Drag
Induced drag is caused by the generation of lift by the wing. Lift generated by a wing is perpendicular to the wing, but since wings typically fly at some small angle of attack, this means that a component of the force is directed 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 drag graph, the left side of the U.
Profile drag is caused by air hitting the wing, and other parts of the aircraft. This form of drag, also known as wind resistance, 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 drag graph's U shape. Profile drag is lowered primarily by reducing cross section and streamlining.
As lift increases steadily until the critical angle, it is normally the point where the combined drag is at its lowest, that the wing or aircraft is performing at its best L/D.
Designers will typically select a wing design which produces an L/D peak at the chosen cruising speed for a powered fixed-wing 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 also important.
Minimising drag is of particular interest in the design and operation of high performance glider (sailplane)s, the largest of which can have glide ratios approaching 60 to 1, though many others have a lower performance; 25:1 being considered adequate for training use.
Read more about this topic: Gliding Flight
Famous quotes containing the word drag:
“Dying smokers sense
Walking towards them through some dappled park
As if on water that unfocused she
No match lit up, nor drag ever brought near....”
—Philip Larkin (1922–1986)
“Executives are like joggers. If you stop a jogger, he goes on running on the spot. If you drag an executive away from his business, he goes on running on the spot, pawing the ground, talking business. He never stops hurtling onwards, making decisions and executing them.”
—Jean Baudrillard (b. 1929)
“Our own theological Church, as we know, has scorned and vilified the body till it has seemed almost a reproach and a shame to have one, yet at the same time has credited it with power to drag the soul to perdition.”
—Eliza Farnham (1815–1864)