Bind-n-Fly ™ (BNF) is a radio control system distributed by Horizon Hobbies which allows one transmitter to control an unlimited number of different models. The "RTF" or Ready-To-Fly models used the same technology, but include a basic BNF compatible transmitter.

The Spektrum transmitters come in several different models which include progressively more channels and model memories to store the trim settings. The DX6i transmitter which I own has 6 channels (one more than is needed for flying the Blade models) and 10 model memories. The DX7 has 7 channels and 20 model memories and a few more control menu options. The Newest addition the DX8 has 8 channels and utilizes SD card storage which allows unlimited model settings to be stored

The Binding Process

Each transmitter has a unique identifier code called the GUID. Before a model is flown for the first time it must be paired or "bound" to that unique transmitter ID code. The binding procedure stores the GUID in non-volatile memory in the receiver in the model which allows it to find its paired transmitter every time it's battery is connected. The command sequences between transmitter and receiver are identified by the GUID which allows up to 32 different people to fly their models simultaneously without interference.

The binding procedure varies, depending on the type of receiver. For the small Blade models equipped with 5-in-1 boards (receiver, esc, servos, mixing, gyro) the transmitter is turned off while the model is powered-up. The indicator on the 5-in-1 board will blink one, go out, then after a few seconds start pulsing rapidly which indicates the model is unbound and looking for a transmitter to mate with. At that point the bind button or switch on the transmitter is held while it is powered-up, then released. When transmitter and receiver establish contact the indicator on the 5-in-1 will go out, then come back on solid a few seconds later. The procedure for larger Spektrum receiver units varies slightly. They require insertion of a "binding plug" on the receiver to perform the pairing. Other than that the procedure is similar: receiver powered on first, then the transmitter with the "bind" switch held.

Once bound together before the first flight with the GUID of the transmitter stored in the memory of the receiver the model will recognize its paired transmitter when it powered on. Because the receiver must find the transmitter it is important to remember to always turn on the transmitter first before powering up the model, and unplug the model before turning off

How the Transmitter Works

With radio systems the design of the antennas for transmitter and receiver is dictated by the wavelength of the signal, with the physical dimension antennas needing to be some fraction of the wavelength. The closer the antennas are to the full wavelength the more efficient they are. The Spektrum system uses the same 2.4Ghz frequency band used by cordless phones and other wireless devices. The wavelength of the 2.4Gz signal is only about 3.9 inches which is what helped make the small micro helicopters possible. The antenna on the 120SR is a full-wavelength monopole (3.9 inches long) antenna and the mSR has a shorter 1/2 wavelength monopole. The antenna in the transmitter is a full-wave monopole. The short wavelength and full and 1/2 wave antennas make the 2.4Gz system very efficient and improve its range.

The signal between transmitter and receiver frequency modulated, with control movements for each function or servos communicated by varying the pulse width of the signal. Each discrete control function of the RC model is assigned a "channel". Depending on the complexity of the the model it may require 3 to 11 channels. The term "channel" is misleading because they represent are meta-data tags for the data-stream sent to the receiver, not different radio frequencies. A six channel transmitter will send a series of six pulses to the receiver with width of each pulse corresponding to where the stick or switch for that function is currently located.

The Blade micro helicopters use five channels: throttle, rudder (tail motor), elevator (forward/backward cyclic), aileron (sideways cyclic) and gyro. The 5th channel designated as GYRO on Spectrum transmitters actually functions as a on/off switch for a dual-rate function on the 5-in-1 board.

How The 5-in-1 Board Servos Work

Blade micro models combine five components on a single circuit board called the 5-in-1. The name denotes the five components - receiver, esc, servos, mixer, gyro - not the five channels used to control the model.

The receiver decodes the sequence of pulses from the transmitter and based on the pulse width computes where the servo should be located over its range of travel. The servos for swash plate movement are linear servos consisting of a link which runs up and down on a rapidly rotating threaded rod called a "lead screw". Tiny contact fingers on the bottom of the moving link move over a resistor strip on the 5-in-1 board. The resistance value tells the servo controller where the link is currently located. If the resistance value does not match the value sent by the transmitter the servo controller fires up the motor, spins the lead screw and moves the link until the resistance value matches what the transmitter ordered.

The servos on the Blade micro models are very fragile, more so on the mSR than the 120SR. During the course of normal flying dirt and grim can get between the resistor strip and fingers of the link. When that occurs the servo will sometimes not be able to find the exact resistance value matching the transmitter request and the servo controller will drive the link back and forth trying to find it. If that "chatter" occurs blowing out the servos will often remedy the problem. In extreme cases it may be necessary to remove the servo from the 5-in-1 board and clean the strip on the board and the contacts on the link, being very careful not to bend the delicate contact fingers of the link. Crashes can also cause the gears driving the lead screw of the servo to become damaged and stick. Replacement gears for the mSR servos are available (Part # EFLR7110)

How Servos on Larger Models Differ

Larger RC models such as collective pitch helicopters use a different type of servo generically referred to as a "control arm" or "control horn" servo. The resistance feedback loop for control is similar to the linear servos on the 5-in-1 board but instead of the servo motor driving a link up and down a lead screw the high-speed motor drives set of clockwork style gears which move a lever arm attached to the drive shaft up and down in an arc. The control arm design is able to generate more power (torque) and is more responsive than the linear lead-screw design and is also more compact. But because the link moves in an arc the alignment of the links relative to the swash plate and control link for the tail rotor pitch on a collective pitch (CP) model is more critical.

CCPM mixing

On a CP model three servos and swash plate arms spaced 120° apart must act in perfect unison over their entire range of travel to keep the swash plate level when changing the collective pitch which moves the helicopter up and down in hover and when moving. When not in a hover the same servos must move at different rates to tilt the swash plate to create the cyclic feathering on the rotor which tilts it. For that reason it is critical to start with three servos which react exactly the same, with there control arms parallel to the swashplate.

CCPM stands for "Collective Cyclic Pitch Mixing". That is the process of the transmitter computing how much each of the three servos controlling the movement of the swash plate must move to execute the control inputs the pilot is making with the left (collective pitch) and right (cyclic pitch) sticks. Given the dynamic forces of the flybar and the two blade rotor that I not an easy task or one that can be done with absolute precision which is the reason that collective pitch model helicopters are much easier to fly that the fixed pitch Blade Models.

5-in-1 Mixing

The fixed pitch Blade models use only two servos and swashplate arms, spaced 90° apart. That 90° spacing put them both 45° from the center line of the mode. The Blade models also use a 45° flybar angle, which differs from the 90° orientation in most other models. This design, in combination with the rotors and other parts which are designed to flex and dampen the destabilizing forces is what allow the Blade models to hover "hands-off" which is quite a remarkable feat, but not one which is unprecedented. Stanley Hiller did the same thing with full-scale helicopters he designed in the 1940s and the flybar control system used in the small Blade models use the same principle for control and stability.

Although not immediately apparent the 5-in-1 controllers of the mSR and 120SR incorporate mixing of inputs to help stabilize flight characteristics. If you hold an mSR in your hand and spin up the rotors and then move the rudder stick sideways you will notice the elevator servo also moves and the throttle changes. That is the automatic mixing of elevator and throttle with rudder input to compensate for the way all helicopters react differently when turning with and against the direction of rotation.