Editing Lift-to-drag ratio (section)

==Lift and drag==
[[File:DragvsSpeed.jpg|thumb|upright=1.2|Drag vs Speed. L/DMAX occurs at minimum Total Drag (e.g. Parasite plus Induced)]]
[[File:Coefficients of Drag and Lift vs AOA.jpg|thumb|upright=1.2|[[Drag coefficient|Coefficients of drag]] C<sub>D</sub> and [[Lift coefficient|lift]] C<sub>L</sub> vs [[angle of attack]].]]
[[File:Polar Curve 2.png|thumb|upright=1.2|Polar curve showing glide angle for the best glide speed (best L/D).  It is the flattest possible glide angle through calm air, which will maximize the distance flown.  This airspeed (vertical line) corresponds to the tangent point of a line starting from the origin of the graph.  A glider flying faster or slower than this airspeed will cover less distance before landing.<ref name="bw">{{cite book |last1=Wander |first1=Bob |title=Glider Polars and Speed-To-Fly...Made Easy! |date=2003 |publisher=Bob Wander's Soaring Books & Supplies |location=Minneapolis |pages=7–10}}</ref><ref name=faa>{{cite book |title=Glider Flying Handbook, FAA-H-8083-13 |date=2003 |publisher=U.S. Department of Transportation, FAA |isbn=9780160514197 |page=5-6 to 5-9}}</ref>]]
Lift can be created when an aerofoil-shaped body travels through a viscous fluid such as air. The aerofoil is often [[camber (aerodynamics)|cambered]] and/or set at an [[angle of attack]] to the airflow. The lift then increases as the square of the airspeed.

Whenever an aerodynamic body generates lift, this also creates [[lift-induced drag]] or induced drag. At low speeds an aircraft has to generate lift with a higher [[angle of attack]], which results in a greater induced drag. This term dominates the low-speed side of the graph of lift versus velocity.
[[File:DargPolarAL.png|thumb|upright=1.2|right|[[Drag curve]] for light aircraft. The tangent gives the maximum {{math|L/D}} point.]]
[[Form drag]] is caused by movement of the body through air. This type of drag, known also as [[air resistance]] or [[profile drag]] varies with the square of speed (see [[drag equation]]). For this reason profile drag is more pronounced at greater speeds, forming the right side of the lift/velocity graph's U shape. Profile drag is lowered primarily by streamlining and reducing cross section.

The [[Drag (aerodynamics)|total drag]] on any aerodynamic body thus has two components, induced drag and form drag.

===Lift and drag coefficients===
The rates of change of lift and drag with angle of attack (AoA) are called respectively the [[lift coefficient|lift]] and [[drag coefficient]]s C<sub>L</sub> and C<sub>D</sub>. The varying ratio of lift to drag with AoA is often plotted in terms of these coefficients.

For any given value of lift, the AoA varies with speed. Graphs of C<sub>L</sub> and C<sub>D</sub> vs. speed are referred to as [[drag curve]]s. Speed is shown increasing from left to right. The lift/drag ratio is given by the slope from the origin to some point on the curve and so the maximum L/D ratio does not occur at the point of least drag coefficient, the leftmost point. Instead, it occurs at a slightly greater speed.  Designers will typically select a wing design which produces an L/D peak at the chosen [[cruising speed]] for a powered fixed-wing aircraft, thereby maximizing economy. Like all things in [[aeronautical engineering]], the lift-to-drag ratio is not the only consideration for wing design. Performance at a high angle of attack and a gentle [[Stall (flight)|stall]] are also important.