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Aviation History
1917
1917 - 0253.PDF
MARCH 15, 1917. METHODS OF MEASURINGAIRCRAFT PERFORMANCES. By Captain H. T. TIZARD, R.F.C. (Continued from page 233.) THE aneroid being what it is, one soon comes to the con- clusion that the only way to make use of it in aeroplane tests is to treat it purely as a pressure instrument. For this reason it is best to do away with the zero adjustment for all test purposes and lock the instrument so that the zero point on the height scale corresponds to the standard atmospheric pressure of 29/9 ins. or 760 mm. of mercury. Every other height then corresponds to a definite pressure ; for instance, the locked aneroid reads 5,000 ft. when the atmospheric pressure is 24/88 ins., and 10,000 ft. when it is 20*70 ins., and so on. If the temperature is noted at the same time as the aneroid reading, we then know both the atmospheric pressure and temperature at the point, and hence the density can be calculated, or, more conveniently, read off curves drawn for the purpose. The observations necessary (after noting the gross aeroplane weight, and nett or useful weight carried) are therefore, (i) aneroid height every 1,000 ft.; (ii) time which has elapsed from the start of the climb ; and (iii) temperature ; to these should be added also (iv) the air speed and (v) engine revolutions at frequent intervals. The observed times are then plotted on squared paper against the aneroid heights and a curve drawn through them. From this curve the irate of climb at any part (also in aneroid feet) can be obtained by measuring the tangent to the curve at the point. This is done for every 1,000 ft. by aneroid. The true rate of climb is then obtained by multiplying the aneroid rate by the correction factor corresponding to the observed temperature. These true rates are then plotted afresh against standard heights and from this curve we can obtain the rates of climb corresponding to the standard heights 1,000, 2,000, 3,000, &c. Knowing the change of rate of climb with height, the time to any required height is best obtained by graphical integration. The following table gives the results of an actual test:— TABLE IV.—Machine Engine >,...-. Date zyth December, 1916. 12.40! 385 8,000 79.16 is.141 345 9,000:76.55)18.20 310 110,000! 74.00J21.61: 280 n,oooJ7i.70:25.41J 245 12,ooo|69.50129.81! 210 o 1,000 2,000 3.000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 13,000 14,000 15,000 At least two climbing tests of every new machine are carried out up to 16,000 ft. or over by aneroid. If time permits three or more tests are made. The final results given are the average of the tests and represent as closely as possible the performance on a standard day, with temperature effects, up-andd-own currents and other errors eliminated. If we produce the rate of climb curve upwards it cuts the height axis at a point at which the rate of climb would be zero, and therefore the limit of climb reached. This is the " ceiling " of the machine. Speeds. His 16,000 ft., or whatever it is, reached, the flyer's next duty is to measure the speed flying level by air speed indicator 13,000 67.3235.13 I4,ooo|65.i7i4i.88 170 130 14,500' 64.11 46.23J 105 at regular intervals of height (generally every 2,000 ft.) fromthe highest point downwards. To do this he requires a sensi- tive instrument which will tell him when he is flying level.The aneroid is quite useless for this purpose, and a " stato- scope " is used- The principle of this instrument is really FIG. 2 Glass jam e and, trips UieO. on BubbLg 5 Bubble5 fcatoscobg T> the same as that of a climbmeter. It consists of a thermos flask connected to a small glass gauge, slightly curved, but placed about horizontally. In this gauge is a small drop of liquid, and at either end are two glass traps which prevent the liquid from escaping either into the outside air or into the thermos flask. As the machine ascends and the atmo- spheric pressure being smaller, and the pressure in the flask being higher than the external pressure, the liquid is pushed up to the right-hand trap, where it breaks, allowing the air to escape. On descending the reverse happens ; the liquid travels to the left, breaks, and air enters the flask. When flying truly level the drop remains stationary, moving neither up nor down. The instrument is made by the British Wright Co. The flyer or the observer notes the maximum speed by the air speed indicator, i.e., the speed at full engine throttle. At one or more heights also, he observes the speeds at various positions of the throttle down to the minimum speed which will keep the machine flying at the height in question. The petrol consumption and the engine revolutions are noted at the same time, as well, of course, as the aneroid height and temperature. Accurate observation of speeds needs very careful flying—in fact much more so than in climbing tests. If the air is at all bumpy observations are necessarily subject to much greater error since the machine is always accelerating and decelerating. The best way to carry out the test seems to be as follows : The machine is flown first just down hill and then just up hill and the air speeds noted. This will give a small range between which the real level speed must lie. The flyer must then keep the speed as steadily as possible on a reading midway between these limits, and watch the stato- scope with his other eye. If it shows steady movement, one way or the other, the air speed must be altered accordingly by 1 m.p.h. In this way it is always possible at heights where the air is steady to obtain the reading correct at any rate to 1 m.p.h., even with light machines, provided always sufficient patience is exercised. The revolutions per minute at this speed are then noted. One difficulty, however, cannot be avoided. If at any height there is a steady up or down air current, then though the air may appear calm, i.e., there may be no " bumps," the air speed indicator reading may be wrong, since to keep the machine level in an up current it is necessary to fly slightly down hill relatively to the air. Such unavoidable errors are, however, eliminated to a large extent by the method of taking speeds every 2,000 ft., and finally averaging the results.XT be concluded.) The Aeronautical Society's Lectures. OWING to the difficulty in obtaining good films in thepresent dull weather, the lecture announced by the Aero- nautical Society for March 21st by Mr. Holt Thomas on 253 " Commercial Aeronautics " has been postponed until May 20th.Mr. M. A. S. Riach will deliver his Paper on " Air Screws " on March 21st, the meeting place, as usual, being the Societyof Arts, John Street, Adelphi, W.C.
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