I apologize, I just noticed that I was logged in with my fathers account. The purpose of my post is to incite a discussion about the true dynamic function of the rudder/ vert. stab on a control line plane. General rules do little to increase understanding and are often full of misinformation. This is only my theory of rudder function. I would love to hear some feedback considering this issue in order to properly answer this fellows question.
The original function appears to have been to "hedge the bets" of the kit manufacturers by making sure that no matter what the builder did, or how much misalignment the airplane may have had when built, that it would stay out on the lines. Although it appears that C/L was invented some considerable time before "tip weight" was invented, and all else being equal, with no tip weight, you better have a lot of rudder offset or learn to run really fast.
As Ted mentioned, the effect of rudder offset was/is to force the nose out, tail in, and create some line tension - which it does in some cases. It creates an aerodynamic yaw torque, and the airplane yaws nose out until the torque created by the offset (which goes down as the angle increases, but very slowly) is balanced by the torque created in the opposite direction by the line tension acting on the leadout guide. In steady level flight this is generally OK, it quickly reached the equilibrium between the two, and then just stays there.
Unfortunately, during maneuvering, this balance is continually changing - the line tension changes, the speed changes, etc.. What happens then is that the "equilibrium" yaw angle keeps changing, and the airplane starts yawing around trying to move to this new and changing angle, which just exacerbates the line tension and speed changes. So you end up wallowing around, which then excites the lines to start whipping around, which keeps it going until you stop.
Worse yet, as you yaw around, the airplane also wants to roll. Essentially any stunt plane will roll as the yaw angle changes - towards the "trailing" wing. Once again, this is livable if you never maneuver. Once you do, the above yaw motion starts, which induces a roll motion, so the entire airplane starts into a pretty random motion.
You might think none of this makes any difference because you are just trying to control pitch. But all this roll/yaw motion makes the line tension vary, too, which definitely does affect the pitch axis. For example, if you hold the handle at a fixed angle, the more line tension there is, the more elevator deflection you get. If it changes 7 times during a corner, the radius of the corner changes 7 times, all without you doing anything intentionally.
You might think that this doesn't have very big effect, but I have seen extreme examples of this from people who post here regularly. Light, straight, powerful airplanes that were close to unflyable because of rudder offset and the resulting wild oscillations in all three directions.
Several people have noted that it is at least hypothetically possible to balance the airplane (aerodynamically and mechanically) to fly with a non-zero yaw angle and whatever rudder offset corresponds to that angle. I haven't seen people having a whole lot of luck with that plan, myself.
That's a brief introduction. This has been rehashed in all the detail any normal human being can stand in threads both here and on SSW, so I would would suggest searching the archives in both spots for the relevant information. If that doesn't answer the question, then there are plenty of people that might help.
Brett
Topic: Rudder Offset (Read 14107 times)