Part b Choose a set of typical aircraft control system param

Part (b)

Choose a set of typical aircraft control system parameters, take photographs or make drawings of appropriate components describing the control methods. Select appropriate components and control methods for at least 2 different types of aircraft control system. (1 page limit, 200 to 400 words).

Solution

Increases in the control surface area required by large aircraft or higher loads caused by high airspeeds in small aircraft lead to a large increase in the forces needed to move them, consequently complicated mechanical gearing arrangements were developed to extract maximum mechanical advantage in order to reduce the forces required from the pilots. This arrangement can be found on bigger or higher performance propeller aircraft such as the Fokker 50. Mechanical Flight Control System • Some mechanical flight control systems use Servo tabs that provide aerodynamic assistance. Servo tabs are small surfaces hinged to the control surfaces. The flight control mechanisms move these tabs, aerodynamic forces in turn move, or assist the movement of the control surfaces reducing the amount of mechanical forces needed. This arrangement was used in early piston-engined transport aircraft and in early jet transports. The Boeing 737 incorporates a system, whereby in the unlikely event of total hydraulic system failure, it automatically and seamlessly reverts to being controlled via servo-tab.

Need for Powered Control System • The Complexity and Weight of the system (Mechanical) increased with Size and Performance of the aircraft. • When the pilot’s action is not directly sufficient for the control, the main option is a powered system that assists the pilot. • The hydraulic system has demonstrated to be a more suitable solution for actuation in terms of reliability, safety, weight per unit power and flexibility, with respect to the electrical system Powered Assisted Control System • The pilot, via the cabin components, sends a signal, or demand, to a valve that opens ports through which high pressure hydraulic fluid flows and operates one or more actuators. • The valve, that is located near the actuators, can be signalled in two different ways: mechanically or electrically • Mechanical signalling is obtained by push-pull rods, or more commonly by cables and pulleys • Electrical signalling is a solution of more modern and sophisticated vehicles Powered Assisted Control System • The basic principle of the hydraulic control is simple, but two aspects must be noticed when a powered control is introduced: • The system must control the surface in a proportional way, i.e. the surface response (deflection) must be function to the pilot’s demand (stick deflection, for instance) • The pilot that with little effort acts on a control valve must have a feedback on the maneuver intensity Powered Assisted Control System • The first problem is solved by using (hydraulic) servo-mechanisms, where the components are linked in such a way to introduce an actuator stroke proportional to the pilot’s demand

The pilot, in normal hydraulic operating conditions, is requested for a very low effort, necessary to contrast the mechanical frictions of the linkage and the movement of the control valve • The pilot is then no more aware of the load condition being imposed to the aircraft. • An artificial feel is introduced in powered systems, acting directly on the cabin control stick or pedals. Powered Assisted Control System • The simplest solution is a spring system, then responding to the pilot’s demand with a force proportional to the stick deflection; this solution has of course the limit to be not sensitive to the actual flight conditions. • A more sophisticated artificial feel is the socalled Q feel. This system receives data from the pitot-static probes, reading the dynamic pressure, or the difference between total (pt) and static (ps) pressure, that is proportional to the aircraft speed v through the air density : Powered Assisted Control System • This signal is used to modulate a hydraulic cylinder that increases the stiffness in the artificial feel system, in such a way that the pilot is given a contrast force in the pedals or stick that is also proportional to the aircraft speed