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Imagine I have an engine failure in cruise flight with a Cessna 152 or 172.

Should I select 10° flap to increase glide angle?
Would it give me more range to reach safety?

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    $\begingroup$ I am curious as to why you think flaps would make for a shallower glide angle or give you more range. The opposite is true in both cases for any aircraft. I hope a flying instructor did not tell you this. $\endgroup$ Apr 16, 2017 at 15:32
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    $\begingroup$ Well not an instructor but I read about L/D ratio (aerodynamic efficiency) and there is written that " with flaps lowered: 1- RANGE will be decreased, 2- GLIDE ANGLE will increase and 3- CLIMB ANGLE reduces. so I've got a bit confused about what it is really mean. (english is not my mother tongue so I am struggling sometimes understanding what exactly means) $\endgroup$ Apr 24, 2017 at 7:55
  • $\begingroup$ My concerns was about GLIDE ANGLE in particular; if it is increased will I flying a longer distance without power? $\endgroup$ Apr 24, 2017 at 7:57
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    $\begingroup$ Increased glide angle means a steeper glide angle, which means flying a shorter distance before reaching the ground. $\endgroup$ Apr 24, 2017 at 9:11
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    $\begingroup$ I got it, I appreciate that. Thank you. $\endgroup$ Apr 25, 2017 at 8:20

2 Answers 2

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You keep it clean: adding flaps adds drag. You can see this in Section 3 of the Cessna POH, which includes a Maximum Glide (distance) diagram. In the C172S POH that I checked, it notes that maximum distance is achieved with:

  • SPEED 68 KIAS
  • PROPELLER WINDMILLING
  • FLAPS UP
  • ZERO WIND

(A tailwind would extend your range, but only in one direction.)

Two other points worth noting:

  • If the engine failure also caused an electrical failure then you won't be able to use the flaps anyway (in models with electrically operated flaps)
  • You might want to reduce your gliding distance if the best landing spot is relatively close by, and extending flaps is one way to do that (along with slipping, S-turns or other maneuvers, and extending the gear)
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    $\begingroup$ To add about landing on closer spots: I do NOT recommend 360s unless you have PLENTY of altitude and are confident you know how much you lose completing a 360 in that aircraft. The last thing you want to do is get 1/2 way through your turn and realize you lack the altitude to hit your target, or worse: over-bank at a low altitude and airspeed and enter a deadly spin. $\endgroup$
    – Bing
    Apr 15, 2017 at 0:41
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In general, the cleanest configuration gives the least drag and hence the maximum range at the optimum gliding speed. Flaps should be lowered (with caution) only a short time before touching the ground, in order to reduce the speed as much as possible...

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    $\begingroup$ And if you have a non-feathering constant-speed prop, pull it to low RPM to reduce drag. $\endgroup$
    – JScarry
    Apr 14, 2017 at 13:41

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