Transmitter

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Overview

Turnigy 9X transmitter

The transmitter (sometimes called the radio) is a hand-held controller with joysticks, knobs and switches that takes the user input and transmits it wirelessly to the UAV's receiver. Transmitters can control a determined number of channels, typically 4 to 18. Transmitters have programming functionality which allows for custom mixing and adjusting of the channels, and some models include telemetry capability for displaying real-time data from the UAV. Almost all transmitter systems currently in the market use 2.4 GHz frequency and spread spectrum technology. While previous transmitter technology was prone to interference as it used a single transmission frequency, spread spectrum systems constantly hop between multiple frequencies. This allows for a practically unlimited quantity of transmitters to be used simultaneously without interference.


Layout and Types

Transmitter modes
Back of transmitter showing removable RF module

Almost all transmitters have the same physical layout and are used in similar ways. The basic setup has two 2-axis joysticks which can be controlled with the thumbs and a trim switch or slider for each joystick axis. The transmitter may also have a number knobs or switches on the top, and an LCD screen with buttons on the bottom which are useful for programming the transmitter. The main difference between brands and models comes down to how many channels they have, if they have programming features or not, how user friendly the programming interface is, how many knobs or switches they have, and other aspects like reliability and quality.

A very important distinction when selecting a transmitter is the transmitter's 'mode'. This simply refers to how the channels are assigned to each joystick axis. The vast majority of pilots use 'mode 2' setup, where the left joystick controls throttle and yaw (rudder), and the right joystick controls pitch (elevator) and roll (aileron). There are three other modes available. Basic transmitters are bought for a specific mode and cannot be changed, but advanced transmitters allow for changing the mode.

Some transmitters also include a 'trainer' functionality. This allows you to connect your transmitter to another one using a cable, and then switch between the active transmitter during flight by using a switch. This is useful for flight training, where the apprentice has the secondary transmitter and the instructor the primary transmitter. The instructor hands control to the apprentice holding the trainer switch, and if there is any problem the instructor can quickly regain control by letting go of the switch.

Nowadays almost all transmitters are modular, and the actual transmitting electronic module (or 'RF module') can be removed from the transmitter and exchanged with another one. However, for the most part modules of a certain brand are only compatible with transmitters and receivers of the same brand. Some third party manufacturers produce RF modules designed for use with specific brand transmitters. Some advanced modules allow for receiving real-time telemetry from the UAV, and displaying it on the transmitter, if the transmitter has this feature. Long-range RF modules are also available.


Setup

Programming

The most challenging task when setting up a transmitter is programming it. Although each transmitter brand has its own user interface, the programming settings are standard. Below is a list of the most common programming settings and how they are used.

  • Reverse: This setting simply lets you reverse the direction of any channel with respect to the transmitter input. For example, if a servo rotates clockwise when you move a joystick to the right, after reversing the channel the servo will rotate counterclockwise when moving the joystick to the right.
  • Subtrim: The subtrim setting allows you to adjust the servo position when the joystick is in neutral position. It has the same function as the regular 'trim' which is controlled using the trim switches or sliders in the transmitter. However, the subtrim cannot be changed during flight, and is meant mainly for adjusting servo neutral positions so all control surfaces are neutral when the joysticks are neutral during transmitter programming. The regular trim buttons add another level of trimming that can be used during flight as required.
  • Travel: The travel feature adjusts how far the servo rotates when a joystick is moved a certain amount, or how sensitive the servo is to the joystick movement. Transmitters typically have two travel values for each channel, one corresponding to each rotation direction relative to the neutral. The travel is set in terms of a percentage. Travel is useful for controlling the sensitivity of a channel, or how much the aircraft will respond to a given input.
  • Dual Rates: Some transmitters have dual rate switches. These switches allow you to easily switch between two different travel settings during flight. For example, you can easily switch between a low-travel setting for fine control and a high-travel setting for acrobatics. Some transmitters have more than one dual rate switch, and each one is assigned to a specific channel, for example an aileron dual rate switch and elevator dual rate switch.
  • Exponential: When the exponential feature is used, a special travel curve is applied to the channel such that the servo travel is low when moving the joystick slightly, and the servo travel is high when moving the joystick close to its limits. This makes the aircraft be less sensitive when doing small movements of the joystick, and very sensitive when doing aggressive movements. This could be useful in an acrobatic aircraft, where you want to have large travels for acrobatic maneuvers, but you still want to maintain precise control for small movements without it being too 'touchy'.

Mixing

Some transmitters also have channel mixing capability. This means that a channel can be controlled by combining more than one transmitter inputs. There is a standard set of mixes that are commonly used and are sometimes included by default. However, some transmitters also allow you to define your own custom mixes. These are the most commonly used mixes:

  • Elevon Mixing: Elevons are control surfaces that double as elevator and ailerons. Flying wings have elevons, where the left and right wing control surfaces can move together up and down to control pitch, and can move opposite to each other to control roll. To move the servos in such a way, the transmitter must mix the two servo signals such that moving the elevator joystick up and down causes the servos to move together, while moving the aileron joystick side to side causes them to move opposite of each other. This is done through elevon mixing.
  • Flaperon Mixing: Flaperons are control surfaces that double as ailerons and flaps. An airplane with flaperons can use control surfaces on its wings as ailerons for roll control, but can simultaneously lower both control surfaces together as flaps. Flaperon mixing mixes the aileron and flap channels to do this task.
  • V-Tail Mixing: A V-Tail is a type of airplane tail with two diagonal stabilizers forming a V shape. By moving the control surfaces in the stabilizers inwards and outwards the aircraft can control pitch, and by moving both side to side the aircraft can control yaw. V-tail mixing mixes the yaw and pitch channels to do this task.

Usage

Spread-spectrum transmitters require binding the transmitter to the receiver so they can communicate. This is usually done by turning on the transmitter while pressing its 'bind button', and then plugging in a 'bind plug' into the receiver and turning on the UAV. After the transmitter is bound to the receiver, the bind plug can be removed. This procedure needs to be done only when the transmitter will be used to control a new receiver unit.

Before the first flight, the transmitter should be set up to provide the correct settings and mixing for that UAV. See the Setup section for more information. Some transmitters can store setup profiles for multiple aircraft on internal memory, so you can use one transmitter with multiple UAVs.

During flight, the joysticks, knobs and switches are used to control the UAV. If the aircraft has a tendency to turn in one direction when the joysticks are left in neutral position, use the trim button or slider corresponding to that joystick axis to trim the UAV and eliminate this tendency.

Be sure to fully charge the transmitter's battery before flight. An alarm or indicator light on the transmitter will alarm you if its battery is too low. If this happens, the UAV should be landed immediately.

Safety

A number of precautions should be followed to prevent injury or loss of the UAV. The main safety issue is that loss of signal, glitches or inadvertently moving a joystick could cause electric motors to turn on unexpectedly. For this reason, an UAV should always be handled as if the electric motors can turn on at any moment. When programming the transmitter, the propellers should be removed from the electric motors or the electric motors should be disconnected.

A common error mistake is to have the wrong aircraft profile loaded, causing some channels to be reversed, causing a crash. To prevent this, the direction of movement of all the channels should be checked before each flight.

Also, the transmitter should be 'range tested', by activating its range test mode, moving a distance away from the UAV, and confirming that the UAV can be controlled. This tests the range of transmission of the transmitter.

Product Listing

Below is a list with the most popular transmitter systems on the market.

Name Brand Channels Switches Knobs LCD Screen Aircraft Memory Telemetry Approximate Price
9x Turnigy 9 7 3 Yes 8 No 55
9xR Turnigy 9 6 3 Yes 16 No 50
HK6S HobbyKing 6 2 2 No 1 No 25
Hobby King 6Ch V2 HobbyKing 6 1 1 No 1 No 30
DX4 Spektrum 4 3 0 No 1 No 53
DX5e Spektrum 5 2 0 No 1 No 60
DX6i Spektrum 6 7 0 Yes 10 No 150
DX6 Spektrum 6 6 0 Yes 250 Yes 230
DX7s Spektrum 7 7 1 Yes 20 Yes 299
DX8 Spektrum 8 8 1 Yes 30 Yes 299
DX9 Spektrum 9 8 1 Yes 250 Yes 419
DX10t Spektrum 10 variable variable Yes 50 Yes 1099
DX18t Spektrum 18 variable variable Yes 250 Yes 945
DX18 Spektrum 18 8 1 Yes 250 Yes 799
XG6 JR 6 2 0 Yes 30 Yes 225
XG8 JR 7 8 0 Yes 30 Yes 429
XG11 JR 11 8 0 Yes 30 Yes 799
XG14 JR 14 8 0 Yes 30 Yes 649
4YF Futaba 4 0 0 No 1 No 99
6J Futaba 6 4 0 Yes 15 No 164
8J Futaba 8 8 0 Yes 20 No 279
10J Futaba 10 8 0 Yes 30 Yes 349
6EX Futaba 6 4 0 Yes 6 No 259
7C Futaba 7 6 1 Yes 10 No 279
10CG Futaba 10 8 3 Yes 15 No 649
14SG Futaba 14 8 2 Yes 30 Yes 549
18MZ Futaba 1 6 3 Yes 0 Yes 2999

External Resources