Aerodynamic explanation of flight speed limits in hawkmoth-like flapping-wing insects

Abstract

It has long been unknown why the hawkmoth's maximum forward flying speed is much lower than the theoretical prediction based on its body mass. Our computational fluid dynamics study revealed that as a hawkmoth's flight speed increases, its wings inevitably generate a significant amount of negative lift during the upstroke, which renders the hawkmoth incapable of sustaining steady forward flight. A similar trend has also been observed for other insects, including fruit flies and bumblebees. However, birds and other flying vertebrates are able to overcome this limitation by flexing their wings during the upstroke.