C* and C*U explained
C* is a computed parameter that combines pitch rate and incremental normal load factor. It was originally developed as a handling qualities metric and then incorporated as the core of maneuver demand control laws.
C* = q + K_C* x delta_Nz
q = pitch rate
delta_Nz = incremental load factor
K_C* = C* crossover gain - chosen to balance q and delta_Nz contributions
At low speed, pitch maneuvers (and thus the C* parameter) are dominated by pitch rate - lots of q with relatively little Nz. At high speed, pitch maneuvers are dominated by incremental load factor - lots of Nz with relatively little q.
An interesting feature of C* is that the variation in C* response between different airplane speeds is much less than the variation in either q or Nz responses. Much less gain scheduling is needed when designing a control law base on C* in comparison with a control law based on q or Nz.
For steady level flight, C* is zero. Pull the 777 column and you are commanding positive C* - the airplane responds with a nose up maneuver such that C* equals the C* command generated by the pilot's pull. The opposite happens for a 777 column push that generates a negative C* command.
Now the transition from C* to C*U.
C*U = C* + K_U x (Speed_Reference - Current_Speed)
Speed_Reference = Trim speed that the airplane will return to in the absence of column input
The 777 with a steady column pull will settle at a speed lower than the reference speed such that C*U is positive matching the C*U command generated via the column. Similarly a steady push will result in flight faster than the reference speed. Note that with C*U for both a column push and a column pull C* itself will be zero in steady state.
When designing the 777, Boeing chose to include positive speed stability as a characteristic of the pitch axis airplane response. This was accomplished by using C*U as the core feature of the augmented longitudinal control law in Normal Mode. The U portion of C*U was developed to be compliant with existing FARs concerning positive speed stability.