FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1951
1951 - 0028.PDF
20 FLIGHT, 4 January 1951 THE HELICOPTER... out torque reaction. While Louis Breguet built a most interesting helicopter of this type in 1936 and more highly developed types are flying today, it is not a very good con- figuration, since its stability is poor, and the rotors have to be separated by a gap to remove all danger of the blades fouling, particularly when starting up, or stopping the rotors in high winds or gusty conditions. (ii) Twin Rotors, Disposed Laterally or in Tandem.—The side-by-side arrangement is unstable longitudinally when the blades are articulated, and the outriggers involve added weight and drag. This arrangement can be made stable longitudinally by using rigid rotors with cyclic blade lift- control. The articulated arrangement, however, in the Focke Achgelis type 61, demonstrated in 1937, gave the first really practical helicopter, which held the following records recognised by the F.A.I.: —altitude, 8,000 ft; time of flight, 1 hr 20 min 50 sec; speed over 20-km course, 76 m.pii.; distance in a closed circuit, 50 miles; maximum distance flown across country, 143 miles. The tandem arrangement would appear to be better, since weight and drag are saved, and it promises to be one of the best configurations for the bigger helicopters of the near future, where the construction of large-diameter single rotors would offer a big structural problem. A variation of the side-by-side arrangement consists in closing the hubs of the two rotors together on inclined" axes, so that the blades intermesh. This reduces dimensions and saves the weight and drag of the outriggers, and is more stable than the normal side-by-side arrangement. Several satisfactory aircraft of this type have been built, both in America and in Germany. (iii) Single Main Rotor, with Auxiliary Rotor at the Tail. —This is the most successful type to date. It was first used by Von Baumhauer in 1924 and developed to a practical stage during World War II by Sikorsky in America. It has given a great stimulus to the helicopter movement. Most helicopters flying today, even up to ten-seater size, are of this type, the Sikorsky S.51, built in this country under licence by Westland Aircraft, Ltd., perhaps being the best-known, and the one with most operational experi- ence behind it. One form of this arrangement, which is intermediate be- tween the helicopter and gyroplane, was suggested by the Cierva Company before the war, and has been called a "Gyrodyne." In this form, which has been built by the Fairey Company, the torque-balancing airscrew is arranged outboard, and contributes to propulsive thrust. It wastes more power when hovering than the tail rotor, but the type is more efficient in high-speed flight, and is less prone to vibration at high speed. (iv) Single Rotor, with Surfaces in the Downwash.— Before the war Hafner proposed a type in which the fuselage was of twisted aerofoil shape and the air forces provided a couple balancing torque-reaction, without any power loss Twin-engined Kellett XR-I0, with interme^hing ff- Cierva W.9, with jtt reaction at the tail for torque compensation. being introduced. Due to the war, this type was not, in fact, built. (v) Single Rotor, with Jet Reaction at the Tail.—In order to eliminate rotating surfaces at the tail a lateral jet reaction device has been proposed. Several aircraft with this feature have been developed, one of them by the Cierva Autogiro Company in this country. (vi) Single Rotor, with Power Units in the Blades.—It has always been recognized that if the rotor could be driven from within itself there would be no torque reaction, and the troublesome correcting devices would be unnecessary. Jet reaction at the blade tips would be the ideal drive, since the tip speed would be high, with corresponding high jet efficiency. Several types are under development, but at the moment, in the absence of suitable jet units, the fuel con- sumption is very high. Even so, up to about two hours' duration, the type can carry as much pay load as its competi- tors with normal power units. Some early helicopters (as for example, Isacco's " Helicogyre ") used normal power units mounted on the blades to achieve this ideal, but engine difficulties were encountered due to faulty carburation and oiling-up troubles caused by the intense centrifugal field, and, in addition, owing to the varying relative velocity the airscrews of the wing engines were operating under poor conditions. Vibration.—One further problem which ought to be mentioned is that of vibration. It must be realized that the aerodynamic forces on a helicopter rotor are periodic in forward flight. The articulated rotor is a particularly bad offender in this respect and this fact explains die develop- ment now taking place on rigid rotors. One of the major sources of vibration is the periodic stalling of the blade tips. As will be appreciated from a later article, when the rotor disc is tilted forward for propulsion there is an added axial flow through the disc. This increases with forward speed and changes the blade angle of attack unequally from root to tip, the least affected being the tip, where the velo- city due to rotation is highest. As a result, when the mean blade angle is increased to compensate for the axial flow, the blade angle at the tip becomes excessive, and at high forward speed the tip section will stall on the retreating blade, where the angle of attack is already high due to blade flapping or cyclic feathering. This periodic stalling not only impairs the efficiency of the rotor, but limits the operat- ing speed due to the vibration which it causes. (To be continued) Piasecki PV3, singie-engined, with tandem rotors.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
