A four-channel radio is needed by small-fixed rc helicopters although micro helicopters using a two-channel infrared control system also exist. Collective-pitch rc choppers require a minimum of five-channels with six being popular. The normal interaction of the various control mechanisms has resulted to advanced radios to include adjustable mixing functions such as throttle/rudder and throttle/collective. The price of radios is in the range of $100 to $2000 USD. The known makers of helicopter specific radio controllers include Futaba, Sanwa, JR, Hitec and Spektrum. The preferred user interface for remote controlled helicopter transmitters during the beginning of the hobby in the early 1970s to the 1990s was the “Knobby” or “single stick” style of Multi-channel remote control transmitter. This possessed a unit primary two-axis joystick and a special rotational, self-centering knob above the single joystick shaft for all three remote controlled helicopters aerodynamic controls. These are integrated into a single primary control mechanism.
The vertical and horizontal movement of such a joystick offers cyclic control, the knob being used to operate the tail rotor control. Examples of such radios were not available when manufacturers made new units in the beginning of the 1990s. There are new units even today being made by electronics rc helicopters hobbyists in North America for person use. Radios used in remote controlled helicopters emit FM signal in two modulations. The PPM modulation is the cheaper mode compared to PCM and is normally used in low-end helicopters. Due to lack of a fail safe, it is more suitable for less dangerous, small model type of remote controlled helicopters. High end radios are characterized by PPM and PCM modulation for a better compatible nature with all radio receivers.
Pulse Code Modulation (PCM).
PCM is a radio scheme used in rc helicopters in which the commanded position for a unit servo is transmitted as a digitally encoded number. Manufacturers use their own proprietary system for encoding the number with several levels of accuracy. The variable numbers is of bits per servo location. Rc helicopters maker, Futaba use PCM-1024 and G3 PCM, JR use Z-PCM then S-PCM. PCM does not broadcast all positions at one time in order to save time. The sending of odd numbered positions is done absolute in one frame with even sent as only differences from their recent values. In the next frame, the opposite action is done. PCM use in remote controlled helicopters radio is inclusive of a checksum at the end of frame for checking the signal’s validity. When interference occurs and arrival of signal is distorted at the receiver, then the checksum is utilized to know its originality. In case there is not interference, a feature called Failsafe is implemented for setting servo locations to a predefined location or holds them to last correct location.
Pulse-position modulation is radio scheme for rc helicopters is whereby the commanded location for a unit servo is transmitted as duty cycle of the pulse transmitted per servo location.
Futaba make use of frequency of 2.4GHZ band instead of the various frequencies in the lower MHZ ranges. The good thing here is that radios no longer use fixed frequency during flight but several frequencies.