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I am reading the FAA Airplane Flying Handbook, chapter 3. It says:

To enter a glide, the pilot should close the throttle and, if equipped, advance the propeller lever forward. (...)

Why do we advance the propeller lever forward? Doing so will increase RPM but will also increase drag, right? Whereas in a glide, we should reduce drag to a minimum. So shouldn't we "feather" the propeller instead to reduce drag?

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  • $\begingroup$ Isn’t that the same thing? $\endgroup$
    – Frog
    Aug 15 at 6:24
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    $\begingroup$ @Frog Propeller lever forward gives a fine blade pitch - the opposite of feather. $\endgroup$ Aug 15 at 6:59

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Don't think of it as resetting propeller pitch. Think of it as resetting the propeller governor to let the engine run at redline RPM when at max throttle. Outside of engine failure situations, glide ratio effected by propeller blade angle is not a consideration.

Going to wide open throttle with the propeller governor set to, say, 2200 RPM does a few things:

  1. The power available is only that which WOT gives at 2200 RPM, maybe 85% of rated horsepower, when you may want all of it.
  2. WOT at low RPM raises Brake Mean Effective Pressure in the cylinders to very high levels and can bring on detonation if the conditions are right.
  3. Lastly if you go to WOT and then follow up with max propeller RPM applied suddenly, the propeller will typically overspeed past redline momentarily as the governor overshoots the RPM setting and has to catch up. Everybody for miles around can hear the result.

There are times when you may leave the prop RPM where it is, say if you are reducing power for an enroute descent, and when you get to the level off altitude, you're just going to bring manifold pressure gently back to its original cruise value. But for any descent where there is a likelihood of needing full power at the bottom of the descent, you ease the prop to max RPM after reducing throttle to ensure that full power is available as soon as throttle is advanced.

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The answer here can be fairly nuanced. But this is a great example of a time when you may need to deviate from the advice in the AFH and follow your POH. Remember- always follow the POH when it disagrees with the AFH, PHAK, or basically any other document!

In a simulated engine out, it makes sense to leave the propeller at fine pitch for a couple reasons. First is that you may need to go around, and you don't want to apply full power when the propeller control is at low RPM. Second is that the idle engine is still producing some power. The pitch being as fine as possible can help offset some of this power and keep you from expecting too much of your glide ratio in a real emergency.

In a bona fide engine out, there are a few possibilities:

  • in some airplanes or with some failure modes you will lose pitch control over the propeller when the engine dies, so it's a moot point
  • if you can feather the propeller, you almost certainly should, as it significantly reduces drag
  • some airplanes do not have the ability to feather the propeller. Putting the propeller to high RPM may cause more drag. But it may also cause the propeller to stop windmilling and give a net reduction in drag! See what, if anything, your POH says to do with the propeller control in an engine out
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Unfortunately the handbook does not give any explanation why they propose the contra intuitive adjustment.

I for myself see two possibilities:

  1. As the handbook states, for a powered plane a glide with the engine stopped is an abnormal situation except the final approach before touchdown. A pilot should try to restart the engine by any chance. In case the propeller was feathered the force to rotate it by the crank might be to high to start the engine. Having the propeller lever forward eliminates this problem.

  2. For a powered plane with constant speed propeller the touchdown configuration is with the propeller lever forward so that, in case of a go around, the plane is in start configuration. Therefore a pilot is used to how the descend of the plane feels with the propeller set to maximum pitch (and drag). This might support an "on point landing" in unknown terrain better then having a slightly longer distance to travel.

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This is a guess, but one possible answer is that at idle power, the engine is not putting out any torque, so it will be spinning at lowest possible RPM for the pitch angle set by the prop governor, so pushing the prop lever forward, (which flattens the pitch) will not cause as much of an increase in rpm (and concomitant increase in drag), as it decreases drag by flattening the pitch.

But this is a guess...

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