Aeromechanics and aeroacoustics of maneuvering hummingbirds
Keywords: Mechanical Engineering.

Nature has bestowed upon hummingbirds and other nectarivorous animals a singular ability to sip energy rich nectar by hovering in front of flowers. However, these flowers are not static: they have different orientations and move in unpredictable ways due to environmental wind as well as self-induced wind from the hovering animal. Hovering is unstable and extraordinarily energetically costly, so these animals must be able to track the moving flowers effectively. To study this phenomenon, we built a robotic flower that moves laterally and longitudinally to investigate how hummingbirds track moving flowers and how this compares to other animals, as it has been shown that hawkmoths can effectively track laterally-moving flowers in natural wind. Sound plays a key role in animal survival, as it is produced naturally during locomotion. Some animals maximize these sounds for communication, while others minimize them for predator avoidance. We performed nearfield acoustic holography measurements on hovering and maneuvering hummingbirds tracking moving flowers, for which we found hummingbirds modulate their wingbeat to track moving objects. We created a first principles acoustic model to further consider how hummingbirds hum and generate sound, which illustrated their hum can be explained by the sound associated with moving aerodynamic forces (lift and drag) acting on each wing. This model is expanded to include other flapping wing animals, ranging from fruit flies to parrotlets, allowing us to investigate effects over a wide range of wingspans, masses, flapping frequencies, and flapping styles. We found an important link between the weight support profile and radiated power: higher harmonic frequency content in the weight support profile means the animals radiates more acoustic power, which is important for some insects like flies which communicate with their wing noise and have a rich amount of harmonic content in their wingbeat. Over the orders of magnitude considered we found that the radiated acoustic power scaled most strongly with weight, then wingspan, and finally wingbeat frequency. While the acoustics can inform us about the forces indirectly through pressure, we would like to directly measure the in vivo lift and drag forces, especially since acoustic methods are unable to measure DC pressure offsets. To accomplish this feat, we developed a new instrument: the 3D Aerodynamic Force Platform (AFP). The flapping animal generates a pressure field which travels to the boundaries of the rectangular flight arena at the speed of sound. Each of the six instrumented plates mechanically integrates the pressure distribution, which is measured by three force sensors on each plate. By using an AFP, we can measure the forces associated with maneuvering while tracking a moving flower. Wing kinematics can be captured in conjunction by using an array of 3D-calibrated high-speed cameras. While flower orientation is important ecologically for the coevolution of flowers and hummingbirds, it is unclear how the forces are associated with that change. While horizontal flowers offer a single entry point for the hummingbird to feed, and through evolution can optimize the location of their stamen to increase the chances of a successful pollination, vertically oriented flowers lack this degree of freedom. These flowers instead have a radially symmetric stamen orientation. Flowers oriented vertically upwards have been shown to be more attractive to nectarivores, while the majority of downwards facing (pendant) flowers are pollinated solely by hummingbirds. We compared the power requirements for hummingbird feedings from horizontal flowers, upwards flowers, and pendant flowers. Hummingbirds incur a 14% increased aerodynamic power requirement for feeding from pendant flowers and a 25% increase for feeding from upwards flowers