Electronics
RC servos explained
By Paul Walsh Last reviewed
Servos move everything that isn't the motor: a car's steering, a plane's control surfaces, a helicopter's swashplate and tail. They're also one of the commonest parts to wear out or break, so it pays to understand what the numbers on the box mean.
How a servo works
Inside the case is a small motor, a gearbox, a position sensor (usually a potentiometer) and a control board. The receiver sends a pulse telling the servo where to be. The board compares that with where the output shaft actually is and drives the motor until they match. That closed loop is why a servo holds its position against a load.
Analogue vs digital
| Analogue | Digital | |
|---|---|---|
| How often it corrects position | About 50 times a second | Hundreds of times a second |
| Holding force | Lower, softer around centre | Higher, firm and precise |
| Response | Slightly slower to start | Quicker and more precise |
| Power draw | Lower | Higher, even when holding still |
| Price | Cheaper | Dearer |
| Best for | Flaps, retracts, light-duty jobs | Helicopter swashplates and tails, car steering, aerobatic planes |
Digital servos aren't automatically "better"; they're better where precision and holding force matter. Their higher current draw means the receiver supply must be up to it, particularly with several servos on a large model.
Reading the specification
Speed is quoted as the time to move 60 degrees, for example 0.10s/60°. Lower is faster.
Torque is quoted in kg·cm (or oz·in): how much force the servo can apply at a set distance from the centre. 4kg·cm means it can hold 4kg at 1cm from the output shaft.
Both figures depend on voltage, so compare servos at the same voltage. A servo might be quoted at 6.0V and 7.4V, with better figures at the higher one.
Sizes
| Size | Typical weight | Typical use |
|---|---|---|
| Nano / sub-micro | Under 5g | Micro planes and small helicopters |
| Micro | 8 to 12g | Park flyers, 250 to 300 class helicopter cyclic |
| Mini | 20 to 30g | 450 to 500 class helicopters, mid-size planes |
| Standard | 40 to 60g | 1/10 cars, larger planes, 600 to 700 class helicopters |
| Large / 1/5 scale | 60g and up | 1/5 cars, giant-scale planes |
Always check the mounting dimensions before buying a replacement.
Gears and motors
- Plastic (nylon) gears: light and cheap, but strip easily in a crash.
- Metal gears: much tougher, slightly heavier, can wear the gears behind them.
- Titanium or steel gears: for high-torque servos in large models.
- Coreless motors respond faster and run smoother than standard brushed motors; brushless servo motors last longest.
High-voltage (HV) servos
HV servos run directly on 2S LiPo or LiFe voltage (around 7.4 to 8.4V), giving more speed and torque. Only use them if your receiver supply is set to match. Plugging a standard servo into an HV supply will damage it.
What different models need
Helicopters. All three cyclic servos should be the same make and model, with low "slop" around centre. The tail servo needs to be fast. Setting the servo arms correctly is covered in swashplate levelling, and how the flybarless unit drives them in gyros and flybarless units.
Planes. Light servos on the surfaces, with enough torque for the speed of the model. Aerobatic models benefit from digital servos.
Cars. The steering servo takes a beating. Upgrading it is one of the most worthwhile changes on a ready-to-run car, as covered in RC car upgrades.
Signs a servo is failing
- Jitter or twitching when the sticks are still.
- Dead spots: it doesn't respond around one part of its travel.
- Not centring back to the same place each time.
- Constant loud buzzing or getting hot.
- Stripped gears: the output shaft turns freely by hand, or clicks.
To test one, plug it into a servo tester or a spare receiver channel and move it slowly through its range. Watch for jumps.
Look after your servos
- Set end points so the servo never pushes against a mechanical stop.
- Make sure linkages move freely. A binding link makes a servo work constantly and burn out.
- Check servo screws and arms after every crash.
If a servo fault is making a helicopter misbehave, the RC helicopter troubleshooting guide helps pin it down. More guides are on the Electronics hub.
Frequently asked questions
Can I mix digital and analogue servos?
Yes, on most models. The exception is a helicopter swashplate, where all three cyclic servos should be identical so they move at the same speed.
Why does my digital servo buzz?
A faint hum while holding position is normal for a digital servo. Loud buzzing usually means it is straining against a binding linkage or an end point set too far.
Can I replace just the gears?
On most decent servos, yes. Gear sets are sold separately and take a few minutes to fit. On cheap micro servos it's often easier to replace the whole servo.
Related guides
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Level the swashplate, set servo arms and zero pitch correctly on a collective pitch helicopter, with or without a levelling tool.
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