While maybe not the greatest lie ever told...

Real planes have rudders, but they don't fly in constant yaw. 

My evidence:
Saw friend flying a non-flapped kick-about CL plane.  I don't remember the exact plane, something like a Flying Clown.   A flip over on landing knocked the rudder off.  "That's OK, control line planes don't need a rudder anyway", was the sentence uttered.  The flight sans rudder was quite chaotic and lasted only a couple of laps.  It was flying fine before that.  The crash left it needing more than a rudder reattached.  So, I kind of concluded they were a good idea.

I had a Super Ringmaster that eventually lost the outboard wing.  Performance was diminished, but it still flew and did loops.  The inboard wing flew "a bit high", and it didn't like hard control inputs.  So, I do understand that flying without appendages can be done.  But it looked dumb and flew better with a wing.


So, with consideration of concepts like Center of Pressure, Weathervane, lead-out rake, Hinging, and such...

 - Do control line planes need a rudder?

 - How much rudder offset is right?  (Just use what the instructions says?)

 - How do you know if you have too much or too little rudder/offset?

    In my opinion, In general, what it needs it to be at least neutrally stable in yaw without counting on the lines. Whether you need a fin/rudder to achieve that depends mostly on how much lateral surface area you have in front of and behind the CG - say a big deep forward fuselage and a stick to hold the stab/elevator, then it will probably be unstable and require a fin/rudder. Short skinny nose and a big turtledeck fuselage is probably stable without a fin/rudder.

   More subtle effects matter too, like how much yaw the prop induces, more stable and the more power/prop diameter you can handle without excessive yaw motion.

    In general, you don't need rudder offset, and airfoiled fin, or anything like that.  You do not want the airplane to want to crab sideways, limited only by the lines pulling it back straight. I think what you want is to fly tangent to the circle, and have as little yaw motion induced in the corners as possible.

    How much offset is the right amount is best determined by test flying, start out with it at zero offset, or at least just enough to make sure it is not nosing you inboard. Set the leadouts where they would naturally fall either by using the LINEII or LINEIII computer program, or using various rules of thumb. Then fly it and adjust the offset until you get the minimal yaw motion in the corners. That will line up the leadout position and rudder offset to be consistent with each other, and if the leadouts are in the right place, you will be flying nominally tangent to the circle.

     Too much/too little rudder offset (for your leadout position) will cause the airplane to yaw as you enter corners, and generally, yaw the same way on both insides and outsides. Too much inboard might cause it to come loose and chase you, or get abnormally light on the lines.

    Most kit plans show large amounts of rudder offset. This seems to be to compensate for too-far-forward fixed leadouts and/or cover the possibility that the airplane is not straight, or just to add additional line tension, at the cost of cornering smoothly.

      Brett

     

     

A rudder might not be needed, but a tail fin helps.

I always include a tail fin in my designs to equalize the side area of the front of the fuselage, even it the rules do not require it. 

Rudder? RUDDER? I don't need no stinkin' rudder!

Bob Hunt

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. 

ken

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.

      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

     

I had a much longer response that somehow got lost...

I think we may be saying the same thing only your explanation is much better.  I basically ignored the roll aspect.   What I am reading into this is that, over simplified, "rudder good, offset bad". 

    Not really. Fin/rudder, aft side area, all impart "passive yaw stability" that opposes any torques that drive it off in yaw WRT the free stream, or, in the example, applies the torque necessary to reorient the angular momentum vector without solely relying on the lines (which would set off line whip). It also damps out any unexpected/unwanted motion.   I find a desired leadout location, then adjust the rudder around that, which, ultimately, gets me flying tangent to the circle with minimum reaction in the corners.

  Of course, this is not the only way to go, it's just the best way I have found. I find gadgets like the Rabe Rudder (which is generally trying to/asserted to do approximately the 'right thing') to be far, far more prone to maladjustment and vary significantly from day to day. I note that Al and I had endless (and fun) arguments over this over the years, and his method for adjusting it absolutely did not yield the least amount of yaw motion, or hold the airplane tangent. Igor's use of a similar mechanism is much different than what Al claimed.

     Brett

Rudder? RUDDER? I don't need no stinkin' rudder!

Bob Hunt

this is the classic case when a picture worht a thousand words & theory. Just check the ammount of hardware the Genesis won with Bob Hunt 
maybe rudder-less is the way to go....
i donīt add rudder deflection to my models anymore

Not rudder so much as side area, thus the Genesis,  I have built two of them.

I am with you with the side area, even the Genesis has added side area via a long thin fin.  Interesting though, all of Bob's newer designs have rudders (really vertical stabs).  I wish I had the time to put huge "rudder" on a plane and whittle away at it till I found the sweet spot.  My Trifecta design has about 50sq in of "rudder", Endgame about 5. Both fly extremely well. Neither of them display any of the problems a rudder is supposed to cause or cure.  Both have -0- offset and both have CAM rudders.  The one with the larger "rudder" stays tight above 45 a bit better, but that could be due to a thousand other reasons.  I corrected that with the timer on the small rudder one.

Ken

Okay, Ty caught me with the disguised side area thing. Attached is a photo of the Genesis LC, which has the disguised side area and a couple of small twin stab tip plates (otherwise known as twin rudders). This thing grooves like a freight train and turns amazingly well; due, I think,  to the lack of spanwise flow off the elevators. Yeah, I know, it's not a new thing...

Later - Bob