Editing Lift-to-drag ratio (section)

===Subsonic===
Mathematically, the maximum lift-to-drag ratio can be estimated as<ref>{{cite web|author=Loftin, LK Jr.|title=Quest for performance: The evolution of modern aircraft. NASA SP-468|url=http://www.hq.nasa.gov/pao/History/SP-468/cover.htm|access-date=2006-04-22}}</ref>
: <math>(L/D)_\text{max} = \frac{1}{2} \sqrt{\frac{\pi \varepsilon \text{AR}}{C_{D,0}}},</math>
where AR is the [[aspect ratio (wing)|aspect ratio]], <math>\varepsilon</math> the [[span efficiency factor]], a number less than but close to unity for long, straight-edged wings, and <math>C_{D,0}</math> the [[zero-lift drag coefficient]].

Most importantly, the maximum lift-to-drag ratio is independent of the weight of the aircraft, the area of the wing, or the wing loading.

It can be shown that two main drivers of maximum lift-to-drag ratio for a fixed wing aircraft are wingspan and total [[wetted area]]. One method for estimating the zero-lift drag coefficient of an aircraft is the equivalent skin-friction method. For a well designed aircraft, zero-lift drag (or parasite drag) is mostly made up of skin friction drag plus a small percentage of pressure drag caused by flow separation. The method uses the equation<ref>{{cite book|last1=Raymer|first1=Daniel|title=Aircraft Design: A Conceptual Approach|date=2012|publisher=AIAA|location=New York|edition=5th}}</ref>
: <math>C_{D,0} = C_\text{fe} \frac{S_\text{wet}}{S_\text{ref}},</math>
where <math>C_\text{fe}</math> is the equivalent skin friction coefficient, <math>S_\text{wet}</math> is the wetted area and <math>S_\text{ref}</math> is the wing reference area. The equivalent skin friction coefficient accounts for both separation drag and skin friction drag and is a fairly consistent value for aircraft types of the same class. Substituting this into the equation for maximum lift-to-drag ratio, along with the equation for aspect ratio (<math>b^2/S_\text{ref}</math>), yields the equation
<math display=block>
 (L/D)_\text{max} = \frac{1}{2} \sqrt{\frac{\pi \varepsilon}{C_\text{fe}} \frac{b^2}{S_\text{wet}}},</math>
where ''b'' is wingspan. The term <math>b^2/S_\text{wet}</math> is known as the wetted aspect ratio. The equation demonstrates the importance of wetted aspect ratio in achieving an aerodynamically efficient design.