15.8.6 Hands-On Throttle and Stick

Other examples of easing a pilot’s workload include the essentials of weapons management and other requirements being incorporated on two controls so that combat pilots can keep their hands on the engine throttle control and the flight control stick. This arrangement of control buttons on the engine throttle and stick is known as HOTAS (see Figure 15.16). The essential control buttons are ergonomically located (Table 15.10). Most modern aircraft have buttons on the flight-control stick for communication, trimming, and so forth.

15.8.7 Voice-Operated Control

Voice-operated control (VOC) through voice recognition – still in the development stage – has been installed in advanced combat aircraft. All voice commands also are visually displayed and are very effective for a pilot operating under severe stress, especially if incapacitated by injury.

All of these advancements help a pilot but the systems still require the pilot’s familiarity. Pilots undergo extensive training and practice to gain familiarity with a mass of information in a rather claustrophobic presentation. A pilot’s workload is nearly an inhuman task. They are a special breed of personnel, rigorously trained for years to face the unknown in a life-or-death situation. It is the moral duty of any combat aircraft designer to enhance pilot survivability as best as possible in an integrated manner, embracing all types of technologies.

15.9 Aircraft Systems

Figure 2.1 shows the aircraft-design process in a systems approach. The definition of system is provided in Section 2.2. In that regard, an aircraft can be seen as a system composed of many subsystems. Chart 15.1 illustrates a typical top-level subsystem architecture of aircraft as a system. The subsystems can be designed in separate modules and then integrated with an aircraft.

Together, the system and subsystem mass is 10 to 12% of an aircraft’s MTOM. Typically, this amounts to nearly a quarter of the OEM. Practically all of the items in aircraft subsystems are bought-out. A better understanding of the subsystems improves weight and cost predictions. It is important for good information about subsystem items at the conceptual design stage for better weight and cost estimation. Designers are continually assessing cost versus performance of the subsystems to obtain the best value for the expense.

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 wirepulley (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 subsonictransport 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.