Accelerometer design Page: 3 of 16
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REPORT No. 100.
By F. H. NORTON and EDWARD P. WARNER.
Langley Memorial Aeronautical Laboratory.
To carry out the work on accelerometry for the Bureau of Construction and Repair, Navy
Department, the first step necessary was to study all previous types of accelerometers, with
the result that an instrument was developed by the technical staff of the National Advisory
Committee for Aeronautics at the Langley memorial Aeronautical Laboratory for the purpose
of recording more accurately than had been done before the accelerations experienced in flight.
The errors due to accelerations acting in other than the required direction and the errors due to
angular accelerations were studied and as far as possible eliminated. The response of the
instrument to shocks of short duration and the damping of the free vibration were analyzed
mathematically, as well as the possibility of resonance with the vibration of the plane or engine.
The results of this work are included in the present report, together with a description of the
THE PROPERTIES DESIRED IN AN ACCELEROMETER.
The ideal accelerometer should have a natural period very high compared with that of any
shocks that it could experience. It should have a large enough deflection to be read with an
error of not more than 0.1 per cent of the maximum acceleration, and it should be so damped
that it will follow the actual acceleration in the closest manner. It may be contended that
such accuracy is unnecessary, as any given maneuver can not be duplicated within several
per cent, and the engineer does not need loads to better than- 5 per cent. On the other hand,
when it is desired to obtain the difference of accelerations in various parts of the airplane in
order to calculate the rotary motions, an accuracy of 0.1 per cent is none too great. The
accelerometer should only record linear accelerations along one axis, and should be unaffected
by any other accelerations. Besides these qualities it should have compactness, ruggedness,
and be simple to operate, and the record should be clear and strong and easy to reproduce.
PREVIOUS TYPES OF INSTRUMENTS.
A type of accelerometer developed by Dr. Zahm' consists of a number of styluses held
against stops a short distance above a moving strip of paper, by springs of different tensions, as
shown in figure 1. Each spring is adjusted so that its stylus is brought into contact with the
paper whenever the acceleration exceeds a certain amount, so that by having a number of graded
springs a curve of acceleration can be traced, as shown in figure 2. This type of instrument
will not trace a continuous curve, and it is not practical to have a sufficient number of springs
to trace the small and rapid vibrations; but, on the other hand, it has no lag, no natural period,
and none of the errors inherent in the free spring instrument. This type of instrument is very
valuable for studying the maximum values of landing shocks, but will not, of course, give a
continuous curve of acceleration against time.
The R. A. F. accelerometer consists of a semicircular quartz fiber, illuminated by a small
incandescent lamp. The deflection of this illuminated fiber is magnified and projected on to a
moving film, as shown in figure 3, thus giving a curve of acceleration against time. The natural
'1Dfevelopment aof an Airplane Shock Recorder, by A. F. Zahm: Jnl. Franklin Inst., August, 1919.
' R. and M. No. 376 of British Advisory Committee for Aeronautics, September, 191Z
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Norton, F. H. & Warner, Edward P. Accelerometer design, report, 1921?~; (digital.library.unt.edu/ark:/67531/metadc65750/m1/3/: accessed January 24, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.