There are 43 different bird orders and more than 10,000 bird species on planet earth but almost 5,000 of them are passerines (belonging to the bird order Passeriformes). According to their flight styles, some of the orders are prominent. Soaring and thermal flight is identified with eagles and vultures while migration is identified with storks, cranes and other large birds, while falcons and hawks are known as high speed flyers. These birds are separated according to genetic science (phylogenetically) which changes their morphology and make each of them a master of their own flight styles.
The biological order of Accipitriformes includes eagles, old world vultures, buzzards, harriers and Cathartiformes includes new world vultures and condors. These birds perform outstanding circling performance in thermals (vertical increasing hot air columns) that ensures low-cost flights because energy conservation is vital during foraging. Staying in the hot air columns properly requires slow speed that provides narrower circling. In this way these birds can fly over the strongest part of a thermal. However, low-speed flight is always difficult during non-flapping flight. For this reason, these birds adapted to low speed flight. They have a low aspect ratio with a large wing area that reduces wing loading, but this brings with it some disadvantages. Due to the wide wings, wingtips are large which increases the correlated wingtip vortices to induced drag in these birds. Other gliding and soaring birds such as seagulls, albatrosses and falcons have different wing morphology. A high aspect ratio and pointed wing tips which reduce vortices can be seen in these birds (Figure-1).
Therefore, Accipitriformes developed a solution, known as winglets in aviation, to reduce wingtip vortices. The concept of winglets was originally developed in the late 1800s by British aerodynamicist F.W. Lancaster, who patented the idea that a vertical surface (end plate) at the wingtip would reduce drag by controlling wingtip vortices. After the cost of jet fuel increased rapidly with the 1973 oil crisis, airlines and manufacturers explored many ways to reduce fuel consumption by improving the operating efficiency of their aircraft. R.T. Whitcomb, an engineer at NASA Langley Research Center, was inspired by an article in Science Magazine on the flight characteristics of soaring birds and their use of tip feathers (primer feathers, thumb feathers) to control flight. He continued on a quest to reduce cruise drag and to further improve aircraft performance and developed the concept of winglets in the late 1970s. The feathers of Harris`s hawk (Parabuteo unicinctus) was clipped by V.A Tucker and the clipped and unclipped wings were tested in a wind tunnel to understand the effects on drag polar. A significant increase in drag can be seen in figure -2.
Apart from the winglet, the alula of the bird provides many advantages allowing the bird to remain in steady flight at lower speeds. The alula is a small structure located at the joint between the hand-wing and arm-wing that is composed of a digit bone and two to six feathers. This structure can be easily seen in eagles, vultures, hawks, buzzards, falcons and pigeons. The common point among these birds is that they execute very short take-offs, have high manoeuvrability and are able to fly at lower speed. Also, falcon species like common kestrel and ospreys use this structure for flapping and non-flapping hover. Its presence is universal in extant flying birds and can also be found in the fossils of several early ancestors of birds. The functions of the alula are similar to that of the extended leading-edge slat in an aircraft, this comparison has often been made as it increases the lift force at high angles of attack and delays the stall (Figure-3).
By flying with slow speed an airplane or a bird must increase the angle of attack to produce enough lift. But this will lead to flow separation. At the separation point, it comes to a complete stop. Then it moves back to the low-pressure region. So, the separation point also moves forward toward the leading edge of the wing. As result, the lift breaks down. To prevent this situation, the solution of birds is a reflux bag. The reverse flow opens the reflux-bags just before separation happens, and that prevents stall. This solution was applied to Stemme motor gliders as eddy-flaps, which increased the angle of attack by 23%.
Despite these solutions that were inspired by observing birds, perhaps the biggest goal for designers is to create morphing wings adapt to conditions seamlessly, as we see with the natural precision of birds in flight. Fowler type flaps are used to change the camber ratio and the wing area in large aircraft. Especially in sailplanes, this type morphing would increase the hybrid flight performance. A larger wing area develops circling performance in the thermal while a lower wing area (high aspect ratio) improves gliding performance. This type of morphological change can be observed in birds of prey and this application has started experimentally in gliders (Figure-5).
With the combination of the natural design of these amazing creatures and technology, many intricate flight challenges have been solved. These days, we can see the inspired design of different types of winglets such as sharklets, spiroid winglets, and the tip turbine which all come from nature’s precise design. Billions of years of evolution holds many secrets, iterations of exquisite adaptation, and humankind has taken different clues from nature. As technology advances and transforms the world around us, solutions in aeronautics will continue to unfold as we revere nature’s aviators soaring high above us in the skies