Servo and control surfaces

The plans say “10 mm up and down”; the servo arm has four holes and the horn three. This works out which pair gives that throw, what the transmitter’s end points and rates do to it, what a mix does to each surface, and whether the servo is strong enough to hold it at speed.

How to use it

Linkage & throw — the servo arm length (axis to the hole you use), the horn height (hole above the hinge line), the hinge offset (how far the hole is ahead of the hinge), the pushrod length, the arm’s tilt at neutral and the servo travel. You get the deflection each way in degrees and in millimetres at the trailing edge (from the surface chord), the differential, and a drawing of the linkage at neutral, pushed and pulled. The Reverse panel finds the arm (or horn) length for the throw you want.
End points, rates & expo — high and low rates with their own expo, end points for each side, sub-trim, and how far your servo turns at 100%. The curves are drawn against stick position, and the table gives the pulse width, servo angle and surface throw at full and half stick.
Mixing — elevons (flying wing, delta), V-tail (or inverted V), flaperons and crow (butterfly), with mix rates, aileron differential and elevator compensation. Move the sliders, or press Use a USB transmitter with the radio plugged in in joystick mode and fly the sticks: each surface is drawn from the side with its % and degrees, and outlined red when the mix asks for more than 100%. Pick which axis is which (and reverse any) — the gamepad tester shows what your radio sends.
Servo torque — surface chord and length, the top speed (in a dive), the throw and the servo travel for it: the hinge moment and the torque at the servo, from the physics and from the rule of thumb many calculators use, against your servo’s rating with a safety factor.

How it works

The linkage is solved as a four-bar: the arm hole turns round the servo axis, the horn hole round the hinge, and the pushrod keeps them a fixed distance apart; of the two places the horn can be, it takes the one nearer neutral. So a horn hole ahead of the hinge, or an arm that isn’t square to the pushrod, shows the differential it gives — and a linkage that would lock over says so. With a square linkage and a long pushrod it reduces to δ = asin(arm·sin θ / horn).

Expo is the EdgeTX/OpenTX curve, y = e·x³ + (1 − e)·x. Mixed outputs use trailing edge down as positive. The hinge moment is Ch·q·S·c with Ch = Chδ·sin δ·cos δ (0.6 per radian is about right for a plain surface hinged at its leading edge; a balanced surface needs less); the servo torque is the hinge moment times the linkage ratio δ/θ. These are estimates for choosing a servo, not for flutter: at high speed, slop-free linkages matter more than torque.

For the wing and the CG, see the model plane designer.

Browser support

The calculators work in any current browser. A USB transmitter needs the Gamepad API, which Chrome, Edge, Firefox and Safari all have; the browser only shows the pad to the page after a stick moves or a button is pressed. EdgeTX and OpenTX radios offer joystick mode when you plug them in; others may need a USB dongle or a simulator cable.

Privacy

Everything runs in your browser. The transmitter is read locally through the Gamepad API; nothing is sent anywhere. Settings are kept in this browser’s local storage, and in the link if you share one.