Understanding the Relationship Between Pitch Agility and Propulsive Aerodynamic Forces in Bio-Inspired Flapping Wing Vehicles

Abstract

Ornithopters, or flapping wing mechanical birds, represent a unique category of aerial vehicles that fill a need for small-scale, agile, long range, and payload-capable flight vehicles. This study focuses on understanding the relationship between the propulsive aerodynamic forces and pitch agility in these flapping wing vehicles. Using analytical methods, the aerodynamic moment acting upon a wing undergoing elastic flapping was calculated. A method to determine the pitch stiffness of the vehicle was then derived using a preexisting stability analysis. This method was used to demonstrate that pitch agility in flapping wing birds is intricately tied to the flapping cycle with different parts of the cycle creating stabilizing and destabilizing effects. The results indicated that pitch agility, and propulsive force generation, have a dependency on the shape of the wing, and that deformations such as bend and sweep are capable of making the vehicle more agile. Contact-aided compliant mechanisms with nonlinear stiffness were designed and inserted into the wing of an ornithopter to induce controlled morphing. These elements have varying stiffness during the upstroke and downstroke parts of the cycle which introduces an asymmetry between the two halves of the flapping cycle. The resulting flapping motion exhibited a two fold increase in horizontal propulsive force over the baseline case. A motion tracking system was used to capture the free flight response of the ornithopter in steady level flight. This information was then used to calculate the pitch stiffness of the ornithopter with a rigid spar, and, one with a nonlinear compliant element inserted into the spar to induce a desired shape change. The results revealed that an upstroke in which the aerodynamic forces are similar in magnitude to that of the downstroke, may be necessary to make the vehicle more agile, and, that there is a compromise between vehicle agility and flight propulsive forces.