If you have purchased an rc helicopter in kit form, you will then need to get a radio for it. The radio system is responsible for sending control signals to your rc helicopter. When you move the sticks on the transmitter the motion is converted to signals and sent out by the transmitter, picked up by the receiver, and relayed to the servos which control the flight of the rc helicopter. There are several types of radios available, and each has different benefits and costs.
A common question is whether or not an rc airplane radio can be used in with an rc helicopter. The answer is yes, but you will have less control options than you would when using an actual rc helicopter radio.
An rc helicopter radio is different from an airplane radio in several ways. The most important is the throttle for an rc airplane is used to control the engine speed, not the collective pitch. In an rc helicopter, both must be controlled. In order to use this kind of radio with an rc helicopter, both the engine throttle and collective pitch servos must be linked together via a Y harness. This will work, but you will have less control than if the pitch and throttle servos were independent of each other. Using this type of system works best in fixed pitch rc helicopters, because they do not use the collective pitch control in the first place.
When shopping for an rc helicopter radio, you will come across the term “channel” often. It has two different meanings, and each is important in your buying decision. It can mean either the number of servos the radio can control (which relates to the number of degrees of freedom the rc helicopter can have), or the specific frequency subset that the radio signal is sent on (the 72MHz frequency band is divided into channels numbered 11 to 90). A two channel radio can control two servos (for example the throttle and rudder servos). A three channel radio could have both throttle and rudder control, as well as elevator control. A rc helicopter will need at least 4 channels, and typically 5 or 6 channels. Some radios have many more channels, but these are not important to the beginner. The basic 5 channels control:
- Collective pitch
The extra channel can be used to control gyro sensitivity or other functions. The cost of a radio system is highly dependent on the number of channels available. When buying your first rc helicopter radio, try to get one with at least 5 channels. More channels will enable you to use more advance features as you progress in the hobby.
Radio systems can also use different methods to transmit their signals. These can be FM, PCM, or PPM, or spread spectrum. The most basic is FM, or frequency modulation. This method is the same type used by FM radio stations, but on a different frequency. Many FM radios can operate simultaneously as long as they are on different frequencies or different channels on the same frequency. If two radios are operating on the same frequency and the same channel, interference will results and one or both of the models flying can crash.
PPM (or pulse proportional mode) radios are better than a normal FM radios because they can operate servos at a higher resolution. The radio transmits by first sending a timing pulse, and following this with the actual command information. The advantage of this system is that the receiver knows what to expect before the command arrives. A different timing pulse is sent for each control channel, and this cycle repeats many times per second. The rate at which these pulses occur is called the pulse rate of the radio. A greater pulse rate gives better control, but getting a high pulse rate radio is not so important for the beginner or casual flyer.
PCM (or pulse code modulation) is similar to PPM except that each pulse is coded. The rc helicopter will only respond to signals with this specific code. This means that it will cope better to interference. Although this technology is helpful, it does not make the rc helicopter immune to all interference. If another pilot turns on a radio using the same frequency and channel, the rc helicopter can still have its signals washed out by the other transmitter and crash.
A synthesized radio lets the pilot transmit on different frequencies. This is helpful when you are flying at a club where there are many other pilots, because you do not have to change the crystal in the transmitter and the receiver to change channels.
The most recent radio technology eliminates the need for channel frequency control, and is immune to almost all forms of interference. Spread spectrum radios can transmit and receive on multiple channels, at a frequency of 2.4 Ghz (2.4 billion cycles per second). This kind of radio automatically scans for two free channels when is is turned on. When it finds them, it uses both to transmit and receive. Spread spectrum technology is becoming more popular, and it will eventually replace the other three kinds of radios mentioned above.
Besides the type of radio, you will have to consider different types of servos to use in your rc helicopter. There are many different types, the broadest categories being: standard, coreless, and digital. Standard servos are the least expensive, but they also provide the least performance. Since the motors they use have coils wound on a rotating iron core placed between stationary magnets, they will not be able to start and stop as quickly as the coreless variety. These servos should really only be used to control the throttle of the rc helicopter. Coreless servos are the same, except that the motor inside them has coils that are rigid and rotate around a stationary magnet without requiring an iron core, and are able to accelerate more quickly. These servos typically have better resolution and more torque than their cored counterparts. Digital servos use a digital amplifier to achieve even better resolution and accuracy. Digital servos can be cored or coreless are better than non-digital servos for several reasons, including:
- More accuracy
- Faster control response
- Greater torque
These are the best type of servo, but they may not be in within the budget of the beginner. Whatever type of servo you choose, make sure that it has ball bearings supporting the output shaft. This will result in both smoother operation, and longer servo life.
Be aware that most radios come with only four servos, and you will have to buy a fifth yourself.
The last major radio component is the battery. The battery is used to power the various radio components. Most batteries are of the four cell variety, and supply 4.8 volts. This is sufficient for most models, but a 5 cell pack producing 6 volts can be useful. The more voltage you have available, the faster the servos will respond. For your first model, you should consult the instructions that came with it to determine what type of battery pack to use.
Several other radio features include, but are not limited to:
Tail rotor compensation:
Tail rotor compensation keeps the nose of the rc helicopter pointed in the same direction. This is accomplished via a gyroscope (which is explained in a previous article, ‘Introduction to RC Helicopter Gyros, Rate & Heading Hold’) telling the tail rotor how fast it needs to spin. When the pilot adjusts the pitch or throttle of the rc helicopter, the torque from the blades causes the nose to move in the opposite direction, and thrust from the tail rotor is required to counteract this.
Exponential allows the pilot to control how sensitive servos are. With this feature we can tell the servos how much to move for a given amount of stick movement on the transmitter. This can allow smoother control for small movements, and sharper control for large movements of the transmitter stick.
With this feature the pilot can control the sensitivity of the gyroscope on board the rc helicopter. This feature can allow for more stable flight, or easier aerobatics depending on the setting.
Electronic digital trim:
Trim can be used to correct undesired motions of a rc helicopter. It controls the adjustments of various servos during flight. For example, if I were flying a model with a slight left bank, I would slide the aileron trim lever in the opposite direction, to apply a right correction. These would cancel each other out, resulting in straight flight.
These are various extra switches located on the transmitter that enable the pilot to control special functions.
Multiple model configuration / switches:
This feature will let the transmitter control multiple models by storing the different settings for each. This is a good feature to have, because you can use one transmitter for several models, no matter how different they are. A memory feature will allow the transmitter to store this information.
An adjustable servo endpoint will allow adjustments to the maximum throw of servos. This can be used as trim to compensate for undesired motions, by reducing the travel in the opposite direction.
Condition / flight mode:
This feature will allow different adjustments to flight parameters for different flight modes. This increases performance in all phases of flight, and is a very good feature to have.
When buying your radio, carefully consider all of the above features. If you’re unsure about your choice, you can ask more experienced pilots for help. If you take good care of your radio, it should last you many years.