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Lifttodrag ratio
In aerodynamics, the lifttodrag ratio, or L/D ratio, is the amount of lift generated by a wing or vehicle, divided by the aerodynamic drag it creates by moving through the air. A higher or more favorable L/D ratio is typically one of the major goals in aircraft design; since a particular aircraft's required lift is set by its weight, delivering that lift with lower drag leads directly to better fuel economy, climb performance, and glide ratio.
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 2D graph. In almost all cases the graph forms a Ushape, due to the two main components of drag.
Lifttodrag ratios can be determined by flight test, by calculation or by testing in a wind tunnel.^{[citation needed]}
Contents
Drag
Liftinduced drag is a component of total drag that arises whenever a threedimensional wing generates lift. At low speeds an aircraft has to generate lift with a higher angle of attack, thereby leading to greater induced drag. This term dominates the lowspeed side of the L/D graph, the left side of the U.
Form drag is caused by movement of the aircraft through the air. This type of drag, also known as air resistance or profile drag varies with the square of speed (see drag equation). For this reason profile drag is more pronounced at higher speeds, forming the right side of the L/D graph's U shape. Profile drag is lowered primarily by reducing cross section and streamlining.
The peak L/D ratio doesn't necessarily occur at the point of least total drag, as the lift produced at that speed is not high, hence a bad L/D ratio. Similarly, the speed at which the highest lift occurs does not have a good L/D ratio, as the drag produced at that speed is too high. The best L/D ratio occurs at a speed somewhere in between (usually slightly above the point of lowest drag). Designers will typically select a wing design which produces an L/D peak at the chosen cruising speed for a powered fixedwing aircraft, thereby maximizing economy. Like all things in aeronautical engineering, the lifttodrag ratio is not the only consideration for wing design. Performance at high angle of attack and a gentle stall are also important.
Glide ratio
As the aircraft fuselage and control surfaces will also add drag and possibly some lift, it is fair to consider the L/D of the aircraft as a whole. As it turns out, the glide ratio, which is the ratio of an (unpowered) aircraft's forward motion to its descent, is (when flown at constant speed) numerically equal to the aircraft's L/D. This is especially of interest in the design and operation of high performance sailplanes, which can have glide ratios approaching 60 to 1 (60 units of distance forward for each unit of descent) in the best cases, but with 30:1 being considered good performance for general recreational use. Achieving a glider's best L/D in practice requires precise control of airspeed and smooth and restrained operation of the controls to reduce drag from deflected control surfaces. In zero wind conditions, L/D will equal distance traveled divided by altitude lost. Achieving the maximum distance for altitude lost in wind conditions requires further modification of the best airspeed, as does alternating cruising and thermaling. To achieve high speed across country, glider pilots anticipating strong thermals often load their gliders (sailplanes) with water ballast: the increased wing loading means optimum glide ratio at higher airspeed, but at the cost of climbing more slowly in thermals. As noted below, the maximum L/D is not dependent on weight or wing loading, but with higher wing loading the maximum L/D occurs at a faster airspeed. Also, the faster airspeed means the aircraft will fly at higher Reynolds number and this will usually bring about a lower zerolift drag coefficient.
Theory
Mathematically, the maximum lifttodrag ratio can be estimated as:
 <math>(L/D)_{max} = \frac{1}{2} \sqrt{\frac{\pi A \epsilon}{C_{D,0}}}</math>,^{[1]}
where A is the aspect ratio, <math>\epsilon</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 zerolift drag coefficient.
Most importantly, the maximum lifttodrag ratio is independent of the weight of the aircraft, the area of the wing, or the wing loading.
It can be shown that the two main drivers of maximum lifttodrag ratio for a fixed wing aircraft are wingspan and total wetted area. One method for estimating the zerolift drag coefficient of an aircraft is the equivalent skinfriction method, which makes use of the fact that for a well designed aircraft, zerolift 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:
 <math>C_{D,0}=C_{fe}\frac{S_{wet}}{S_{ref}}</math>,^{[2]}
where <math>C_{fe}</math> is the equivalent skin friction coefficient, <math>S_{wet}</math> is the wetted area and <math>S_{ref}</math> is the wing reference area. The equivalent skin friction coefficient accounts for the 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 lifttodrag ratio, along with the equation for aspect ratio (<math>b^2/S_{ref}</math>), yields the equation:
 <math>(L/D)_{max}=\frac{1}{2} \sqrt{\frac{\pi \epsilon}{C_{fe}}\frac{b^2}{S_{wet}}}</math>
where b is wingspan. The term <math>b^2/S_{wet}</math> is known as the wetted aspect ratio. The equation demonstrates the importance of wetted aspect ratio in achieving an aerodynamically efficient design.
Supersonic/hypersonic lift to drag ratios
At very high speeds, lift to drag ratios tend to be lower. Concorde had a lift/drag ratio of around 7 at Mach 2, whereas a 747 is around 17 at about mach 0.85.
Dietrich Küchemann developed an empirical relationship for predicting L/D ratio for high Mach:^{[3]}
 <math>L/D_{max}=\frac{4(M+3)}{M}</math>
where M is the Mach number. Windtunnel tests have shown this to be roughly accurate.
Examples
The following table includes some representative L/D ratios.
Flight article  Scenario  L/D ratio 

Virgin Atlantic GlobalFlyer  Cruise  37^{[4]} 
Lockheed U2  Cruise  ~28 
Rutan Voyager  Cruise^{[4]}  27 
Albatross  20^{[5]}  
Boeing 747  Cruise  17 
Common tern  12^{[5]}  
Herring gull  10^{[5]}  
Concorde  M2 Cruise  7.14 
Cessna 150  Cruise  7 
Helicopter  100 kts speed  4.5^{[6]} 
Concorde  Approach  4.35 
House sparrow  4^{[5]} 
In gliding flight, the L/D ratios are equal to the glide ratio (when flown at constant speed).
Flight article  Scenario  L/D ratio/ glide ratio 

Modern sailplane  Gliding  40–60 (depending on span) 
Hang glider  15  
Gimli Glider  Boeing 767200 with fuel exhaustion  ~12 
Paraglider  High performance model  11 
Helicopter  Autorotation  4 
Powered parachute  Rectangular/elliptical parachute  3.6/5.6 
Space Shuttle  Approach  4.5^{[7]} 
Wingsuit  Gliding  2.5 
Northern flying squirrel  Gliding  1.98 
Space Shuttle  Hypersonic  1^{[7]} 
Apollo CM  Reentry  0.368^{[8]} 
See also
 Lift coefficient
 Thrust specific fuel consumption the lift to drag determines the required thrust to maintain altitude (given the aircraft weight), and the SFC permits calculation of the fuel burn rate
 thrust to weight ratio
 Range (aircraft) range depends on the lift/drag ratio
 Inductrack maglev has a higher lift/drag ratio than aircraft at sufficient speeds^{[citation needed]}
 Gravity drag rockets can have an effective lift to drag ratio while maintaining altitude
References
 ^ Loftin, LK, Jr. "Quest for performance: The evolution of modern aircraft. NASA SP468". Retrieved 20060422.
 ^ Raymer, Daniel (2012). Aircraft Design: A Conceptual Approach (5th ed.). New York: AIAA.
 ^ Aerospaceweb.org Hypersonic Vehicle Design
 ^ ^{a} ^{b} David Noland, "Steve Fossett and Burt Rutan's Ultimate Solo: Behind the Scenes", Popular Mechanics, Feb. 2005 (web version)
 ^ ^{a} ^{b} ^{c} ^{d} Fillipone
 ^ Leishman, J. Gordon. Principles of helicopter aerodynamics p230, Cambridge University Press, 24 April 2006. Accessed: 27 February 2012. ISBN 0521858607. Quote: The maximum lifttodrag ratio of the complete helicopter is about 4.5
 ^ ^{a} ^{b} Space Shuttle Technical Conference pg 258
 ^ Hillje, Ernest R., "Entry Aerodynamics at Lunar Return Conditions Obtained from the Flight of Apollo 4 (AS501)," NASA TN D5399, (1969).
