Definition
A pendulum-style device mounted on a helicopter main rotor hub that uses freely swinging weights suspended on two parallel pins to counteract in-plane vibrations produced by the rotating blades. The weights move in response to rotor-induced forces and produce an out-of-phase reaction that cancels much of the vibration before it reaches the airframe.
Plain English
A set of weights hung from the rotor hub on two pins that swing in a way that cancels out shaking caused by the spinning blades, so the helicopter rides smoother.
Context Anchor
Seen in aircraft engine maintenance, crankshaft design, and powerplant discussions about controlling engine vibration.
Derivation
Bifilar comes from the Latin 'bi-' meaning two and 'filum' meaning thread or wire. The name reflects the device's design: each weight hangs on two pins (originally two threads or wires), allowing it to swing freely in a controlled path.
Why Pilots Care
Reduces airframe stress and improves cabin comfort by limiting vibration transmitted from the main rotor.
Analogy
It is like a small weight that moves at just the right time to calm a shake, the way a swinging counterweight can steady a moving machine.
Grounding Statement
As the crankshaft twists back and forth during engine operation, the absorber’s weight moves in the opposite timing to reduce that twisting motion.
Intuition Check
A bifilar vibration absorber does not absorb all engine vibration. It is designed to reduce a specific twisting vibration in the crankshaft.
Example Sentence 1
After replacing the bifilar vibration absorber weights, the maintenance crew tracked and balanced the rotor to confirm the cabin vibration levels were back within limits.
Example Sentence 2
After rotor track and balance, the bifilar vibration absorber kept cabin shake within limits.