Cool, I'm glad I was able to help. As a side note, what the datum line does is give us a common reference point for all the measurements. I think we all understand this now but here's something to keep in mind. One of the things you're trying to do when you trim the plane for straight and level flight is establish a good presentation of the fuselage. Ideally, the "datum line" will be at 0 degrees relative to the horizon. You CAN trim the plane to fly straight and level with the nose high or even with the nose low. In that case, the datum line will NOT be at 0 degrees relative to the horizon. The datum line depicts the "desired" attitude of the fuselage in straight and level flight. That's what Matt was talking about. He likes to set his planes up so that the stab. is effectively flying at 0 degrees in straight and level flight with no up/ down thrust. You will however see some planes with deliberate downthrust (i.e stab. is set at 0 degrees, thrust is at 1-2 degrees down referenced to the datum line). In this case, the designer has flown the airplane and determined all of the recommended settings based on flight testing. This makes your setup task much easier than it is when starting from scratch with an undocumented ARF. Keep in mind - different airplanes react to trim adjustments in slightly different ways. You can't set 2 totally different airplanes up with the same thrust angle, wing and stab. incidence and expect them to behave EXACTLY the same way. Like I said, the final adjustments are made when you fly and trim the plane. That's another story...

John Pavlick
Team Black Magic, Tech-Aero Designs