Slide 27 -
|
Aircraft Hydraulic System Design Peter A. Stricker, PE
Product Sales Manager
Eaton Aerospace Hydraulic Systems Division
August 20, 2010 Purpose Acquaint participants with hydraulic system design principles for civil aircraft
Review examples of hydraulic system architectures on common aircraft Agenda Introduction
Review of Aircraft Motion Controls
Uses for and sources of hydraulic power
Key hydraulic system design drivers
Safety standards for system design
Hydraulic design philosophies for conventional, “more electric” and “all electric” architectures
Hydraulic System Interfaces
Sample aircraft hydraulic system block diagrams
Conclusions Introduction As airplanes grow in size, so do the forces needed to move the flight controls … thus the need to transmit larger amount of power Ram Air Turbine Pump Hydraulic Storage/Conditioning Engine Pump Electric Generator Electric Motorpump Flight Control Actuators Air Turbine Pump Hydraulic system transmits and controls power from engine to flight control actuators 2 Pilot inputs are transmitted to remote actuators and amplified 1 3 Pilot commands move actuators with little effort 4 Hydraulic power is generated mechanically, electrically and pneumatically 5 Pilot Inputs Introduction Aircraft’s Maximum Take-Off Weight (MTOW) drives aerodynamic forces that drive control surface size and loading
A380 – 1.25 million lb MTOW – extensive use of hydraulics
Cessna 172 – 2500 lb MTOW – no hydraulics – all manual Controlling Aircraft Motion Primary Flight Controls Definition of Airplane Axes 1 Ailerons control roll
2 Elevators control pitch
3 Rudder controls yaw 1 3 2 Controlling Aircraft Motion Secondary Flight Controls High Lift Devices: ►
Flaps (Trailing Edge), slats (LE Flaps) increase area and camber of wing
permit low speed flight
Flight Spoilers / Speed Brakes: permit steeper descent and augment ailerons at low speed when deployed on only one wing
Ground Spoilers: Enhance deceleration on ground (not deployed in flight)
Trim Controls:
Stabilizer (pitch), roll and rudder (yaw) trim to balance controls for desired flight condition Example of Flight Controls (A320) REF: A320 FLIGHT CREW OPERATING MANUAL CHAPTER 1.27 - FLIGHT CONTROLS Why use Hydraulics? Effective and efficient method of power amplification
Small control effort results in a large power output
Precise control of load rate, position and magnitude
Infinitely variable rotary or linear motion control
Adjustable limits / reversible direction / fast response
Ability to handle multiple loads simultaneously
Independently in parallel or sequenced in series
Smooth, vibration free power output
Little impact from load variation
Hydraulic fluid transmission medium
Removes heat generated by internal losses
Serves as lubricant to increase component life HYDR. MOTOR TORQUE TUBE GEARBOX Typical Users of Hydraulic Power Landing gear
Extension, retraction, locking, steering, braking
Primary flight controls
Rudder, elevator, aileron, active (multi-function) spoiler
Secondary flight controls
high lift (flap / slat), horizontal stabilizer, spoiler, thrust reverser
Utility systems
Cargo handling, doors, ramps, emergency electrical power generation Flap Drive Spoiler Actuator Landing Gear Nosewheel Steering Sources of Hydraulic Power Ram Air Turbine AC Electric Motorpump Maintenance-free Accumulator Engine Driven Pump Mechanical
Engine Driven Pump (EDP) - primary hydraulic power source, mounted directly to engines on special gearbox pads
Power Transfer Unit – mechanically transfers hydraulic power between systems
Electrical
Pump attached to electric motors, either AC or DC
Generally used as backup or as auxiliary power
Electric driven powerpack used for powering actuation zones
Used for ground check-out or actuating doors when engines are not running
Pneumatic
Bleed Air turbine driven pump used for backup power
Ram Air Turbine driven pump deployed when all engines are inoperative and uses ram air to drive the pump
Accumulator provides high transient power by releasing stored energy, also used for emergency and parking brake Power Transfer Unit Key Hydraulic System Design Drivers High Level certification requirement per aviation regulations:
Maintain control of the aircraft under all normal and anticipated failure conditions
Many system architectures* and design approaches exist to meet this high level requirement – aircraft designer has to certify to airworthiness regulators by analysis and test that his solution meets requirements
Hydraulic System Architecture: Arrangement and interconnection of hydraulic power sources and consumers in a manner that meets requirements for controllability of aircraft Considerations for Hydraulic System Design to meet System Safety Requirements Redundancy in case of failures must be designed into system
Any and every component will fail during life of aircraft
Manual control system requires less redundancy Fly-by-wire (FBW) requires more redundancy
Level of redundancy necessary evaluated per methodology described in ARP4761
Safety Assessment Tools
Failure Modes, Effects and Criticality Analysis – computes failure rates and failure criticalities of individual components and systems by considering all failure modes
Fault Tree Analysis – computes failure rates and probabilities of various combinations of failure modes
Markov Analysis – computes failure rates and criticality of various chains of events
Common Cause Analysis – evaluates failures that can impact multiple components and systems Principal failure modes considered
Single system or component failure
Multiple system or component failures occurring simultaneously
Dormant failures of components or subsystems that only operate in emergencies
Common mode failures – single failures that can impact multiple systems
Examples of failure cases to be considered
One engine shuts down during take-off – need to retract landing gear rapidly
Engine rotor bursts – damage to and loss of multiple hydraulic systems
Rejected take-off – deploy thrust reversers, spoilers and brakes rapidly
All engines fail in flight – need to land safely without main hydraulic and electric power sources
Civil Aircraft System Safety Standards (Applies to all aircraft systems) Examples
Minor: Single hydraulic system fails
Major: Two (out of 3) hydraulic systems fail
Hazardous: All hydraulic sources fail, except RAT or APU (US1549 Hudson River A320 – 2009)
Catastrophic: All hydraulic systems fail (UA232 DC-10 Sioux City – 1989) System Design Philosophy Conventional Central System Architecture Multiple independent centralized power systems
Each engine drives dedicated pump(s), augmented by independently powered pumps – electric, pneumatic
No fluid transfer between systems to maintain integrity
System segregation
Route lines and locate components far apart to prevent single rotor or tire burst from impacting multiple systems
Multiple control channels for critical functions
Each flight control needs multiple independent actuators or control surfaces
Fail-safe failure modes – e.g., landing gear can extend by gravity and be locked down mechanically LEFT ENG.
SYSTEM 1 SYSTEM 3 RIGHT ENG.
SYSTEM 2 ROLL 1 PITCH 1 YAW 1 OTHERS PTU ROLL 2 PITCH 2 YAW 2 OTHERS ROLL 3 PITCH 3 YAW 3 LNDG GR EMRG BRK NORM BRK NSWL STRG EDP Engine Driven Pump
EMP Electric Motor Pump
ADP Air Driven Pump PTU Power Transfer Unit
RAT Ram Air Turbine
Engine Bleed Air OTHERS System Design Philosophy More Electric Architecture Two independent centralized power systems + Zonal & Dedicated Actuators
Each engine drives dedicated pump(s), augmented by independently powered pumps – electric, pneumatic
No fluid transfer between systems to maintain integrity
System segregation
Route lines and locate components far apart to prevent single rotor or tire burst to impact multiple systems
Third System replaced by one or more local and dedicated electric systems
Tail zonal system for pitch, yaw
Aileron actuators for roll
Electric driven hydraulic powerpack for emergency landing gear and brake
Examples: Airbus A380, Boeing 787 LEFT ENG.
SYSTEM 1 RIGHT ENG.
SYSTEM 2 ROLL 1 PITCH 1 YAW 1 OTHERS ROLL 2 PITCH 2 YAW 2 OTHERS ROLL 3 ZONAL PITCH 3 YAW 3 NORM BRK EMRG BRK LNDG GR NW STRG EDP Engine Driven Pump
EMP Electric Motor Pump
GEN Electric Generator
RAT Ram Air Turbine Generator
Electric Channel OTHERS ELECTRICAL ACTUATORS LG / BRK EMERG POWER System Design Philosophy All Electric Architecture “Holy Grail” of aircraft power distribution ….
Relies on future engine-core mounted electric generators capable of high power / high power density generation, running at engine speed – typically 40,000 rpm
Electric power will replace all hydraulic and pneumatic power for all flight controls, environmental controls, de-icing, etc.
Flight control actuators will like remain hydraulic, using Electro-Hydrostatic Actuators (EHA) or local hydraulic systems, consisting of
Miniature, electrically driven, integrated hydraulic power generation system
Hydraulic actuator controlled by electrical input Fly-by-Wire (FBW) Systems Fly-by-Wire
Pilot input read by computers
Computer provides input to electrohydraulic flight control actuator
Control laws include
Enhanced logic to automate many functions
Artificial damping and stability
Flight Envelope Protection to prevent airframe from exceeding structural limits
Multiple computers and actuators provide sufficient redundancy – no manual reversion Conventional Mechanical
Pilot input mechanically connected to flight control hydraulic servo-actuator by cables, linkages, bellcranks, etc.
Servo-actuator follows pilot command with high force output
Autopilot input mechanically summed
Manual reversion in case of loss of hydraulics or autopilot malfunction
BOEING 757 AILERON SYSTEM PILOT INPUTS AUTOPILOT INPUTS LEFT WING RIGHT WING Principal System Interfaces Design Considerations Hydraulic System Aircraft Hydraulic Architectures Comparative Aircraft Weights Aircraft Hydraulic Architectures Example Block Diagrams – Learjet 40/45 MAIN SYSTEM EMERGENCY SYSTEM MTOW: 21,750 lb
Flight Controls: Manual
Key Features
One main system fed by 2 EDP’s
Emergency system fed by DC electric pump
Common partitioned reservoir (air/oil)
Selector valve allows flaps, landing gear, nosewheel steering to operate from main or emergency system
All primary flight controls are manual
Safety / Redundancy
Engine-out take-off: One EDP has sufficient power to retract gear
All Power-out: Manual flight controls; LG extends by gravity with electric pump assist; emergency flap extends by electric pump; Emergency brake energy stored in accumulator for safe stopping REF.: AIR5005A (SAE) Mid-Size Jet Aircraft Hydraulic Architectures Example Block Diagrams – Hawker 4000 MTOW: 39,500 lb
Flight Controls: Hydraulic with manual reversion exc. Rudder, which is Fly-by-Wire (FBW)
Key Features
Two independent systems
Bi-directional PTU to transfer power between systems without transferring fluid
Electrically powered hydraulic power-pack for Emergency Rudder System (ERS) Safety / Redundancy
All primary flight controls 2-channel; rudder has additional backup powerpack; others manual reversion
Engine-out take-off: PTU transfers power from system #1 to #2 to retract LG
Rotorburst: Emergency Rudder System is located outside burst area
All Power-out: ERS runs off battery; others manual; LG extends by gravity Super Mid Size REF.: EATON C5-38A 04/2003 Aircraft Hydraulic Architectures Example Block Diagrams – Airbus A320/321 MTOW (A321): 206,000 lb
Flight Controls: Hydraulic FBW
Key Features
3 independent systems
2 main systems with EDP 1 main system also includes backup EMP & hand pump for cargo door 3rd system has EMP and RAT pump
Bi-directional PTU to transfer power between primary systems without transferring fluid
Safety / Redundancy
All primary flight controls have 3 independent channels
Engine-out take-off: PTU transfers power from Y to G system to retract LG
Rotorburst: Three systems sufficiently segregated
All Power-out: RAT pump powers Blue; LG extends by gravity Single-Aisle REF.: AIR5005 (SAE) Aircraft Hydraulic Architectures Example Block Diagrams – Boeing 777 LEFT SYSTEM Wide Body RIGHT SYSTEM CENTER SYSTEM MTOW (B777-300ER): 660,000 lb
Flight Controls: Hydraulic FBW
Key Features
3 independent systems
2 main systems with EDP + EMP each
3rd system with 2 EMPs, 2 engine bleed air-driven (engine bleed air) pumps, + RAT pump
Safety / Redundancy
All primary flight controls have 3 independent channels
Engine-out take-off: One air driven pump and EMP available in system 3 to retract LG
Rotorburst: Three systems sufficiently segregated
All Power-out: RAT pump powers center system; LG extends by gravity REF.: AIR5005 (SAE) Aircraft Hydraulic Architectures Example Block Diagrams – Airbus A380 Wide Body MTOW: 1,250,000 lb
Flight Controls: FBW (2H + 1E channel)
Key Features / Redundancies
Two independent hydraulic systems + one electric system (backup)
Primary hydraulic power supplied by 4 EDP’s per system
All primary flight controls have 3 channels – 2 hydraulic + 1 electric
4 engines provide sufficient redundancy for engine-out cases REF.: EATON C5-37A 06/2006 Conclusions Aircraft hydraulic systems are designed for high levels of safety using multiple levels of redundancy
Fly-by-wire systems require higher levels of redundancy than manual systems to maintain same levels of safety
System complexity increases with aircraft weight Suggested References Federal Aviation Regulations
FAR Part 25: Airworthiness Standards for Transport Category Airplanes
FAR Part 23: Airworthiness Standards for Normal, Utility, Acrobatic, and Commuter Category Airplanes
FAR Part 21: Certification Procedures For Products And Parts
AC 25.1309-1A System Design and Analysis Advisory Circular, 1998
Aerospace Recommended Practices (SAE)
ARP4761: Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment
ARP 4754: Certification Considerations for Highly-Integrated or Complex Aircraft Systems
Aerospace Information Reports (SAE)
AIR5005: Aerospace - Commercial Aircraft Hydraulic Systems
Radio Technical Committee Association (RTCA)
DO-178: Software Considerations in Airborne Systems and Equipment Certification (incl. Errata Issued 3-26-99)
DO-254: Design Assurance Guidance For Airborne Electronic Hardware
Text
Moir & Seabridge: Aircraft Systems – Mechanical, Electrical and Avionics Subsystems Integration 3rd Edition, Wiley 2008
|