RPM problem

JKF159 · #1 12-02-2012, 07:18 AM

Hi all, I'm new to the forum.
I'm flying an RV6A that my Dad built 18 years ago. It has an O320 with a Bernie Warnke 70X74 that was installed during the build. It did not turn up what he thought was enough RPM so it was cut to 68 inches per Bernies recommendation. It gained 100 RPM just like Bernie said it would. It now turns 2300 static. The problem is it will only turn 2300 at 8000'. It trues at about 168 MPH at this altitude. Climb at gross was timed with a stop watch at about 1000 fpm at 1000 feet and 550 fpm at 7000'. 9 minutes 23 seconds from 250 to 7000 feet.
We are curious as to why others seem to get static RPM plus 400 to 500 RPM at altitude. I can get about 2500 RPM at low altitude WOT but I dont fly WOT at low alt so I dont have any numbers for that.
Two weeks ago I flew a 400 mile XC at 7500' ind alt and about 52 degrees at altitude and WOT it burned 8.6 GPH. This was exactly 2 hours at altitude on one full tank so this is an accurate fuel burn. I dont have a fuel flow gauge or manifold pressure so I dont have any real info on what percent power I'm running except fuel burn after the fact.
It seems that the engine is developing enough power but why the narrow RPM range?
Any thoughts on this setup or these numbers?

gerrychuck · #2 12-02-2012, 11:37 AM

Welcome to VAF! You will find this forum a fantastic resource moving forward. I'm sure much more knowledgeable people will chime in quickly with more detailed info and theories, but what is immediately obvious is that your engine is not producing the power that it should at altitude. With a fixed pitch prop producing that static rpm, you should be right around redline at WOT and 8000 ft. Having said that, your fuel burn is about right for an 0-320 at 75%, although that is obviously very dependant on how you lean as well.

Beyond this simplistic observation, I don't have much (intake restriction, maybe?) except that it shouldn't have anything to do with the prop itself. I'm sure someone with much more expertise will be along soon. Good luck troubleshooting!

Russ McCutcheon · #3 12-02-2012, 11:53 AM

I flew one of these props on a 160hp RV a while back and had the exact same result as you are having, 2250 static and the same 2250 max in the air and it liked 1900 to 2000 in cruse. I believe the Warnke prop un-twists under static load allowing your static RPM to be 2300 and when the load is reduced wile in flight (it?s making less thrust in flight therefore less load) the prop goes back to its built in higher twist/pitch. So it?s like having a cruse prop that un-twists under take off load and re-twists under cruse load. I thought it seemed to work ok but you will never get top speed out of your plane because you will never get full power.

Mike S · #4 12-02-2012, 12:32 PM

Quote:

Originally Posted by JKF159

Hi all, I'm new to the forum.

Jeff,

to the good ship VAF.

Steve Barnes · #5 12-02-2012, 12:39 PM

Typically if you have a static of 2300 you should get a slight over speed at cruise WOT. I am not familiar Bernie's props. I can remember in the 80's someone came up with an "Almost CS Prop" that would change the twist at different loading. I can't remember if this was a Warnake or not.

Steve

Mike S · #6 12-02-2012, 12:47 PM

Quote:

Originally Posted by Steve Barnes

Typically if you have a static of 2300 you should get a slight over speed at cruise WOT. I am not familiar Bernie's props. I can remember in the 80's someone came up with an "Almost CS Prop" that would change the twist at different loading. I can't remember if this was a Warnake or not.

Steve

Yes, that was what Bernie called his props.

JKF159 · #7 12-02-2012, 02:02 PM

Quote:

Originally Posted by gerrychuck

Welcome to VAF! You will find this forum a fantastic resource moving forward. I'm sure much more knowledgeable people will chime in quickly with more detailed info and theories, but what is immediately obvious is that your engine is not producing the power that it should at altitude. With a fixed pitch prop producing that static rpm, you should be right around redline at WOT and 8000 ft. Having said that, your fuel burn is about right for an 0-320 at 75%, although that is obviously very dependant on how you lean as well.

Beyond this simplistic observation, I don't have much (intake restriction, maybe?) except that it shouldn't have anything to do with the prop itself. I'm sure someone with much more expertise will be along soon. Good luck troubleshooting!

Yes leaning will make a difference for sure. On this flight I leaned to roughness, enrichened to smoothness then 3/4 turn in. This is a little richer than necessary probably but I don't know.

JKF159 · #8 12-02-2012, 02:06 PM

Quote:

Originally Posted by Russ McCutcheon

I flew one of these props on a 160hp RV a while back and had the exact same result as you are having, 2250 static and the same 2250 max in the air and it liked 1900 to 2000 in cruse. I believe the Warnke prop un-twists under static load allowing your static RPM to be 2300 and when the load is reduced wile in flight (it?s making less thrust in flight therefore less load) the prop goes back to its built in higher twist/pitch. So it?s like having a cruse prop that un-twists under take off load and re-twists under cruse load. I thought it seemed to work ok but you will never get top speed out of your plane because you will never get full power.

Yes the Warnke was indeed advertised as the almost constant speed prop. Until now I've never heard any static verses cruise numbers. When you say that it liked 1900 to 2000 what exactly do you mean? TAS? Altitude?
I get about 140mph IAS at 2100 RPM down low. That is my instrument approach speed.

Russ McCutcheon · #9 12-02-2012, 02:13 PM

Quote:

Originally Posted by JKF159

Yes the Warnke was indeed advertised as the almost constant speed prop. Until now I've never heard any static verses cruise numbers. When you say that it liked 1900 to 2000 what exactly do you mean? TAS? Altitude?
I get about 140mph IAS at 2100 RPM down low. That is my instrument approach speed.

RPM,,, about 155 MPH indicated and 8 GPH at about 2000 RPM and 24" MP at 2000 MSL.

ccsmith51 · #10 12-02-2012, 02:32 PM

Interesting thread. I have a O-360 RV-4 with a Sterba 70x82 wood prop. I recently did a static and WOT test. Static at 1,000' DA and leaned was 2,400 RPM. WOT leaned at 5,000' DA was 2,800 RPM and 180 MPH IAS. I felt that speed was a bit slow, but I also think that my plane is draggy... I don't know if this info is of any benefit to you, just thought I'd throw it in.