Passively morphing ornithopter wings constructed using a novel compliant spine: design and testing

Abstract

Ornithopters or flapping wing uncrewed aerial vehicles (UAVs) have potential applications in civil and military sectors. Amongst the UAVs, ornithopters have a unique ability to fly in low Reynolds number flight regimes and also have the agility and maneuverability of rotary wing aircraft. In nature, birds achieve such performance by exploiting various wing kinematics known as gaits. The objective of this work is to improve the steady level flight performance of an ornithopter by implementing a continuous vortex gait using a novel passive compliant spine inserted in the ornithopter’s wings. This paper presents an optimal compliant spine concept for ornithopter applications. A quasi-static design optimization procedure was formulated to design the compliant spine. Finite element analysis was performed on a first generation spine and the spine was fabricated. This prototype was then tested by inserting it into an ornithopter’s wing leading edge spar. The effect of inserting the compliant spine into the wings on the electric power required, the aerodynamic loads and the wing kinematics was studied. The ornithopter with the compliant spines inserted in its wings consumed 45% less power and produced an additional 16% of its weight in mean lift compared to the same ornithopter without the compliant spine. The results indicate that this passive morphing approach is promising for improved steady level flight performance.

Flight Testing of Novel Compliant Spines for Passive Wing Morphing on Ornithopters

“…Further data analysis shows that when the mean acceleration over one flapping cycle is computed, the ornithopter with Comp 24PM and Comp 4PM inserted in its wing reduced the body's center of mass positive acceleration by 69% and 5%, respectively. The positive acceleration reduction translates into overall lift gains, which would confirm previous bench test results 7 .…”

“…The data collected were used to assess the effect of the compliant mechanism on wing kinematics. Wing kinematics data were previously collected on a bench top where the ornithopter fuselage was clamped to a six degree of freedom (DOF) load cell 7,8 . The purpose of the flight test described in this paper is to understand the effect of the compliant mechanisms during free flight and to determine whether the previous bench test performance relates directly to free flight performance, as well as to increase the understanding of the flight physics and dynamics of flapping wing unmanned vehicles.…”

“…The two letters in the name refer to the loading condition under which this compliant spine was Table 2 lists the compliant spine designs, their loading condition, the number of compliant hinges, and the total mass of the ornithopter with these spines inserted in its leading edge and including all the components that was previously present in the solid configuration. Design 4TL was previously tested during the bench test 7 and therefore was included in the free flight test to aid in the comparison between the bench and free flight testing results. Table 3 shows all four compliant spine designs and their predicted stress distribution at mid upstroke.…”

Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates

“…(d ) A robot with artificial feathers uses asymmetric morphing for roll control [157]. (e) The UMD big bird folds the wings during the upstroke to increase efficiency [169]. (f ) The Nano-hummingbird controls its flight by adjusting flapping wing parameters and requires no tail for flight stability [13].…”

“…These wings typically have flexible membrane-based wings with simple up and downstroke kinematics inspired by insect flight [93,171,172]. Others offer more vertebrate-like features such as a hinge in the spar enabling wing folding during the upstroke, and increasing efficiency by reducing drag (figure 8e) [169]. Alternatively, a wrist oriented in the sweep direction enables recovery from obstacle impact [173].…”