Boeing 787 Dreamliner - Mark Wagner [56]
Boeing was able to take advantage of the 787’s Honeywell-developed fly-by-wire (FBW) flight control system (FCS) to make the aircraft thousands of pounds lighter as well as to reduce cruise drag and improve safety. Electrically signaled in the pitch axis like the 777, the 787 was also fly-by-wire in the roll and yaw axis, giving control in three axes. The 787 system was therefore more sophisticated and integrated, and gave designers more flexibility to tailor the aircraft’s structure and flight control responses.
“We’ve taken all the lessons learned from the 777 and applied them to the new aircraft, as well as taken advantage of the FBW technology we didn’t fully do with the 777,” said Sinnett.
The 787 FCS combined a control law called P-Beta (P being the aerodynamic term for roll rate), with the 777 flight control law called C*u (pronounced Cee star u), which governed speed stability rather than pitch—or pointing—stability. This meant that if the speed of the trimmed aircraft changed, the pitch would change, to return it to the set speed. In roll (wing down/up) and in yaw (nose left/right), control was via direct electronic signals to the control surfaces.
Boeing 787 chief test pilot Mike Carriker said “sideslip angle β [beta] is the angle between the direction the wind is coming from and the direction the nose is pointed. Generally P, roll rate, is controlled by the rotating of the control wheel, and β is created by stepping on a rudder pedal. When the pilot ‘rolls’ the airplane, 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. The hard part is that these two terms have an effect on each other. Get some β, and you get some roll. Create some roll, create some β. Sometimes you really want β, like a crosswind landing, but most of the time you don’t want any. Getting this part correct is the hard part.”
Flight tests of the new control laws were made as early as 2006 in a leased American Airlines 777-200ER in which the FCS could be switched on and off in flight. More authority, or “gain” in the P-Beta laws was gradually introduced to evaluate the interplay between the control inputs. Dubbed the CV/RR (control verification/risk-reduction) test bed, the flight tests also included simulation of the 787’s drooped ailerons as well as a drag-reducing feature called the trailing edge variable camber (TEVC) function.
Boeing expected that the TEVC could cut cruise drag and save the equivalent of 750 to 1,000 pounds in weight, and took advantage of the all-new wing and flight control surface design. The fully automatic system, which was the first practical commercial application of in-flight variable camber, operated by deflecting the trailing edge flaps in 0.5-degree increments while in cruise. The system could be moved through a 3-degree arc, with the trailing edge being set up and down by as much as 1.5 degrees on either side of a neutral position.
Rockwell Collins’s WXR-2100 MultiScan weather radar was designed to give up to a 320-nautical-mile-range strategic weather picture and was protected by a 7-foot-diameter, electromagnetically transparent cover developed by Ohio-based Saint-Gobain Flight Structures. The unit was the largest quartz radome structure the company has ever designed and manufactured for commercial aviation. Guy Norris
As the 787 fly-by-wire system was “full authority,” acting in the longitudinal as well as the lateral axis, it also was used to help reduce the structural weight of the outboard wing by shifting the lift distribution inboard. This was achieved by the basic shape and twist of the wing itself, as well as by using maneuver load alleviation control laws to move ailerons, spoilers, and flaperons (multi-function control surfaces that acted as both flaps and ailerons).