One of our glider club's tow planes is a Cherokee 235 with a constant speed prop. Recently it had a partial loss of power on a tow. Everyone got back OK. Pilot reported takeoff was OK but shortly after takeoff he started hearing a low, bass, "distant" rumbling and noticed they weren't climbing as usual. Engine was running smoothly but he needed to lower the nose to keep flying. Thinks the needles for RPM and manifold pressure were about right but he didn't look closely because he was busy flying the plane. Observer on the ground reported seeing smoke, possibly light colored. (I think I should repeat that the pilot reported that the engine continued to run smoothly while this was happening; that strikes me as an important bit of information).
Subsequently, we've done a compression test and all the numbers came out OK. Looked for blockages in the air intake system but didn't find any. Apparently there was soot on one of the plugs in each cylinder but not on the other.
Owner asked me to do some full power ground tests. With the wheels chocked I opened up the throttle. RPMs only came up to 2500 at 28" MP and then rose as high as 2550 when I leaned. There was about a 50-75 rpm drop when checking each mag. However, the tach never got to red-line (2575, I think) in this static test and certainly didn't come close at lower MP. Owner thinks it should get to red line around 22" MP, at which point the governor should kick in. Engine ran very smoothly at all powers and repeatedly started very easily.
Two questions:
1- Any idea of what the problem should be?
2- Should a cherokee 235 with constant speed prop get to max RPM at about 22" MP when it's stationary on the ground?
Any advice would be appreciated.
Thanks,
Larry
Northern California Soaring Association
Subsequently, we've done a compression test and all the numbers came out OK. Looked for blockages in the air intake system but didn't find any. Apparently there was soot on one of the plugs in each cylinder but not on the other.
Owner asked me to do some full power ground tests. With the wheels chocked I opened up the throttle. RPMs only came up to 2500 at 28" MP and then rose as high as 2550 when I leaned. There was about a 50-75 rpm drop when checking each mag. However, the tach never got to red-line (2575, I think) in this static test and certainly didn't come close at lower MP. Owner thinks it should get to red line around 22" MP, at which point the governor should kick in. Engine ran very smoothly at all powers and repeatedly started very easily.
Two questions:
1- Any idea of what the problem should be?
2- Should a cherokee 235 with constant speed prop get to max RPM at about 22" MP when it's stationary on the ground?
Any advice would be appreciated.
Thanks,
Larry
Northern California Soaring Association
