Trim examples from this weekend.
By the way, one thing I told my group at the Chumley Memorial Clinic, criticism or comment sof how your pattern looks or how your airplane flies IS NOT A REFLECTION OF YOUR WORTH AS A HUMAN BEING. Plenty of people take it that way, fortunately, we don't have a lot of those guys around here. I referred to someone's square 8 yesterday as "catastrophic", that doesn't mean that the pilot is a bad person!
Airplane #1: Sound design, good apparent craftsmanship, and finish, competent and powerful tuned pipe engine (although the bearing sounded like they had been running sand through the engine). Pilot is on the border between very good advanced and new Expert class pilot. He could certainly get through decent patterns with it, but with repeated odd issues like high bottoms on round maneuvers, unexpected and incorrect direction changes, that sort of thing.
He noticed that the tension was "light" at the tops of the loops. I flew it and had it set intentionally a little slow to better expose the trim issues. If you are flying 4.8 second laps on a 65-ish ounce airplane, you can have massive trim issues and still get through with it pulling your arm off. Sure enough, the tension got very light at the tops of maneuvers, particularly the inside round loops, more so than the outside. It was slightly better as far as hinging goes in the square corners, but with wild yawing. The airplane was *very sensitive* even with very slow controls. Flying level in nearly dead calm, the outboard wheel was about 1/2 inch or so ahead of the inboard wheel, but the wings were level. Excess tip weight caused very large hitches in the intersections of the round 8s, but kept it alive by vectoring the thrust out. Revealingly, the square corners did not hinge nearly as much as I expected, and there was wild yawing motion instead. Overhead 8, there was *no* tension whatsoever in the intersection, and then it came on and pulled more-or-less OK at the ends of the loops with the airplane rolled out.
Diagnosis, just from flying it - inboard rudder, leadouts too far forward, way too much tio weight (which I expect was ALL that had kept the airplane alive to this point), and CG too far aft. The pilot had correctly diagnosed lack of line tension, and added mucho tipweight to compensate for the other issues, giving him just enough line tension to keep it from chasing him around. A 65-ish ounce airplane with a 60-sized tuned pipe engine with excess tip weight should just about pull you over even at 5.4 second laps, and with +-15 degrees control motion for full handle deflection should be very sluggish. Instead the tension was light to nonexistent, and the airplane was so sensitive that even I couldn't reliably keep it going straight in the straight sections and the control loads were tiny. I bailed on the hourglass, I got to the point of the turn and there was nothing so I just flew out of it. The lack of hinging in squares despite clearly excessive tip weight indicated that the airplane was yawing in on every corner, causing the outboard wing to swing forward in the corner, and cause an inboard roll torque that was compensating for the excess tip weight causing an outboard roll torque. Same thing did not happen on rounds due to lack of tension in level flight, so it hinged severely. That indicated that the leadouts were too far aft for the amount of aerodynamic yaw offset.
Checked the fin/rudder, and sure enough, the fin was inset a bit (even though it was, very unfortunately, airfoiled on one side, making it very difficult to see or measure the alignment). The reasoning was that it had massively too much tip weight, apparent in round loops, but not in level flight and much less than expected in corners. This suggested that the airplane was yawed in, and sure enough, the relative position of the wheels confirmed it. The leadouts being too far forward gave it overall weak line tension, and just encouraged the inboard yaw angle. In round maneuvers the airplane hinged at the bottoms of the loops, but under low load factors at the tops of the loops, the roll angle was reduced, the reduced lift reduced the magnitude of the vector component of lift along the line axis, so it no longer assisted much in compensating for the inboard yaw.
I asked for about 1/16" of offset in the movable rudder and about 1/4" of aft leadout movement. In this case, 1/16" of rudder motion would have suggest a large aft movement of the leadouts, so, effectively, 1/4" aft was more like moving them forward relative to the other change. That is recommended only for experienced fliers, one change at a time is a lot easier to keep track of.
I flew the airplane again, speeding it up a bit to make sure I could get through safely. The changes made the line tension somewhat better and with reduced variation, with the wheels more-or-less lined up all the time. The hinging in the rounds was increased slightly, the outboard wing hung down a bit upright and inverted, and the airplane began to hinge substantially in the corners.
What I suggested was for the pilot to move the rudder some more, move the leadouts back even further, and add noseweight. What I expect to happen is that the outboard wing will begin to hang down more in level flight, the hinging in the corners will increase dramatically, and the wild yawing will abate. The line tension should go up a large amount. At that point, THEN, start removing tip weight to bring the hinging under control.
I didn't suggest it, but maybe it would be a good idea, to cut the fin/rudder off the airplane, make a new one that is symmetrical and mostly flat, and then align it with extreme care (long rulers or rods on either side of the fin that extend up to the nose) to be *exactly* straight ahead. Then start with about 1/16 or rudder offset, LINEII leadout position, and start over from there.
In questioning, and apropos of this thread, the pilot knew something was wrong with the line tension, but not where to look for the cause. He did some trim adjustments (tip weight and speed) to make it safe to fly. But he did not have the experience to recognize the yaw problem, caused by an underlying alignment issue. Of course, with the line tension varying all over the place, and wildly rolling and yawing at every control movement, plus the sensitivity, would have made attempting competitive patterns an exercise in frustration. He could not see or recognize the yaw angle. With nothing to compare to, the sensitivity issue would have been invisible, maybe that's the way it is supposed to fly.
Paul's excellent trim flowcharts aren't going to solve the issue if you can't see or don't know how to diagnose the inboard yaw angle in level flight, or that that the line tension was abnormally light. Of course, Paul's fin would have been on straight to begin with.
The formulaic approach with "LINEII" to set the leadouts would have pretty immediately discovered the fin misalignment, when it took lots of forward movement of the leadouts to remove the yaw reactions in the corners, which were far more obvious. Careful inspection would have done the same. One could easily calculate that the line tension should be around 10 lbs in level flight in the absence of aerodynamic "enhancements"/manufactured line tension, about 7.5 lbs at the tops of round loops (instead of nearly zero), and around 6 lbs instead of 10 overhead (instead of absolutely zero). The pilot made a similar assessment based on feel.
In any case, stuff like inboard fin misalignment, along with similar issues with fuselage or tail skew, are potential killers. There may have been some good fortune in this case that the airplane survived the first flew flights, or maybe the pilot started with greatly excessive tipweight to hedge his bets - I didn't ask.
I note that he DID NOT attempt to excuse or otherwise claim this was his "personal preference", he knew for sure that something was wrong but not how to go about it, hence, this situation:
I am not saying that this is what is going on here, but far too often, "personal preference" is used as a cover for not trying to improve, not knowing what to do, or far more often, not knowing there is anything wrong in the first place.
Brett
Topic: Trimming your model for flight characteristics or personal style? (Read 4122 times)