Editing Lift-to-drag ratio (section)

{{Short description|Measure of aerodynamic efficiency}}

[[File:Airfoil lift and drag.svg|thumb|upright=1.2|[[Lift (force)|Lift]] and [[aerodynamic drag|drag]] are the two components of the total [[aerodynamic force]] acting on an aerofoil or aircraft.]]

In [[aerodynamics]], the '''lift-to-drag ratio''' (or '''L/D ratio''') is the [[Lift (force)|lift]] generated by an aerodynamic body such as an [[aerofoil]] or aircraft, divided by the [[aerodynamic drag]] caused by moving through air. It describes the aerodynamic [[efficiency]] under given flight conditions. The L/D ratio for any given body will vary according to these flight conditions.

For an aerofoil wing or powered aircraft, the L/D is specified when in straight and level flight. For a glider it determines the [[Lift-to-drag ratio#Glide ratio|glide ratio]], of distance travelled against loss of height.

The term is calculated for any particular airspeed by measuring the lift generated, then dividing by the drag at that speed. These vary with speed, so the results are typically plotted on a 2-dimensional graph. In almost all cases the graph forms a U-shape, due to the two main components of drag. The L/D may be calculated using [[computational fluid dynamics]] or [[computer simulation]]. It is measured empirically by testing in a [[wind tunnel]] or in free [[flight test]].<ref>{{cite journal | doi=10.1177/1729881418766190 | title=Accurate calculation of aerodynamic coefficients of parafoil airdrop system based on computational fluid dynamic | date=2018 | last1=Wu | first1=Wannan | last2=Sun | first2=Qinglin | last3=Luo | first3=Shuzhen | last4=Sun | first4=Mingwei | last5=Chen | first5=Zengqiang | last6=Sun | first6=Hao | journal=International Journal of Advanced Robotic Systems | volume=15 | issue=2 | doi-access=free }}</ref><ref>[https://www.diva-portal.org/smash/get/diva2:342732/fulltext01.pdf Validation of software for the calculation of aerodynamic coefficients] Ramón López Pereira, Linköpings Universitet</ref><ref>[https://www.mdpi.com/2226-4310/8/2/43/pdf In-flight Lift and Drag Estimation of an Unmanned Propeller-Driven Aircraft] Dominique Paul Bergmann, Jan Denzel, Ole Pfeifle, Stefan Notter, Walter Fichter and Andreas Strohmayer</ref>

The L/D ratio is affected by both the form drag of the body and by the induced drag associated with creating a lifting force. It depends principally on the lift and drag coefficients, [[angle of attack]] to the airflow and the wing [[Aspect ratio (aeronautics)|aspect ratio]].

The L/D ratio is inversely proportional to the [[Dissipation|energy required]] for a given flightpath, so that doubling the L/D ratio will require only half of the energy for the same distance travelled. This results directly in better [[fuel economy in aircraft|fuel economy]].

The L/D ratio can also be used for water craft and land vehicles. The L/D ratios for hydrofoil boats and displacement craft are determined similarly to aircraft.