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A Microfabricated Actuator for Active Flow Control on Aircraft

Final Report Summary - MEMFAC (A microfabricated actuator for active flow control on aircraft)

As part of the European Union (EU) funded 'Clean Sky, Smart Fixed Wing' programme, the BAE Systems Advanced Technology Centre (ATC) is further developing a micro-fabricated actuator for active flow control. The main objectives are as follows: develop a micro-fabricated pulsed jet flow control actuator capable of delivering a jet velocity of 200 - 300 m/s from a 1.0 mm diameter hole at frequencies up to 1 kHz and a 20 % - 80 % duty cycle. There is also a target of 80+ % manufacturing yield. The current project produced a micro-fabricated pulsed jet actuator with an exit velocity of about 200 - 220 m/s from a 1.0 mm hole at up to 200+ Hz operating frequency, with a duty cycle variable between 0 and 100 %. The fabrication yield achieved was 70+ %.

Project context and objectives:

The context of the project was to meet the Clean Sky Joint Undertaking JTI-CS-2010-1-SFWA-01-015. That is, the development and test of a fluidic actuator prototype using MEMS technology of dimensions suitable for incorporation into an aircraft. The envisaged application is to use a number of these actuators near the leading-edge of the wing to generate a series of air-jets which then roll up into a series of stream-wise vortices which energise the boundary layer to prevent flow separation further downstream on the wing.

The main objectives of the project were as follows: develop a micro-fabricated pulsed jet flow control actuator capable of delivering a jet velocity of 200 - 300 m/s from a 1.0 mm diameter hole at frequencies up to 1 kHz, with a duty cycle of 20 - 80 %. A target of 80+ % manufacturing yield was also set. The actuator design was based on one developed under a previous EU RTD project AEROMEMSII and was based on a bimorph piezo-electric cantilever beam, consisting of two piezo-electric layers separated by a titanium shim. In the 'off' position, the piezo-electric beam is straight and covers the hole preventing the pressurised air from escaping. When suitable voltages are applied to the three electrodes (upper piezo-electric surface, shim, and lower piezo-electric surface) the beam bends away from the horizontal because the upper piezo-electric layer is expanding and the lower piezo-electric layer is contracting and so the hole is uncovered allowing the pressurised air to escape through the hole creating a high velocity jet. The actuator is fabricated using a combination of silicon micro-manufacturing techniques, high precision grinding, and laser machining. It is the refinement of these processes to remove previously identified shortcomings that forms the major part of this project, as the existing design of actuator (AEROMEMSII) has already been demonstrated capable of achieving two out of three of the stated objectives (jet velocity and frequency). This leaves only the objective to achieve this with a 1.0 mm orifice diameter outstanding.

Project results:

The current project produced a micro-fabricated pulsed jet actuator with an exit velocity of about 200 - 220 m/s from a 1.0 mm hole at up to 200+ Hz operating frequency and with a duty cycle that can be varied between 0 and 100 %. Between about 200 and 1500 Hz the actuator acted as a modulator rather than a valve. The fabrication yield achieved was 70+ % (13 out of 18 devices) for the 1.0 mm actuator manufacture compared to a target of 80+ %, the shortfall was largely due to the re-work required after the PZT to silicon bond failed. It is anticipated that any future production runs would meet the required target. Improvements in error checking on the laser cutting processes would also probably push the yield up to 90+ %. The other key conclusions from the project are as follows:

- Actuator deflection closely matched prediction, but the actuator did not open against as high a pressure as designed.
- The quality of the actuators manufactured in the current project was a significant improvement over the previous project.
- One of the completed actuators was tested to 10.4 million cycles without noticeable degradation in measured performance.

Four areas have also been identified that need further work to improve the manufacture of the actuators. First, the wafer grinding is a relatively expensive process, so if a large number of actuators were needed it would be worthwhile to switch from using a 2 inch diameter PZT wafer to a 4 inch diameter wafer as this would at least quadruple the yield for the same cost of grinding. Second, the laser cutting process is currently too slow, and hence expensive per device. Since at the moment most of the beam power available is not effectively used, it makes sense to develop the optics used so that multiple beams can be used in parallel, which would increase speed at least by an order of magnitude with only a moderate amount of investment. Third, although good electrical connections have been made manually in this project a semi-automatic process needs to be developed if a large number of actuators are going wired up and incorporated into test arrangements. It would also be desirable to improve the design of the connections further, so that all the connection points were flush and on the plenum or bottom side of the device. Fourth, a good and inexpensive method of making the exit holes needs to be found as laser drilling is good but expensive, and conventional drilling is cheap, but of marginal quality and difficult for slanted holes. electrical discharge machining (EDM) is probably the best candidate.

Although not directly part of the current project, an important aspect of any flow control actuator will be its suitability for flight applications. This means both its survival of the flight environment, and the requirement that it does not unduly influence any other part of the aircraft systems. Survivability of this type of actuator in the flight environment has already been considered under a previous United Kingdom project. Electromagnetic compatibility has not yet been considered as this is dependent on both the actuator, its housing, and how it is integrated into an aircraft, so it cannot be considered in the 'stand-alone' approach that has been used in the tests to date. The lessons learnt from the previous environmental tests have been incorporated into the current actuator designs and due consideration of likely electromagnetic compatibility effects has been made during the design process.

Potential impact:

Socio-economic impact

The MEMFAC project has contributed towards BAE Systems maintaining the employment of a strong team capable of developing both MEMS and flow control technologies. This in turn contributes the company's competitiveness in a global market. By the MEMFAC project being part of the Clean Sky consortium, the overall competitiveness and technological excellence of the European Aerospace Industry is also enhanced. By developing and maturing technologies appropriate to flow control on aircraft the project has also made a contribution to making commercial aviation more environmentally friendly in the future by reducing fuel burn and noise around airports, which should benefit all the citizens of the European Union (EU).

Section A: Dissemination

All the deliverable reports have been made available to the Clean Sky Consortium. It is intended that a public paper be presented on the MEMFAC work at the SMART Control based on Flow Control Manipulation Symposium, 24 - 26 June 2013, Torino, Italy.

Section B: Exploitation

It is intended that the foreground material generated under MEMFAC will be initially exploited in the work proposed under work package 2 of the AFLoNext programme recently submitted to the EU (November 2012). As well as aerospace applications BAE Systems will also seek alternative uses for the piezo-electrically driven micro-valve. For example, in areas such as medicine, where the ability to control the flow of a gas precisely within a small volume could be potentially very useful.