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Aviation History
1911
1911 - 0245.PDF
mounted bodily to and fro operates the elevator, which is formed by two triangular flap extensions of the fixed horizontal tail-plane. The triangular shape of these planes gives room for the free movement of a rudder-plane mounted vertically between them, and, like the elevator, forming an extension of a fixed tail-plane. The rudder is operated by foot through the agency of a pivoted bar. Of the wings themselves, it is interesting to remark that they each weigh 70 lbs. complete and are very strongly con structed, in spite of the fact that the spars by which they are attached to the body have the appearance of being very light. Inside the wings themselves, however, these spars are of f- section, i\ ins. in width and 7 ins. in depth. In addition to their top and bottom guy wires, the wings are also stayed by wires leading forwards to the projecting skid. The camber of the wings varies from the shoulder to the tip, the chord having a positive angle of incidence of about 5° adjacent to the body and being horizontal at the outer ex tremities. The thickness of the plane at the shoulder is 8 ins., but diminishes i\ ins. at the extremities and the chord itself also diminishes from 6 ft. to 5 ft., although this latter change is brought about more or less abruptly by obliquely cutting away the trailing edge from a point about 5 ft. from the extremities. NATIONAL PHYSICAL LABORATORY REPORT FOR 1910. A GENERAL description of the equipment provided for aero nautical research was given in the report for 1909.* During 1910 considerable additions have been made, chiefly to the motor-testing plant, of which details are given below. The following researches have been carried out during the year:— 1. Experimental Wind Channel.— («) Determination of the various positions of the centre of pressure on a curved airship rudder corresponding to different inclinations. (6) Determination of the curves of lift and drift of four model sets of lifting planes for a dirigible. (c) Measiirement of the horizontal and vertical components of the wind pressure on eleven model balloon sheds. (i) Determination of the resistance of thin wires in a current of air (1) when steady (2) when vibrating at high frequencies. (e) Determination of the air resistance of radiators of honeycomb section. 2. Experimental Water Channel. — (a) Determination of the total resistance of a large number of model dirigibles. (b) Experiments on the lift and drift of a model dirigible at various inclinations to the current and determination of the line of action of the resultant force. (c) Experiments on the stabilising action of fins of varying areas on a model dirigible. 3. Whirling Table.—The calibration of the dynamometer for corrections due to air pressure and friction has been completed and the following tests have been made :— Complete tests of three Ratmanoff model propellers for the balloon factory. Test of model propeller for a private firm. Tests of five model propellers for the Admiralty. Determination of the amount of the Air Swirl in the Neighbourhood of the Whirling Arm.—In addition to the work already referred to, a careful investigation has been made of the extent to which the air in the whirling table shed is set in motion by the whirling arm. The velocity of this air current was originally estimated, by means of a Pitot tube, as about 2 miles per hour when the end of the arm was travelling at 35 m.p.h., and it was considered neces sary to measure the swirl velocity with an accuracy of 10 per cent, so as to give the velocity of the arm relative to the air correctly to i per cent. As no instruments capable of measur ing speeds less than 1 m.p.h. directly were available, the method of observation hart to be modified in consequence. A 6-in. Biram anemometer, kindly lent for the purpose by Messrs. Davis and Son, and found suitable for speeds above 1 m.p.h., was calibrated by fixing it to the arm and moving the arm slowly. For this purpose the air swirl was neglected, as the arm speed never exceeded 4 m.p.h. The anemometer was then fixed to the floor, first above and then below the rotating arm, and readings taken. At a mean reading of 1 m.p.h. the instrument sometimes stopped completely. For the higher speeds this did not occur, and the reading errors were not great. It was found that the velocity at the same radius depended on the position of the anemometer in the room, and finally readings were taken at eight points in the half circle and a mean obtained. This gave a swirl velocity of 2-1 m.p.h., and the swirl was very nearly proportional to the arm speed. As a check on this, the anemometer was fixed to a pole and carried round the room in the direction opposite to that of the motion of the arm. The new series of values obtained in this way agreed very well with the previous determination. It may be noted that the velocity measured in this way * See FLIGHT, Mar. 26th, 1910. gives the mean velocity of the air swirl, whereas for a pro peller the velocity required is that of the air into which the propeller is entering, this being somewhat lower than that determined. A series of observations was made to determine the retardation of velocity corresponding to a given swirl speed with the arm stopped. The correction found was small, and reduced the swirl velocity to 2-0 m.p.h. at 35 m.p.h. A second method of determining the swirl velocity was then attempted. If a tube rotating about a point O has its outer end bent at right angles to its length so as to form a Pitot tube facing the direction of motion, and rotates so that the end moves with a velocity v, the pressure at O is that due to the velocity, v, i.e., fpv2, less the centrifugal head, which is also $pir, acting in the opposite direction. The change in density in calculating the centrifugal head amounts to o-1 per cent, and is neglected. If, however, the velocity relative to the air is not v but V, then the velocity head is ipV, and the centrifugal head Jpy2, and the total pressure at O is $p (V2—v-). If therefore the tube at O is connected to a gauge, the reading is a direct measure of the swirl, and in the gauge used a swirl of 2 m.p.h. at 35 m.p.h. would give a reading of about 100 divisions. The rotating tube was one of the ordinary lead pipes laid along the arm to the mercury seal at the centre, and was connected to a Pitot tube mounted at the end of the arm. The static pressure was taken from a tube lying on the floor and protected from draughts, the position being directly below the circle traced by the Pitot tube. The readings so obtained were very definite, but only gave a velocity of i-6 m.p.h. at an arm speed of 35 m.p.h. This is 0-4 m.p.h. less than that obtained by the anemometer, the difference being twice as great as was considered satisfactory. After repeating the experiments with the same results it was suspected that in the neighbourhood of the arm mean con ditions did not exist, and that the estimation by the second method was probably the one required. To check this conclusion measurements of the swirl velocity 2 ft. from the floor and 4 ft. below the arm were taken with the anemometer, giving 1-5 m.p.h. at an arm speed of 35 m.p.h. The Pitot tube measurement at the same place by the second method gave i-6 m.p.h., i.e., was in sufficiently good agreement. Having obtained this agreement, one of the Laboratory Standard Dines tubes was calibrated in this position, and its constant obtained from the known value of the air swirl in this position. The Dines tube was then placed on the arm in the same position as the original Pitot tube, and the pressure indications obtained for that position. From the calibration as determined above the air swirl was deduced, and it was found to agree with the Pitot tube determination of the air swirl. It was therefore concluded that the air swirl correction for test on the arm was i-6 m.p.h. at 35 m.p.h., and was approximately proportional to the arm speed. 5. The Testing of the Strength and Elasticity of Fabrics for Balloons and Airships.—The investigation into the best form of test-piece for simple tensile tests has resulted in the adoption, as a temporary standard, of a rectangular specimen 2 ins. wide and 6 ins. iong between the grips. The reasons for this decision were that specimens longer than about this size gave the same results, while smaller ones gave higher values. For these tests the Avery fabric-testing machine, which was installed during the year, is now used. It has been noticed that the local variations of strength in a fabric are generally gradual, whereas the variations over large areas are considerable ; in consequence of this, when 247
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