Modal Testing of Vertical Tail of F/A-18 Hornet

The goal of the study was to identify experimentally dynamic vibration characteristics of the aft fuselage and especially the Vertical Tail (VT) of a fighter aircraft. The object of the study was F/A-18 Hornet of the Finnish Air Force. Vibration properties, i.e., natural frequencies, modal damping factors and mode shapes were determined experimentally by impact testing. Special focus was on the modes around about 15 Hz and 45 Hz. Based on earlier analysis of the measured (in-flight) acceleration and strain data, it has been found that the VT experiences high vibration levels around these frequencies due to flow-induced excitations.
The plane was standing on the landing gears during the measurements. The left and right Vertical Tails were excited by an instrumented impact hammer having a soft plastic tip. Both VTs were excited separately and responses from both VTs and other locations were measured during all tests. Frequency Response Functions (FRFs) were calculated between measured input excitation force and acceleration responses. To identify closely spaced double modes, separate measurements were conducted where both VTs were excited simultaneously randomly at random locations of the VT surface by two impact hammers having soft plastic tips. Time histories of acceleration responses were recorded for Operational Modal Analysis (OMA). Natural frequencies, modal damping factors and mode shapes were identified using both conventional experimental modal analysis and Operational Modal Analysis methods.
Lowest elastic natural modes of the VT of the F/A-18 Hornet were identified experimentally successfully. It was found that due to symmetry of the structure, main VT modes are divided into symmetric and antimetric modes having close natural frequencies. Random impact excitation technique for Operational Modal Analysis was demonstrated to be applicable for identification of very closely spaced modes.