CH-47D Airframe Vibration Reduction Through Airframe Stiffening

Lightweight, local airframe stiffening has been identified by a powerful new analysis process to reduce vibration and airframe fatigue loads in the Chinook. This has strong potential to reduce maintenance costs and may also allow the elimination of one or more of the cockpit vibration absorbers. Chinook airframe vibrations and loads are concentrated mainly at 3/rev and 6/rev. The 3/rev levels are amplified by two nearby modes of the forward pylon, a longitudinal/pitch mode and a lateral/roll mode. At 6/rev there are a number of airframe modes which contribute to vibration, mainly two aft pylon pitching modes. The new analytical approach has been used to identify an efficient way to move all of these modes farther away from the rotor forcing frequencies and significantly reduce airframe vibration.