This is part seven of a series of blog posts concerning the MIL-STD 810 Vibration Section. This blog was written with reference to MIL-STD-810G w/Change 1 dated 15 April 2014. DES has the experience and expertise to help you determine what profiles are appropriate for your product and to run your MIL-STD-810 vibration test. For more information, please check out our Vibration Testing services page and our other MIL-STD-810 vibration testing blog articles:
Category 15 of Method 514.7 Vibration testing describes the different vibratory environments aircraft stores may experience in an aircraft. Three environments are detailed in category 15; captive flight – external carriage, captive flight – internal carriage and free flight.
Captive flight – external carriage include stores carried externally on a jet aircraft. The overall vibration profile arises from four sources; engine noise, in-flight aerodynamic turbulence, vibration transmitted through attaching structures and internal store vibration. Engine noise is highest at the boundary layer of the exhaust jet plume. At this point there is turbulence between the exhaust air and ambient air and is at its maximum during takeoff. In-flight aerodynamic turbulence do not greatly affect overall store vibration, however, they may cause other structures such as tailfins to produce vibrations that are then transmitted to the store. These vibrations, as it states in category 15 of method 514.7, are a “low frequency system” and are often characterized as buffet vibration, Figure 2. “Buffet vibration is typically concentrated between 10 and 50 Hz” and “is dominated by store structural resonances.” Finally stores are also susceptible to internal vibration from elements such as rotating machinery.
Captive flight – internal carriages generally don’t experience harsh vibration levels unless the bay is opened during flight. “This event is referred to as a cavity resonance and results in high levels of turbulence inside the bay.” This vibration is characterized as a wide band spectrum with high spikes across it. Low frequency spikes (<100 Hz) are not expected to be of issue for most stores, however, high frequency spikes can be very dangerous.
Free flight vibration will typically be experienced by stores that are deployed from the aircraft. Free flight vibration is comprised of engine noise vibration, onboard vibration and varying turbulence similar to that of captive flight.
Figure 1 below, shows the profile for store vibration exposure on jet aircrafts. Figure 2 shows the vibration profile for buffet vibration response. W0, W1 and W2 in Figures 1 and 2 are calculated from rather complicated tables and formulas within Category 15. The vibration testing durations are determined from the Life Cycle Environment Profile.