Butterflies fly using efficient propulsive clap mechanism owing to flexible wings

Area of Science:

• Aerodynamics
• Biomechanics
• Insect Flight

Background:

• Butterflies possess unique wing morphology, with large, broad wings relative to body size.
• Unsteady aerodynamic mechanisms, particularly the upstroke wing clap, are hypothesized to enhance butterfly flight.
• Quantitative aerodynamic data for the wing clap in freely flying butterflies has been lacking.

Purpose of the Study:

• To provide quantitative aerodynamic performance estimates for the butterfly wing clap.
• To investigate the role of wing flexibility in the wing clap mechanism.
• To offer a mechanistic hypothesis for butterfly wing morphology and its implications for flight.

Main Methods:

• Quantitative flow measurements were conducted behind freely flying butterflies during take-off.
• A mechanical clapper was used to simulate and analyze the wing clap.
• Aerodynamic performance was estimated using data from both free flight and mechanical simulations.

Main Results:

• The butterfly wing clap generates forward thrust, with the downstroke supporting weight.
• Flexible butterfly wings, when cupped during the upstroke and clap, significantly increase useful impulse (+22%) and efficiency (+28%) compared to rigid wings.
• The study provides the first quantitative aerodynamic measurements of the wing clap in freely flying butterflies.

Conclusions:

• Butterflies have evolved a highly effective wing clap mechanism, leveraging wing flexibility for enhanced propulsive performance.
• The findings provide a mechanistic explanation for the evolution of butterfly wing morphology.
• The study's insights could inform the design of more efficient, bio-inspired flapping drones.