Bill Boyd
Well Known Member
It might even be a useful something - not sure.
Due to some surprisingly high CHT's on a hot-day climb to 9500 a few days back, I decided I'd better do some sealing up of neglected air leaks in the baffling and check the GAMI spread again to see if ol' #6 had gotten leaner than the rest, making it more susceptible to running hot. As part of that plan, I thought I should calibrate this latest (my 3rd) Red Cube's K-factor, rather than assuming it was "close enough" out of the box.
Since my plane is not based at an airport with a fuel pump to simplify the fill-tanks, fly, refill tanks and see if the fuel pump agrees with the EFIS totalizer routine, I thought I'd try the calibration on the ground. My red cube is between the servo and the spider, so I disconnected the line to the spider and routed it via a clear PVC tube into a graduated pitcher. Plan was to run a gallon at a time into the pitcher with the boost pump and then compare 3 gallons actual with the reported fuel used and calculate the correction needed for the K-factor.
That's where the surprises came: it was immediately apparent that the Cube was way off (3 gallons pumped showed as 2.3 on the totalizer) and the fuel stream in the clear plastic line was *full* of air bubbles. This was true for either tank, leading me to think the air leak must be on the suction side, after the Andair valve but before the pump. But I couldn't figure for the life of me why a leak that allowed so much air to be entrained was not causing at least a sniffable fuel leak in the tunnel from the static pressure in the tanks.
I tried disconnecting the line from the mechanical pump to the servo and emptying the fuel into the jug, and to my surprise/relief, there were no bubbles at all! I could only deduce that the air was somehow getting entrained into the fuel inside the servo, while under positive pressure. This seemed very counter-intuitive but I could see no other explanation.
I ran all this past my mechanic, who being on a first-name basis with Don Rivera at Airflow Performance offered to call him and get his opinion. Don realized where this was going before Mike could even get it all out, and assured him that the regulator in the servo under these conditions could indeed introduce a stream of bubbles in the fuel stream when the line was not terminated by the spider and nozzles. So I abandoned the effort and will do my flow calibration the conventional way - at the airport fuel pump after my next X/C.
Since the results were unreliable (no doubt a stream of bubbles is not what the red cube wants to see), I'm shelving my worry that my full-rich climb burn could actually be 32 gph and my LOP economy cruise 16 gph.
Inspection of the nozzles showed #5 was visibly clogged, and #6 had a tiny speck of debris at the edge of the orifice. I found a bunch of baffling light leaks in the dark hangar last night and slathered them with RTV. Hopefully I won't see #6 hit 473F in climb anymore - that has never happened before. If it does, I will bump the timing advance back on the CPi2 in addition to dropping the nose for a less aggressive cruise climb.
I feel like I know what else to pursue for the CHT issue (front dams, etc) but just thought I'd report the unusual finding of air bubbles in the hose between servo and spider and that The Don himself says this was expected. Random factoid now filed away in case I ever need it
Due to some surprisingly high CHT's on a hot-day climb to 9500 a few days back, I decided I'd better do some sealing up of neglected air leaks in the baffling and check the GAMI spread again to see if ol' #6 had gotten leaner than the rest, making it more susceptible to running hot. As part of that plan, I thought I should calibrate this latest (my 3rd) Red Cube's K-factor, rather than assuming it was "close enough" out of the box.
Since my plane is not based at an airport with a fuel pump to simplify the fill-tanks, fly, refill tanks and see if the fuel pump agrees with the EFIS totalizer routine, I thought I'd try the calibration on the ground. My red cube is between the servo and the spider, so I disconnected the line to the spider and routed it via a clear PVC tube into a graduated pitcher. Plan was to run a gallon at a time into the pitcher with the boost pump and then compare 3 gallons actual with the reported fuel used and calculate the correction needed for the K-factor.
That's where the surprises came: it was immediately apparent that the Cube was way off (3 gallons pumped showed as 2.3 on the totalizer) and the fuel stream in the clear plastic line was *full* of air bubbles. This was true for either tank, leading me to think the air leak must be on the suction side, after the Andair valve but before the pump. But I couldn't figure for the life of me why a leak that allowed so much air to be entrained was not causing at least a sniffable fuel leak in the tunnel from the static pressure in the tanks.
I tried disconnecting the line from the mechanical pump to the servo and emptying the fuel into the jug, and to my surprise/relief, there were no bubbles at all! I could only deduce that the air was somehow getting entrained into the fuel inside the servo, while under positive pressure. This seemed very counter-intuitive but I could see no other explanation.
I ran all this past my mechanic, who being on a first-name basis with Don Rivera at Airflow Performance offered to call him and get his opinion. Don realized where this was going before Mike could even get it all out, and assured him that the regulator in the servo under these conditions could indeed introduce a stream of bubbles in the fuel stream when the line was not terminated by the spider and nozzles. So I abandoned the effort and will do my flow calibration the conventional way - at the airport fuel pump after my next X/C.
Since the results were unreliable (no doubt a stream of bubbles is not what the red cube wants to see), I'm shelving my worry that my full-rich climb burn could actually be 32 gph and my LOP economy cruise 16 gph.
Inspection of the nozzles showed #5 was visibly clogged, and #6 had a tiny speck of debris at the edge of the orifice. I found a bunch of baffling light leaks in the dark hangar last night and slathered them with RTV. Hopefully I won't see #6 hit 473F in climb anymore - that has never happened before. If it does, I will bump the timing advance back on the CPi2 in addition to dropping the nose for a less aggressive cruise climb.
I feel like I know what else to pursue for the CHT issue (front dams, etc) but just thought I'd report the unusual finding of air bubbles in the hose between servo and spider and that The Don himself says this was expected. Random factoid now filed away in case I ever need it
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