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Inspired by bat flight performance, we explore the advantages of wing twist and fold for flapping wing robots. For this purpose, we develop a dynamical model that incorporates these two degrees of freedom to the wing. The twist is assumed to be linearly-increasing along the wing, while the wing fold is modeled as a relative rotation of the handwing with respect to the armwing. An optimization scheme parameterizes the wing kinematics for 2, 5 and 8 m/s forward flight velocities. The intricate interplay between wing orientation, effective angle of attack and the ensuing lift and thrust generation are discussed. The results show that wing twist and fold alleviate negative lift and thrust in the upstroke, and in some cases producing persistent positive thrust throughout cycle for handwing. As a result, power consumption drops precipitously compared to the base case of a rigid flat plate. Another crucial realization is the relative importance of wing twist and fold in achieving efficient flight strongly depends on speeds. At slow flight, twist is significantly more effective in minimizing the power, but becomes energetically inefficient for fast speeds. The results also show that a 45° wing fold during upstroke is energetically beneficial for all speeds. The synergy of wing twist and fold are most prominent at slow flight. These findings provides useful guidelines for designing flapping wing robots. 

The paper presents a novel closed-circuit ultra-compact wind tunnel with an 8:1 contraction ratio and high flow quality. Its key design features include a 2D-type main diffuser, a minimum length contraction, and a screened expanding vane cascade in the last corner. These features reduce the overall tunnel footprint area to less than half of the area of a conventional tunnel design with the same test section size and same contraction ratio. The test section turbulence level is a very low 0.03% over a 4Hz–8500Hz bandwidth. 

The kinematics of hipposiderid bats (Hipposideros pratti) in straight and level flight has been deconstructed into a series of modes using proper orthogonal decomposition, to determine the relative importance of each mode in the overall force dynamics. Simplified kinematics have been reconstructed using different combinations of modes, and large eddy simulations were performed to compare the forces generated for each case. The first two modes (0,1) recovered only 62% of the lift, and manifested a drag force instead of thrust, whereas the first three modes (0,1,2) recovered 77% of the thrust and, unexpectedly, even more lift than the native kinematics. This demonstrates that mode 2, which features a combination of streamwise and chordwise cambering and twisting during the upstroke, is critical for the generation of lift, and more so for thrust. Detailed flow analyses reveal that the leading edge vortex and the trailing edge vortex hold the key to understanding this phenomenon. Such reduced order modeling of bat flight could provide guidelines for designing autonomous micro air vehicles which require a detailed understanding of the associated forces for the preservation of structural integrity.