Scale Effects Of Insect Flapping Flight On Flight Dynamics,Stability And Control

Flying insects in nature generate lift forces by rapidly flapping their wings.Due to the extraordinary maneuverability and stability of flapping-wing insects,development of biomimetic flapping-wing micro-air-vehicles(MAVs)has draw interests of many research groups recently.However,current man-made flapping MAVs are still unstable and unpractical.Research about flight dynamics and stability of realistic insects of different scales may unveil generalized characteristics of flapping flight and further reveals their stabilization strategies,which may provide bioinspiration to flapping-wing MAVs.Most previous studies about insects’ flight dynamics,stability and control are based on simplified or even linearized single rigid body dynamic models,thus wing inertial effects on flapping flight’s aerodynamic performance and flight stability still remain poorly understood yet.In this study we address a numerical study with an integrated CFD(computational-fluid-dynamics)model of hovering flight by coupling unsteady aerodynamics and nonlinear multi-rigid-body dynamics,and conduct a systematic analysis on three insect models(fruit fly,honeybee,hawkmoth)of different scales and wing-to-body mass ratios(WBMRs).Computed results demonstrate that,for the three insects,their realistic wing mass tend to suppress body pitch oscillation to a minimized level.We further derive a scaling law to estimate the optimal WBMR and its association with power consumption,and propose a novel method to predict the power for sustaining hovering flights.Analysis about flight stability shows that different insects have similar stability characteristics: their attitude angles are unstable under longitudinal and lateral perturbations.Moreover,we find that for insects with large WBMRs like hawkmoth,the wing inertia can play a crucial role in enhancing roll stability.Simulations above are carried out considering insect body’s 6 degree-of-freedom(DOF).However,some researches point out that DOF may also exist in insect wings.So,this paper further addresses a study about a bumblebee’s 8 DOF flight stability,considering each wing’s passive rotations.Results show that the passive mechanism has little effect on longitudinal and vertical stability while lateral stability is greatly enhanced,because compensation torque will be generated by asymmetric wing angle of attack to offset the disturbance torques.Although insects may utilize some passive mechanisms to improve their flight stability,total stable hovering still need closed-loop controls.PD control has been widely proposed to explain insects’ single-axis stabilization,but whether it will work in multi-DOF still remains unclear.Here we further apply a three-axis PD-controller model in the CFD simulation of a bumblebee hovering.A simplified flight dynamic model is developed as a fast model for control parameter tuning.Our results demonstrate that the stabilizing control model is capable of achieving the hovering stabilization.A further sensitivity analysis of the control parameters reveals that yaw control via manipulating pitch angle of the wing is mostly sensitive.