How Does a Bat Fly?

Introduction

Bats are the only flying mammals in the entire world. Moreover, they can be incredibly dexterous fliers, able to wheel and pivot on short notice in order to capture insects in mid-flight. Since they're mostly nocturnal, they need to do it in the dark to boot. How do they accomplish that? The answer lies in the way their body structure adheres to principles of aerodynamics.

Anatomy

Bats' flight abilities stem from the structure of their wings, which is extremely efficient and can be operated with very little expenditure of energy. Their lightweight forelimb bones are elongated, with a very flexible wrist bone and a very small ulna which is strongly fused to the pelvis. The pelvis bones themselves are fused more strongly than they would be in other mammals. That provides strong structural support for the wings, while allowing them to be folded and unfolded very easily by using just a single muscle. That lets bats flap their wings extremely quickly and stay in flight for extended periods of time.

Lift

Different types of bats flying in different ways. Larger fruit bats, for example, tend to fly in straight lines, seeking out their food and then returning the way they came. Smaller insect-hunting bats, on the other hand, will dart and weave in the air in pursuit of prey. In all cases, however, their wings operate on the principle of lift. A bat's wing structure is curved, similar to an airplane, which means that air passing above it moves faster than the air passing below it. That reduces the air pressure above the bat's wings, while increasing the air pressure below it, pulling the bat upward.

Thrust

Flapping also answers the second half of the flying equation, which is thrust. As the wing moves, the angle causes it to press against the air, similar to the way an oar presses against the water. That drives the bat's movement forward, allowing it to truly fly. Thrust is typically created on the downstroke of the wing, when it is broadest and flattest against the air. Lift is generated on the upstroke when the wing is narrower. The bat can adjust the particular tilt and angle of its wing in order to trade off between lift and thrust, thus controlling the speed and direction in which it flies.

Size Factors

The size of the bat and its wings plays a further role in the precise way in which it flies. The larger the bat's size, the more weight it has to get off the ground and the greater the length its wings must be. The slower it needs to fly, the faster it needs to flap, so smaller bats with short wings are able to fly much more slowly (and thus turn, pivot and hover more easily) than larger bats with long broad wings. Flight dynamics also means that bats can only reach a certain size and still be able to fly, limiting their overall size.