Downloaded from https://www.everyspec.com MIL-STD.81OD 19 July 1983 a. Vector distance (D). The vector distance from the muzzle of the gun to the mean distance between equipment support points as shown in figure 519.3-2. For configurations involving multiple guns, the origin of vector D is determined from the centroidal point of the gun muzzle as shown in figure 519.3-3. b. Gun standoff distance (H). The distance normal to the aircraft’s surface as shown in figure 519.3-4. c. Depth parameter (R~). The distance normal to the aircraft’s skin to the equipment location inside the aircraft. . If R= is unknown, use R~ three inches (see figure 519.3-2).
The vibration peaks bandwidths should be baaed on teat data if possible. When data are not available, the vibration peaks’ bandwidths can be calculated as: m Bhk@ = ~ . Where: Bw3dB = the bandwidth at a level 3dB below the peak PSD level F . the fundamental frequency (Fi) or one of the harmonics: fl, f2, fa, or f4 For cases where the gun firing rate changes during a development program, it is desirable to perform sinusoidal sweeps within the proposed bandwidth for the fundamental and each harmonic. o 1-3.2.2 D~ti. The duration of the gunfire test in each of the three axes should be equivalent to the expected total amount of time the equipment will experience the environment in actual usage.
This duration may be conservatively estimated by multiplying the expected number of aircraft aortiee in which gunfiring will occur by the maximum amount of time that gunfiring can occur in each sortie. The number of sorties in which gunfire will occur will be tied to planned aircraft training and combat utilization rates, but will generally be in the vicinity of 200 to 300 sorties. The maximum gunfire time per sortie can be determined from table 519.3-11 by dividing total rounds per aircraft by the firing rate.
When a gun has more than one firing rate, the test shculd be run using both firing rates, with test time at each firing rate based on the expected proportion of time at each firing rate in service. The guns carried by an aircraft are generally fired in short bursts that a . few seconds and testing should be performed accordingly. For example, vibra- tion should be applied for two seconds followed by an eight- to ten-second rest period during which no vibration is applied. This two-second-on/eight-to-ten- second-off cycle is repeated until the total vibration time equals that deter- mined for the aircraft type.
This cycling will prevent the occurrence of > unrealistic failure modes due to vibration isolator overheating in continuous vibration. Intermittent vibration can readily be achieved by several means including interrupting the shaker input signal and storing acceleration time history inputs on magnetic disc or tape. o METHOD 519.3 19 July,1983 519.3-3