July 9, 2013
A light sport aircraft taxies by as you are tying down your trainer after the day’s flight lesson.
“Notice anything unusual about that airplane?” asks your instructor.
Given the question, you know the answer has to be “yes,” so you give the aircraft an extra careful inspection as it rolls past.
“Winglets,” the CFI says, pointing to small vertical airfoils at the tips of the airplane’s wings.
Frequently a design element of large jet and turboprop aircraft, winglets—designed to address aerodynamic inefficiencies of the airflow around wingtips—have begun to appear as original equipment or as aftermarket installations on smaller aircraft. The basic idea is that winglets can counteract a loss of lift resulting from the phenomenon of wingtip vortices.
“The high-pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top,” explains the Pilot’s Handbook of Aeronautical Knowledge. “This action creates a rotating flow called a tip vortex. The vortex flows behind the airfoil creating a downwash that extends back to the trailing edge of the airfoil. This downwash results in an overall reduction in lift for the affected portion of the airfoil. Manufacturers have developed different methods to counteract this action. Winglets can be added to the tip of an airfoil to reduce this flow. The winglets act as a dam preventing the vortex from forming. Winglets can be on the top or bottom of the airfoil.”
Designs have advantages and drawbacks, so designers are further subdividing winglets as either passive or active devices.
“Passive winglets such as those found on airliners and business jets today impart a bending moment to the wing, due to the extra lift they provide at the very tip,” explained this October 2012 AOPA Online article. “Either the wing connection to the fuselage must be strengthened, or the winglets must be small enough to avoid extra strain.” By contrast, active winglets tested by AOPA are designed to “turn off” the extra lift when wing loads are high.
More benefits were suggested when recent testing of winglets on a Piper Aerostar twin-engine aircraft demonstrated better handling during stalls, and increased safety margins in slow-speed flight.
Winglets are even finding their way into future propeller design for the light sport and experimental aircraft market, striving for the dual benefits of increased efficiency and reduced noise.
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