Introduction to RC systems

The radio system is your link between you and your model. They may seem complicated at first but with a little study, all aspects of the radio system can be easily understood. We will attempt to introduce you to the radio system here and explain a few of the features found on many of the systems available today.

Radios are separated into two groups, those used for model aircraft and those used for surface models. Although the operation, electronics, and mechanics for both types of systems are virtually identical, they operate on different sets of frequencies. The separation is stipulated by law and it is to protect the safety of the modeller and those in the vicinity. A flying aircraft can be dangerous if it becomes uncontrolled and the frequency separation helps avoid an aircraft being interfered with by someone operating a car or boat.

In addition to a difference in frequencies, some surface radios are also available with a pistol grip control which is ergonomically easier when controlling cars and boats. A pistol grip for aircraft would be impractical.

The first criteria one usually looks for when choosing a radio is the number of control functions or channels. (Note that the term channels here refers to the number of controls and does not have anything to do with the frequency on which the radio operates.) Generally modern radio systems are available with anything from 2 to 10 channels.

Surface Radios

Radios used for surface models generally have from 2 to 4 channels with 2 channel units being the most popular. One channel would be for the steering of the model while the other would be for control of the speed and direction (forward or reverse) of the model. Pistol grip radios are quite often more comfortable in controlling surface models.

Glider Radios

Gliders usually require 2 channels of control, one for rudder and one for elevator. Additional channels may be utilized for ailerons, flaps, spoilers, etc. on more sophisticated models.

Aircraft Radios

Model aircraft may require anything from 2 to 8 or even 10 channels of control, depending on complexity. The average aircraft will generally require at least 4 channels of control, one for rudder, one for elevator, one for ailerons and one for throttle. Simpler models may omit the ailerons and some even the throttle (common with smaller 1/2A models). With no throttle in an aircraft, the model would be flown with full throttle until the fuel has run out. It would then be glided in for a landing without power. Additional radio channels may be used for things such as retractable landing gear, operating flaps, bomb drop, camera actuation, glider release, etc.

Helictoper Radios

Model helicopters usually require different radios than model aircraft. Their controls are different with more mixing functions required. Usually a helicopter will operate with a minimum of 5 channels with the throttle and collective pitch channels both being controlled by one movement of the throttle control stick.

What Comes With Your Radio System?

Generally you will get just about everything you need when you purchase your radio system. Most aircraft and helicopter systems (of four or more channels) come with transmitter, receiver, three or more servos, rechargeable NiCad batteries for both the transmitter and reciever, battery charger, switch harness, servo trays, aileron extension, frequency flag and extra servo control arms. Surface radio systems do not usually come with batteries or charger and only have two servos.

Transmitter — The transmitter is the control box which you hold that converts your human control movements into electrical impulses and sends them via radio waves to the receiver in your model.

Receiver — The receiver is the small electronic unit in your model which converts the radio signal from your transmitter into electrical control signals which can be sent to your servos.

Servos — Servos are the devices in the model which actually produce the control movements. They convert the electrical signals from your receiver into physical movement to control your model. A different servo is required for each control function or radio channel.

Batteries — Two and three channel radio systems, for the most part, do not come with batteries and additional Alkaline (AA-type) cells are required. Again, these systems are generally used in surface vehicles. Virtually all 4 channel and greater systems come complete with NiCad rechargeable battery packs (for both transmitter and receiver) and charger.

Types of Radio Modulation — When shopping for a radio, you will find that people refer to different types of modulation. They are referring to the way the electronic control information is sent from your transmitter to the receiver over radio waves.

  • AM — AM, or Ampilitude Modulation, was the primary means of modulation in R/C until recently. The control information is transmitted by varying the amplitude of the signal.
  • FM — FM, or Frequency Modulation, is now the common method and is less prone to interference than AM. Information is transmitted by varying the frequency of the signal.
  • PCM — In PCM, or Pulse Code Modulation, FM is still utilized, however the control information is in the form of a digital word rather than just a pulse width as in standard AM or FM. Using PCM adds additional protection against interference from various sources.

 

Additional Functions and Features

The following is a list of other functions and features which may be available on various radio systems:

Servo Reversing — This feature allows the modeller to reverse a servo's rotation direction at the flip of a switch. Servos can be mounted in the most convenient way without concern for their rotation direction. The proper movement can then be selected when the installation is completed.

Dual Rates (D/R) — Dual Rate allows the modeller to choose between two different control sensitivities. With the dual rate switch in the "OFF" position, 100% servo throw is available for maximum control response. In some more sophisticated systems this "OFF" position may be adjusted to provide anywhere from 30% to 120% of normal full throw. In the "ON" position, servo throw is reduced and the control response is effectively desensitized. The amount of throw in the Dual Rate "ON" position is usually adjustable from 30% to 100% of total servo movement. The modeller can tailor the sensitivity of his model to his own preferences.

Exponential Rate — Exponential Rate is where the servo movement is not directly proportional to the amount of control stick movement. Over the first half of the stick travel, the servo moves less than the stick. this makes control response milder and smooths out level flight and normal flight maneuvers. Over the extreme half of the stick travel, the servo gradually catches up with the stick throw, achieving 100% servo travel at full stick throw for aerobatics or trouble situations.

Variable Trace Rate (VTR) — This radio function is similar to exponential except it uses two linear responses to determine the servo sensitivity on the first and second half of the control stick movements.

Fail Safe (FS) — An Electronically programmed mechanism in most PCM radios which automatically returns a servo or servos to neutral or a preset position in case of radio malfunction or interference.

Adjustable Travel Volume (ATV) — Frequently referred to as End Point Adjustment, ATV lets you independently preset the maximum travel of a servo either side of neutral.

Adjustable Function Rate (AFR) — Similar to ATV, AFR allows end point adjustment independent of Dual Rate or Exponential settings.

Sub Trim — A radio function which allows very precise electronic centering of servos.

Direct Servo Controller (DSC) — Allows full function of an aircraft's servos via an umbilical cord. This permits adjustment of radio functions without switching on the RF portion of a transmitter.

Programmable Mixing — Electronic coupling of one channel to another. This one control input will yield output to two different servos.

Aileron/Rudder Mixing — Adds rudder control when aileron is input from the transmitter aileron stick.

V-Tail Mixing — Used when there is a V-Tail on the aircraft rather than the conventional elevator and rudder. Each control surface of the V is connected to a separate servo. Operating the elevator control stick will move both surfaces up for back stick or both surfaces down for forward stick. Moving the rudder control stick left will move the left surface of the V down and the right surface up. Moving the rudder control stick to the right will move the left surface of the V up and the right surface down.

Flaperon Mixing — Mixes the Flap and Aileron functions so that when each aileron is connected to a separate servo (one servo plugged into the aileron channel and the other plugged into the flap channel), the surfaces will act as both ailerons and flaps, depending on the position of the controls.

Elevon Mixing — Mixes the Elevator and Aileron functions, especially useful for delta-wing models where the elevator and ailerons are the same control surfaces. Each surface is connected to a separate servo (one servo plugged into the aileron channel and the other plugged into the elevator channel), the surfaces will act as both ailerons and elevator, depending on the position of the controls.

Flap/Elevator Mixing — Couples the Flaps and Elevators such that when the flaps are lowered, the elevator will be automatically adjusted to prevent pitching of the model.

Elevator/Flap Mixing — Couples the Elevators and Flaps such that when control is input to the elevators, the flaps will move in the opposite direction. This permits the model to perform tighter maneuvers in the pitch attitude.

Crow Mixing — Primarily used in gliders for spoiler action by mixing the flaps and ailerons. It is necessary for the ailerons to be using separate servos, plugged into separate channels and the flap servo to be independent of both aileron channels. Upon applying Crow Mixing, the flaps go down while both ailerons go up.

Differential Ailerons — This type of mixing is accomplished by having separate servos on each aileron, plugging one into the aileron channel and the other into another unused channel. The two channels can be programmed to both operate from the aileron control stick, however the travel volume for each aileron may be adjusted separately giving more deflection in one direction (usually up) than in the other.

Mode I — The control stick configuration with the rudder and elevator being controlled by the left stick while the right stick controls the throttle and ailerons.

Mode II — The control stick configuration with the ailerons and elevator being controlled by the right stick while the left stick controls the rudder and throttle.

Mode III — The control stick configuration with the rudder and elevator being controlled by the right stick while the left stick controls the ailerons and throttle.

Dual Conversion — Dual Conversion refers to the method in which the receiver processes the incoming signal. Generally a Dual Conversion receiver is less prone to outside interference and is the preferred type of receiver.

Trainer System — The trainer system feature allows two transmitters of similar design to be connected together via a cord (trainer cord) so that one transmitter may be used by an instructor and the second one by a student when teaching to fly. The instructor simply has to hold a switch on his transmitter to give the student's transmitter full control. If the student gets into trouble, the instructor can release the switch and he has full control of the model.

Snap Roll Button — This feature is found on more complex radios and is used to perform a snap roll maneuver by simply pressing one button. The function is usually programmable to give a combination of rudder, elevator and aileron control.

Radio Accessories

The following is a list of some of the more common accessories that are available for radio systems:

Aileron Extension — The Aileron Extension (also known as a servo extension) is a cable with connectors on either end which goes between the receiver and a servo. This allows the servo to be placed at a greater distance from the receiver than the cable that comes on the servo will allow. It also permits easier removal of a wing when the servo that controls the aileron is mounted in the wing and the receiver is in the fuselage (which is usually the case). One aileron extension is usually included with a radio system of four or more channels. Aileron Extensions of various lengths are available from different manufacturers. Please note: long aileron extensions can sometimes cause radio interference problems unless "noise traps" are used.

Dual Aileron Extension or Y-Harness — The Y-Harness is a cable which plugs into a single channel in a receiver and two servos. This allows both servos to be operated from the same channel.

Noise Traps — A Noise Trap is a small electronic device which is wired into a long servo extension to reduce radio interference and to boost the control signal going to the servo. These are recommended for use where long servo leads are necessary.

Switch Harness — The switch harness is mounted in your model and it connects between your receiver and the NiCad battery pack. It provides a power ON/OFF switch to the radio in your model and it also allows your charger to be connected to your model's battery pack for charging.

Frequency Flag — The frequency flag is a marker that is mounted on your transmitter to indicate what frequency your system is operating on to alert other modelers so as not to cause interference. See the section on frequencies below for more information on radio frequencies.

Servo Trays — A Servo Tray is a plastic tray which facilitates mounting your servos easily in your model. The tray is molded to hold your servos securely and ensure positive control to your control surfaces. Different trays may hold anywhere from one to four servos and are shaped for different uses and servo positions in your model.

Servo Control Arms — Servo Control Arms are the plastic output horns which are mounted to the output shaft on your servos. These come in various sizes and styles for different control applications. Most servos will come with an assortment of arms so you can customize to your own specific control needs.

Frequencies — As mentioned earlier, there are different frequencies used for controlling aircraft and surface models. Government has allotted one aircraft frequency band (72 MHz) and one surface vehicle band (75 MHz) for use in controlling models. The separation is for public safety. Each band has quite a number of individual frequencies. In addition, some equipment may be available on 27 MHz which is the CB band or on 50 and 53 MHz which is for Amateur Radio operators and a Radio Amateur's operating license is required to use equipment on these frequencies. These Amateur Radio frequencies may only be available on some equipment and would always be on a special order basis only. Delivery is usually in the order of 6 to 8 weeks.

At East Coast Model Center we generally carry a good selection of frequencies, however, if you require a certain piece of radio equipment on a special frequency and we do not currently have it in stock, we would be more than happy to order it for you.

Frequencies have been given channel numbers (not to be confused with the function channels of a radio system). The following link is a chart of radio frequencies which are currently allotted for model control use in Canada along with their channel number.