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RADIO CONTROL BY BOB YOUNG The art of slope soaring This month, we will look at some aspects of slope soaring or the art of staying up all day with no motor. What you need is plenty of wind, a spectacular hillside and quick reflexes. It was 23:59 on a cold night in Kiama NSW on a date long since forgotten; probably somewhere around late 1969. Any resi­ dent of Kiama NSW foolish enough to venture outside on that cold, blustery night and who bothered to glance in the direction of Saddleback Mountain would have been startled to see a brilliant searchlight burst forth from the top of the ridge. Had they held their gaze steady on the cone of light, they would have seen at precisely 12 midnight, a model aircraft leave the ridge and weave backwards and forwards along the ridge, transfixed in that beam of light. Had they been robust or foolhardy enough to watch this eerie spectacle for another 25 minutes or so, they would have seen the light blink out as the cloud base descended below the ridge. This enveloped the model, searchlight and ground-crew in a very wet blanket indeed. So ended Bill Marden’s second attempt at the Australian sailplane duration record. The model flew off inside the cloud, we knew not where. Despite an intense aerial and ground search, it did not show up that following day. We finally found it several weeks later, about a kilometre behind the ridge. Have a look at the photo on this page. It was taken at Otford, just near Stanwell Tops, a popular hang-glider location. That cliff is a very civilised baby compared to Saddleback. Any Photo 1 (left): Dennis Bailey hand launches a slope soarer out into the wild blue yonder at Stanwell Tops on the NSW south coast. A 2-channel radio is used and there is no rudder control, turns being by ailer­ons only. August 1998  67 cessful outcome. We had waited for months for a southerly buster to come through on a Wednesday or Thursday so that the three day tail-out would give us ideal conditions on a Friday or Saturday night. On the third day after a southerly, the wind settles to a strong, steady blow which is quite predictable in strength and direction for at least one to two days. World record In Germany, the winds blow up the mountains for months on end. In the 1920s and 1930s, full-size sailplane records tumbled one after the other as pilots pushed themselves to the limits of physical endurance. In the end the record stood at something like 72 hours and people were killing themselves attempting to better these times. This type of record was finally abolished because it simply became a test of a pilot’s physical endurance rather than a test of his flying skill. On the 30th of September, 1978 Brian Laging of Victoria established the current Australian R/C glider endurance record which still stands at 28 hours and 28 seconds. Such is the art of slope soaring, one of the most interesting branches of the sport of R/C flying. What is slope soaring? Photo 2: Dennis Bailey poses with his slope soarer. It has a wing span of 1.3 metres and the short stubby wings are fitted with ailerons. mistake or bad luck really tests one’s physical fitness. Climbing up and down these sorts of cliff faces is not for the cardiac-challenged (newspeak for an unfit slob), so you very rapidly learn that the best place to land your model is at the top of the hill, not the bottom. We scoured Saddleback for days looking for that model and thought nothing of it; up and down that mountain a half a dozen times. Oh to be young again! Months earlier, Bill had successfully set a new Australian R/C sailplane duration record of 11 hours, 8 minutes and 1 second at the same 68  Silicon Chip site. That was a purely daylight flight, taking off at dawn and flying until the wind ran out 11 hours later. The model was built and flown by Bill Marden. My part in the opera­tion was to provide the technical support. This was in the days when I did everything in my power to help modellers set records. The midnight takeoff was necessary because this time we were going for 24 hours or better. There was a good chance we would have got it too, if the cloud had not come down so low that night. But that is the nature of record attempts. Luck, skill and preparation all have to come together for a suc- Slope soaring makes use of the energy contained in the stream of air which is deflected upwards when winds encounter a barrier to their natural direction of flow. Fig.1 shows the concept. As the air near the slope has to travel over a longer path than the higher smoother wind-stream, it speeds up as it nears the crest and finally rolls over the crest and breaks up into turbulent, highly disturbed air on the lee side. This is often exacerbated by obstacles on the top of the rise such as trees, houses and so on. So not only do we get an upward deflection from the barrier but we also get an accelerated airstream into which we may launch our heavier-than-air machine; the stronger the wind, the greater the lift. Provided the wind keeps blowing, there is an almost unlimited amount of energy available to overcome the force of gravity. From this simple fact arise the vast differences between aircraft designed for winch-launching and slope-soar- ing and indeed the attitudes of the people who fly them. The slope brigade are the otters of the soaring community. These are fun creatures who spend their days gliding up and down the slopes, cavorting amongst the trees and fighting with each other in wild aerobatic duals, often using specially designed combat aircraft which can stand the impact of a full blown crash. Not for them the concerns that plague the flatlanders. The problems of launch stresses, wing efficiency, variable geometry and sniffing therm­als are all minor concerns. The hill dwellers have vastly different concerns. Can you land this model on a cluttered hilltop in a 20-knot breeze? Does it do three rolls in one millisecond or less? Can it loop inside the diameter of a 20-cent piece? Will it withstand the impact of flying into the slope at full speed or a midair collision? Such are the concerns of the aerial otters. This is not to say that the highlanders do not concern themselves with matters aerodynamic but with almost limitless energy available to keep the aircraft flying, the level of aero­ dynamic finesse required is shifted into other areas. For the slope brigade, manoeuvrability is more important than efficiency and this results in a markedly different approach to the design of their aircraft. Typical slope soarer Photo 1 shows a typical aerobatic slope soarer. It is small and compact with the most noticeable feature being the low aspect ratio wing which is usually fitted with ailerons. Quite often the model has no rudder control, relying entirely on the ailerons for directional control. A high rate of roll calls for a short, stubby wing fitted with ailerons. A small diameter loop calls for a short moment arm (distance between the wing and the tailplane). Inverted flight demands a symmetrical or semi-symmetrical wing section and no dihedral. All of these features are in complete opposition to the types of concerns we have discussed in the last two or three columns. Structurally, the models are vastly different. There is no need for the exotic materials used in the high aspect ratio, winch-launched models. The short, broad chord wing is very robust Fig.1: slope soaring depends on the considerable lift produced by wind being forced up and over an obstacle. On the lee side of the hill there is danger though, because of turbulence and down draughts. even when built from conventional materials and with no launch stress to worry about, they do not need to be anywhere near as strong. The handlaunch used in slope soaring places no structur­al demands on the airframe whatsoever. The model shown in photo 1 is an own design (Pylobat) and belongs to Dennis Bailey of the NSW Slope Soaring Association (NSWSSA). The NSWSSA fly at Otford near Stanwell Tops, just south of Sydney and can be contacted on (02) 9547 2277. The Pylobat has a wingspan of 1.3 metres and weighs in at 650-750g, depending on the amount of ballast carried. This type of model requires a fairly strong breeze to per­form at its best. For those days when Dennis desperately needs a flying fix and the weather is not very cooperative, he keeps a second model in the car. This is pictured in photo 3 and for those who have followed the series so far, is instantly recognis­able as one of our old friends, the 2-metre glider. In this case it is a modified Spirit built from a kit. Weighing in at 850g, here is a model that is concerned with aerodynamic efficiency. In light winds, this model will outperform its more aero­batic counterparts but with the penalty of reduced manoeuvrabil­ity. Notice that it is fitted with the conventional 2-channel, rudder/elevator control configuration. Slope soarers make great use of 2-channel systems and often the lack of rudder control is dictated by this factor. 2-channel systems are popular with the slope fliers because they are cheap and therefore more expendable than the four and 6-channel sys­tems. Due to the nature of the flying sites, there is a consider­ able risk of lost aircraft. A sudden change of wind direction, a drop in wind speed or a radio failure can result in the model being lost in dense undergrowth or worse still, the ocean. Hazards of the ocean Ten minutes in the ocean with the battery still connected will result in the copper on the circuit board being electrolyti­ c ally removed and the receiver is useless thereafter. If you do fly near salt water, carry a bottle of fresh water and a bottle of methylated spirits with you. Disconnect the battery as quickly as possible, wash the electronics immediately and thoroughly in fresh water and then with metho. The spirits will absorb the fresh water and evaporate off, leaving the electronics clean and dry. But you must be quick. Whilst the emphasis may be on simple and inexpensive mod­els, there are other streams of slope activity which call for vastly different types of approaches to the model design. One which we have already mentioned is the endurance slope soarer. This type of model does need to concern itself with aerodynamic efficiency for during a period of 24 hours or August 1998  69 Photo 3: For light wind conditions, when the lift is insufficient for slope soarers, enthusiasts often use a standard 2-metre glider, such as pictured here. Now where did that first glider get to? more, all types of wind speeds may be encountered. For example, during Bill Marden’s 11-hour flight, he took off in 20 knots of wind, suffered through a period of gusting winds of up to 50 knots that nearly drove the model back over the ridge and finally had to land because the wind dropped to almost nothing in the afternoon. Obviously a very clean, variable geometry aircraft would have a distinct advantage in this type of task. Scale models Another very popular branch of slope soaring has evolved around scale models. Here we see not only scale models of popular full-size gliders, but scale models of aircraft such as Spit­fires, Mustangs, MIG-15s and so on. In Europe where the big model has taken off in a big way, we see monster gliders that look like Boeing 747s and other airline types. Modellers are ingenious and it nev70  Silicon Chip er fails to amaze me what they will come up with next. One final note before closing: during the foregoing discus­sion it may have appeared that slope soaring is a snap and that there is little skill compared to the problems facing the flat-earthers. This is not the case. On an ideal slope it is true that it is fairly easy to stay up for long periods. However, all slopes are not equal. Some are more equal than others (if you will forgive the borrowed phrase) and the less equal can be pretty daunting. Rocky outcrops, gullies, trees and other disturbances can interfere with the airflow and create pockets of turbulence, downdrafts and heavily disturbed air. Likewise, warmer or cooler conditions will result in more or less thermal activity and so on. Thus, all of the subtle signs encountered in thermal soaring are there for the experienced slope pilot but with added spice. The slope pilot has to learn to land the model in high winds, often on rock-strewn hillsides, with the model continually trying to lift off the ground and keep flying. If the unwary or tyro pilot is foolish enough to let his model drift behind the hill, then all hell breaks loose as the model enters the turbulent lee-side, downward moving air. No, never make the mistake of thinking slope soaring is easy. It is a highly skilled art, just as all types of R/C flying are a particular form of the overall art. Each branch of the hobby has its master craftsmen and women who have taken the time and trouble to study their chosen field very carefully and who now exhibit a level of understanding that makes it all look so easy. But look at that lead photo once more and imagine yourself sitting on that hilltop on a glorious day, with a model sailing serenely above you in a clear blue sky. I can think of few better SC ways to spend your day. Bob Young is principal of Silvertone Electronics. Phone (02) 9533 3517. Their web site is at: www.silvertone.com.au