Bird, bat and insect wings are complex structures that are moved in stereotypical ways to generate lift and thrust. It was once thought that animal flight could simply be understood by assuming that animals were no different from aeroplanes. The claim that "bumblebees can't fly" is based on this assumption. Clearly bumblebees can fly. The truth is that bats, birds, and especially insects, use unconventional aerodynamic mechanisms for generating the forces necessary for flight. We have recently begun to visualize and understand the aerodynamic tricks that these animals use to generate lift and thrust. This research is valuable not only in terms of our understanding of animal flight mechanics, but also for the development of new technologies, such as micro-air vehicles and improved propeller designs, which have significant engineering applications.
In this essay, we will briefly explain how animal flight is different from aeroplane flight, how animal flight is typically studied, and present some of the emerging theories and applications of this work. The complexities of biological wings and wing motions present many technical challenges for studying flight. Here, we use the term "flight" broadly and note that it applies to many behaviours including gliding, soaring, hovering, parachuting, manoeuvring, and even take- off and landing. This essay is not limited to the work of our own research group, but hopefully will convince the reader why it is valuable, and necessary, to look to animals for aerodynamic insight.
Conventional aerodynamic theory
Let us begin with aeroplane wings and a basic understanding of how they generate lift. Structurally, aeroplane wings are rounded at the leading edge, sharp at the trailing edge and are often cambered, meaning they have a slight curvature when viewed in cross section. An aeroplane wing generates lift when the airflow becomes separated at the leading edge, and the air moves faster over the upper wing surface than along the lower surface. This causes a pressure difference to develop between the upper and lower wing surfaces because, in accordance with Bernoulli's principle, fast-moving fluid has a lower pressure than slow-moving fluid. It is the pressure difference above and below the wing that causes lift.
The amount lift produced by aeroplane wings depends on many factors. These include the relative air velocity, the camber of the wing, the area of the wing, and the angle of attack relative to the direction of the oncoming air. Aeroplanes manoeuver by manipulating some of these parameters during flight with control surfaces such as rudders and ailerons. To turn, the pilot adjusts the ailerons on the wing surfaces, changing the lift produced by the left and right wings and causing the plane to roll, and therefore turn. During steering, the pilot will also adjust the angle of attack of the rudder for yaw control and elevators on the tail for pitch control. Lastly, an example of changing the wing area and camber occurs when a passenger jet lands. The engines are throttled back as the plane approaches the runway, and the slower speeds would decrease the lift produced by the wings. However, the pilot extends the trailing edges (flaps down) increasing the wing area and camber. This preserves the lift at the slow speeds necessary for a smooth touch-down.
Unconventional aerodynamics and the flapping wing
Bird, bat and insect wings are quite different from aeroplane wings. Most biological wings tend to have sharp leading edges and textured surfaces. The surface properties of biological wings can include ridges (for example bones in bat wings and veins in insect wings), folds, gaps, and other microscopic features on their surfaces. Biological wings often change shape and are frequently deformed during flapping. Birds and bats shorten their wing span by drawing them toward the body during the upstroke and extend their wings during the downstroke. Similarly, soft wing membranes can deform like the material in a sail, creating a dynamically changing surface.
The other consideration in our comparison to aeroplane wings is in the flapping motion of biological wings. An animal wing is constantly changing velocity as it flaps, slowing down and stopping at the ends of the downstroke and upstroke, and then accelerating into the next halfstroke. Furthermore, the wing base will always be moving slower than the wing tip, meaning that the wing velocity increases from base to tip.
Approaches to studying biological flight
Typically, studies begin by observing an animal in flight and quantifying the wing and body motion, or kinematics. About 20-30 pictures per wingbeat are needed to see the flapping wing motion clearly, so high-speed (digital) cinematography is needed for the rapid wingbeats of insects. Videography (using conventional video recorders) is limited to 25 or 30 frames per second and hence is suitable only for large, slow flapping birds and bats. Accurately measuring the wing movements is best accomplished by using two cameras, which allows the 3-D coordinates of objects (e.g. wing tips, other wing features, body position) to be calculated using stereophotogrammetric techniques. From accurate position data and the temporal resolution of the measuring system, velocities (and accelerations) can be determined from the captured sequences. The wings and body are also usually photographed in isolation to accurately determine wing shape, body shape, and the animal's centre of mass. Isolated wings or wing models are placed in a wind tunnel and used to measure lift and drag, which characterizes the animals’ capacity for flight.
Mathematical models can then be constructed using the measured kinematic and morphological data. The blade-element theory, for example, divides the wings into a number of strips, each with its own velocity and area. The forces acting on each 'blade-elements' are calculated, and these are summed to give the total forces for the flapping wings. The blade-element analysis can be greatly simplified by invoking the quasi-steady assumption: that the aerodynamic forces on an element are the same as those acting on a fixed wing at the same instantaneous velocity and angle of attack. This assumption is probably reasonable for the slowly flapping wings of large birds, but it seriously underestimates the lift produced at the high wingbeat frequencies of insects. An inappropriate use of the quasi-steady assumption is certainly one way to ‘prove’ that bumblebees cannot fly.
To understand the origin of the fluid forces we can visualize the airflow around the moving wings. Flow visualization is important for observing vortices, turbulence and other disturbances to the flow of air. All flow visualization techniques rely on "seeding" the air with small particles that can be photographed, filmed or observed directly during the wingstroke. Smoke has traditionally been used and can be easily generated by vaporizing oil on a hot wire placed in front of an insect or vertebrate beating its wings in a wind tunnel. Modern techniques use lasers to illuminate particular areas of the flow (planes) and digital cameras to capture images during flapping. Software is then used to determine the movement of the particles. This technique is called particle image velocimetry (PIV) or particle tracking velocimetry (PTV). Visualizing the flow of air over the surface of insect wings has provided great insight into the mechanisms that insects use to generate lift.
Using mechanical models of insects and animals
Those who have attempted to work with animals in a laboratory know that, despite great efforts to design and organize experiments and control all extraneous variables, the study animal will usually do whatever it damn well pleases. Furthermore, real animals can be difficult to work with because of their size. Simply put, insects are stubborn, small and beat their wings very quickly. However, as long as the proper relationship between flapping frequency, fluid viscosity, and wing size is conserved, we can use a more cooperative and larger mechanical model; it's a simple matter of physics that the flow patterns over the wings will be the same. In other words, the airflow over a real hawkmoth wing is the same as that flowing over a mechanical hawkmoth ten times as large flapping its wings at 1/100th the frequency of the real insect. This scaling offers a powerful tool for studying the aerodynamics of flapping flight provided that the challenge can be met of building mechanical/robotic versions of real animals. It was with such a mechanical model, the ‘Flapper’, that we discovered in 1996 how insect wings produce much more lift than could be explained by conventional aerodynamics.
Spiral Leading-Edge Vortex
As air passes around the sharp leading edge of an insect wing, it breaks away from the wing and rolls up into a leading-edge vortex (LEV). You can see this effect yourself by moving a spoon broadside through a cup of coffee, and watching the swirling motion from the edges. In both cases the fluid moves along a circular path, demonstrating a lower pressure at the centre of the vortex. (To swing a weight around on a string you have to exert a centripetal force by pulling it towards the centre, and the fluid is similarly sucked towards the centre by the low pressure.) So the LEV is a region of low pressure above the wing, and this provides an extra suction that increases the lift. The only problem is that the flow continues to feed into the LEV. This would normally cause the vortex to grow so large that it breaks away from the wing, ruining the lift and stalling the wing. However, we discovered that the flapping motion causes the LEV to spiral out to the wingtip, siphoning off the vortex and delaying stall. The augmented lift, coupled with the delayed stall, is the principle mechanism that insects use for generating lift. Spiral LEVs are not new to aerodynamics, and indeed they keep delta-winged aircraft like Concorde up in the air. However, those spiral LEVs are generated passively by the swept leading edge of the wing. What is unexpected and interesting about insect flight is that the spiral LEVs are created and stabilised by the flapping motion itself.
Rotational lift
When an insect reaches the end of the upstroke, it must rotate its wings to place them at the correct angle of attack for the start of the downstroke. Similarly, the wings must flip over between the downstroke and upstroke. Ellington first suggested in 1984 that these rapid rotations could produce extra lift, drawing on some experimental and theoretical results for aeroplane wings with rapidly increasing angles of attack. Michael Dickinson's group, working at Berkeley Dickinson
Wake recapture
Insects generate lift by producing and shedding vortices from their wings. These vortices move with the wake as spiralling masses of air that slowly decay and disappear, rather like the tip vortices of aeroplanes. For insects with high wingbeat frequencies, such as flies, the vortices move only a short distance before the wing returns in the cycle, and they can use this as a point of leverage for generating additional lift. This process of ‘wake recapture’, described by Dickinson
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