Low RPM, Slow RV-6

Mark7986 · #1 09-07-2006, 11:03 AM

Some details: New rv owner, 0-320 150HP, 78 degree pitch prop, internal timing of engine OK, carb heat box works properly, getting full though on throttle,compressions are good, 2100 rpm static, 2450 rpm flat out at 8000 feet 145kts, prop does not unload very fast - takes a long time to get to 2450, 9.3 gph @ 75% power, ball is one ball width to right at cruise, wheel pants and gear fairings installed. Suggestions will be very welcome.

Mark Craven
Fairmont, MN

pierre smith · #2 09-07-2006, 01:08 PM

Hi Mark,
Our prop is only 76" pitch on our 180 HP 6A and that's a lot of pitch, intended for max cruise speed and we do 175+ TAS at 7500 and it turning 2700 RPMs.

Your prop seems overpitched by comparison with ours. Can you borrow a flatter pitched prop? It doesn't seem like you're leaning the engine enough if you're burning 9.3 GPH in an 0-320. We burn 9.5 at 7500'/2700 RPM, leaned.
Regards,

Mel · #3 09-07-2006, 04:03 PM

You don't say what kind of prop. If it is wood, then pitch cannot be compared other than to another prop of the same manufacturer and same diameter. If you are cruising with the ball that far off, you need to address trim problems first.

Mark7986 · #4 09-07-2006, 07:14 PM

Thank for the info. I have been thinking about getting the prop repitched to a 75 what do you think.

Mark

Mark7986 · #5 09-07-2006, 07:15 PM

It a sensenich metal prop

Mark

Leonard_Smith_nz · #6 09-07-2006, 07:29 PM

Quote:

Originally Posted by pierre smith

Hi Mark,
Our prop is only 76" pitch on our 180 HP 6A and that's a lot of pitch, intended for max cruise speed and we do 175+ TAS at 7500 and it turning 2700 RPMs.

Your prop seems overpitched by comparison with ours. Can you borrow a flatter pitched prop? It doesn't seem like you're leaning the engine enough if you're burning 9.3 GPH in an 0-320. We burn 9.5 at 7500'/2700 RPM, leaned.
Regards,

Pierre, double check the 76". Your numbers would make sense for 86"....

Mark,

I've collected a few 'samples' of RVs with the FP Sensenich and have a rough rule of thumb for their performance. I believe your TAS sounds about right. What you may need to look into is your description of the motor 'labouring' to get to 2450 RPM.

sprucemoose · #7 09-07-2006, 08:49 PM

Quote:

Originally Posted by Leonard_Smith_nz

Pierre, double check the 76". Your numbers would make sense for 86"

No, he's right on. I've got a 76" Catto on my 180/ RV-6 and I'm pretty close to those numbers. As Mel suggested, a 76" Catto and a 76" Sensenich are two very different things.

78" is the recommended pitch for a Sensenich for a 150HP RV-6. Have you checked the accuracy of your tach?

Mark7986 · #8 09-07-2006, 09:10 PM

I checked the tach, on the money

Mark

Leonard_Smith_nz · #9 09-07-2006, 09:11 PM

Quote:

Originally Posted by sprucemoose

No, he's right on. I've got a 76" Catto on my 180/ RV-6 and I'm pretty close to those numbers. As Mel suggested, a 76" Catto and a 76" Sensenich are two very different things.

78" is the recommended pitch for a Sensenich for a 150HP RV-6. Have you checked the accuracy of your tach?

Jeff,

My mistake...assumed both were talking about FP Sensenich.

SteinAir · #10 09-07-2006, 09:14 PM

Before I took the 320/FP sensenich off of my latest RV6 I had numbers a little higher than that (and yes, mine was a 150)...

My engine was brand new (built by me), and my Sensenich was 79". I could pretty easily pull it up close to redline (2600 on that prop). I REALLY would NOT consider going all the way down to 76" on a 320. That redline is a hard number. Better to be just under than poking around 26.

Like others have said, pitch numbers on wood props are useless when compared to the metal ones, and even useless when compared to other mfgrs of wood props. Totally depends on the airfoil, blade length, etc......

I'd be more inclined to check a few other things before repitching the prop. Typically these metal sensenich's yield around 150RPM / Inch, so losing 2 inches in pitch from your 78 to 76 would probably give you well over the redline....NOT a good thing on that prop!

I'd check the airplane, tach, asi, rigging, etc...get a good GPS, calculator and do a bunch of runs to really ensure your numbers are accurate.

Anyway, don't know what else to tell you except that 78" should be perfect on that engine.....

Cheers,
Stein.

RV6's, Minneapolis

P.S., for an apples comparision the 180 on our other -6 has an 85" sensenich prop on it, and I can pretty easily wind the engine up to 2850RPM..........but the engine is good & strong, and the plane is clean & light!