Crankcase Vent, oil on belly & speed test CC vent in/out of cowl
My RV-7a firewall forward sports a Barrett I-O 360 (with ~80 hours), a horizontal intake, a Sam James Cowl with a Sam James Plenum, Vetterman 4-into-2 cross-over exhaust, and Robert Paisley's EFII (dual ecu's). I'm very happy with this setup, and the engine runs strong. However, the belly is oil-coated at the rate of about 1 qt every 5-6 hours, and that's with only starting at 6.5 qts per change/fill. I reached out to Allen Barrett a couple of weeks back, and his suggestion was that the oil is being "pulled" or expelled out somehow, and to look into that first.
After several hours of searching/reading the many posts on VAF about oil on the belly, the one post that seemed more likely to be a match for my situation was this excellent post by Tom Martin a number of years back.
To determine if I have the same issue, I enlisted the help of my good friend Lee (the crazy Hawaiian dude who's also a great A&P as well), where we set up three measuring locations using his magnahelic: Upper cowl (inside plenum), lower cowl (at CC vent) and crankcase (by way of oil filler tube). After checking to make sure all test locations were secure, and could handle full power runs, I took some measurements. All measurements were at 7500 MSL, 2600 RPM with 23" MP, using a 2-axis auto-pilot to hold things steady while I swapped out the tubes on the magnahelic. A few tests were done during climb, but the data was not significantly different, so I'm only posting the level flight measurements. Also, the process of making a change, and re-testing spanned a couple of days, so the OAT's were not identical from test-to-test, but all speeds are measured in Knots True from my trusty Dynon (with OAT probe). Prior to taking these measurements, I cleaned the airplane which includes a spotless belly.
This first picture shows the location and configuration of the CC vent (along with a diffuser on the measurement line) before any changes. Basically Van's stock setup (but without the recommended slant-cut on the end of the tube -- I missed that subtle detail..).
Test Case 1: No changes, original setup
Location/HG Measurement DATA:
Crankcase: OAT 65F,171KTS,7500MSL,2600RPM,23MP,2.7"WC
Bottom: OAT 65F,171KTS,7500MSL,2600RPM,23MP,2.5"WC
Top: OAT 65F,171KTS,7500MSL,2600RPM,23MP,10+"WC (pegged the 0-10 HG Magnahelic)
From the above data I was encouraged that my scenario is matching what Tom Martin saw (however, my pressures were lower, but still relative relationship-wise), where the crankcase pressure was slightly higher than the pressure measured low in the cowl at the CC vent.
This next picture is where I chose to take a baby-step, and change the cut on the end of the CC vent tube to match Van's plans. This is the only change made between these tests.
Test Case 2: Slant cut on CC vent tube
Location/HG Measurement DATA:
Crankcase: OAT 72F,171KTS,7500MSL,2600RPM,23MP,2.6"WC
Bottom: OAT 72F,171KTS,7500MSL,2600RPM,23MP,2.7"WC
Top: OAT 72F,171KTS,7500MSL,2600RPM,23MP,10+"WC (pegged the 0-10 HG Magnahelic)
Interesting numbers here, and I suppose I could have stopped with my changes to see if the oil on the belly would be reduced simply by the slant cut on the tube end. I decided to keep testing, because I really wanted to see what would happen by moving the CC vent outside of the cowling -- which these next two pictures show....
Now, I was quite surprised by these numbers:
Test Case 3: CC vent tube outside cowling
Location/HG Measurement DATA:
Crankcase: OAT 72F,175KTS,7500MSL,2600RPM,23MP,-1.3"WC
Bottom: OAT 72F,175KTS,7500MSL,2600RPM,23MP,-.5"WC
Top: OAT 72F,175KTS,7500MSL,2600RPM,23MP,10+"WC (pegged the 0-10 HG Magnahelic
Wow! A 4KT increase in speed! (it actually was bouncing between 175-176KTS). I wasn't going for that, but needless to say, I was very pleased. Immediately after this test I inspected the belly for oil, and also removed the test tube from the Oil filler tube, and double-checked the oil level -- yep, same as where it started. I've since put 2 more hours on the aircraft and I am checking the oil level before/after each flight, as well as inspecting the belly. So far, still clean on the belly. In addition, I called Mr. Allen Barrett and shared these findings with him, and to get his opinion if there is any risk to continuing the testing with a negative pressure on the CC vent. He saw no reason to not continue, but was cautious to keep checking the oil levels before and after each flight. So, I will keep flying/checking, and will report back!
Michael
My RV-7a firewall forward sports a Barrett I-O 360 (with ~80 hours), a horizontal intake, a Sam James Cowl with a Sam James Plenum, Vetterman 4-into-2 cross-over exhaust, and Robert Paisley's EFII (dual ecu's). I'm very happy with this setup, and the engine runs strong. However, the belly is oil-coated at the rate of about 1 qt every 5-6 hours, and that's with only starting at 6.5 qts per change/fill. I reached out to Allen Barrett a couple of weeks back, and his suggestion was that the oil is being "pulled" or expelled out somehow, and to look into that first.
After several hours of searching/reading the many posts on VAF about oil on the belly, the one post that seemed more likely to be a match for my situation was this excellent post by Tom Martin a number of years back.
To determine if I have the same issue, I enlisted the help of my good friend Lee (the crazy Hawaiian dude who's also a great A&P as well), where we set up three measuring locations using his magnahelic: Upper cowl (inside plenum), lower cowl (at CC vent) and crankcase (by way of oil filler tube). After checking to make sure all test locations were secure, and could handle full power runs, I took some measurements. All measurements were at 7500 MSL, 2600 RPM with 23" MP, using a 2-axis auto-pilot to hold things steady while I swapped out the tubes on the magnahelic. A few tests were done during climb, but the data was not significantly different, so I'm only posting the level flight measurements. Also, the process of making a change, and re-testing spanned a couple of days, so the OAT's were not identical from test-to-test, but all speeds are measured in Knots True from my trusty Dynon (with OAT probe). Prior to taking these measurements, I cleaned the airplane which includes a spotless belly.
This first picture shows the location and configuration of the CC vent (along with a diffuser on the measurement line) before any changes. Basically Van's stock setup (but without the recommended slant-cut on the end of the tube -- I missed that subtle detail..).
Test Case 1: No changes, original setup
Location/HG Measurement DATA:
Crankcase: OAT 65F,171KTS,7500MSL,2600RPM,23MP,2.7"WC
Bottom: OAT 65F,171KTS,7500MSL,2600RPM,23MP,2.5"WC
Top: OAT 65F,171KTS,7500MSL,2600RPM,23MP,10+"WC (pegged the 0-10 HG Magnahelic)
From the above data I was encouraged that my scenario is matching what Tom Martin saw (however, my pressures were lower, but still relative relationship-wise), where the crankcase pressure was slightly higher than the pressure measured low in the cowl at the CC vent.
This next picture is where I chose to take a baby-step, and change the cut on the end of the CC vent tube to match Van's plans. This is the only change made between these tests.
Test Case 2: Slant cut on CC vent tube
Location/HG Measurement DATA:
Crankcase: OAT 72F,171KTS,7500MSL,2600RPM,23MP,2.6"WC
Bottom: OAT 72F,171KTS,7500MSL,2600RPM,23MP,2.7"WC
Top: OAT 72F,171KTS,7500MSL,2600RPM,23MP,10+"WC (pegged the 0-10 HG Magnahelic)
Interesting numbers here, and I suppose I could have stopped with my changes to see if the oil on the belly would be reduced simply by the slant cut on the tube end. I decided to keep testing, because I really wanted to see what would happen by moving the CC vent outside of the cowling -- which these next two pictures show....
Now, I was quite surprised by these numbers:
Test Case 3: CC vent tube outside cowling
Location/HG Measurement DATA:
Crankcase: OAT 72F,175KTS,7500MSL,2600RPM,23MP,-1.3"WC
Bottom: OAT 72F,175KTS,7500MSL,2600RPM,23MP,-.5"WC
Top: OAT 72F,175KTS,7500MSL,2600RPM,23MP,10+"WC (pegged the 0-10 HG Magnahelic
Wow! A 4KT increase in speed! (it actually was bouncing between 175-176KTS). I wasn't going for that, but needless to say, I was very pleased. Immediately after this test I inspected the belly for oil, and also removed the test tube from the Oil filler tube, and double-checked the oil level -- yep, same as where it started. I've since put 2 more hours on the aircraft and I am checking the oil level before/after each flight, as well as inspecting the belly. So far, still clean on the belly. In addition, I called Mr. Allen Barrett and shared these findings with him, and to get his opinion if there is any risk to continuing the testing with a negative pressure on the CC vent. He saw no reason to not continue, but was cautious to keep checking the oil levels before and after each flight. So, I will keep flying/checking, and will report back!
Michael
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