As aircraft increase in size and "weight, their con- trols become more difficult to operate and systems must be used to aid the pilot. The power-boosted control system is similar in principle to power steering in an automobile. A hydraulic actuator is in parallel with the mechanical operation of the controls. In addition to moving the control surface, the normal control movement by the pilot also moves a control valve that directs hydraulic fluid to the actuator to help move the surface. A typical boost ratio is about 14, meaning that a stick force of one pound will apply a force of 14 pounds to the control surface.
TYPES OF CONTROL SYSTEMS
The problem with a power-boosted control system is that during transonic flight shock waves form on the control surfaces and cause control surface buf- feting. This force is fed back into the control sys- tem. To prevent these forces reaching the pilot,
many airplanes that fly in this airspeed region use a power-operated irreversible control system. The flight controls in the cockpit actuate control valves which direct hydraulic fluid to control surface actuators. Since the pilot has no actual feel of the flight loads, some form of artificial feel must be built into the system that will make the control stick force proportional to the flight loads on the control surfaces.
As mentioned earlier, if a mechanical system exists that links the pilot's controls with tabs on the control surfaces so that the pilot can control the airplane in the event of a complete hydraulic failure, the airplane is said to have "manual rever- sion" capability. If sufficient redundancy is built into the system so that loss of one or more hydraulic systems will not seriously affect the pilot's ability to control the airplane, manual reversion is not necessary.
BOEING 747 CONTROL SYSTEM
The flight control system for the Boeing 747 jet transport aircraft is typical for large aircraft.
Rotation about the three axes is controlled by con- ventional ailerons, rudders and elevators. These pri- mary controls are assisted by spoilers which also double as speed brakes, by an adjustable horizontal stabilizer and by both leading and trailing edge flaps. [Figure 1-63]
The primary control surfaces are moved hydraulically by dual-tandem irreversible actuators. Hydraulic power for each primary flight control is supplied by the four independent hydraulic systems. Manual or electrical inputs from the cockpit direct hydraulic pressure to the control surface actuators. There are no trim tabs installed on any control surface and there is no manual reversion capability such as found in other aircraft like the Boeing 727. Manual reversion is not necessary due to hydraulic redundancies and duplication of control surfaces. Control system feel forces are computed and generated artificially. The flight controls are programmed so that the airplane response to control inputs is the same regardless of speed, center of gravity or gross weight.
Figure 1-63. The flight controls for each axis of a Boeing 747 are powered by all four hydraulic systems and are designed so that the airplane remains controllable even after failure of any two hydraulic systems.
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ROLL CONTROL
The 747 has two sets of ailerons and six spoilers on each wing. During high-speed flight, the inboard ailerons and flight spoilers provide roll control and an outboard aileron lockout system isolates the out- board ailerons from the lateral control system. When the trailing-edge flaps are extended out of the full up (retracted) position, the outboard ailerons are unlocked and both sets of ailerons and the flight spoilers operate together.
The five outboard spoilers on each wing operate in flight. Their movement is proportional to aileron displacement and they decrease lift on the wing the pilot wants to move down. When the speed brake control is actuated in flight, the four inboard sec- tions on each wing are raised, and they produce a great deal of drag. The innermost spoiler on each wing rises only when the speed brake control is at its maximum flight setting. If the control wheel is rotated while the spoilers are deployed as speed brakes, they will move differentially, decreasing the lift on the wing that should go down, and decreas- ing some of the drag on the wing that should rise. When the airplane lands, the ten flight spoilers and the two ground spoilers automatically extend to the fully raised position to dump lift and slow the air- plane in its landing roll.
Both control wheels in the cockpit are intercon- nected so that movement of either wheel provides an input through a trim, feel and centering unit to two central control actuators. The pilot's control inputs are hydraulically augmented by the central control actuators and transmitted to the aileron pro- grammers and a spoiler mixer. The central control actuators also receive and augment inputs from the autopilot.
The aileron programmers control the movement of the ailerons through individual dual-tandem actua- tors which are powered by two separate hydraulic systems. The design of the system is such that loss of any two hydraulic systems will cause only one of the four ailerons to become inoperative. The ailerons are trimmed by an electric motor which adjusts the neutral position of the trim, feel and cen- tering unit.
PITCH CONTROL
Four elevators, powered by all four hydraulic sys- tems, provide pitch control. Elevator movement is accomplished by inputs from either pilot's control column through a system of cables and pulleys to the inboard elevator control valves. When the inboard elevators move, they mechanically position
the opposite outboard elevator control valves to move the outboard elevator. The inboard elevators are powered by two hydraulic systems and the out- board elevators by a single system. The autopilot controls the elevators through transfer valves which operate a common linkage to both inboard elevator control valves.
Elevator feel is provided by a computer which receives airspeed, hydraulic and stabilizer trim inputs. The computer varies control column pres- sure relative to airspeed and stabilizer position. Pitch trim is provided by hydraulically positioning the horizontal stabilizer. Pilot or autopilot inputs move the stabilizer through two independent hydraulic motors and a gearbox which rotates a trim jackscrew to move the leading edge of the stabilizer up or down. Arming and directional valves control the hydraulic motors and these valves are actuated electrically by trim switches on the pilot's control wheels or by the autopilot. Cables from a trim lever in the cockpit provide backup manual control of these valves.
YAW CONTROL
The Boeing 747 has two independent rudders, an upper and lower, to provide yaw control. Each rudder is positioned by a hydraulically powered control unit and is powered by two separate hydraulic systems. The upper rudder is powered by systems 1 and 3, and the lower rudder by systems 2 and 4. Inputs from the pilot's rudder pedals control a trim, feel and centering unit which is connected to two rudder ratio changers. The rudder ratio changers control the amount of rudder movement as a func- tion of airspeed. As airspeed increases, maximum rudder displacement decreases. Rudder trim is accomplished by changing the zero position of the trim, feel and centering unit.
Swept-wing airplanes have a tendency towards a combined rolling and pitching oscillation known as dutch roll. Dutch roll is generally of low magnitude and, while objectionable from a flight comfort standpoint, it is usually not a serious flight condi- tion.
Two full-time yaw dampers provide compensation for dutch roll. Yaw damper computers sense the oscillation and provide signals to the rudder control valves to cancel it.
FLAPS
Each wing of the Boeing 747 is equipped with two triple-slotted Fowler flaps on the trailing edge. The
leading edge is equipped with three Krueger flaps on the section between the fuselage and the inboard engines and unique, variable-camber leading edge flaps from the inboard engines out to the wingtips. The trailing edge flaps are actuated by hydraulic motors and have an emergency backup system of electric motors that will raise or lower them.
There are detents in the cockpit flap selector control at 0, 1, 5, 10, 20, 25 and 30. These numbers are ref- erences only and do not represent the flap extension in degrees. Gates are located at the Flaps 1 and Flaps 20 positions to remind the pilot to check air- speed before extending or retracting the flaps fur- ther. Flaps 10 and 20 are used for takeoff and Flaps 25 and 30 are used for landing.
The Fowler flaps extend out of the trailing edge of the wing to nearly their full chord before they deflect downward an appreciable amount. This pro- vides a large increase in wing area and lift, with a minimum amount of drag, and this position is used for takeoff. When the flaps are extended to the land- ing position, they deflect fully downward to pro- vide the increased lift and drag needed to slow the airplane down for landing.
Both the Krueger flaps and the variable-camber leading edge flaps alter the shape of the wing's lead- ing edge to provide the added camber needed for high lift at slow speed. Operation of the leading edge flaps is controlled automatically by the posi- tion of the trailing edge flaps. The leading edge flaps are normally powered by pneumatic motors with electric motors providing a backup means to extend or retract the flaps. [Figures 1-64, 1-65]
Figure 1-65. The flexible, composite construction of the cen- ter section of the variable camber leading edge flaps allows them to retract into the relatively thin wing leading edge, yet assume the most effective aerodynamic shape when extended. These flaps are used from the inboard engines out to the wingtip.