A 40-inch RC plane needs 9 g metal-gear servos on ailerons and elevator, and a standard servo on rudder if the load is light. The MG90D digital metal-gear micro servo is the common pick at this size. This guide compares servo sizes, explains torque and gear types, and shows the arms, extensions, and tester that finish the install.
What size servo fits a 40-inch plane?
A 9 g micro servo fits a 40-inch foam or balsa sport plane. The SG92R analog servo weighs 9 g, and the MG90D digital servo weighs about the same.
A 25 g mini servo such as the 1250MG suits a larger control surface. A standard servo such as the MG996R suits a 60-inch plane, not a 40-inch one.
How much torque does a control surface need?
Torque is measured in kilogram-centimeters, written kg-cm. A 9 g servo gives about 1.8 to 2.2 kg-cm, which moves a 40-inch aileron with margin.
A 12 kg digital servo such as the SPT4412LV suits a 1/10 drift car steering rack. It is far too strong and heavy for a 40-inch plane.
Metal gear or plastic gear servos?
Metal gears survive landing shocks and strip less often than plastic gears. The MG90D uses metal gears and weighs 9 g, while the SG92R uses plastic composite gears.
Plastic gear servos cost less and work for a light rudder. Use metal gear servos on ailerons and elevator, where a stripped gear means a crash.
Analog or digital servos?
Analog servos update at 50 Hz and center well enough for a sport plane. Digital servos update faster and hold center tighter under load.
The DSM-44 digital metal-gear servo gives finer centering for ailerons on a fast airframe. An analog SG90 works for a slow trainer.
How do you test a servo before install?
An RC servo tester drives the servo through its full travel without a receiver. It shows dead spots, jitter, and weak centering in seconds.
Test each servo at 5 V and at 6 V if your BEC supports it. A servo that buzzes at center under no load has a worn gear or a bad potentiometer.
Which arms and extensions finish the job?
A 25T spline arm fits most micro servos, and a 2.25 inch metal arm gives more throw. Match the arm spline count to the servo output shaft.
Heavy-duty universal servo extensions in 12 inch and 18 inch lengths reach the tail on a 40-inch plane. Use 22AWG wire for extensions over 12 inches.
Quick comparison
| Servo | Weight | Gear | Use on a 40-inch plane |
|---|---|---|---|
| SG92R | 9 g | Plastic composite | Light rudder |
| MG90D | 9 g | Metal | Ailerons, elevator |
| DSM-44 | 9 g | Metal, digital | Fast ailerons |
| 1250MG | 25 g | Metal | Large surfaces |
Frequently asked questions
What does kg-cm mean on a servo?
It is torque: the weight in kilograms the servo can hold at 1 centimeter from the shaft. More kg-cm means a stronger servo.
Can I mix analog and digital servos on one plane?
Yes. Each servo runs on its own channel, so mixing is fine. Keep digital servos on the surfaces that need tight centering.
Why does my servo buzz at center?
The servo is fighting a load, or the linkage binds. Check for a bent pushrod or a tight control horn before replacing the servo.
How do I pick a servo arm length?
A longer arm gives more throw and less torque at the surface. Start with the stock arm length, then change it only if the throw is wrong.
Parts used
SG92R Analog Plastic Composite Gear Micro Servo, $8.00. US stock, ships fast from NY, USA. Towerpro SG92R Analog Plastic Composite Gear Micro Servo. Torque: 2.5 kg/cm at 4.8V; Speed: 0.1 sec/60deg at 4.8V; Gears: plastic composite.
MG90D Digital Metal Gear Micro Servo, $14.00. US stock, ships fast from NY, USA. Towerpro MG90D Digital Metal Gear Micro Servo. Weight: 13g; Gears: metal; Digital: True.
DSM-44 Digital Metal Gear Micro Servo, $22.00. US stock, ships fast from NY, USA. Power HD DSM-44 Digital Metal Gear Micro Servo. Torque: 1.6 kg/cm (18.0 oz/in); Speed: 0.07 sec; Dimensions: 22 x 8.7 x 22 mm.
1250MG Metal Gear 25g Mini Servo, $18.00. US stock, ships fast from NY, USA. Power HD 1250MG Metal Gear 25g Mini Servo. Torque: 3.5 kg-cm (48.6 oz/in) at 6.0V; Speed: 0.14 sec; Dimensions: 31.0 x 16.3 x 29.5 mm.
SA-44 4.4g Metal Gear Sub-Micro Servo, $10.00. US stock, ships fast from NY, USA. Torkstar SA-44 4.4g Metal Gear Sub-Micro Servo. Torque: 0.8 kg/cm (9.3 oz/in); Speed: 0.12 sec; Dimensions: 20 x 26 x 8.0 mm.
Micro Servo SG90 / SG92R / MG90S, $13.99. Micro servos: SG90, SG92R, or MG90S. Pick your model. Ships from NY, USA.
MG996R Metal Gear Servo, $15.99. MG996R metal gear standard servo. Ships from NY, USA.
RC Servo Tester, $15.99. Bench servo tester for RC servos.
4-Point Aluminum Servo Arm Horn Set with Rudder Angle Ball Link, RC Aircraft, 2-Pack, $8.37. 4-point aluminum alloy servo arm horn set with rudder angle ball link for RC aircraft, racing drones and airplanes. Two triangle horns plus glass fiber plate, ball link and hardware. Ships from NY, USA.
25T Extended Metal Servo Horn, $7.99. Extended 25T metal servo horn arm for RC car, boat, and airplane servos. Red or black. Ships from NY, USA.
2-1/4in Metal Servo Arm, 25T, $12.00. 2-1/4in metal servo arm for 25-tooth standard servos. Maximum throw for big control surfaces. Fits Futaba and compatible brands.
Heavy-Duty Universal Servo Extension 18in, 22AWG, 5-Pack, $5.00. Five heavy-duty 18in universal servo extensions on 22AWG wire. For the longest runs in big wings and fuselages.
Control Linkage Hardware Kit, $21.95. Control horns, pushrod linkage stoppers, and push rods for RC airplane control linkages.
FPV Pan/Tilt Servo Gimbal Mount (SG90 / MG90S), $8.09. Nylon dual-axis FPV PTZ gimbal for RC planes and drones. Anti-vibration, fits 9-12g servos. With or without SG90/MG90S servos. Ships from NY, USA.
Servo Arms, Extensions and Testers
Related buying guides on Quality RC Parts
Micro Servos, Servos and Soldering Irons for RC Builds in the USA
Servo Extensions, Arms and Testers for RC in the USA
Browse all the guides on the Quality RC Parts Drone Guides hub, or start at the Quality RC Parts store.
Sources
Quality RC Parts stocks parts in New York, USA and ships in-stock orders in 1-2 business days.
Last updated: 2026-11-10.