Editing Flight (section)

==== Lift-to-drag ratio ====
{{Main|Lift-to-drag ratio}}
[[File:Drag curves for aircraft in flight.svg|thumb|Speed and drag relationships for a typical aircraft]]
Aerodynamic lift is created by the motion of an aerodynamic object (wing) through the air, which due to its shape and angle deflects the air. For sustained straight and level flight, lift must be equal and opposite to weight. In general, long narrow wings are able deflect a large amount of air at a slow speed, whereas smaller wings need a higher forward speed to deflect an equivalent amount of air and thus generate an equivalent amount of lift. Large cargo aircraft tend to use longer wings with higher angles of attack, whereas supersonic aircraft tend to have short wings and rely heavily on high forward speed to generate lift.

However, this lift (deflection) process inevitably causes a retarding force called drag. Because lift and drag are both aerodynamic forces, the ratio of lift to drag is an indication of the aerodynamic efficiency of the airplane. The lift to drag ratio is the L/D ratio, pronounced "L over D ratio." An airplane has a high L/D ratio if it produces a large amount of lift or a small amount of drag. The lift/drag ratio is determined by dividing the lift coefficient by the drag coefficient, CL/CD.<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html</ref>

The lift coefficient Cl is equal to the lift L divided by the (density r times half the velocity V squared times the wing area A). [Cl = L / (A * .5 * r * V^2)] The lift coefficient is also affected by the compressibility of the air, which is much greater at higher speeds, so velocity V is not a linear function. Compressibility is also affected by the shape of the aircraft surfaces.
<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/liftco.html</ref>

The drag coefficient Cd is equal to the drag D divided by the (density r times half the velocity V squared times the reference area A). [Cd = D / (A * .5 * r * V^2)] 
<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center https://www.grc.nasa.gov/www/k-12/airplane/dragco.html</ref>

Lift-to-drag ratios for practical aircraft vary from about 4:1 for vehicles and birds with relatively short wings, up to 60:1 or more for vehicles with very long wings, such as gliders. A greater angle of attack relative to the forward movement also increases the extent of deflection, and thus generates extra lift. However a greater angle of attack also generates extra drag.

Lift/drag ratio also determines the glide ratio and gliding range. Since the glide ratio is based only on the relationship of the aerodynamics forces acting on the aircraft, aircraft weight will not affect it. The only effect weight has is to vary the time that the aircraft will glide for – a heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time.<ref>The Beginner's Guide to Aeronautics - NASA Glenn Research Center
https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html</ref>