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Autopilots for radio controlled model aircraft Everyone is familiar with the concept of autopilots for aircraft. They take over control of the aircraft and let the pilot have a rest from the humdrum of normal flight. But the concept of an autopilot for a radio controlled model aircraft is quite different. By BOB YOUNG The difference between an autopilot for a jetliner and one for a model aircraft is that while a full-scale aircraft always has a skilled human pilot on hand to intervene, an autopilot for a model aircraft lets an unskilled operator do the flying. A real aircraft’s autopilot controls all flight functions, while a model aircraft’s autopilot controls only aileron and elevator, as we shall see. The story of how autopilots became a viable proposition harks back to early Russian attempts at biological control of crop pests. If that seems like an odd precursor, read on. The development of autopilots for model aircraft goes back a long way and probably began with the first R/C models. However, the first serious commercial attempt to create a viable low cost autopilot would probably be the device designed by the renowned American R/C pioneer Maynard Hill. Maynard set several altitude records for R/C models in the early 1960s, some reaching nearly 10,000 metres. Now the problem with high altitude flying is that of keep­ing the model in the correct attitude to achieve best rate of climb. At any sort of altitude it becomes almost impossible to tell if the model is climbing or diving, let alone tell if it is at the best climb angle. It is also very easy to tear the wings off a model if the pilot is unaware of speed build up in a dive. It is here that an autopilot becomes invaluable. Maynard’s device used radioactive isotopes, such as those found in some smoke detectors, in the sensors and these were mounted on the wing tips, nose and tail of the model. The princi­ple of operation of this device was extremely clever and relied upon the gradient of the electrostatic field surrounding the Earth. The voltage of this field diminishes with altitude and so if the model raised or lowered its wing tips or raised or lowered its nose or tail, a voltage differential was detected by the sensors. This was amplified and used to apply the appropriate corrections to the model aircraft’s flight controls. While this device worked very effectively, the radioactive components caused concern and it never found its way into popular usage. Russian development This photo shows the original crop spraying model aircraft which was fitted with an optical autopilot so that unskilled users could fly it. 4  Silicon Chip The autopilot described here had its beginnings in 1975, in Russia, when Igor Tsibizov, fresh from military service, arrived at the SKB-AM (Stu- This view of the crop sprayer shows the hole at the end of the wing spar through which the paper balls were ejected. dent’s design office for aeromodelling) and began work there. Igor was soon approached by A. S. Abashkin, the chief of a mechanisation department at the Kishinev Institute of Biological Methods of Plant Protection. His brief was in regard to the development of model aircraft to scatter wasp larvae over crop fields. It appears that the USSR was amongst the first countries in the world to recognise that the large-scale use of chemicals in farming was not a wise practice. They therefore embarked on an extensive program of biological methods of pest control and this became very large in relation to the rest of the world. In 1990 alone, the USSR claimed to have treated 27.6 million hectares with a parasitic wasp (Trichogramma) that lays its eggs inside the larvae of crop pests. Now the cost of delivering the wasp larvae was, and still is, a serious concern. Normal methods of delivery include trac­tor, aircraft and helicopters, with rates of treatment ranging from 100 to 250 hectares per hour. Divide 250 into 27.6 million and you get a lot of hours. It turns out that this Trichogramma wasp is very tiny and this means that the aircraft are flying with a very peculiar cargo, about 2kg of tiny paper balls! There have been over 70 spe- cies of Trichogramma used around the world but of these only about 20 species have been mass-reared for field use. And this in itself is a very interesting story. In the project that Igor worked on, the biological plants cultivated the larvae to the chrysalis phase, at which point they were placed in darkness, whereupon their development was suspend­ed. The transformation of the chrysalis into an adult wasp can only take place in the presence of light. The chrysalises were then packed into paper balls about 10mm in dia­ meter, without any food. Still without light, the chrysalis remained in suspended development. Just prior to being dropped over the fields, the paper balls were pierced with a sharp instrument, thus letting in sufficient light to allow the wasp to resume development. Within 24 hours of being dropped, the adult wasp would emerge from the paper ball and immediately look for a suitable host for its eggs. The eggs develop into larvae which eventually kill the host, thus achieving the pest control function. Approximately 400 balls were dropped per hectare and in tests conducted in Moldavia and Krasnodarskiy Krai, the system worked well. But clearly, the balls weigh practically nothing and so a full size aeroplane is flying almost empty, merely carrying air in the balls. With aircraft and helicopters being very expen­sive to run, it becomes obvious that there are great savings to be had using model aircraft to deliver the wasps. However the real saving comes about if the farmers can manage the model themselves and it is here that the autopilot is not a luxury but an absolute necessity. Solving the problems The development of a suitable model aircraft was a major project. It was immediately apparent that the highest level of automation was required and all of this had to fit into a model aircraft of modest dimensions. Remember here that all of this took place from 1975 onwards. Miniaturisation was only just beginning and the autopilots available in those days were con­fined to Maynard Hill’s electrostatic system and some small military gyroscopes, which were far too big and bulky. Maynard’s device proved to be unsuitable because the flying took place at an altitude of no more than 3 metres and changing atmospheric and Earth field conditions caused serious in­stab­ili­ty. And the one thing you do not need when cruising at 3 metres and 100km/h is instability! Igor went through an intense period of trying all sorts of devices, ranging from the simple to the exotic, before April 1999  5 Flying only a few metres above the crop, the plane would release hundreds of tiny paper balls over each hectare. Each paper ball was pierced at release so that the developing wasp inside could escape and release its eggs. finally settling on an optical system of sensing. In the optical system an array of four photodiodes was arranged to “look” in four directions, to the left, right, front and rear. In effect, the diodes “look” at the horizon and they sense the line between the bright sky and the darker ground. Operating principle The operating principle is quite simple. As long as the outputs from all four photodiodes are equal, the output from the autopilot is zero. If the model begins to drop its nose, for example, the rear diode will “see” the bright sky and the front diode will “see” the darker ground. This will develop an error voltage across the sensor array and this voltage is fed to the processor in the autopilot. The processor then sends a correction to the elevator servo which results in an UP elevator correction being applied. As the nose comes up, the error voltage diminishes until equilibrium is re-established. The more clearly defined the horizon is, the better the system works. Two obvious disadvant­ag­es to this system are that no night flying is possible and snow and haze can cause serious resolution problems. However, for most conditions the system works well. 6  Silicon Chip It is important to note that this autopilot will not con­trol altitude or direction (yaw). It is merely a device to main­tain level (horizontal) flight. However, this is the hard bit and it leaves the pilot plenty of time to concentrate on direction and height. The original agricultural aircraft was a rather unusual looking model fitted with some very unusual mechanics. The fuse­ lage was a basic fibreglass shell with one former upon which almost all of the mechanical components, engine, scatter mechan­ ism, main undercarriage and struts, were mounted. The engine was a standard 10cc 2-stroke, attached via a shock-absorbing mount. The fuel tank was under the engine and thus used a fuel pump. The wing was foam covered with polyester film. The wing spars were made of titanium pipe 18mm in dia­ meter and also acted as spray ducts for the paper balls. The scatter mechanism was driven from a small turbine mounted in the air collector. It took the balls, punched a hole in them and then delivered the ball along with a portion of air to the hollow wing spars. Thus the balls shot from the wing tips. An additional outlet shot balls directly downwards. Up to 2000 balls could be carried per flight. In operation, the aircraft treated approximately 100 hec­tares per hour and was flown successfully by unskilled operators; all in all a significant achieve­ment. Present day autopilots As a footnote to this biological control story, the patents to the autopilot were sold overseas and form the basis of the HAL-2100 and PA-1 autopilots now available in most model shops. It is also sold as the Graupner AP-2000. Included in this article is a photo of the latest version, soon to be marketed by Silvertone Electronics. As can be seen from the photo, there is a mushroom-shaped module and this houses the four photodiodes. The module is mounted under the model in a very precise manner. There a number of important points in the installation. Briefly, they are the alignment of the sensor head in the correct sense; ie, the front diode of the fore/aft pair is actually pointing to the front of the model. To assist in this, the housing is marked with two small arrows, one for the “+” mode and one for the “X” mode. In the “+” mode, the diodes point to the front, back and to the two sides. In the “X” option (45° to the line of flight), the photodiodes point to the left/front, right/front, left/rear and right/rear. This mode is available because it sometimes helps eliminate shading from wing-mounted undercarriag­es and mufflers on side mounted engines. A DIP switch is used to select this option. The second point in the installation is that the alignment of the diode array must be perfectly horizontal in relation to the tailplane. Finally, a trainer installation should have 2-3° pitch offset which will result in a gentle climb. It is not recommended that the module be mounted on the top of the model because bright sunlight can cause serious problems. Often the model will turn towards the Sun and possibly enter a shallow dive. The photodiodes in the array are set well back in a small tube in the sensor head moulding to provide additional shielding from bright sunlight. Before each flight it is important to check that these holes have not been blocked by dirt, grass or other debris. A blocked or dirty hole will cause a serious imbalance in the sensor input. Side mounted motors present a This is the current model autopilot which is micropro­cessor controlled. The mushroom-shaped module contains the four photodiodes which look at the horizon. This model will be marketed by Silvertone Electronics. particular problem here because the oily exhaust gas is sprayed over the sensor head; this installation is not recommended. Photodiode memory One interesting point in regard to the photodiodes is the problem of memory. If the sensors have been left in the dark for a length of time or the model has been stored, initially they may not work correctly. A simple analogy would be if a person is kept in the dark for several hours and then brought out into bright daylight. It then takes some time for that person’s eyes to adapt to the higher light levels. Thus, it is recommended that the module is left in daylight in a bright t Shop soiled bu ! HALF PRICE area for at least 12 hours after removal from prolonged darkness. This allows the module to adjust to the light levels and balance itself. It is important to ensure that all sides of the module are exposed to the same light levels. The output of the photodiode array is fed into a micropro­cessor which then applies the appropriate corrections to the two main flight controls, aileron and elevator. In essence, this is similar in action to the in-line mixer published in the July 1997 issue of SILICON CHIP, in that the output from the receiver goes into the autopilot and the servos plug into the auto pilot, making it an in-line device. However, in this case the mixing occurs between the light source inputs and the channel inputs, not between channels. As the transmitter control sticks are moved off-centre, the effects of the autopilot corrections are reduced. However, when the sticks are returned to neutral, the full effect of the auto­ pilot control corrections are applied to the controls and the model returns to horizontal flight. Thus if a beginner is using the system on a model aircraft and he gets into difficulties, then all he need do is let go of the controls and the model will return to horizontal flight auto­ mati­cally. An additional channel is required to adjust the gain (or sensitivity) from the transmitter for the most successful opera­tion of the optical autopilot. The gain control sets the amount of correction the autopilot will apply to the flight controls and the gain may be set 14 Model Railway Projects THE PROJECTS: LED Flasher; Railpower Walkaround Throttle; SteamSound Simulator; Diesel Sound Generator; Fluorescent Light Simulator; IR Remote Controlled Throttle; Track Tester; Single Chip Sound Recorder; Three Simple Projects (Train Controller, Traffic Lights Simulator & Points Controller); Level Crossing Detector; Sound & Lights For Level Crossings; Diesel Sound Simulator. Our stocks of this book are now limited. All we have left are newsagents’ returns which means that they may be slightly shop-soiled or have minor cover blemishes. SPECIAL CLEARANCE PRICE: $3.95 + $3 P&P (Aust. & NZ) Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. April 1999  7 in flight using a proportional channel. Gain control is needed to prevent the model from “over control­ling”, a situation where the autopilot applies too much correc­tion to the flight controls and quiet literally shakes the model. It is very useful in different flying conditions. The extreme low gain setting switches the autopilot off completely. Some autopilots feature a programming function that will allow a preset reduction in gain when flying without the extra gain control channel (4channel systems). There is one very important final point when setting up a model with an autopilot. It makes good sense to install a throt­tle fail-safe device such as that published in the June 1997 issue of SILICON CHIP. Because the model will now fly perfectly well by itself, a radio failure becomes a serious business. The model can fly long distances under perfect control from the auto pilot and could land goodness knows where. The throttle fail-safe will shut off the motor upon loss of signal and the autopilot will bring the model down safely in close proximity to the field. Learning to fly Learning to fly with an autopilot fitted is an interesting experience and it certainly speeds up the process remarkably as well as increasing the life span of the models and improving safety all round. In a model helicopter, the autopilot would be used to control the “cyclic pitch”, thus keeping the rotor disc horizontal. Combined with a tail rotor gyro, this device can take most of the pain out of learning to fly helicopters. Acknowledgments These cross-sectional drawings show the construction of the crop-spraying model aircraft and the hollow tubing used as wing spars and spray outlets. 8  Silicon Chip • My thanks to Dmitry Bernt, Moscow, for bringing this story to my attention and providing the translations. • To Alan Westcott of the Elizabeth Macarthur Agricultural Insti­ t ute, Menangle, NSW for his assistance in providing information and advice. • Worldwide Use of Trichogramma for Biological Control on Different Crops: A survey. Li-Ying Li. Guang­ dong Entomological Institute, Guang­ zhou. China. • University of California, Riverside. http://insects.ucr.edu/tricho/ SC tricho.html