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3 Answers

Why the L/D is same to the glide ratio?

Asked by: 1238 views Aerodynamics

Why the L/D is same to the glide ratio when the engin is dead and we keep the same airspeed?

3 Answers



  1. Wes Beard on Mar 01, 2013

    Let’s think about this for a minute. The L/D curve shows you how airspeed will affect both lift and drag. You want to find an airspeed that has minimal drag and maximum lift. This speed is called L/Dmax.

    When you glide, you are converting potential energy (altitude) to kinetic energy (airspeed). You would want to find an airspeed that gives the most efficient transfer of energy. You want to find an airspeed that will allow the airplane to stay in the air the longest amount of time. This sounds to me like you want to have maximum lift and minimum drag. The same as L/Dmax.

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  2. Hongxu Chen on Mar 02, 2013

    But what if the L/D is not max? Why they are still the same if you maintain the airspeed after engine die?

    Here is the question I meet.
    When straight and level, V=75kts L/D=12. If engine die. V is still 75kts, and we have 25kts head wind. What is the glide ratio?

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  3. Brian on Mar 02, 2013

    “When straight and level, V=75kts L/D=12. If engine die. V is still 75kts, and we have 25kts head wind. What is the glide ratio?”

    You’re issues a point of reference question. Relative to the ground you’re glide ratio got worse, or would be 8 if my mental math is right. Relative to air passing over the wing it remains the same.

    However, glide ratio is given in a no wind condition. It’s up to the pilot to take the wind conditions into account when calculating a glide distance.

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