B737 pitch trim description
Please allow another try at describing pitch trim on the 737. Notice that I chose the word "pitch" trim rather than "speed" trim to be more general. I will address the speed trim system more directly below.
One of the sources of confusion (at least for me and maybe others) is that the word "trim" is often used to describe any motion of the horizontal stabilizer. This gets confusing in that "trim" or "pitch trim" also carries the notion of "moving the horizontal stabilizer so as to offload column forces" as in getting to a state where steady column input is not required to continue flying at the desired steady state condition. It is unfortunate that the word "trim" has been used for both of these.
Now we need to consider the FAR speed stability requirement. In general terms, the FAR states that an airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots. That means that once trimmed (i.e., no column input required for steady flight) reducing speed by 10 knots (without a throttle or configuration change) should require 3 pounds of pull force to maintain steady flight. Similarly increasing speed (again without a throttle or configuration change) should require 3 pounds of push force to maintain steady flight. In this manner, the airplane (absent a column input) will tend to nose up/down appropriately to resist changing speed. If an airplane does not exhibit sufficient speed stability to meet this requirement, one solution is to have a control system function that moves airplane surfaces as needed to provide sufficient pitching moment as a function of speed changes to require the specified levels of column input to balance moments.
Meeting the stick force per knot FAR is the motivation for the speed trim system on the 737. For a limited portion of the flight envelope the bare airplane without any stability augmentation did not meet this FAR. To compensate, the 737 speed trim system moves the stabilizer in response to speed changes to augment the stability. Note that while this system is only needed to help meet the FAR at a limited set of conditions, it is for simplicity sake designed to respond to speed changes regardless of CG or weight. As a result, the speed stability is increased over a much wider range of conditions than just those where the airplane would not meet the FAR without it. For instance, at aft CG where the function is needed it will require just enough column when speed is changed. At forward CG where the airplane already has sufficient inherent speed stability, the STS will add more and could be seen as being an unnecessary nuisance.
As many have noted in previous entries to this thread, the STS "trims" (i.e., moves) the stabilizer opposite the direction that the pilot has to move to the stabilizer to relieve column forces. In a sense, the STS "un-trims" the airplane thus requiring the pilot to re-trim it. Whether one sees this as an unnecessary bother or a tool providing the flight crew with positive awareness of speed deviations is a matter of opinion. This gets to the heart of the A vs. B differentiation between their respective C* and C*U pitch augmentation systems - another topic that has been discussed in other PPRUNE threads over the years.
It is important to recognize that there are a number of other factors that impact pitch trim for which the speed trim system does not take any consideration. Changes to thrust, flap position, speedbrake setting, and gear position will generate pitching moment changes that must be balanced via the elevator (i.e., column when flying with autopilot disengaged) and followed up via pilot pitch trim inputs (i.e., movements of the stabilizer) to allow releasing the column. The 737 speed trim system essentially adds an increment of stabilizer motion in the positive speed stability direction as a function of a change in airspeed. It does not maintain a target airspeed that it seeks to return to regardless of other pitching moment disturbances that the airplane may experience. Other airplanes with higher levels of pitch augmentation (777 and 787 for example) do provide control surface inputs to counter such pitching moment disturbances and thus return to a specific airspeed, but that is another story for another thread.
Last edited by FCeng84; 7th November 2018 at 02:35.
Reason: correct error: STS is a function of thrust but not weight