With the rudder (master channel) stick at full deflection (above left), the curve type Pmix outputs the maximum programmed mix value of 4 percent to the elevator (slave channel). This is accomplished by programming output values
have at least one 5-point curve Pmix avail- able, and five or more 9-point curves are be- coming pretty common on higher level transmitters.
On the JR 12X, I programmed a curve Pmix with 8 points using the rudder as the master (input) channel and the elevator as the slave (output) channel. When centered, the rudder stick input value is 50 percent; with 100 percent equaling full left rudder stick, and 0 equaling full right rudder stick. I programmed mix points at 40 percent (right of center) and 60 percent (left of cen- ter), and set the Pmix output values to 0 at those points. This results in no elevator mix for small rudder inputs and low beta during which Yuri does not exhibit any pitch coupling.
I programmed additional mix points at 26 percent (about half right rudder) and 74 per- cent (about half left rudder), and 2 more mix points at 13 (about three-quarters right rud- der) and 87 percent (about three-quarters left rudder). Most transmitters with curve Pmixes automatically have mix points pro- grammed at full throw. In the case of the JR 12X, 0 percent, or point L (Low) is full right rudder and 100, or point H (High) is full left rudder.
I initially used mixing value percentages of 5 at full throw with 3 at three-quarters throw and 1 percent at half throw. After it- erative flight testing and adjusting of the mix values, I settled on 4, 3 and 2 percent, respectively. The maximum mix value of 4 percent equates to approximately 1⁄16-inch of elevator deflection when the rudder is at full throw. As with the prior Pmixes setup for Yuri, the rudder > elevator Pmix is very small, but for the discriminating pilot, they do enhance the degree of precision to which Yuri can be flown.
While Pmixes are not generally as effec- tive in addressing roll and pitch coupling as aerodynamic fixes (i.e., adjusting wing dihe- dral), Yuri is extremely well behaved for all maneuvers in the current pattern schedules through the preliminary P15 sequence flown in the FAI class. For the more extreme ma- neuvers found in the finals F15 sequence in the FAI class, coupling is minimal if air- speed is managed properly.
As context, a modern Pattern plane de- signed for FAI, such as Yuri, has vastly
FLYING MODELS
more knife edge capability than planes of just 5 years ago. Using a low rate rudder of 50 percent, at which point only the smallest bit of coupling is being mitigated by the Pmixes, Yuri is capable of all maneuvers in Masters and the P15 sequence.
A great number of articles have been writ- ten over the years on the topic of aerobatic trimming. Much of the information is du- plicitous among them, even when the pres- entation of the material can vary substan- tially. Knowing that multiple sources advocate a specific trim technique to fix a given problem allows a degree of confidence to tackle a trim problem. However, knowing a trim technique works is one thing; know- ing why it works and why it is needed re- quires a different level of understanding. When I began to write about the trim process I use, I first wanted to make the pri- orities/goals of the trimming process very clear. Secondly, I wanted to include descrip- tions of the mechanical and aerodynamic forces at work to facilitate a greater under- standing of why various trim techniques are needed and why they are effective. Since the building phase of Yuri was com- pleted, this is the seventh column written on the topics of control throws, incidences, de- sired trim condition (i.e., the “ideal” trim condition), center of gravity, aerodynamic forces at work, and mechanical forces at work. I have yet to broach what are seem- ingly more basic topics such as lateral bal- ance and basic trimming for loop tracking.
While this may appear to be an oversight, it is very intentional on my part.
There is nothing wrong with the tradition- al starting point of a geometrically straight aircraft; i.e., wings and stabs are parallel to each other and perpendicular to the fuselage centerline, all surfaces are free of warps, all hinge gaps are equal and sealed, and lead- ing edge radii and airfoils match. In truth, the geometrically straight aircraft is the best starting point, as it is common ground among all planes, and a good reference to start from and fall back on (if needed). The problem with a geometrically straight aircraft is that it cannot be expected to fly “straight” when it is powered by an asym- metric propulsion system (single propeller) driving a twisted column of air over the air- craft. That is why my trimming process starts with finding the best settings for right thrust (to compensate for spiral airflow), throttle to rudder Pmix (to compensate for spiral airflow), throttle to aileron Pmix (to compensate for torque), and asymmetric in- cidence in the wings and stabilizers (to com- pensate for spiral airflow). Torque sharing counter-rotating propeller systems are, of course, an exception. I predict they will be- come much more popular for high level aer- obatics in the future.
With asymmetric forces mitigated to the extent possible, next month the trimming process will move forward with an eye to- wards loop tracking and various influencing factors.
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(above right) of 4 percent for mixing points L (Low) and H (High) along with 2 percent for points 2 and 5, and 3 percent for points 1 and 6 on the JR 12X. The mix value of 4 percent is barely visible, as shown below.
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