Breaking Down Bird Flight
Natural flyers have lead to the birth of pioneering ideas
The observation of flight in nature by birds, bats and insects has motivated the human desire to fly, and ultimately lead to the development of aircraft. Some of the earliest designs of flying vehicles by Leonardo da Vinci involved the use of flapping wings and control mechanisms to generate lift and forward thrust. However, the first manned-powered flight by the Wright brothers used fixed wings, unlike natural flyers.
Engineers are fascinated by the integration between the structural and functional aspects that characterizes the wings of birds. These wings can be morphed into a wide range of wing configurations, each of which is effectively used for a particular flight task. Even in complex urban environments, birds can rapidly change shapes to transition from efficient cruise to aggressive manoeuvring and smooth descents. Similar to an aircraft, wing loading and aspect ratio play important roles in the determination of the ability and type of bird flight. Bird flight is achieved by the primary and secondary feathers on its wings. The primary feathers are attached to its bones and are on the hand while the secondary feathers are inserted along the arm and are mainly responsible for the generation of lift. These feathers act as a set of flaps. The inner part of a bird’s wing remains stationary and acts as an aerofoil.
Birds can remain airborne by three means: gliding, hovering or flapping.
In gliding flight, the wings are not flapped and are extended out completely. Lift is produced due to the relative pressure difference between the upper and lower surfaces of the wing. However, the drag might cause the bird to slow down ultimately to a point where it might not be able to produce enough lift. Thus the bird slightly tilts downwards to use the loss of its gravitational potential energy and gain the required thrust. Soaring flight is when the bird uses energy from the atmosphere (such as rising air current due to thermal gradient). Birds can maintain or gain height during this type of flight. Birds such as eagles, hawks and kites, among several others, glide and soar.
Hovering is mostly performed by smaller flyers, though some larger birds also hover for a shorter period of time. True hovering is achieved by generating lift through flapping alone, without any airflow due to forward motion. Some large birds hover by flying into a headwind. This headwind generates lift without any flapping.
Flapping flight is more complex than fixed-wing flight because of the continuous movement of the lift producing device and the unsteady aerodynamics around it. Birds use a complex combination of tail and wing modification, such as altering the shape, camber and surface area, as the environment requires dealing with winds gusts, prey, enemy attack, collision avoidance etc. Flapping birds also simultaneously twist their wings to achieve varying angles of attack (AoAs) at different sections of the wing; similar to wing wash-out and wash-in in fixed-wing aircrafts. Twisting the wings to different degrees produces aerodynamic effects on the wings similar to that produced by ailerons on a conventional fixed-wing aircraft. For example, one wing is twisted downward (pronated) to reduce the AoA and hence lift; while the other wing is twisted upward (supinated) to increase the AoA and consequently lift. Hence, the bird is able to roll.
Axes of Flight:
The flapping wing can have three distinct motions with respect to three axes-
- Flapping: Up and down plunging motion of the wing about the longitudinal axis. A majority of the bird’s lift and thrust is produced by flapping. It also has the largest degree of freedom.
- Feathering: Pitching motion of wing about the lateral axis. It can vary along the span.
- Lead-lag: In-plane lateral movement of the wing about the vertical axis.
Generation of Lift:
Birds, bats and insects all have different anatomy and flapping patterns. Consequently, they use different mechanisms for forward thrust and lift generation. The generation of lift by most birds while flapping is not continuous; rather in regular intervals. One lift generation (flapping) cycle consists of two phases: one complete downstroke and one complete upstroke.
- Downstroke (Power stroke): During the downstroke, the primary feathers are all closed together to form an airfoil and the AoA is increased. Thus the air pressure acting on the wing during downstroke is greater than that acting during the upstroke. Therefore, there is more lift and drag during the downstroke.
- Upstroke (Recovery stroke): During the upstroke, the primary feathers are separated, allowing more air to pass through the feathers and thus reducing drag. The AoA is decreased during the upstroke. The upstroke can be viewed as a recovery phase to conduct the next power stroke. During the upstroke, the outer part of the wing must return with as minimum drag as possible, usually with zero angle of attack. However, during hovering, as in the case of hummingbirds, the wing is configured such that the upstroke also produces lift.
It must be noted that there is very little movement of the wing at the root so that it continues to provide lift at all times. This ensures that lift is not reduced to zero in between downstrokes. The movement of the wing may get more and more vigorous across the span from the root to the tip. For larger birds that cannot extensively twist their wings, the lift is primarily produced in the downstroke, while for smaller birds and insects, the lift is produced during the entire wing stroke.
Although lift in fixed-wing aircrafts is not generated in cycles, it is still important to understand the physics underlying the flight of natural flyers. Understanding the science behind them is essential to make a profound impact on the future of man-made flying devices. This will further expand the diversity, durability and applications of micro-air vehicles such as ornithopters and entomopters. An amalgamation of work from several different backgrounds is needed to develop new mathematical models, simulate and experiment with different techniques, engineer new designs and concepts, and finally bring newer bio mimicking technology to life.
