RV-6 Glide Variation with Prop Pitch

[Ironflight]

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.....

[scsmith]

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.

[Mel]

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.

[Ironflight]

Originally Posted by scsmith

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…..

[Finley Atherton]

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

[rvbuilder2002]

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.

[mike newall]

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.

[rvmills]

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

[Mikeyb]

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

[BillL]

Originally Posted by scsmith

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!!