Flight, October 1925

A NEW TYPE OF FLYING MACHINE
The Cierva "Autogiro" Demonstrated at Farnborough

   ALTHOUGH it cannot be regarded as an entirely new machine, a description and illustration of it having been published in FLIGHT more than two years ago, the de la Cierva "Autogiro" which was demonstrated over Laffan's Plain on Monday of this week, October 19, incorporates some principles not hitherto applied to flying machines. Reports of flights with this machine have reached this country from Spain from time to time, but it is one thing reading rather vague reports of flights with a new type, and quite a different thing actually to witness such flights. It was therefore with a good deal of eagerness that one proceeded to Farnborough on Monday to see Capt. Frank Courtney fly the "Autogiro" at the Royal Aircraft Establishment. In order to make sure of arriving in good time we took an earlier train than that which had been described by the Air Ministry as "suitable," and arrived at Farnborough shortly after 10 a.m. Knowing his apparently inexhaustible energy, one was not in the least surprised to discover that the Director of Civil Aviation, Air Vice-Marshal Sir W. Sefton Brancker, had also decided upon an early start, and the small party was soon within the sacred precincts of the R.A.E., where a visitors' book was duly signed, credentials presented and other small formalities seen to.
   It was discovered that the "Autogiro" was out in the middle of the aerodrome, and what was worse, was being towed slowly but with maddening persistency farther and farther away in the general direction of Laffan's Plain. One waited for Sir Sefton to give a hint as to what was the proper procedure under the circumstances, and had a few momentary, although as it proved, futile, visions of a conveyance heading Laffan-wards. But the D. of C.A. scorned such modern aids to locomotion and set off hot-foot (at first) in pursuit of four curious flapping wings discernible in the dim distance. The small party had not gone far before experiencing how abominably wet the grass on Laffan's Piain can be, and the guess was ventured that this, was a special kind of R.A.E. experimental grass, specially produced to dissuade curious onlookers from trespassing on the sacred ground over which all our hush-hush machines make their first secret flights. On this occasion there was only a large single-engined biplane fitted with a Rolls-Royce air-cooled "Vulture" engine, and Leitner-Watts all-wood propeller on the ground, and as the engine was idling over, it was suggested that this was done to prevent visitors from seeing the secret propeller.
   Finally, the "Autogiro" came to a standstill opposite the pavilion on the Plain, and our party slowly approached it, all blessing the very wet wetness of the grass in hearty undertones. All except Sir Sefton Brancker that is. The Director of Civil Aviation looked as if he always walked three miles in wet grass after breakfast, and if his indifference was not genuine it was splendidly simulated.
   On reaching the "Autogiro" a detailed inspection was at once commenced, and owing to the utter simplicity of the mechanism, it did not take long to master the details, as far as the working gear was concerned. The "Autogiro" portion, of the machine consisted of four wings, single-spar affairs, pivoted around horizontal hinges to a steel tube pillar rising from the fuselage. Near the top of this pillar were substantial shackles holding short lengths of rubber cord, which prevented the wings from drooping below a certain level. Just below the shackles were a couple of ball thrust bearings and that seemed to be about all. The rest of the curious bird was an Avro 504K fuselage with normal tail, a le Rhone rotary engine, and a more or less normal Avro undercarriage. Apparently in order to avoid a too violent departure from normal, a couple of spars projected laterally from the fuselage, each carrying a fairly large aileron. It was later learned that these served mainly to cheer up the pilot and were in no way necessary. Almost as soon as the inspection of the machine was completed, Mr. Courtney and Senor de la Cierva arrived, and one was able to ask questions concerning such points as were less obvious from an examination of the machine. Thus it was ascertained that the overall span of the "Autogiro" is in the neighbourhood of 36 ft., while the mean chord of the wings is 2 ft. 7 in., and the area of each wing approximately 40 sq. ft. (a total of 160 sq. ft. in all). At full speed the windmill lifting surfaces revolve at something like 140 r.p.m., or about 2-3 revs, per second. At first this speed appears to be very low, until a rapid estimate indicates that it actually means a tip speed of about 260 ft. per second, or 180 miles per hour. That having been realised, one begins to understand whence the "Autogiro" obtains its lift.
   Although the official demonstration flight was timed to take place at 12 o'clock, Mr. Courtney decided to make a trial flight, and the two Spanish mechanics consequently commenced to "wind up" the windmill. This process is necessary at present in order to reduce the length of run required for the machine to take off. As soon as the whole of the cable had been wound off the wood blocks on the wings, Courtney opened out his engine and the machine commenced to taxi. It was noticed that after running some distance along the ground the wings commenced to rise at a relatively pronounced angle, this evidently being caused by the action of the lift on the wings before they had reached sufficient speed for centrifugal force to bring them back to the horizontal.
   As the machine ran along the ground the windmill gathered speed, and after a run of approximately the normal required by the standard Avro, the "Autogiro" rose slowly into the air. Courtney made several turns to right and left, showing that the machine was under perfect control, and finally he came in and landed. Approaching the aerodrome at what appeared a fairly normal angle, he carried on until about 50 or 60 ft. up, when he pulled back the stick. The tail of the machine dropped, and a descent was commenced which seemed to be very nearly vertical. Actually we believe the horizontal speed at the time was between 10 and 15 m.p.h. The machine touched the ground, ran a distance of rather less than its own length, and came to rest, the windmill continuing to rotate for quite a long period afterwards. From the short flight it was perfectly obvious that in the "Autogiro" one has a machine which is capable of flying horizontally under perfect control; is able, if desired, to glide down at about the normal angle for an ordinary machine, or is capable of descending at a very low horizontal velocity indeed. Throughout the machine was evidently under perfect control and showed no tendency to "stall" in the ordinary sense of the term. Actually, when descending in a nearly vertical flight path, the rotating wings are, of course, following a helical path so that, although the machine itself is descending almost vertically, the lifting surfaces are actually gliding at fairly high speed, the only difference being that they follow a helical path while the machine itself follows a straight one.
   Shortly after the conclusion of the test flight a very distinguished party arrived, including the Secretary of State for Air, Sir Samuel Hoare; the Chief of the Air Staff, Air Chief-Marshal Sir Hugh Trenchard; the Air Member for Supply and Research, Air Vice-Marshal Sir Geoffrey Salmond, and a number of other officials prominent in Government aviation circles, conducted by Mr. W. Sydney Smith, Superintendent of the Royal Aircraft Establishment at Farnborough. Capt. Courtney and Senor de La Cierva briefly explained the principle of the "Autogiro" to an interested assembly, and Capt. Courtney then made several flights on the machine. On one occasion he came down in a very steep glide from about 600 ft , flattening out about 100 ft from the ground, and flying off again without landing. Throughout the machine appeared to be under perfect control.
   The action of the "Autogiro" has already been described in FLIGHT, but a brief outline may be of assistance, and may serve to fix ideas. The "Autogiro" is, in effect, a four-bladed airscrew mounted on an approximately vertical shaft, and having its blades pivoted around horizontal axes on this shaft. This "windmill" is not geared to the engine in any way, but is caused to rotate slowly by the air forces acting upon it. Perhaps the best way of realising the reason for its rotation is to picture the "windmill" as being stationary, while the machine is travelling forward. Disregarding for the moment the two blades which are in a fore and aft position, it will be obvious that the starboard blade is meeting the air leading edge foremost (the direction of rotation being anticlockwise when viewing the windmill from above), while the port blade is meeting the air trailing edge foremost. The resistance of the blade travelling leading edge foremost is smaller than that of the blade travelling trailing edge foremost, with the result that the windmill begins to rotate.
   It will be realised that during flight the blade which is travelling forward is at a greater relative velocity than that travelling back, the one having the machine's speed added to its own velocity and the other having it subtracted therefrom. In a rigid airscrew the result would be that there would be more lift on one side than on the other. By reason of the blades being hinged, however, they are free to move up and down, and in doing so virtually increase and decrease their angle of incidence, thus equalising the lift on the two sides. The special feature of the "Autogiro" is that centrifugal force is relied upon to keep the blades outstretched against the action of the lift, and by slightly arching the blades the designer has attempted to ensure that all the stresses are purely tensile ones (with, presumably, a slight torque or twisting stress caused by the travel of the centre of pressure. As the chord is small and the wing section carefully chosen, presumably this travel is small, and consequently the torque is small also.)
   This, very briefly, is the principle of the "Autogiro." A number of interesting problems naturally arise, but with these doubtless the inventor, Senor de la Cierva, will deal in the paper which he is reading tonight before the Royal Aeronautical Society.