All RC airplanes and helicopters are controlled by servos - small, electromechanical devices that allow everything from controlled flight to payload releases. So what are servos? How do they work? And how do you choose the ones that will work best in your model? We’ll answer all these questions, and take you through everything from the basics of servo operation to their technical details.
What is a Servo?
A servo is a device that can rotate to an arbitrary position, as set by the user. Servos usually consist of a small DC (direct current) electric motor, several gears, and a head where an arm or wheel can be attached. When the user tells the servo what angular position to move to, the servo rotates and holds that position until further input is specified. The servo holds position because external forces are always interacting with the aircraft, and would set control surfaces to undesired positions unless stopped. Servos exert a torque on external forces, that prevents them from changing the position of any control surface.
How Servos Work
A servos job is to convert the angular movement of a servo arm to the linear movement of a control surface. This is done by attaching linkages, called control rods to the servo arm and the associated control surface. When the servo head rotates, it pushes the control rod back and forth. The rod is linked to a control surface, and can move it up or down as the servo rotates.
Servos are controlled by three wires: two to provide the DC power that the motor needs, and one that sends the signal, controlling the servo. The signal wire works by sending the servo a series of pulses, which are interpreted by its internal circuitry. By varying the timing of each pulse, the servo knows exactly which position to move to.
RC Servo Motors
The motors that drive RC servos come in several different types. Here’s a list of the most common varieties, and some information on each to help you decide which ones to use:
- Coreless – Conventional motors use copper wires wrapped around metal cores to form electromagnets. In a coreless motor, there is a metal mesh that rotates around the permanent magnets. Coreless motors respond more quickly than conventional motors, because they don’t have to overcome the momentum associated with heavy metal cores.
- Brushless – Servos can be powered by brushless motors, giving them longer life, faster response time, and more torque.
- 3 Pole and 5 Pole – Electric motors have permanent magnets, called poles, that electromagnets are attracted to. Servo motors can have either 3 or 5 poles, with more poles providing better torque. If you’re new to RC or have a regular sport model, you probably won’t notice the difference between 3 pole and 5 pole servos.
Choosing the Right Servo
Servos have a number of defining properties that make them suitable for different applications:
- Torque – This is a measure of the servos strength, or how much “push” it has. More precisely, torque is the product of force and the radius at which it acts. This is shown graphically in the figure on the right. Bigger planes need high torque servos to move their large control surfaces. In general, servo size goes up with rated torque.
- Dimensions – Servos come in many different sizes, which you can choose from depending on your application.
- Weight – The weight of a servo depends on several variables. Most often recorded in grams, the weight of a servo is always reported on the package.
- Bearings – There are two ways to support the output shaft of a servo – bearings and brushes. Brushes are cheaper, but bearings last longer and operate more smoothly. Very small and very cheap servos tend to be brushed, while high end and very large servos generally have bearings. It’s possible to upgrade a brushed servo to bearings, with several upgrade kits being available on the internet.
- Gears – Most hobby grade servos use nylon gears, while higher end servos use metal gears. Metal gears add more weight, but their advantage is that they can’t “strip”, causing an RC helicopter or airplane to crash. Metal gears wear over time, which can cause “slop” in their rotation, but the gears can be replaced somewhat economically. In general, nylon servos are adequate for sport flying. If you’re particularly worried about losing a model in a crash, or are flying intense aerobatics, a metal geared servo is probably the right choice.
- Speed – Speed measures how fast the servo can move from one position to another. Different RC airplanes and helicopters will need servos with different speeds. For example: a trainer doesn’t need to change control surface positions rapidly, but a 3D helicopter or plane does. High speed servos are many times more expensive than standard ones.
- Digital / Standard – Servos can be of two types: digital, or standard. Both digital and standard servos can be used with a normal receiver, the real difference is performance. All servos use a series of short pulses as signals that determine what angular position they should maintain. This series of signals is usually very fast, somewhere around 50 pulses per second maximum. On a standard servo, this rate is so fast that small movements of the control sticks may not have an affect. This means that there’s a small deadband on the control sticks, in which no servo movement takes place. Although it’s not a problem on trainers and most sport class models, the deadband becomes a significant issue with 3D aircraft. Even a small delay with a 3D aircraft could cause a crash.
Digital servos remove the deadband by speeding up the rate at which it receives pulses. Usually, this is increased from around 50 to 300 pulses per second. This increase in resolution allows the servo to operate much more precisely.
All RC kits and ARFs will specify the type and brand of servo required. Generally, you should adhere to these recommendations.
Now that you know what servos to get for your model, you can browse the large number of servos available on our website.
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