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  #11  
Old 08-26-2017, 07:40 AM
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WingedFrog WingedFrog is offline
 
Join Date: Mar 2010
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Quote:
Originally Posted by DaveWelch View Post
My understanding has been that, although perhaps counterintuitive, the best strategy is to try to get the prop stopped (stalled) if you have a dead engine. Reason is that it takes energy to keep it turning (think of the effort needed to start by hand propping). That energy is coming from the air against the prop (drag) and is stealing your airspeed and altitude at the worst possible time while you're trying to get max glide. Seems like I recall at least one POH that said if no luck restarting, then pull up to slow down in an attempt to let the prop stop. Kind of a twofer, you (might) gain some altitude (don't stall!) and get a better glide after it stops.
Excellent point, Dave and convincing argument. I just wonder if I would think of it in the quite stressing situation of engine loss? Well, I am not too much concerned, I am flying behind a Rotax 912, not a Lycoming or Continental engine!
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  #12  
Old 08-26-2017, 07:45 AM
bruceflys bruceflys is offline
 
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I experienced the same effect in my twin engine days. Feathering a windmilling prop stopped it and produced a noticeable surge forward.

If unable to stop windmilling in a single-engine with a constant speed prop, pulling the control full out results in less drag.
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  #13  
Old 08-26-2017, 10:31 AM
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Snowflake Snowflake is offline
 
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Quote:
Originally Posted by Tommycat View Post
But which does create the most drag with a damaged engine? (No power)
A freewheeling prop...or one that's stuck in one position?
We did the math in University to figure this out, and then I tried it in flight after I got my license and verified it... On a Cessna 150 with a metal prop, and a homebuilt with a wooden prop.

A windmilling, fixed-pitch prop creates about the same drag as a flat disk the same diameter as the prop circle. A stopped prop creates the same drag as a 2x6 the length of your prop. The difference *is* significant, so if you can get it stopped, you'll be better off.

However: Unless you have a wood prop, or maybe composite, it'll be hard to get it to stop turning. A wood prop will stop with only a little loading via pitching up. The metal prop on the Cessna took a very high deck angle and low speed to get the blades to stop moving.

Once it's stopped, you have to rely on engine compression keeping it there. If you've blown a cylinder, or cracked a shaft, and have little or no compression available, you *can't* stop it. If you have a constant speed prop that's gone to coarse pitch, it'll be hard to keep it there as well. And if you get your RV up over 100mph or so in the descent, it's likely that airflow over the blades will start it spinning again.

So yes, there are benefits to stopping it, but there are a lot of practical limitations that might make it not worthwhile.
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  #14  
Old 08-26-2017, 11:31 AM
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Mel Mel is offline
 
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Some years ago ABC WideWorld of Flying did a demonstration with a C-182.
He demonstrated both glide ratios and minimum sink with engine at idle, engine idle with prop in course and fine positions and with the prop stopped.
Results were very impressive. If I'm remembering correctly, delta between engine at idle, prop at fine pitch and prop stopped, the glide improved something like 26%.
I'm pretty sure I still have the tape. I could dig it out and run it again.
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  #15  
Old 08-26-2017, 11:48 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Since this is the RV-12 forum, I will suggest that the discussion of stopping it doesn't really matter because as is the case with pretty much all RV's with a light weight (wood or composite) propeller it will stop all by itself if the engine quits unless the the airspeed is kept quite high (not likely if you are using best glide speed).

This can be seen in THIS VIDEO.
He mentions slowing to 65 so that the prop will stop, but with the geared engine and low mass (low rotational inertia) prop, I am pretty sure it will still stop at quite a bit higher airspeed.
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