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Prop-er descents.

Is there any advantage to reducing prop speed in a descent for those with constant-speed props? ("How to Fly Cheaper," November 2008 IFR). I got this idea from my instructing days, but haven't run any comparisons. I just do it.

As an instructor, we used to amaze students transitioning to high-performance aircraft by demonstrating that you could stretch an engine-out glide with the prop control. We'd pull the prop all the way back. Assuming we had oil pres sure, the act of forcing the prop to coarse pitch (approaching feather) caused the airplane to accelerate. I thought the same dynamics might save fuel in a descent, but Mr. Bowlin didn't mention it.

In our Glasair, we plan nine miles for every 1000-foot de scent. If terrain, airspace and smooth air allow, we leave the throttle wide open, but back the prop down to 2100 rpm. The Glasair will indicate 160 knots in a 500-fpm descent.

Given that drag is proportional to velocity squared, I'm thinking anything I can do to reduce drag should save fuel. I mean, if I could feather the prop and disengage a clutch to keep the engine running, wouldn't that be the ideal when going downhill? For example, it's illegal in most states to drive downhill in neutral because your car will gain too much speed without the drag of the engine. Isn't reducing prop rpm, whether full throttle or throttled back, like reducing the drag of the engine?

I know I save fuel by reducing raw horsepower by reducing rpm. But what do you think about reducing rpm in a descent to save drag?

Mike Palmer

Phoenix, Ariz.

Good points. We can't say with authority, but we'd reckon the amount of savings would depend on the situation. At idle throttle when the prop is windmilling, clearly a slower windmill should reduce drag and save fuel that's just running through the engine. But this is a low burn to begin with.

At high power, it would be more complex--beyond our math skills. That said, a higher mp and lower rpm combination is usually more efficient for a given horsepower.

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Title Annotation:READBACK
Author:Palmer, Mike
Article Type:Letter to the editor
Date:Jan 1, 2009
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