tive) pitch results in more thrust being pro- duced on the right side, and a yaw to the left. When trimmed for straight and level flight, most modern aerobatic designs have the propeller disc directed straight ahead (neglecting right thrust), or slightly nega- tive (top of disc forward) due to downthrust. In either case, the amount of asymmetric thrust produced in level flight is negligible. During positive corners, the yaw force is to the left, and during negative corners, when the top of the disc is tilted forward, the yaw force is to the right. As the alpha (angle of attack in pitch) increases, the amount of asymmetric thrust (and yaw force) also in- creases. As the magnitude of the yaw force is variable depending on the nature of the ma- neuvering, it is not practical to address p- factor with mechanical setup or a Pmix. In practice, the solution is pilot application of variable amounts of right rudder during positive loops and left rudder during nega- tive loops.
P-factor can also affect knife edge flight, most noticeably at high beta (angle of attack in the yaw axis), when the direction the fuselage is pointing is substantially differ- ent than the flight path followed by the cen- ter of gravity (c.g.). During right rudder knife edge flight, a pitching force towards the canopy (effectively up elevator) is pro- duced, and similarly left rudder knife edge flight produces a pitching force towards the belly (effectively down elevator). Spiral airflow, propwash, or spiral slip- stream is the characteristic of the air accel- erated by the propeller to rotate around the fuselage in the same direction as the rota- tion of the prop. The spiraling airmass im- pacts the fin/rudder on the left side above the thrustline, and on the right side below the thrustline. As the majority of the fin/rudder area is above the thrustline, the result is that the tail is pushed to the right (turning the nose to the left).
At low speeds and high power settings, the spiral airmass is the most compact/force- ful. At high speed, the spiral form/shape elongates and the amount of force (on the fin/rudder) is reduced. Designs with an abundance of side area that is both aft of the c.g. and below the thrustline will require less right thrust, and quite possibly none. Unlike P-factor, which can adversely steer the plane in any direction, spiral airflow will always (at a minimum) impart a yaw to the
left. During straight and level flight, the spi- ral airmass impacts each stabilizer half at different angles, resulting in different effec- tive alphas for each stabilizer half. The al- pha differential is difficult to predict, as it is influenced by the shape of the fuselage, the vertical location of the stabilizer, and rela- tive position of the stabilizer with respect to the fin and rudder. In normal flight (no beta), the alpha differential is accounted for/balanced with the use of elevator trim (whatever is needed for straight and level flight).
When beta is introduced, such as knife edge flight, the alpha differential can mani- fest as a pitching element. In knife edge flight, the “top” stabilizer is partially blanked by turbulence from the fuselage. I.e., when in right rudder knife edge flight, the inboard section of the right stabilizer is blanked while the left stabilizer is flying in clean air. The alpha of the left stabilizer be- comes dominant and controls the pitch of airplane. Typically, right rudder knife edge will pitch to the canopy while left rudder knife edge will pitch to the belly. This is a re- sult of both spiral airflow and p-factor. Trim techniques for pitching in knife edge will be covered in a future column. If I’ve done a good job describing the na- ture of torque, gyroscopic precession, p-fac- tor, and spiral airflow, it should be clear that the built-in right thrust (about 2 degrees) for Yuri is present to counter the effects of spi- ral airflow. As with torque, spiral airflow is not a problem at idle, and the negative ef- fects increase as the power level increases. At first glance, the seemingly simple and di- rect solution is to trim the rudder for straight flight at idle (slight descent as if on a fast landing approach), and progressively add right thrust to have straight flight at full throttle. Planes that track straight in yaw at any throttle setting in any attitude using that approach are rare.
Modern Pattern airplanes (and other aer- obatic models) have extremely high power to weight ratios, and the amount of right thrust needed to maintain a straight flight path at full power (straight and level or in a vertical climb) often becomes impractical. Depending on the design, any more than 2 to 4 degrees of right thrust will begin to cause roll differ- ential problems; axial rolls simply become impossible. At that point, the remedies are pilot application of right rudder when high
power settings are used, or using a Pmix similar to the throttle > aileron Pmix. A throttle > rudder Pmix can be done sev- eral ways. The first option begins by trim- ming the rudder for vertical climbs at full throttle. This option will likely result in a drift to the right at reduced power settings, and most certainly at idle. The drift to the right is addressed with a Pmix using a throt- tle (master) > rudder (slave) mix with an off- set point. The offset point is the throttle po- sition below which the plane begins to track to the right in straight and level flight. The left rudder mix value is set to achieve straight flight at idle (slight descent). As the throttle is advanced (from idle), the left rud- der will gradually diminish until the offset point is reached. Above the offset point, the rudder trim does not change.
The second throttle > rudder Pmix option is essentially the reverse of the first option. The rudder trim is set to achieve straight flight at idle (slight descent). As power is in- creased, at some point the plane will start tracking to the left, and that throttle posi- tion is the offset point. The right rudder per- centage is set to achieve a straight vertical climb at full power. As the throttle is re- duced (from full power), the right rudder gradually diminishes until the offset point is reached. Below the offset point, the rudder trim does not change. Yet another option is to trim the rudder for straight and level flight at “cruise” speed, and implement both of the above described Pmixes as needed. I know of no magical formula for estab- lishing the “correct” amount/balance of right thrust and rudder trim; it is simply the re- sult of iterative flight tests. While I typically favor adding right rudder at higher power settings, Yuri flew better with the rudder trimmed for high power settings and the ad- dition of left rudder at low power. The final Pmix values I settled on were 3% left rudder at idle with an offset value of –140%, which is effectively 15% throttle. This equates to approximately 1⁄32 inch of left rudder at idle. It is worth reiterating that the throttle > aileron (compensating for torque) and throt- tle > rudder (compensating for spiral air- flow) Pmixes are very small, and while ben- eficial, require careful attention to detail to set up properly. Such detailed mixes I often fine tune over the course of several flying sessions, and they are specific to the pro- peller being used.
As described in this month’s text, an offset throttle > rudder Pmix is used to add a small bit of left rudder (3%) at low throttle settings (above left) while the
FLYING MODELS
rudder trim is unaffected at higher throttle settings (above right) (above 15%, or –140 offset). These values were established by iterative flight tests.
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