Gliding, Props and Arithmetic

FLYING Staff

Saturday, January 12, 2008 at 10:00 AM ET

Verified

Key Takeaways:

Propeller pitch significantly affects glide performance during an engine-out scenario; coarse pitch substantially increases gliding distance (e.g., 20% in testing) by reducing the power absorbed by the windmilling engine compared to fine pitch.
The notion that a stopped propeller always yields a better glide than a windmilling one is often untrue for modern, faster aircraft; a windmilling prop in coarse pitch is generally more efficient, as stopped prop drag increases disproportionately with speed.
Initial tests also indicate that an open throttle, when combined with coarse propeller pitch, provides the best glide performance, a finding that the author acknowledges is somewhat puzzling and warrants further investigation.
The article provides a detailed test procedure for pilots to replicate these observations and measure the effects of prop pitch and throttle position on glide rates in their own aircraft.

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I’ve mentioned a couple of times lately that an airplane with a control-lable-pitch propeller will glide farther with the propeller in coarse pitch than in fine pitch.

These terms coarse and fine aren’t exactly intuitive, but they are analogous to screw threads of coarse and fine pitch. A screw with fine threads needs more turns to advance a certain distance; likewise, an airplane engine turns faster, at a given forward speed, when the prop is in fine pitch. Fine pitch is also called flat pitch. “Flat” may give a clearer idea of the position of the blade, but unfortunately it does not have a suitable antonym. Tilted pitch? Bumpy pitch? You hear “steep” pitch sometimes, but that’s confusing because a more steep pitch gives a less steep climb or descent.