I have been given a NIB WM40S Ultimate. It is a cute little thing and I have either a small four stroke or a .46SF I plan to install though at this time not ruling out some sort of electric menagerie. I saw the threads on the airplane and rigging the ailerons came up. It seems as provided the upper ailerons move less than the lower, especially downward. Which, given my past experience with biplanes would be correct. That being, slightly less incidence (angle of attack) on the upper wing compared to the lower and slightly less aileron, especially down aileron the upper wing. This to keep the upper wing flying even if the lower tries to stall to prevent dropping a wing.

But, in those threads it seems they were suggesting equal movement of ailerons top and bottom. I have not checked the wing rigging boards that were provided to see if they offer slightly lower incidence on the upper wing. It seems the WM40S might be slightly prone to dropping a wing or snapping. My main experience with an Ultimate bipe was the CGM Ultimate which I set up regardless of what the instructions said, lower upper wing incidence, four aileron servos and slightly less travel on the upper set. The airplane flew beautifully, no tendency to snap or drop a wing, unless I wanted it to.

So, I guess my aerodynamics question, is there a rule of thumb or general practice for rigging model biplane ailerons and wings, equal ailerons, equal incidence or unequal for either or both? What have you done with biplanes, in particular performance bipes.

Even on monoplanes I have often set slightly less aileron down travel, especially on tapered wings with no twist. Like the GP Cap 21 (kit) which mine flew wonderful but many had reported a tendency to tip stall or snap. And subsequent rekitting.

Reduced throw on the upper wing and differential (less down than up) are both correct for scale type biplanes. Differential really makes a difference with long wings and flat bottom airfoils. It helps to avoid adverse yaw and can help with dropping a wing, particularly at low speeds. The Ultimate is different from the usual scale model biplane in that it has a symmetrical wing. I had a look at the manual online. It looks like, as you would expect that the model is set up as an aerobatic plane with all incidences set to zero. It seems likely that there is some right thrust angle built in based on the illustrations but it isn't specified. The instructions didn't make any distinction between the upper and lower wings for aileron throw. Given what it is, the Ultimate should fly the same upside down as right side up so you wouldn't expect much deviation from the zero-zero-zero setup. Some of this is going to depend on how you want to fly the airplane. If you are flying around, shooting touch and goes with the occasional loop or roll then yes, mild throws with reduced throw on the upper wing will work well for you. If you are wringing out the plane and pushing it to its maximum ability you are going to want full throws on every surface. You can mitigate the effect on handing and stall characteristics with things like dual rates and exponential but the Ultimate is by nature a high performance airplane and some of those flight tendencies that yo are hoping to avoid come with that performance. You have to adjust your landings accordingly and practice things like using the rudder to pick up a low wing

Biplanes can be tricky. I would say it all depends on what your expectations are. Having some aileron differential is beneficial to eliminate adverse yaw. With an Ultimate, you won’t need much. Personally I’m not a fan of the 0-0-0 setup. Most are fooled into thinking that it lend the airplane to fly inverted the same as upright. Not exactly accurate. With a reasonable CG the 0-0-0 setup you need to force the wing(s) into a positive AOA to produce lift with some up trim. That works fine for upright flight but you can imagine what it does for inverted flight but it will also adversely affect knife edge and up lines. My preference on aerobatic biplanes is 1 degree positive incidence on the bottom wing with engine and stab at zero. The 0-0-0 setup works well if you are running the CG fairly aft. I also set up all 4 ailerons with the same throw and differential. I in fact accidentally had more throw on the bottom once and could not get a rolling combination to look good no matter what. After I got home and started measuring things I found and corrected the issue. The airplane then performed as expected. Getting all ailerons in sync is easy with separate servos, with link rods the connections must be right on the TE of each aileron and not offset from the center.

Thank you both for the very complete answers.

Here is a good example of what I was talking about. My latest build of my F3A design Divergent. The motor has zero up or down thrust but has 3.5 degrees of right thrust. The upper wing and stab are at zero degrees in reference to motor. The bottom wing has 1 degree positive incidence. The four ailerons have exactly the same travel. I measure them at high rate which is 25mm up and 23mm down. That small amount of aileron differential fives me very nice axial rolls.