Modelling the flight of the bat

Consider the wings of large moth, a bat, and a hummingbird. Which do you think would require the least amount of energy to keep in flight? If you guessed a bat, you were right! In terms of how much energy they must expend in order to stay in flight, bats have the most efficient design. Scientists hypothesize that this is because of the many bones of the bat's wings and the flexible membrane stretching between the bones.

Bat wings are complex, much more complex than those of insects and birds. They consist of 25 specialized joints in the wings. Bats also use their hind limbs, back feet and fingers to control the shape of the wing and the angle of their flight. Their stretchy wings allow them to capture air on the down stroke like a parachute, and to release that air on the upstroke. This makes a bat wing two to three times more efficient than the rigid airfoil wing of an airplane. To gain speed when they fly, bats pull the thin flexible membrane of their wings so that the entire wing is shorter and straighter. This reduces drag, which is the friction between the wing and the air through which it travels. The specialized structure of bat wings also makes it possible for them to carry relatively heavy loads, fly long distances, and to dodge through tight spots between tree branches.

Engineering Bat Flight

Engineers are interested in learning more about how bats fly so that they can build flying machines that are more flexible and able to withstand impact. One of their goals is to one day use what they learn about bat flight to build bat-sized drones. Scientists and engineers at Brown University are collaborating to reveal some of the secrets of bat flight. They have observed the motion of bat wings by placing sensors on the underside of the wings of bats and filming them with high-resolution cameras. Using the information they gathered about how bats move in flight, they have designed a "robobat" which is a simplified version of a bat wing, consisting of only seven joints. They used a 3D printer to build the skeleton, and a thin sheet of rubber for the membrane. The 20-cm long wing folds and expands just like a bat's wing.

In the engineering process of building a model, the scientists learned as much when the model failed as they did when it worked. For example, they learned that the elbow joint of the wing had to be held together by very strong ligaments in order to withstand the forces from flight. They also learned how to mimic the strong and stretchy wing membrane of the bat using a network of elastic fibers.

Robot motors that flap the wings measure power output and the energy required for each flap.

Scientists observe the robobat in a wind tunnel to measure the aerodynamics of bat flight, including the lift and the thrust generated by the motion of the wing. Since it is easier to observe and work with than a live bat, the robobat helps scientists understand the aerodynamics of a bat's flight, and will hopefully help scientists to build better flying vehicles.

• Describe the characteristics of a bat wing that make it especially adapted for flight.

• Compare and contrast the methods that scientists and engineers use for performing research on bat flight.