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What causes aircraft engine detuning

All six-cylinder Lycoming and Continental, and some Lycoming four cylinder, engines employ bifilar mounted pendulum absorbers. These are incorrectly but commonly called "counterweights". Both Continental and Lycoming use a term called "detuning" when describing failure of the absorbers to function properly. Just what is and what causes detuning?

back-end of Lycoming crankshaft showing two "counterweights"

It is the function of pendulums to negate the harmful effects of crankshaft torsional vibrations caused by combustion forces. Crankshaft counterweights (pendulum absorbers) are tuned to absorb torsional( twisting) vibrations from the crankshaft at critical vibration frequencies. Detuning occurs when the pendulums are unable to absorb the crankshaft's vibratory torque.

Crankshaft torsional vibration happens because each power stroke slightly twists the crankshaft. The crankshaft then untwists between power strokes. If the frequency of this twisting and untwisting coincides to the natural frequency of the crankshaft then the twisting is amplified by a phenomenon called resonance. Each crankshaft design has a natural torsional frequency like the note of a ringing bell or sound of a vibrating guitar string. If this natural frequency coincides with the torsional frequency of the power pulses then the twisting is amplified through resonance. It is the function of the pendulum absorbers to counteract torsional twisting at the natural frequency of the crankshaft.

The pendulum absorber swings in opposition to the crankshaft's vibratory torque energy. It is the acceleration of the pendulum's mass that counteracts the twisting motion of the crankshaft. The pendulum acts like a child on a swing. Since daddy pushes at just the right time (frequency), and with the right force, the child swings smoothly. If daddy pushes too hard, the rope slackens, and the swing jumps slightly as it comes back down.

The pendulum acts in the same manner. If you feed it too much energy it stops swinging and starts jumping. Once it starts jumping, it stops absorbing the crankshaft's twisting energy. The increased energy reinforces the pendulum's jumping such that the pendulum stabilizes and doesn't want to return to swinging. The jumping counterweight hammers against the attachments leaving "impingement marks".

The pilot usually doesn't feel anything different in his engine. Torsional vibrations are rapid reversals in crankshaft rotation superimposed onto normal crankshaft rotation. Vibrations are not transmitted out of the engine because the crankshaft rotates in the bearings and doesn't move against the crankcase. Only if the crankshaft or propeller bends in response to the twisting force do vibrations leave the engine. The propeller and accessories, however, feel the torsional vibrations as rapid reversals in direction that cause very high stress loading.
The following is a listing of engine problems that have been caused by detuned counterweights:

Impulse coupling failure - vibrations in the gear train cause the impulse pawls to vibrate.
Magneto oil seal failure - vibrations in the gear train cause the magneto shaft to flex.
Oil pump stress and damage in Lycoming engines - again caused by gear train vibrations.
Crankshaft cracking.
Propeller failure.

It is interesting to note that the more wear that occurs to the counterweight bushings the easier it is to detune the counterweights. One reason I do not recommend operating past engine TBO.

What causes detuning? It's normally impossible to detune a counterweight when operating within the engine's recommended power settings. Combine rapid throttle movement, high engine torque settings (high manifold pressure, low rpm), counterweight bushing wear, and cylinder misfiring and you get closer to detuning.

 

Pendulum dampners (counterweights) Continental IO-520 engine

 

Close-up of Lycoming counterweight showing plates (the part with 3 holes in it) that hold the pins in place. Pin diameter determines the pendulum length and thus the frequency.

Close-up of hanger and counterweight bushings. Polishing and impingement damage to the sides of the hanger indicate counterweight detuning.

  If it's water, this presents no problem at all, as one wave is mighty like another. Parachute Sense US NAVY 1944

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