Am posting this here mainly because Brett and Ted seem to visit once in a while and would like their input.
LineIII is about ready to release and need some opinions on a paragraph I am getting ready to add to the help files. Below is a synopsis of information I have gathered from several sources, it makes sense to me but want to make sure I am not saying something not exactly correct.
PS: Linedrag2003.PDF will be included in the zip file of the release.
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LineIII accurately calculates the theoretically correct leadout position based on an extremely complex equation that first calculates the drag and arc of the lines. For an in depth review of how LineIII works see Pete Soules Line Drag article (Linedrag2003.PDF). This calculation should be within 1/4 inch of final flight trim or something else like engine offset and/or rudder offset is effecting the models flight attitude.
Engine/Rudder offset will make the model fly crabbed out instead of tangent to the circle, this situation requires the leadouts be set further forward to compensate. The net result is two forces fighting each other wasting engine power. Also, anytime the lines go a little slack the model tries to resume its crabbed-out flight attitude. This whole situation gets more complicated because any sudden yaw change will also result in a roll. If your pride and joy is waging it’s wings this could very well be the reason.
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Hi Bob,
Brett is better at this stuff than me, but I do think the second paragraph should probably be eliminated or slightly restated. What you say is more or less true but only begs further questions.
Brett would probably say that rudder offset, for instance, would actually need the lines further aft in order to optimize the ship for flights "as is". The use of a forward leadout location to try to force the ship back to tangent would only work under a given set of conditions -- i.e. level flight at a given airspeed. Any change from that condition would result in greater yaw oscillations as the various physical and aerodynamic fixtures/parameters fight for dominance.
I suspect Brett's position will be that if you do use rudder offset (he would stoutly argue you shouldn't in the first place) that the leadouts should be further aft so that the offset rudder is aerodynamically "neutralized" i.e. The yaw allowed by the leadout position will match that which would result from the offset rudder. Thus, there would be no tension between the two at that given state. Minor deviations in speed during maneuvering would therefore have a less dramatic effect on the yaw attitude of the airplane (still wrong from a drag point of view but less yawing back and forth as the physical demands of the the leadout position and the aerodynamic desires of the rudder fight one another).
The ideal location of the leadouts (to the extent that it can be ideal given the change in drag, gravity, etc. on the lines as speed and aircraft attitude change during maneuvering) is such that the fuselage will be as close to tangent to the circle as possible throughout the maneuver cycle. Excessive yawing can be minimized even with rudder offset but only at the cost of excessive drag due to the yaw of the fuse to the circular flight path.
Further, because that drag is greater there will be more speed changes during maneuvering which will, once again, aggravate the yaw/roll situation.
Does any of that make sense?
Ted