Actuators are critical engine and flight control components used for motion and fuel controls. Actuators control thrust vectoring of the main engines during powered ascent, movement of the aerodynamic control surfaces, and the positioning of propulsion system geometry and fuel/air control valves. All aircraft today contain three primary types of actuators: electromechanical actuators (EMAs), electrohydraulic actuators (EHAs), and hydraulic actuators.
EMAs consist of an electric motor and geartrain to reduce speed, translate motion, and provide appropriate load torque. EHAs are self-contained systems that combine the benefits of an electric system with those of hydraulic systems. EHAs use an electric motor to drive a hydraulic pump, which develops hydraulic pressure to act on a cylinder to provide the mechanical actuation energy. Hydraulic actuators use a centralized hydraulic pump that supplies the required pressure.
EHAs avoid the operability issues associated with a central hydraulic supply and distribution system. They also have weight and integration benefits. Aerospace actuation has historically been dominated by hydraulic and fluid power systems. Sales of hydraulic actuation systems today accounts for more than several billion dollars per year of business for the major vendors. These systems comprise about 19% of the cost of a commercial aircraft.
However, as entrenched as hydraulics are in flight applications, the emergence and maturation of electrical actuation promises to encroach significantly on hydraulic technology over the next several decades.
The primary benefit of the EMA for the air vehicle is eliminating the hydraulic system and associated pump hardware and fluid and distribution lines. This architecture is being achieved in incremental steps on current designs that are using the EMA in combination with other technologies for redundancy. The Boeing 787 hydraulic systems feature a primary electric-driven hydraulic pump on each engine plus a third, slipstream-driven RAT (ram air turbine) to operate the actuators in an emergency.
On the engine, there will always be a high-pressure (1500 psi) fuel system to supply the combustion flow. In this application, the choice of EMA or other technology will be driven by consideration of the optimized engine and air vehicle performance requirements and appropriate cost-benefits. The combined use of electric and hydraulic technology may also yield an optimal design with appropriate failure modes as an alternative to a single choice of EMA, EHA, or hydraulic technology. When new actuation technology increases the maintainability, reliability, efficiency, and safety of engine and aircraft, the dependence on hydraulically powered actuators may not be maintained if any reasonable alternatives like EMAs can be demonstrated.
EMAs and hydra-mechanical actuators (HMAs) provide better and faster response time than pneumatic actuators, but EMAs have significant advantages in terms of size, weight, and power requirements than other actuation systems.
In terms of ease and flexibility of installation, EMAs have several advantages over hydraulic actuators, and the cost is the primarily reason. A hydraulic actuation system requires extensive hydraulic fittings and component seals. EMAs are finding increased applications for variable cycle turbine engine and flight control applications. HMAs historically have been used successfully for different applications.
There is a trend toward more electric aircraft, and also hotter engine operation requires a careful examination for their use. EMAs still lack the knowledge base accumulated for aerospace applications, particularly with regard to fault detection and characterization. The advantages and disadvantages for both types of actuators, and their specific applications, needs to be examined carefully before a decision can be made for their use.
Analysis of critical failure mode via Failure Modes and Effects and Criticality Analysis (FMECA) must be done first, so that reliability assessment can be accomplished. In addition, ease of replacement, redundancy management, and onboard prognostics are important considerations for choosing the proper actuation systems.
For EMAs, the controllers in a distributed architecture include a microprocessor and memory to monitor operational characteristics, and to allow performance history to be included for health monitoring. This will permit hardware to be used over a greater portion of its operational life as compared to current procedure of actuator replacement based upon a fixed amount of flight/engine hours. For both types of actuators, the extended use based on performance history information has a potential to save hardware and labor costs for repair and replacement.
This article is based on SAE technical paper 2010-01-1747 by Alireza R. Behbahani and Kenneth J. Semega, Air Force Research Laboratory.