Winged animals, ranging in size from insects to birds, effect remarkable maneurerability and stability while turning during flight. How is this achieved, and is there a common mechanism at work here?
Scientists at the University of North Carolina and the University of Delaware have investigated these questions. They have found that winged animals utilize a common passive mechanism to slow down while turning, regardless of body size.
Low-speed yaw turns.
The scientists focused their efforts on low-speed yaw turns. These are turns about the vertical axis, and are the most commonly recorded type of turn among flying animals.
These turns include two stages. In the first stage, the animal speeds up, and in the second, the animal slows down.
While accelerating during a turn (first stage), a flying animal must actively generate a twisting force to speed up. This may be by either wing flapping or body asymmetry.
While decelerating during a turn (second stage), a flying animal must make use of either active or passive deceleration. In other words, they must either actively generate a twisting force in the opposite direction of flight, or utilize passive friction, to slow down at the end of a turn.
Large animals are often thought to require active deceleration to slow down. Small animals are often thought to require no more than passive deceleration to slow down.
Velocity and twisting force during symmetric wing flapping.
In contrast, these scientists have investigated the possibility that all winged animals make use of passive deceleration to slow down at the end of a turn. Their hypothesis is based on the use of symmetric wing motion to passively generate slow-down twisting forces.
When animals hover or slowly fly straight ahead, during an active wing upstroke, a force is generated down and towards the head. During an active wing downstroke, a force is generated up and towards the other end of the body.
The situation is different when an animal rotates during flight. Here, velocity increases on the inside wing during upstroke, and on the outside wing during downstroke.
Thus, while flapping symmetrically, there is an asymmetry in flapping velocity (since velocity is a measurement of both speed and direction). This causes an asymmetry in twisting force.
Comparing mathematical models to observations.
The result of this asymmetric twisting force resulting from symmetric wingbeats during the execution of a turn is that the animal's rotation is slowed down. The scientists mathematically modeled the slow-down twisting force, with either symmetric or asymmetric wingbeats.
The prediction of the mathematical models is that, for winged animals of similar relative dimensions (but of different overall size), similar wingbeat magnitude, and similar air flow motion, decelerating twisting force should behave in one of two manners. It should either decelerate very quickly (exponentially), or decelerate more slowly (linearly), at the end of the turn.
Exponential slowdown is consistent with passive slowdown by symmetric wingbeats. Linear slowdown in consistent with active slowdown by asymmetric wingbeats.
They then compared these modeling predictions to measurements of yaw turning in insects and vertebrates, from 0.1 milligrams to 285 grams weight. The scientists found that different species decelerated at widely differing rates.
However, all were observed to exhibit similar maximum twisting forces along the turn. The scientists' conclusion is that winged animals passively slow down during turns with symmetric wingbeats, and do not actively slow down with asymmetric wingbeats.
Overall conclusions.
Winged animals can effect enhanced maneuverability and stability during flight by increasing wingbeat frequency. Thus, maneuverability and stability are not opposing concepts.
Flying animals execute flying maneuvers in addition to turns about the vertical axis, that are unstable and require active control. However, slowdown during simple slow-motion turns clearly only requires passive control through symmetrical wingbeats, regardless of the species.
for more information:
Hedrick, T. L.; Cheng, B.; Deng, X.
Wingbeat time and the scaling of passive flapping flight.
Science 2009, 324, 252-255.