I have never compared them (on the same design to actually test for differences), but in my opinion a rudder with adjustable offset is better than a rudder with an airfoil. I don't like anything that influences trim as much as a rudder to be "fixed".

On the last couple of planes, I haven't used any rudder offset and no adjustment. Just have to make sure that thing is on straight. But I agree with Mike. If you are going to do it at all, make it adjustable.

Hi Bill,
   I have never heard of the rudder only being effective when the power is off??? Hmmmm......

   As long as there is airflow over the rudder it is effecting the flight path. If the airspeed changes the rudder will create more or less line tension/yaw. I may be wrong, but I believe it can contribute to a wiggle in the corners (along with leadout position). This is also due to the change in airspeed. 

I have mentioned this before, I lost part of the rudder on the Time machine and it was not fliable in the overhead stuff, so I am 100% convinced it does have an affect. Now at 3 sec laps anything will fly.

I totally agree Randy, my Time machine is modified to look like a 30s air racer, not much flat side area.

Hi Guys,
years ago I chose to not spend time on finishing and a lot more time on experimenting with design stuff. I flew slower 5.4-5.5 lap times on 64 ft center handle to center of plane line length on a classic size 600 sq"-52" span plane.
I made 3 interchangeable rudders and fin combination.
1. airfoiled  fin & rudder  2.  flat adjustable rudder
3. flat offset fin and rudder. ( this means that the entire fin& rudder  are  straight but are mounted at an offset  angle on the fuselage.)

I then flew multiple days of flights with all 3 options mounted and kept a flight log on effects on line tension. I did not change anything else as I had already discovered the best leadout position for all 3 options with the earliest tests.
The results:

1. The airfoiled fin/rudder and the flat-adjustable fin/rudder set straight were
    the poorest performers in providing additional line tension. I was suprised
    so I took several more days to just test just the airfoil vs the straight flat-
    adjustable fin/rudder and after at least 40 flights I came to the conclusion
    that the airfoil fin/rudder is no more effective than a flat fin/rudder .
    Paul Walker and others have used a flat stab/elevator as opposed an
    airfoiled stab/elevator so it may explain my results.

2. The flat adjustable fin/rudder seemed to help line tension with 1 to 2
   degrees of offset but too much offset caused yaw and hurt line tension.

3. The flat fin/rudder only needed to be offset 1 degree or a little less to
    equal the results of 2 degrees of rudder offset. 

The net results of my tests is I have again never bothered with the extra work of airfoiling the fin/rudder on any airplane ever again and even though
an offset fin/rudder probably produces less drag with a lower offset angle I
have chosen to use a adjustable rudder because it is adjustable.

I don't claim that my tests are any kind of absolute answer for everybody but they were objective empirical tests in which I tried minimize variables in my test proceedures to results I could use. I am now too old to take on this kind of energy consuming project but I am glad I did it because we didn't have all the analysis & information that is available today and I used what I learned in all my planes.
                                                                   Till next time,
                                                                   Pat Robinson
                                                                   
     

Hi Pat,

I whole heartedly agree with your findings.  In fact my last 6 airplanes have had a flat (1/8" thick) rudder with 1 to 3 degrees permanent offset.  The reason for the thin rudder is that it actually flexes and in my opinion somehow is self adjusting to keep the tail of the airplane flying on a straight path.  The idea of a semi flexible self adjusting rudder comes from the stone ages of RC when rubber escapements were used to power/move control surfaces (lightweight servos had not been invented yet).  OK, that might seem kinda of crazy reasoning but it sure works on my fleet of Divas.

Phil

Here is what I learned about rudders.Lets look  at a high speed competition free flight with a rudder the size of a Nobler. With the rudder straight a small tab 1/4 inch wide one inch tall offset 1/16 inch will turn the plane any way you want to go under power. In the glide it has no effect. One day flying a stock Flight Streak the rudder came off during flight and the plane flew better until the motor quick and then there was no line tension. Learned these two lesson in the late 50's. I have used almost no rudder offset on controline models ever since to gain line tension under power. Some models need a rudder for the glide portion but no offset is needer if the line rake is OK. Planes with a lot off line rack need offset to get the rudder back straight into the airflow. I think miss adjusted planes need offset.