Boeing FBW Pitch Feel
Microburst - glad to take a crack at your questions - feel free to ask follow-ups as you see fit.
When talking of controller feel it is important to distinguish between force generated via changing the force/displacement characteristics of the controller and forces that are generated via the pilot finding it necessary to hold the controller out of detent. Both of these concepts come into play with the Boeing FBW pitch control system.
As you are probably well aware, C* is a combination of pitch rate and the normal load factor increment from that for level flight. To get to C*U, Boeing has chosen to add a third, speed based term. The U in C*U is a command that is a function of the difference between the current airspeed and a reference airspeed. If speed is higher than the reference speed, the U term commands airplane nose up response. Similarly, if the speed is lower than the reference speed, the U term commands airplane nose down response.
The C*U control system forms the sum of measured C*, the U term based on speed, and the C*U command that is a function of column position. In steady state, the C*U control system drives the elevators to command pitch such that the sum mentioned above is zero. If speed is equal to the reference speed, the pilot will find that zero column commands zero pitch rate and normal load factor for constant flight path angle. If speed is higher than the reference speed, the pilot will find that a push displacement of the column is needed to generate sufficient C*U command to offset the system's nose up U term. Similarly, if speed is lower than the reference speed, the pilot will have to pull in steady flight to keep the U term from commanding the nose down. Note that all of this is achieved without changing the force/displacement feel characteristics of the column.
Now to the question of what happens when starting at a trim condition the pilot pulls and holds a small amount of column. The initial response will be nose up pitch rate / increase in normal load factor. As speed bleeds off, the U term will command more and more nose down in an effort to return to the reference speed. Eventually the speed difference will become large enough that the pilot's nose up command due to column pull is balanced by the nose down command due to speed error. If held long enough and allowed to damp out a new equilibrium will be reached where the pilot is holding constant pull force and the airplane is steady at a speed the is the corresponding amount slower than the reference speed. For a push, the converse happens leading eventually to steady flight at a speed higher than the reference speed.
The Boeing system operates with minimum reference speed limit of top of amber band on the speed tape and a maximum reference speed limit at Vmo/Mmo. The C*U reference speed cannot be set outside of those limits.
The system is designed to feel intuitive to the pilot who has learned to fly a conventional, unaugmented airplane. The basic operation is to fly the airplane to the desired condition activating pitch trim as needed to remove steady column force (i.e., the need to hold column out of detent). On an unaugmented airplane the pitch trim input drives the stabilizer (or elevator trim) up/down. With C*U these same inputs command the reference speed down/up (polarity choice here intentional) to yield the same flight deck effect.
Boeing does vary the stiffness of the column feel (i.e., the force displacement relationship) for other purposes, but not as part of the basic C*U control law at a given airspeed. Column feel is stiffened at high speed and softened at low speed to provide better speed awareness and feel similar to an airplane with mechanical linkage between pilot an elevator such that column force is proportional to elevator hinge moment.
Last edited by FCeng84; 6th Feb 2015 at 19:07.
Reason: Correct misleading final statement