The ability to change the shape of an aerofoil allows adapting it to different flow conditions and is typically used for improving the performance of aircraft wings. Morphing techniques have been explored to seamlessly change the shape of aerofoils between different geometries. A major difficulty in their implementation is represented by the size, weigh, complexity, and power demanded by appropriate actuators (hydraulic, pneumatic, electric motors, or even some smart materials like shape-memory alloys). From a system-integration point of view the performance increase offered by the morphing must out-weight its associated penalties.
The macro fiber composite (MFC) actuators, originally developed at NASA, are thin, light, and flexible piezoelectric actuators, Fig. 1, that appear promising for implementing different types of aerofoil morphing, especially if limited shape changes are required. When embedded in a surface or attached to flexible structures, the MFCs provide distributed deflection and vibration control. They can also be used as sensors to measure the structural strain under applied loads.

This research uses MFC actuators for changing the shape of aerofoils as required for improving their performance or reducing their undesirable aerodynamic characteristics. To this aim I have designed and constructed different aerofoil models with flexible skins whose shape can be changed by MFC actuators bonded to their inner side. Asymmetric (cambered) models have been initially used for testing the change of the shape of the upper skin only, Fig 2.

The skin deflects inward when a positive voltage is applied to the MFCs, whereas it deflects outward when a negative voltage is applied, Fig. 3. The deflection causes the skin to have a small variation in the longitudinal direction which is accommodated by allowing it to slide in a thin pocket at the trailing edge. The geometry of the aerofoil without MFC actuation is close to that of the NACA 4415 type.

Symmetric aerofoil models have also been made that are capable of changing the shape of both the upper and the lower skins, Fig. 4.

Clicking on the following figure shows a movie of the model above which changes its shape and "dances" in a windtunnel (smoke is used to visualize the flow).

Wind-tunnel measurements of these models have been taken at freestream velocities up to 15 m/s for values of the angle of attack ranging from -16° to 16°. In this range of velocities and angles of attack the skins of all the models did not exhibit any vibration or anomalous deformation and maintained a good response to actuation.

Figure 6 shows the lift over drag ratio of the model above. Changing the shape allows controlling the aerodynamic characteristics and larger values of the lift over drag ratio can be obtained by increasing the camber of the aerofoil at angles of attack typical of aircraft cruise conditions. The values for negative angles of attack can be obtained by mirroring the data of Fig. 6 about the two axes such that the data of the positive-camber case would mirror into the data of the negative-camber case, and vice-versa.

Other aerodynamic characteristics, not shown here, indicate that such performance is achieved without deteriorating the static stability if applied to an aircraft. These data validate the idea that shaping an aerofoil by MFC actuators can be useful for tailoring or augmenting its aerodynamic performance. In particular, this technique could be very beneficial to broaden and stabilize the useful aerodynamic characteristics of high-performance aerofoils that quickly deteriorate at off-design conditions. The long term goal is developing smart wings and rotating blades capable of autonomous shaping for self-adjusting to changing flow conditions.

Additional information on this research can be found in:

• Jones G., Santer M., Debiasi M. and Papadakis G., “Control of flow separation around an aerofoil at low Reynolds numbers using periodic surface morphing”, Journal of Fluids and Structures, Vol. 76, pp. 536-557, January 2018. [DOI: 10.1016/j.jfluidstructs.2017.11.008]
• Debiasi M., Chan W. L. and Jadhav S., “Measurements of a Symmetric Wing Morphed by Macro Fiber Composite Actuators”, AIAA Paper 2016-1565, 54th AIAA Aerospace Sciences Meeting, San Diego, California, 4-8 January 2016. [DOI: 10.2514/6.2016-1565]
• Jones G., Santer M., Papadakis G. and Debiasi M., “Active Flow Control At Low Reynolds Numbers By Periodic Airfoil Morphing”, AIAA Paper 2016-1303, 54th AIAA Aerospace Sciences Meeting, San Diego, California, 4-8 January 2016. [DOI: 10.2514/6.2016-1303]
• Jones G., Debiasi M., Bouremel Y., Santer M. and Papadakis G., “Open-Loop Flow Control At Low Reynolds Numbers Using Periodic Airfoil Morphing”, AIAA Paper 2015-1933, 533d AIAA Aerospace Sciences Meeting, Kissimmee, Florida, 5-9 January 2015. [DOI: 10.2514/6.2015-1933]
• Bouremel Y., Chan W. L., Jones G. and Debiasi M., “Measurements of a Symmetric Airfoil Morphed by Macro Fiber Composite Actuators”, AIAA Paper 2014-3246, 32nd AIAA Applied Aerodynamics Conference, Atlanta, Georgia, 16-20 June 2014. [DOI: 10.2514/6.2014-3246]
• Debiasi M., Bouremel Y., Lu Z., and Ravichandran V., “Deformation of the Upper and Lower Surfaces of an Airfoil by Macro Fiber Composite Actuators”, AIAA Paper 2013-2405, 31st AIAA Applied Aerodynamics Conference, San Diego, California, June 2013.
• Debiasi M., Bouremel Y., Khoo H. H., and Luo S. C., “Deformation of the Upper Surface of an Airfoil by Macro Fiber Composite Actuators”, AIAA Paper 2012-3206, 30th AIAA Applied Aerodynamics Conference, New Orleans, Louisiana, June 2012.
• Debiasi M., Bouremel Y., Khoo H. H., Luo S. C., and Tan E. Z., “Shape Change of the Upper Surface of an Airfoil by Macro Fiber Composite Actuators”, AIAA Paper 2011-3809, 29th AIAA Applied Aerodynamics Conference, Honolulu, HI, June 2011.