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Question ID 810172

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  • Question ID 810172

    Question states that Minimum drag of an aeroplane in straight and level flight occurs at : Maximum CL-CD ratio.

    My concern is, question is about Mimimum Drag and not Minimum drag speed which are two different points on a polar curve

    So why is not the answer: at minimum Cd value ?


  • #2
    Check the mathematical definition of Cd in QID 811370 and its explanation.
    Now look at the total drag curve (ATPdigital PoF Lesson 4 page 10)

    Vmd (and therefore best lift / drag ratio) are both of course found at the bottom of the curve.
    But, as we accelerate beyond Vmd, the curve is relatively flat and we can increase speed by quite a large percentage with only a small percentage increase in total drag.
    So, mathematically, the ratio of total drag to speed (in other words Cd) will continue to fall for a while - until the slope of the curve gets significantly steeper.


    • #3
      I agree with matj1986, question asks about Minimum Drag not Vmd.


      • #4
        The minimum value of drag occurs when we are flying at Vmd. So both refer to the same situation.
        At Vmd we are achieving the best CL CD ratio.
        The formula for the total value of drag is:
        Drag = CD x Q x S.
        So the value of drag is not only dependent on CD. Speed (Q) has an influence as well.

        Take another close look at the polar curve in ATP digital PoF lesson 4 pages 15 and 16.
        As we head down the blue line from position B towards A, (as speed increases) the value of CD continues to decrease a little.
        So the minimum value of CD is NOT found when the value of drag is at its minimum, Vmd.
        The reason for the increase in total drag when CD is decreasing is because the influence of increasing speed is greater than the influence of the small decline in CD (V is squared inside the abbreviation Q).