ON THE AIR
When Dr. Walter Good and his twin brother, Bill, first flew their free fall glider with an rc receiver in 1937, they had no idea their flight experiments would turn into the hobby we know today as radio control (rc) aviation. During the course of this blog, we'll trace the development of radio control aviation from the more bulky and primitive sets in operation during its early days to the compact and sophisticated units of today.
Along with the Good brothers, other pioneers in the late 1930's began to advance the science of radio control flying. In 1938 Leo Weiss designed an eight-channel, audio-tone reed system for remote control aircraft. Later that year a new tube design was introduced, which led to the RK-62 tube produced by Raytheon. The RK-62 tube was a miniature gas filled thyratron used as a combination high sensitivity detector and relay tube in a self quenching super-regenerative detector circuit. While single gas-tube receivers were the principal receiver units until the mid 1950's, due to their being relatively inexpensive and of simple construction, they had a major drawback as their useful life was between 4-25 hours of use, during which their characteristics would change, requiring circuit adjustments. To achieve longer lasting and more reliable control systems designers turned to hard (vacuum) tubes and multiple gas tubes instead of single gas tube receivers. Howard McEntee, a noted radio control modeler and pilot, built a twin-frequency transmitter in 1939. This was of prime importance at the time, because during the 1930's the only radio frequencies available for rc modelers were those designated for amateur radio use. To obtain an amateur radio license, the operator had to pass a test in Morse code and radio theory. Thus, amateur radio operators were able to use their knowledge and interest directly to the development of rc models. The early rc modelers were supplied with electrical components from the expanding commercial radio industry. It is difficult to know which radio control model aircraft was the first to fly, since both the United States and Germany had active radio control model programs at the time. However, two German engineering students were reported to have flown a radio control glider at a competition held in Rhoen Germany in May 1936. This was later authenticated by a magazine article in Flug and Modelltechnik22 in 1957, which was confirmed by German aviation author Matthaus Weidner in 1987.
One of the earliest publications of a multifunction, single-channel rc system was by Thracey Petrides and Leon Hillman in 1941. During World War II, the U.S. Army used rc planes (radioplanes) as artillery target drones. As a sidebar to the wartime drone communication technology, the Federal Communications Commission Order 130-C went into effect on March 1, 1946, which established the 6-meter band allocation for amateur service as 50 to 54 MHz. With training in radio theory and Morse code, an rc pilot could fly on the 6-meter band, which was almost exclusively allocated to them. In the late 1940's to mid 1950's simple on-off transmitters were in use. From these evolved rc aircraft using complex systems of relays to control a rubber powered escapement's speed and direction. A more sophisticated version, developed by the Good brothers, called TTPW encoded information by varying the signal's mark/space ratio (pulse proportional). Commercial versions of these systems soon became available. The tuned reed system brought new sophistication, using metal reeds to resonate with the transmitted signal and operated one of a number of different relays. In the late 1950's rc hobbyists had mastered tricks to manage proportional control of the flight control surfaces by rapidly switching on and off reed systems, often referred to as a skillful blipping or a nervous proportional technique.
Transistors had replaced tube systems by the early 1960's with electric motors becoming more common. The first inexpensive proportional systems did not employ servos, but used a bidirectional motor with a proportional pulse train that consisted of two tones, pulse-width modulated (TTPW). This system was driven with a pulse train that caused the rudder and elevator to flutter through a small angle, which had no drastic effect upon the performance of the rc aircraft due to the low angle caused by the pulse and the high speed of the aircraft, the average position determined by the positions of the pulse train. A more sophisticated and unique proportional system was developed by Hershel Toomin of Electrosolids Corporation called the Space Control. This touchstone system used two tones, pulse width and rate modulated to drive 4 fully proportional servos. The Space Control system was manufactured and refined by Zel Ritchie, who ultimately gave the technology to the Durhams of Orbit in 1964. Though this system was widely imitated, analog proportional radios were very expensive at the time, placing them out of the price range of the average rc modeler. Though at a greater expense to the enthusiast, single channel transmitters eventually succumbed to multi-channel units with various audio tones driving electromagnets affecting tuned resonant reeds for channel selection. Speaking of channel selection, crystal oscillator superheterodyne receivers came out in the 1960's. These receiver units offered better frequency selectivity and stability, making control equipment more capable at a lower cost. As superheterodyne receivers became more common, their reduced weight enabled more applications to be added to rc model controls.
Another benefit of superheterodyne receivers was they could be operated more closely to other rc control units, due to less frequency interference while blocking signals from nearby Citizen Band voice transmissions. In 1962 Doug Spreng developed the first digital pulse-width feedback
servo. Shortly thereafter he and Don Mathis developed and sold the first digital proportional radio, called the Digicon.
Due to the speedy growth of model radio technology during the 1960's, single-signal circuit design had become outdated with newer radios using coded signal streams, which a servomechanism could interpret. Each stream replaced two of the original control channels with the streams themselves referred to as channels. For example, an old on/off 6 channel reed control system which could drive the rudder, throttle and ailerons of an rc model was replaced with a new proportional 3-channel transmitter doing the same job. Control of all of the primary aspects of flight of a powered aircraft (ailerons, elevator, rudder and throttle) was recognized as a full-house control. A glider could be considered under full-house control with only three channels. With the full-house proportional radio control firmly established in the 1970's, in addition to the lighter and smaller sized transmitter units, a highly competitive market began to form. The two common types of radio control systems are pulse-width modulation (PWM) and pulse-position modulation (PPM) with both followed by spread-spectrum technology. Pulse -width modulation is where transmitter controls change the width (duration) of the pulse for that channel between 920 pps and 2,120 pps with 1.520 pps being the center (neutral) position. The pulse is repeated in a frame of between 10 and 30 milliseconds in length. Off-the-shelf servos respond directly to servo control pulse trains of this type using integrated decoder circuits. Pulse-position modulation is where the amplitude and width of the pulses are constant while only the position of the pulses is varied. Pulse displacement is directly proportional to the sampled value of the message signal. In recent years, Pulse-Code Modulation (PCM) characteristics have entered the marketplace, which provide a digital bit-stream signal to the receiving device instead of analog type pulse modulation. Advantages include bit error checking capabilities of the data stream and fail-safe options such as motor throttle down and other automatic actions based on signal loss. The primary disadvantage of pulse-code modulation is more lag due to less frames sent per second, due to the bit checking process. Pulse-Code Modulation devices can only detect errors and either go to the last verified position or go into fail-safe mode.
In the early 2000's, 2.4 gigahertz (GHz) began to be utilized on the more upscale model aircraft. This frequency range offers a number of advantages. Because the 2.4 GHz wavelengths are so small (around 10 centimeters), the antennas on the receivers do not need to exceed 3 to 5 cm. Electromagnetic noise, such as that from an electric motor, is not detected by 2.4 GHz receivers, largely due to the noise frequency, which ranges from 10 to 150 MHz. The transmitter antenna only needs to be 10 to 20 cm. long with receiver power usage much lower, prolonging battery life. However, the primary drawback to the 2.4 gig transmitters is they must be used in a line of sight mode since the short wave length tends to disperse at longer distances of flight. If there is a loss of power or signal for even a few milliseconds, it may take the receiver a few seconds to re-sync-time enough for a crash.
RC aviation has advanced greatly from the one channel transmitters of a few decades ago. Todays rc craft not only fly, but incorporate a number of features, some of which are now considered standard, such as elevator (horizontal stabilizer), rudder (vertical stabilizer), ailerons (roll control) and throttle control (motor speed and thrust). In recent years additional functionalities have increased the capability of radio control models, such as landing gear control, flap control, auxiliary control (additional channels), as well as miscellaneous channels, which control items such as bomb bay doors, remote camera shutter operation, gyro-based stabilization, GPS location hold and auto return home. Since 1967 most rc aircraft in the United States have utilized a 72 MHz frequency band for communication. Six of these were actually on the 72 MHz band at 80 KHz separation from each other, with one additional isolated frequency at 75.640 MHz. These remained legal to use until the 1983 FCC reform, which introduced narrowband rc frequencies, which were at 40 KHz separation from 1983 to 1991, changing to a
20 KHz separation from 1991 to present. Currently there are fifty frequencies on the 72 MHz band, but this could change due to the proliferation of commercial drones.