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  #11  
Old 11-30-2022, 09:48 AM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by Finley Atherton View Post
A general word of caution.

I know it sounds obvious but if you are practicing forced landings (especially to low altitude) with the prop control full coarse remember to go full fine at least a few seconds before applying power for the climb as it will take this time to go fine. It is easy to forget and the engine will not be happy with high power and a coarse prop plus you will not climb till the prop goes fine.

Fin 9A
You’ll note that I was doing this about 6,000 AGL…for a reason! I do a lot of flight testing….. But for those who might be playing with this for their airplane, yes - test your engine/prop’s response, and don’t expect instant power, so don’t be low.


Quote:
Originally Posted by rvbuilder2002 View Post
As Steve hinted, the result is dependent on how low of an RPM a particular gov is able to control the propeller.
We did some testing at Vans with two airplanes a couple years ago, with the engines fully shut down but windmilling. We couldn’t measure any difference between the control full fwd or full aft.
Point… it won’t hurt in an emergency but don’t assume it will help unless you have confirmed it wit testing.
Interesting Scott - makes me want to go out and do this again using various “low rpm” settings to see the shape of the curve - plot descent rate versus rpm (at idle). I’m drawing a straight lien through two data points here - there’s no reason to think the curve is linear!

Quote:
Originally Posted by rvmills View Post
Paul,

Is Smart Glide a new product, or is it resident in Garmin EFISs or navigators? (Sounds like the EFIS). A few questions (which you'll likely cover in the article ):


How did you settle on, 93 KIAS? Previous best glide testing?

How did you select that in Smart Glide? Setup selection in the box?

Did you test both fine first then course, and course first, then fine, to see if decreasing altitude impacted or skewed performance? I noted that the altitudes weren't that far apart, so this may not be an issue. I ask, because as you know, I have some glide testing coming in the not-too-distant future.

When I lived in your 'hood, and went from my D-twist to my Hartzell BA, I tested for best glide, and for relative performance in descent from a high key at Stead, to see what high key altitude I needed to make the runway. At fine pitch (blue knob in, high RPM) it required crossing the numbers at high key at 1800' AGL. At coarse pitch (blue knob out, low RPM), it required 1200-1300'. It was pretty significant. That's the Super Six with a heavy motor, clipped wings and an 80" prop. I'll bet your light -6 will require less. I add a bit for margin, figuring I'd want to cross the numbers at 2000' at high key to easily make the field, with some margin...and just in case the oil pressure had gone to zero, and the prop would not go coarse...or the meat-servo was less than perfect in execution

Neat test results amigo!

Cheers,
Bob
Smart Glide has been around for a couple of revs in the G3X Touch software Bob - it is a super-slimmed-down version of Garmin’s Autoland. It will pitch up to best glide and point you at the nearest available airport, and when you get to 2,000’ AGL, tell you to disconnect the autopilot and land. Since there’s no auto throttle, it can’t do a full landing, and it won’t maneuver you to touchdown - just get you to an airport. If no airport is within the glide ring (most often the case in this area), it just goes wings level and best glide. That lets you put your attention on trying to get a re-light, or “prepare the cabin for an off-airport event”…..

I determined 93 knots as best glide in this airplane a couple of weeks back in testing - did an airspeed sweep at idle (about six times - need to filter out bad data) and reduced the data front eh EFIS on the ground later. Both the -6 and the -3 have best glide between 92 and 95 KIAS….

You set up the SmartGlide parameters in G3X setup - its a pain to do in flight (intentionally, I think) - by setting best glide speed and descent rate in the “aircraft Parameters” tab. That also determines the size of the “Glide Ring” on the map. The dilemma now is whether to set it for fine or course pitch conditions…..

I did the test as described both times - fine first, then coarse - but ignored the data during the transition. If I go out gain, I’ll reverse to see if it makes a difference, but honestly, what I was trying to do was see if there was a significant difference, and yes - with the very low RPM, the difference is significant!
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Last edited by Ironflight : 11-30-2022 at 10:29 PM.
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  #12  
Old 11-30-2022, 10:16 AM
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riseric riseric is offline
 
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Default Zero Thrust? and Engine out vs. Throttle open?

Thanks Paul for your testing.
I recently did the gliding speed tests as per the EAA test manual.
Best distance with throttle at idle and prop full coarse was found at 95 KIAS.
I was solo and approximately 1500 lbs.

I recall (but did not note down) RPM was around 1150

In light twins, when practicing one engine out procedures, there was a "Zero Thrust" setting that was used to closely simulate a stopped engine and featered prop.

Would it be useful to find such zero thrust data/setting on our constant speed prop equipped RV's to practice/find best gliding speeds?
Yeah, most of our props don't feather...

I understand that it's quite hard to actually stop the windmilling prop at 95 KIAS, so maybe it would be useless unless a hard internal break in the engine actually stopped the prop.

And another thing (that may already be discussed previously somewhere here), in case of an real engine out with prop windmilling at coarse/min RPM, since the relative wind is now pushing on the prop, creating some drag, would it be best to open the throttle fully so the pistons/prop turn more freely, thus helping a wee bit to glide further?
Me thinks so.
What's the take on this from the collective brain trust??
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  #13  
Old 11-30-2022, 01:43 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by riseric View Post

And another thing (that may already be discussed previously somewhere here), in case of an real engine out with prop windmilling at coarse/min RPM, since the relative wind is now pushing on the prop, creating some drag, would it be best to open the throttle fully so the pistons/prop turn more freely, thus helping a wee bit to glide further?
Me thinks so.
What's the take on this from the collective brain trust??
I am sure there is a small benefit, but MOST of the resistance is from the compression stroke and not the vacuum from a closed throttle on the intake stoke. Many diesels let you lock the valves open to avoid the compression resistance. I used to use this on the sailboats diesel to get it to start when the battery was low.
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  #14  
Old 11-30-2022, 03:21 PM
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riseric riseric is offline
 
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Default Good point

Quote:
Originally Posted by lr172 View Post
I am sure there is a small benefit, but MOST of the resistance is from the compression stroke and not the vacuum from a closed throttle on the intake stoke. Many diesels let you lock the valves open to avoid the compression resistance. I used to use this on the sailboats diesel to get it to start when the battery was low.

Good point !!
Didn't think about the compression stroke...
Sheeesh...
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  #15  
Old 11-30-2022, 04:25 PM
agent4573 agent4573 is offline
 
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We've done some similar testing in the 7 with the PCU5000 and the whirlwind 300 series. A 95 KIAS descent is around a 30 degree dive for us with the prop forward. We ended up down around 70-75KIAS best glide with the prop forward, and around 80KIAS with it full coarse. A lot of people post best glide speed in the 90-95KIAS range, and I was surprised we were so much slower than the standard range. On the plus side, power off 180's are a breeze. You can feel the plane get lighter and accelerate when you pull the prop back. It works better than some speed brakes.

What glide ratio are you programming into the Garmin? Do you put the conservative full forward glide ratio and hope you can make a further out airport if you still have prop control or do you put the coarse ratio and decrease your glide range mentally if you can't pull the prop back for some reason?
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  #16  
Old 11-30-2022, 04:58 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by lr172 View Post
I am sure there is a small benefit, but MOST of the resistance is from the compression stroke and not the vacuum from a closed throttle on the intake stoke. Many diesels let you lock the valves open to avoid the compression resistance. I used to use this on the sailboats diesel to get it to start when the battery was low.
I disagree. Immediately after the compression stroke is the power stroke, and, if there is no leakage past valves and rings, you get all that energy back. I think it is completely about throttling losses (pumping air from lower pressure back up to outside pressure. (In your diesel, if you can lock open the valves, your ‘air pump’ leaks like a sieve, and doesn’t pump air at all.)
As to throttle open or closed, this is tricky. The power extracted will go something like delta P times RPM, where delta P is the pressure loss in the intake system. Opening the throttle decreases delta P, but increases RPM. I suspect which is better or worse depends on prop efficiency - in which case are you closer to the prop’s max L/D?
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  #17  
Old 11-30-2022, 05:52 PM
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Ironflight Ironflight is offline
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Quote:
Originally Posted by agent4573 View Post
What glide ratio are you programming into the Garmin? Do you put the conservative full forward glide ratio and hope you can make a further out airport if you still have prop control or do you put the coarse ratio and decrease your glide range mentally if you can't pull the prop back for some reason?
To be honest, that is a matter of debate in our household right now, and we haven’t settled on which way to go. Out here in the Great Basin, it is rare to have ANY airport in glide range for a short-wing RV, so its more likely the glide ring will be used to find a lakebed or highway, so the question of setting a conservative or “on the edge” glide ratio is still open. We’ve got two G3X Touch airplanes, and we have been playing with both options on both planes, and haven’t come up with a definitive answer yet.

Paul
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  #18  
Old 11-30-2022, 08:11 PM
scsmith scsmith is offline
 
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Quote:
Originally Posted by riseric View Post

And another thing (that may already be discussed previously somewhere here), in case of an real engine out with prop windmilling at coarse/min RPM, since the relative wind is now pushing on the prop, creating some drag, would it be best to open the throttle fully so the pistons/prop turn more freely, thus helping a wee bit to glide further?
Me thinks so.
What's the take on this from the collective brain trust??
We-all talked about this once before. I remember thinking that with the throttle closed there would be less pumping work, since it couldn't pull air in from the intake, so the overall pressures would be lower thoughout the cycle. But I forgot that with low pressures, you can actually suck air in backward through the exhaust, so it will find the air it craves one way or the other.

So....

I went out and tested, and I don't remember which turned out to be better for glide, but I do remember that whatever I had thought about it at the time was wrong.
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  #19  
Old 11-30-2022, 09:17 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Ironflight View Post
Interesting Scott - makes me want to go out and do this again using various “low rpm” settings to see the shape of the curve - plot descent rate versus rpm (at idle). I’m drawing a straight lien through two data points here - there’s no reason to think the curve is linear!
It is likely related to the particular prop. gov. that are installed on the two airplanes.
Neither will cycle the prop on the ground below 1800 RPM.
The RPM in the engine off glide was well below that so pulling the prop control made no noticeable difference.
BTW in case anyone is wondering... The mentioned flight testing was done during a scheduled overnight trip to the Alvord Desert... Many square miles of flat surface to land on should an engine not come back to life.
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  #20  
Old 12-01-2022, 09:16 AM
lr172 lr172 is offline
 
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Originally Posted by BobTurner View Post
I disagree. Immediately after the compression stroke is the power stroke, and, if there is no leakage past valves and rings, you get all that energy back. I think it is completely about throttling losses (pumping air from lower pressure back up to outside pressure. (In your diesel, if you can lock open the valves, your ‘air pump’ leaks like a sieve, and doesn’t pump air at all.)
As to throttle open or closed, this is tricky. The power extracted will go something like delta P times RPM, where delta P is the pressure loss in the intake system. Opening the throttle decreases delta P, but increases RPM. I suspect which is better or worse depends on prop efficiency - in which case are you closer to the prop’s max L/D?
We can agree to disagree on that. If your hypothesis is correct, why does it take 150 - 200 amps (1800-2400 watts) to get one of these engines spinning at 200 RPM? In my experience, idle vs WOT makes very little difference in the power required to get them spinning. Cylinders are made to hold pressure at high RPMs. At very low RPMs, they leak like a sieve. I can drain my pancake compressor in a minute or two when doing leak down tests.

Not an engineer, so won't debate the science behind it. Only working off real world experience over the years. My belief is that the majority of the pressure created on the upstroke is consumed by bleeding out of the gaps and therefore very little energy left to push the piston back down.
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Last edited by lr172 : 12-01-2022 at 09:28 AM.
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