How to Choose Servos for an RC Plane: Torque, Size and Speed
To choose servos for an RC plane, match them to the load on each control surface. Bigger surfaces, faster planes and larger throws need more torque. As a rough guide: micro 9 g servos for park flyers up to about 1 m, mini or standard servos for 1–1.5 m sport planes, standard digital servos with metal gears for 1.5–2 m planes, and high-torque servos for giant scale. Then consider speed, precision (digital), gear material, voltage and the space available in the wing.
By Kyle HilsonUpdated: October 20263 min read
Sizing guide
| Plane | Servo class | Typical torque |
|---|---|---|
| Indoor and micro | Sub-micro, 2–5 g | 0.2–0.8 kg·cm |
| Park flyers up to ~1 m | Micro 9 g | 1.5–2.5 kg·cm |
| Sport planes 1–1.5 m | Mini or standard | 3–8 kg·cm |
| Planes 1.5–2 m | Standard digital, metal gears | 8–15 kg·cm |
| Giant scale and 3D | High-torque, HV | 20–40+ kg·cm |
Rough guide; follow the plane maker’s recommendation where given. Small servos are covered in micro servos for RC airplanes.
Factors that matter
- Surface size and speed: load rises with area and the square of speed.
- Throw: 3D throws load servos far more than sport throws.
- Linkage geometry: horn lengths change force and travel.
- Speed: faster servos for aerobatics and 3D.
- Digital vs analog: digital holds and centres better.
- Gears: metal for main surfaces on bigger planes.
- Voltage: HV servos give more torque on 2S LiPo/LiFe.
- Size: thin wings need slim wing servos.
Installing
- Centre servos with the radio before fitting horns.
- Mount with grommets, or glue into foam pockets securely.
- Use short, stiff pushrods and no slop.
- Set end points to avoid stalling at full throw.
- Check the power supply can handle all servos moving at once.
Power
Many servos moving together can draw high current. On larger planes, use a separate BEC or receiver battery with a switch rated for the load. Brownouts (receiver resets) are a common hidden cause of crashes. Servo basics are in the RC servo guide.
Avoiding flutter
Flutter is a rapid vibration of a control surface at speed that can destroy it in seconds. It’s caused by slop in linkages, weak servos or flexible surfaces. Use servos with enough torque, eliminate play, and keep hinges tight, especially on fast planes.
Checklist
- Torque with margin for each surface.
- Speed suited to the flying style.
- Metal gears on main surfaces of larger planes.
- Fits the space available.
- Power system can supply them.
Common mistakes
The most common mistake is using servos that are too weak for big surfaces, especially on 3D planes and fast warbirds; the plane flies but controls feel soft and flutter risk rises. Another is mixing very different servos on paired surfaces, such as two ailerons, which then move at different speeds. A third is running high-voltage servos on a low-voltage supply, which wastes their torque, or the reverse, which can burn out standard servos. Matching servos to the job and to each other avoids all of these.
Brands and budget
Cheap servos work on simple foam planes, but for valuable planes, branded servos with good centring and metal gears are worth the money. A servo failure is one of the most common causes of crashes, so this isn’t the place to save.
FAQ
How much torque do I need?
From about 2 kg·cm for park flyers to 20+ kg·cm for giant scale.
Digital or analog?
Digital for precision; analog is fine on simple planes.
Do I need metal gears?
On main surfaces of larger or faster planes, yes.
What causes servo jitter?
Power problems, interference or worn servos.
More on this topic: the complete RC Airplanes guide covers the basics behind this article.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
