Downloaded from https://www.everyspec.com MIL-STD-810D 19 July 1983 to the “vibration the vibration conditions apecified. The fixture shall be designed by utilizing actual racka, panels or platform structures of the helicopter to minimize the introduction of unrepresentative response within the test item. Low-level (0’.5-g)5 to 500 Hz sweepa to identify the resonant and transmissible frequenting the item shal be conducted before and after its exposure to the test ‘vibrationenvironment. Significant changea in resonant properties shall be considered as failures even though the item still functions to specification levels. c. lest se tUQ.
The test item shall be attached exciter by means of a fixture capable of transmitting I-3.2.7 ~egorv 7A - Assembled externa1 stores. let airce. Assembled jet aircraft stores will encounter three distinct vibration environments: captive flight, buffet maneuver, and free flight. Gictive f= : Extensive measurement programs have ahown that the vibration experienced by an externally carried store on a jet aircraft arises from three distinct sources: (1) Aerodymmic boundary layer turbulence. (2) Buffet maneuvers. : (3) Aircraft-induced vibration.
In general, store vibration is primarily caused by broadband aerodynamic boundary layer turbulence and ia relatively independent of the carrying aircraft and mounting location on the aircraft. Instead, vibratory excitation is Mostly influenced by store shape, mcunting configuration and dynamic pressure. This source of vibration is distributed along the entire surface of the atore and is difficult to simulate by point input of vibration, euch as from a vibration shaker, unless the store is relatively stiff. Therefore, an acoustic teat (method 515) ia recommended for this environment.
The lower-frequency portion o f the aasembled store vibration spectrum comes from either aircraft-induced vibration or buffet maneuvers. Aircraft-induced vibration generally is present during the entire captive flight phaae for a store. Ita frequency range is covered by the response threshold spectrum shown in figure 514.3-30. ~. Recent flight test programs on the F-16 and F-15 with varioua externally carried stores have shown intense vibration associated with buffet maneuvers. Other similar aircraft, such aa F-14, F-18, or next generation fightera, have the potential to produce intense vibrations during high- performance maneuvers.
The buffet maneuver envelope is generally bounded by speeds of O.8 to 0.9 MACH and altitudes of 3 to 10 kilometers (9,84o to 32,800 ft). Although the vibration levels during high-performance maneuvers are very intense, they generally do not laat for more than 10 seconds, reaching their peak in less than a second and rapidly deteriorating in 5 to 10 seconds. For the purpose of establishing test durations, a comionly used assumption is that an aircraft store may experience as much aa 30 seconds of maneuver buffet vibration for each hour of captive-carriage flight (figure 514.3-31). 19 July 1983 514.3-U5 METHOD 514.3