Control surface linkage

About the control surface linkage

What this calculator is for

This calculator sizes the linkage between a servo and a control surface on an RC plane. It gives the length of the servo horn (A) or of the control horn (B) that produces the throw you want (F) with the servo travel (E), then the settings to enter in the radio: end points on each side, low rate (D/R) and expo. In “Check” mode, it gives the throw obtained with parts already fitted.

The right throw comes from the geometry, using the servo's full travel: the control is then as precise as possible. The radio is then only used to fine-tune.

The calculator also handles differential, when the control surface has to go further one way than the other (often on ailerons): through the geometry, or through the radio.

Four setups are possible: servo parallel to the hinge (elevator, rudder), perpendicular servo (servo lying flat in the wing), central servo for two ailerons, and cable linkage (pull-pull).

Why an exact calculation

The rule “throw ≈ servo travel × A / B” only holds at small angles. The servo horn and the control horn turn along arcs: the control surface does not follow the servo proportionally, and depending on where the servo sits, it does not go as far one way as the other.

The calculator solves the exact geometry. It gives the throw obtained on each side, and a correction to get the same movement both ways: offset horn hole or angled servo horn.

The curve of the movement acts like expo: the calculator puts a number on it and gives the expo to set in the radio.

What the calculator gives

The length of the part to make and the throw obtained on each side.
For the central servo: the mounting angle of the servo horn, with or without differential.
The end points, the low rate (D/R) and the expo to set in the radio.
The dimensioned drawing of the part, a 1:1 PDF plan to print, and the 3D model already set up (STL for 3D printing, DXF for cutting).

Tips

Place the servo so that the linkage does not work at an angle.
Check that there is no binding over the full travel, and that nothing gets in the way of the moving linkage.

Related calculators and parts

Servo arm (indoor) ›Custom servo horn that sits on top of the servo's stock horn: one arm or two opposite arms, straight or angled. It gives the arm length that makes the servo work over its full travel; an angled arm, or two arms of different lengths, gives differential without any radio setting.Control horn (indoor) ›Control horn that fits into a slot in the control surface: one arm on one side for a pushrod, or two arms on either side for a cable linkage (pull-pull). Solid base or tabs, adjustable linkage hole to put it in line with the hinge.

Servo / HornControl surface / HornPushrod

Servo mounting

Parallel servoServo axis parallel to the hinge (e.g. elevator, rudder).

Perpendicular servoServo axis perpendicular to the hinge (e.g. servo flat in the wing).

Center servoOne servo for both ailerons.

Pull-pullTwo cables between the servo and the control surface.

Part to calculate

Servo horn known: control horn length calculated.

AServo horn length

CServo / hinge distance

DServo / hinge height

HControl horn hole lead

H: offset of the control horn hole from the hinge axis, positive ahead (toward the servo), negative behind; B is then the hole height from the hinge. An offset hole creates differential, shown in the Result; the fix can then only be made on the servo horn.

EServo travel at radio (±)

FSurface throw (±)

Result

BComputed control horn40,39 mm

Pushrod length101,18 mm

Gearing ratio0,60

Asymmetric motion

Actual travel▲ 26,76° · ▼ 25,00°Can be fixed by moving the control horn hole or tilting the servo horn.

Build

Dimensioned drawing of each part, as a 3D model or a full-scale plan.

Control horncomputed40,39 mm

DifferentialDifferent up and down throws.Off

With 30 %, the small side is 70 % of the large one; F remains the target ▲ throw.

Radio setupRates and expo, allowing for the linkage.End points ▲ 93 % · ▼ 100 %Expo ▲ +4 % · ▼ +11 %

The linkage is not linear: it already acts like expo (▲ −4 % · ▼ −11 %, sharper around neutral). Radio expo adds to it: the "to set" values allow for this, for each rate and each direction.

In the radio, servo travel = D/R × end point. High rate: D/R at 100 %, the ▲ / ▼ end points carry the wanted angle and correct the difference between both directions.

High rate

Target angle (F)±25°Taken from F: to change it, edit F.

Wanted expo

End point to set (radio)▲ Up93 %▼ Down100 %Expo to set (radio)▲ Up+4 %▼ Down+11 %Resulting travel▲ Up25,0°▼ Down25,0°

High rate curve

TargetWithout correctionWith suggested expo

Here, positive expo softens the centre. Some radios use the opposite sign: check at the stick that the centre does get softer. The two directions of the surface do not respond the same way: set one expo per direction.

Control surface linkage

1SetupParallel servo · pointing up · control horn calculatedServo mounting and part to calculate.

2PartsA 25,00 mmLength of the known part.

3LayoutC 100,00 mmD 0,00 mmServo position relative to the hinge.

4DeflectionsE 45,00 °F 25,00 °Each way, from neutral.

Result

Build