The photo above was taken as we made a full power ground run in Pensacola. This was actually the second run in the past two weeks. Here's what has been going on:
If you read the past few months' posts, you'll recall we have been working several issues and questions on Doll. For clarity, I had flown on March 9 and had the fuel selector valve issue, had re-built it and reinstalled it and operationally checked it and inspected it for leaks, and figured we were past that issue. Maybe not.
You'll also recall we had tightened and re-torqued and pressure tested our cooling system and figured we were past that issue. Maybe not.
On Sunday, May 14th, I had Doll ready to fly, I thought, and started my takeoff roll on Runway 35 in Pensacola. At about 70% of the way into the acceleration, I got a secondary acceleration, which happens occasionally and is deemed to be normal. Other S51's exhibit this phenomenon, going all the way back to Jim's prototype. Correspondence between Jim and Hartzell theorized that the prop blades stayed partially stalled due to torque and relative angle of attack of the blades, and that the secondary acceleration we get is due to the blades gaining "full" efficiency as the stalled portion begins to develop thrust. Normal stuff.
I rotated at between 70 and 80 knots, but soon after airborne, got what I perceived to be another acceleration, then another, which could also be described as a series of surges, which had never happened before. I elected to abort the takeoff, which I think was the proper move. The drawback to the situation was that I had gotten high enough and fast enough that I was not able to get her back down and stopped before using more runway than Pensacola has pavement. I slowly rolled past the end of the runway and whacked the four foot tall MASLR lights at the end of the pavement with the left wing leading edge before I could get her stopped. Not really any kind of adrenalin event or anything: Just disappointed that I had dented the airplane.
We then began to investigate over the next few days and weeks what exactly had caused the engine surge. We thought about prop governor, oil pressure, fuel flow, and several other possibilities. Let's go back first to the fuel flow possibility.
Recall my fuel selector valve rebuild and reinstallation. Harry Stenger theorized that it was possible I had screwed something up with that operation. I had run my operational check at 3,000 RPM with ten gallons in each tank, and had pronounced Doll's fuel system healthy and ready. We decided to do a full power run to see if anything might show up similar to the surging I got on that takeoff attempt. We strapped her down and ran her full power for two minutes on each tank, using one fuel pump as normal, and on both fuel pumps for periods. Nothing abnormal showed up. No surging, nothing.
I did decide that I was going to take one more look at the fuel selector valve before checking that possibility off the list. I removed enough interior again to get at it and, sure enough! There was dripping at the connection of the fuel line from the valve to the firewall; the supply line to the engine. I had failed to tighten it properly. I couldn't believe it, but there it was.
Realize that the engine had run the day before at full power with the loose line and no hiccups, yet we had to conclude that there was the possibility that the difference between the full power static ground run vs. the 80 to 90 knot full power dynamic condition were enough to reveal my mistake. As Harry had predicted, it was probably sucking enough air through my faulty connection that I got the surging. I tightened the connection and made another full power run, which, again, was normal.
Why had my 3,000 RPM operational run and leak check from a month earlier not revealed my failure? Our theory here is that my putting only ten gallons per side for that leak check had the fuel level low relative to the fuel selector valve, such that there was no fuel at the level of the connection. When we ran, the leaky connection might have sucked air, but not dripped fuel. Once topped off, the fuel did begin to show itself, but I had closed up and not inspected after topping off later. The 3,000 RPM run vs. the full power 4,200 RPM run did not show the leak either way. Had I run at 3,000 RPM but had full tanks, I might have revealed the leak a month ago.
Now. The coolant system: The day after the last full power static run, I went back and did another pressure test on the cooling system. Pumping up to 16 lbs., she lost 3 lbs. in 50 minutes. Not OK. I resolved to do a borescope inspection.
I removed the front plug from the #8 cylinder (left front from the cockpit), as we had been seeing evidence of coolant in that area (recall the head stud coolant leak). Our thinking was that we would check the condition of the cylinder walls and valves and whatever else we could see. The plug had coolant on it whan I removed it. I then stuck a paper towel into the spark plug hole to see what I could wick out. The photo below tells the story.
