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ments to the wing incidence of each panel. Yuri had the smallest bit of right aileron trim, so I decreased the incidence of the right wing panel (dropped the leading edge) and increased the incidence of the left wing panel by an equal amount to induce a small amount of right roll. The small amount of in- duced right roll was then compensated for with a small amount of left aileron trim, ul- timately resulting in the ailerons being per- fectly centered with the wing panels. I made a similar adjustment with the sta- bilizers as Yuri was flying with the slightest bit of down elevator trim. I increased the in- cidence (raised the leading edge) of each sta- bilizer, which caused a slight dive in level flight, and this was compensated for by a small amount of up trim, which centered the elevators to the stabilizers.


These adjustments, while minor, are im- portant. In general, the incidence of the main surface (wing or stabilizer) will domi-


nate at lower speeds, while the trim of the control surface (aileron or elevator) will dominate at higher speeds. Having the con- trol surfaces centered exactly will generally result in an airplane that is stable in trim over a broader speed range.


Roll coupling is a very common condition, but it is one that is undesirable with aero- batic planes. The goal is to have zero re- sponse in the roll axis when any amount of rudder is applied at any airspeed. Yuri con- sistently demonstrated a small amount of adverse roll (left rudder yielded right roll, and right rudder yielded left roll). The best fix for adverse roll is to slightly increase the dihedral angle; however, this generally re- quires a new set of wings (built with in- creased dihedral). Not entirely practical. Enter the programmable mix (Pmix). Af- ter adjusting the mix values over the course of a couple of short flights involving knife edge passes and knife edge loops, I settled on


a linear mix of 6% left aileron (slave channel) with full left rudder (master channel) and 7% right aileron with full right rudder. This Pmix applies a small amount of aileron the same direction as the rudder input, and vir- tually eliminates all roll coupling. While the mix values of 6% and 7% are generally higher than I would like to see (higher mix values increasingly impart un- intended consequences), the outcome of this Pmix was quite effective without introduc- ing any undesired behavior.


Classically, Pmixes (such as rudder to aileron, and rudder to elevator) for knife edge maneuvers have been assigned to a switch so the mix could be turned on only for knife edge maneuvers. While this makes a certain amount of sense, the plane simply does not know what maneuvers are being performed, and the laws of aerodynamics do not change for different maneuvers. The adverse roll behavior of Yuri will oc-


cur anytime (well, most anytime) the rudder is used; it is simply more visible in some ma- neuvers and attitudes. Therefore, I leave the rudder to aileron Pmix active all the time, with one exception. I used a “stick switch” or “logic switch” to turn off the Pmix when the throttle is close to idle.


To compensate for changes in roll trim caused by motor torque, a programmable mix (above) in the JR 12X is used. At idle, approximately 1⁄64 inch of right aileron occurs (below) and is gradually turned off at 4 clicks above idle using a stick switch value of 10.


The reason for this is when completing spins, I really don’t want an aileron input opposite of the rudder. And when doing stall turns (very low airspeed), Yuri did not ex- hibit adverse roll tendencies. So with the use of the stick switch, the Pmix is automat- ically turned off at low throttle. The next trim item I addressed was one due solely to the presence of the rotating mass at the front of the plane; the spinning powerplant and propeller. While airplanes are well damped in the roll axis, there is no overcoming the basic laws of physics. In this case, the torque of the motor in lev- el flight causes a slight roll to the right, which is balanced by a slight amount of left aileron trim. Or, more precisely, as I matched the ailerons’ trim to the wing pan- els, a slight bias in the incidence of the wing panels (left panel slightly negative compared to the right panel). While this bias keeps the airplane from rolling at moderate and high power settings, at idle, the torque of the mo- tor is removed, and the aerodynamic bias causes a very slow roll to the left. Save the use of a counter rotating propeller system to eliminate torque, the only improvement that can be made is the use of another Pmix. The throttle is used as the master channel with the ailerons as the slave channel. Over the majority of the throttle range, the ailerons are not affected. However, at the lower end of the throttle range, the Pmix be- comes active and the smallest amount of right aileron is introduced to keep the plane from rolling as the torque from the motor is removed. This Pmix cannot be perfect be- cause while it is linked to the torque condi- tion of the motor, the Pmix cannot take into account the actual airspeed of the plane. Consider the condition of the plane just af- ter a stall turn; the airspeed is low, the torque is low, and the Pmix is active, and the plane is gradually accelerating to terminal downline speed. As the plane accelerates, the small bit of aileron trim will become in- creasingly effective. I program this Pmix value to be correct for the average downline speed in normal maneuver flying as this is how the Pmix will provide an accurate cor- rection the most times per flight.


FLYING MODELS 39


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