Bifilar damper on UH-60

[NickLappos]

I helped develop the bifilars on two of our models, and understand them a bit:

They are simple swinging masses that are resonent near the N/rev frequencies (4 per revolution in the Black Hawk and S-76)- they love to hum along at the same beat frequency as the blade passages. They are actually tuned to N-1 and N+1 because they are in the rotating system, where the blade passage frequency is made up of the sum of those two frequencies.

The bifilar is much lighter than other vibration absorbers, and right at the source. Also, there is no spring on a bifilar, they achieve their frequency by CF, so they automatically tune to the rpm they are at, allowing large rpm changes without large vibration changes.

Bifilars work in-plane, the most effective direction. Generally, if only one is needed, the one bifilar is tuned to 3 per rev (N-1) which is the bigger, more objectionable component. Nobody removes bifilars by themselves, because they are part of the airworthiness of the machine, not an option. The vibrations at many airframe stations, not just the people, depend on them, so just tinkering with them might make some nice piece of equipment very sad without showing up on the normal maintenance gear. We want our engines to be very very happy, don't we?

Bifilars were invented for piston engines decades ago, and are not rocket science.

The orientation of the arm over the space between the hub arms is specifically designed to allow ease of maintenance and to prevent interference with blade flapping. There is no issue with the exact orientation, the weights all act to absorb the in-plane vibrations.

The only vibration they act on is 4 per rev. NO other frequency is helped by the bifilar. If one weight is badly slung, and does not swing properly, some odd frequencies can be flet, because the other weights conspire to upset the vibration balance. Typically, a 3 per rev is felt if the bifilar is badly maintained.

Any connection between the bifilar and hydarulic pressure drop exists only as coincidence, there is no inter-connection between the two.