Getting a slope model away means reaching a reasonable flying speed asap so throw it like it just spat in your face!
So you fancy R/C model flying, but without a motor, soaring like the birds and pitting your aeromodelling wits against Mother Nature? It’s a fascinating branch of our wonderful hobby that many power fliers never experience. At its most basic level, R/C model gliding can be split into three distinct disciplines:
- Slope Soaring
- Thermal Soaring
- Dynamic Soaring
All three types of soaring are quite different, yet inextricably linked. Slope soaring makes use of air rising upwards over inclined terrain, thermal soaring exploits columns of warm air rising through the surrounding cooler air, and dynamic soaring extracts energy from two adjacent air masses moving at different velocities. The latter is a specialised form of soaring requiring strengthened models flown by expert hands and as such is a topic outside the bounds of this article.
The objective of slope and thermal soaring is to attain an elongation of the model’s flight time beyond that which would be possible in so called ‘dead air’ by flying in air that’s moving naturally upwards faster than the model is gliding downwards. This permits height gain and acquires potential energy that can be used to generate extra speed, perform aerobatics, cover more ground or just simply stay aloft for longer.
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Find a slope facing into a decent breeze and the right toy for the conditions.
The air in which we fly our models is never still. Some of it is always going up and, similarly, some of it is going down; rising air is known as ‘lift’, falling air ‘sink’. To prolong a model glider flight, lift must be exploited and sink must be avoided. The challenge to R/C glider pilots comes in finding lift and flying around sink, and in knowing where they both are and how to differentiate quickly between the two.
Slope lift requires movement of air by wind. In finding obstructions in its path such as a hillside, building, cliff face or even a dense bank of trees, such wind will endeavour to climb over it as shown in Fig. 1.
This demonstrates slope soaring in its most basic form. The topography of the obstacle in the path of the wind defines the position, strength and the extent of the usable lift for a given wind condition, as does the type, size and weight of the glider being flown.
Thermal soaring is a much trickier discipline to accomplish well, and while it’s common to experience thermals on the slope as the warmer air of the rising thermal hits the slope side, we generally regard thermal soaring as a flat field activity. It’s trickier to do well, as the position of any lift isn’t at all obvious. Also, unlike slope soaring, where on a reasonably windy day on a windward facing slope there’ll be some element of lift available, thermal lift must be sought out, recognised and tracked downwind in order for the model to gain height (see Fig. 2).
OUT TO LAUNCH
So, now we know about the two main types of R/C soaring, but how do we go about achieving it? Well, for both classes the first thing to do is to get the model into the air, and in this respect slope soaring is significantly easier. On the face of it, launching a slope soarer seems to be as easy as hand-launching out into the area of lift, and in practice it usually is. However, one of the main sources of crashes upon launching at the slope is trying to do so from a position that’s too far back from the edge of the hill. It’s not uncommon for an area of turbulence or ‘rotor’ to be at the lip of a good slope, and trying to launch the model at this point will almost certainly see it promptly crash in an uncontrolled manner, usually behind the pilot.
You can tell if you’re in the wrong spot by holding the model aloft over your head. If it feels as though it’s rocking violently from side to side, move nearer to the edge of the hill or perhaps slightly down over it until the rocking stops. Here you can release the model with a good throw to quickly achieve a healthy flying speed. No limp-wristed, half-hearted girlie throws please – throw the model as if it had just spat in your face!
With heavier aircraft or in light conditions, a throw down the hillside followed by a prolonged dive to a safe flying speed will help tremendously. We’ll expand on what to do next after we’ve looked at getting a thermal soarer into the sky.
Launching models for flat-field thermal soaring is entirely different. There are many tried and tested ways of doing this, depending on the size and type of model. The list of techniques includes, but is not solely restricted to:
A monofilament. Dacron or Dyneema ‘string’ is hitched onto a tow-hook just forward of the model’s centre of gravity. The other end is held by a towman either directly, or staked to the floor following a pass through a single pulley that the towman retains. When the towman starts to run away from the glider (into wind) the line tightens and then stretches, the model is released, flown ‘up the line’ and separated by the pilot at the apogee (highest point) of the tow. Alternative, two man pulley tows can be vicious and exert wild forces on a model’s wings.
This is similar to the pulley tow but with the pulley retained at the upwind end of the field and the winch located near the pilot, who can control the speed of the tow with a foot-operated pedal. Full-power winch launches can easily fold an unsuitable glider wing, and this launching method is usually the reserve of competitive gliding events.
- Bungee (or Hi-start)
Prior to an increase in the performance of electric powered gliders, bungee launches were certainly the most popular method for a lone thermal flier to get his glider airborne. The method is predominantly the same as the straight towline launch but with the towman replaced by a strong section of bungee elastic at the upwind end of the towline. This elastic is typically in excess of 25m (82’) long and is staked at one end. Bungee elastic can be cotton-covered (hard, fast tow), solid uncovered rubber (initially fast but with a smoother tow) or tubular surgical tubing (a smooth and steady tow requiring an initially long stretch). The model is flown up the line keeping the elastic in tension or perhaps even stretching it further. As the pull on the line is uncontrolled, bungee systems have a small parachute at the model end to resist a sudden recoil on releasing the line from the model and to help in laying the line back downwind for easy retrieval. (Fig. 3).
Like a bungee launch but using only the elastic section (often in multiple strands) and usually reserved for heavier models and launching to lower heights. Catapult launching is overly popular and often quite dangerous due to the forces involved and the proximity to the operator.
The Discus Launch is a violent affair so models are designed to withstand the forces involved.
- Electric assist
Now more popular thanks to Li-Po batteries and brushless motors, these systems are worthy of a book on their own. The initial height for thermal hunting is achieved with the model in electric flight mode, whereupon the motor is switched off and the model transitions to gliding flight. It’s a method considered by many (including me) as not true gliding, for the means to prolong flight by height gain is contained within the model. Electric assist soaring is banned at many slope sites and on all National Trust land.
Here the glider is towed behind a conventional, powered model. A fussy method of launching R/C thermal soarers, this requires good teamwork between tug and glider pilots and is popular with scale glider enthusiasts flying very large models.
- Hand / side arm / discus
Small but efficient gliders spanning up to 1.5m (60”) are lofted manually skywards before setting off on thermal search patterns. With a hand-launch the glider is thrown up like a spear or javelin. Side arm launching utilises an underarm swing of the model (held at the wing tip) to throw it skywards, and is still popular today for smaller models such as the Dreamflight Alula.
Discus Launch Gliders (DLGs) are built from composite materials designed to withstand the forces of what is a very violent but extremely efficient launching technique. The pilot will usually hold the model by a peg embedded into the wing tip of the model. Launch height can reach upwards of 60m the pilot running forward while spinning through one complete turn as if throwing a discus in athletics. He releases the model into wind as it reaches maximum rotational velocity where its very design enables a straight, arrow-like climb to be achieved to apogee. Many pilots have been injured through inadequate stretching and warming up before launching with this method! DLG is now the most popular method of launching small field thermal soarers. It doesn’t require the space needed to lay out a towline or bungee and flight patterns tend to be set closer to the pilot due to the restricted size of the models; these are well set for exploiting low level lift not usable by other types of glider. DLG models are also popular on the slope on windless days as the launching method allows a significant flight time before a landing must be made.
Great – so your soarer is now well and truly airborne, but what do you do with it now?
On the slope there’s the ever-present wind, and usually the more wind there is, the better the flying will be; maybe much more wind than the power pilot is used to flying in. Here’s the rub, of course – most of this howling gale will effectively be a crosswind in the mind of a power pilot – as he stands to fly, the wind will be in his face rather than his ear. This catches out many pilots who try to make a turn towards the slope before they’ve acquired the experience to do so. On the slope a turn of this nature will point the model downwind so it’s not unusual to see novice slope pilots chasing models blown away over the back of the hilltop. Turns towards the hillside should be avoided therefore. With the model launched and flying out into lift, a turn to the left or right can be made so that the model is flying parallel to the edge of the slope (See Fig. 4). Better, stronger ‘compression’ lift exists close in to the slope if it has a well-defined edge; smoother, more widespread lift exists further out if the slope has a bowl shape.
At the end of the first pass the pilot should turn the model away from the slope (into wind) and through 180° until it’s coming back in the opposite direction. Passing the pilot and flying in the other direction, another turn can be made in a similar manner, again into wind, to bring the model back again. These ‘S’ turns form the basic slope soaring circuit.
Experienced R/C power fliers can struggle with the elevator. One can’t just yank in a handful of ‘up’ to climb the model away from the slope. In most cases the opposite is true, and down trim may have to be added to accelerate the model and get the wing to generate more lift. Think about your elevator and airspeed management when flying a powered model dead-stick, and you’ll be in the right ballpark. Energy needs to be managed efficiently on the slope and some element of height gain followed by a dive will be required for most aerobatics, unless the lift is good.
Thermal soaring requires a totally different approach, and if the flight is to be prolonged then some lift must be found. This particular discipline is about covering, and flying through, lots of air, preserving height until lift is found, and the pilot’s focus can change from searching to tracking, exploiting as much lift as possible. What he then chooses to do with the potential energy of the height gain is up to him.
A standard thermal search pattern (Fig. 5) sees the pilot coming off the line at altitude and continuing upwind towards the boundaries of the flying site or beyond. Just as on the slope, turns are made away from the pilot with ‘legs’ running across the wind. In tracking the model back and forth with minimal control disturbance, the pilot can use its behaviour to spot the effects of thermal lift. With the elevator set at a position known as ‘best glide’, ground can be covered easily for minimal height loss, and then when in lift the pilot can re-trim with up elevator and perhaps wing cambering to a minimum sink setting. Here the model’s glide is flattest and slower, but it won’t cover ground so well. Nor would you want it too! Having just found the lift you’ll want to keep with it as long as possible by tracking it as it travels slowly downwind.
Thermals are columns of rising warm air formed by uneven heating of the air by the sun. Thermal ‘generators’ are areas that characteristically warm sooner or easier than their surrounding land mass and as a consequence warm the air in their vicinity. Ploughed fields, car parks, roads or even large roofs can be thermal generators, as can virtually any area where a spot sheltered from the wind allows the sun to warm the still air – behind a bank of trees, for instance; even the transition between long and short grass can produce minor thermal activity. Now, you may think this implies that thermal soaring can only take place on sunny days, but this isn’t the case. Only a slight warming of the air need take place to cause its density to decrease in comparison to the air around it. The resultant bubble of air can be carried along the ground for some way before it meets a trigger that will launch it skyward. Good triggers can be trees, hedgerows, buildings or hills.
The art of thermal soaring is in knowing when the glider is in lift and when it’s in the corresponding sink (which follows a thermal’s departure as cooler air rushes in to replace the rising warmer mass). Sink is much more obvious and may lead to an early landing! So how do you know when the model is in lift?
It’s easy to put the suddenly erratic behaviour of a lightweight thermal soarer down to a gust of wind or a radio glitch! In reality there’s very little that will suddenly disturb the flight path of a model glider other than some kind of air movement. That sudden lifting of a wing tip or a distinct buffet as if it’s being tossed around by the wind is much more likely to be the result of flying through buoyant air.Thermal pilots are always on the lookout for signs of lift. A sudden change of wind direction, for instance, can indicate a thermal breaking away in the vicinity. A drop in temperature, seeing small birds feeding on flies and insects lifted aloft by air breaking away, and birds circling at higher altitudes are all indicative of ongoing thermal activity. If your glider suddenly lifts a wing tip, this could be due to the rising air of a thermal. Try a smoothly controlled circular turn towards the lifting wing tip, remembering that any lift will drift downwind and expand in size as it gets higher (due to lower air pressure); watch the model carefully for signs of rising with the moving air. If in doubt try another turn. It should soon become obvious if there’s something there. If not, you may of course have been mistaken or maybe missed the thermal as it passed by. Try to search for it again further downwind if you have available height, but beware the sinking air that follows a thermal by flying quickly away from the scene at a good angle to the wind if you decide to bug out.
With practice, signs of ongoing thermal activity will become more obvious to you, as will avoidance of sink, and you’ll soon find that you can extend your flight, if just by a small amount.
Electric assist soaring is very popular yet isn't considered by some to be 'proper' soaring although it does provide a very simple way of getting a model into the sky.
WHAT GOES UP…
With your model safely flying away, the next thing to consider is how to get it back on the ground. For thermal soaring this is relatively simple, and a landing can be executed in the same manner as a power model. For slope soaring, though, there’s the ever-present problem of your model blowing away downwind. I’d love to tell you how to succeed with landing a slope soarer but the methods involved are many and dependant on the hillside, areas of lift, obstacles and, of course, the type of model. Suffice to say you should land with commitment and not pussyfoot around trying to float your glider into the gentlest of touch-downs. Land safely but with authority to avoid damage by rotor or a sudden stall.
You might think that one glider is much the same as any other, but this isn’t the case. Thermal soaring gliders tend to be light in weight for their size compared to slope soaring aircraft. Slope models tend to be more rugged as conditions in the landing areas on the slope are generally rougher than at a flat field site. Slope lift is usually much stronger than thermal lift so slope soarers tend to be smaller with higher wing loadings. Of course, many thermal soarers can perform perfectly well on the slope given suitable conditions, but this practice isn’t normally reversible.
There’s no doubting the part that Expanded Poly Propylene (EPP) has played in the development of virtually indestructible slope soaring models. The ability to crash as a result of a simple mistake yet fly for the rest of the day has helped new pilots stay up for longer. As its use results in models that are heavier than those using more traditional materials, EPP aircraft are usually more successful on the hillside. Often they take the form of a flying wing, although whilst this configuration is great for experienced pilots it can have quirky characteristics near to the stall that total novices struggle with. Certainly aircraft such as the Soar Ahead Sailplanes Wildthing, the Trick RC Zagi range, Canterbury Sailplanes Eaglet and, perhaps, the Perkins Bullet are EPP slope trainers of distinction and shouldn’t be discounted as a first-time model. Whilst not EPP, the Multiplex Easy Glider has proven to be an exceptional first model for slope abuse though it does start to get blown out at wind speeds exceeding 25mph, even with ballast on board. I heartily recommend it nonetheless.
ARTF models are widespread on the slopes and almost any thermal-orientated model will get you into the air quickly, though you may find your flying time restricted due to the higher wind speeds found on the hill tops. Do try and avoid moulded sailplanes for your first efforts – they’re easily damaged. Ripmax produce some heavier-loaded ARTF models, the Stargazer 2 and the Coyote, which are both worth a look.
For pilots preferring to build from a traditional kit there’s one trainer that outshines all others, and has done so now for many years. The Chris Foss Middle Phase has taught many of today’s competition pilots to fly on the slope, and all hold it in high regard.
Faster (heavier) models like my Jart benefit from a good chuck down the hill to start the flight safely.
For thermal soaring the key is to learn with a model that’s light, carries little inertia (crashes gently) and has reasonable handling ability. There are lots of ARTF models on the market, and again the aforementioned Easy Glider stands out. Supplied with its own towline, tow hook and even apparatus for aerotowing, you’re sure to get into the air quickly. Lightweight, large span airframes are readily available in ARTF or kit-built form, and those suitable for the novice include the Albatross 100, Dynaflite Bird Of Time, Goldberg Gentle Lady, Graupner Amigo, and, from Great Planes, the Spirit or the new Fling series.
As with any aspect of our hobby, search out and seek the advice of local fliers and follow their lead on models, flying sites and local rules. Most important of all, fly safely and enjoy yourself – and wrap up warm if you’re headed up the hills… Who knows, maybe I’ll see you up there!
- Read Andy's On The Edge column in RCM&E.
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