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Aviation History
1917
1917 - 0280.PDF
MARCH 22, 1917. ised afterwards from the knowledge that the image musthave passed over the centre line simultaneously in the two cameras. The main disadvantage is that somewhat elaborate appa-ratus is necessary, but this is of not much importance in a permanent testing station.There are often periods in war time, however, when an aeroplane has to be tested quickly, and low cloud layers andother causes prevent the camera test from being carried out. It is then necessary to rely on measurements of speeds nearthe ground for the calibration of the air speed indicator. In this method the aeroplane is flown about 10 ft. off the ground,and is timed over a measured run. There are two observers, -FIGURE 5 — AT I6OOO ULCOO 18OOO 4O0O \ \ N\ \ V \ so 90 /'OO too IXOOno one at each end of the course. When the aeroplane passesthe starting point the observer sends a signal and starts his stop-watch simultaneously ; the second observer starts bis' Stop-vratch directly he hears the signal, and in his turn sends a signal and stops his watch when the aeroplane passes thefinishing point. By this double timing errors due to the so- called " reaction time" of the observers are practicallyeliminated, for the observer at the end of the course tends to start his watch late, while the first observer stops his late.The mean of the two observations gives the real time. Four runs, two each up and down the course, are done at eachair speed, the pilot or his observer noting carefully the average air speed during the run. Observations of the atmosphericpressure and temperature from which the density can be obtained are also taken. The average strength and directionof the wind during each trial are noted from a small direct reading (or receding) anemometer and the speed correctedin the K inc way as in the camera tests. If there is a strong cross wind the aeroplane may have to be pointed at a con-siderable angle to the course, and this makes the test a very difficult one to carry out well. Generally speaking, it is onlyreliable when the wind is quite light, not more, at any rate, than 10 m.p.h. Even this is too strong if it is a crosswind. A further difficulty is that at high speeds, over 100 m.p.h.,an aeroplane may take quite a considerable time to accelerate up to a steady speed, and so it must fly level for a long dis-tance each end before reaching the actual course. At the testing station previously alluded to the course is a mile long,and there is a clear half mile or more at each end ; but it is doubtful whether even this distance is enough for the machineto attain steady speed before the starting point. Finally, the flyer of a single-seater is generally too busy watching theground to do more than glance at his air speed indicator more than a few times during the run. Doubtless it wouldbe better in such a case to use some form of recording air speed instrument, although then other difficulties wouldarise. Having got the true air speed from camera or speed coursetests, and knowing the density at the height at which the test was carried out, we obtain what the air speed indicator shouldhave read by multiplying the measured air speed by the square root of the density. By comparing this with theactual reading of the indicator we obtain the necessary correction. The whole procedure may be shown best by atable giving part of the results of a camera test made at the beginning of the year. TABLE V.—Calibration Test oj Air Speed Indicator Nu. on Machine. Date December 24th, 1916. ~ a. a 3 g <T3 O d, S > '—' O "vi '< „_ S8,s i-li. O< : O z6 O o ^ C uo c U ii p.h- 59'1I33"4 62 o m.p.h.| 31-0 i 161-5 286 ! 5,100: 32-3 31 5'S 93'7 J5»IO0 31168-5 93-8 i 5,050; 31 2i'o 95'615,000; 31 0-879 : 0-879 0-881 0882 80-o 85-0 85-0 860 87-81x2-8 881 x3l 88-8 I x 2-8 Mean x 3-1 -FIGURE €" I IXOOO k K8OOO vj 400O 1 -A V\ IASt>£E£>S \ *& \ AT h \ \ j \ \ \ 90 900 /oo /coo //o //OO /so - ? P.M." TABLE VI.— Air Speeds at Heights. December ijlh, fl • c c ; en T3 • -- (3 _M O u 5 '53 "8 a Final results from curve. 0 ' T3 S 3,OOO 5,000 7,000 i 9,2OO| IO,8OO I2,8OO I3,8OO I5,2OO 39 35 30 24 19 17 12 8 "935 2.900 95 •875; 4>9ooj 93 •821 6,900 i 88 •767 9,000 81 i 731 10,400 80 •682 112,600 72 •664 13,400 68 •636 14,800 65 ; 98 ! 96 ,I02| 91 ;ioo|| 84 i 96 I 83 97 I 76 ! 92 72 88J 69 86i ,280 ,280 3,000 6,500 ,240 10,000 ,220 13,0001 ,220 15,000] ,200 ,180 ,160 103 100 96 94 86 •0 •5 •s •5 •0 1,290 1,250 1,180 1,160 A summary of the complete speed tests may now be given.Firstly, the air speed and engine revolutions are noted riymg level at full throttle every 2,000 ft. approximately by aneroid.From the aneroid reading and temperature observation at each height the density is obtained. The reading of the air.speed indicator is then first corrected for instrumental errors by adding or subtracting the correction found by calibrationtests over the cameras or speed course. This number is then again corrected for height by dividing by the square root of thedensity. The result should give the true air speed, subject of course, to errors of observation. The numbers so obtainedare plotted against the " standard " heights, i.e., the average height in feet corresponding to the density during the test. /A smooth curve is then drawn through the points and the air speeds at standard heights of 3,000, 6,500, 10,000, 13,000and 16,500 read off the curve. These heights are chosen because they correspond closely with 1, 2, 3, &c, kilometres.The indicated engine revolutions are also plotted against the standard heights, because these observations form a checkon the reliability of the results ; also the ratio of speed to engine revolutions at different heights may give valuableinformation with regard to the propeller. Table VI gives complete results of one of our tests of airspeed at heights. The table refers to the same machine as Table V, which gives the results of calibration tests of the airspeed indicator. Fig. 5 shows the smooth curve drawn from the calculated data, the actual air speeds calculated from theobservations being shown by dots, while the observed engine revolutions at the same heights are marked in by dots. Fig. 6 280
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