The motors inside of our servos turn much faster than we would ever need the servo to move. For that reason, every servo has a set of gears that reduces the speed of the output shaft from the speed of the motor. As with any gear reducer, this also causes the torque of the output shaft to be much higher than the torque of the motor.
Most servo gears are made from a type of plastic called nylon. Sometimes under excess torque these plastic gears will strip out. If you're lucky, you will get some warning signs before having a catastrophic failure that destroys your airplane. But there have been a many of crashes due to stripped servo gears.
Many large scale or 3D pilots will buy RC servos with metal gears. These are bit more expensive and they weigh a little more. But they will withstand a lot more torque than standard plastic gears. The only downside, besides the added weight, is that metals gears will wear over time and this leads to slop in the servo. This wear happens slowly and gives you plenty of warning, as opposed to set of nylon gears that has the potential of stripping any time it is overloaded.
Replacement gears sets are readily available for both metal and plastic. So when you destroy the gears in your servo, it is cheaper to replace the gears rather than buy a new servo.
Digital Servo vs. Standard Servos
The physical working portion of a digital servo is exactly the same as a standard RC servo. Both have the same plug and both can be used by a standard receiver. The only difference is the digital servo has a microprocessor that modifies the frequency of the control pulses to the motor.
If you're not familiar with how a standard servo operates, you may want to read this page that explains pulse code modulation in order to have a better understanding of the following discussion.
Lets say you only want to move the servo a tiny bit, so you barely move the transmitter stick off center. A standard servo will receive a series of extremely short full voltage pulses 50 times a second. These very short length instantaneous voltage pulses occur so infrequently that the motor doesn't even react. The amount of transmitter stick movement from center position that is required to make a servo start moving is called the servo's deadband
If you're flying a trainer or sports RC airplane, the deadband isn't a huge deal. But if you're a helicopter or 3D pilot you need every bit of the stick movement to count. Too much deadband could lead to a dead plane!
A digial servo's microprocessor will convert the signal to shorter length pulses at a frequency of 300 times per second as opposed to 50 times per second of the original PPM signal. The motor is able to respond to smaller stick movements because it is receiving those very short pulses 6 times more often. This also gives a digital servo much stronger holding power because it reacts much more quickly when pushed out of position.
Digital servos react to your stick movements much more accurately than standard RC servos. You can think of this like the frame rate of a video. A video will appear much smoother if the frames are shorter and appear more often, as opposed to longer frames that occur less often. This same logic can be applied for understanding why digital servos give you much more precise control over your RC airplane.
This is a good article from Futaba if you want a more in-depth explanation of how a digital servo works.
If you've been shopping around for a servo you have probably noticed the different types of servo motors available. You will see 3P, 5p, coreless and brushless. So which one should you choose?
3P and 5P
Most RC servos on the market are either 3 pole(3p) or 5 pole(5p) motors. All DC motors have a set of permanent magnets that the electromagnets(windings) are attracted to. These permanent magnets are called poles.
The torque of the motor can be reduced if the electromagnets happen to be precisely in between two poles when the servo is trying to hold position. In other words, 5P motors will have more holding torque with smoother operation than 3P motors. If you're a beginner, there probably isn't enough difference that you would even be able to tell.
In a conventional motor, windings are wrapped around a metal core to form the electromagnets. A coreless motor does not have a core. The windings consist of a wire mesh that rotates around the outside of the permanent magnets.
This eliminates the problem with reduced torque at certain positions as discussed above with standard "pole" motors. A coreless motor will also respond much quicker to stick inputs because it doesn't have to overcome the momentum of a metal core when changing directions.
Brushless Servo Motors
Futaba has recently developed a brushless servo motor. So you get all the benefits of a brushless motor incorporated into your servo. This translates into longer servo life, faster response time, smoother operation and resistance to vibrations.
Servo Plugs and Wires
All RC servos will have three wires. One wire is positive lead (+) from the battery. One wire the ground (-) from the battery. The third wire is the control input from the receiver.
It is important to note that you can't just swap the pos.(+) and neg.(-) to reverse the servo. Doing so may damage your servo and receiver. You can buy connectors that act as a servo reverser by reversing the control signal. A Y connector is commonly used with two ailerons servos that share the same receiver output. The Y connector reverses the signal to one servo so that the two aileron servos act opposite of one other.
Futaba's "J" connector is a plug with a little plastic tab, or key, that prevents you from plugging it in backwards.
Futaba's J plugs, Airtronic's Z plugs, Hitec's S plugs, Hobbico's U plugs, and JR's connectors are all compatible as long as you make sure the wires are in the correct order within the plug. If you want to plug a male Futaba plug into another brand you will have to shave the safety tab off.