mately, it is far more important to align the plane to fly aerodynamically straight. Yuri falls in the category of needing minimal sta- bilizer twist; less than 0.1 degree of differ- ence in left/right stabilizer incidences. Now, the previous several paragraphs as-
sumed an ideal fuselage that produced –1G lift when in a yawed condition. If the fuse is not ideally shaped, in terms of producing ex- actly –1G, the “correct” amount of stabilizer twist will result in a plane that has symmet- rical behavior in knife edge flight; an equal amount of pitching or pulling. With the same behavior exhibited with both right and left rudder, several mechanical trim options are available.
This vertical strake is minimal in size, but with optimal placement (above), has a positive effect on rudder response. This Pegasus PH-II (below) uses a traditional style and sized T-canalizer to enhance yaw performance. Bernd Beschorner uses pairs of small strakes (bottom) just aft of the canopy to produce effects similar to a much larger T-canalizer.
For an airplane that pulls to the canopy in knife edge, potential solutions are mov- ing the CG aft, increasing the wing inci- dence, and reducing the amount of down- thrust. All three of these changes will result in a small climb in level flight. The addition of a small bit of down elevator trim to negate the climb will also impart a pitching moment to the belly in knife edge flight. For airplanes that pitch to the belly in knife edge, the potential solutions are reversed; move the CG forward, increase downthrust, and reduce wing incidence.
Please note that these mechanical trim- ming techniques are limited in effective- ness, and only work within a range of pa- rameters. A grossly noseheavy plane will always pull to the canopy in knife edge; no amount of positive incidence or downthrust will produce a clean knife edge.
Fuselage enhancements While changing the fuselage shape may initially seem to be a daunting proposition, there are actually a couple fairly simple modifications
that are commonplace on
modern F3A planes. The first of which is the “T-canalizer,” which broadly includes a vari- ety of horizontal and vertical wings/strakes, located generally on the top of the fuselage near the trailing edge of the wing. It is generally accepted that the T-canaliz- er reduces turbulence from the canopy bump and promotes smoother airflow on the turtle deck, fin, and rudder; increasing the effec- tiveness of this fuselage area. Adding side area aft of the CG and above the thrustline will impart a pull to the canopy in knife edge flight. Conversely, adding a ventral strake or subfin will impart a pitch to the belly in knife edge flight.
In the final configuration I prefer for F3A sequences, Yuri has the smallest of vertical strakes on the canopy (approximately 1 inch tall by 7 inches long), and a larger ventral fin measuring approximately 1.25 by 13 inches, extending forward from the rudder hinge line. I use a 1-inch wide piece of clear tape to protect the fuselage, and then attach the 6mm Depron fins with foam safe CA. These additions are very simple, light and surprisingly durable.
The effect of the top fin was to produce a more balanced rudder feel/response in rolling circles, with the slight adverse effect of increasing the pull to the canopy in knife edge. The effect of the ventral fin was to in- crease yaw damping and, impart a pitch to the belly in knife edge. In fact, at small to moderate beta, Yuri exhibits absolutely no pitch coupling.
Next month, the knife edge trimming will continue, with an eye towards more extreme knife edge loops.
FLYING MODELS 35
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