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ier to see than small “bumps” in yaw, and the actual culprit (lack of yaw stability) is not evident.


As I have stated before, my preferences


are geared toward the FAI F3A maneuver schedules which contain a substantial num- ber of integrated rolling/looping elements and ¼ rolls. If I specifically set up a plane for AMA Masters, Advanced, Intermediate, or Sportsman, my bias progressively shifts to- wards greater margins of stability (basically a more forward c.g.).


A BJ Park design, John Ford’s Episode evidences an abundance of forward side area for maximum responsiveness in yaw. The design includes the use of a canopy mounted “canalizer” and side force generators (SFGs) on the stabilizers to achieve the desired yaw characteristics.


So, if a plane meets the criteria of the ideal trim condition, is capable of all maneuvers, and is relatively easy to fly, does it earn the title of the ideal/best Pattern plane? Maybe, maybe not; there are still more attributes re- quired. The mission of a Pattern plane is very demanding indeed! The ideal/best Pat- tern plane will retain all the aforementioned requirements in virtually all climatic condi- tions; primarily wind/turbulence. Principally, the plane needs to be very set- tled in turbulence; minimally disturbed in yaw, pitch, and roll. This is more commonly described as “lock” or “groove”. The ideal plane in calm air is quite limited if it de- mands a very high workload on the pilot to maintain the desired track in windy condi- tions. Increased groove is generally the re- sult of increased margin of stability (in both pitch and yaw), and comes at the expense of responsiveness and efficiency during ma- neuvering. Most certainly the degree of groove is a personal preference and quite possibly is influenced by the maneuver schedule being flown.


Related to how well the plane flies in tur- bulence is how well the plane flies in a cross- wind condition in which a crab angle is pres- ent. Crab angle being defined as the direction the nose of the plane is pointing in comparison to the actual track of the plane across the ground. Assuming a homoge- neous airmass (constant wind direction and velocity), the plane never flies in a cross- wind; it rotates (in yaw) about the c.g. until the air pressure on both sides of the fuselage is equal, just as a weathervane always points directly into the wind. When flown in a heterogeneous airmass (variable wind direction and velocity), every change in wind is matched by a change in crab angle (as viewed from the ground). However, the change in crab angle is not in- stantaneous because the plane has mass and inertia and requires time to accelerate and decelerate in the yaw axis.


Whether this time lag results in a plane that iteratively increases crab angle (rela- tive to the average wind direction) or de- creases crab angle (relative to the average wind direction) is a product of the side area distribution of the fuselage in front of and behind the c.g. The distribution of side area becomes increasingly critical as the margin of yaw stability is reduced.


With the prior paragraph as context, my preference is for a plane that has slight posi- tive stability in yaw and exhibits relatively neutral behavior in turbulent crosswind con- ditions. My definition of neutral behavior be- ing that rudder corrections will be 50% into the wind (upwind) and 50% with the wind (downwind) to maintain the desired track.


FLYING MODELS


The margins of stability for pitch and yaw should be similar such that angles of attack in pitch (alpha) and yaw (beta) are smoothly translated in rolling circle and rolling loops. Such a balance will produce smooth transla- tions of crab angle between pitch and yaw in a variety of wind conditions, which means minimal (if any) corrections are needed when the presentation of the plane (to a crosswind) changes from pitch to yaw during ¼ roll elements. A number of pilots prefer a plane that always drifts downwind (relative- ly less side area behind the c.g.), or always steers into the wind (relatively more side area behind the c.g.) because it makes the needed rudder corrections more predictable (as opposed to a 50/50 split).


What is very often overlooked is when the c.g. is changed to alter pitch behavior, the yaw behavior will also be affected. Moving the c.g. alters the ratio of side area fore/aft of the c.g. which changes the margin of stability in yaw and the yaw behavior in crosswinds. Yet another effect, is that shifting the c.g. aft can, in some instances, result in a plane that is less stable in the roll axis in turbu- lence. If in turbulence the plane is “wig- gling” in yaw (rotating about the c.g.), the wing halves are advancing/retreating with respect to each other. The advancing wing generates more lift while the retreating wing generates less lift, and this bias of lift is manifested as a small “bump” in the roll axis. From the typical perspective of the pi- lot, small turbulence “bumps” in roll are eas-


Quite simply, I find the benefits of greater stability in turbulence to be of a greater ben- efit than the minor translational losses of pitch/yaw angles. More often than not, a plane that I find to be optimally trimmed for Intermediate will need to be re-trimmed (to an extent) to be optimally trimmed for Ad- vanced. Trimming for the lower classes is generally easier, as the variety of maneu- vers are reduced.


Hopefully what I have conveyed thus far makes it clear that a number of desired be- haviors for the ideal Pattern plane are at odds with each other, and quite possibly the ideal Pattern plane is an unachievable goal. In large part, that is simply because “easy to fly” and “stable in turbulence” are not ab- solute terms; “easier” and “more stable” are always possible, and if one is accomplished without compromising the other, progress has been made. In some instances, compro- mising one or more of the ideal trim condi- tion criteria to a minor extent in exchange for a substantial reduction in the workload of the pilot is worthwhile.


My bottom line remains that the best air-


plane (and trim setup) is the one that re- quires the fewest control inputs to complete a maneuver sequence in the majority of weather conditions. While aerodynamic fix- es are preferred, even programmable mix- ing (Pmix) is fair game, so long as the result is a reduced workload for the pilot. Most certainly the best state of trim will vary based on the specific priorities/preferences of the pilot.


Next month I will present some details on the trim characteristics of Yuri at the time of the test flight, and the changes I made to better suit my personal preferences.


Ron Lockhart’s stock Bravo (top) flown in the Masters class is very stable in crosswinds, whereas Dave’s Bravo (middle) uses a T-Canalizer and slightly further aft c.g. for increased knife edge power for the F3A class. Yuri (bottom) in the final flight configuration for Dave’s preferred flying style sprouted additional side area in the forms of stabilizer SFGs, ventral fin, and the smallest of strakes on the top of the canopy.


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