project pattern
W
Redux
ith the business end of Yuri ad- dressed, it is time to take a more detailed look at the servos being used. As the aileron and eleva-
tor servos are mounted on ribs within the wings/stabs, the servos were specified prior to production such that the ribs were built to the correct mounting dimensions during the construction of Yuri. Virtually all rudder servos used in the modern day Pattern air- craft are of the “standard” size (+/- 1.5 inch- es long and 0.75 inch wide), and the rudder servo mounting area is accessible, should modifications be needed. To roughly parallel a real estate phrase,
the three most important factors for good Pattern servos are centering, centering, and centering. In truth, the best Pattern servos are accurate and precise, even when placed under heavy loads. The resolution is very high with “zero” deadband or <1 microsec- ond. Deadband ratings are not always easy to find, but in practice it means a servo that will respond to every single “click” of trans- mitter trim, even with the transmitter trim set to the smallest increment (1 microsecond on most modern day transmitters suitable for Pattern). Critical in achieving this level of perfor-
mance are geartrains with minimal slop and a sufficient amount of torque to enable im- mediate response. Speed itself is also im- portant, but quite meaningless if the servo is inaccurate or suffers from blowback under load. Suitable servos will assuredly be of the digital variety, with coreless or brushless motors, ball bearing supported output shafts, and may have nylon, composite, or metal gears. For the modern day 2-meter Pattern ship,
torque requirements are fairly modest as the control surfaces are reasonably sized and control throws are not excessive. Addi- tionally, the same control linkage setup used to take advantage of the full resolution of the servo will also give the servo good me- chanical advantage. That said, the target
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
PHOTOGRAPHY: DAVE LOCKHART
The servos chosen for Yuri include the mini sized JR DS3517 (elevators) and mid-sized JR DS9411 (ailerons). Also shown is the standard size JR DS8411SA, similar in size to the JR DS8711 selected for the rudder.
torque levels are 40+ ounce-inches per ele- vator, 80+ ounce-inches per aileron, and 150+ ounce-inches for the rudder. Speed is also a factor in picking good Pat-
tern servos, but should not take priority over centering and torque. Target servo speed (transit time for 60 degrees travel) is 0.15 second/60 degrees, or less. Matching servo speeds for each control function is a very minor consideration, as the native re- sponse characteristics of the plane are dif- ferent in roll, pitch, and yaw. Powering the airborne system (servos and receiver) is a very important aspect, and could impact the choice of servos used. I strongly recommend the airborne power sys- tem be of the redundant or backup variety, as Pattern planes frequently accumulate a
large number of flights (1,000+ is not un- common), placing a strong demand on relia- bility. I also strongly recommend regulating the voltage to the servos such that the volt- age to the servos is constant throughout the course of a practice session. Achieving the goal of either redundant or backup regulat- ed airborne power can be accomplished in many ways, only a few of which will be dis- cussed herein. Starting with my preferred method, Yuri
will use a pair of Tech Aero Flex Reg PLR5- E adjustable voltage regulators (Vregs, 0.4 ounce each). Each Vreg is independently plugged into the Rx and independently pow- ered by a Thunder Power 2S1P 480 Prolite lithium polymer (Li-Po) battery (1 ounce each). The Vregs are set to a common volt-
PHOTO: MARK ATWOOD
Servos suited for Pattern applications (above) from Airtronics include the mini sized 94761 (elevators), standard sized 94774 (ailerons), and standard sized 94780 (rudder). The “budget” friendly Spektrum line of servos (at right) are high performance by most measures, and only slightly less accurate than premium servos from JR, Airtronics, and Futaba.
36 SEPTEMBER 2012
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