Flight Control for Rotorcraft

Flight Control for Rotorcraft Tim Clarke Senior Lecturer in Avionics, Department of Electronics, University of York, Heslington, York YO10 5DD UK

ABSTRACT In this chapter, we consider some of the practical issues necessary to take idealised flight control laws, perhaps developed using classical or modern control engineering design methodologies, to their implementation on a real flight vehicle. We restrict the discussion to the conventional, single main rotor, tail rotor configuration. key words: helicopter

Helicopter, rotorcraft, flight control systems, implementation issues, single rotor

1. INTRODUCTION This is about the practicalities of flight control systems for helicopters. We shall restrict the treatment to the single rotor helicopter used for civil or military operations, but not small one/two seater aircraft popularly used for sports flying.

2. SETTING THE SCENE In setting the scene for the subject of rotorcraft flight control, we shall first consider what must be achieved in order for a pilot to be able to fly safely and effectively. We shall then consider the control problem, without mathematical rigour, yet in terms that reflect the magnitude of the fundamental problem. Following this, we must look at examples of best practice - if this is what current practice represents. Next we shall look at issues that can be best defined as fitness for purpose. If the augmentation of a helicopter moves problems of the flying task somewhere else such that the pilot’s workload is fraught with other difficulties, then the main objective of the exercise is missed. But why have an AFCS in the first place? • To compensate for lack of inherent stability. Most helicopters exhibit instabilities within their flight envelope. Later we shall conduct a simple analysis of a disc tilt model of a helicopter, trimmed for straight and level flight over a range of airspeeds. From this a feel for the variability of basic stability can be deduced. Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °

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ENCYCLOPEDIA OF AEROSPACE ENGINEERING

• To improve handling or ride qualities. Good handling and ride qualities impact positively on pilot workload and hence on mission effectiveness. This is especially the case if the helicopter is used in a role that necessitates aggressive manoeuvring and precision flying. • To enable manoeuvres not otherwise possible. There are some manoeuvres that are very difficult to perform safely and effectively in poor cueing environments. An appropriate autopilot mode may enable them to be performed more easily and consistently. • To reduce pilot workload. A pilot who is relaxed and has good situational awareness is a much more effective pilot. • To enable IFR certification. For full effectiveness, a helicopter is required to fly in all weather, day and night. To achieve IFR certification, an AFCS is an absolute necessity in order to achieve appropriate, good flying qualities. It is universally held that achieving all of the above by appropriate design of the basic flying vehicle is impossible. There will always be design compromises and consequent deficiencies which can be addressed through an AFCS. 2.1. An Operational Perspective The perspective we must take here is that of the professional pilot. In assessing workload and effectiveness, we first consider the wider objectives. The pilot has to make decisions about the progression of the mission - considering what needs to be done in order for successful completion. The term ’mission’, at first glance, has a military connotation. However, the concept is equally applicable to civil operations. For the civil pilot, the ’mission’ may be to deliver passengers safely to their destination, without giving them any cause for concern, or discomfort. However, these considerations, military or otherwise, must be balanced against the need to survive and continue flying under adverse conditions - hostile activity in a combat situation, adverse atmospheric disturbances, degraded visual cues, precipitation or icing, or system failures, for example. To this end, we must consider the pilot’s ’office environment’ and the importance of the sensory and other ’cues’ received to promote mission effectiveness. The cues support situational awareness. This can be defined as a pilot’s continuous cognizance of some key factors, discussed below, plus an appreciation of the capabilities of the helicopter at that instance and, importantly, the physical environment. In some critical operating conditions, mostly associated with wind speeds and directions, available movement of flying controls, especially aft and lateral cyclic and rudder pedal, may become restricted. A pilot’s awareness of these control margins must come from a familiarity with the limits of the primary flying controls, aided by the control feel system. In some conditions, such as near-hover flight, when power settings can be particularly high, the margin between available and required power can be severely eroded. Similarly, in adverse ambient conditions (such as hot and high), with a maximum take-off weight, or after a critical engine failure, the pilot must be able to demonstrate good airmanship. From an engineering viewpoint, limitations on torque applied to the transmission system ’trump’ power levels. Engine power is usually under closed loop control to meet constant rotor speed demands. However, if the total power demanded by the rotor system exceeds that which is available, rotor speed will droop. If the engine control system attempts to maintain the power output, transmission torque increases. Prescribed over-torque limits may permitted Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °

FLIGHT CONTROL FOR ROTORCRAFT

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only transiently, and only for exceptional circumstances, with consequent inspection of the transmission system. In this busy environment, it is essential that any control system does not mask or distort cues that inform situational awareness or misdirect the pilot. Many of the above factors are prone to the effects of a poorly designed control system. Moving closer to a specification for a flight control system, we can identify key attributes a pilot requires from the flying controls themselves. There should be logical, or instinctive, relationships between control movements and forces, and the sense and magnitude of the helicopter responses, with a smooth, progressive build up of forces with displacement. Necessarily small break-out forces, such as those required for self-centering and grounding in displacement control runs, and force gradients should be well-matched. Irreversible control runs should remain so, with no objectionable control forces felt, or tendency for a flying control to move when any other control is moved, for example as a result of control interference or control mixing. The cyclic pitch channels, at least, should return to their trimmed preset (datum) positions when released from a transient displacement against a spring-feel system. There should always be harmonisation between the control forces and displacements in all channels, and the associated helicopter responses. Responses to pilot commands should be such that manoeuvre about a given flight path should be a natural flying process. 2.2. A Systems Perspective We now turn to the plant, itself. What is the magnitude of the control problem? It is an oftrepeated that the unaugmented helicopter is unstable, non-linear and highly cross-coupled. Taking as an example, a single-rotor utility helicopter with starboard advancing main rotor blades and a port-side pusher tail rotor. Using a simple, non-linear, disc-tilt rotor based model that has been trimmed for straight and level flight∗ at a range of airspeeds from the hover up to 126 knots (233 km/hour or 145 miles per hour), Figure 1 shows the migration of the fuselage motion poles of the resulting linearisations at these operating points. In fact, the helicopter is unstable throughout the speed range. At higher speeds, a mildly unstable oscillatory mode dominates - a longitudinal pitch mode akin to the fixed-wing Phugoid in nature, but with very different flight mechanics. At lower speeds, lateral instabilities also occur, giving rise to what is sometimes termed the ’falling leaf’ mode - a divergent oscillation coupling translational velocities with fuselage rotations. The period of the oscillations primarily increases with increasing speed instability, but is decreased by increased rate damping. These are affected by forward airspeed. The mode can be reduced by reducing static speed stability, and increasing rate damping. From the viewpoint of the primary design of the helicopter, this can be effected by increasing blade inertia, effective flapping hinge offset, rotor height above the centre of gravity, and reducing the total helicopter moment of inertia. If an increase in pitch damping at speed is required, it may be achieved by increasing tailplane size. However, there is a potential cost, in terms of weight, vibration and compromised low speed handling. The alternative is to employ a control

∗ To

hold this trim condition in the hover, the helicopter will have to adopt starboard roll attitude to balance the tail rotor thrust and often a nose-up pitch attitude to accommodate any main rotor shaft tilt and an aft centre of gravity. At low speeds, the pilot will trim for zero side slip and accept a small roll attitude. Above about 50 knots, the pilot will adopt a wings level attitude and endure a small side slip angle. Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °

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Open loop pole locations against forward speed 0 - 126 knots

Imaginary axis

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Real axis Open loop pole locations against forward speed (origin of upper plot) 3

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Figure 1. Pole Migrations for a Single Rotor Utility Helicopter as a Function of Airspeed

strategy, such as appropriate rate feedback. But what are the major issues in adopting an AFCS appropach? 2.3. Fitness For Purpose A major challenge in procuring an AFCS is in defining the requirements in such a way that will ensure that what is produced will be fit for purpose. A requirements specification will define all the required attributes of a system but this may still not guarantee a system which is fit for purpose. The control laws and the operation of the inner loop stabilisation, in particular, will greatly affect the way in which the aircraft performs. The employment of a flight control system should not detract from the pilot’s natural flying of the aircraft, which would otherwise impinge upon cockpit workloads. Autopilot mode selection, and control channel engagement switches, should be logically grouped, with unambiguous indications of which modes and channels are engaged at any particular time. Crucially, the pilot must be able to override the automatic system through the use of her flying control system, at any time, without any conscious switching action to disengage any part of the system. So, from an operational perspective, the design of a good flight control system for a helicopter is particularly non-trivial - especially when considered against traditional fixed-wing aircraft. Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °

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2.4. Approaches to Flight Control Pilot workload is high for two main reasons. Firstly, the effects of each of the three controls, cyclic, main rotor collective and tail rotor collective, are not normally restricted to one particular axis. For example, an increase of collective induces a heave response whilst, at the same time, causing the fuselage to yaw. Therefore, the pilot needs to apply an appropriate amount of tail rotor collective, via the pedals, to compensate for this torque reaction and prevent the aircraft from rotating in yaw. A common alternative way to compensate for this effect in a helicopter is to employ an appropriate control interlink. In the example above, this interlink will automatically move the yaw pedals as the collective is raised in order to increase the tail rotor pitch. Control interlinks are common in all but the most basic helicopters, with collective/yaw and collective/pitch being the most common forms of mixing. It can be considered as a form of feed-forward control. Secondly, the helicopter is fundamentally unstable, so the pilot would be continually required to make adjustments to the controls to maintain the flight condition, without the aid of a flight control system. Automatic stabilisation of the helicopter is effected by sensing the aircraft motion and making adjustments to the flying control demands. A basic Stability Augmentation System (SAS) uses feedback of fuselage angular rates (predominantly roll and pitch) to improve stability, disturbance rejection and basic response characteristics. As part of an Automatic Stabilisation Equipment (ASE) body attitude and heading are additionally used within feedback loops. The ASE allows a pilot to fly with hands off the controls for extended periods of time. However, the pilot is still required to control the helicopter flight path. An Autopilot system reduces the pilot workload further by controlling the flight path as required. A basic autopilot could, for example, hold the aircraft on preselected combination of airspeed, height and heading. An AFCS, therefore, will consist of the SAS or ASE for basic stabilisation and the Autopilot for flight path control. 2.4.1. SAS A SAS is provides rate damping. It can also provide short term attitude stabilisation. However, it reacts to pilot input, so requires some care in design. In simple fixed wing form, the yaw damper is a SAS, and is used to improve Dutch roll characteristics (Figure 2).

rcomm(s)

Rudder Actuator

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Aircraft Dynamics

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Washout Filter

Rate Gyro

Figure 2. The Fixed-Wing Yaw Damper as an SAS

A wash out filter ensures that transient behaviour, sensed by a rate gyro, is transmitted through the feedback path of the yaw damper, whereas persistent pilot command responses Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °

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are washed out. The characteristics of the washout filter are best seen by considering the transfer function and the frequency response. The transfer function is: s s+a The frequency response for H(s), where a = 2 is shown in Figure 3. H(s) =

(1)

Magnitude (dB)

0 −10 −20

Phase (deg)

−30 90

45

0 −1 10

0

1

10 10 Frequency (rad/sec)

2

10

Figure 3. The Frequency Response for a Washout Filter

Note that the low frequency gain is small, so signals with low frequency content, such as pilot commands, are blocked, whereas higher frequency components, such as, in this case, Dutch roll oscillations are subject to the effects of the yaw damper, and are damped out as appropriate. Some advantages can be claimed for a SAS: • SAS control laws are simple and robust. All that is required is a rate sensor which is simple and reliable (although sensor drift must be addressed). • A SAS is able to cope with changes of flight conditions and/or centre of gravity location - there is no requirement to provide trim inputs. • When there is no body rate, SAS outputs tend to zero - whatever the attitude of the helicopter. • A SAS will not affect long term relationships between stick position, attitude and speed - the system is transparent to the pilot. Strategies have been adopted that are flavours of SAS. In the main, these employ mechanisms to reduce the impact of the SAS on pilot inputs. Others allow for improvements in hands off performance. A SAS will retain attitude as long as rate gyro drift is negligible - which is not a practical reality. If the integral of the rate signal is also incorporated into this simple control law, attitude holding is moderately improved. To ensure that any attitude error term is short term, the integral action must be short term. This may be achieved by the simple expedient of using a Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °


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relatively long time constant 1st order (low pass) filter. To ensure that this does not ’fight’ the pilot commands, it must be disabled when triggered by a stick displacement switch. This all requires some mode logic and switching, which complicates the system. An alternative is to apply a feedforward signal, proportional to stick displacement which shaped by passing it through a washout filter with appropriate time constant and gain tuning. 2.4.2. ASE SAS use rate feedback as the primary sensor input to augment stability. Rate gyros are prone to long term drift, which is the primary reason why a SAS is only able to hold attitude for a relatively short term. For longer term attitude hold, a system with an attitude sensor as its input, such as a vertical gyro, is sensible. However, without appropriate augmentation, the system changes the helicopter behaviour, resulting from pilot inputs, from a rate response to an attitude response† . In terms of good handling qualities, for up and away flight, a rate response is mandatory. Therefore, it is necessary to switch out the attitude signal and retain a stabilising rate feedback signal under larger pilot-initiated manoeuvres. This may be done by, for example, • shaping (scheduling) the attitude gain with stick input or hard limiting the gain output. • using a signal proportional to stick input to ’back off’ the term generated by the attitude gyro (sometimes termed a stick-canceller) Also, we must also consider the concept of datums. A helicopter, flying in a particular trim configuration, if manoeuvred under pilot command, will either return to its original attitude under attitude feedback, or will be required to retain a new attitude at the completion of the manoeuvre. A datum is, in control parlance, a set-point. Under some circumstances, the pilot may wish to manoeuvre and return to datum (attitude); at others, the datum is changed either during the manoeuvre, or once the new flight condition is attained. Whatever the case, an ASE requires some means to manage datums, and to cope with pilot inputs which would otherwise be treated as system disturbances. If properly designed, the main benefits of an ASE system can be summarised as follows: • An ASE enables long term attitude hold, which is retained independently of time and manoeuvre history. • During a manoeuvre, if the pilot becomes disoriented, an ASE will tend to return the helicopter to datum, if the stick is released. 2.4.3. Autopilots With artificial stability and short term hands off flying capability established through the SAS and/or ASE, the pilot’s task is eased. We now consider the role of Autopilots. They can be considered to provide three kinds of functionality: • long term stabilisation with respect to, for example, airspeed or height (hold modes). • automation of a specific flight path manoeuvre such as the transition between forward flight and hover at a datum height, or turn co-ordination. • navigational modes such as automatic ILS/MLS approach or VOR, TACAN and GPS navigation.

† Unaugmented

fixed wing aircraft have a rate response. So, for example, a longitudinal stick input will generate a pitch rate the magnitude of which increases with stick input size. Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °

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2.5. Hold Modes Autopilots provide a control function, which goes beyond the stabilisation action of SAS and ASE. They often function alongside them. With an Autopilot hold mode engaged, the pilot may fly hands off in that axis, but may select and adjust appropriate datums, via a control unit or via switches on the collective and cyclic controls. Hold modes may be engaged at the desired flight condition (e.g. airspeed, height and/or heading), or through an acquisition or capture process. In both cases, the nature of the helicopter creates choices for the AFCS designer. We consider height hold as an example. Height hold may be implemented using barometric (normally away from the ground and under non-terminal air traffic control) or radio height (for prolonged precise control close to the ground) and it may be controlled through main rotor collective or fore-aft cyclic. The range of possible air speeds (hover to high speed forward flight with the complications of low speed lateral or backwards flight) coupled with lateral manoeuvres and a desire for additional features to reduce further the pilot’s workload, can necessitate complications to the implementation:

• Vertical speed and/or vertical acceleration can be used to improve heave damping or improve the precision and transient behaviour of the helicopter under controlled vertical manoeuvre. • Introducing bank angle into a height hold control law can compensate for the variation in normal acceleration and consequent height variations, whilst the helicopter is rolling into and out of turns. • It may be a requirement to enable the height datum to be altered via a beep button, usually on the collective lever.

Height control whilst flying over water, especially in higher sea states, can induce degraded performance and ride qualities. It is then necessary to smooth out the height measurement - a compromise between ride quality and low level performance. Similarly, barometric measurements suffer from manometric lags and high frequency noise. Complementary filtering, Kalman filters and other data fusion techniques are often used to combine low frequency height measurements with higher frequency inertial acceleration measurements to produce smoothed height measurements. 2.6. Transition Control Transition modes are important types of mode that provide flight path control. They enable the automatic transition down from forward flight to the hover and transition up from a hover to forward flight. Transition modes are primarily naval modes, used for example in the repetitive dipping of sonobuoys in anti-submarine operations. However, they have been adapted for Search and Rescue operations. There are good operational arguments for the transitions to be as fast as possible; but safety and crew comfort put constraints on this. Other issues that would have to be addressed include risks of entering autorotation, compromised recovery from engine failure, entering the vortex ring state and actuator saturation due to excessive control demands from the control laws. Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °


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2.7. Navigation Modes With hold modes available in height, heading and airspeed, navigation modes usually operate through the roll axis. They can provide automatic routing along a track or via way points, with navigation data from, for example, VOR, TACAN, DME, ILS, MLS or GPS. The relationship between steering (AFCS) and navigation (typically a Flight Management System (FMS)) is not consistent across the aerospace industry. However, there are some important issues that must be addressed in the design and implementation of all Autopilots. They can be summarised as mode availability, mode compatibility, display and transparency. Availability depends upon whether the helicopter is in an appropriate flight state (flight envelope), if the appropriate flight date is available (sensor status), if the actuators have the necessary capacity (authority), and if the autopilot itself is on a serviceable state (error status). Compatibility requires the entry conditions for a mode or sub-mode (perhaps a hold) are consistent and non-contradictory, and the correct sequence for their engagements can be met. The pilot should be given, in a clear and unambiguous manner, the complete picture of what modes and sub-modes are engaged at any time. This can be through illuminated button captions, on the main instrument displays (EFIS) or on separate annunciator panels. Finally, transparency is the ability of the AFCS to handle pilot interventions in a manner that enables the aircraft to return to the original flight condition or retain a new flight condition following stick release, and does not interfere with the pilot’s stick cues during the intervention manoeuvre.

3. Conclusions The steps taken in designing the actual flight control laws for a helicopter are just the start of a much wider and major process. This process has been introduced through the enunciation of some fundamental implementation issues. We have looked at the motivations for any AFCS on a helicopter - a fundamentally pilot-unfriendly air vehicle if flown without augmentation and one which is characterised by high workloads, wide operating conditions with equally wide variations in potentially adverse flying characteristics. Instabilities, non-linearity and significant cross-couplings make a strong argument in favour of the AFCS. However, from such a remote starting point, it is absolutely essential to construct a consistent and transparent system to manage the modification of these characteristics and enable a safe product. In this chapter, some of the more pressing issues that exercise the mind of the flight control design engineer have been identified. Full exploration of these alone would necessitate many more chapters, and even more issues would need to be addressed.

FURTHER READING

Newman S. The Foundations of Helicopter Flight Edward Arnold 1994 Padfield GD. Helicopter Flight Dynamics. Blackwell Science Ltd 1996. Prouty RW. Helicopter Performance Stability, and Control. Robert E. Krieger Publishing Company 1990; Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °

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Prouty RW. and Curtiss HC. Helicopter Control Systems: A History. Journal of Guidance, Control and Dynamics 2003; 26(1):12–18.

Encyclopedia of Aerospace Engineering. Edited by Richard Blockley and Wei Shyy. c 2010 John Wiley & Sons, Ltd. °