Some cool projects over the years!

Where are my wings?

The Wright brothers dawned the aviation age from keen observation of bird flight. Now in 2024, five decades after the moon landing, with interplanetary travel on the horizon, there is still no aerial robots that remotely compete with birds or bats. The irony of rocket science outpacing the seemingly simpler science of animal flight casts an unsatisfying shadow over the progress we have made. The reality is that birds and bats have deceived us all by their effortless grace, as bio-inspired mimicry and mechanics are highly interdisciplinary and challenging studies. It demands a fundamental understanding of unsteady aerodynamics, robust robotic design and flight control, and the appreciation towards reduced-order modeling for nonlinear dynamics, which lends itself to data-driven approaches. To make matter more complicated, all these efforts are deeply coupled, and need to weave together to create faithful bio-inspired, autonomous robots.

Bio-inspired jet propulsion

Wing articulation is critical for efficient flight of bird- and bat-sized animals. Inspired by the flight of Cynopterus brachyotis, the lesser short-nosed fruit bat, we built a three-degree-of-freedom flapping wing platform with variable wing folding and twisting capability. In late upstroke, the wings ”clap” and produce an air jet that significantly increases lift production, with a positive peak matched to that produced in downstroke. We used multiple approaches – quasi-steady modeling, direct force/power measurement, and PIV experiments in a wind tunnel – to understand critical aspects of lift/power variation in relation to wing folding magnitude over Strouhal numbers between 𝑆𝑡 = 0.2 − 0.4. While lift increases monotonically with folding amplitude in that range, power economy (ratio of lift/power) is more nuanced. At 𝑆𝑡 = 0.2 − 0.3, it increase with wing folding amplitude monotonically. At 𝑆𝑡 = 0.3 − 0.4, it features two maxima – one at medium folding amplitude (∼ 30◦), and the other at maximum folding. These findings illuminate two strategies available to flapping wing animals and robots – symmetry-breaking lift augmentation and appendage-based jet propulsion. (This work is in revision as requested by Journal of Royal Society Interface, arXiv link: http://arxiv.org/abs/2411.01434)

Subsequently, we found this jet can be directed by controlling the wing twist at the moment of clapping, which leads to greatly enhanced cycle-averaged thrust, especially at high 𝑆𝑡 or low flight speeds. Additional benefits of more thrust and less negative lift are gained during upstroke using wing twist. Remarkably, less total actuating force, or less total power, is required during upstroke with wing twist. These findings emphasize the benefits of critical wing articulation for the future flapping wing/fin robots and for an accurate test platform to study natural flapping wing flight or underwater vehicles. (This work is presented at IROS 2024, with the my talk recording below. The arXiv link: https://www.arxiv.org/abs/2408.15577)

Flapping wing robot Kestrel

The plan of the ornithopter, called kestrel, was patented by an inventor called Andrew Kinkade, in the 2001. I built it as a hobby. It weighs around 430 grams, and mounts a BLDC motor that drives the wing flapping up and down via a four-bar linkage mechanism. There are two servos that controls the tail to pitch and roll

This video was taken on August, 25th, 2018, in front of a lawn of the stadium in Sichuan University, China In this video, right after launching, Kestrel started to descend, then I immediately increased the throttle, which effectively increased the flapping frequency. It started to ascend, albeit slowly.

Then, I steered it to the right by controling the tail surface to roll to right. Towards the end of the video, the carbon rod snapped from the shoulder joint, potentially due to siginificant shear force at the base of the wing root.

Computational fluid dynamics on flapping flight

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.