Optimizing Servo Control Surface Movement 1 Optimizing Servo

Optimizing Servo & Control Surface Movement 1

Optimizing Servo & Control Surface Movement Making an RC airplane fly smoothly requires that: § All control surfaces move in the proper direction called for by the transmitter § Control surface movement must be adjusted to kit specs § Servos must be centered § Linkage settings must allow near or full servo travel § Linkages must not bind § Programmable transmitters must be properly set § The airplane must be balanced fore/aft and side-to-side 2

What we will cover in this session: § § § § § Control surface movement specifications End Point Adjustment (EPA) High/Low/Triple Rates Rx channel assignments (Futaba) Importance of trim Dual vs single aileron servos Servo reversing Exponential (control sensitivity) Mixing (programmable radios) Differential (more up than down) 3

Kit Manufacturer Control Surface Specifications: § Control surface movement is specified in inches or in degrees and includes the following: § Aileron deflection (up & down) § Rudder deflection (right & left) § Elevator deflection (up & down) § Flap deflection (where applicable) § Dual/Triple Rates (where applicable § Throttle travel (where applicable) 4

Four-Star 60 Control Surface Specifications: Aileron (Standard) 5/8 in. up & down Aileron (High Rate) 7/8 in. up & down Rudder (Standard) 1 in. left & right Rudder (High Rate) 1. 25 in. left & right Elevator (Standard) ¾ in. up & down Elevator (High Rate): 1 in. up & down Important: Sig specifies only “standard” throws. Medium or High Rates are determined by the kit builder. 5

RC Airplane Channel Assignments (Futaba): CHANNEL 1 2 3 4 5 6 7 8 FUNCTION Aileron 1* Elevator Throttle Rudder Aux Aileron 2* Voltwatch Aux Use for gear, flaps, smoke, bomb drop etc. * This airplane uses the ailerons as “flaperons”. Channel #1 is dedicated to one aileron servo and channel #6 is dedicated to the other aileron servo. Flaperons selected using Tx switch G. 6

Initial servo set up: § Ensure the servo being set up is connected to the correct Rx channel. § Remove the servo arm from the servo being set up. § Turn on the transmitter and then the receiver. § Center the trim control for the servo being set up. For programmable radios “reset” the trim (zero it) in the “Trim” menu. § Cycle the Tx stick for the servo being set up. § Install the servo arm 90 degrees to push rod direction. § Install the push rod and adjust the clevis so the control surface is at it’s neutral position. Get this as close as possible visually. § Check control surface throw against kit manufacturers specifications. § Check both “Dual” and “Triple” rates if applicable. 7

Importance of Trim: • • Few airplanes fly straight and level on their maiden flight. • Trim controls on the transmitter provide “electronic” adjustment of servo position. • In some cases linkage adjustments are necessary because the trim controls still can’t achieve the desired result. • You know linkage adjustment is necessary when the trim control is at full up or down and airplane still needs more correction. TRIM adjustments “fine tune” the rudder, elevator and ailerons to achieve hands-off flight characteristics. 8

Single vs Dual Aileron Servos: Single Aileron Servo: • • One servo operates both ailerons in opposite directions. Cannot have flaperons. Requires only one receiver channel Can incorporate differential (more up then down) Dual Aileron Servos: • • • One servo for each aileron. Requires only one receiver channel using “Y-cable”. Can incorporate flaperons with computer radio. Requires two channels for flaperons (programmed mixing) Can incorporate differential with programming 9

Servo Reversing: • Check for proper control surface and throttle movement before attempting servo and linkage adjustments • Check Throttle/Rudder and Elevator/Aileron stick movement for corresponding servo and control surface movement. • If a control surface moves opposite what the control stick is calling for, the servo needs to be reversed. • • Even “basic” transmitters have servo reversing switches. On programmable transmitters, there is a menu where individual servos can be reversed. 10

Typical control horn configuration: Typical Elevator Proper Horn Alignment Typical Elevator Incorrect Horn Alignment 11

Typical servo arm movement: Typical Servo Arm Max travel – Min Power Min travel – Max Power 40º Typical Servo 0º 40º 12

What is “End Point Adjustment (EPA)? § A typical servo rotates the shaft that the servo arm is secured to about 90 degrees (45 degrees ccw and 45 degrees cw). § Each servo has a certain amount of “over-travel” incorporated into it’s design. § A transmitter that has adjustable servo end points can electronically adjust the amount of servo travel – e. g. , its “end point” anywhere from 20% to about 140%. § This feature can be used in conjunction with linkage adjustments to fine tune control surface movement. 13

Servo Travel and Leverage: Less force, more movement More force, less movement Difference in “throw” § A push rod in the outer-most hole moves farther than a rod place in the inner-most hole. § The rod in the outer-most hole exerts less force than a rod placed in the inner most hole. § A servo is most effective when it moves nearly full travel. § Selecting the correct hole in the control horn allows best servo movement without over- or under-movement of the control surface. 14

Which holes to use? Typical Elevator § In this example all three rods are the same length and regardless of position, results in neutral elevator. § This is true provided hole spacing is identical on both the servo arm and the control horn. 15

Max/Min linkage comparisons: Typical Elevator Max throw/min power Rods are the same length Typical Elevator Min throw/max power 16

Counter effects on servos: Hi forces acting on elevator This is why aerobatic airplanes generally require high-torque servos. Max pressure against servo Low forces acting on elevator Min pressure against servo 17

Adjusting for overly sensitive controls: § A “twitchy” airplane has too much control surface movement and very likely -- too little servo movement. § Change the linkage to make the servo move farther to make the control surface move the same distance. § Move “A” and/or “B” to slow the movement (see next picture). § The combination must allow the control surfaces to reach the kit manufacturers recommended throws with the servo at or near its limit. Typical Elevator A Most aggressive setup B 18

Adjusting for overly sensitive controls: § Moving the push rod to a lower hole in the elevator horn (B), requires the servo to move farther to make the elevator move the same amount as before. § The red push rod allows the same elevator movement but requires even more servo movement. § Experiment with the linkage to make the servo move as close to its limits while allowing the control surfaces to reach the kit manufacturers throw recommendations. A “OR” B 19

Adjusting for insensitive controls: § A “lazy” airplane has too little control surface movement regardless of the amount of servo travel. § Change the linkage to ensure the servo moves at or very near its limit and the control surface is at the kit manufacturers recommendations. § Some airplanes are “lazy” by design with the standard control throw recommendations. Linkage modifications on these airplanes will not likely provide significant improvement. A “OR” B 20

What is “Exponential”? § Exponential is the opposite of Linear or Proportional. § Linear/Proportional: 50% stick movement = 50% servo movement. § Non-programmable radios are Linear/Proportional. § Exponential can be a plus or minus value and is available only on some programmable, computer-based radios. § Minus 25% Expo results in less than 25% control surface movement in the first 25% of stick movement. § From 25% to 100% control surface movement is still a curve but more linear as stick movement approaches 100% § Exponential can also be a positive value with the opposite effect. § Stick sensitivity around neutral can be made more responsive (positive expo) or less responsive (negative expo) using this feature. 21

Linear/Proportional Movement 100% Control Surface Movement 50% 0 50% 100% Stick Movement 22

Exponential Control 100% Control Surface Movement 50% 40% 10% Positive Expo Negative Expo 0 25% 50% 100% Stick Movement 23

Mixing and Differential Applies to programmable radios only! Mixing is the ability to combine the functions of two or more channels. Examples are: aileron and elvator, aileron and rudder, flaps and elevator. When deployed, flaps tend to cause the airplane to pitch up. Mixing some down elevator keeps the airplane at the desired pitch angle. Differential is the ability to incorporate more movement in one direction than the other. To reduce drag on a glider for example, one may wish to have more up aileron on one wing than down on the other. 24

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