Downloaded from https://www.everyspec.com MIL-STD-810D 19 July 1983 0 Most current turboprop aircraft and many turbine engines are constant-speed machines. This means that rpm ia held constant.and power changes are made through fuel flow changes and variable-pitch bladea, vanes, and propellers. These machines produce the fixed frequency spikea of figure 514.3-25. These spikes have an asscciated bandwidth because there is miner rpm drift and because there is miner rpm drift and because the vibraticn is not pure sinusodial (I-4.5). . There are.indications that future turboprop cr propfan enginea will net be ccnstant-speed machinea.
All reciprocating engines and many turbine engines are net constant-speed. Also modern turbofan engines usual1y have two and sometimes three mechanically independentrotors operating at different speeds. The spectra of figure 514.3-25 must be modified if used for these cases. These vibration environments can be practically approximated in the laboratory by the source’dwell test described in I-4.2.2. Many vibraticn problems in this type of environment are associated with the coincidence of equipment vibration modes and the excitation spikes. The notches between spikes are used in intelligent design as safe regicna for crit’icalvibration modes.
Thus source dwell ‘teatsminimize the likelihood that equipment will be overstressed at non- representative conditions and that reasonable design provisions will net be subverted. . a. ~. Whenever possible, flight vibraticn measurements should be used to develop vibration criteria for laboratory tests. Appendix 1 prcvidea guidance for the development of test criteria from field data. In the absence of flight measurement, the test levels of table ‘514.3-11 can be used with the spectra of figure 514.3-25. The turbcprcp levels are based on data from varioua C-130 and P-3 aircraft measurements and are fairly representative of the environments of these aircraft.
The decline of spike acceleration spectral density with frequency is based on relatively recent data analyzed in a spectral density format. Engine levels are based cn data measured cn several current Air Force aircraft enginea. b. Test durationa should be developed from fright measurements or field data. Refer to appendix 1 for”guidance in developing test durations from field data. If field data are not available for development of the test durations, then tests ,ghouldbe conducted for 1 hour per axis at the test levels listed in table 514.3-II, modified according to the guidance in I-4.3, I-4.6, 1-4.7, 1-4.8 and I-4.9.
These levels represent maximum actual operating conditions and are functional test levels. C. ~. The test time shall be installed in a vibration fixture which simulates the actual application configuration. To the extent practical, the vibration test setup shculd incorporate actual mounting and isclation prcvisiona from the carrying aircraft. Fixture designs which utilize the maximum amount of platform structure possible will allcw the test item tc respcnd to the laboratory excitation in a manner more closely duplicating its response in the actual field environment.
However, all.equipment items protected from vibration by these means should also pass the minimum integrity test requirements of I-3.2.12 with the test item bard-mounted to the fixture. METHOD 514.3 19 July 1983 514.3-33