Winglet (source code)
= Winglet
A \b[winglet] is a small wing-tip lifting surface, often inclined upward out of the main <wing planform>. By changing the three-dimensional loading and flow near the tip, it can reduce <lift-induced drag>. Its <winglet cant angle>, shape and attachment must be considered with the entire <aircraft wing>; it is not a universal bolt-on reduction in total <drag>.<A Design Approach and Selected Wind-Tunnel Results at High Subsonic Speeds for Wing-Tip Mounted Winglets by Richard T. Whitcomb>[NTRS author abstract, geometry and measured aerodynamic effects]{ref}
<Richard T. Whitcomb>'s 1976 report compared measured <forces>, moments and loads on a representative transport wing with and without wing-tip devices. A particularly useful comparison was a tip extension giving approximately the same increase of bending moment at the wing–<fuselage> junction as the winglets. That comparison asks how much aerodynamic benefit is obtained for a similar structural penalty, rather than comparing only nominal span.<A Design Approach and Selected Wind-Tunnel Results at High Subsonic Speeds for Wing-Tip Mounted Winglets by Richard T. Whitcomb>[Abstract, matched bending-moment increment for tip-extension comparison]{ref}
An original coefficient budget illustrates why component and total benefits differ. Keep the same reference <wing area>, <dynamic pressure> and <lift coefficient>. Suppose initially
$$
C_D=0.020+0.010=0.030,
$$
where the second term is the <lift-induced drag> contribution. If a proposed modification reduces that term by $20\%$ but adds $0.001$ to other <drag coefficient> contributions, then
$$
C_{D,\rm modified}=0.021+0.008=0.029.
$$
The \b[total reduction is about $3.33\%$, not $20\%$]. This is an invented budget, not a prediction or a measured winglet efficiency. Increased loading, additional wetted area and an off-design operating point can change the result.
A convincing <wind tunnel> comparison measures total <drag> at matched <lift>, records moments and loading, and documents how the mounting correction and reference area are handled. A photograph of a weaker-looking <wingtip vortex> is not a measurement of total <drag>. Conversely, reducing <lift-induced drag> does not require all three-dimensional trailing vorticity to disappear.
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