Analyzing the Aerodynamic Performance of Flexible Finite Wings

Deliverables

Premise

Within the Aerodynamics Laboratory (AAE 33401) curriculum, students are able to design their own experiments to conduct, so long as they relate to the field of aerodynamics. For this experiment, I chose to do a study on the aerodynamic performance of flexible wings alongside my lab group. The flexible wings in question were additively manufactured with thermoplastic polyurethane (TPU) and had varying stiffnesses depending on the amount of shells and infill. The study intends to determine whether a flexible wing, like one made of TPU, could be a viable wing material for small-scale UAVs.

TPU provides excellent impact resistance, making it ideal for creating a durable UAV. However, it's unclear how the wing would behave in high freestream speeds because it would be prone to deformation from the aerodynamic forces. Therefore, this study is necessary to determine how that potential deformation can impact the aerodynamic performance of the wing.

Development

Four different wings were additively manufactured, with all four using the NACA 0012 airfoil profile for their cross-section with a special adapter design created in Autodesk Inventor to attach it to wind tunnel pylons. The first wing acts as the control group; it was intended to be a brittle wing with no flexibility, so it was manufactured with polylactic acid (PLA) filament. The next three wings were the experimental group, being manufactured out of TPU. The wings were given differing wall loops (shells) varying from 1-3 loops while the infill percentage ranged from 5-7%. For example, the most flexible wing had only 1 wall loop and 5% infill.

A Bambu Lab A1 Mini was used to manufacture these wings in two separate six-inch long male/female halves that were fused together via a pin connector. Then, the wing assemblies were placed inside of a wind tunnel located inside of Purdue University's Aerospace Sciences Laboratory. Each wing faced a freestream velocity of around 12 m/s, while their angles of attack ranged from -4° to 16°. An additional test was performed on the most flexible wing at a freestream velocity of approximately 21 m/s to investigate structural anomalies that the wing might face at high velocities.

Outcomes

A report was written to highlight the findings of this experiment. The report highlighted the surprising finding that the most flexible wing actually yielded the highest lift coefficient out of all of the wings, although it had the largest drag penalties due to stall. My theory for why this might have been the case was due to the deformation of the trailing edge of the wing. In particular, the trailing edge is forced downwards at high angles of attack, allowing it to effectively behave the same as flaps. This was further proven not only by our additional high speed (21 m/s) test of the least flexible wing, which showed that the trailing edge experiences an oscillating, fluttering effect from the structure yielding to the aerodynamic forces, but also through a rudimentary finite element analysis done on the wing, which confirmed that the trailing edge could deform by at least 0.07 inches when aerodynamic forces are applied to it.

Nevertheless, this experiment proved that in subsonic, incompressible flight regimes, a flexible wing, like one made of TPU, could provide sufficient aerodynamic properties for a low-speed UAV or RC plane while also offering excellent damage mitigation during crashes.