DETERMINATION OF AH–64 AIRFRAME IN–FLIGHT MODAL FREQUENCIES USING ROTOR CONTROL–INDUCED EXCITATIONS

Flight tests were performed to determine the natural frequencies of the major airframe modes of the McDonnell Douglas AH–64A helicopter and the variation thereof with flight condition and vehicle mass distribution. Two wing stores configurations were tested in hover and in forward flight. An electronic controller connected to the control actuators provided the exciting forces for the modes of interest by oscillating main and tail rotor controls both at prescribed frequencies and for frequency sweeps. NASTRAN finite element analysis was used to support the test planning and for comparison with the test results. In addition, the flight–measured frequencies were compared to those measured by suspended shake test for assessment of the effects of the rotating system on the major modes. Analysis of the test data provided for conclusive identification of a number of major airframe modes for each of the aircraft configurations and for each flight condition flown. These results are presented here along with a description of the flight test methods used. Conclusions are drawn as to the effectiveness of the test methods and data analysis techniques as well as for the necessity of determining airframe natural frequencies in flight as opposed to using suspended shake test methods.