Aerodynamic efficiency explains flapping strategies used by birds.

Area of Science:

• Biomechanics
• Aerodynamics
• Robotics

Background:

• Birds adjust wingbeat kinematics to meet changing aerodynamic demands during flight.
• The relative efficiency of different kinematic strategies for force generation remains poorly understood.

Purpose of the Study:

• To evaluate the efficiency of various wingbeat strategies for adjusting the thrust-to-lift ratio.
• To investigate the relationship between propulsive efficiency, Strouhal number, and overall force generation efficiency.

Main Methods:

• Utilized a robotic wing system for controlled kinematic experiments.
• Employed quantitative flow measurements to analyze aerodynamic forces and efficiency.

Main Results:

• Modulating wingbeat elevation was found to be the most efficient strategy for altering the thrust-to-lift ratio.
• Propulsive efficiency may peak at a Strouhal number of 0.3, but overall force generation efficiency decreases with increasing Strouhal number.
• The optimal Strouhal number appears to be dependent on the thrust-to-lift ratio, not a fixed value.

Conclusions:

• Wingbeat elevation adjustments offer an efficient mechanism for birds to adapt to varying flight conditions.
• Findings challenge the concept of a single optimal Strouhal number for flapping flight, suggesting it is context-dependent.
• Results provide insights into avian flight variations and suggest novel flapping strategies for enhancing drone performance and range.