Tough time turning!
 

https://cimg8.ibsrv.net/gimg/www.rcu...1cfbe25215.jpg
https://cimg0.ibsrv.net/gimg/www.rcu...fdf020b55a.jpg
This is my new scratch built 3-channel plane that is very reluctant to turn using the rudder. I put in gobs of dihedral and I shaped the rudder to produce most of its power down low. It doesn't have the strong yaw/roll coupling that I wanted. I built another like it many years ago that would do tight barrel rolls with rudder only. It had a 36" wing with a 4" flat center section with polyhedral tips (a broken, thin section, undercamber glider wing that I shortened and reassembled). This older one was shaped very much like this one, both low wing. I added more rudder area and retried it and it is better, but still barely turns. Looking for help in figuring out my next move to make this into a good flyer. Thanks in advance!

First 3 thoughts-
Enough rudder area? Not enough servo throw? Servo too wimpy to move the rudder far enough to do any good?

Servo blowback was a thought of mine also. CG too far forward could be a cause as well.

In both views the vertical tail seems to be shielded or blanketed by the unusually shaped fuselage and the big canopy, which both might make for a lot of turbulence. That might render the rudder ineffective, if that is what you are observing (that is too little yaw and not lacking roll effect despite enough yaw).

You could try a canalizer. :)

Thanks for your input fellows, The servo strength and the rudder deflection seem OK but I'll check again. The canopy and yes, the sloping "hood" have an
https://cimg5.ibsrv.net/gimg/www.rcu...dd03bf0e8d.jpg
effect on shadowing the vertical stabilizer. Darn. I can maybe pop that canopy off. My original airplane, with the orange fuselage, had less fuss on it's dorsal side and we can see it has a bigger vertical stab. Can you name the manufacturer and the name of the white one here?

To make the rudder area bigger, I did this to it. I know, it's awful, but I'm trying stuff out. This change made the model steerable, but not nearly good enough. Do you think that a thin, undercamber wing would make the plane more responsive than a fatter, flat bottom?
https://cimg5.ibsrv.net/gimg/www.rcu...f8e07f4a57.jpg

Flat-bottom wing is a tried and proven concept, especially with RE models. I don't think undercamber would help.

If fuselage shape and canopy really shadow the vertical tail I'd try to get the airflow to it. That's why I mentioned a canalizer but maybe even a row of pins put into the kink on the fuselage's top (just in front of the canopy) could help. At least it would be an easy experiment.

P.S.: As an afterthought, I find the dihedral not overly big. I measured 7° (on each side) and that's less than my 8° rule-of-thumb for shoulder-wingers.

It looks like the white one might be an Ace "Mach None"

Yes, Proptop, it is the Ace Mach None! It had only ailerons and elevator and we had to be careful that the fuel didn't run out while you're too far down wind to glide it home! This picture dates from approximately 1986 or 87, so your rememberer is working very well. What a fast, fun, flies-on-rails that plane was!

Ustik, I hadn't heard of a canalyser before and I just investigated what it is. Very interesting! Thanks for introducing me to the concept!

If you're set on flying it with rudder and elevator I'd try increasing the dihedral. Lots of "Old-Timer" free-flight inspired designs have small tails and tiny rudders, but turn very well because of massive amounts of dihedral.

Or you could add ailerons...

Thanks Grognard, I think I will take your advice and cut the wing in half and re-do it with more dihedral angle, I want this one to remain rudder and elevator. The next one will have a more sensibly shaped fuselage and use four channels with maybe a forward swept wing. Going off and designing one's own planes is way more fun than building kits!

Let us know how it works out!

For the next one, remember that wing sweep contributes to "dihedral effect". That is, sweeping a wing back is equivalent to adding dihedral - and sweeping it forward is equivalent to anhedral. So, you'll need more dihedral than usual with a forward swept wing!

Sweeping a wing forward also moves the "mean aerodynamic chord" forward - which affects the desired CG location. So, you'll either need ballast, an extended forward fuselage - or to shift the wing root backward.

Your weak rudder to roll might be a couple of things. First off flying speed makes a difference. Try flying a touch faster and see if things perk up. Next is that comparing the new blue and yellow model to the orange and patches (?) one in the old picture makes it seem like the new one has less dihedral. And that might be part of the issue.

Thank you Mr. B., I'm going to cut the wing and add more dihedral. I measured it and the dihedral angle is 13.3°, less than 7° per side. Ustik above measured well from the picture! I'm disappointed, but it will fly well soon. I'll report back when changes have been made.

Somehow, I "found the time" to get back into repairing this wing. I cut it in half on the table saw and stuck some aileron stock into the gap and got 19.25° dihedral angle, or 9.6° per side.
I'm not going to glue anything together until I get a hold of some low wing model airplane design criteria that will be easy to find on the net.
One wing panel suffered a buckle that has to be strengthened, too. The project is still alive and I''m looking forward to doing barrel rolls and low & slow flying.
https://cimg7.ibsrv.net/gimg/www.rcu...c563402bcc.jpg

Somehow, I "found the time" to get back into repairing this wing. I cut it almost
https://cimg7.ibsrv.net/gimg/www.rcu...53bd43ac42.jpg
in half on the table saw and stuck some aileron stock into the gap and got 19.25° dihedral angle, or 9.6° per side.
Now the angle is 9.6° per side and that has to be enough! I'm not going to glue anything together until I get a hold of some low wing model airplane design criteria that will be easy to find on the net.
One wing panel suffered a buckle that has to be strengthened, too. The project is still alive and I''m looking forward to doing barrel rolls and low & slow flying.