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RC Helicopter Radio Systems Explained

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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. An 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:

  1. Aileron
  2. Elevator
  3. Throttle
  4. Collective pitch
  5. Rudder

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 advanced 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 result and one or both of the models flying can crash.

PPM (or pulse proportional mode) radios are better than a normal FM radio 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 with 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 it 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 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 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:

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.

Gyro gain:

With this feature, the pilot can control the sensitivity of the gyroscope onboard 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 an 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 a straight flight.

Programmable switches:

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.

Servo endpoint:

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.

Basic RC Helicopter Radio Functions

When getting started in flying radio-controlled helicopters, there are a lot of new words to learn, many of which have to do with settings on your radio.

It can often be intimidating hearing seasoned pilots talk in terms that you just don’t understand. By learning what the basic RC helicopter radio functions are and what they do, you can not only improve your understanding of what others are talking about but also fine-tune them to your flying and fly better.

While there are many different radio functions that you will have to learn, there are some basic RC helicopter radio functions that are most commonly used. It is also important to note, that some radio functions can vary depending on the type of heli you are flying, as well as the type of radio you are using.

The most basic RC helicopter radio functions are:

  • Trim – A fine adjustment, usually controlled by small levers or buttons near the control sticks on the transmitter. Trim is for fine-tuning control surface center points or adjusting engine idle.
  • Sub trim – A finer division of trim, it is usually adjusted in the transmitters software or control menus.
  • Expo – Makes the feel of the controls around the center stick less sensitive, while still giving 100% travel of your control surface. Expo will help to soften the feel of your helicopter without reducing it’s aerobatic capability. Expo can be assigned to a switch to change the feel of your helicopter to suit multiple flying styles.
  • Throttle hold – Brings your engine to idle while still allowing full control of collective pitch. It’s always best to start your helicopter in throttle hold to prevent a hot start from occurring. A hot start is when the helicopter starts at a throttle position high enough to engage the clutch. In the event of an inevitable crash, you should always use throttle hold to help cut power to the main rotor blades, which can help to reduce the amount of damage that is incurred.
  • Throttle Cut – Simply kills the engine. It is normally used for turbine, nitro, or gas helicopters and is not applicable to electric helicopters.
  • Servo Reverse – Reverses the direction of servo wheel travel relative to control input.
  • Gyro gain – Is used to increase or decrease the sensitivity of the gyro. The gyro is a device that is usually used for yaw control, which is the movement around the vertical axis of the helicopter. There is a fine line between setting your gyro gain to sensitive or not sensitive enough, so you will have to experiment to find a setting that best suits your flying style and abilities.
  • Endpoints/ATV – Adjustments that control the total amount of servo and/or control surface deflection or travel.
  • Pitch Curve – Controls the degree of the collective pitch at each point along collective stick travel. Collective pitch is the angle of attack of the main rotor blades.
  • Throttle Curve – Governs how much of the power of the engine is transferred to the main rotor blades at each point in the collective range.
  • Idle Up – Allows you to maintain a constant rotor speed whether your using positive or negative collective pitch. Maneuvers such as flying inverted would not be possible without idle up.
  • AFR (D/R) – Allows you to adjust your control deflection to suit your flying style or flying ability. It reduces the total amount of control deflection. It is usually assigned to a switch, so you can change between lower rates, which produces a softer and slower feel, to higher rates which can produce more aerobatic and agile handling aircraft.

By having an understanding of the most basic RC helicopter radio functions, you will greatly increase the amount of success you have when flying. It will allow you to fine-tune both your helicopter and your radio to best match both your flying skills and your abilities.

Which radio system?

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 with an RC helicopter. The answer is yes, but you will have fewer control options than you would when using an actual RC helicopter radio.

It is highly important to have good quality electronics for your helicopter. You can make a sub-standard helicopter fly well with good electronics, but it’s difficult to make a good helicopter fly well with sub-standard electronics.

It’s my opinion that you should buy the best quality electronics you can afford, then buy the helicopter afterwards. If this means settling for a slightly lesser helicopter in order to get good electronics, then so be it. I myself tried to save money on electronics and then ended up paying twice when I finally relented and purchased good electronics.

Good electronics will transform your flying. They did for me. Today’s R/C systems allow huge amounts of fine tuning that allow you to get on with the business of flying enjoyably. If you’re fighting with sub-standard electronics you won’t enjoy your flying as much.

Plus, in the end, buying good electronics from the outset ends up cheaper as you don’t have to buy good stuff later on. If I were to do it all over again, that’s what I’d do.

When you’re looking to buy a radio set, you’ve got to take into account a number of things. Some of these are:

  • How much it costs. Obviously important in any purchasing decision.
  • Local knowledge. While these systems may look complicated (and they are!), they’re not too hard to use. However, if you know someone who already uses the system you are thinking of buying and can help you set it up, it’s much easier.

Anyway, I’ll step down from my soapbox now and explain some of the electronic wizardry that these little heli’s need to fly.

Transmitter

The transmitter is the control box that you use to command the helicopter. Nowadays these have become very comprehensive computer controlled devices.

You set up a lot of how your helicopter performs via the transmitter.

Ideally, you want a minimum of 8 channels for your transmitter, but you can use 6 channels if need be. I recommend getting at least an 8 channel radio, not so much for the number of channels, but for the extra added features you get with these radios.
There is a lot of debate about what kind of transmitting system is best, either PCM, or PPM. My opinion is that you should get the PCM system because you can always use PPM on it if need be. You can’t do it the other way around. Plus, some helicopters, like gassers, require PCM modulation. Remember you’re planning for the future here, you never know, you might wanna get a gasser later on!

The more popular makes are JR and Futaba. Both make excellent quality products. I recommend getting either the JR 3810 (or 8103 if you live in the US), or the Futaba Super 8. If you really want to splash out, get the JR PCM10x, or the Futaba 9Z. Those things will pretty much cook your breakfast for you.

There are two popular types of control layouts with transmitters. They are Mode One and Mode Two. This refers to which sticks look after the various controls of the helicopter.

Mode one is when the left stick controls the fore/aft cyclic and the rudder controls. The right stick controls the collective/throttle and left/right cyclic.

Mode two is when the left stick controls the collective/throttle and rudder controls. The right stick controls the left/right cyclic and the fore/aft cyclic.

In New Zealand, Mode Two is the more popular layout. However, it is important to get the same layout that the people who will be teaching you use. That way they can fly your machine if need be.

Receiver

If you purchase a new radio set, you’ll most likely get the corresponding receiver with it too. The receiver just act’s upon the signals the transmitter produces and commands the appropriate servos/gyro etc accordingly.

There’s not really much more to say about receivers.

Servos

Servos are the little gadgets that actually do the work in the helicopter. They move the little arms that make your helicopter move.

You can get all types of different servos. The difference is mainly in the speed of the servo (transit time) and the amount of power it produces (torque).

When you’re first beginning to learn and you’re not thrashing the helicopter round, basic servos that come with your radio kit (such as Futaba 3001’s etc) will be fine cause you’re not putting them under large loads. However, when you start throwing your machine in around, or if you upgrade to a larger machine such as a 60 or a gasser, you’ll want to upgrade your servos to a higher torque.

There are also two types of servo, the Digital servo and the non-digital. Digital’s are designed to center more accurately and also come to full power from stop. Non-digitals are less expensive, but some do not center as accurately, and also don’t have full strength in minute movements.

Batteries

Your radio set will often come with a battery for the electronics in the helicopter. A lot of them are around 1000mah. These are fine, but you might want to consider upgrading to a larger capacity in the future. Some gyros and servos really chew up battery time. 1700+mah batteries are good, allowing you to fly pretty much all day with high capacity servos and gyros without having to recharge.

Final Words

Get the absolute BEST radio system you can afford. If it means going with a lesser heli than you were wanting so be it. Helicopters come and go, but your radio gear can be transferred between them. I can’t emphasise enough how much a good radio will help your flying.

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