From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical description of recording accelerometers, their operation, and methods for analyzing the recorded data.
| Identifier | ExFiles\Box 138\2\ scan0183 | |
| Date | 19th July 1939 guessed | |
| RECORDING ACCELEROMETERS. Fig. 4. 11 x 5 x 10 inches. Fig. 5. Accelerometer making Two Simultaneous Records. FOR some purposes it is convenient to have a record of the accelerations in two planes at right angles, e.g., to obtain simultaneous records of horizontal and vertical accelerations or, alternatively, accelerations in two directions at right angles in a horizontal plane. The instrument shown in Figs. 4 and 5 has been designed to meet this requirement. It is of similar design to the single Accelerometer, Fig. 1, but is fitted with two weights, M, M₁, (Fig. 5), mounted on flat steel strips, so that they can only move in planes at right angles to each other. Movements of the weights relative to the instrument are transmitted to two styles, Q, Q₁, situated side by side, and moving over the celluloid film in a similar way to that already described, both movements being critically damped by means of the electromagnets P and P₁. Time or other datum lines are marked by means of a third stylus operating on the back of the film. Fig. 4 shows the external view, while Fig. 5 shows diagrammatically the principal parts of the mechanism, in elevation. By courtesy of the Chief Mechanical Engineer, L.N.E. Railway, an enlargement of a record taken with this Accelerometer is reproduced in Fig. 20. Combined Accelerometer and Air Speed Recorder. In connection with aerodynamic research work, it is frequently desirable to obtain permanent records of the speeds attained during a flight, and also to record simultaneously the acceleration along some axis of the aeroplane. The instrument shown in Fig. 6 has been designed for this purpose. It comprises a Recording Accelerometer of the type illustrated in Figs. 1 to 3 and an Air Speed Recorder, mounted together in a closed case. The Accelerometer may be used for recording the component of the acceleration in any direction in relation to the aeroplane. The air speed is measured by a Pitot tube of the standard aircraft pattern, mounted in a position clear of slipstream and wing disturbance. The dynamic tube is connected to the interior of a differential manometer of the aneroid box type, mounted within the case of the Recorder, while the static tube is simply connected to the inside of the case, which is rendered as airtight as possible by means of rubber packing. Movements of the manometer, due to the fluctuating differences between the static and dynamic pressures caused by variations in the air speed, are communicated to a recording stylus, which makes a record upon the same celluloid strip as the acceleration and time styles. The instrument can be adjusted for any practicable range of speed and, if carefully calibrated with its Pitot tube, has an accuracy of at least 0·5 per cent at the maximum speed. Fig. 6. 9 x 6 x 8 inches. Time Marker. For marking the time-datum lines on the celluloid records, a convenient form of time marker has been designed, fitted with a starting and stopping switch, and provided with terminals for connection to the Accelerometer. The time marker comprises a clock mechanism arranged to make and break an electric circuit at regular intervals of 0·1 second, causing definite "kicks" of the time-marking stylus at these intervals. The characteristic form of the markings is clearly shown in the lower portions of Figs. 16 and 17. 4 RECORDING ACCELEROMETERS. Fig. 7. 72 x 18 x 9 inches. Records. THE records made by the Accelerometers are of small dimensions, and are permanent. The lines forming a diagram are clearly defined, and will bear considerable magnification. They can be examined at once by means of a suitable microscope, or direct enlargements can be obtained by photographic projection upon a screen and tracing. A convenient enlarging camera (Fig. 7) has been designed to facilitate direct photographic enlargement of the records. The celluloid record strip is held by a clip A in the optical axis of a projecting system comprising a pointolite lantern B, a condenser C, and a focussing lens D.{John DeLooze - Company Secretary} A shutter E is provided for regulating the exposure. The time of exposure depends upon the speed of paper used; an exposure of a few seconds on ordinary bromide paper is generally sufficient. The bromide paper is attached to a stand F which can be moved along the wooden frame H which extends from the base of the camera, the limits of movement permitting enlargements up to 15 diameters to be obtained. By means of milled heads G, G₁, attached to the rod K, the lens can be readily focussed to obtain a critically sharp image. A series resistance (not shown) for the pointolite lamp is included in the outfit. Alternatively, the records can be examined at once by means of a light and portable microscope (Fig. 8) fitted with a graticule. With this microscope, readings on the film can be repeated to an accuracy of 0·01 mm. (0·0004 inch). Fig. 9 shows the appearance of the celluloid strip, actual size, with a record of the accelerations of a shaping machine ram. Fig. 10 shows the same record as viewed through the microscope, while in Fig. 11 is reproduced a photographic enlargement. Direct photographic enlargements of some additional records are reproduced on pages 7 and 8. It should be noted that each record consists of a fine black line enclosed between two white lines; this triple line is due to an optical effect produced by the indentation of the celluloid surface by the spherical point of the stylus. It enables any point on the enlarged diagram to be read with much greater accuracy than is possible when examining purely photographic records obtained in the usual manner. Fig. 8. 17 x 2 x 3 inches Advantages. From the manipulative point of view, the small celluloid record has many advantages over records taken on a photographic film, or paper. The records can be cut from the celluloid strip and examined immediately after they are made, without chemical treatment or the use of a dark room. There is thus a considerable saving in time, and the danger of spoiled records due to premature exposure or imperfect development is entirely eliminated. The records are durable and are capable of withstanding the worst conditions of dirt and wet without special handling precautions. The length of celluloid strip provided is sufficient for the production of a large number of records, while a new strip can be readily and easily inserted in the instrument. Six spare lengths of film are provided with each instrument. The amount of recording material required is extremely small as compared with that necessary when photographic methods are adopted. A record from an aeroplane in flight has been taken over a period of three hours, using only 15 ft. of film; if a photographic method had been employed, about 1800 feet of material would have been required. As the optical magnification in the instruments is small, inertia effects are reduced to a minimum. The instruments are compact and light in weight. 5 | ||