An Instrument for Measuring Low Frequency Accelerations in Flight

Abstract

Instruments for measuring the vibratory motions and strains which are directly associated with power plant operation in aircraft have been in service use for some time but equipment for studying the low frequency, high amplitude motions of wings and control surfaces has not been generally available. The measurement of such motions introduces certain problems of instrument design which are discussed in the present paper. An instrument which offers a practical solution of these problems is described and the results of performance tests are discussed. A seismographic system consisting of a mass connected to the vibrating member by means of an elastic coupling and a damper is an essential part of an instrument for measuring vibration in aircraft. The performance characteristics of. such systems are discussed with the aid of non-dimensional curves. By means of these curves the frequency ranges for satisfactory operation of instruments for measuring vibratory displacements, velocities, and accelerations can be quantitatively determined for various degrees of damping. I t is shown that an accelerometer is the most suitable type of equipment for measuring low-frequency, high-amplitude vibrations. The experimental accelerometer developed during the work reported in the present paper uses an oil damped seismographic system with an effective natural frequency of about fifty cycles per second. A variable reluctance system changes the inductance of two coils used as arms of an inductance bridge and thus controls the bridge output. The bridge is supplied from a 600cycle, governor-controlled, motor generator set and feeds into a vacuum tube amplifier which in turn operates an oscillograph. The accelerometer operates with substantially constant sensitivity from zero frequency up to some high-frequency limit which depends upon the damping medium and may be as high as thirty cycles per second under proper conditions. A table is given which summarizes the effect of oil temperature and viscosity on the operating range.