Transport Research and Innovation Portal

Background & policy context

Recent developments in advanced manufacturing technology have made possible the fabrication of Microfabricated-Electro-Mechanical-Systems (MEMS) enabling sensors and actuators, having dimensions of a few hundreds of microns, to be integrated with controlling electronics. One potential application of MEMS is the active control of the thin boundary layer flow that exists on the aerodynamic surfaces of aircraft and their propulsion systems. The development of these boundary layer flows directly affects the performance of the aircraft since they give rise to skin friction drag and flow separation, which leads to buffet and limits maximum achievable lift.

MEMS technology offers, in the medium to long term, a means whereby these boundary layers can be actively controlled during certain phases of flight to achieve a performance benefit whilst not incurring penalties at other stages as is the case with more conventional passive flow control systems. An EU 4^th Framework project (AEROMEMS) undertook a “basic research” study to assess the viability of applying MEMS for boundary layer control on aircraft. The project demonstrated that:

• MEMS can be used to delay boundary layer separation in simple laboratory experiments.
• MEMS actuators and sensors could meet the necessary full-scale requirements.
• Medium term applications are aircraft high-lift systems, intakes and engine components.

Objectives

AEROMEMS II aimed to undertake industrial-scale wind-tunnel demonstrations and engineering integration assessments of MEMS flow separation control technology applied to improving the performance of wing high-lift systems, intake ducts and turbo-machinery components. A target objective was to demonstrate the ability of MEMS flow control technology to increase maximum lift by 10-15%.

Development of prototype MEMS flow sensors and actuators had to be undertaken to address the issues of robustness associated with engineering integration. Finally, aerodynamic prediction tools had to be developed and validated for use during future full-scale development. This programme was focussed on the industrial development, demonstration and assessment of the technology benefits required prior to full–scale development.

Methodology

The major work packages addressed the following areas:

• The optimisation of appropriate flow separation control actuation strategies and configurations through experimental and numerical studies.
• The development of prototype MEMS flow sensors and actuators to meet the requirement of application in large-scale wind tunnel demonstration experiments and full-scale application including addressing some of the issues of robustness associated with engineering integration.
• The undertaking of large, industrial scale experiments to demonstrate the application of MEMS flow separation control at realistic Reynolds and Mach numbers.
• The undertaking of industrial cost/benefit assessments with respect to the application of the developed technology for improving the high-lift performance of aircraft and the performance of intakes and turbomachinery components.

The studies focussed highly on the requirements for the industrial scale demonstration and assessment of the technology benefits required prior to full–scale concept development. Significant progress was made during the project, and many of these developments were embedded into the final demonstration experiments.

Research Programme

FP5 - GROWTH - KA4 (AERONAUTICS) - New Perspectives in Aeronautics

Leading institution(s)

Public institution:

European Commission, Directorate-General for Research (DG Research)

Type of funding

Public (EU)

Key results

The highlights of the project were:

• Optimisation of actuator concepts for the use of MEMS scale devices to provide flow separation control for full-scale application by undertaking detailed experimental measurements.
• Undertaking of industrial-scale wind-tunnel demonstration tests on high-lift applications of the technology to quantify the performance benefits that are realisable. These demonstrations were undertaken on configurations representative of contemporary design philosophy and made use of real MEMS sensor and actuator hardware where possible.
• The results of the industrial wind-tunnel demonstration tests were extrapolated to full-scale flight conditions. This was undertaken using numerical simulation tools.
• Prototype MEMS sensor and actuator hardware concepts were developed towards the realisation of commercially viable, robust, and reliable systems. This involved the development of two micro-valve actuator concepts to maximise their efficiency and ability to deliver high amplitude outputs. The issues of industrial packaging and the structural/electrical/pneumatic integration of the devices within an aircraft or engine were also given consideration.
• Basic level research was undertaken to explore applications of MEMS flow separation control concepts to intake ducts and engine compressors.
• Completion of industrial cost/benefit assessments and engineering integration studies. When considering benefits, not only aerodynamic improvements were explored but also the benefits resulting from simpler or more effective structural designs made possible by MEMS flow control allowing substantial savings in weight, complexity and manufacturing costs. A number of specific target applications (aircraft and engine configurations) were selected and used as the basis for an overall industrial engineering cost/benefit assessment.

The major results of the project were:

• A concept for a MEMS scale flow actuator for the control of high Reynolds number, adverse pressure gradient, turbulent flow separation has been optimised and validated in basic tests conducted in a large boundary layer wind tunnel.
• The flow actuation concept has been demonstrated to be able to achieve the nearly full reattachment of separated flow on the upper surface of a deflected trailing edge flap on a three dimensional wing/body configuration at moderate Reynolds numbers.
• The ability of MEMS scale actuation has been demonstrated to significantly control flow separation in an engine intake duct in a simple wind tunnel experiment. Reductions in flow distortion at the engine face in excess of 40% have been achieved.
• The ability of MEMS scale actuation has been experimentally demonstrated to be capable of increasing the surge margin of a turbomachine compressor stage in excess of 10%. Microfabricated hot-film flow sensors and associated miniaturised controlling anemometer electronics were successfully developed and tested. Viable industrial packaging concepts were identified and assessments of commercial fabrication methods and costs were made.
• Two prototype microfabricated flow actuator concepts were developed and viable industrial commercial fabrication methods were assessed. The actuators are microvalve devices that can be used to modulate a compressed air supply to provide “pulsed jet” actuation through an array of micro holes. The MEMS valve actuators operate on two principles one is an electrostatic device and the other operates on piezoelectric principles both are capable of controlling supply pressure differences of up to the order of 100 kPa.
• The developed flow actuation concepts has been demonstrated to be able to achieve control of the separated flow on the upper surface of a deflected leading edge slat on an unswept, wing high-lift configuration at moderate Reynolds numbers.
• The developed flow actuation concept has been demonstrated to be able to achieve the nearly full reattachment of separated flow on the upper surface of a deflected trailing edge flap on a swept, three dimensional wing/body configuration at near to flight scale Reynolds numbers.
• The overall performance gains of MEMS based flow separation control technology have been quantified for commercial high lift systems and studied for novel civil aircraft wing planforms. Through the amalgamation of the results from separate tests on the application of MEMS scale actuation to delay flow separation on leading and trailing edge devices it has been concluded that it is reasonable to anticipate overall gains in maximum lift coefficient of the order 0.15 to 0.2 and increases in Cl of the order of 0.4 in the pre-stall lift coefficient
• The effectiveness of the studied flow separation control technique is robust to variations in the definition of the flow actuators (size, spacing and location) and flow conditions.
• Small scale failure of MEMS systems has a limited impact on effectiveness for the studied flow separation control applications.
• The potential performance gains of MEMS flow separation control applied to intakes and turbo-machinery has been identified and demonstrated in preliminary low-cost facilities.
• MEMS flow sensor and actuator hardware prototypes and packaging concepts suitable for the next step towards the development of industrially oriented application concepts are now available. Estimates of size, mass, power consumption and cost have been made for commercially produced pulsed jet flow actuators. However, further work is required to demonstrate the required level of robustness and to develop full mechanical integration schemes for flow actuator devices within a practical airframe construction to allow for considerations of certification, failure, replacement and maintenance.
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• Practical engineering and systems integration concepts have been developed and key certification and operational issues were identified.

Technical implications

Future studies should focus on:

• Further developing and demonstrating MEMS flow actuator hardware to meet the challenging robustness requirements required for full scale application.
• Adopting a holistic approach to high-lift system design whereby, instead of applying flow separation control as a retrofit to an existing configuration, the entire wing is redesigned to take advantage of flow separation control. This may require the flow separation control devices to be applied to all elements of the wing high-lift system (slat, main body and flap) and the aerofoil geometry and leading edge/trailing edge device geometry and settings to be re-optimised to take full advantage of the benefits of flow control. Such an approach is potentially likely to lead to significant increases in performance to those identified in this study and ultimately lead to substantial reduction in weight and high-lift system complexity.

Partners

PROJECT CO-ORDINATOR : BAE SYSTEMS

PARTNERS: Dassault Aviation

Airbus – Germany

EADS – Military aircraft

Snecma

Auxitrol

ONERA

DLR

CNRS – LPMO

CNRS - LML

Manchester University

University of Lille (USTL-LML)

University of Warwick

TU Berlin

Cranfield University

NTUA

Contact for further information

Clyde Warsop
BAE Systems

Tel: (+44) 117 302 82 42