stevemcd
Well-Known Member
Arrow II with IO360-C1C, JPI EDM 830 (secondary gauge for OP). On flights at 2100 RPM, OP is steady. At 2400 RPM, OP starts at "normal" (80-ish), then drops slowly but steadily in flight. Most flights it only goes into 70s, but sometimes much lower, even as low as 50s and high 40s. Rate of drop varies. Analog gauge seems to match though not easy to tell except for the cases where OP went to half, in which case the analog gauge showed top of yellow range.
There appears to be a correlation to oil quantity. Starting each flight at 6QT seems to eliminate or defer the OP drop. Starting with less oil seems to cause it to happen sooner though this correlation is not absolute. Oil consumption is normal.
Yesterday, I was cruising at 2400 RPM, watching OP slowly drop on 830. Started at 76, worked its way down to 71 after about 1.5 hours. (OP numbers jump around +/-2 on display but trend is clear. Data log shows only changes of 5 PSI so not exactly reflecting displayed values, but step function is clear.)
I climbed 2000'. As soon as I started the cruise climb (no power changes), OP jumped from 71 to 75, then dropped down to 68 after levelling off.
I dropped RPM to 2100. OP jumped to 75 and remained there until descent.
Next 3-hour flight I did at 2100 and OP didn't budge from 75.
Since the analog gauge does show some drop, it doesn't seem to be an indication problem but investigation hasn't turned up a cause. OP regulator has been examined, adjusted. Savvy Analysis suggested cavitation as a cause but not clear how that would happen. A&P is sceptical but can't think of another reason. I am considering replacing transducer (cheap in airplane terms) but again, analog gauge suggests that the OP drop is real.
Ideas?
There appears to be a correlation to oil quantity. Starting each flight at 6QT seems to eliminate or defer the OP drop. Starting with less oil seems to cause it to happen sooner though this correlation is not absolute. Oil consumption is normal.
Yesterday, I was cruising at 2400 RPM, watching OP slowly drop on 830. Started at 76, worked its way down to 71 after about 1.5 hours. (OP numbers jump around +/-2 on display but trend is clear. Data log shows only changes of 5 PSI so not exactly reflecting displayed values, but step function is clear.)
I climbed 2000'. As soon as I started the cruise climb (no power changes), OP jumped from 71 to 75, then dropped down to 68 after levelling off.
I dropped RPM to 2100. OP jumped to 75 and remained there until descent.
Next 3-hour flight I did at 2100 and OP didn't budge from 75.
Since the analog gauge does show some drop, it doesn't seem to be an indication problem but investigation hasn't turned up a cause. OP regulator has been examined, adjusted. Savvy Analysis suggested cavitation as a cause but not clear how that would happen. A&P is sceptical but can't think of another reason. I am considering replacing transducer (cheap in airplane terms) but again, analog gauge suggests that the OP drop is real.
Ideas?
