According to Cessna 172 POH the speed for max range with power off, or glide speed, is 65, and achieved at the best L/D ratio. However with power on the speed for max range will be around 75. Why is there a difference between speeds for max range with and without power?

Edit- Is this because with power off we are just looking for the lowest point on the drag curve, and with power on we are willing to accept the increase in drag from speed of 65 to 75 since I guess lift increases more then drag between those speeds?

  • $\begingroup$ Different vertical speeds and altitudes? $\endgroup$ Commented Aug 19, 2020 at 4:28
  • 1
    $\begingroup$ Because max range isn't the same as max endurance. See this question: aviation.stackexchange.com/questions/36440/… $\endgroup$
    – Ralph J
    Commented Aug 19, 2020 at 4:52
  • $\begingroup$ A- what altitudes? B- what does endurance have to do with anything? That wasn’t mentioned anywhere in the question $\endgroup$
    – leha007
    Commented Aug 19, 2020 at 4:56
  • $\begingroup$ @leha007 Flying at best L/D will give you max endurance, but (as the linked Q/A explains) that isn't the same as max range. Regardless of the altitude you're at. $\endgroup$
    – Ralph J
    Commented Aug 19, 2020 at 13:57
  • $\begingroup$ Ralph J- according to TC flight manual- “maximum range is achieved when the aircraft is being operated at the angle of attack giving the greatest ratio of lift to drag”. Maximum range- not maximum endurance. $\endgroup$
    – leha007
    Commented Aug 19, 2020 at 14:43

1 Answer 1


Maximum range depends not only on the variation of lift to drag versus airspeed, but also on the efficiency of the engine and propeller versus airspeed. The C172's fixed pitch prop, fixed ignition spark advance timing, fixed valve lift and closure timing and other characteristics all have effects on the miles per gallon attained at different indicated airspeeds along with the lift to drag of the airframe.


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