Aircraft fuel efficiency is directly affected by the magnitudes of drag airliners experience during flight. Apart from the drag due to the body of the aircraft (parasite drag), a large portion of the aerodynamic drag results from the airflow rolling from the higher-pressure (lower) side of the wing to the lower-pressure (upper) side, causing wingtips to shed vortices.
These vortices are whirlpools of air that spiral downstream of the wing (behind the airplane), creating drag.
Wingtip devices, also known as winglets, improve the aircraft’s performance by managing the pressure distribution as the high-pressure flow wraps around the wingtips. The lift-induced drag is reduced by partial recovery of the tip vortex energy through winglets. Depending on the type of device and flight conditions, winglets can improve fuel efficiency by 2-5%.
A bird’s flight
During the second and third centuries BC, scientists and researchers extensively studied a bird’s flight to understand the aerodynamic characteristics of a lifting body. The early researchers questioned the perching of birds, their speed in the air, and wing movements. They aimed to identify the relationship between lift (achieved through wings) and drag (air resistance theorized by Isaac Newton).
The aspect ratio of a wing is defined as the ratio of the wingspan to its chord. The early research on sized lifting plates rotating at different angles revealed that higher aspect ratio plates result in lower drag and increased lift. In other words, the lift-induced drag was less dominant on gliders having broader wings. Wingtip vortices have minimal effect on the overall lift generated by the wing. This is due to the formation of vortices furthest away from the wing root.
At the onset of the 20th century, Wright Brothers built a homemade wind tunnel to experiment with different wing designs. They tested over 200 airfoil designs using the weight balance function. NASA and the US Air Force (USAF) began their research on extended-wing technology (winglets) in the early 1970s.
The first winglet test flight took off from NASA Dryden Flight Research Center on July 24, 1979. Over the course of 48 test flights, winglets proved to reduce lift-induced drag and increased the aircraft’s fuel efficiency by 6% to 7%.
Why are winglets curved upwards?
Similar to the (upward) curling of feathers by birds at low speeds, winglets on modern aircraft enable greater efficiency during flight. The upturned ends of the wings are designed to control the wrapping of the airflow from underneath the wing. The upward-positioned winglets dissipate high-pressure air in the direction of the flow, thereby minimizing the size and strength of the vortices.
Winglets reduce energy loss by gradually stemming the airflow down the tip of the winglet. A considerable amount of fuel can be reduced through the use of winglets, resulting in lowered carbon emissions.
Winglets began appearing on commercial and business jets in the early 1990s. Today, almost all modern jets are equipped with some kind of wingtip device to increase fuel efficiency. Until a new and revolutionary wing design is introduced that could further reduce lift-induced drag, upward-curved winglets are here to stay.
Source: simpleflying.com