Abstract: There are many theories behind the colors of a bird’s feathers. Many of these theories point to the color’s purpose to attract mates and hide from predators. Some recent investigations concluded that the dark colors of birds help in reducing the drag force during flight. A new theory is presented in the current research, which states that a bird's dark color not only reduces the drag, but the color pattern also improves the overall flight performance, and each color pattern has a different type of flight performance improvement. This difference in improvement is a result of variation in hot and cold surfaces on the bird skin as a result of the variation between light and dark feather colors. To prove this new theory, thermal images were captured of real bird wings under the effect of infrared waves. Also, a novel wind tunnel wing with the ability to adjust the temperature in desired locations and patterns on the wing’s surface was manufactured and tested to evaluate the effect of aerodynamics forces as a function in the surface temperature and the hot–cold regions. The collected data from this wing showed potential flight efficiency improvements of 20%, comparing the lift-to-drag ratio for specific heating cases, which could increase the flight range. Individually considering lift and drag, there were specific heating cases with corresponding angles of attack in which these parameters improved by up to 20% and 7%, respectively. Some heating cases could increase the lift at a low angle of attack, which is helpful in cruise flight performance, while some cases could increase the maximum lift coefficient by 6%. This is very helpful in lowering stall and the minimum flight speeds. Furthermore, some cases could increase the lift-to-drag ratio, which led to an increase in the flight range. To better understand the effect of the various patterns, computational fluid dynamics (CFD) simulations were conducted on the wing. The new theory was proved based on the CFD results and verified through the successful results from the wind tunnel experiments.

Key words:  , Aerodynamic loads , · CFD , · Drag , · Heated boundary layer , · Surface temperature