project pattern
Redux
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
PHOTOGRAPHY: DAVE LOCKHART
With the rudder (master channel) stick partially deflected (above left), the curve type Pmix does not output to the elevator (slave channel) (below). This is
L
ast month’s report on the F3P Team Trials interrupted Project Pattern Redux, which left off with an ongo- ing discussion of the factors influ- encing behavior in knife edge flight and var- ious
trimming techniques to improve
aircraft response in knife edge flight. With the use of a rudder > aileron programmable mix (Pmix) and some small fins to better control airflow on/around the fuselage, Yuri exhibited no coupling in pitch at small to moderate beta (yaw) angles.
Previously, Project Pattern Redux has de- tailed the setup of several types of (Pmixes) including: • Rudder > Aileron Pmix (linear mix) to mitigate unwanted roll coupling with rud- der inputs; • Throttle > Rudder Pmix (linear mix with offset) to mitigate yaw trim changes as- sociated with changes in spiral airflow re- sulting from changes in throttle settings; • Throttle > Aileron Pmix (linear mix with offset) to mitigate aileron trim changes associated with changes in torque resulting from changes in throttle settings. All of the Pmixes used thus far are of the linear type; the amount of the output for the slave channel is always the same for a given amount of input from the master channel. Put another way, each increment of control input from the master channel would yield “X” output for the slave channel, where “X” is the programmed Pmix value (in percent- age). In the case of a rudder > aileron Pmix, if a 10 percent rudder stick input yielded 1 percent aileron output, 100 percent rudder stick input would yield a 10 percent aileron output. Linear Pmixes are the most common type of mix, and generally very effective for a broad range of applications.
However, many aerodynamic relation- ships are not linear. The lift and drag result- ing from a fuselage with increasing beta (an-
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accomplished by programming output values of 0 for mixing points 3 and 4 (10% to each side of center) as shown in the screen shot on the JR 12X (above right).
gle of attack of the fuselage resulting from rudder inputs) is a good example of non-lin- ear response. Specifically, with Yuri, small rudder deflections and low beta are all that is needed to perform knife edge flight, point rolls, and large diameter rolling circles at cruise or high speed. Yuri does not exhibit any pitch coupling during performance of the previously described maneuvers. For more demanding maneuvers like loops with one integrated roll and knife edge loops, increased beta (resulting from larger rudder inputs) is required to generate enough “side lift.” The increased beta also generated a component of lift toward the canopy of the plane, resulting in a pull to the canopy that is not present at low beta. A lin- ear Pmix could be used to counter the pull to the canopy (at increased beta), but then the
Pmix would also cause a pitch to the belly at low beta. In this situation, a curve type Pmix is very useful.
To mitigate the pull to the canopy during knife edge loops and other similar maneu- vers, I used a curve type Pmix with rudder as the master channel and elevator as the slave channel. As with the previously em- ployed rudder > aileron Pmix, I employed a “stick switch” to turn off the rudder > eleva- tor mix when the throttle stick is close to idle. Turning off the Pmix close to idle elim- inates the possibility of adversely affecting spins and stall turns, during which time the airspeed is very low and pitch coupling be- havior is generally not evident. Depending on the transmitter being used, the availabil- ity of curve type Pmixes can vary substan- tially. Most modern mid-level transmitters
JUNE 2014
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