Downloaded from https://www.everyspec.com MIL-STD-81OC 3.2.1 Primary structure. Primary structure craft which comprises the main 1oad carrying aircraft. Examples of primary structure are stringers and includes those consoles structurally integrated with the aircraft outer walls. is that structure of the air- members or elements of the skin, frames, rings, bulkheads, 3.2.2 Secondary structure. Secondary structure is that structure to which equipment is attached onto or centained in and whose mounting points termin- ate at the outer frame, skin, stringers, bulkheads, floors, spars or cast framing of the primary structure.
Examples of secondary structure are instrument panels, trays, racks, shelves, trusses, beams, and consoles. 3.3 Determination of test spectrum. This method requires that the test spectrum for primary structure be determined first and then; if applicable-- that is, for those equipments associated with secondary structure; the pri- mary spectrum be modified to represent the final test spectrum. The test spectrum consists of low frequency sinusoidal (basic gunfire rate and the second, third and fourth harmonics) superimposedon a centin- uous random vibration spectrum (see figure S19. 2- 1).
The vibration prediction surfaces from which the spectrum for the primary structure is derived are shown in figures 519.2-2 through 519.2-4. Figure S19.2-2 represents the prediction surface for the sinusoids. Figure 519.2-3 is the surface used to obtain the high frequency random part of the spectrum. Figure 519.2-4 is the surface used to define the low frequency portion of the random spec- trum. All surfaces are three dimensional isometric projections in which the surface shape varies,as a function of the no~alized frequencY Par~eters f/fo, f/fo, and the vector distance, D. The frequency parameters are pre- sented in this form since they are treated as variables.
The distance parameter, D, represents the vector distance measured from the gun muzzle to the mean distance between equipment support points. Note that the amplitude (vertical)scale is represented as a dB scalar (10 10glo); thus, all values of the surfaces are referenced tO O dB. TO utilize these surfaces it is necessary to: first establish the magnitudes of the sine and the random vibration at the O dB level and then proceed to the D = x slice of the surface and pick off the required spectral values, relative to O dB. METHOD 519.2 519.2-2