Final Report Summary - OREAT (Open Rotor Engines Advanced Technologies for rotor blade pitch change mechanism)
Executive Summary:
Snecma's planned SAGE 2 demonstrator pusher contra-rotating open rotor engine requires independent blade pitch-change mechanisms (PCMs) for its forward and aft rotor blades. Project OREAT had the objective of studying potential PCM concepts, down-selecting the best concept, and preparing design schemes and bills of materials for the chosen concept. Working from Snecma’s PCM requirements and their SAGE 2 propulsor cross-section, PCM concepts based on electric and hydraulic actuation, with linear and rotary, single and multiple actuators, mounted inboard and outboard of the gas path and with and without blade counterweights were considered. Electric actuation and outboard actuator location were eliminated early on as impractical, and blade counterweights were made baseline to avoid the additional complexity of pitch-lock mechanisms to prevent control failures resulting in hazaradous high drag and overspeed. A rigorous comparative evaluation of the several remaining potential PCM concepts was conducted using a scorecard process taking into account reliability, safety, weight, efficiency, controllability, maintainability, durability, maturity and cost. A final selection was made for two independent PCMs, one for each rotor stage, each with a single, co-axial, linear hydraulic actuator using engine oil driving each blade via a collective yoke, individual link rods, a crank lever and a torqueshaft connecting to the blade root or hub. Both PCMs are mounted inboard of the engine gas path in the propulsor sump, with the torqueshafts linking to individual blades through hollow struts in the rotating propulsor exhaust duct. The main architectural difference between the two PCMs is that the forward rotor annular hydraulic actuator is mounted on the adjacent static structure and transfers its force to the rotating collective yoke via a load-transfer ball bearing (LTB), while the aft rotor PCM, having no adjacent static structure, is fed with its actuation high-pressure oil via the static frame upstream of the power turbine through the centre of the propulsor to its rotating hydraulic actuator via an Oil Transfer Bearing (OTB). Despite the extended schedule, there was insufficient time to design two important elements of the PCMs, the flight-fine pitch stops and blade angle sensors, so these features will have to be included in the scope of a follow-on project.
The project concluded not with a preliminary design and bill of materials for the down-selected PCM as intended, but with architectural design schemes because the SAGE 2 propulsor design is still evolving. Snecma’s intention in the next Clean Sky Call for Proposals was to move directly from the PCM concept study to detailed design, manufacture and qualification of the chosen concept for the SAGE 2 engine. However, the OREAT study concluded, using established reliability data, that even the selected best concept would not satisfy the certification safety requirement for in-flight-shutdown probability. Snecma therefore accepted the recommendation from OREAT and inserted an intermediate PCM risk reduction testing phase that became a new Clean Sky Call for Proposals in 2011 and was subsequently awarded to the OREAT consortium as OREAT II, Project 296515.
Project Context and Objectives:
A principal contributor to affordable and sustainable public air transportation will be the introduction of more fuel-efficient propulsion systems, one of which is the Contra-Rotating Open Rotor Engine concept. Unlike turbofans, open rotors must vary blade pitch to accommodate power and aircraft speed range, so a key element to the viability of the concept is the actuation and control of the blade pitch via the Pitch Change Mechanism (PCM). Open rotors are essentially the same as variable-pitch, contra-rotating propellers, and like variable-pitch propellers must also incorporate independent safety features to prevent overspeeds and excessive drag. The two PCMs for SAGE 2 must each incorporate a low blade angle stop for this reason. Whilst the PCMs of variable-pitch propellers for FAR/EASA Part 25 transport aircraft do meet safety and reliability requirements, they achieve this using very simple mechanisms due to the small number of blades (up to six) and single rotation. (There are no western Type Approved contra-rotating propeller systems.) Dowty Propellers, part of GE Aviation Systems and Project Coordinator for the OREAT project, holds Type Certificates for many modern certified propeller systems and all have a PCM with a linear hydraulic actuator which is essentially a single moving part - a hydraulic piston on a shaft attached to a collective yoke that engages offset operating pins on the inner surface of each blade root. It is the simplicity of this concept that enables safety and reliability requirements to be satisfied. The challenge of OREAT is the inevitable increase in complexity due to contra-rotation and very high blade complements, together with a high temperature environment in the case of the pusher SAGE 2 engine. The main objective is to design the simplest, most rugged and reliable PCM systems, which in Clean Sky means satisfying the Airbus target of being no less reliable than current generation high-bypass ratio turbofan engines, which in turn are an order of magnitude more reliable than typical modern turboprop engines.
Project Results:
Unlike turbofans with their ducted rotors, which isolate the blades from the free airstream velocity, open rotors must vary blade pitch like variable-pitch propellers do to accommodate the variation of power and aircraft speed. The main elements of the challenge are:
1. Complexity: Compared to a fan which is a single rotating structure with no moving parts, a CROR propulsor comprises two independent rotors with all the blades mounted in rotatable pitch-change bearings and varying their pitch collectively on each rotor but independently between rotors by means of two independent PCMs, each of which inevitably has a quantity of moving parts largely in proportion to the quantity of blades. The aft rotor has the added challenge of having no adjacent stationary structure. This multiplicity of mechanisms brings with it an inevitable durability and reliability burden, which must result in increased maintenance costs and potentially worse safety than offered by current turbofans. In addition, the PCMs must incorporate blade angle feedback sensors for control and flight fine pitch stops required for safety by propeller regulations that link directly with the movement of the actuation system.
2. Environment: Unlike conventional turboprop propellers, the pusher CROR architecture favoured by Airbus and made a requirement on the SAGE 2 engine necessitates proximity of the PCM to the hot gas path exhaust from the engine. The mechanical parts must be kept cool and lubricated, and leakage of the hydraulic and lubricating oil into the gas path must be prevented as it would have potentially hazardous consequences. The blade pitch-change bearings and the interface connection to the PCM are beyond the reach of positive lubrication.
3. 1P Loads: Like propellers, but unlike turbofans, exposing the CROR blades to the free airstream with its inevitable velocity variation around the plane of rotation causes the thrust from each blade to vary cyclically at rotational frequency. In a fixed, non-uniform velocity field, every blade will vary its thrust in the same pattern in space, causing a moment that is fixed in space, the 1P moment, and an associated fixed side force due to the variation of the torque to drive each blade around its path. Even with wing-mounted tractor propellers with little upstream disturbance to deal with, 1P moments and blade cyclic bending dominate propeller design and testing, but the rear pusher installation favoured by Airbus will be much more challenging and variable. The PCM actuator will experience the 1P moment, and the individual blade actuation mechanisms in the collective management of blade pitch will experience load variation at rotational frequency.
4. Blade Flutter: PCMs must be very stiff to avoid the risk of blade flutter, which is difficult to predict accurately in the very variable conditions of an open rotor. The necessity of crossing the gas path with the pitch change mechanism using torque shafts brings with it an inevitable stiffness deficit as the shafts and the hollow struts they pass through must be kept as slender as possible to minimise exhaust duct losses.
Working from Snecma’s PCM requirements and their SAGE 2 propulsor cross-section, PCM concepts based on electric and hydraulic actuation, with linear and rotary, single and multiple actuators, mounted inboard and outboard of the gas path and with and without blade counterweights were considered. Electric actuation and outboard actuator location were eliminated early on as impractical, and blade counterweights were made baseline to avoid the additional complexity of pitch-lock mechanisms to prevent control failures resulting in hazaradous high drag and overspeed. A rigorous comparative evaluation of the several remaining potential PCM concepts was conducted using a scorecard process taking into account reliability, safety, weight, efficiency, controllability, maintainability, durability, maturity and cost. A final selection was made for two independent PCMs, one for each rotor stage, each with a single, co-axial, linear hydraulic actuator using engine oil driving each blade via a collective yoke, individual link rods, a crank lever and a torqueshaft connecting to the blade root or hub. Both PCMs are mounted inboard of the engine gas path in the propulsor sump, with the torqueshafts linking to individual blades through hollow struts in the rotating propulsor exhaust duct. The main architectural difference between the two PCMs is that the forward rotor annular hydraulic actuator is mounted on the adjacent static structure and transfers its force to the rotating collective yoke via a load-transfer ball bearing (LTB), while the aft rotor PCM, having no adjacent static structure, is fed with its actuation high-pressure oil via the static frame upstream of the power turbine through the centre of the propulsor to its rotating hydraulic actuator via an Oil Transfer Bearing (OTB). Despite the extended schedule, there was insufficient time to design two important elements of the PCMs, the flight-fine pitch stops and blade angle sensors, so these features will have to included in the scope of follow-on project.
1.4 Conclusions and Recommendations
The project concluded not with a preliminary design and bill of materials for the down-selected PCM as intended, but with architectural design schemes because the SAGE 2 propulsor design is still evolving. Snecma’s intention in the next Clean Sky Call for Proposals was to move directly from the PCM concept study to detailed design, manufacture and qualification of the chosen concept for the SAGE 2 engine. However, the OREAT study concluded, using established reliability data, that even the selected best concept would not satisfy the certification safety requirement for in-flight-shutdown probability. Snecma therefore accepted the recommendation from OREAT and inserted an intermediate PCM risk reduction testing phase that became a new Clean Sky Call for Proposals in 2011 and was subsequently awarded to the OREAT consortium as OREAT II, Project 296515.
Potential Impact:
A principal contributor to affordable and sustainable public air transportation will be the introduction of more fuel-efficient propulsion systems, one of which is the Contra-Rotating Open Rotor Engine (CROR) concept. CROR engines offer the potential of a 20% reduction in emissions compared to turbofan engines due to the inherent propulsive efficiency that results from accelerating a larger air mass through a smaller velocity increment, an advantage shared with conventional aircraft propellers but further improved by the use of contra-rotation to remove most of the wasted swirl energy in the efflux and extending the benefits to higher airspeeds. The key technologies that must be developed to enable CROR engines to displace increasingly efficient and highly reliable turbofan engines are the aero-acoustic design of the propulsor and the blade pitch change mechanism. Earlier EU RTD programmes such as NACRE and DREAM have included projects concentrating on the aeroacoustic design, establishing confidence in the performance that can be achieved and allaying earlier concerns from similar 1980s studies about the excessive wake-interaction noise. Arguably, the blade pitch change mechanism is probably the greatest remaining challenge to CROR engines, and overcoming the issues involved could be the gateway to the introduction of these radical engines into commercial use. OREAT has highlighted the issues and made some useful steps along this path.
Snecma's planned SAGE 2 demonstrator pusher contra-rotating open rotor engine requires independent blade pitch-change mechanisms (PCMs) for its forward and aft rotor blades. Project OREAT had the objective of studying potential PCM concepts, down-selecting the best concept, and preparing design schemes and bills of materials for the chosen concept. Working from Snecma’s PCM requirements and their SAGE 2 propulsor cross-section, PCM concepts based on electric and hydraulic actuation, with linear and rotary, single and multiple actuators, mounted inboard and outboard of the gas path and with and without blade counterweights were considered. Electric actuation and outboard actuator location were eliminated early on as impractical, and blade counterweights were made baseline to avoid the additional complexity of pitch-lock mechanisms to prevent control failures resulting in hazaradous high drag and overspeed. A rigorous comparative evaluation of the several remaining potential PCM concepts was conducted using a scorecard process taking into account reliability, safety, weight, efficiency, controllability, maintainability, durability, maturity and cost. A final selection was made for two independent PCMs, one for each rotor stage, each with a single, co-axial, linear hydraulic actuator using engine oil driving each blade via a collective yoke, individual link rods, a crank lever and a torqueshaft connecting to the blade root or hub. Both PCMs are mounted inboard of the engine gas path in the propulsor sump, with the torqueshafts linking to individual blades through hollow struts in the rotating propulsor exhaust duct. The main architectural difference between the two PCMs is that the forward rotor annular hydraulic actuator is mounted on the adjacent static structure and transfers its force to the rotating collective yoke via a load-transfer ball bearing (LTB), while the aft rotor PCM, having no adjacent static structure, is fed with its actuation high-pressure oil via the static frame upstream of the power turbine through the centre of the propulsor to its rotating hydraulic actuator via an Oil Transfer Bearing (OTB). Despite the extended schedule, there was insufficient time to design two important elements of the PCMs, the flight-fine pitch stops and blade angle sensors, so these features will have to be included in the scope of a follow-on project.
The project concluded not with a preliminary design and bill of materials for the down-selected PCM as intended, but with architectural design schemes because the SAGE 2 propulsor design is still evolving. Snecma’s intention in the next Clean Sky Call for Proposals was to move directly from the PCM concept study to detailed design, manufacture and qualification of the chosen concept for the SAGE 2 engine. However, the OREAT study concluded, using established reliability data, that even the selected best concept would not satisfy the certification safety requirement for in-flight-shutdown probability. Snecma therefore accepted the recommendation from OREAT and inserted an intermediate PCM risk reduction testing phase that became a new Clean Sky Call for Proposals in 2011 and was subsequently awarded to the OREAT consortium as OREAT II, Project 296515.
Project Context and Objectives:
A principal contributor to affordable and sustainable public air transportation will be the introduction of more fuel-efficient propulsion systems, one of which is the Contra-Rotating Open Rotor Engine concept. Unlike turbofans, open rotors must vary blade pitch to accommodate power and aircraft speed range, so a key element to the viability of the concept is the actuation and control of the blade pitch via the Pitch Change Mechanism (PCM). Open rotors are essentially the same as variable-pitch, contra-rotating propellers, and like variable-pitch propellers must also incorporate independent safety features to prevent overspeeds and excessive drag. The two PCMs for SAGE 2 must each incorporate a low blade angle stop for this reason. Whilst the PCMs of variable-pitch propellers for FAR/EASA Part 25 transport aircraft do meet safety and reliability requirements, they achieve this using very simple mechanisms due to the small number of blades (up to six) and single rotation. (There are no western Type Approved contra-rotating propeller systems.) Dowty Propellers, part of GE Aviation Systems and Project Coordinator for the OREAT project, holds Type Certificates for many modern certified propeller systems and all have a PCM with a linear hydraulic actuator which is essentially a single moving part - a hydraulic piston on a shaft attached to a collective yoke that engages offset operating pins on the inner surface of each blade root. It is the simplicity of this concept that enables safety and reliability requirements to be satisfied. The challenge of OREAT is the inevitable increase in complexity due to contra-rotation and very high blade complements, together with a high temperature environment in the case of the pusher SAGE 2 engine. The main objective is to design the simplest, most rugged and reliable PCM systems, which in Clean Sky means satisfying the Airbus target of being no less reliable than current generation high-bypass ratio turbofan engines, which in turn are an order of magnitude more reliable than typical modern turboprop engines.
Project Results:
Unlike turbofans with their ducted rotors, which isolate the blades from the free airstream velocity, open rotors must vary blade pitch like variable-pitch propellers do to accommodate the variation of power and aircraft speed. The main elements of the challenge are:
1. Complexity: Compared to a fan which is a single rotating structure with no moving parts, a CROR propulsor comprises two independent rotors with all the blades mounted in rotatable pitch-change bearings and varying their pitch collectively on each rotor but independently between rotors by means of two independent PCMs, each of which inevitably has a quantity of moving parts largely in proportion to the quantity of blades. The aft rotor has the added challenge of having no adjacent stationary structure. This multiplicity of mechanisms brings with it an inevitable durability and reliability burden, which must result in increased maintenance costs and potentially worse safety than offered by current turbofans. In addition, the PCMs must incorporate blade angle feedback sensors for control and flight fine pitch stops required for safety by propeller regulations that link directly with the movement of the actuation system.
2. Environment: Unlike conventional turboprop propellers, the pusher CROR architecture favoured by Airbus and made a requirement on the SAGE 2 engine necessitates proximity of the PCM to the hot gas path exhaust from the engine. The mechanical parts must be kept cool and lubricated, and leakage of the hydraulic and lubricating oil into the gas path must be prevented as it would have potentially hazardous consequences. The blade pitch-change bearings and the interface connection to the PCM are beyond the reach of positive lubrication.
3. 1P Loads: Like propellers, but unlike turbofans, exposing the CROR blades to the free airstream with its inevitable velocity variation around the plane of rotation causes the thrust from each blade to vary cyclically at rotational frequency. In a fixed, non-uniform velocity field, every blade will vary its thrust in the same pattern in space, causing a moment that is fixed in space, the 1P moment, and an associated fixed side force due to the variation of the torque to drive each blade around its path. Even with wing-mounted tractor propellers with little upstream disturbance to deal with, 1P moments and blade cyclic bending dominate propeller design and testing, but the rear pusher installation favoured by Airbus will be much more challenging and variable. The PCM actuator will experience the 1P moment, and the individual blade actuation mechanisms in the collective management of blade pitch will experience load variation at rotational frequency.
4. Blade Flutter: PCMs must be very stiff to avoid the risk of blade flutter, which is difficult to predict accurately in the very variable conditions of an open rotor. The necessity of crossing the gas path with the pitch change mechanism using torque shafts brings with it an inevitable stiffness deficit as the shafts and the hollow struts they pass through must be kept as slender as possible to minimise exhaust duct losses.
Working from Snecma’s PCM requirements and their SAGE 2 propulsor cross-section, PCM concepts based on electric and hydraulic actuation, with linear and rotary, single and multiple actuators, mounted inboard and outboard of the gas path and with and without blade counterweights were considered. Electric actuation and outboard actuator location were eliminated early on as impractical, and blade counterweights were made baseline to avoid the additional complexity of pitch-lock mechanisms to prevent control failures resulting in hazaradous high drag and overspeed. A rigorous comparative evaluation of the several remaining potential PCM concepts was conducted using a scorecard process taking into account reliability, safety, weight, efficiency, controllability, maintainability, durability, maturity and cost. A final selection was made for two independent PCMs, one for each rotor stage, each with a single, co-axial, linear hydraulic actuator using engine oil driving each blade via a collective yoke, individual link rods, a crank lever and a torqueshaft connecting to the blade root or hub. Both PCMs are mounted inboard of the engine gas path in the propulsor sump, with the torqueshafts linking to individual blades through hollow struts in the rotating propulsor exhaust duct. The main architectural difference between the two PCMs is that the forward rotor annular hydraulic actuator is mounted on the adjacent static structure and transfers its force to the rotating collective yoke via a load-transfer ball bearing (LTB), while the aft rotor PCM, having no adjacent static structure, is fed with its actuation high-pressure oil via the static frame upstream of the power turbine through the centre of the propulsor to its rotating hydraulic actuator via an Oil Transfer Bearing (OTB). Despite the extended schedule, there was insufficient time to design two important elements of the PCMs, the flight-fine pitch stops and blade angle sensors, so these features will have to included in the scope of follow-on project.
1.4 Conclusions and Recommendations
The project concluded not with a preliminary design and bill of materials for the down-selected PCM as intended, but with architectural design schemes because the SAGE 2 propulsor design is still evolving. Snecma’s intention in the next Clean Sky Call for Proposals was to move directly from the PCM concept study to detailed design, manufacture and qualification of the chosen concept for the SAGE 2 engine. However, the OREAT study concluded, using established reliability data, that even the selected best concept would not satisfy the certification safety requirement for in-flight-shutdown probability. Snecma therefore accepted the recommendation from OREAT and inserted an intermediate PCM risk reduction testing phase that became a new Clean Sky Call for Proposals in 2011 and was subsequently awarded to the OREAT consortium as OREAT II, Project 296515.
Potential Impact:
A principal contributor to affordable and sustainable public air transportation will be the introduction of more fuel-efficient propulsion systems, one of which is the Contra-Rotating Open Rotor Engine (CROR) concept. CROR engines offer the potential of a 20% reduction in emissions compared to turbofan engines due to the inherent propulsive efficiency that results from accelerating a larger air mass through a smaller velocity increment, an advantage shared with conventional aircraft propellers but further improved by the use of contra-rotation to remove most of the wasted swirl energy in the efflux and extending the benefits to higher airspeeds. The key technologies that must be developed to enable CROR engines to displace increasingly efficient and highly reliable turbofan engines are the aero-acoustic design of the propulsor and the blade pitch change mechanism. Earlier EU RTD programmes such as NACRE and DREAM have included projects concentrating on the aeroacoustic design, establishing confidence in the performance that can be achieved and allaying earlier concerns from similar 1980s studies about the excessive wake-interaction noise. Arguably, the blade pitch change mechanism is probably the greatest remaining challenge to CROR engines, and overcoming the issues involved could be the gateway to the introduction of these radical engines into commercial use. OREAT has highlighted the issues and made some useful steps along this path.