Flight Control Systems

Transfer inovácií 26/2013 2013 FLIGHT CONTROL SYSTEMS doc. Ing. Naqib Daneshjo, PhD. powered surfaces began to be used with Ing. Cristian Dan Stratyinski hydraulically powered actuators boosting the efforts Ing. Andreas Kohla of the pilot to reduce the physical effort required. Ing. Christian Dietrich This brought another problem: that of ‘feel’. By Technická univerzita v Košiciach divorcing the pilot from the true effort required to Katedra leteckého inžinierstva fly the aircraft it became possible to undertake Rampová 7, 041 21 Košice manoeuvres which could overstress the aircraft. e-mail: [email protected] Thereafter it was necessary to provide artificial feel [email protected] so that the pilot was given feedback representative [email protected] of the demands he was imposing on the aircraft. The need to provide artificial means of trimming Abstract the aircraft was required as Mach trim devices were The development of basic aircraft systems developed. has not stood still. We can check this simple fact by A further complication of increasing top looking at the wing size of a modern passenger speeds was aerodynamically related effects. The aircraft and see that its size is reducing while the tendency of many high-performance aircraft to lifting power of the wing is still increasing. This is experience roll/yaw coupled oscillations – a measure of improvements now capable of being commonly called ‘dutch roll’ – led to the made in wing design which in turn are dependent introduction of yaw dampers and other auto- on ‘Systems’ capable of developing the maximum stabilization systems. For a transport aircraft these performance from the minimum of weight, hence were required for passenger comfort whereas on fly by wire. There is nothing new in all this: aircraft military aircraft it became necessary for target performance has ever been about the relationship tracking and weapon aiming reasons. between power and weight. Indeed, until adequate power at the right weight was available, sustained 2 PRINCIPLES OF FLIGHT CONTROL manned flight was not possible. All aircraft are governed by the same basic principles of flight control, whether the vehicle is Key words: Aircraft systems, flight control, Flight the most sophisticated high-performance fighter or control surfaces, Experimental Aircraft Programme the simplest model aircraft. The motion of an (EAP) aircraft is defined in relation to translational motion and rotational motion around a fixed set of defined 1 INTRODUCTION axes. Translational motion is that by which a vehicle travels from one point to another in space. Flight controls have advanced For an orthodox aircraft the direction in which considerably throughout the years. In the earliest translational motion occurs is the direction in which biplanes flown by the pioneers flight control was the aircraft is flying, which is also the direction in achieved by warping wings and control surfaces by which it is pointing. The rotational motion relates to means of wires attached to the flying controls in the the motion of the aircraft around three defined axes: cockpit. Such a means of exercising control was clearly rudimentary and was usually barely pitch, adequate for the task in hand. The use of articulated flight control surfaces followed soon after but the roll, use of wires and pulleys to connect the flight and yaw. control surfaces to the pilot’s controls persisted for many years until advances in aircraft performance This figure shows the direction of the rendered the technique inadequate for all but the aircraft velocity in relation to the pitch, roll and simplest aircraft. When top speeds advanced into yaw axes. For most of the flight an aircraft will be the transonic region the need for more complex and flying straight and level and the velocity vector will more sophisticated methods became obvious. They be parallel with the surface of the earth and were needed first for high-speed fighter aircraft and proceeding upon a heading that the pilot has then with larger aircraft when jet propulsion chosen. If the pilot wishes to climb the flight became more widespread. The higher speeds control system is required to rotate the aircraft resulted in higher loads on the flight control around the pitch axis (Ox) in a nose-up sense to surfaces which made the aircraft very difficult to fly achieve a climb angle. Upon reaching the new physically. The Spitfire experienced high control desired altitude the aircraft will be rotated in a forces and a control reversal which was not initially nose-down sense until the aircraft is once again understood. To overcome the higher loadings straight and level. 33 Transfer inovácií 26/2013 2013 In most fixed wing aircraft, if the pilot 3 FLIGHT CONTROL SURFACES wishes to alter the aircraft heading then he will The requirements for flight control need to execute a turn to align the aircraft with the surfaces vary greatly between one aircraft and new heading. During a turn the aircraft wings are another, depending upon the role, range and agility rotated around the roll axis (Oy) until a certain bank needs of the vehicle. These varying requirements angle is attained. may best be summarized by giving examples of two In a properly balanced turn the roll altitude differing types of aircraft: an agile fighter aircraft will result in an accompanying change of heading and a typical modern airliner. while the roll angle (often called the bank angle) is The Experimental Aircraft Programme maintained. This change in heading is actually a (EAP) aircraft is shown in Fig. 1 and represents the rotation around the yaw axis (Oz). The difference state-of-the-art fighter aircraft as defined by between the climb (or descent) and the turn is that European manufacturers at the beginning of the the climb only involves rotation around one axis 1990s. The EAP is similar to the European fighter whereas the turn involves simultaneous co- aircraft (EFA) being developed by the four nation ordination of two axes. In a properly coordinated Eurofighter consortium comprising Alenia (Italy), turn, a component of aircraft lift acts in the BAE SYSTEMS (UK), CASA (Spain) and Daimler direction of the turn, thereby reducing the vertical Chrysler (Germany). component of lift. If nothing were done to correct this situation, the aircraft would begin to descend; therefore in a prolonged turning manoeuvre the pilot has to raise the nose to compensate for this Primary flight control loss of lift. At certain times during flight the pilot Primary flight control in pitch, roll and may in fact be rotating the aircraft around all three yaw is provided by the control surfaces described axes, for example during a climbing or descending below. Pitch control is provided by the moving turning manoeuvre. canard surfaces, or foreplanes, as they are The aircraft flight control system enables sometimes called, located either side of the cockpit. These surfaces provide the very powerful pitch the pilot to exercise control over the aircraft during all portions of flight. The system provides control control authority required by an agile high- surfaces that allow the aircraft to manoeuvre in performance aircraft. The position of the canards in relation to the wings renders the aircraft unstable. pitch, roll and yaw. The system has also to be designed so that it provides stable control for all Without the benefit of an active computer driven parts of the aircraft flight envelope; this requires a control system the aircraft would be uncontrollable and would crash in a matter of seconds. While this thorough understanding of the aerodynamics and dynamic motion of the aircraft. As will be seen, may appear to be a fairly drastic implementation, additional control surfaces are required for the the benefits in terms of improved manoeuvrability enjoyed by the pilot outweigh the engineering specific purposes of controlling the high-lift devices required during approach and landing phases of required to provide the computer controlled or flight. ‘active’ flight control system. 34 Transfer inovácií 26/2013 2013 P- Primary controls S- Secondary controls Fig. 1 Example of flight control surfaces – EAP (BAE SYSTEMS) Secondary flight control 4 COMMERCIAL AIRCRAFT High-lift control is provided by a An example of flight control surfaces of a combination of flaperons and leading-edge slats. typical commercial airliner is shown in Fig. 2. The flaperons may be lowered during the landing Although the example is for the Airbus Industrie approach to increase the wing camber and improve A320 it holds good for similar airliners produced by the aerodynamic characteristics of the wing. The Boeing or other manufacturers. The controls used leading-edge slats are typically extended during by this type of aircraft are described below. combat to further increase wing camber and lift. Pitch control is exercised by four elevators The control of these high-lift devices during combat located on the trailing edge of the tailplane or may occur automatically under the control of an horizontal stabilizer. Each elevator section is active flight control system. The penalty for using independently powered by a dedicated flight control these high-lift devices is increased drag, but the actuator, powered in turn by one of several aircraft high levels of thrust generated by a fighter aircraft hydraulic power systems. This arrangement is usually minimizes this drawback. dictated by the high integrity requirements placed The EAP has airbrakes located on the upon flight control systems. The entire tailplane upper rear fuselage. They extend to an angle of section itself is powered by two or more actuators around 30 degrees, thereby quickly increasing the in order to trim the aircraft in pitch. In a dire aircraft drag. The air brakes are deployed when the emergency this facility could be used to control the pilot needs to reduce speed quickly in the air; they aircraft, but the rates of movement and associated are also often extended during the landing run to authority are insufficient for normal control enhance the aerodynamic brake effect and reduce purposes.

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