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Aviation History
1951
1951 - 0027.PDF
British-built Isacco helicogyre (1930). The blade engines were Cherubs, the nost engine a Genet. Note separate control rotor. THE HELICOPTER... tural problem more difficult, due to me periodic bending moments at the blade roots, (f) Use can be made of articu- lated blades, Le., blades mounted on hinges, so that they are free to "flap" and so automatically adjust their effective angle of incidence. The advancing blade flaps upward, so reducing its incidence and the air load on it, and the re- treating blade flaps downwards. This principle was known in the early days of rotating-wing development, but it was not until Cierva rediscovered it and applied it to his Autogiro that its importance was appreciated. Unfortunately, it intro- duces periodic forces in the plane of rotation, with attendant vibration troubles, and it is necessary to introduce a second hinge, permitting freedom of movement in the plane of rotation. In spite of its drawbacks, however, it was the articulated blade which gave us the first practical helicopter and even today all helicopters in operation have this type of blade system. It has the additional merits of relieving the blade roots of the bending moment due to the air loads, and of reducing the effect of gusts. Control.—In the early helicopters, control was often sought by having surfaces hanging in the downwash from the rotors, or by auxiliary variable-pitch airscrews mounted on outriggers about appropriate axes. The first arrangement is not good, because it introduces extra weight and drag and because the controls become inverted in the forced-landing case when the rotor is in auto-rotation and the airflow is reversed, being now upwards through the disc instead of downwards as in normal power-on flight. The second alter- native, while giving good control, is poor because of the extra weight and drag and the mechanical complexity. Here again, the gyroplane showed the way, either as in Cierva's Autogiro, where the whole rotor was tilted in the desired sense, or in the form developed by Hafner, in which the lift vector is tilted not by tilting the whole rotor, but by a cyclic change of the blade angles, which by tilting the tip- path plane achieves exactly the same result. The latter method is more appropriate to the helicopter, where the rotor is permanently driven from the power unit, since the tilting head in this case involves considerable mechanical difficulties. With rigid rotors using cyclic lift-control the lift vector, instead of being tilted, moves eccentrically, so giving a very powerful control. Stability.—Stability is a big subject. It will suffice to say here that the ordinary single articulated rotor is not very good as regards its stability characteristics. These make it very tiring to fly on instruments under blind-flying conditions. The stability problem is now beginning to be better understood and stable rotors can now be designed. The superimposed type of twin rotor is unstable, while the articulated side-by-side, or Cierva C.I 9 Mk. IV two-seater Autogiro, circa 1931. Asboth's fourth helicopter (1930). The machine is seen coming in to land, with the author at the controls. tandem, arrangements are stable about the axis parallel to the line of the two rotors, but relatively unstable about the axis at right angles. The side-by-side type, using cyclic blade lift-control, is stable about all axes. The Forced Landing.—Besides the articulated rotor and tilting-head control, Cierva's other great contribution to rotating-wing aircraft was the discovery of the high para- chutal value of an auto-rotating rotor with the blades at small positive angles. In the event of power-unit failure in a helicopter all that is necessary is for the blade angles to be reduced, when the rotor will remain rotating, the aircraft then becoming a gyroplane, and a safe landing can be effected by using ordinary auto-rotative technique. From this simple resume^ it will be seen that Cierva's Autogiro had almost all the elements necessary for a practical helicopter. Indeed, in its final form—the so-called " direct take-off " type—it was a helicopter for a few seconds at take- off; when the blades were put at small angles and over- rewed by the engine, the engine then declutched, and the blade angles suddenly increased. The aircraft then jumped vertically as a true helicopter, using up the kinetic energy of the rotor. At the top of the jump the blades were auto- matically returned to the normal auto-rotative incidence and the aircraft continued in flight as a pure autogiro. Hafner's gyroplane was nearer to the helicopter in that the pilot was given manual control over the blade angles. In this case a helicopter landing, as well as take-off, could be effected by suddenly increasing the blade angles in the glide when a few feet from the ground, and so using the kinetic energy of the rotor to touch down with zero velocity and no run. Other than arranging to put the whole of the engine power into the rotor, and providing control over blade angles, so that the pilot can choose the angle appropriate to any desired flight regime (or arranging for some automatic device, such as a constant-speed unit, which would maintain the blades at the correct angle whatever be the applied power), only one other thing remains—to derive a helicopter from the basic gyroplane. This involves torque correction. Torque Correction.—Whenever we have a single rotor driven through a transmission, we have a torque reaction, as a result of which the fuselage tends to rotate about the rotor axis in the opposite sense to the rotor. The methods which can be used for balancing this torque reaction are characteristic and can be used to classify helicopter types: — (i) Two Co-axial Rotors.—Most early helicopters were of this type, which has the double merit of eliminating the rolling moment due to forward flight, and balancing- Briguet Gyroplane of 1936—the first to set a real standard of helicopter performance. • .
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