whats needed in order to have a design with zero coupling of control surfaces?

im thinking about symetrical airfoil, wing+stab on the thrust line

what else?

Vertical and rudder with equal area above and below the longitudinal centerline of the fuselage.

Dean Pappas writes about this in the latest issue of Flying Models. Basically the airplane should have all areas balanced about the centerline. There are benefits to this type of model except that the presentation to the judges can be less effective. We are conditioned since birth to seeing the tail on top of the fuse. Going to the equal tail looks a little strange.

For power considerations a counter rotating prop assembly would be nice to get rid of the torque/prop problems but is a mechanical nightmare. Another way is to go to a twin engine setup with opposite crankshaft rotation.

Even couterrotating props won't get trid of torque effects very easily, as the rear prop is operating in the swirl of the front one - means that you have to tweak the two pitches very carefully to get rid of torque, and even then it only works perfectly at one speed. The pod-mounted twin with counterrotating props would work, though.

Getting rid of yaw-pitch coupling is even harder though, because even a symmetric plane has asymmetry in flight, due to the downwash of the wing. This is especially evident in the transient response, but will also show up in steady-state response to rudder inputs. This is because the vortices shed by the fuselage when in sideslip get "bent" by the wing's downwash field, causing pressure differentials on the fuselage, and unpredictable effects on the horizontal stabs. You can even get some yaw-roll coupling this way.

The longitudinal centerline of what:
The fuselage geometric center?
The fuselage mass center?
The thrust line?
The main lifting wing(s) aerodynamic center?
The center of drag?
Yes to all the above?

A bi-plane seems like it would do good and look normal at the same time. However, my Ultimate has proverse roll and up-pitch with rudder. It seems like the aircraft mass centerline is the important one. I'm just guessing though.

I also heard that things like dihedral can increase or decrease coupling. I once saw a model Citabria with anhedral (drooping wings). It was intended to decrease yaw coupling with ailerons.

StarFire - agreed, but you have to start somewhere. If the idea is to minimize as much as possible the interactions then the full symmetry is the way to go. If the idea is to get an airplane that has the same interactions upright as inverted then full symmetry is still good. At least rudder causing pitch coupling due to the sideslip/downwash mixing is going to be the same whether the belly or canopy is up. As a matter of fact the only factor that is left that won't at least be consistent in coupling effects is gravity.

ilikeplanes - you want the fuselage longitudinal areas, mass, thrust and all of the stuff to be all lined up. The idea is to give the forces no moment arm to couple about the CG. You have to have the wing lift and the tail lift there with moments to stabilize the airplane but in a case like the Citabria the wing is quite a bit above the CG and is positioned above the tail, rudder and fuselage. It does make for a lot of coupling.

The Citabria has a negative Clbeta term due to the high wing. This means for a left rudder input you get a positive sideslip (nose left) and the rolling moment the Clbeta term produced is negative (left wing down). The anhedral in the wing lowers the magnitude of the Clbeta term. Lowering the wing does the same thing. Distribution of area about the CG can effect this also but to a lesser extent.

Basically you can adjust wing position and dihedral to give a zero Clbeta term for a small range of angle of attacks and sideslip angles but then with high angle of attacks or sideslip angles you get variations in Clbeta also.

It is a big balancing act in which you try to minimize the inputs.

Certainly, one must start somewhere - I wasn't criticizing.

After a bit more thought, I think the only way to totally decouple an airplane would be through a CAS (Control Augmentation System). It would have to be a very complicated system, with lots of very expensive sensors, too.

That said, the symmetric airplane proposed here will get very close to being decoupled, and with a lot of time tweaking mixes on the higher-end computer radios, could probably be made to fly so close to completely decoupled as to not be nticeably different from the real thing.

With careful work on the S+C, and a neutral airplane, one could get a plane that handles identically upright and inverted, as well.

I didn't take it as criticism (I even had to look up the word:-). This stuff is interesting isn't it. No you made a good point about the mixing flows and their relationships.

Just think how good things could be if we could get rid of gravity. In my youth I used to imagine flying in a big bubble in orbit. You could draw some nice figures in space. Then I saw the insides of the space station and the concept of a smooth bubble burst pretty bad.

We did work (when I used to work) on some control systems for unstable aircraft. One of them that wasn't too practical used the set of wind tunnel data that was tweaked with flight test data. The system did a look up of where you were, where you wanted to be and figured up how to get there using the data set to work out control deflections. Worked great on a monster computer we had but at the time the memory requirements were impossible for actual use (this was a long time ago). Then we tried to reduce it to curve slopes which kind of missed when the non linear things were happening (most of the time) Then I luckily got transferred.

With the modern electronics and memory it would be possible in a model but like you said it would just add a layer of stuff to go wrong. Plus we are trying to find out who the best pilot is and which is the best airplane.

It would be interesting to make an airplane that had easily changable wings, tails, etc. locations so you could change the configuration at the field. Three horizontal tail and wing positions, vertical tail profile changes, etc. With modern materials it might be kept light enough to give some answers that would be useful.

Too many interesting problems and not enough time. Even being retired I found the hobbies expanded to fill all the extra time.