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Aviation History
1910
1910 - 0267.PDF
APRIL g, IQIO. psl THE WHIRLING TABLE AND OTHER APPARATUS AT THE NATIONAL PHYSICAL LABORATORY. ON March 26th we published abstracts from the annual report of the National Physical Laboratory, which is the headquarters of the Government Flight Office, and now doubtless it will be of interest to our readers to gain some slight idea of the sort of apparatus that is being used there. The most important device in point of view of size, costliness, and also, it may be said, utility, is the new whirling table that has just been erected. This apparatus occupies a large shed entirely by itself, and yet its purpose is merely to test models of propellers which only measure a few inches in diameter. It is, or at any rate should be, well known that in order to test a propeller accurately, it must be observed under conditions that are analogous to those under which it will actually work in practice, and one of the most of the test is not to see how fast a propeller will push an object through the air, but to find out what propeller is best suited to provide a predetermined thrust at some predetermined speed of revolution and flight. The thrust of the propeller is indicated by the automatic illumination of a signal lamp, mounted near the inner end of the whirling arm so as to be more conveniently under observation. The signal lamp is controlled by electric contact operated by the axial movement of the propeller- shaft in its journal. The various instruments are so arranged as to enable a simultaneous observation to be « \ ''-i-SUSNAL. LAMP tOMTftCTS END View oF WHIRMNG ARM • "Flight" Copyright. The Whirling Tabic at the National Physical Laboratory.— Model propellers are tested on this apparatus. COMPRESSED AIR. -resT PIECE. "Flight" Copyright. Sketch illustrating the National Physical Laboratory apparatus for inflating a dirigible envelope until it bursts. " Flight " Copyright. Sketches illustrating how aeroplane fabric is tested at the National Physical Labora tory. important features associated with the practical use of a propeller on a flying machine is that the propeller moves bodily through the air with the flyer itself. It is therefore useless to test a propeller as if it were a fan by allowing it to remain stationary in one spot; it must be made to move through the air at a speed that corresponds to the speed of flight. A whirling table is an apparatus that enables a propeller to be tested under such conditions, for the whirling table consists of a long arm mounted on a vertical column that is rotated at any desired speed by an electric motor. The whirling arm has to be of very great radius in proportion to the diameter of the propeller in order to neutralise the effect of the curved path. The propeller to be tested is mounted on the extremity ofthe whirling arm, and is independently driven by another electric motor carried on the whirling arm itself. The action of the propeller has nothing to do with the rotating of the whirling arm, although the position is such that the whirling arm would rotate under its action were sufficient force developed for the purpose. The object made of the speed of revolution and speed of flight at the moment the signal lamp is illuminated. The speed of revolution is obtained by calibrating a voltmeter that is in circuit with a small dynamo direct driven from . the propeller-shaft. The propeller-shaft itself is driven through a spring so arranged as to enable an automatic record of the torque to be made on a revolving chart. Among other interesting but less elaborate testing appliances at Bushey House are those used for investi gating the strength of aeroplane fabrics. Ordinary tension tests are obtained by stretching a strip of fabric two inches wide between two massive wooden clamps, but there is also an ingenious and very simple apparatus for simultaneously stretching the material along the warp and the weft, with a definite ratio of pull in each direction. In order to graduate the application of the applied load, sand is admitted at a uniform rate into the scale pan. For testing the bursting strength of the fabric for the envelopes of dirigibles, cylindrical test pieces are inflated inside a box that has sliding scales so arranged as to measure the stretch of the material. 263
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