Design And Aerodynamic Analysis Of Compliant Mechanism Based Morphing Wings
Authors: Sharp Stephen D. 
Published: May 2021

Aircraft today use discrete control surface, typically mounted using pin and sliding joints. These designs can lead to high part-count assemblies and backlash within the assemblies that require lubrication and frequent maintenance. These wing designs also feature fixed dimensions and do not allow for geometry changes mid-flight. These limitations lead to a compromised design that must work relatively well in all situations. This causes inefficiencies in all stages of flight.

The Wright brothers, who achieved the first successful powered flight did not use these techniques. Instead they used a system on cables to apply tension and bend the wings to changes their angle of attack. They called this technique wing warping. As aviation advanced it quickly moved from the wing-warping technique towards the discrete element control surfaces. However, there is renewed interest in techniques such as wing warping as the idea of morphing wings becomes more prevalent in aerospace research. Morphing wings would allow for changing major characteristics, such as camber, span, sweep, etc. of the wing mid-flight and allow for continuous optimization through all stages of its mission.

The design covered in this thesis was centered around camber morphing of the wing in flight. Biomimicry played a large role in the design, with research into the skeletal systems of birds and fish used to dictate the rib structures. This bio-inspired path led to the use of compliant mechanisms for the ribs. This choice allowed for a low part-count and zero-backlash design that would require no maintenance and have a very long service life due to an extremely low amount of fatigue. Several design iterations were tested with different common desktop 3-D printing materials. The final rib design was made of PETG and whose compliant shape was directly inspired by the skeletal structure of the spine of a fish. The design proved to be extremely reliable and robust.

Skin design has long been one of the biggest hurdles of morphing wing design. Most research reviewed in this paper used an elastomer style skin that was pre-stretched to reduce buckling under compression. Through testing it was found that this method is difficult and unreliable to maintain a smooth and continuous surface. Even when pre-stretching, the elastomer would fatigue and buckle under compression. The final design was a PETG panel with a web and flange that would interact with the rib structure and was able to translate chordwise along the rib as the wing altered its camber. The skin had built-in flexures to reduce bending actuation forces. The wing also featured a rigid leading-edge skin panel with which the other skin panels would be able to slide under to maintain skin coverage under both extension and compression of the wing surfaces. This however led to aerodynamic problems that were discovered in the CFD analysis.

The wing was prepared for CFD using finite element analysis to produced morphed wing bodies for a 0, 10, 20, and 30-degree trailing edge deflection angles. A model was also produced of the same base airfoil (NACA 0018) with a hinged flap of 30% chord length deflected by the same amount to serve as a performance benchmark for the morphing wing. The main criteria used to evaluate the performance were the lift, drag, and lift-to-drag ratios. For the 0⁰ tests, the morphing wing had up to almost 29% higher drag at high speeds. The results showed that the 10⁰ deflection tests found up to a 115% increase in lift over the hinged flap design and a lift-to-drag ratio of up to 161% higher for the morphing wing. The 20⁰ and 30⁰ tests saw the lift advantage of the morphing wing decrease but on average across all tests, the morphing wing had a lift coefficient higher than the hinged flap by 43%. Additionally, for the large deflection tests the hinged flap had up to a 60.5% advantage in lift-to-drag ratio.

The computational fluid dynamic analysis showed that due to the larger effective angle of attack and the step-down in the skin of the morphing wing, at larger deflection angles the flow would separate much earlier along the chord. Therefore, based on the analysis, the morphing wing would create a substantial performance and efficiency gains when wing trailing edge deflection was kept below 20⁰. This meant it would be suitable for stages of flight such as takeoff and climb.

Planned future work aims to reduce the 0⁰ drag of the morphing wing as well as the early flow separation at high angles of deflection. It is assumed, that by scaling up the wing, the proportion of the step size will decrease dramatically and as a result would improve the flow characteristics. Additionally, the placement and rotational limits of the flexures can be tested further to optimize the morphed shape to reduce the severity of the adverse pressure gradient along the upper surface when in high deflection states. With continued work on improving the flow separation, this design proves promising for even high-deflection cases. Overall the V4 rib design and the accompanying compliant skin panel design were very successful for their initial tests.