Hispano-Suiza radial, is mounted in the forepart of the “Gyroplane” and drives the rotors which, as there is no conventional airscrew, provide the forward propulsion as well as the means of vertical ascent.
The framework of the “Gyroplane” is of steel and the wheels of the exceptionally wide-track undercarriage are carried on steel-tube outriggers projecting from either side of the fuselage. The tail-unit, carried on a boom, has plywood-covered vertical fins and rudder. Here also are two horizontal stabilizing planes, the ends of which are upturned, with sharp dihedrals. Directional and lateral control are also ensured by the rotor blades themselves, each of which is mounted on a universal joint and is, therefore, free to adjust itself up or down during each revolution according to whether it is advancing into or receding from the relative wind pressure created by the machine’s forward movement.
In its present experimental form, the Breguet “Gyroplane” weighs slightly over 2 tons, of which nearly 1,500 lb. represent special recording instruments and apparatus which could be dispensed with in a standard production model. Without this unnecessary weight, the Breguet’s performance, both in load-carrying and m speed, would show considerable improvement. It is the inventor’s claim that the “Gyroplane”, as a type, gives promise not only of carrying greater useful loads than any aeroplane of equivalent power, but also of attaining much higher speeds, without impairing its ability to take off and descend vertically.
It is too early yet in the development of the Breguet “Gyroplane” and of the Focke-Wulf helicopter to adjudicate between them, but these two different designs undoubtedly represent the most successful direct-lift aircraft yet built, and the comparative progress of each will be watched with interest.
THE BREGUET-DORANO “ GYROPLANE ” won several prizes in December 1935 in a helicopter performance competition organized by the French Government. It flew 1,625 feet in a closed circuit at an average height of about 100 feet and attained a speed of 62 miles an hour in a straight line. The power-driven rotor unit comprises a pair of two-bladed rotors mounted on a pylon one above the other and turning in opposite directions. A 350 horse-power Hispano-Suiza engine is mounted in the forepart of the “ Gyroplane.”
Their wide divergence in design shows, however, that finality in the fundamental design of helicopters is not yet in sight. It is possible that one of the many different designs of the past, or some modern adaptation of it, may yet achieve still greater success and become the type pattern of the future.
Of the earliest helicopter designs little need be said, for the majority never progressed beyond the model-making stage. It is at once the special attraction and greatest disappointment of the helicopter that virtually any model, upon almost any principle, can be made to ascend with surprising ease. It is only when full-scale experiments, with their far greater weights of structure and motive power, are attempted that the true difficulties of vertical flight are encountered.
Thus the early story of the helicopter is a record of a long series of models built and flown by inventors all over the world. In France, as early as 1768, Paucton, a French mathematician, revived da Vinci’s idea of the ornithopter (see the chapter “Experiments With Ornithopters”) and designed a model, which he called a Pterophore. In this model two aerial screws, one to sustain and one to propel, were to be operated by the power of human muscles.
The first practical application of the helicopter is, however, generally attributed to Launoy, a naturalist, and to Bienvenu, an engineer. These two men in 1784 jointly built an apparatus with two superposed screws mounted upon the same vertical shaft and arranged to rotate in opposite directions — a pattern which, with individual modifications, has since been imitated by innumerable inventors. These early model helicopters were usually driven by an elastic substance such as whalebone, or by clock springs. In 1796 Sir George Cayley built a device which rose to a height of some 90 feet.
The development of the steam engine gave a new impetus to inventors. The first successful steam-driven helicopter model stands to the credit of W. H. Phillips, who in 1842 produced a two-bladed miniature which could ascend vertically and make short horizontal flights.
A Helicopter Balloon
Other inventors, seeking a new line of approach, tried a combination of lighter-than-air and heavier-than-air craft. In 1861 Henry Bright took out a patent for a helicopter to be suspended beneath the car of a balloon and worked by man-power to alter or maintain any desired altitude without expenditure of gas or ballast. According to the inventor, rotation of the horizontal airscrews in one direction would give ascent; rotation in another direction would give descent.
The American inventor Thomas Alva Edison was one of the first to apply electricity as a motive power to a model helicopter. This was in 1880, but the results were disappointing. Helicopters remained little more than amusing toys until 1907, when Paul Cornu, a Frenchman, demonstrated the practicability of a full-sized, man-carrying vertically-ascending aircraft — a combination which many scientists had, by that time, come to regard as impossible of achievement.
SUPPORTED ON A FOUR-WHEELED CHASSIS, the Pescara helicopter had a vertical shaft with two superposed rotors which were turned in opposite directions by one 180 horse-power Hispano-Suiza engine. Each rotor consisted of six sets of small biplane wings, with an area of about 270 square .feet and radiating like wheel spokes from a central shaft. The rotors revolved at a speed of about 200 revolutions a minute In May 1924 the Pescara helicopter made a straight flight of 2,550 feet in 4 minutes 11 seconds at a height of about six feet from the ground.
Cornu succeeded in remaining in the air for only about sixty seconds at a time, but he made several flights, alone and with a passenger, and his achievement was unique. His machine was lifted by two oppositely-revolving screws, each about 20 feet long and connected by a belt to a single petrol engine, and he relied upon a new principle to give his helicopter horizontal as well as vertical propulsion. A set of vanes of variable angles was fitted beneath the rotors so that their down-draught would react against the vanes in a horizontal plane. By this ingenious method, Cornu was able to achieve a forward speed of 10 feet a second, equivalent to nearly seven miles an hour. Cornu’s success was closely followed by Louis Breguet’s still greater achievement of a controlled flight of 64 feet in 1908. In the following year America produced a craft powered by two engines, each geared directly to two-bladed airscrews revolving in opposite directions. The joint inventors were Emile Berliner and J. Newton Williams. On three occasions the machine, loaded to a total weight of 610 lb, succeeded in lifting Williams clear of the ground.
These three successes inspired inventors anew, but it was not until midway through the war of 1914-18 that another full-sized helicopter managed to leave the ground. This was a particularly ambitious attempt — no less than a military helicopter armed with a machine-gun and with accommodation for several occupants. The inventors were Lt. Stefan Petroczy, of the Austro-Hungarian Army, and Professor von Karman.
Their machine had three 120 horsepower Le Rhone petrol engines and weighed 2,900 lb. The machine-gun was mounted in a ring over the observer’s seat and a huge parachute was carried, upon which the occupants had to rely for the safe descent of the machine and themselves in the event of engine failure. No free flights were ever made by this machine but, picketed to the ground by ropes, it made numerous successful ascents to heights up to 160 feet carrying a maximum load of four men, and on several occasions remained aloft for as long as an hour.
Later, the same inventors designed an electrically-driven captive helicopter attached to the ground by a cable which carried the current for the machine’s 225 horse-power Daimler electro-motor. This machine, which differed slightly from the previous design, comprised a steel framework, an observer’s car and four horizontal rotors. The entire apparatus weighed about 1,400 lb.
A £50,000 Competition
Post-war interest in vertical flight was aroused by a helicopter competition inaugurated by the British Air Ministry in 1923. Among other conditions, the rules stipulated that the winning helicopter, carrying pilot, an hour’s fuel and a 150-lb. load, had to rise vertically to 2,000 feet, remain hovering over a specified area for thirty minutes, attain a speed of not less than sixty miles an hour and descend, undamaged, without using its engine.
Despite the optimism of nearly twenty inventors who submitted entries for the contest, no machine came anywhere near qualifying for the prize of £50,000. Most of this sum the Government later spent in financing the fruitless development of a British helicopter which had been designed by Louis Brennan, inventor of the Brennan torpedo and of the gyroscope monorail.
The Brennan experiments were carried out in conditions of great secrecy, and even now little is known about the design of the machine. It was stated to have made several tethered flights inside a balloon shed at Farnborough, Hampshire.
The French Air Ministry meanwhile was also showing official interest in helicopters and had offered a cash prize for the first kilometre circuit (five-eighths of a mile) to be flown by a helicopter. The prize was claimed on May 4, 1924, by Etienne Oehmichen, designer and pilot of a rotating-wing aircraft of extraordinarily complicated design, which succeeded in making a circular flight of a little under two miles in 7 minutes 40 seconds.
Oehmichen’s helicopter experiments had begun in 1921 with a revival of the old idea of combining lighter-than-air and heavier-than-air principles of flight. A metal framework, carrying two horizontal rotors and an engine, was attached to a gas-filled envelope. Several ascents were made which, however, owed more to the natural lift of the gasbag than to the propulsive power of the rotors. Oehmichen then abandoned the gas envelope and produced in 1923 a much more elaborate design in the form of large, unequal-span cross-arms. Lifting screws were placed at the four extremities of the cross and were driven, at the rate of 145 revolutions a minute, through shafting, by a 120 horse-power Le Rhone engine mounted at the centre of the cross. In addition to this lifting mechanism, there were two belt-driven propulsive airscrews placed halfway out on a lateral arm of the cross and — complicating the design still further — five auxiliary propellers of variable pitch, driven from the main shaft. These were attached at the extremities of the arms to provide control about every axis. A large flywheel was incorporated in the driving mechanism so that its gyroscopic effect would give stability in calm air and help to damp out rolling in gusty weather.
D’ASCANI O S HELICOPTER made a vertical ascent of over 20 feet in 1930, as well as an endurance flight lasting nearly nine minutes and a closed-circuit flight of one kilometre (five-eighths of a mile). D’Ascanio also succeeded in hovering in mid-air for one and a half minutes. His machine consisted of a slender metal framework with a central vertical shaft supporting two horizontal superposed rotors revolving in opposite directions. A 95 horse-power Fiat engine was carried in the lower framework.
The whole structure weighed 2,200 lb. That was possibly the reason why the inventor dispensed with a normal undercarriage and used, instead, six inflated footballs mounted on shock-absorbing struts.
The Oehmichen helicopter was able to fly, and when in April 1924 the Federation Aeronautique Internationale officially recognized helicopter records it was Oehmichen who became the first holder of a world helicopter record. On April 17, 1924, he made a horizontal flight of 1,722 feet. The following month he won the French Air Ministry’s cash prize, though by then he had lost his
world record to the Marquis Pateras de Pescara. Pescara had made a straight-line flight of 2,550 feet in 4 minutes 11 seconds at a height of about six feet above the ground.
Pescara’s success was well merited, for he had already spent several years and a small fortune in vertical flight experiments and had produced five helicopters of varying designs. The one on which he achieved his record flight was supported on a four-wheeled chassis and had in the centre a vertical shaft with two superposed rotors which were turned in opposite directions by one 180 horse-power Hispano-Suiza engine.
Six Sets of Small Wings
The special feature of the two rotors was that each consisted of six sets of small biplane wings, with an area of about 270 square feet, and radiating like wheel spokes from the central driving shaft. In action, the helicopter resembled twelve miniature biplanes revolving in two superposed groups of six at a speed of about 200 revolutions a minute and with a tip speed of about 200 feet a second.
Despite its initial success, the Pescara helicopter did not make much further progress, as the old problem of adequate control in flight once more proved an insurmountable obstacle to development.
Meanwhile, Berliner in America had resumed the experiments that he had begun in 1908. His first new successes were a series of brief vertical ascents in 1922. Though these greatly impressed the official Army and Navy observers, Berliner himself realized that in the event of engine failure his machine would at once fall to earth like a stone. He therefore withdrew his helicopter and spent the next two years in modifying his designs to permit of a free and sustained glide if the engine stopped.
His new machine appeared in 1924, and somewhat resembled a triplane, with two lifting propellers rotating horizontally at the centre line of the wings. These lifting propellers could be so altered in their angle of rotation as to give forward flight as well as vertical ascent. Vertical flight was assisted by a variable-pitch propeller mounted at the tail. In the event of engine failure, the machine would be sustained in a glide by the lifting surface of the three fixed triplane wings.
Powered with a 200 horse-power Bentley rotary engine and carrying a pilot, the Berliner weighed 1,950 lb., but despite, or perhaps because of its elaborate design, it proved a failure. Its maximum duration in flight was 1 minute 35 seconds and the greatest height it could manage to reach was but 15 feet.
SEVERAL WORLD RECORDS were set up uy this Focke-Wuit helicopter in June 1937, a year after it had made its first free fl.ght. These records included a speed over 20 kilometres (12’4 miles) of 76 miles an hour, a height of 8,000 feet and a flight duration of I hour 20 minutes 50 seconds. The Focke-Wulf helicopter, which was designed by Professor H. Focke, has a normal aeroplane fuselage with a fin and rudder at the tail and a 160 horse-power Bramo engine mounted in the nose. Instead of wings, two upwards-inclined pylons on either side of the fuselage carry on their apexes two three-bladed rotors.
A Spanish engineer, Senor Juan de la Cierva, now entered the field of vertical flight experiment with a new type of rotating-wing aircraft which he called an “Autogiro”. The “Autogiro” aircraft is not a helicopter, as its rotors are not power-driven in flight, but it introduced an entirely new principle of flight and showed the way towards a solution of several problems which had for long been baffling helicopter designers.
The “Autogiro” aircraft, by its first successful flights in 1923, demonstrated how rotating planes, freely revolving in an upward airstream, would sustain a machine in flight and allow an engineless descent under full control. It proved also that the overturning tendency set up by rotors in horizontal flight could be overcome by the full articulation of the rotor blades at their roots.
The first to appreciate the importance of these discoveries and to apply them to the development of the helicopter was Vittorio Isaaco, an Italian engineer who had spent several years collaborating in the construction of the Pescara helicopters. Isaaco called his machine a “Helicogyre”, and it resembled a four-bladed “Autogiro” aircraft, with the fundamental difference that whereas the “Autogiro” aircraft’s rotors are wind-driven in flight, Isaaco fitted an engine to each of his four rotor blades, each engine driving a small vertical tractor airscrew.
His first machine, built for the French Government in 1927, had a 50 horsepower Anzani engine mounted midway along each of the four rotor-blades. The “Helicogyre” succeeded in leaving the ground several times. A second machine was then built having only two rotor-blades.
This model was less successful. When the British Government ordered an experimental machine in 1930, Isaaco reverted to his original four-bladed design, using Bristol Cherub engines as on the two-bladed model, to drive the blades round.
Considerable ingenuity had to be exercised by the inventor in devising means for supplying fuel to and controlling each of the engines rotating with the blades. It was probably because of this complication that no further development of the design took place after 1930. Yet the Isaaco “Helicogyre” was a most interesting example of an entirely new method of overcoming the long-standing problem of autorotation — that tendency in a helicopter for the whole fuselage itself to rotate in the opposite direction to that of the power-driven lifting screws.
Broke All Existing Records
Another Italian inventor, Corradino D’Ascanio, made the next step forward. There was no doubt about the promise of his design, for in 1930 he broke all then existing records for helicopters with a vertical ascent of over 20 feet, an endurance flight of eight and three-quarter minutes and a closed-circuit flight of one kilometre (five-eighths of a mile). D’Ascanio even succeeded in hovering stationary in mid-air for one and a half minutes. Unlike Isaaco’s original conception, D’Ascanio’s machine was of the now almost conventional helicopter design — a slender metal framework with a central vertical shaft supporting two horizontal superposed rotors revolving in opposite directions. His power plant carried in the lower framework, was a 95 horse-power Fiat engine installation.
D’Ascanio’s records stood unbeaten for nearly three years. In 1933, Nicholas Florine, a Belgian, captured the duration record with a flight of 9 minutes 58 seconds. The Florine helicopter was of more original design. It had the usual two sets of rotors, but instead of mounting these one above the other, Florine installed one set at either end of his long framework and drove them both through shafting from a 200 horsepower engine set midway between the two. So confident was the inventor of his machine’s ability to make consistently vertical descents that he dispensed altogether with a wheeled undercarriage and mounted his helicopter on four metal stands.
Yet another modern type of helicopter, of which much was expected, is the direct-lift aircraft designed by Oskar von Asboth, a former director of the Central Aviation Experimental Station in Austria. The basic design of the Asboth is a wingless fuselage with one or more horizontal rotors mounted on a central pylon and driven from an engine in the body, which also drives a propulsive tractor airscrew in the nose.
The original Asboth helicopter was built in 1928, and is stated to have made a total of 182 flights, each beginning with a vertical ascent. On one occasion the machine is said to have remained in the air for fifty-three minutes. Three more improved models were built and flown. In 1934 it was announced that arrangements had been made for the construction of a fifth machine.
BUILT IN 1923 BY ETIENNE OEHMICHEN, this helicopter had the form of a cross. Lifting screws were placed at the four ends of the cross and were driven through shafting by a 120 horse-power Le Rhone engine mounted in the centre of the cross. In addition, there were two belt-driven airscrews and five auxiliary propellers. A large flywheel incorporated in the driving mechanism provided gyroscopic effect, to give stability and to prevent rolling. Instead of an undercarriage, the machine used six inflated footballs mounted on shock-absorbing struts.