Upstroke wing clapping in bats and bat-inspired robots offers efficient lift generation

Area of Science:

• Aerodynamics
• Bio-inspired engineering
• Robotics

Background:

• Efficient flight in birds and bats relies on complex wing articulation.
• Ventral wing clapping, observed in avian flight, has potential aerodynamic benefits yet to be fully understood.

Purpose of the Study:

• To investigate the aerodynamic impact of variable wing folding and clapping on flapping wing platforms.
• To explore lift and power economy variations with wing folding magnitude and Strouhal numbers.

Main Methods:

• Developed a two-degree-of-freedom flapping wing platform with variable wing folding.
• Employed quasi-steady modeling, direct force/power measurements, and particle image velocimetry (PIV) experiments.
• Conducted wind tunnel tests across a range of Strouhal numbers (St = 0.2-0.4).

Main Results:

• Wing clapping during the upstroke generates an air jet, significantly increasing lift.
• Lift increases monotonically with wing folding amplitude.
• Power economy shows complex behavior, with optimal folding strategies varying by Strouhal number.

Conclusions:

• Wing folding and clapping offer distinct strategies for lift augmentation and propulsion in flapping flight.
• Findings provide insights for designing more efficient bio-inspired flapping wing robots.
• Identified symmetry-breaking lift augmentation and appendage-based jet propulsion as key mechanisms.