In order to maneauver an RC plane, control surfaces on the airplane need to move based on the input from the pilot holding the transmitter. This is achieved when the signals from the transmitter are received in the RC plane by the receiver, which in turn send a signal on to the servo. The servo is attached to a control surface and when the servo moves based on the input signal the control surface moves accordingly.
At least two servos are needed in a electric RC plane, one for elevator and one for rudder alternatively aileron. The most common beginners RC planes utilizes 2-3 servos.
When talking about servo gears it is often the material used that is meant. There are three common material, all with their own pros and cons.
Nylon gear servos are the most common types and the cheapest servos can often be found in this category. They are commonly lightweight which makes them very suitable for e.g. foam planes. On the downside they break much easier than especially metal gear servos. Since they are cheap you can always afford to have a couple of spare servos at hand.
Carbonite gear servos are stronger than the nylon servos but not as strong as metal gear servos. They are more wear resistant than nylon servos but they are also more expensive.
Metal gear servos are the number one when it comes to strength. Due to the metal gears they can handle much more force without breaking. The downside of metal gear servos would be that they are slightly heavier than nylon and carbonite gear servos. But if you can handle that extra weight metal gear servos are the servos you want for the future.
Analog and digital servos
A servo can have an analog motor or a digital motor. A analog servo is controlled by a series of pulses based on the transmitters frame rate. The signal is compared to a potentiometers position and the resulting signal is then sent to the servo motor. Between each pulse the motor in the servo is virtually not moving.
A digital servo on the other hand has a small built-in CPU calculation the signal to the motor. A higher frequency than the transmitters frame rate can be used and the result is a servo with a better abillity to hold torque.
When looking at the specifications of a servo some of the most important parameters are the weight of the servo, how strong it is or how much torque it can handle and how fast it moves.
Different servos has different weights. A larger servo is heavier and more powerful than a smaller servo. The weight of the servos needs to be considered in order to know the total weight of the RC plane. This is important when choosing the power system for your plane. Servos are commonly categorized based on their weights. To mention some categories 0-5g servos are called sub-micro, 5-10g servos are called micro and 11-20g are called mini and so on.
The torque value refers to how much power the servo is able to produce. This is measured in kg/cm. It is important that the servo that you choose can handle the force needed to control your RC plane.
How fast the servo can move is often determined by measuring the time it takes for the servo to rotate 60 degrees. A servo specified at 0.12 needs 0.12 seconds to rotate 60 degrees.