Crankcase Vent, oil on belly

[n707sm]

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

[bret]

So, did a slight vacume on the crankcase help to seal the piston rings and make more HP?

[n707sm]

@Bret,

I'm not an engineering-type, nor do I have any training or experience in that area -- but, from my research, and reading the various discussions on this forum, the general thinking appears to be a) Lower windage in the CC with negative pressures, and b) ring flutter at the bottom is reduced/eliminated.

My theory is that with the rings tight against the cylinder wall at the bottom, you get a complete compression stroke without having to loose valuable stroke distance (at the bottom, on the way back up) getting the rings seated again.

I would love to hear from any experts that wouldn't mind chiming in...

Michael

[bret]

This makes total sense now, during combustion, the gases go down the outside of the piston crown, to the gap on top of first piston ring land and and top of ring, to the inside of piston ring and outside of inner wall of piston ring land and seals on the bottom of the top ring and bottom surface of top piston ring land and pushes the piston ring out to the cylinder wall, if there is any pressure on the bottom surface of the top ring it COULD unseat the ring from the wall, causing more blowby ?........I think ?.......someone check my insanity please....

[DanH]

A few accuracy notes....

If you did not connect the aircraft static system to the static port of the gauge, the gauge pressure is the difference between cockpit pressure and probe pressure. The actual pressures at the probe locations may thus be higher or lower than indicated. As an aside, cabin air vent open or closed can also change the indication.

If you did connect the static port of the gauge to aircraft static, you could easily affect your airspeed indication; the probe hanging behind the cowl exit is probably in a lower pressure region than the fuselage static ports. That's why it's necessary to fly a NTPS GPS three-leg if valid airspeed is important to the test...the recorded airspeed data is independent of the aircraft's pitot static system.

There is a HP gain with negative case pressure, but I'd be surprised if it was enough to pick up 4 knots, in particular if it was only 1.3" H2O negative.

A valid test is repeatable.

[n707sm]

@Dan,

Very good points. I was using cabin pressure, with the same configuration of cabin vents (between tests), but I do understand what you mean about the accuracy is in question. Between test case #2 and #3 there was roughly 3 hours time on the same day with relatively the same conditions. Both tests were in the afternoon, and both tests flown over the same area for time and distance. Test case #1 was the previous day in slightly cooler conditions.

Whether I will see a speed gain (or not) over time is secondary to my primary objective of keeping the oil in the crankcase and off of the belly, but I should be able to tell over the next 10 hours or so if this plane is noticeably faster than before.

Very much appreciate the feedback!
Michael

[MarkW]

I am more interesting in hearing how you lose less oil through the breather with a lower crankcase pressure. Seems backwards to me.

[n707sm]

I know! ...it's a head-scratcher for sure ;-) I will report back once I have more time with this new configuration.

[DanH]

Originally Posted by n707sm

Whether I will see a speed gain (or not) over time is secondary to my primary objective of keeping the oil in the crankcase and off of the belly, but I should be able to tell over the next 10 hours or so if this plane is noticeably faster than before.

Like I said, a valid test is repeatable. Re the speed increase, just move the breather inside and outside the exit, and fly some 3-leg laps.

http://www.ntps.edu/images/stories/d...pec-method.doc

http://www.ntps.edu/images/stories/d...ts/gps-pec.XLS

[n707sm]

Thanks Dan for the links to the reference doc, and spreadsheet. I will see about doing the 3-leg testing with vent both in and outside of the cowling. The weather is forecast to get rainy here later this week and over the weekend, so it may be a bit before I find decent conditions (with a stable/consistent air mass) over (or near to) Tahoe any time this week.