| 20KG·CM STALL | 0.030S/60° NO-LOAD | 12V RATED |
| 78G WEIGHT | StainlessGEARTRAIN | 360°TRAVELCustomizable |
Overview
The BLS8120H is a tail servo, and the spec sheet reads like one: a 510–1010 µs pulse window with 760 µs neutral rather than the usual 1000–2000 / 1500. That narrow window is what lets a tail gyro command fine corrections without the resolution loss of a full-range channel.
The case is the standard 20 × 40 × 38 mm aluminium block at 78 g, with a stainless steel geartrain on two ball bearings and a 25T-φ5.96 mm spline. Built-in motor is brushless, and the lead is 300 mm of 22 AWG.
This page covers the TTL build: the same mechanics, commanded over a serial bus so several servos can share one signal line and be addressed individually. Useful when the actuator count outgrows the PWM channels available.
Performance at each supply voltage
| Supply | No-load speed | Peak stall torque | Idle current | Stall current |
|---|---|---|---|---|
| 7.4 V | 0.050 s/60° (1200 °/s) | 12.5 kg·cm (1.23 N·m / 174 oz·in) | 320 mA | 2.8 A |
| 8.4 V | 0.044 s/60° (1364 °/s) | 15 kg·cm (1.47 N·m / 208 oz·in) | 370 mA | 3.5 A |
| 12 V | 0.030 s/60° (2000 °/s) | 20 kg·cm (1.96 N·m / 278 oz·in) | 440 mA | 4.0 A |
Control specification
| Item | Specification |
|---|---|
| Command signal | TTL serial |
| Amplifier | Digital controller |
| Dead band | 1 µs |
| Rotating direction | CCW (510–1010 µs) |
| Travel | 360°, customizable |
Mechanical specification
| Item | Specification | Item | Specification |
|---|---|---|---|
| Dimensions | 20 × 40 × 38 mm | Case material | Aluminium, CNC |
| Weight | 78 g | Geartrain | Stainless steel |
| Bearings | 2 × ball bearing | Motor | Brushless |
| Output spline | 25T-φ5.96 mm | Horn | Plastic, POM |
| Lead / connector | 300 mm, 22 AWG | Mechanical limit | 210° |
| Operating temperature | −15 °C to 65 °C | Storage temperature | −20 °C to 70 °C |
| Ingress protection | Splash resistant (customizable) | Travel | 360°, customizable |
What’s in the box
- 1 × XQ-BLS8120TTL servo
- 1 × POM horn set
- 1 × mounting screw pack
Optional: metal horn, sold separately.
How to size it
The pulse window is not the standard one. 510–1010 µs with 760 µs neutral means a default 1000–2000 µs output will drive it to one end stop — set the radio or gyro to the narrow tail window before the first power-up.
Torque at 8.4 V is 15 kg·cm at 0.044 s/60°; on 12 V it is 20 kg·cm at 0.03 s/60°. Stall current stays at or under 4 A, so a 5 A BEC is enough for one, but not for a tail plus three swashplate servos on the same rail.
FAQ
Why is the pulse range 510–1010 µs?
It is a tail-servo window, not a standard channel. Set the gyro or radio to the narrow tail pulse range — feeding it 1000–2000 µs will drive the output against the mechanical limit.
What is the neutral position?
760 µs, i.e. the centre of the 510–1010 µs window. Do not assume 1500 µs.
Is the pulse width table valid for this unit?
No. The microsecond window printed on the inspection sheet belongs to the PWM build of the same servo. This unit is commanded over the serial bus — ask for the command set and register map before writing firmware.
How many can share one bus?
The limit is the address space and the current budget of the shared supply, not the signal pair. Keep the bus ground common with the power ground at a single point.
Other interfaces
Downloads
- XQ-BLS8120TTL inspection specification (PDF, 846 KB)
Torque, speed and current at each supply voltage, mechanical data, control specification, mounting dimensions. - 2D dimension drawing (image)
Mounting hole positions, case depth and overall height; the same drawing is inside the datasheet.
