Project Highlight

Low Speed Aerodynamics

Airfoil morphing by MCF actuators

The ability to change the shape of a wing allows adapting it to different flight conditions. Thus much research is devoted to optimize different types of wing morphing for improving the aircraft performance. A major difficulty in implementing wing morphing 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 wing 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.

Figure 1: MFC actuator [image courtesy of Smart Material Corp.].

The main scope of this research is exploring the use of MFC actuators integrated to the skin of a wing for changing its shape to the degree required for tailoring its performance. To this aim different airfoil models have been designed and constructed 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.

Figure 2: NACA-4415 airfoil model for testing the change of shape of the upper skin: assembled model (left) and inner side of the upper skin with MFC patches (right).

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 airfoil without MFC actuation is close to that of the NACA-4415 type.

Figure 3: Schematic diagram of upper-surface actuation by MFC patches.

More recently symmetric airfoil models have been made that are capable of changing the shape of both the upper and the lower skins, Fig. 4.

Figure 4: NACA-0015 airfoil model for testing the change of shape of both the upper and the lower skins: assembled model (left) and inner side of one skin with MFC patches (right).

Clicking the play button of 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).


Figure 5: Airfoil model performing an excerpt of the "Dance of the Flowers" from Tchaikovsky's The Nutcracker.
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Preliminary wind-tunnel measurements have been done 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 response to actuation comparable to that observed in still air. Figure 6 shows the lift over drag ratio of a model similar to that of Figs. 4 and 5. 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 be the mirrored data of the negative-camber case, and vice-versa. As expected, larger values of the lift over drag ratio can be obtained by increasing the camber of the airfoil at angles of attack typical of cruise conditions.


Figure 6: Lift over drag ratio of airfoil model without actuation (symmetric) and with actuation to obtain positive and negative camber.

Other aerodynamic characteristics, not shown here, indicate that such performance is achieved without deteriorating the static stability of an aircraft. These data validate the idea that shaping of the surfaces of an airfoil 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 airfoils that quickly deteriorate at off-design conditions. The ultimate goal of this ongoing research is to develop adaptable wings that provide increased aircraft endurance, range, higher maneuverability, or a combination of these characteristics.

Additional information on this research can be found in:

  • 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.
  • 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., 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.