The 777 uses a flight control law called C*u to fly the aircraft in the pitch (nose up/down) axis. In C*u law, the aircraft’s speed is governed by speed stability rather than pitch – or pointing – stability, which means that if the aircraft’s trimmed speed changes, the pitch will change to return it to the set speed. In roll (wing down/up) and in yaw (nose left/right), control is via a direct electronic signal to the control surfaces. C*u, (pronounced cee star u), also incorporates feedback to the flight control computer to assist the pilot in flying.
Boeing 787 chief test pilot Mike Carriker says “in the 787, we will use C*u for the pitch axis, and in roll yaw, we will use a control law named P-Beta. P is the aerodynamic term for roll rate. Side slip angle, ß (normally spelled out Beta) is the angle between the direction the wind is coming from and the direction the nose is pointed. Generally, P is controlled by the rotating of the control wheel, and ß is created by stepping on a rudder pedal. When the pilot ‘rolls’ the aircraft, a command is sent to the flight control computers for a roll rate, and the computers figure out how much control surface is used to meet the command. When the pedal is pressed, it is a command to establish an angle of sideslip, ß. The hard part is that these two terms have an effect on each other. Get some ß, and you get some roll. Create some roll, create some ß. Sometimes you really want ß, like a crosswind landing, but most of the time you don’t want any. Getting this part correct is the hard part.” Flight test evaluations of the P-Beta laws have been undertaken in a leased 777-200ER.
Source: Flight International
