Originally Posted by Francesco
Overstability in launch is generally bad, it causes oscillations which cause the plane to overshoot the optimal angle of attack and enter a pre-stall condition. To overcome this you either use your thumb or settle for a less than optimal trim.
Reducing longitudinal stability via the towhook position is an easy fix. A plane like the Supra also gets some residual stability in launch from the pylon-mounted wing.
I think we started this thread from that - too much stability causes funny things as plane accelerates in line. But it is not black/white. If you trim your plane is absolutely neutral stability in launch, it is also very critical to proper throw (tension & trajectory). I think in practice we want something in between (depends on your throw / your helpers). Also in F3J I see no problem in sacrificing a wee bit of tension for stability and speed (look for slightly lower lift coefficient in launch, just a little forward towhook).
Your point about Supra pylon is good one. I did not think of that, but common sense tells me that Supra pylon gives some leverage for rotation after less than perfect trow? My point was to have 2 launch phases. First one trimmed for getting the quickly to optimum climb (plenty of stability), second phase for steady climb in longer launch (and neutral stability).
The last point we have not discussed is wing flex. It is always bad for consistent launch trim, and specially so if wing twists torsionally. If plane suddenly gets unstable in strong wind launch, it might be also due to structural reasons. With adequate structure, there is no need to change launch trim even in very strong winds, just go to thicker line.
Finally, we get large % of total altitude from zoom. The time we spend in line is preparation for high zoom - plenty of tension and absolutely straight up.