Aircraft Control Subsystem

Chapter 12 discusses the analytical consideration of aircraft motion having six degrees of freedom and its control. Figure 3.8 is a Cartesian representation of the six degrees of freedom, consisting of three linear and three rotational motions. This sec­tion describes the associated control hardware and design considerations (see also [9] and [10]). An aircraft’s control-system weight is about 1 to 2% of the MTOW.

The three axes (i. e., pitch, yaw, and roll) of aircraft control have evolved con­siderably. The use of trim tabs and aerodynamic and mass balances alleviates hinge

moments of the deflecting control surfaces, which reduces a pilot’s workload. Some operational types are as follows:

1. Wire-Pulley Type. This is the basic type. Two wires per axis act as tension cables, moving over low-friction pulleys to pull the control surfaces in each direction. Although there are many well-designed aircraft using this type of mechanism, it requires frequent maintenance to check the tension level and the possible fraying of wire strands. If the pulley has improper tension, the wires can jump out, making the system inoperable. Other associated problems include dirt in the mechanism, the rare occasion of jamming, and the elastic deformation of support structures leading to a loss of tension. Figure 15.19 shows the wire – pulley (i. e., rudder and aileron) and push-pull rod (i. e., H-tail) types of control linkages.

2. Push-Pull-Rod Type. The problems of the wire-pulley type are largely over­come by the use of push-pull rods to move the control surfaces. Designers must ensure that the rods do not buckle under a compressive load. In general, this mechanism is slightly heavier and somewhat more expensive, but it is worth installing for the ease of maintenance. Many aircraft use a combination of push – pull-rod and wire-pulley arrangement (see Figure 15.19).

3. Mechanical Control Linkage Boosted by a Power Control Unit (PCU). As an aircraft size increases, the forces required to move the control surfaces increase to a point where a pilot’s workload exceeds the specified limit. Power assistance by a PCU resolves this problem. However, a problem of using a PCU is that the natural feedback “feel” of control forces is obscured. Therefore, an artificial feel is incorporated for finer adjustment, leading to smoother flights. PCUs are either hydraulic or electric motors driven by linear or rotary actuators (there are several types). Figure 12.16 is supported by a PCU.

4. Electromechanical Control System. In larger aircraft, considerable weight can be saved by replacing mechanical linkages with electrical signals to drive the actuators. Aircraft with FBW use this type of control system (see Figure 12.16). Currently, many aircraft routinely use secondary controls (e. g., high-lift devices, spoilers, and trim tabs) driven by electrically signalled actuators.

5. Optically Signaled Control System. This latest innovation uses an optically sig­naled actuator. Advanced aircraft already have fiber-optic lines to communicate with the control system.

Modern aircraft, especially the combat aircraft control system, have become very sophisticated. A FBW architecture is essential to these complex systems so that aircraft can fly under relaxed stability margins. Enhanced performance require­ments and safety issues have increased the design complexities by incorporating various types of additional control surfaces. Figure 15.20 shows the typical subsonic- transport aircraft-control surfaces.

Figure 15.21 shows the various control surfaces and areas as well as the system retractions required for a three-surface configuration. As shown in the figure, there is more control than what most modern civil aircraft have. Military aircraft control requirements are at a higher level due to the demand for difficult maneuvers and a possible negative stability margin. The F117 is incapable of flying without FBW. Additional controls are the canard, intake-scheduling, and thrust-vectoring devices.

Figure 15.20. Civil aircraft control sur­faces

Fighter aircraft may use stabilators (e. g., the F15) in which the elevators can move differentially to improve roll capability. Stabilators are used collectively for pitch and differentially for roll control. Also, the aileron and rudder can be interconnect­ing. There also can be automatic control that parallels the basic system.

15.9.1 Engine and Fuel Control Subsystemsч

In this section, the engine and fuel control subsystems are addressed together. The engine and fuel/oil control subsystems must have a fire-extinguishing capabil­ity. A better understanding of the engine and fuel/oil control subsystems improves weight – and cost-prediction accuracy. The dry-engine weight supplied by an engine manufacturer is accounted for separately. The earliest aircraft were piston-engine – powered. Piston engines use petrol (i. e., AVGAS). Diesel-powered engines were introduced recently. Figure 15.22 shows a basic fuel system for a small piston – engine-powered aircraft.

Piston Engine Fuel Control System (The total system weight is approximately 1 to 1.5% of the MTOW)

• ignition and starting system

• throttle to control fuel flow

• fuel storage (tank) and flow management: This must incorporate fuel refueling and defueling and venting arrangements. High-wing, smaller aircraft may have

Figure 15.22. Piston engine fuel system

a gravity-fed fuel supply to the engine, but most aircraft use fuel pumps. Aero­batic aircraft should be capable of flying in an inverted position for at least a minute

• mixture control to adjust air-density changes when the altitude changes

• propeller-pitch control (see Chapter 12); smaller aircraft may have a fixed pitch

• engine-cooling system

• engine anti-icing system

• oil system

• fire protection system

• instrumentation and sensor devices

Smaller aircraft can store fuel in the wing. Although a few aircraft store fuel in the fuselage, it is not recommended. Fuel in the fuselage can affect a larger CG shift and, in the case of a crash, the occupants may get doused by leftover fuel. Fuselage fuel tanks are an optional installation in order to increase range.

Gas turbine engine control at the pilot interface is simpler in that it does not require mixture control by a pilot. For turbofans, there are no propellers; hence, there is no pitch control by a pilot. The turbofan engine/fuel control system is described as follows and shown in Figure 15.23.

Turbofan Engine Fuel Control System (The total system weight is approximately 1.5 to 2% of the MTOW)

• ignition and starting system

• throttle to control fuel flow (thrust adjustment); larger jets have thrust reversers

• fuel storage (tank) and flow management: This must incorporate fuel refueling and defueling and venting arrangements. Some combat aircraft need mid-air refueling. In an emergency, aircraft should be able to dump (i. e., jettison) fuel (this is an environmental hazard and is discouraged)

• engine-cooling system

• engine anti-icing system

• oil system

• fire-sensing and protection system

• built-in tests for the fault-detection system; there should be flight and ground crew interface

• instrumentation and sensor devices

Figure 15.23. Turbofan engine fuel control system

Modern military aircraft and commercial aircraft engine control is microprocessor – based and known as FADEC. It is linked with FBW using air data to respond cor­rectly as demanded by a pilot. A typical turbofan and fuel/oil control system (see Figure 15.23) is discussed in more detail in the following paragraphs.

Fuel Storage and Flow Management

The fuel supply to an engine must be made smoothly and accurately. There must be adequate fuel-storage capacity to meet the mission profile with mandatory excess as a reserve. These requirements are an important part of the study during the concep­tual design phase. Fuel management is complex: Fuel weight is a significant percent­age of an aircraft MTOW and consumption from full to empty has the potential for major movement of the CG, affecting the aircraft’s stability. It is important for fuel consumption to be managed for the least shift in the CG. In a demanding situation, this is achieved by an in-flight fuel transfer.

A typical commercial aircraft tank arrangement is shown in Figure 15.23. Stor­age of fuel is located primarily in the cavity of the wing box that extends from the

wing root to close to the wing tip (a tapered wing tip has a lower volumetric capac­ity). Fuel storage in the wing is advantageous because it is close to the aircraft’s CG, which lowers the range of the CG shift. If the volume available in the wing is not adequate, especially for thin-winged combat aircraft, then the fuselage space can be used for storage. Typically, fuel storage in the fuselage can be above or below the floorboards and forward, rearward, and/or at the center of the wing. When there are several tanks, it is convenient to collect fuel at a central location before delivering it to the engines. Fuel from tanks at various locations is pumped into a centrally located collector tank following a transfer schedule that minimizes the CG shift. A symmetrical fuel level in the wings also is important. Note the compartmentaliza – tion of the wing tank; surge tanks are provided at the wing tips and internal baffles restrict fuel-sloshing. Some long-range aircraft have volume available in the stabi­lizer to balance the CG shift through an in-flight fuel transfer.

The fuel-tank arrangement for a thin-winged aircraft (i. e, the supersonic type) is complex because there is insufficient volume available for the mission. Therefore, fuel is provisioned in the fuselage wherever space is available. The Concorde exam­ple, shown in Figure 15.23, carries a substantial amount of fuel and the CG shift is minimized by in-flight balancing through a fuel transfer from the forward and aft trim tanks. The military aircraft fuel-tank arrangement is similar to the Concorde. There can be as many as sixteen tanks, all interconnected to meet the fuel require­ments of the mission.

Fuel tanks can be either rigid, made of metal or composite material, or flexible, made of a rubber-neoprene-like material. Tanks are installed during component assembly. Flexible-tank maintenance requires a change of tanks, which can be a laborious task. Most modern aircraft have wet tanks, in which the skin at the joints is treated with a sealant. A wet-tank system is lighter and more volume-efficient; however, leakage is problematic and these aircraft require strict inspection, espe­cially older aircraft. Sealant technology has improved and wet tanks are favored.

Heat generated in stagnant regions of an aircraft flying faster than Mach 2.4 can be cooled by recirculating cold fuel around the hot zones before being fed to the engine. The preheating of fuel also helps in the combustion process.

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