Pitch Change Mechanism Conceptual and Preliminary Design Activities. 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 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).
GE Aviation Systems Ltd (trading as Dowty Propellers) is engaged in specifying requirements and concept evaluation for PCMs for direct-drive pusher open rotors in the EU 7th Framework DREAM SP3 work package 3.1. Dowty Propellers has also participated in similar studies with GE Aviation Engine division to assess the viability of a number of PCM control and actuation concepts. GE Aviation proposed to build on this background to support the SAGE 2 engine PCM. The work packages proposed via this application will focused on developing the critical technologies associated with the selected actuation system concept. The proposal describes the activity supporting functional, architectural, failure mode, mass, and performance analyses, the creation of a preliminary bill of material and definition of system performance and safety verification.
The deliverables from the programme were the definition of the preferred PCM concept with 3D geometry, performance, functional, safety and reliability assessments, interface definition and power plant control system integration architecture proposal. The work packages were closely aligned to the engine demonstrator requirements, in preparation for the launch of demonstration hardware.
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, downselecting 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 hazardous 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 torque shaft connecting to the blade root or hub. Both PCMs are mounted inboard of the engine gas path in the propulsor sump, with the torque shafts 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 downselected 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.