ORIGINAL: jrpav1
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
Actually John and Tianci,
My stab will fly at slightly negative incidence because the wing AOA will dominate and seek it's own equilibrium. The fuse will follow where the wing equilibrates. Thrust is the dominant force under all conditions past 1/4 throttle or so, therefore you don't want a whole bunch of negative in the engine. Remember that the powerplants of today's pattern models push back tons of air not just a few pounds. That's the reason why I don't use any more delta than about 1 1/2 degrees total between the main vectors in question. Right thrust is another matter and it complicates things dramatically. I try to minimize right thrust also. I use the minimum possible to get the desired flight character....it usually works out to around 2 degrees right for a conventionally designed pattern model.
(My new design is not conventional and will use about 1/4 degree of right and zero down. At least my experiments on a smaller model worked this way. I'm not certain that it will work as well on the full 2 metter size)
My final settings always are done after flying, of course. On occasion I've had to increase dowthrust slightly to improve the powered vertical upline without compromise to the downline (increased by -1/2 degree at most). This wasn't always required and depended to some degree on fuse shape and most importantly, fuse length. It's the spiralling air stream hitting the stab again and all the strange stuff that causes
Keep in mind that most pattern models are shaped very similar nowadays so these settings will hold pretty much across the board (for pattern models). If you are working with some other type of model, then other settings may be required.
regards
MattK