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Lift and 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 generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air.
Drag and lift coefficients for the NACA 63 3 618 airfoil. Full curves are lift, dashed drag; red curves have R e = 3·10 6, blue 9·10 6. Coefficients of lift and drag against angle of attack. Curve showing induced drag, parasitic drag and total drag as a function of airspeed. Drag curve for the NACA 63 3 618 airfoil, colour-coded as opposite plot.
The power is equal to the drag force times velocity. For aircraft in cruise flight the lift is equal to the weight (L=mg) and the engine thrust is equal to the drag (T=D). Hence, ϵ = P / ( m g v ) = D / L = 1 / f {\displaystyle \epsilon =P/(mgv)=D/L=1/f} , with f=L/D the lift-to-drag ratio , so the specific resistance of airplanes is roughly ...
The ratio of the distance forwards to downwards is called the glide ratio. The glide ratio (E) is numerically equal to the lift-to-drag ratio under these conditions; but is not necessarily equal during other manoeuvres, especially if speed is not constant. A glider's glide ratio varies with airspeed, but there is a maximum value which is ...
For many applications the Clark Y has been an adequate airfoil section; it gives reasonable overall performance in respect of its lift-to-drag ratio, and has gentle and relatively benign stall characteristics. The flat lower surface is not optimal from an aerodynamic perspective, and it is rarely used in modern designs.
The Eta is an example of a trend in glider development in which private pilots initiate the development of new open class gliders. The private development of the Concordia sailplane promises a further elevation of the max lift-to-drag ratio to slightly over 75 at 137 km/h (85 mph; 74 kn). [2]
In the drag force case, the relative wind speed decreases subsequently, and so does the drag force. The relative wind aspect dramatically limits the maximum power that can be extracted by a drag-based wind turbine. Lift-based wind turbines typically have lifting surfaces moving perpendicular to the flow.
It is used for near-supersonic flight and produces a higher lift-to-drag ratio at near supersonic flight than traditional airfoils. Supercritical airfoils employ a flattened upper surface, highly cambered (curved) aft section, and greater leading-edge radius as compared to traditional airfoil shapes. These changes delay the onset of wave drag.