THE AUTOGIRO

Evening Post, Volume CX, Issue 135, 4 December 1925, Page 9

THE AUTOGIRO

WORKING DESCRIBED

EXTRAORDINARY MECHANISM

SOMETHING NEW IN AVIATION,

A few weeks ago the cable news from London recorded the astonishment created in aviation circles by the official demonstrations of the "Autogiro," a novel flying machine invented by a Spanish engineer, Senor de la Cierva. Since then, further particulars have been received, and the following matter, in which an attempt to explain 'tg operation, is taken from "Flight," the well known aero-transport journal, a representative of which witnessed -he trials. In this issue will be found a photograph of the machine. The description given in "Flight" indicates that, scanty as the visible details in the picture are, there was little more to see.

Although it cannot bo regarded as an entirely new machine, a description and illustration of it haviug been published more than two years ago, the de la Cierva "Autogiro" which was demonstrated over Laffan's Plain on 19th October incorporates some principles not hitherto applied to flying machines (says "Flight"). 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 actualiy to witness such flights. It was therefore with a good deal of eagerness that one proceeded to Farnborough to see Captain Frank Courtney fly the "Autogiro" at the Royal Aircraft Establishment. . .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, ps far as the working gear was concerned. THE ESSENTIAL FEATURE. The "Autogiro" portion, of the machine consisted of four wings, single-spar iiffairs, 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 tf 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 lie la Cierva arrived, and one was able to asfc questions concerning such points ••'& W6T> itss obvious from an examination of th# machine. Thus it was ascertained that the overall span of the "Autogiro" is in the neighbourhood of 36ft. while the mean chord of the winc;s :s 2ft 7in, and Hie area of each wins approximately 40 sqnaro. feet (a total of 160 squari feet in all). At full speed the. windmill lifting surfaces vevnlvc >-t something like IK) r.p.pi., or nr>out 2.3 revs, per spronci. At first tfiis spe°rl apr>eavs to bo very low. until a rapid estimnlo Ttidjmfes that 't nchtnl'y means a tin sr-wfl of n'^nt 260f nw' Funnel, or 1.80 '"ips per hour. That Win"' nppn r^n lanu. /tip bo^ins. to nmlr jrsf;i^o! whence the "Atifosiro" nlilniivi its lift, . , . . THE MACHINE TAKES OFF. 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 cablo 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 oeins* caused by the action of the lift rii the wings before they had reached sufficient speed for centrifugal fovce to bring them bank to the horizontal. .An the machine ran along the ground the windmill gathered speed, and aftei 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 l.c came in and landed. Approaching the aerodrome •at what appeared a fairly normal angle, he carried on until about 50 or 60 feet up, when ho 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 *he 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 "sta'l" in the ordinary sense of the term . . Captain Courtney made several flights. Shortly after the conclusion of the test flight a very distinguished party arrived. . . Captain Courtney then made several flights on the machine. On one occasion he came down in a very steep glide from about 600 feet, flattening out about 100 feet from the ground, and flying off again without' landing. HOW IT WORKS. The "Autogiro" is, in effect, a fourbladed airscrew mounted on an approximately vertical shaft, and having ■ its blades pivoted around horizontal axes ort this shaft. This "windmill" is rot geared to the engine in any way, but is caused to rotate solely by the air fofces acting upon it. Perhaps the best way of realising the reason for its rota- j tion is to picture tho "windmill" as be- I ing stationary, whiie the machine is travelling forward. Disregarding ft>r the moment the two blades which are in a fore and aft position t it will be obvious that the starboard blade is meeting the air leading edge foremost (the direction of rotation being anti-clock-wise 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 bavins; it subtracted therefrom. In a.rigid airscrew. .Hie result would !>o that there would-bo more lift on one side than on.(lie other. l>y reason of the blades being hinged, however, tlioy are freo to move up and down, and in doing so virtually increase and decrease their ancle 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 l<y slightly arching the blades tho 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 wins; section carefully chosen, presumably this travel is small. and consequently the torque is small also). A MECHANICAL PUZZLE. A contributor to "The Post" who has taken a keen 'interest in tho reports regarding the "Autogiro" and has endeavoured to see how it works, confesses himself unable to understand it. Ho writes: It may be suspected by some people that tho machine is not quite as. simple as it looks; for example, that the four-bladed supporting system is really driven by an engine and is not merely a passive affair, which ia rotated by the wind created by the forward motion of the whole machine. There is plenty of evidence.that this is not the case; but even if it were, de la Cierva would be able to claim success*in doing what nobody else has done yet; he would have made a successful helicopter, and would be able to lift a very healthy cheque.

There are several/ curious points which are not explained by "Flight" (continues the contributor). It appears that the four-Waded rotor from which the support of the "Autogiro" is derived, revolves simply because it

is blown round. The flying speed of the machine is about 60 miles an hour, but the wings rotate at such a speed that the velocity of their tips is about 180 miles per hour. The force which rotates the wings is, of course, the pressure of the air upon thoso which are at the moment moving rearward; and if there were no wind pressure on those moving forward, it can be supposed that the wing-tip velocity might exceed the flight speed of the machine. The actual velocity would depend on several factors, including the position of the "centre of pressure" of the wind on the wing, and the distribution of the resistance of the wing. But as the pressuro of the wind is also applied to the wiiiga, which are moving towards the front, the speed of rotation must be reduced; and the first puzzle is therefore how such a high speed of rotation is obtained. The wings may be regarded as a windmill. The normal windmill runs fastest •when the wind blows directly in the face of its "disc," and if presented edgewise to the wind, it stops. In th& "Autogiro" we have the wings rotating at a very high velocity when the "disc" is presented almost edgewise It is true that the relative wind due to tho flight velocity has at right angles to the wing-disc a component due to the fact that tho machino is virtually falling through the air all the time under the force- of gravity, its measured fall being reduced to zero (in horizontal flight) by tho tilting of the wing system. But it will be seen that the wind velocity at right angles to tho wingn will tent! not to make them run faster, but to check them, and it only adds to the puzzle. PERFORMANCE OF THE WINGS. I That the wings do rotate as describ- ' cd, however, must bo accepted; it 5s proved by performance. The next peculiarity is the very irregular distribution of the lift of,the wings. Taking for example a moment when two of the wings are fore and aft, these two will have a velocity through the air, at tho tips, of approximately 180 miles an hour, and will give a certain amount of lift. The other two wings have very different velocities. If the flight speed is 60 miles an hour, the tip of one wing will have an air velocity of 240 miles an hour, and the o^hcr velocity of 120 miles an hour, or exactly half.' The lift at'the tips of these wings, if they were not hinged, would differ much mora than their I velocities; the faster one would have ! four times the lift of the slower. 1 But at a point one-third of the length 'of the wing from the axis, one wing j will have a velocity of 120 miles an j hour relative to the «ir, and the other [ wing will.have no. velocity at all, and 1 therefore no lift. Owing to the auto- , matic flapping motidh'of tho wings, | ami the consequent change of the ! angles of incidence of the wings, these j discrepancies will bo somewhat reduced; but it is apparent that thcro is ' a great difference between the total lift on one side and the total lift on the other, and it seems that consideralbc importance attaches to the rather i large ailerons shown in the photographs of the "Autogiro" in their function of keeping the machine on an even j lcccl., The ailerons, of course, lose I their efficacy as the flying speed of | j the machine" is. reduced in a steep descent, but the unbalance also becomes less.

The "flapping" action of the wings, like their rotation, is a passive effect. The wings flap vertically, and are restrained in the extent of the flap solely by the great centrifugal force which tends to keep them in the same plans, and which converts the loose wings into a rigid structure from the centre of which the body of the "Autogiro" hangs by the shaft. In view of the varying speed of the elements of the wing from the shaft outwards, the centre of pressure on each wing is weir out towards the end, and a consideration of this fact and the consequence that the heavy machine is really supported by loose Wings on these widely-separated centres of pressure gives one a new respect for centrifugal force. . ■ . The description of the '' Autogiro" and its performances impresses the writer, like Maeterlinck's famous writing on the Eberfelt horses and their marvellous calculations. He thoroughly sympathises with the spectator of the flights, who said: "I've seen it, but ! I don't believe it."