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RV-6 Glide Variation with Prop Pitch

Ironflight

VAF Moderator / Line Boy
Mentor
I have been doing a lot of Garmin SmartGlide testing recently (yup - look for a possible future article in Kitplanes....), and today I took the RV-6 out in very smooth air to quantify the difference in descent rate in a glide with the prop full forward versus full aft.

Testing was solo, about half fuel, RV-6 with standard O-360 and a paddle-blade Hartzell (from the Jurassic, but recently overhauled...). Temperature on the ground about 40 degrees F, and ground elevation 4,400 msl (which is why I test so high).

Methodology was simple – I climbed to 10,200’, stabilized on autopilot with prop pitch full fine (forward, high RPM). Pulled the throttle to idle, engaged Smart glide to control speed at 93 knots, and let it descend, with the autopilot holding speed and heading. The EFIS captured steady state data between 9,800 and 9,200 msl. Then at 9,000, I pulled the prop all the way back to coarse (full aft, slow RPM), and let it stabilize. Descent rate immediately slowed. Got steady state data between 8,800 and 8,200. All data was recorded on the EFIS card, and I analyzed that at home, taking averages over the periods of interest, when airspeed was steady. Repeated the tests, and got good correlation between the two tests.

Prop Full Forward

RPM 1790
Descent 1400 fpm

Prop Full Aft

RPM 1030
Descent 910 fpm

It was amazing to see the see the difference - and the jump in size of the "glide zone" on the EFIS map.....
 
Thats really cool, great data.

Unfortunately for those of us with Hartzell governors, they only govern down to about 1550 RPM, so pulling the prop lever back farther than that does nothing. If windmill is 1790, I guess I can slow it down a little.
 
I have a video made MANY years ago by the "Wide World of Flying" on this very subject. They used a C-182. Each segment was from same altitude timed to 1,000' lower. Fine pitch, coarse pitch, and with prop stopped The results were quite impressive. Especially with the prop stopped.
 
Thats really cool, great data.

Unfortunately for those of us with Hartzell governors, they only govern down to about 1550 RPM, so pulling the prop lever back farther than that does nothing. If windmill is 1790, I guess I can slow it down a little.

All my planes have the PCU5000…I wasn’t quite sure how low it’d go, and the first pullback it sounded like it was going to fully feather….. :eek:
 
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
 
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.
 
Did similar a couple of years ago on our RV7 with an IO-360, MT 3 blade hydraulic and reported here.

As long as the prop is turning and generating oil pressure, the governor will work. We tried hard to get the prop to stop, but couldn't.

Also, if you think it is failing, running to full coarse will probably prevent the prop stopping and give you just a little more time.
 
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
 
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Not a -6 but another data point on the governor. I did a similar test with my-8 with a Hartzell Composite prop and PCU 5000X governor.
1440 lb solo
5500-3500 ft altitude

Procedure:
Level flight at 88 MPH to Idle.
Maintain 88MPH in decent
No flaps

Prop full forward
1700 RPM
1100 fpm average rate of decent.

Prop full aft
1000RPM
600 fpm average rate of decent.
Mike
 
I need some thinking on this one . . .

Thats really cool, great data.

Unfortunately for those of us with Hartzell governors, they only govern down to about 1550 RPM, so pulling the prop lever back farther than that does nothing. If windmill is 1790, I guess I can slow it down a little.

I suppose the prop blade torque is a factor in pushing rather than pulling, but this is not obvious to me. It only takes 30 psi oil pressure to go full coarse on my hartzell composite when sitting on the ground, and that is in the range of oil pressure.

I would think you will find that governing under power is different from drawing power. Let us know to get our thinking adjusted. And say what prop you are using.

BTW: I used mixture cutoff for testing. And . . . duplicated testing for generation of the glide speed graphs at max and min prop pitch. I'll have to find the data to add for comparison.
Thanks!!
 
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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.


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!

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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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??
 
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.
 
Good point

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... :rolleyes:
Sheeesh...
 
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?
 
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?
 
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
 
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.
 
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.
 
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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While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

An alternative would be some sort of brake fabricated to work against the flywheel to stop the prop which would likely further improve glide characteristics.

Seeing stopped vs. windmilling prop glide data would be interesting to see.
 
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

An alternative would be some sort of brake fabricated to work against the flywheel to stop the prop which would likely further improve glide characteristics.

Seeing stopped vs. windmilling prop glide data would be interesting to see.

BillL said that it only takes 30 psi to push the prop to coarse pitch against the spring when no aerodynamic forces on it. So at coarse pitch, so I think the evidence and practical experience is that the windmilling prop can turn the engine fast enough to generate enough oil pressure to maintain coarse pitch (assuming the governor will control to that low an RPM at all)

Kevin Horton published some prop-stopped glide performance numbers years ago. I think he was planning a flight to a remote island, and wanted to figure out how high he would have to be to be able to glide half way across (or turn around and glide back). IIRC, he found that he essentially had to stall to stop the prop, and the altitude lost in that maneuver was worse than the saving in the glide, unless you were really high and were going to glide for a loooong way.
 
BillL said that it only takes 30 psi to push the prop to coarse pitch against the spring when no aerodynamic forces on it. So at coarse pitch, so I think the evidence and practical experience is that the windmilling prop can turn the engine fast enough to generate enough oil pressure to maintain coarse pitch (assuming the governor will control to that low an RPM at all)

So if catastrophic engine failure occurs where no rotation is possible how would one get 30psi?

Windmilling is the problem...at higher RPMs there is more far induced drag from the thrust the windmilling prop is producing. Thats why the prop needs to be stopped. Feathering a blade is one way to stop it, a brake is another. Feathering obviously has a bit less drag.
 
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

Quite a few years ago my 182 partner did this experiment. He really wanted to see how hard it was to stop the prop. He started with a power at idle-prop in coarse pitch glide. Then he pulled the mixture. The prop stayed in coarse pitch.
(He also found, as others have, that it was hard to stop the prop without stalling the plane. But, once stopped, it stayed stopped.)
Obviously things may be different in an actual engine out, where something might be broken, or there is no oil left, etc.
 
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.

If you carefully watch your ammeter during start, I think you’ll see that it draws much more current during the first 180 deg of rotation (and is much more likely to ‘hang’) than it does once the prop is spinning. That’s because there’s no other cylinder already full of compressed air on its ‘power stroke’. Yes, at very low RPM the losses past the rings are noticeable. But at 800 rpm or more, they’re quite small. If this was not the case, any prolonged idling would result in your oil becoming very thin - from all the raw gas pushed out on the compression stroke.
 
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

That aligns with the results of testing we did at Van's with engines fully shut down but the prop. windmilling.
 
things may be different in an actual engine out, where something might be broken, or there is no oil left, etc.

Yep, and nothing beats real life experience...
January 3rd, 2008, con rod failure on my Falco #1. An exciting ROD with a windmilling prop... luckily my guardian angels were working overtime, thank you all :D

top-hole-780.jpg


Now regularly practising the "impossible turn" with my 3 bladed MT demonstrates a net worth in reduction in ROD if the blue lever is pulled ASAP. Oil P does help indeed.
 
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While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

Fourteen years ago I tested this with a dead engine and the prop would go coarse just like if the engine was idling. Hartzell, MT governor, O-320.

At 70 kts with the mixture at idle cut off (dead engine) and the prop control full out (coarse):
Oil pressure 50 psi
RPM 900

Fin 9A.
 
While it is certainly true pulling the prop back increases glide performance on an idling motor, pulling the prop back in a real engine-out situation would not sustain governor oil pressure where it could work against the spring in the prop. In a short time the prop would go back to fine pitch. Unless one had a prop with an accumulator off of a twin.

An alternative would be some sort of brake fabricated to work against the flywheel to stop the prop which would likely further improve glide characteristics.

Seeing stopped vs. windmilling prop glide data would be interesting to see.

From my research, it is the speeder spring that is the big issue and not oil pressure. Once the engine RPM gets below a certain level (around 1600-1800 on a Hartzel S series) there is not enough energy to move the spring that controls the flow of oil out to the prop. Not surprising that different governor models have different designs and therefore different minimum control levels. I suspect feathering governors have a very different design, but not sure.
 
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Prop Governor in Windmilling Engine

Fourteen years ago I tested this with a dead engine and the prop would go coarse just like if the engine was idling. Hartzell, MT governor, O-320.

At 70 kts with the mixture at idle cut off (dead engine) and the prop control full out (coarse):
Oil pressure 50 psi
RPM 900

Fin 9A.

Yes, I’ve had similar results with true power-off glide tests. My MT governor on my IO-360A1B6 will govern to a low enough speed that it will cycle the prop to full coarse pitch when cranking the engine with the starter and the upper plugs pulled out! In actual glide tests with the mixture pulled to ICO, the governor has no problem governing the prop down to 600 RPM or so. This was tested through about 2000’ of glide, multiple times. If the prop is windmilling the oil pump is pumping. Now as posted above, if the engine failure is the result of a catastrophic internal failure that causes a loss of oil pressure, the governor will not work so this option might not be available in a real emergency.

Also, the windmilling RPM of the engine is dependent on the actual glide airspeed.

Skylor
 
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