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I was looking for the best glide speed of a DA42 with both engines out, but it seems it's not in the POH. In the emergency checklist, it only mentions a speed of 85 knots to maintain on final approach with landing flaps.

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  • $\begingroup$ In case no information can be derived from the usual sources, (as the POH) the best glide speed can be obtained by finding the maximum endurance speed (the speed that gives the minimum variometer reading when gliding with the engines at idle) and multiplying it by 1,32 $\endgroup$
    – xxavier
    Jul 27 '17 at 14:29
  • $\begingroup$ @xxavier, where did you get the number 1.32? The ratio between best gliding range and best gliding time speeds depends on the lift/drag ratio. $\endgroup$
    – Jan Hudec
    Jul 27 '17 at 20:48
  • $\begingroup$ @Jan Hudec The ratio of 1,32 (more precisely, the fourth root of 3) is the quotient maximum range speed/maximum endurance speed. Hence, if you determine experimentally the maximum endurance speed by watching the variometer while gliding with the engines idling, you can derive the maximum glide speed just by multiplying the maximum endurance speed by 1,32... $\endgroup$
    – xxavier
    Jul 27 '17 at 21:01
  • $\begingroup$ @Jan Hudec I forgot giving the references. A derivation can be found here web.mit.edu/16.unified/www/FALL/thermodynamics/notes/… And another one here cafe.foundation/v2/pdf_tech/MPG.engines/… $\endgroup$
    – xxavier
    Jul 27 '17 at 21:08
  • $\begingroup$ @xxavier, that only applies to powered best range and endurance speed and only valid for piston-powered propeller aircraft. Best gliding time speed is the same as best endurance, but best gliding range speed is different from best powered range speed. $\endgroup$
    – Jan Hudec
    Jul 29 '17 at 8:58

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