Analog vs digital servos
A servo is a motor with a gearbox and control electronics that rotates to a commanded position based on the signal from the receiver. Analog servos read the control signal at about 50 Hz (every 20 ms). Digital servos process the signal at 200 to 300 Hz, giving faster response to position changes and stronger holding power under load.
Digital servos offer higher resolution and lower latency but draw more current. In models with many digital servos (for example six servos in an aerobatic model) a stronger power supply is needed (a higher-capacity BEC or a dedicated receiver pack).
For beginning RC modellers, analog servos are sufficient in trainers and simple sport models. Digital servos become important in fast, aerobatic and scale models where control surface precision and speed of response are critical.
Servo torque vs speed
The two key servo specifications are torque and speed. Torque is given in kg/cm (or oz/in) and represents the force the servo produces at a 1 cm lever arm. Speed is given in seconds per 60 degrees (for example 0.12 s/60 degrees) and represents the time to rotate the horn by 60 degrees.
Trainer models need servos with moderate torque (3 to 6 kg/cm) and average speed. Aerobatic models need fast servos (under 0.10 s/60 degrees) with high torque (8 to 15 kg/cm). RC gliders need servos with high torque but not necessarily high speed.
It is important to match torque to the control surface size and flight speed. A larger surface and higher flight speed produce greater aerodynamic forces on the servo horn. A servo that is too weak will flutter or fail to hold the commanded deflection.
Gears: metal vs plastic
Servos with plastic gears are lighter and cheaper but less resistant to impacts and high loads. Plastic gear teeth can wear or strip under sudden loads (for example a hard ground takeoff or a wing tip strike).
Servos with metal gears (MG, Metal Gear) are more durable and handle heavy loads better, but they are heavier and more expensive. A common compromise is to use MG servos on critical control surfaces (ailerons, elevator) and plastic-gear servos on less loaded ones (rudder on a trainer).
In free flight models with RC power (for example F1Q), weight is critical, so lightweight micro servos (5 to 9 g) with plastic gears are used. In models weighing more than 2 kg, MG servos are practically standard.
Receivers: PWM, PPM, SBUS
The receiver (RX) picks up the signal from the transmitter (TX) and converts it into control signals for the servos and ESC. Traditional PWM receivers have a separate output for each channel (one cable per servo). The system is simple but requires many wires.
PPM (Pulse Position Modulation) sends all channels over a single wire but is more susceptible to interference and has limited resolution. PPM is sometimes used in older systems and simple applications.
SBUS (Serial Bus) is a digital protocol that carries up to 16 channels on a single wire with high resolution and low latency. Most modern RC systems use SBUS or similar digital protocols. SBUS servos are connected in a daisy chain, which reduces wiring inside the fuselage.
When choosing a receiver, make sure it is compatible with your transmitter (same manufacturer and same radio protocol). Check the channel count (minimum 4 for a trainer: throttle, ailerons, elevator, rudder) and range (standard range is 1 to 2 km, long-range systems can reach 10 km and beyond).
ESC selection and parameters
The ESC must handle the motor's maximum continuous current with at least 20% headroom. If the motor draws a maximum of 30 A, choose an ESC rated for at least 36 A, preferably 40 A.
Pay attention to the supported LiPo cell count. An ESC labeled 2S to 4S will not work correctly with a 6S pack. Also check the BEC voltage and its maximum output current. For a model with four analog servos a 5 V / 2 A BEC is sufficient. For a model with six digital servos you need a 5 V / 5 A BEC or more, and it is better to use a separate UBEC.
Programmable ESC features
Most ESCs allow settings to be changed via a programming card or transmitter signals. The key parameters are:
- Brake type: off, soft, hard. The brake stops the propeller when throttle is at zero. In electric RC gliders a brake is desirable so that a folding propeller folds correctly.
- Motor timing: affects efficiency and power. The factory setting is suitable for most applications.
- Cutoff voltage: protects the LiPo pack from deep discharge. Set it to 3.2 to 3.3 V per cell. Too low a threshold risks pack damage, too high causes premature motor shutdown.
- Cutoff mode: soft (gradual power reduction) is safer than hard (immediate cutoff) because it gives time for a safe landing.
Matching electronics to the model
To wrap up, several practical rules for component selection:
Always check the total current draw of all servos and compare it with the BEC capacity. If the total exceeds 70% of the BEC output, add a separate UBEC or a dedicated receiver pack.
- Electric trainer 1.2 to 1.5 m: 9 g analog or digital servos (torque 2 to 4 kg/cm), 6-channel receiver, 30 to 40 A ESC with BEC.
- Sport model 1.5 to 1.8 m: standard digital MG servos (torque 5 to 10 kg/cm), 6 to 8 channel SBUS receiver, 40 to 60 A ESC.
- Aerobatic model 1.5 m and above: fast digital MG servos (torque 8 to 15 kg/cm, speed under 0.08 s/60 degrees), 8-channel receiver or more, 60 to 80 A ESC with external UBEC.