Design and analysis of aerodynamic force platforms for free flight studies

Abstract

We describe and explain new advancements in the design of the aerodynamic force platform, a novel instrument that can directly measure the aerodynamic forces generated by freely flying animals and robots. Such in vivo recordings are essential to better understand the precise aerodynamic function of flapping wings in nature, which can critically inform the design of new bioinspired robots. By designing the aerodynamic force platform to be stiff yet lightweight, the natural frequencies of all structural components can be made over five times greater than the frequencies of interest. The associated high-frequency noise can then be filtered out during post-processing to obtain accurate and precise force recordings. We illustrate these abilities by measuring the aerodynamic forces generated by a freely flying bird. The design principles can also be translated to other fluid media. This offers an opportunity to perform high-throughput, real-time, non-intrusive, and in vivo comparative biomechanical measurements of force generation by locomoting animals and robots. These recordings can include complex bimodal terrestrial, aquatic, and aerial behaviors, which will help advance the fields of experimental biology and bioinspired design. NOTE Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

Biomechanics of hover performance in Neotropical hummingbirds versus bats

“…Capacitive sensors [MicroSense model 8800 electronics module with model 2805 probes (resolution, ~0.8 nm), National Instruments USB-6210 DAQ, Lenovo ThinkPad T440s, and MATLAB R2015b] sampled spring displacements at 10 kHz, which were then converted into forces using the respective spring stiffness. The instantaneous sum of forces on the top and bottom force plates is equal to the instantaneous vertical aerodynamic force generated by the hovering animals ( 14 , 35 , 36 ). Hummingbird flights were recorded at 2000 frames per second (fps) using Phantom Miro M310 and LC310 cameras.…”

“…First, step responses from the perch and feeder during takeoff and landing were used to determine hummingbird weights. Next, aerodynamic forces were filtered offline at 180 Hz (about six times their wingbeat frequency) using an eighth-order digital low-pass Butterworth filter to isolate animal frequencies from structural frequencies of the setup ( 35 ). Temperature drift caused flexural springs to slowly expand and contract during recordings.…”

The aerodynamic force platform as an ergometer

“…All these parameters are informative for evaluating the in vivo external work performed by flight muscles in the context of the overall flight mechanics of the animal, which no model can currently offer with comparable accuracy and precision. For an optimal implementation to evaluate the aerodynamic performance of flying animals, we recommend following the AFP design guidelines in Hightower et al (2017). The current AFP volume is a smaller design, meaning that the aerodynamic power exerted by the animal is lower than during free flight in the atmosphere as a result of wall effects (Hightower et al, 2017).…”

“…For an optimal implementation to evaluate the aerodynamic performance of flying animals, we recommend following the AFP design guidelines in Hightower et al (2017). The current AFP volume is a smaller design, meaning that the aerodynamic power exerted by the animal is lower than during free flight in the atmosphere as a result of wall effects (Hightower et al, 2017). However, when the main goal of the study is to evaluate muscle function using the AFP as an ergometer, our method succeeds in accurately measuring the aerodynamic power produced by the bird as it flies in the AFP, and this smaller volume design has the benefit of providing better time resolution.…”

“…Thus, they sum up to the time-resolved net aerodynamic force generated inside by the dove (Lentink, 2018;Lentink et al, 2015). The pressure and shear stress distributions on the walls result from the motion of the dove, which generates airflow inside that is updated with the speed of sound (Hightower et al, 2017;Lentink, 2018;Lentink et al, 2015). The associated phase delay due to the traveling time of the acoustics waves in the volume is on the order of 1.5 ms (for 0.5 m distance from the wall and a speed of sound of 340 m s −1 ).…”