MIL-STD-81OD Downloaded from https://www.everyspec.com 19 July 1983 I-3.1 -Choiceof test Droce&reS. There’is only one procedure in this method. I-3.2 c.h~ 1 t . . 1-3.2.1 Test sDec trum. Onboard gunfire produces vibration from three different sources: a. the overpressure pulses emanating from the gun muzzle, b. the recoil kick.of the gun on its mounts, and c. the motion of ammunition-handling mechanisms as the gun is firing. ., Of these, the one having the greatest effect at most equipment locations is the first. Therefore this test focuses on the vibration caused by.the overpressure pulses from the gun muzzle.
The vibration environment that occurs at a specific equipment location is a complex combination of broadband random vibration and intense narrowband random vibration or sinusoidal peaks at specific frequencies. (See method 514.3, paragraph I-4.2.2.) This vibration is a combination of forced and free vibration of the vehicleta structure. The narrowbsnd vibration--that is,”the pressure pulaea from a firing gun--is very intense and likely to have severe resonance effects on the aircraft’s structure and equipment. These effects are hard to predict from a model and are better discovered by direct measurement.
In the ease of equipment that is structurally coupled to the gun or its mounting structure, test spectra should always be derived from measured data. The significant vibration occurs at frequencies between 10 and 2000 Hz. I-3.2.1.I . It is recommended that meaaured data be taken at the equipment location of interest while the onboard guns are fired. These data should cover a frequenoy range of at least 10 to 2000 Hz.
The data should be analyzed using no greater than a 1O-HZ constant bandwidth filter up to 500 Hz and a 20-Hz constant bandwidth filter from 500 Hz to 2000 Hz with a statistical degree of freedom of at leaat 50.’ These data should be recorded with the aircraft in actual flight because ground tests I could result in data that aren‘t representative of actual conditions. 1-3.2.1.2 d.. aunflre lbra t. n s~ . If maaaured data are not available, teat’spectra may be calculated using the following technique. NOTE: This tachnique may over-predict those frequencieswhere the structure or equipment responses become significantly nonlinear.
Likewise, for those cases in which the structure or equipment resonances coincide with the frequencies ii the gunfire, the vibration response could be under-predicted (re90nance on resonance) Figure 519.3-1 shows a generalized vibration spactrum fOr sunfire-induc?d vibration. It is characterized by four vibration peaks and broadband random. The vibration peaka are at frequencies that correspond to the nominal gunfire rate and its first three harmonics. The specific values for each of the parameters shown in figure 519.3-1 can be determined from table 519.3-I using the following distance factors. METHOD 519.3 19 July 1983 519.3-2