Design > Stunt design
A Rudder isn't necessary for a Control Line plane.
Bob Hunt:
Rudder? RUDDER? I don't need no stinkin' rudder!
Bob Hunt
Steve Thompson:
Thank you all for the excellent information.
Ken Culbertson:
Rudders are a steering device. Many of what we call rudders are actually vertical stabilizers which, in tethered flight serve no real purpose other than balancing side area aft of the CG, esthetics or giving you a place to mount a rudder. Yaw is controlled by other means. A VS produces drag which has become an issue we never paid much attention to before because we didn't have a battery constantly reminding us. Aside from side area, the only thing a rudder can do is counter yaw and add drag. We tend to use it to cover up not selecting the right lines, leadouts, CG and all the other trim items that cause loss of line tension and for most of those, engine offset is more effective. A lot of yaw issues are isolated. If we use rudder to fix an isolated issue, then what do we do for the rest of the pattern to unfix it? Other than the Cam, I don't have an answer for that one. Bob did, he designed a plane that didn't have one, so it was not there to let him cheat on the trim although, I doubt that was the reason he tried it.
I am a product of the 60's and I can tell you unequivocally that no matter what the trim issue was there were only two fixes, add nose weight and more rudder offset. Life was good. LL~
ken
Colin McRae:
My first build a couple of years ago was a Brodak Shark 402 stunt trainer. OS 25LA powered. Being a rookie builder and not knowing any better, per the Brodak plans, I put in rudder offset as well as used washers to produce a bit of engine offset. Since I was only able to do some basic beginner maneuvers, the model flew OK.
But over the past 2 years I have read a lot about this rudder offset topic, and that it is basically not needed, or can even hurt model flight performance. I also noticed that expert flying buddies had no rudder offset on their models.
So, I decided to redo the rudder on my Shark 402 and also remove the engine offset. I am a convert to the no rudder offset design or designing the model with an adjustable rudder. My Shark flies better than before, and still has plenty of pull.
My most recent build is a SIG Skyray 35 with no engine or rudder offset. (I did build it with an adjustable weight box and adjustable lead outs.) It is my best flying model to date.
Brett Buck:
--- Quote from: Ken Culbertson on December 16, 2023, 08:31:02 AM ---Rudders are a steering device. Many of what we call rudders are actually vertical stabilizers which, in tethered flight serve no real purpose other than balancing side area aft of the CG, esthetics or giving you a place to mount a rudder. Yaw is controlled by other means.
--- End quote ---
I was mostly with you earlier, but a fin/rudder is *critical* to controlling yaw, fixed rudder and no offset notwithstanding. There are plenty of other disturbing forces in yaw that you have to deal with somehow, products of inertia and/or precession, not to mention the kinematics during corners - it's yawing with a angular momentum vector pointing vertically, you do a 90 degree corner, and it needs to be yawing with angular momentum pointing horizontally a 1/4 second later.
That means something has to apply a fair bit of torque to something, and ignoring any tricks you might want to try with products of inertia, the only thing you have working for you in the right direction is the fin/rudder trying to line it up with the direction of flight. Note that if you do nothing, and it was neutrally stable, the *lines* would have to straight it out for you, which guarantees that you are going to whip up the lines.
The yaw from this is relatively benign, but it also couples into roll, and affects the pitch response since it also causes the line tension to vary, altering your control gains.
I will grant that most of the differences people are concerned with at the earlier stages of skill primarily about having lots of rudder offset. That's where the "I knocked the fin off and it flew better!" stuff came from - because most airplanes of the early era had lots of offset and when you got rid of it, it flew better. But beyond that, you have to also deal with these other issues, which very strongly suggest you want some (or in my case, a lot) of yaw restoring force.
Brett
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