It's been already more than a month since my last post and I could not make much progress since then mainly due to work and small issues with the paint job on the fuse structure. I'm currently in Salina, KS for work and still won't be able to work on the Pitts until August so I thought posting some details on my fuel system would help my readers keeping interest!
Those who read this blog from the very beginning will remember that the day I discovered the crack in the fuse structure, I was actually removing panels to make a change to my fuel system. Here are the two reasons why:
Reason #1
When I bought the airplane, the seller briefed me on a little issue with the fuel system when filling the tanks: if I overfilled, the vents would get full of fuel and siphon the whole tank on the tarmac! The only way to stop it was to quickly grab a plastic tube, connect it to the vent and blow the fuel back up... Not cool. Very not cool. I have been careful in not overfilling the tanks after it happened to me twice. I promised myself to investigate and fix this problem during winter. It turned out I had higher priority items to work on during the first winter so I decided postpone this fix until the next winter.
Reason #2
During the ferry flight from my last contest of 2014, I was in formation flight with my friend Luc in his S2C when I had sudden drops of rpm. I checked my mixture, fuel valve and turned on my boost pump and it reduced the magnitude of the drops by about 50%, but they were still more than 500 rpm drops. Although I could maintain my 7000ft altitude, I was mainly focusing on finding a suitable field with the help of Luc. At one point I realized my wing aux tank was empty but my x-feed valve was still open, so I shut it off. Immediately the drops in rpm stopped and the engine behaved normally so I decided to continue the ferry paying special attention to my fuel pressure.
A few days after getting back to my home airport, I decided to perform a little test with my fuel system: with the aux tank empty and the valve closed, I climbed to 5000ft over the airfield and then opened up the wing tank valve. My fuel pressure gradually went down until I got the same rpm drops again. My suspect was confirmed being guilty: there was a problem when the wing tanks was empty and the valve open. I didn't have this problem when the aux tank was still transferring fuel in the main; only when it got empty.
Analysis
After this test, it was already the end of the season so I only made a few practice flights before grounding my airplane and perform my investigation. I was kind of anxious to understand the reasons of both problems but with a wing tank and a header tank, the inverted/upright vent/fuel transfers are not easy to understand without actually drawing a sketch. I therefore took the time to analyze it and sketching my whole system. That's what it looked like after cleaning it up in visio:
After drawing the sketch, the reason for problem #1 became obvious: the header tank vent was the one filling up and creating a siphon when overfilling the main. Problem #2 was a bit more tricky but it seemed like air was sucked into the wobble pump inlet when the transfer valve was open.
While having the airplane teared down, I took the opportunity to give my main tank to my welder so that he adds two new ports on it: one to install a vent interconnect between the header tank and the main tank which addresses problem #1:
And another one dedicated to the aux tank fuel transfer, which I am confident will solve problem #2:
So that's about it for the story behind the changes to my fuel system. Will write a new post as soon as I start the reassembly process!
