Lift/Drag Ratio

The lift-to-drag ratio (L/D) is the amount of lift generated by a wing or airfoil compared to its drag. The L/D ratio indicates airfoil efficiency. Aircraft with higher L/D ratios are more efficient than those with lower L/D ratios. By plotting the induced and parasitic drag relative to speed, a ‘U’ shaped profile emerges as shown below. This profile is known as total drag and is simply the sum of the induced and parasitic drag.

The most important concepts to remember regarding induced, parasitic and total drag are highlighted below

  • In flight at low speeds, total drag is predominantly composed of induced drag
  • As speed increases, parasitic drag increases, but induced drag decreases at a faster rate. Therefore total drag decreases. At one particular angle of attack total drag is at a minimum. This point corresponds to the maximum lift to drag ratio (L/D)MAX. Because of the particular manner in which parasite and induced drags vary with speed, the minimum total drag occurs when induced and parasite drag are equal.
  • At high speeds, induced drag is an extremely small part of the total drag and parasite drag predominates.

Many important items of airplane performance are obtained in flight at (L/D)MAX Typical performance conditions which occur at (L/D)MAX are:

  • Maximum range of propeller-driven airplanes; and,
  • Maximum power-off gliding distance

An unbelievable feature of gliding performance is the effect of airplane gross weight. Since the maximum L/D ratio of a given airplane is an intrinsic property of the aerodynamic configuration, the gross weight will not affect the gliding performance.

For example, if an aircraft has an (L/D)MAX of 15, it can obtain a maximum of 15 miles horizontal distance for each mile of altitude. This would be true of this particular airplane at any gross weight if the airplane is flown at the angle of attack for (L/D)MAX. Of course, the gross weight would affect the glide airspeed necessary for this particular angle of attack but the glide ratio would be unaffected.

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