THE AUTOGIRO ADVANCES. TWO NEW PRODUCTION MACHINES WITH MECHANICAL ROTOR STARTER AND FOLDING BLADES.
IT is eleven years since Senor Don Juan de la Cierva designed and built the first Autogiro, and in the intervening period, in spite of many set-backs–of which popular prejudice against the unconventional was not the least serious —this remarkable aeroplane has slowly but surely advanced in development until to-day it has reached so practical and satisfactory form, in the opinion of its manufacturers, that they have decided
to place it on the market.
The Autogiro is undoubtedly one of the most outstanding breakaways from the orthodox in the history of aeronautics, and even though it has been widely demonstrated both in England and abroad (probably more than any other individual aircraft) its extraordinary appearance and performance arouses very great interest wherever it is seen. And small wonder, for it is unlike anything else which ever went into the air. When Cierva designed his first machine he did so with the main object in view of producing an aeroplane which would not stall under any circumstances, and prolonged thought and study brought him to the conclusion that in reaching this end the aeroplane as is now known must be ruled out. It is true that the modern ‘plane of normal design is safe in the sense that it is reliable and has none of the tricks” which characterised machines of by-gone days ; an accumulation of experience and data now permits the designer to build a machine which is devoid of any structural weakness and which has a respectable performance in regard to climb, speed and manoeuvrability. Engines, too, are wonderfully reliable. But the conventional type of aeroplane can still be a dangerous vehicle in the hands of the unskilled and the unwary since it becomes uncontrollable below a comparatively high speed. The novice who cannot sense that he is aproaching the period of the stall, or a
worried pilot intent on getting down in a sudden emergency, even in these days, can find to his alarm that the controls have “gone sloppy,” and a wing has dropped, or worse still, he has gone into a spin. Provided he has sufficient height and knows how to act, this losing of flying speed need not end in disaster. But accidents still happen, and a big proportion of aeroplane crashes are due to the stall and unrecovered spin. With the Autogiro no amount of mishandling can get the pilot into such difficulties, since by its design and working principle the necessity of maintaining a relatively high forward speed in order to keep stability is eliminated. How this is brought about can be best explained as follows :— The blades of the rotor, which are mounted so as to revolve freely on the pylon, are in effect aerofoils of small chord ; they are hinged at their attachment to enable them to have freedom in both the vertical or ” flapping ” plane, and in the rotational plane. Once energised they maintain their rotation by aero-dynamic action, and in revolving, induce lift to the machine. It will be readily grasped that the rotation of the blades is not dependent on the forward movement of the aircraft, and that even when the air speed is nil, other forces are acting upon them to keep them revolving, and thus, so long as they are moving, the rotor disc acts as the lifting medium. In other words, although the ma
chine itself may have no forward speed, its wings are still travelling through the air at a high velocity, and thus giving support.
In the matter of stability, the hinged arrangement of the rotor blades to the shaft allow any difference of lift to be taken up by the movement of the blade Ii an upward or downward direction on the hinged root, so that all lift forces must pass through one point at the centre, and so supply complete stability.
It was in 1920 that Don Cierva completed the construction of his first experimental craft. This, as in the present Autogiros, had a normal fuselage, but the lifting surface comprised two four bladed windmills, mounted in a vertical axis and revolving in opposite directions. A normal type rudder, fin and elevator were used. Machine No. 1 proved a disappointment, and although various modifications were carried out, it was a failure.
Undeterred, the designer produced a second craft, and this had a rotor consisting of a single windmill made up of three cantilever blades, and a device was included in the general arrangement which allowed the pilot to alter the angle of incidence, so as to vary the amount of hit to the right or to the left. Preliminary trials afforded some degree of encouragement, but in the end Cierva came to the conclusion that his theories in regard to this machine were at fault, and although it was reconstructed and altered nine times, he finally scrapped it.
In the third Autogiro, the design embodied a windmill with five blades rigidly braced to the axis with steel wires, and although this machine was promising as far as lift was concerned, stability was lacking, and after being crashed several times, it was abandoned, and a fourth type was built.
The Articulated Blade Solution,
It was at this stage that Cierva tried out the principle of articulated blades in the rotor, which was subsequently to prove the solution of the problem. In his fourth design the rotor had four blades hinged at the roots, so that they were enabled to move freely up and down in the vertical plane, and in addition, a device was used so that the rotor column could be tilted to the right and the left by a control worked by the pilot. The idea of this was to maintain lateral control, but in practice it proved too difficult to operate. Incidentally, this machine (No. 4) was crashed no less than fifteen times
An Intensive Two Years.
In all, the designing, building and trying out of these experimental models took about two years, and although a certain amount of -success had been achieved, the Autogiro was far from being a practical or safe aircraft.
In January, 1923, however, Don Cierva constructed yet another ‘plane which ultimately flew in a closed circuit for four minutes at Cuatro Vientos aerodrome, near Madrid, under official observation, and so the first definite stage had been reached in the evolution of his original aircraft.
Thus encouraged, he continued experimenting, and later met with such success that the Spanish Government authorities came forward with assistance, and under their auspices yet another machine was made. This particular Autogiro was built up on an Avro 504 fuselage, and the standard undercarriage, engine (110 h.p. Le Rhone) and tail unit were retained. It was flown quite extensively, and was demonstrated for the ‘first time outside Spain, when in 1925. its designer brought it to the R.A.E. at Farnborough, and also flew it before French aeronautical experts at the aerodrome at Villacoublay.
Later it was shown and flown at the R.A.F. Display by Frank Courtney, who subsequently carried out a lot of test work for the Cierva Autogiro Company which was formed in this country in 1926. From that time until now, the company.
has worked incessantly towards the perfection of the Autogiro and something like twenty-one different types have been constructed and tried out. At the same time, demonstrations have been constantly made all over the British Isles and on the Continent, while in the U.S.A. the Pitcairn Autogiro Company of America Inc., have been doing work of a like nature.
And now, after 11 years of effort, the Autogiro has been placed in regular production in direct competition with light aircraft of the conventional type. There are two models, the C.24 and the C.19 Mark IV, and each is now standardized. The former is the latest design, and is a cabin machine with a three bladed rotor. Designed by the Cierva Company, the C.24 was made by the de Havilland Co. This latest Autogiro has many interesting and striking features, Of which the most startling is the rotor. In the earlier editions, the blades (of which there were four) were connected by lengths of stranded cable which served to prevent any tendency of any one blade to close up or ” catch up ” with the preceding one while rotating ; in the C.24 rotor, this device is dispensed with and in its place stops are interposed between the roots of the blades at the hinges. The blades can also be folded back, to facilitate housing the machine. Another modification is found in the absence of the supporting cords which were used formerly to prevent excessive droop of the blades when at rest. The latter are now full cantilever, and as a result the rotor as a whole is much cleaner and neater in form and outline. A further development, which is a marked advance on previous practice, is that the rotor is now set in motion by mechanical means ; readers who have followed the progress of Don Cierva’s machine, may remember that in the early models the rotor was started by using a rope wound round the shaft, and this was pulled by about half -a-dozen
men—ti somewhat crude device. Then experiments were tried with rockets attached to the tips of the blades. Later it was found that the initial rotations could be set up by taxiing the machine about for some minutes. But this, of course, was a slow and rather elementary business. The next step was to incorporate a novel form of tail unit in the Autogiro’s general lay-out in which the elevator could be raised to a vertical position so that it deflected the slipstream from the propellor on to the rotor, and set it in motion in that way. But now the engine is utilised to do the job, through a straightforward system of shafts, gears and a clutch. The cabin Autogiro has a metal fuselage of more or less normal design, and this is constructed with welded steel tubes with the rotor pylon built into it. In order to keep down the frontal area the cross-section is narrow and the en
closed cockpit contains two seats in tandem. The upper part of the rotor pylon is faired off in good streamline form which tapers down to terminate in a fin of large area at the rear end. The tail unit is of normal type, except that two fixed fins are attached to the ends of the tailplane. The rudder and elevator are of orthodox pattern, and are operated by the usual rudder-bar and stick. The main ‘plane with upturned tips and narrow chord which has been a common feature of all the more recent types of Autogiro is retained in the C.24, and faired struts are affixed to the spars and to the main members at the top of the cabin. An ordinary form of axleless “undercarriage with a small ” booted” wheel in front, and a well-cowled D.A. ” Gipsy ” Mark III engine complete the new model’s specification.
Actual test of the C.24 shows that it has a maximum speed while flying level of 120 m.p.h., and that it can be flown without loss of height at 28 m.p.h. It can be landed, too, without any difficulty at no more than 7 to 8 m.p.h. The incorporation of the mechanical starting now makes the initial rotation of the rotor a matter of about 30-40 seconds, and the actual take-off occupies no more time than is found with machines of normal design and with approximately the same horsepower.
The other production model Autogiro— the C.19, Mark IV—is an open two-seater similar in general arrangement to the type which has been flown so extensively by Mr. R. A. C. Brie in the past twelve months. It is fitted with a 100 h.p. Genet Major engine, and like the C.24, has the new mechanical starting gear for the rotor. Various details have been cleaned up, and the craft, while still appearing unusual, looks compact, sturdy and workmanlike. The performance figures of the C.19 are given by the makers as follow :— Maximum speed, 105 m.p.h. ; minimum speed (without loss of height), 28 m.p.h. ; cruising speed, 90 m.p.h.; take-off run, 30 yards ; ceiling, 14,000 feet ; range (at 90 m.p.h.), 250 miles. It has been abundantly shown by expert pilots, such as Mr. Rawson and Mr. Brie, that the Autogiro does all that its designer claims for it ; that it takes off as quickly and flies as quickly as the average, ordinary aircraft of the same power. It has also been proved that it can be landed in such places and in a way that would be impossible with other aeroplanes. And now that the ” windmill” ‘plane is available for the private purchaser it should be possible to gauge soon how far
its unique quality of foolproofness will go in overcoming the inevitable prejudice which the unconventional has to combat before being accepted as a practical proposition.
NrIsrroRs to the Humber factory at Coventry are generally very curious to know why a model aeroplane is suspended prominently in the entrance hall. Few people realise that several years before the war a Humber monoplane was actually produced and was one of the first machines ever to fly. The model at the factory, however, is an example of Humber’s war-time production, when no fewer than twenty-five complete aeroplanes were turned out every week in addition to a vast quantity of other war material.
Humber’s 25 Aeroplanes per Week.