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Aviation History
1948
1948 - 0182.PDF
FLIGHT FEBRUARY 5TH, T04S Photograph of trailing vortex system shed from main rotor blades. Itie convolution is not due to tail rotor effect. «• FLIGHT TESTING of HELICOPTERS A Summary of Farnborough Experiments in Test Flying ••-•;; m- - ..---•• .•-- the Sikorsky R-4B. ^:,^.:.^-.,;^.:wT^L.^^ IT is only within recent years that the helicopter hasemerged from the experimental aircraft category to aposition in which it can be considered as an effective operational and commercial means of aerial transport. Thedevelopment of the helicopter to its present stage is due to intensive work in many countries, particularly inAmerica, and it is unfortunate but unavoidable that Britain was unable to take a larger share in this work. A small research group dealing with flight investigationson the helicopter was formed in the Aerodynamics Depart- ment of the Royal Aircraft Establishment, Farnborougji,at the end of 1945. It is mainly with the work of this group that Mr. W. Stewart, B.Sc, dealt in his paperdelivered to the Royal Aeronautical Society, on Thursday, January 29th. With the initiation of flight research into the aenvdynamic characteristics of the helicopter, the first essen- tial in each section of the work was?to establish satisfactorytesting techniques. Not only have'the methods to be made applicable to the helicopter as a type of flying machinebut also to the specific type of helicopter. The Sikorsky R-4B1 on which most of the flight testing has been done,both in America and in this country, has several undesir- able features from the point of view of its use as a researchhelicopter. Lack of power, which renders it unable to hover in free air at normal loading; the vibration in thepilot's control stick, which makes the measurement of control behaviour very difficult; and the " unaerodyna-mic " fuselage, all produce serious difficulties to the pilot and to the research technicians in flight testing. In helicopter flight testing, as in all forms of researchwork, the use of proper instrumentation is. of major import- ance, and the use of an automatic observer was indispens-able in many of the tests. The extensive use of instruments- is essential if the pilot's impressions of the flying charac-teristics are to be understood and rationalized in scientific form. Several new instruments and techniques havealready l>een used for present flight-testing work, and further developments will be required for the many prob-lems which still remain unsolved. The rotor has three distinct working conditions, definedin general terms as (a) propeller-working state, (b) vortex- ring state, and (c) windmill-brake state. The flight tests described were initiated to investigatethese operating conditions for the full-scale helicopter. While it is comparatively easy to derive the existing con-ditions in general terms, the accurate measurement of the conditions presents many difficulties, mainly becauseof the very small order of the terms involved. This makes it essential to measure the flight conditions very accurately and to maintain these conditions absolutely steady during the test runs.. The obvious start for an investigation of the flow con- ditions through a helicopter disc is in hovering flight. In this condition the aircraft is stationary, the flow is directed downwards through the rotor, and the radial distribution is practically similar for any azimuth position of the blade. Attempts to measure the velocities close to the disc by the hot-wire anemometer method were made in America. Even for the closest practicable position, the correct disu tribution is not maintained because of the rapidity with which the slipstream develops. In the present tests, the method of measuring the velocities downstream was used. In addition, velocity measurements were made at several distances below the disc to investigate the slipstream development. The induced velocity from the rotor is very small, and to obtain the required accuracy some form of magnifica- tion is necessary. The flight tests were made with a double venturi in which the differential pressure between the total- head pitot and the static pressure in the centre throat gave an amplification of thirteen in terms of p V2. The venturi instrument was carried on a pole projecting radially from the top of the port undercarriage leg (4.7ft below the flapping-hinge plane). Measurements could be obtained for radial positions 0.28 to 1.1 of the blade radius. - Provision for vertical adjustment of the instrument to cover a range of 4ft above and below the pole was made. Smoking to Some Piirpose A special technique of streamline visualization by smoke filaments was developed to enable measurements at any point in the slipstream to be referred to the appropriate radial position at the rotor disc. It was found that the smoke stream released from a generator continued through the disc without any serious dispersion of the smoke. Hovering flight presents peculiar difficulties since the helicopter rotor must be at least 40ft above the ground to eliminate the ground cushion effect. It then becomes difficult to find a means of positioning the smoke generator close to and above the rotor disc. The practical solution to this problem was to attach it to the end of a jib on a 60ft crane, and to hover the helicopter under this. Ground signals to the pilot were necessary to achieve the required helicopter positioning. The development of the slipstream for hovering flight as obtained from the smoke filaments is shown diagram-
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