Isolation of Rotor Induced Vibration with the Bell Focal Pylon-Nodal Beam System

The stringent vibration requirements of the military and the desire of the civil customer for a smooth ride has led Bell to develop the NODAMATIC isolation system. The NODAMATIC system consists of a focal pylon to isolate inplane hub shears and hub moments and a nodal beam to isolate vertical hub shears. Presently this system is used on two production helicopters, the Bell Models 214 and 206L, on the Model 222 8-place commercial helicopter now in development testing, and on the U. S. Army YAH-63 Advanced Attack Helicopter. Flight tests have demonstrated that crew station and cabin vibration levels of less than .05g at blade passage frequency can be attained with the NODAMATIC system.

The technical design considerations of tuning for isolation over a wide rpm range, minimization of system weight, redundancy and crashworthiness, and the influence of the isolation system on rotor stability are discussed. Mechanical design requirements for the system are also discussed. Operational experience with the 214 and 206L NODAMATIC systems is reviewed.

The paper concludes with a discussion of design requirements for an isolation system that could provide the helicopter with a “jet smooth” ride.