Over the past several years there has been an increasing interest in the development of miniature air vehicles (MAVs) with flapping wings. To allow these MAVs to adjust to changes in wind direction and to maximize their efficiency, it is desirable to monitor the deformation of the wing during flight. This paper presents a step in this direction, demonstrating the measurement of strain on the surface of the wing using minimally invasive compliant piezoresistive sensors. The strain gauges consisted of latex mixed with electrically conducting exfoliated graphite, and they were applied by spray coating. To calibrate the gauges, both static and dynamic testing up to 10 Hz were performed using cantilever structures. In tension the static sensitivity was a linear 0.4 Ω με−1 and the gauge factor was 28; in compression, the gauge factor was −5. Although sensitivities in tension and compression differed by a factor of almost six, this was not reflected in the dynamic data, which followed the strain reversibly with little distortion. There was no attenuation with frequency, indicating a sufficiently small time constant for this application. The gauges were thin, compliant, and light enough to measure, without interference, deformations due to shape changes of the flexible wing associated with generating lift and thrust. During flapping the resistance closely tracked the generated thrust, measured on a test stand, with both signals tracing figure-8 loops as a function of wing position throughout each cycle.

Currently, flapping wing unmanned aerial vehicles (a.k.a., ornithopters or robotic birds) sustain very short duration flight due to limited on-board energy storage capacity. Therefore, energy harvesting elements, such as flexible solar cells, need to be used as materials in critical components, such as wing structures, to increase operational performance. In this paper, we describe a layered fabrication method that was developed for realizing multifunctional composite wings for a unique robotic bird we developed, known as Robo Raven, by creating compliant wing structure from flexible solar cells. The deformed wing shape and aerodynamic lift/thrust loads were characterized throughout the flapping cycle to understand wing mechanics. A multifunctional performance analysis was developed to understand how integration of solar cells into the wings influences flight performance under two different operating conditions: (1) directly powering wings to increase operation time, and (2) recharging batteries to eliminate need for external charging sources. The experimental data is then used in the analysis to identify a performance index for assessing benefits of multifunctional compliant wing structures. The resulting platform, Robo Raven III, was the first demonstration of a robotic bird that flew using energy harvested from solar cells. We developed three different versions of the wing design to validate the multifunctional performance analysis. It was also determined that residual thrust correlated to shear deformation of the wing induced by torsional twist, while biaxial strain related to change in aerodynamic shape correlated to lift. It was also found that shear deformation of the solar cells induced changes in power output directly correlating to thrust generation associated with torsional deformation. Thus, it was determined that multifunctional solar cell wings may be capable of three functions: (1) lightweight and flexible structure to generate aerodynamic forces, (2) energy harvesting to extend operational time and autonomy, and (3) sensing of an aerodynamic force associated with wing deformation.

Flapping wing aerial vehicles (FWAVs) may require charging in the field where electrical power supply is not available. Flexible solar cells can be integrated into wings, tail, and body of FWAVs to harvest solar energy. The harvested solar energy can be used to recharge batteries and eliminate the need for external electrical power. It can also be used to increase the flight time of the vehicle by supplementing the battery power. The integration of solar cells in wings has been found to alter flight performance because solar cells have significantly different mechanical and density characteristics compared to other materials used for the FWAV construction. Previously, solar cells had been successfully integrated into the wings of Robo Raven, a FWAV developed at the University of Maryland. Despite changes in the aerodynamic forces, the vehicle was able to maintain flight and an overall increase in flight time was achieved. This paper investigates ways to redesign Robo Raven to significantly increase the wing area and incorporate solar cells into the wings, tail, and body. Increasing wing area allows for additional solar cells to be integrated, but there are tradeoffs due to the torque limitations of the servomotors used to actuate the wings as well changes in the lift and thrust forces that affect payload capacity. These effects were modeled and systematically characterized as a function of the wing area to determine the impact on enhancing flight endurance. In addition, solar cells were integrated into the body and the tail. The new design of Robo Raven generated a total of 64% more power using on-board solar cells, and increased flight time by 46% over the previous design. They were also able to recharge batteries at a similar rate to commercial chargers.

Flapping wing air vehicles offer many useful flight characteristics due to their versatility, as proven by flying animals. Wing design significantly influences the performance. However designing successful wings presents significant challenges. Efficient matching of the drive motors to the flapping wings is necessary to overcome the highly constrained weight budget. Simulating detailed information about the force response due to flapping is challenging due to complex fluid-structural interactions of the wings resulting in non-linear force response to flapping motion. To overcome this challenge, we conducted an experimental study of flapping wings to provide detailed temporal force response data for flapping wings. A prototype was built by synthesizing lightweight manufacturing techniques with the results of the experimental study. Our experimental investigations enabled us to select the flapping angle range and flapping frequency.

Flapping wing unmanned air vehicles (UAVs) are small light weight vehicles that typically have short flight times due to the small size of the batteries that are used to power them. During longer missions, the batteries must be recharged. The lack of nearby electrical outlets severely limits the locations and types of missions that these UAVs can be flown in. To improve flight time and eliminate the need for electrical outlets, solar cells can be used to harvest energy and charge/power the UAV. Robo Raven III, a flapping wing UAV, was developed at the University of Maryland and consists of wings with integrated solar cells. This paper aims to investigate how the addition of solar cells affects the UAV. The changes in performance are quantified and compared using a load cell test as well as Digital Image Correlation (DIC). The UAV platform reported in this paper was the first flapping wing robotic bird that flew using energy harvested from on-board solar cells. Experimentally, the power from the solar cells was used to augment battery power and increase operational time.