Experienced the propped-stop glide once...
Back when I was CFI'n in college (OK, way back...sorta
), I experienced an engine failure in a C-150, while prepping a student for her check ride. Had the chance to see the glide rate both with the prop windmilling and with it stopped that day.
As Mel said, it was a marked increase in glide capability (reduction in sink rate), such that I quickly felt pretty high, and had to slip the aircraft pretty aggressively (though carefully) on base and final to hit my spot on the (appropriately enough) glider airpark that we happened to be above when it let go.
The gist of it is that while climbing out after GRMs to do high work, the engine started sounding a bit "loud", so I had the student climb and get high enough for crossing the SF Bay back to home field safely, just in case...and at about 2500' AGL a piston pin failed. The rod sounded like it was coming through the firewall, so I shut it down. At best glide, we did 2 360's to land at the glider airpark. The prop stopped on the second downwind (without any effort or slowing on my part...I was not even considering trying to stop it...just fly best glide and git 'er on the ground). It really became a glider at that point (it was a 150, not an RV of course). The change in glide/descent rate left a lasting impression on me, though at the time, I was looking out at the landing field, and not at the VSI, so unfortunately I can't give a quantitative VSI change. But it was a significant change in glide, from what I saw and felt.
My RV is CS, so I'd be very interested in seeing the descent rate delta in prop forward versus back...and it sounds like a very good step in the emergency engine-out procedures. I'd be interested in quantifying it, or seeing info from those that do...but if it happens for real, my guess is it won't be about what VSI numbers you get, it'll be about getting to best glide speed, doing what you can to improve the glide rate (ex., prop back) and flying the airplane to get to a safe landing spot.
I'm
not advocating slowing to intentionally stop the prop...no internet instruction here...and I've personally not seen that put out as a single-engine airplane engine-out procedure anywhere. Just relating the experience.
As for the reduction in drag and increase in glide capability, I suspect perhaps Alan and Mel may both be correct. Though I've studied aero, I'm not an aerodynamicist, but I was taught that the windmilling engine will extract energy from the air, which translates to drag, as Alan stated. I've also seen it described as "flat-plate" drag of a windmilling prop somewhere, so it may be that the drag of a windmilling prop can be equated to an equivalent flat plate drag amount, and perhaps that's just an easy way to visualize it. Could be a combination of energy extraction and the drag of the disc, as Mel called it. Either way, when it stops, you do glide farther!
The note about the throttle is of interest...I just got that mag, so I looked it up...Peter Garrison wrote, "the airplane glides farther with the throttle open, because the engine offers less resistance to the propeller". My question would be, if the engine is really dead and windmilling, how does throttle position have an impact? I get the prop pitch thing, and I'll probably do a V-8 salute when someone explains the throttle part, but the brain is not connecting the dots on this one for now. PG also said that the difference between a windmilling CS prop in coarse pitch (low RPM) and fine pitch (high RPM) was about 10% in his plane, and he estimates that the delta between a CS prop in coarse pitch and a stopped CS prop might not be that large, and would be much less than the change with a fixed pitch prop (windmilling vs. stopped). All interesting stuff.
Cheers,
Bob Mills
"Rocket" RV-6
N600SS
4SD