Motors come in two variants: Brushed motors, which have small brushes that switch the power through the coils, and brushless motors, which need an external controller to do the switching for them.
Brushless motors are usually more efficient, can handle higher speeds and torques, can handle huge amounts of power, and are longer lasting because they do not have brushes to wear out. they usually come in out-runner or in-runner styles. Outrunners have the permanent magnets on the outside, and their housing spins with the axle. Inrunners have the permanent magnets on the inside and have a non-rotating housing. Outrunners have more torque, but less speed, and are usually used with larger propellers. Inrunners have less torque, but can reach very high speed, and are therefore used in geared applications, such as RC cars, or with small propellers.
Motors in the RC world often have an interesting rating called Kv. Although one would expect this to mean Kilovolts, it actually means rotations per minute per volt. A 300Kv motor would, in theory, rotate at 900rpm at 3 volts. the lower the Kv rating, the more torque but the less speed a motor has, and the higher, the more speed but less torque a motor would have.
When looking for a suitable propeller, the prospective buyer may be confronted with such things as "1247" or "12x4.7". These define the propeller's size and pitch. in this instance, one would be looking at a 12 inch diameter, 4.7 inch pitch propeller. Pitch is defined as the distance a propeller would move in the air under ideal conditions in one full rotation. Propellers are also often classified into "slow fly" and normal propellers. Slow fly propellers are usually for geared applications or low speed applications, and have a curved leading edge for more efficiency. Normal Propellers have a straight leading edge for better efficiency, and are usually used with faster motors.
Gears in the RC world are usually only sold as a replacement for RC car gearboxes. they are never sold by diameter, but only by the pitch (spacing of the teeth) and the number of the teeth. A Spur gear is usually a large gear, that is turned by the pinion, a smaller gear directly attached to the motor.
Motor controllers, also known as ESC's (Electronic Speed Controller) are very simple to use. Their purpose is to take a low power input signal in the form of ppm, and the battery voltage, and convert it into something useful for the motor. They also have a voltage output through the signal wire, and are often used to power the receiver.
There are essentially two types of motor controllers: brushless and brushed controllers. Brushed motor controllers only work with brushed motors, and send them a PWM, a Pulse Width Modulated (not PPM, Pulse Position Modulated) power input. These are simple and cheap, and can also be used to control lights and anything else that requires DC power. Brushless motor controllers are quite different. They switch power between the three combinations of two of the three poles of a brushless motor, and modulate the speed of this according to the PPM signal.
The difference between Car ESC's and Airplane ESC's is that car ESC's usually come with a power switch, heat sink and a reversing ability, while airplane ESC's have no reversing capability (Switch any two of the motor wires to reverse spin of the motor), no power switch and usually only a minimal heat sink.
ESC's are also programmable, usually through a programming card or through the reciever and transmitter. This will vary depending on the ESC, but usually they have programmable braking, starting, and battery voltage cutoff settings.
Batteries come in many different Shapes and sizes, chemistries, and ratings.
The two most common chemistries for RC applications are NiMH or NiCd and Lithium polymer batteries.
NiMH (Nickel metal hydride) and NiCd (Nickel-Cadmium) batteries are heavy, weak and outdated. the only benefit is that they may be marginally cheaper, and less sensitive to shock, vibration, overcharging, and deep discharge.
Lithium Polymer Batteries are lighter, smaller, can deliver huge currents, and have a very large capacity for their size. A 5000mAh Lipo battery is about as big and heavy as 2500mah worth of NiMH batteries. however, if pierced, discharged too far, or charged too far, they will "puff", or expand. This means that the battery is close to exploding. do not use this battery anymore, and dispose of it safely. If you keep going, it will most likely erupt in a large ball of flame and launch chemicals and burning things everywhere. Never short these batteries, they will most likely explode.
Charging: get yourself a good charger, I can recommend the Turnigy Accucell-6. Most chargers do not come with a built in power supply, so make sure you have an adequate power supply for your charger. Follow all the charging instructions that come with the charger, and place your battery in a fireproof container or bag. Never leave it unattended while charging.
Batteries have a number of different ratings:
mAh or Ah: Stand for milliamp hour or Amp hour. this is the capacity of the battery. 1Ah=1000mAh, which means a battery can supply one amp for one hour (or 1000milliamps for an hour)
S and P: Stand for number of Series cells and number of parallel cells. the number of cells in series determines voltage, parallel determines capacity.
C ratings: This is how much current the battery can give you. Multiply the battery's capacity in amp hours by this rating to get the current in amps. There are usually two specified. the lower one is the continuous rating, and the higher one is the pulse discharge rating.
I usually use one standardized connector for all of my batteries, so that I can use any battery with any device, and can interchange them easily. It doesn't matter what you use, I use the XT60 connectors, just make sure that you have one type of connector for all of your batteries.
Get some silicone wire for your power connections, because it is a flexible, multi-stranded wire that will in general make your life many times easier.
Also often abbreviated as Tx, this is the thing that actually takes your commands and sends your receiver the commands for the servos and the motors.
There are many Transmitters available on the market nowadays, and the low-cost ones start around $30. However, remotes for RC airplane or helicopter pilots, with programmable features, model memory, and computer simulators can start around $150 and go to nearly $3000.
All RC Remotes have a certain frequency that they operate on. Older, analog remotes operate on lower frequencies, which were determined by a crystal. Modern, digital, remotes usually operate in the Gigahertz Range, often at 2.4GHZ.
RC airplane and helicopter remotes have control sticks, which are mostly self explanatory. Usually, one of the two control sticks on an airplane remote is the throttle control stick, and is not spring loaded in the vertical direction, which means it will not spring back when you let go. Remotes are usually available with this stick on either the left or the right side, and are sold as mode 1 or mode 2. RC car remotes have a trigger and a knob to control different things.
Most RC remotes have something next to the Control sticks called the trim. this basically slightly alters the position of your control sticks, and is useful for finely altering the position of the servos, or the starting point of a motor controller. Transmitters also often have reversing, which reverses the output signal of the control sticks. this is usually controlled by a series of switches at the bottom of the transmitter.
The number of channels a remote has shows how many things it can control. For example a three channel remote could control three things, such as three servos, three motors, or a combination of any of those. Motor controllers and servos receive the same type of signal.
Expo, Telemetry, Limited travel and Model memory are all things seen on better, more expensive transmitters, but are not necessary for the average project. Expo stands for exponential rates, so that linear movement of your control sticks will result in an exponential output signal curve. Telemetry transmits information from the vehicle (such as battery voltage, altitude, motor temperature, etc.) back to the transmitter and displays these on the transmitter. Limited travel means that by moving your control sticks to their maximum will only send out a 30% signal, or whatever you set it to. both of these features are beneficial to sensitive mechanisms and airplanes, so that one can maintain better control over them. Model memory simply allows you to store your trim settings in the transmitter, so that you can recall them for later use, or eliminate the need of having to readjust with every time if you have multiple devices. RC car remotes are often equipped with dual rates, expo, memory, and even telemetry.
Most Transmitters need to bind to your Receiver to ensure proper communications. How to do this depends on your Transmitter/receiver.
Many transmitters need 12v to operate, usually supplied by 8AA batteries. this is 8 batteries too many for me, so i got a small, 0.8Ah battery and hooked it up to my remote, so that i only have one battery to worry about, instead of 8 small ones.
A cheap, beginner-level Transmitter that i can recommend from personal experience is the HobbyKing 6S.
The receiver, abbreviated as Rx, receives the signals of the transmitter, and sends them out to the individual channels through PPM, or Pulse Position Modulation commands.
The Receiver must operate on the same frequency as the transmitter, and the two will usually be sold together. Replacement or extra receivers are commonly available, so that they do not need to be switched out if one has more than one devices requiring a receiver.
The receiver usually receives power from a motor controller, simply by plugging it into one of the signal ports. Servos receive power from the receiver.
The Polarity of the plugs is usually found out by trial and error, in one way it will work, and the other way it probably will show a flashing light or something similar. Make sure to never connect one wire one way and another with opposite polarity to that wire, because depending on what those two things are, they will most likely be damaged, because one is going to be reverse polarity.
Some servo or motor controller connectors have a small plastic tab on them, and can therefore only be used with RC car receivers that can easily be removed with wire cutters.
Be careful with the antennae of digital receivers, because they are rather difficult to reattach.