Maidened the P-39 this morning. Went to a spot at the club where there's a small downslope followed by some long grass and weeds (makes a pillow for failed launches). I needed it...upon release, the plane pitched down and corkscrewed into the weeds. Retrieved the plane, put in about a dozen clicks of uptrim, tried again. Second launch was still tricky, but I got her successfully underway. And then I had to take out all that elevator trim. And trim the ailerons a little. And I had to flip down to low rates, which I had set per the throw gauge on the plans. The higher rates I launched with were too sensitive (the throw gauge rates are good!). The plane definitely had too much downthrust. If I slowed to near stall, then goosed the power, it would pitch down.
Also, it wasn't as fast as my Corsair funfighter with the same power system, and it was making a funny noise, like something was rubbing. After I got the plane down and inspected it, my conclusion was that under power, the "give" in the pod mount combined with the downthrust angle must have been pulling the motor can into contact with the inside fuse lip (which, like I said before, was only a miniscule clearance to begin with). So I borrowed a hobby knife from another club member and cut a chunk out of the lower fuse lip. Then I flew a second flight. After another 'tricky' launch, which I barely salvaged, the plane settled down to better behavior again, and was quieter and faster (so I guess that verified that the motor had been rubbing the inside fuse on the first flight). I enjoyed doing the kind of low fast passes back and forth that are the stock-in-trade for funfighters.
I tried for a third flight, but after two more failed launch attempts with the plane diving for the ground each time, it broke the prop. In hindsight, I should have gone back to putting in a bunch of "uptrim" for launch, to counter the downthrust. I guess I was in a mindset that the plane is trimmed up now, it should launch better. Something I noticed before one launch is that if you hold the plane in your hand and run the power up, you could feel it pulling downward and twisting.
So apparently the optimal thrust angle is different for the power system that I'm using. (but oddly enough, that is not the case with my Corsair funfighter, which flies neutral with either power system) So I'm going to make a new power pod with only a small amount of downthrust and try again.
She should still have a respectable showing, but she deserves more. Propping on 4S does get interesting, however -- particularly if you have a fuse and spinner stealing useable length from the prop.
