Uplinker, megan, gums, et al.,
Given the title of this thread and your combined references to balanced surfaces and/or control tabs I feel the need to comment on what balance/tabs can do and how that would not be sufficient for the issues faced by the 737 control system designers. Balanced surfaces and/or tabs can help address control forces when they could otherwise be too heavy (or even too light for that matter). These control system design tools cannot, however, compensate for aerodynamic characteristics that yield control forces with the wrong polarity. For instance, if an airplane has a tendency to pitch down with increased airspeed such that the pilot has to pull to maintain the desired speed the inherent speed instability cannot be solved via elevator surface balancing or design of an elevator tab. Similarly if an airplane exhibits pitch up characteristics with increased AOA that pitch maneuver instability cannot be addressed by balancing or tab design. For both of these characteristics there needs to be compensation through one or more surfaces capable of generating pitching moment to yield net pitch response in the desired, stabilizing direction given a variation in speed or AOA.
The undesirable pitch characteristics described above (speed instability and AOA instability) are both present in some parts of the 737 flight envelope. Insufficient speed stability has been a 737 issue reaching back over a number of minor models. Need to mitigate speed stability issues gave rise to the 737 Speed Trim System (STS). When the 737MAX was found to also have issues with AOA stability the control system features that implemented STS were expanded in the form of MCAS.
Please excuse my going over ground already covered within PPRUNE over the past three months. I just felt the need to make the point that elevator balance and tab design alone would not be sufficient to mitigate the undesirable open-loop 737 pitch characteristics we have been discussing.
Regards,
FCeng84