In aerodynamics, the lift-to-drag ratio, or L/D ratio ("ell-over-dee", as opposed to "ell-dee" in the US, "ell-dee" in the UK), is the amount of lift generated by a wing, compared to the drag it creates by moving through the air. A "better" L/D ratio is one of the major goals in wing design, since a particular aircraft's needed lift doesn't change, delivering that lift with lower drag leads directly to better fuel economy, climb performance and glide ratio. This article does not cite any references or sources. ... The lift force, lifting force or simply lift is a mechanical force generated by a solid object moving through a fluid. ... A Laughing Gull with its wings extended in a gull wing profile Aircraft wing planform shapes: a swept wing KC-10 Extender (top) refuels a trapezoid-wing F/A-22 Raptor A wing is a surface used to produce lift and therefore flight, for travel in the air or another... An object falling through a gas or liquid experiences a force in direction opposite to its motion. ... Glide ratio is an aviation term that refers to the distance an aircraft will move forward for any given amount of lost altitude (the cotangent of the downward angle). ...

The term is calculated for any particular speed by measuring the lift generated, then dividing by the drag it causes. These vary with speed, so the results are typically plotted on a 2D graph. In almost all cases the graph forms a U-shape, due to the two main components of drag on the wing.

Induced drag is caused by the generation of lift by the wing. Lift generated by a wing is directed straight up from the wing, but since wings typically fly at some small angle of attack, this means the force is directed both up and to the rear. The rearward component of this force is seen as drag. 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 low-speed side of the L/D graph, the left side of the U. In aerodynamics, lift-induced drag, or more simply, induced drag, is a drag force arising from the generation of lift by wings or a lifting body during flight. ... In this diagram, the black arrow represents the direction of the wind. ...

Profile drag is caused by air hitting the wing itself. This form of drag, simply another name for wind resistance, varies with the square of speed (see drag equation). For this reason profile drag is only a real factor at higher speeds, forming the right side of the L/D graph's U shape. Profile drag is lowered primarily by using thinner wings, but such a shape often leads to less low-speed lift, and thus higher induced drag. In aerodynamics, form drag, profile drag, or pressure drag, is a component of parasitic drag created by wind hitting the body. ... Wind resistance is overall drag on a body due to its interaction with the atmosphere. ... In physics, the drag equation gives the drag experienced by an object moving through a fluid. ...

The drag curve

It is the bottom point of the graph, the point where the combined drag is at its lowest, that the wing is performing at its best. For this reason designers will typically select a wing with its L/D peak at the chosen cruising speed of the 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 high angle of attack and a gentle stall are often considered more important, and for this reason easy-to-fly wing designs like the Clark-Y continue to be used even though many more efficient wings have since been designed. Better version of Image:Drag Curve. ... Better version of Image:Drag Curve. ... Aerospace engineering is the branch of engineering concerning aircraft, spacecraft and related topics. ... In this diagram, the black arrow represents the direction of the wind. ... In aerodynamics, a stall is a condition in which an excessive angle of attack causes loss of lift due to disruption of airflow. ... Clark-Y is the name of a particular aerofoil profile, widely used in general purpose aircraft designs, and much studied in aerodynamics over the years. ...

As the aircraft fuselage and control surfaces will also add drag 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 descent to its forward motion, is exactly equal to the aircraft's L/D. This is especially of interest in the design and operation of high performance gliders (called 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 sailplane'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 altitude lost divided by distance traveled. 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, gliders are often loaded with water ballast, which increases induced drag, and moves the best L/D position to the right in the graph. The fuselage can be short, and seemingly unaerodynamic, as in this Christen Eagle 2 The fuselage (from the French fuselé spindle-shaped) is an aircrafts main body section that holds crew and passengers or cargo. ... Glide ratio is an aviation term that refers to the distance an aircraft will move forward for any given amount of lost altitude (the cotangent of the downward angle). ... Gliders are un-powered heavier-than-air aircraft. ... Gliders are un-powered heavier-than-air aircraft. ...

For maximum range, one should fly at the point on the graph with minimum drag. (The speed should decrease a bit during the flight because the optimal speed decreases as the plane uses up fuel and becomes lighter.) Because this theoretical speed may still be slightly exceeded without siginificant losses in efficiency, the "long range cruise speed" is normally slightly higher than the maximum range speed. There is a trade-off between saving fuel and saving time. The upper limit of speed is dictated by available (continuous) thrust and is not shown on the graph.

Mathematically, the maximum lift-to-drag ratio can be estimated as:

^[1],

where A is the aspect ratio, ε is the aircraft's efficiency factor, and C_D,0 is the zero-lift drag coefficient. The low aspect ratio wing of a Piper PA-28 Cherokee In aerodynamics, the aspect ratio is an airplanes wings span divided by its standard mean chord (SMC). ... In aerodynamics, the zero-lift drag coefficient is a dimensionless parameter which relates an aircrafts zero-lift drag force to its size, speed, and flying altitude. ...

References

1. ^ Loftin, LK, Jr.. Quest for performance: The evolution of modern aircraft. NASA SP-468. Retrieved on 2006-04-22.