Yasa Flight Control

Introduction

Fly By Wire (FBW) is the general term used to describe a system where the primary flight control actuators are electrically signaled. These command signals are generated by a flight control computer which has inputs from the pilot and other flight control parameters. The system is safety critical and therefore uses redundancy as a method of maintaining the overall safety of the aircraft. Whilst the actuator is electrically signaled, nearly all servos are hydraulically powered. As with the electrical systems the hydraulic systems need to contain redundancy to account for hydraulic system failures. FBW systems have a number of advantages over conventional mechanical control. Mechanical systems limit the design and operation to a fixed set or relationships between the pilots movement and that of the flight controls. This represents a significant compromise given the potential operational envelope of the aircraft. The integration of a digital flight control computer allows the aircraft to be controlled over a larger range of operating conditions and automate functions to reduce pilot workload. FBW systems also have a general advantage that wires offer more flexibility in the design of the aircraft packaging.There are currently around 20 aircraft types that adopt a FBW architecture. Most of these employ Electro-Hydraulic Servo Valves (EHSV) as the primary means of converting electrical signal to hydraulic control. Roughly half of these are where the FBW system requires full operation to maintain safe flight. That is, there is no 'fail safe' position for the actuator. There is no consensus within the aviation industry on the best way of implementing a safety critical FBW system. Each air-framer has sought to generate architectures based on their individual requirements and history. This document describes a FBW actuator which has been designed to achieve the highest level of safety integrity together with greatest simplicity and lowest weight. It uses Additive Manufacturing (otherwise known as 3D printing) techniques to assist in meeting these objectives. The architecture adopted also aims to simplify the methods of failure detection which need to be embodied within the flight control computer. A typical use for this flight control would be a main rotor control for a medium sized civil helicopter.

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Failure Management

Safety critical flight control actuators are complex items. They have to manage multiple failure scenarios of both the systems that they interface (electric and hydraulic), but also internal features (jams and breakages). The aim is to make the sum of those probabilities to

-7not exceed a failure rate of >1x10 failures per flight hour. The actuator has been designed to achieve the following best practice with respect to flight control failure management:

1. To be inherently safe. That is, to operate safely without FCC intervention, following first credible failure (ie all failures with a probability of occurrence

-8 of > 1x10 /FtHr)2. Long observation period (>250ms) before FCC isolation of first failure to permit transient filtering.3. Failures are primarily determined through a voting strategy and reduce the reliance on in-line monitors.4. No single intervention of the FCC should lead to an unsafe condition.5. Safety related dormant failures (as used in reversionary systems) should be screened but preflight test.

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EHSV or DDV ?

Electrohydraulic Servo Valve

Direct Drive Valve

Two types of valve technology have been employed by FBW servo actuators; EHSV or Direct Drive Valves (DDV). The key difference between the technologies is the power to control the main control valve. For EHSVs this is high pressure hydraulic fluid which is directed by a low electrical power torque motor. With a DDV the main control valve is directly coupled to either a linear or rotary permanent magnet motor. The trades between the two technologies with respect to a flight critical actuator are as follows:

1. Technology employed for the majority of FBW applications2. Typically requires 3 or more valves per critical control surface based on failure rate of 1st stage close to failure rate of hydraulic system3. Synchronisation between systems challenging as a result of poor null stability4. Difficult to implement a second stage valve redundancy therefore requiring means of bypass on 1st or 2nd stage failure5. High parasitic leakage6. Ultra low electrical power consumption

1. Traditionally larger and heavier than EHSV equivalent2. Separates' electrical and hydraulic failure associated with 1st stage3. Improved low temperature performance4. Low parasitic leakage5. Excellent null stability and ability to software null adjust

In general, the trades between the technologies are complex. With respect to the desire to achieve the safety design goals and favouring lightweight and low cost solutions, the DDV technology represents a clear winner.

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Actuator ArchitectureThe flight control architecture is shown in Figure 1.

Figure 1 - Control and Redundancy Architecture

The actuator is a dual hydraulic, quadruplex electric, primary valve jam redundant actuator. The main jack is a dual hydraulic tandem configuration. The control valve is a rotary motor, rotary spool, dual hydraulic arrangement with independent valve jam redundancy on each system. All elements within the valve are pressure and flow balanced in order to reduce frictions and hysteresis. Valve feedback angle feedback is provided by quadruplex Hall effect transducer. The torque motor comprises a quadruplex, 3 phase, permanent magnet motor with full mechanical separation between the respective control lanes. Outer loop feedback can be either through mechanical linkage or electrical transducer. Figure 2 shows the coupling between the torque motor and the respective hydraulic systems.

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Figure 2 - Control Section

sleeve

The command is applied simultaneously to the 4 motor stator coils. The summation of currents produces a torque on the motor rotor. The system is designed to operate from any combination of energised coils and therefore can tolerate a maximum of 3 failures within a given exposure time. The torque on the rotor produces a rotary acceleration, leading to a velocity and change in angular position of the rotor. This has two consequences, a change in flow from each of the main control valves, together with a change in the voltage from the sensing element of each of the Hall effect transducers. Two feedback mechanisms now come into play; the inner loop controlling the motor rotor angle and the outer loop controlling the position of the main actuator. The inner loop feedback is provided by Hall effect transducers. The outer loop by either mechanical or electrical control. The control elements are arranged to minimize nonlinearities such as frictions and backlash. In this way the rotor is directly coupled to the main control spool. Phasing between the two hydraulic systems is achieved by an adjustment between the secondary sleeves and the body. Figure 3 shows a section of the main hydraulic jack. The jack is conventional in nature employing the best practice for a modern flight control servo. It is a semi balanced arrangement where a balance tube provides partial balance to the rear system. The primary dynamic seals are all dual dissimilar. Inter seal pressure is managed by a combination of the type of seal employed and use of venting to the hydraulic return, where appropriate. Primary lock of parts is external to the oil cavity with a positive shear lock employed where critical. The jack has facility for electrical feedback.

Figure 3 - Jack Section

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Complete Flight Control ActuatorFigure 4 shows the external view of the completed servo.

ConclusionThe objective has been to produce an actuator of exceptional performance in terms of safety, simplicity and weight. In terms of safety it conforms not only to the latest airworthiness requirements, but also meeting best practice in it's general design and construction. AM technology has significantly benefitted the meeting of these objectives. It can accommodate a range of external and internal failures whilst still maintaining safe performance. These include; loss of hydraulic system, loss of up to 3 electrical control lanes, and jam of primary control valve. In terms of simplicity and weight the FBW servo is a substantial improvement on the mechanically signaled servo it replaces on many aircraft. It is both lighter and has fewer parts. With respect to weight, it offers it is around half the weight of the equivalent mechanical servo and less than half the weight of any FBW alternative.

Figure 4 - Flight Control External

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