PHOTOS: PAUL OSINSKI
Yuriexhibits a smooth and stable takeoff (above left). A properly set up Pattern plane will require the fewest amount of control inputs to complete maneuvers.
er, various trim techniques can be employed to reduce the effects.
2) When trimmed for straight and level
flight, with any degree of pitch stability (more on this later), the wing will be produc- ing “1G” of lift. “1G” being the amount of lift produced by the wing to counteract the ef- fect gravity and provide level flight. Given the symmetrical airfoils used on Pattern planes, the wing must fly through the air at a positive angle of attack (AOA) to generate lift.
When trimmed for straight and level flight, 1G of lift results from the net sum of forces from engine thrust, wing incidence, stabilizer incidence (and c.g.). Barring a change in airspeed (which affects the amount of lift), throttle setting (which af- fects the thrust vector, and/or airspeed), or wing/stabilizer incidence (or c.g.), the 1G of lift never changes, no matter what the orien- tation of the airplane is. Changing the orien- tation of the plane to gravity does not change the 1G of lift being produced by the wing. This is key as to why planes tend to pull to the canopy in uplines, downlines, and in knife edge flight.
3) When full scale pilots plot cross-country flights, they calculate a wind correction an- gle to compensate for the effects of cross- winds and achieve the desired flight path, or track, to destination. The only factors consid- ered in the calculation are the airspeed of the plane and the crosswind angle and velocity. This simple calculation is effective be- cause once airborne, the aircraft is flying di- rectly into the air mass. It is not flying at a crab or yaw angle into the air. The only crab or yaw angle is that which could be observed from a fixed point on the ground. This idea would suggest that when flying a Pattern airplane in a crosswind condition, the wind
“1G” of lift is always present on a properly set up Pattern plane (like Yuri) (above right), thus requiring the slightest bit of down elevator for inverted flight.
correction angle (or crab angle observed from the ground) would not need to be ad- justed once set correctly.
In practice, I find this is only possible if the wind is very steady without any gusts or turbulence (essentially homogeneous). In variable winds and gusty conditions (non- homogeneous), the degree of yaw stability becomes quite critical, and the relative bal- ance between pitch and yaw stability is also critical, increasing so as the degrees of sta- bility are decreased.
Pitch stability
More appropriately, I am referencing the degree or margin of pitch stability. As the c.g. moves forward of the neutral point (aerodynamic center), the margin of stabili- ty increases (positive stability). The same concept applies to the yaw axis. Positive stability increases lock/groove, and makes the plane more resistant to changes in pitch (and yaw) from wind gusts in turbulent conditions.
The downside to positive stability is that the plane is also more resistant to direction changes from control inputs, less efficient, and incurs greater drag in maneuvers. In- creased pitch stability generally results in greater tendencies to pull to the canopy in uplines, downlines, and knife edge flight. Planes setup with positive stability (nose
heavy) require some amount of down eleva- tor to maintain level flight when inverted. The classic analogy is a marble placed in a bowl; when the bowl is disturbed (i.e., a con- trol input or wind gust), the marble will move, but it will always try to return to the bottom of the bowl.
A plane with the c.g. matching the neutral
point is neutrally stable, requires no down elevator in inverted flight, and will react to
a change (control input or wind gust) with- out any tendency to return to a prior posi- tion. Neutral stability is analogous to a mar- ble on a flat plate; it will move when the plate is tilted, and stop moving when the plate is level (ignoring inertial effects of the marble), remaining at the new location. When the c.g. is aft of the neutral point (tail heavy), planes exhibit negative or di- vergent stability, where changes in pitch and yaw are accelerated. The analogy for a tail heavy plane is placing the marble on an inverted bowl. It is possible to place the mar- ble at the peak of the bowl, but any distur- bance (control input) will result in the mar- ble rolling off the peak, and it will continue to roll until an opposing disturbance (control input) sends it back to the peak of the bowl. With context established, my preference is for a plane that requires a small amount of down elevator in inverted flight and has a margin of yaw stability very similar to the margin of pitch stability. At the same time, I like the yaw response to be similar to the pitch response. When achieved, the result is an airplane that does rolling loops and rolling circles with similar amounts of stick movement for the elevator and rudder, and smoothly translates angles of attack and crab angles between pitch and yaw. My preference most certainly prioritizes
the high KFactor integrated looping/rolling maneuvers found in the F3A finals and un- known sequences. For flying the prelimi- nary sequence, and AMA classes, my “ideal” setup shifts to favor greater margins of sta- bility, especially in yaw. Exactly how I go about achieving the desired balance will be explored in more depth in a future column, but next month will be the return of Brian Clemmons with more tips on the latest YS powerplants.
Typical of the modern day Pattern plane, even slow speed knife edge passes (at left) only need partial rudder throw. Having a correctly set up Pattern plane makes practice that much more productive and even landings (above) will consistantly be a thing of beauty.
FLYING MODELS 57
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