USING RESILIENT CONNECTIONS FOR HELICOPTER AIRFRAME MODAL FREQUENCY PLACEMENT

Critical to the design of rotorcraft is the placement of the major airframe natural frequencies for sufficient separation from the frequencies of the excitations generated by the rotating components. Often, however, an existing platform which has good frequency characteristics is required to undergo significant modifications which adversely impact the airframe natural frequency placements. A design approach is necessary which provides for re–tuning the modified airframe to return its dynamics characteristics to the quality of the original design. For this purpose, preliminary design studies have been performed for utilizing resilient connections for airframe components to favorably adjust airframe natural frequencies. Specifically, the need arose for frequency re–tuning of the McDonnell Douglas AH–64 helicopter due to planned airframe modifications. A preliminary design was developed for mounting the empennage to the tailboom using elastomeric cartridges to tune the frequency of the fundamental airframe mode away from the main rotor rotational frequency (1P). The stiffness of the elastomeric elements was tailored to provide the desired frequency separation. Consideration was given to the effect of the resilient mounts on other major airframe modes, and the design tailored to minimize that effect. The preliminary studies indicate that the approach is very effective for frequency tuning, that it provides acceptable fatigue life for the components involved, that it can be implemented cost–effectively as a retrofit, that it can be designed to have minimal effect on the operational characteristics of the aircraft, and that it can be effectively and efficiently fine–tuned for various current and future aircraft derivatives, as necessary.