TM 1-6625-724-13&P0071 000071 00-37TASK 5 – ISOLATE IRREGULAR VIBRATIONSThere are two main sources of vibration in helicopters. The rotors produce low frequency vibrations, 200-5000 rpm or3 to 80 cycles per second (Hz), at multiples of rotor revolution, and the drive train-transmission produces mid, 5000 to100,000 rpm or 80 to 1700 Hz, to high, above 100,000 rpm or 1700 Hz, frequency vibrations. Unusual low frequencyvibrations are usually a human factors issue and don't indicate a safety of flight problem, but not always. Irregular “Hi-Freq” vibrations usually indicate an impending failure of some drive train component and so are a safety concern.Figure 17 is an example of a spectrum and some of the frequencies that you might see. No limits exist for any ofthese levels except main rotor 1/REV and tail rotor 1/REV.Figure 17. Spectrum ExampleThe AVA is capable of acquiring accelerometer measurements and generating vibration spectra from thosemeasurements. This measurement mode is referred to as “PROBES”. A vibration spectrum is a plot of vibrationmagnitude, usually in inches per second (ips) or in g’s, versus frequency, usually in Hz or revolutions per minute(rpm). In general, the frequency of a response generated by a rotating component (e.g. rotors, fans, compressors,engines, etc.) is equal to the frequency or an integer multiple of the frequency of rotation of the component which iscausing the vibration. In this way, the rotating component causing an abnormal response can be identified.SPECTRUMSWhat is a spectrum? If an accelerometer is mounted at a particular location on an airframe, a spectrum will show howthat point is responding to the forces generated by the rotating components of the helicopter. A peak at a particularfrequency associated with a rotating part is the amplitude of the response, the vibration, of that point to the forcesgenerated by that part. The higher the peak the more forces that part is generating. Low frequency vibrations aregenerated by the rotors, Mid frequency vibrations are generated by drive-shafts, generators, pumps, oil coolerblowers, and engines. High frequency vibrations are generated by gear meshing and fan, turbine and compressorblade passage. The AVA allows the collection of spectrums at appropriate location by mounting an accelerometer onthe airframe and use the PROBES flight plan or the FFT Aircraft Type. The “PROBES” flight plan allows vibrationdata to be taken on accelerometers 1 through 4 for one flight plan, and the range is from 0 to 30,000 rpm (0 to 500Hz). This range is usually sufficient to trouble shoot most of the vibration problems on the AH-64 series. The FFTAircraft Type has many different ranges, but it only allows measurements to be taken on ACC1. The only reason thatthe FFT Aircraft Type needs to be utilized is if it is known that the source in question is above 30,000 rpm (500 Hz).This data can be used for troubleshooting “Hi-Freq” vibration problems. However, there currently are no set limits onwhat is good and what is bad. That will be discussed further in this section.Main Rotor 1/REVMain Rotor 4PTail Rotor 1/REVMain Rotor 8PTail Rotor Drive Shaft 1/REVTail Rotor 4P
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