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Aviation History
1950
1950 - 0531.PDF
16 March 1950 331 De-icing flight trials are performed with a Halifax on which is carried a spray-frame ahead of the surboard inner airscrew. Com- prehensive test instrumentation is used to register performance characteristics and, in addition, photographic records are taken. QUEST for EFFICIENCY . . . first sight to be relatively insignificant in the sequence of testing, but as it is a safety requirement it is treated very seriously. The overshoe adhesion and fluid (or heat) dispersion under flight conditions, particularly in the case of contra-rotating airscrews, frequently demands a great deal of attention. In the case of braking airscrews, 50 landings using the reversed-thrust feature must be carried out on each specific installation, regardless of any previous testing which might have been done. Flight strain-gauge testing is left until last because, although in the past it has not been a regular requirement, it is now generally regarded as essential since variations do exist between the results obtained in the tunnel tests and those obtained in flight. It is, of course, the vibratory stresses which are the primary concern, for these can vary as a result of fuselage interference, airflow conditions and so forth, and can only be determined by actual flight trial. There is, however, a particularly difficult form of vibration which is sometimes encountered and cannot easily be remedied, i.e., that due to aerodynamic excitation. This can be induced by the environment of the airscrew, and is most usually created or accentuated by the proximity of any structural excresence. It may be such that there is no reaction in a transverse or torsional direction and it will be detected only in airscrews having more than Many foreign aircraft come to Hatfield for airscrew trials ; one ofthe visitors during recent months was the S.O. 30P Bretagne, three blades. This particular form of vibration is almost entirely '' airscrew-contained '' and it is, therefore, prefer- able to re-check the exact magnitude of the stresses under actual flight conditions—since they may be within per- missible limits—rather than to embark upon extensive modifications which might reduce aerodynamic efficiency. With the completion of the flight strain-gauging tests, the sequence of prototype airscrew approval is concluded and it is no exaggeration to state that it has few equals, either in length or complexity. Airscrew approval clearance is never a cut-and-dried process; development and approval testing inevitably proceed together and they are rarely ahead of an engine / aircraft project whilst, inevitably, they are always unsettled until the last. Bearing this in mind, the true nature of the de Havilland Propeller Company's achievement in putting through 265 distinct types of air- screw for development during the past year can be appreciated. One cornerwith which vi >rately tests can be carried oot^in tlte
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