TBE - "Toilet Bowl Effect"
TBE, Toilet Bowl Effect, describes the behavior of a model helicopter which moves in a small, usually clock-wise circle like water in a toilet when flushed instead of hovering in one spot. The cause of TBE is often difficult to detect because there are usually no overt visible causes. But given the design of the controls which change the feathering pitch of the rotor blades the culprit is one or more factors which move the links to rotors.
The underlying TBE forces are always there, but other helicopter design features dampen it in the same way shock absorbers on cars prevent the car from continuing to bounce up and down when it hits a bump. For example, the main rotor blades should "dish" under load. As you power up the tips should be able to rise up and form a shallow V shape. Tightening the blades in the grips too tightly will prevent the nominal amount of dishing to occur and also cause TBE, or more accurately prevent the dishing from dampening it and the imbalance in the rotors.
The two blades of the rotor are at equilibrium when hovering. But when control input or other factors changes their opposite feathering pitch (one +, the other -) things occur that are not visible to the eye because the rotor is a spinning blur. On the blade where the + pitch on cyclic input occurs the blade tip on that rotor slows and rises relative to the other one where the pitch has decreased by the same amount causing its tip to speed-up and drop. That lead/lag and flapping action is something all helicopter rotors do, and why on the Blade models the rotor blades need to be loose enough to move.
In hybrid Bell-Hiller 45° flybar the 120SR the flybar, which both amplifies and dampens control inputs is a likely suspect when TBE occurs after a crash. It does not take much servo pressure to move the flybar because when tilts the airfoil wings on its ends fly it into its new attitude and change the cyclic feathering pitch of the blades -- that's the Hiller-Matic Paddle system effect. The Bell hybrid part, and difference from a pure Hiller design is the heavy mass on the ends of the flybars which acts as a gyroscopic stabilizer which makes it more stable and self correcting.
The fact the flybar sits 45° to the rotor, not 90° as in the original Bell stabilizer bar and Hiller flying paddle designs makes balancing the light long rotor and short heavy flybar a difficult task. Anything that throws the flybar out of its nominal 45° phase with the rotor, such as getting bent up in a crash will induce TBE. Since the links also move the flybar, which amplifies the movement like power steering (the advantage of Hiller vs. Bell) any slop in the linkage will move change the feathering of the rotor.
If you have persistent TBE that adjusting the blade grips looser doesn't fix, try replacing the flybar which might be cracked or bent in a way you can't see when still but is creating an out-of-phase problem when it spins.