1

Optimizing Servo & Control Surface Movement

2

Optimizing Servo & Control Surface Movement

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

Making an RC airplane fly smoothly requires that:

3

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)

What we will cover in this session:

4

Control surface movement is specified in inchesor 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)

Kit Manufacturer Control Surface Specifications:

5

Four-Star 60 Control Surface Specifications:

Aileron (Standard) 5/8 in. up & downAileron (High Rate) 7/8 in. up & down

Rudder (Standard) 1 in. left & rightRudder (High Rate) 1.25 in. left & right

Elevator (Standard) ¾ in. up & downElevator (High Rate): 1 in. up & down

Important: Sig specifies only “standard” throws. Medium orHigh Rates are determined by the kit builder.

6

RC Airplane Channel Assignments (Futaba):

1 Aileron 1*2 Elevator3 Throttle4 Rudder5 Aux6 Aileron 2*7 Voltwatch8 Aux

CHANNEL FUNCTION

Use for gear,flaps, smoke,bomb drop etc.

* This airplane uses the ailerons as “flaperons”. Channel #1 isdedicated to one aileron servo and channel #6 is dedicated tothe other aileron servo. Flaperons selected using Tx switch G.

7

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.

8

Importance of Trim:

• Few airplanes fly straight and level on their maiden flight.

• TRIM adjustments “fine tune” the rudder, elevator and aileronsto achieve hands-off flight characteristics.

• Trim controls on the transmitter provide “electronic” adjustmentof servo position.

• In some cases linkage adjustments are necessary because thetrim controls still can’t achieve the desired result.

• You know linkage adjustment is necessary when the trim controlis at full up or down and airplane still needs more correction.

9

Single vs Dual Aileron Servos:

• One servo operates both ailerons in opposite directions.

• Cannot have flaperons.

• Requires only one receiver channel

• Can incorporate differential (more up then down)

Single Aileron Servo:

• 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

Dual Aileron Servos:

10

Servo Reversing:

• Check for proper control surface and throttle movementbefore attempting servo and linkage adjustments

• Check Throttle/Rudder and Elevator/Aileron stick movementfor 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.

11

Typical control horn configuration:

Typical Elevator

ProperHorn Alignment

Typical Elevator

IncorrectHorn Alignment

12

Typical servo arm movement:

Typical Servo

TypicalServo Arm

Max travel – Min Power

Min travel – Max Power

0º40º 40º

13

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.

14

Servo Travel and Leverage:

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.

Difference in “throw”

Less force, more movementMore force, less movement

15

Which holes to use?

Typical Elevator

In this example all three rods are the same lengthand regardless of position, results in neutral elevator.

This is true provided hole spacing is identical on both theservo arm and the control horn.

16

Max/Min linkage comparisons:

Typical Elevator

Typical Elevator

Rods are the same length

Max throw/min power

Min throw/max power

17

Counter effects on servos:

Max pressure against servo

Hi forces acting on elevator

Low forces acting on elevator

Min pressure against servo

This is why aerobatic airplanes generallyrequire high-torque servos.

18

Adjusting for overly sensitive controls:

Typical Elevator

A

B

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.

Most aggressive setup

19

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

B

“OR”

20

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

B

“OR”

21

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.

22

Linear/Proportional Movement

0

100%

100%50%

50%

Stick Movement

Control Surface Movement

23

Exponential Control

Negative ExpoPositive Expo 0

100%

100%50%

50%

Stick Movement

Control Surface Movement

25%

40%

10%

24

Mixing and DifferentialApplies 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 somedown elevator keeps the airplane at the desired pitch angle.

Differential is the ability to incorporate more movement in one directionthan the other. To reduce drag on a glider for example, one may wish tohave more up aileron on one wing than down on the other.

25