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RADIO CONTROL BY BOB YOUNG Radio-controlled gliders While most people think of radio-controlled aircraft as powered models there is a whole branch of radio control devoted to gliders. These can be large, fast and very acrobatic models which are great to watch and even better to fly. In this series of articles we will look at the subtle art of glider flying, including slope and soaring models. As well, we will look at some of the technology used in this very demanding aspect of R/C modelling. We will begin with the two metre class of soaring glider and continue up through slope soaring to the exotic international F3B competition class. The two metre class glider is an officially recognised class of model which is sanctioned by the Model Aeronautical Association of Australia (MAAA). The rules are deliberately aimed at producing a simple model which is ideal for introducing mod­ ellers to the rigorous art of competition glider flying. As a result, clubs run regular competitions for this class) often with a yearly point score) and they are quite popular. The simple model places few demands on the radio equipment and a This close-up view of one of the home-made winches clearly shows the high standard of the workmanship involved. Long metal stakes are used to anchor the winch to the ground, while a car starter motor drives the pulley. Power comes from a heavy-duty 12V car battery. 70  Silicon Chip basic two-channel system will suffice. These simple models usually take little time to build, unlike their ultra-complex cousins, the F3B models. These place extraordinary demands on the radio and require a high level of manual skill and the innovative application of materials technology to produce a competitive model. The two metre and F3B models are primarily intended to be used on flat fields, using hand-tow, bungee or winch-launching and therefore must be designed to withstand the quite consider­ a ble launch stress. Thus these types of models are essentially thermal or soaring models, unlike the slope soaring models which are designed to ride the wave lift from hills. The two types of models have completely different design parameters and we will look at slope soarers in due course. There are various methods of scoring for soaring competi­tions but one simple and popular method is the MAAA thermal soaring point system. Under these rules, each pilot is allowed eleven minutes working time with a maximum eight-minute flight in this period. Thus, there is sufficient time for a re-launch if required. There is no limit to the number of re-launches. Landing points are scored by measuring the distance from the model’s nose to a marked spot after the model has come to rest. One point is given for each second of flight time up to the maximum eight minutes. If the flight exceeds eight minutes then the clock starts to run backwards with one point being deducted for each second over eight minutes. If the flight exceeds eleven minutes then there are no landing points allowed. If the model lands more than one hundred metres away from the marked spot, then the flight does not score at all. It’s not meant to be easy and contests are won on mar­gins of one or two points. Any reader wishing to know more about the fine points of the rules should refer to the MAAA rules book. Reading the sky Everyone has heard about thermals but what are they? Ther­mals are bubbles of hot air which originate on the ground due to local differences in heat absorption which may be due to ground texture, colour or composition. Thus a bitumen car park located in a field covered in moist green grass will generate a local hot spot. The air over the bitumen will gradually warm up and a hot bubble of air will form over the bitumen. This will increase in size until the bubble eventually becomes large enough to break away through the colder air above it. The bubble will start to rise, increasing in size as the air pressure drops with altitude. As the bubble leaves the ground, it often sucks up grass seeds and small insects along with it. Birds quickly find these bubbles and feed off the debris, thus signalling to the alert pilot that here is a thermal to be used when you are ready. Ultimately, the bubble reaches an altitude (and therefore a tempera­ture) at which the originally warm air can no longer hold its moisture and the familiar puffy little cumulus-nimbus cloud begins to form. Thus when you look across a clear blue sky and see those little cotton puffs scattered around, you are looking at the end of a great little thermal in each cotton puff. Flying soaring models requires a very high level of under­ standing on the part of the pilot in regards to the formation of thermals, weather patterns, local conditions and all of the very subtle information available on any one field at any given time. It is no accident that the same small group of pilots dominate soaring competitions. They are usually the very experienced pilots who have learned their craft well and can read the subtle signs available to all on the field but observed only by the few. For example, birds soaring in a thermal are a dead givea­way. Small birds zooming around feeding on the debris give anoth­er clue. Wind-puffs, changes in wind direction and lots of equal­ly subtle pointers are there like Taking the strain – Peter Abel about to launch an F3B model built by Phil Bird. Note that the winch line has been stretched over his shoulder in order to take the strain off the model. signposts in the sky for the experienced glider pilot, all pointing the way to victory at competition level. It takes years to absorb and you have enough learning to deal with just this aspect of glider flying without having to cope with a complex model as well. Thus the two-metre class competition uses simple models which are relatively easy to fly. Launch methods The original and simplest launch method is hand-tow. It requires one or more runners to run across the field, leaping ditches, fences and fallen trees while keeping an eye on the model at all times to ensure the tow speed is not too high. If the tow speed is too high, the model will try to move sideways, resulting in the familiar kite-type looping circle, often striking the ground in the process if the model cannot flick off the line beforehand. The strain of excessive launch speed can also tear the wings off the model. An additional hazard is stalling of the tailplane which will result in a frantic small looping circle which can wrap the towline around the model with no hope of it releasing. This can be a sad spectacle, as you might imagine. Obviously hand tow is only for the young at heart or the very fit. It can result in excellent launches as the tow team can move around the field, keeping the model on the line until a thermal comes through, at which time the model is released. The drawback with hand-tow is that it is difficult to fly the model and tow at the same time, which means that another person or persons are required. This can lead to inconsistency in the launch if the same people are not available each contest. It also requires May 1998  71 Bob Young (foreground) watches as his own-design glider is launched for round 5 of the Heathcote Cup. According to Bob, it was his “last chance” to redeem himself but as it turned out, this flight was no better than his previous efforts. a very skilled tow team to launch a large model and into a thermal to boot and skilled people of this order are not easy to find. A more popular method is the bungee launch in which a fixed length of nylon line is attached to a length of bungee rubber. This is drawn back to a suitable tension and the model is re­ leased. The rubber snaps back to its original length and the model climbs away to launch height. This method is simple and consistent and one man can operate the system quite comfortably. However, the most popular method for contest work is the winch launch. In this method a turn-around pulley is placed at one end of the field and the towline looped though the pulley and returned to the winch. Fig.1 shows the basic con­cept. The line is fitted with a small parachute and a tow ring. The tow-ring is attached to the top of the parachute so that, during the launch, the tension on the line keeps the parachute closed. When the model is released the chute opens, slowing the decent of the line and allowing the winch to wind in the line before it reaches the ground. The rules call for the turn-around pulley to be placed 200 metres from the winch, with a maximum of 400 metres of line on the drum. Winch launches are very spectacular, especially for the larger models, and the glider climbs away vertically at a speed that takes your breath away for the first few launches. Driving these powerful winches takes a deal of skill as it is very easy to rip the wings off the model if you launch too hard. However, used correctly, winch­es result in one-man launches of great consistency and excellent height. A typical winch system consists of an automotive starter motor with solenoid and foot switch, a winch drum with ratchet and a 12V car battery. The foot-switch applies voltage to the solenoid which in turn switches the starter motor. The motor drives the drum directly and the ratchet allows the line to be recovered after the launch. The footswitch is used by the pilot to power Fig.1: winch rules call for the turnaround pulley to be placed 200 metres from the winch with a maximum of 400 metres of line on the drum. 72  Silicon Chip the winch during launch and takes a little getting used to. Depending on the wind speed, the foot-switch may have to be pulsed on and off to adjust the launch speed. There are very strict rules governing the design of the winches for it is here that real advantages can be obtained by a lavish application of technology and money. In the late 1980s, those techno-junkies, the Germans, developed a series of winches for their F3B team which were microprocessor controlled. The winches were mounted on hydraulic rams and the microprocessor sensed the ram pressure which reflected the towline tension. It then adjusted the winch speed to compensate for the wind speed. The cost of these engineering marvels? – $10,000 each and the German team had four of them! The rules now call for a winch motor of no less than 15mΩ armature resistance powered by a 12V battery of a designated size and capacity. No electronics are permitted in the winch switch­ ing, only electromechanical switches. Most winches use a simple foot-switch to actuate one or more solenoids. Parallel solenoids are sometimes used for extra reliability. Once the glider has been launched it is incumbent upon the pilot or winch operator to wind in his line as quickly as possi­ble to prevent any crosswind component blowing the line across the other winch lines on the field. In a contest there may be many winch lines laid out in parallel and there is nothing worse than attempting a re-launch with the clock ticking away, only to find that someone has laid their line right across the top of your line. Once the model has left the line, the real work begins. In still air with no lift of any kind, or worse still sinking air, it is possible to be back on the ground inside three minutes, even from an excellent launch. This means a re-launch if the pilot considers that he can improve his position by doing so. In good conditions a glider can stay up for hours but this is of no benefit in a contest. The rules call for the model to be back on the ground in eight minutes or the clock starts to run backwards. So every trick in the book is used to milk eight minutes from each flight and it is a measure of the pilot’s skill to place the model on Another of the winches is shown here and once again the high standard of the workmanship is evident. The wheels and the handle at the top make it easy to move the unit from one place to another. The launch winches come in all shapes – it all depends on the ingenuity of the builder and the materials to hand. This was the line-up for the Heathcote Cup, held back in March. There are very strict rules governing the design of winches. the ground as close to eight minutes as possible and with the nose resting on the spot landing marker. If all has gone to plan and the pilot wins the round he is given the maximum number of points (1000) and the battle begins anew. There are usually four to six rounds flown in a contest, depending on the number of competitors and the time available. Contest glider flying is good fun and calls for a high level of team effort to ensure a successful day. It can be very physical, chasing towlines and retrieving gliders, and it’s all done against a ticking clock and in a spirit of good-natured competi­ t iveness. For those who enjoy the intelligent application of technology mixed with outdoors exercise and a real and subtle appreciation of nature, I can strongly recommend it. SC May 1998  73