Roll and Pitch Coupling By James Goss       My last fun fly plane I built is known as Flubber and I built it to do mainly one thing, flat spin. It will flat spin as well inverted as it will upright and come out of it as soon as you release the sticks. It spins as flat as a pancake even with reduced throttle and I believe it will do a flat spin landing and survive. I have attempted a flat spin landing with other planes and had pretty good luck some of the time. At least it is an unusual approach to a landing and doesn't take much distance for the approach. Flubber is also a great landing plane even though it is very tail heavy. I have it tail heavy for good flat spins. It will slow down to a crawl without stalling and kiss the ground with ease. It will also hover well with the .91 os four stroke up front.  I built this plane with the horizontal stab setting a full inch above the center line of the wing and much longer than most planes this size. The wing is 41 inches long and the stab is xxxxxxx inches long with a very large elevator and rudder. Knife-edge is another story because it will not hold a good knife-edge at all.      When you apply the rudder while in knife-edge position the plane will either roll out or pitch to one side. This was shocking when I first flew Flubber because I was hoping for good knife-edge characteristics. I have had Flubber at the field several times now and have seen all of its little idiosyncrasies. My next step is to correct the pitch and roll coupling. To find out if your plane has either pitch or roll problems is simple. Get your plane in a knife-edge position and let go of your aileron and elevator stick, the right stick in mode two. Your plane should fly straight and true. If it has roll problems it will either roll back toward an upright position or to an inverted position. If it has a pitch problem it will travel toward either the belly of the plane or the canopy. Here is how you correct these unwanted aerobatic maneuvers. If your plane develops roll coupling you will have to use aileron control to counter it. You can do this by mixing your rudder and ailerons on your computer radio. You set it up so the rudder becomes the master control surface and the ailerons become the slave control surface. This means that if the master surface is moved by your command the ailerons will also move. If you command the ailerons to move the rudder will not move. How this coupling is programmed will be found in your manual that came with your radio. With a computer radio this manual is like gold, be sure not to loose yours.      If your plane has pitch problems and travels toward the belly of the plane or towards the canopy while in a knife-edge, here is how to correct it. You will need to mix the rudder and elevator this time. Again the rudder will be the master and this time the elevator will be the slave. When you activate the rudder the elevator will also move. The direction and amount of movement will be set during the programming sequence. This mixing is only on when you want it to be on. On my T6XA Futaba radio the mixing is activated when the elevator dual rates switch is in the low position. I will probably make these corrections to Flubber the next time I am at the flying field. I believe it will make this plane one of the best I have ever owned.      I believe what makes a plane have the tendency to have roll and pitch coupling is the fact that it has such a large rudder. When you activate the rudder to the maximum position like we do in a knife-edge, the air over the horizontal stab is upset on one side more than on the other. In other words the air current and pressure on one side of the stab is reduced while the other side is normal. This is when the roll and or pitch come into play. Having the elevator halves with different airflow over them will create the same effect as will the ailerons when activated, a rolling action. Also changing the airflow over the elevator will reduce the efficiency of the elevator and allow it to create less up or down control that allows the plane to travel toward its belly or canopy. A plane that has its rudder both above and below the horizontal stab will have less pitch and roll coupling. With the rudder in this position it will have the tendency to maintain the same pressure above and below the elevator instead of a differential pressure that allows the plane to rotate or pitch.      Flubber is a prototype so I think on the next version I will divide the rudder so half will be on top of the horizontal stab and half will be on the bottom. It will be interesting to see the difference. The thickness of the wing on Flubber is a full three inches. It seems that the thicker the wing is the better and slower the plane will fly. I haven't seen any fun fly plane with a wing any thicker than this. I guess the next step is to go with a four inch thick wing, or maybe go on up to six inches thick. We won't know until we try it, but this concept may be a valid approach to a new fun fly style. Remember that the concept of the profile was slow to catch on, so who knows what the future holds. At first I didn't think a three-inch thick, 40-inch wing would perform as well as it does. It amazes me how fast these thick wings will fly; I thought they would be slow and sluggish with all that drag. So I think my next Flubber will have a six-inch thick wing.