Boeing 787 FBW Lat/Dir Concept
Aztec Kid - thanks for your response and inquiry about the lateral directional handling of the 787. As I mentioned in my earlier entry, the 777 has maneuver demand augmented control in the pitch axis provided by the C*U control law. This has been carried over to the 787 with little modification. The biggest HQ difference between 777 and 787 is the maneuver demand augmentation introduced on the 787 for lat/dir control.
The 787 includes a control function that Boeing calls "P-Beta" referring to the common nomenclature for roll rate (p) and sideslip angle (beta). This system drives the wing roll surfaces and the rudder to achieve a desired roll rate / sideslip angle combination as commanded via the wheel and pedals.
For wheel inputs alone (feet on the floor - no pedal input) the system commands to a roll rate proportional to wheel displacement. For bank angles in the +/- 30 degree range turn coordination and compensation is provided such that no column or pedal inputs are needed to fly constant flight path angle coordinated turns. At higher bank angles the turn compensation is removed such that column pull is required to keep the flight path from decreasing. In addition, the Boeing Bank Angle Protection (BAP) function comes active at when bank exceeds 35 degrees generating a wheel force that the pilot must overcome to continue to command roll in the direction of increasing bank angle. (Note that BAP was introduced on the 777 and is essentially the same on 787.)
For the 787 augmented airplane response to pedal Boeing decided to stick with the philosophy of designing the augmented response to be similar to that of a well behaved unaugmented airplane. It was decided that pedal inputs should generate both sideslip and roll rate. This gives rise to the need for cross control when encountering cross winds. It was felt that pilots expect this characteristic and would find the airplane unnatural if it were removed. Retaining roll response to pedal input also helps with ground-to-air and air-to-ground transitions as the control system on ground provides direct coupling (wheel to wing roll surfaces and pedal to rudder). As such, on ground the pilot needs to make both wheel and pedal inputs to keep wings level while maintaining centerline during takeoff and landing. The intent is to provide a system that requires approximately the same amount of wheel / pedal cross control on-ground and in-air to minimize takeoff and landing transients.
The intent of the FBW augmentation designs for the 777 and 787 is to present the flight crew with airplane response characteristics that are intuitive and feel natural. The goal has been to have pilot techniques developed on earlier Boeing models translate seamlessly to these models with augmented manual control modes.
Pilot feedback regarding handling qualities on 777 and 787 would be most appreciated and helpful. Note that the 777X now under development will use the 787 control system arrangement as its baseline. This 777 derivative will have both the C*U pitch and the P-Beta roll/yaw control laws delivering maneuver demand augmented control in all three axes.