RC glider in thermal sailing

When looking at the aircraft model record tables, I sometimes wonder why RC glider records for the duration of the flight, sailing exclusively in thermals, are not actually set and registered. (As is well known, all previous multi-hour flights have been carried out thanks to the use of mainly slope wind.) At first glance, it may seem that under favorable meteorological conditions it is not a problem to fly in thermals for almost any length of time. In reality, however, long-term thermal sailing is more difficult than downhill sailing. However, due to the considerable popularity of RC gliders in our country, I believe that attempts at record flights in thermals could also find their supporters.
For a start, let's set the following rules for example:

Model: Wing span maximum 4 m, everything else arbitrary.
Start with a hand tow rope with a maximum length of 200 m: or a rubber slingshot of the same length, the elastic part of which is a maximum of 25% of the total length. It starts from a 100x100 m square, in which the pilot must stay during the tow and the entire flight.

Upslope flow or flying in a so-called long wave must not be used for sailing. The pilot must not use any optical devices to monitor the model or receive any information about it from another person.

Landing in the above square, in which the model must be completely flat and undamaged after stopping. In case of landing outside the square, the entire attempt is cancelled.

The time from the release of the tow cable to the stop of the model in the square counts towards the record attempt. This time can be measured either by the pilot himself or by another person.

The following discussion is intended for the less experienced, seasoned fighters of the RC V2 category competitions should skip the next few paragraphs.
The formation of rising air currents, called thermals, is based on the uneven heating of different parts of the earth's surface by solar radiation. A wet surface (dew meadow, unripe grain, water surface) heats up longer than a dry one (sand, plowed field, concrete or asphalt of an airport runway). Surfaces with a more perpendicular impact of the sun's rays also heat up faster. Above these places, a region of warmer air is formed, which expands and then "breaks away" from the ground and, due to its smaller specific weight, rises upwards. The surrounding colder air descends in its place, and after it warms up again, the whole process repeats itself. Warm air thus rises in the form of individual bubbles or - especially at higher altitudes - more continuous streams, called chimneys (Img. 1). On the ground, the rate of ascent is generally low and increases with altitude. Under average conditions, it is less than 1 m/s at heights up to 100 m. The air movement here is rather disordered and chaotic ("ground turbulence"), especially when the wind is blowing. Here, the only hope for a fit in the thermals is mainly loose models with a weight per unit area (area load) below 15 g/dm2 and with a drop of less than 0.3 m/s. At heights of 150 to 200 m, the speed of the updraft reaches an average of 1 to 3 m/s, and then it usually rises slightly (but it can also fall quite strongly depending on the state of the air balance). However, these are already speeds sufficient for sailing even heavier gliders with a load of 25 g/drrr or higher and a descent rate of 0.4 to 0.5 m/s. Real gliders can also take hold here and, in good weather, climb to below the bases of cumulus clouds, where the air rises at a speed of 5 to 6 m/s. Up to 30 m/s is reported under cumulonimbus!

Img 1: Formation of an updraft in calm and windy conditions

Let's try to recommend proven principles on the basis of the above knowledge and many years of experience with RC gliders and draw attention to possible pitfalls on the way to efficient thermal sailing.

The model for sailing in thermals should be characterized by a small descent (0.35 to 0.4 m/s), so it must be light, with an area load of 20 to 25 g/dm2, and aerodynamically clean. Good flight characteristics (maneuverability, stability, easy handling) are also desirable for flight in a turbulent environment. It must therefore be controlled by at least a rudder and an elevator; ailerons can be recommended if they are made in such a way that they do not significantly impair the aerodynamic qualities and weight of the model. A sometimes underestimated necessity is the strength and resistance of the structure, especially the wing and tail surfaces, against flutter. From the speed polar of an average RC glider, we find that to escape from an updraft of, for example, 2 m/s, it is necessary to give the model a forward speed of 13 to 16 m/s, and this does not take into account various yaws caused by turbulence, gusts of wind and incorrect pilotage. Effective braking shields are therefore very suitable for escape from strong thermals. By extending them, the descent is substantially increased, without the need to accelerate the model too much, so the structure is not stressed so much. If the model is not equipped with them, the conflicting requirements for strength and at the same time low weight can be met by using Construction materials (carbon, Kevlar composites, etc.). When covering with iron-on foils, it is sometimes forgotten that despite its flexibility, it is not able to ensure sufficient rigidity of the frame. Therefore, the surfaces must already show the necessary torsional strength before coating. You will stop seeing the model with the white transparent coating at 250 to 300 meters. An opaque orange or red color is better. Above 400 to 500 m, only enlarging the model to the mentioned four meters will help.

Terrain: It is clear from the mechanism of formation of thermals that it mainly occurs over a surface that is broken and diverse in terms of shape and color. So we will not look for it, for example, over a flat meadow or unripe grain, especially if they lie on a slope facing away from the sun. However, it is possible that even over such areas the air rises, because the thermal bubbles were blown here by the wind from places where conditions are more favorable for their formation. However, they will be higher the farther these places are from our starting point. A common case from practice is shown in Img. 2: At greater distances from airport buildings, the take-off height is no longer sufficient for the model to enter the thermal bubble. Similar situations are quite common in different variants, and therefore it is worthwhile to take into account the configuration of the terrain, the direction and speed of the wind when choosing the starting place.

Img 2: Correctly (A) and incorrectly (B) chosen starting point

Flight technique and tactics: The first prerequisite for a successful thermal flight is a sufficient starting height. At least 150 m can be considered as such in case of not very significant thermals. Experience shows that RC gliders that do not reach this height in weaker conditions descend relatively quickly and are caught only a few tens of meters above the ground, where they fly in ground turbulence for several minutes. At the same time, it would be enough to get some 50 meters higher at the start for a much longer flight. Frequent proof of this was the flights of colleagues training nearby with models of the RC V2-PM category. When they gained a height of 200 to 250 m in powered flight, they found themselves in a vast ascending field, where it would not be a problem to fly "until evening".

Getting to the required height with a hand tow or rubber sling is not a problem if the wind is right. Optimum speeds can be considered 2 to 3 m/s for light gliders, 4 to 5 m/s for heavier ones. Otherwise, the lifter will sweat a lot, and the rubber sling - no matter how strong and long - can't do it at all. Conversely, when the wind is too strong, the start is easy, but the subsequent piloting is all the more difficult. The wind speed increases considerably with height (remember: at levels 150 to 300 m it is twice or three times higher than at ground level), so the model is constantly trying to move away from the wind. If they were in a thermal bubble or chimney, they are carried away with it, disappearing quickly in height and distance. Its return during this constant battle with wind and thermals is difficult even for a more experienced pilot, it may not be possible at all with a light model. In addition, strength and flutter resistance testing occurs under these conditions, so the flight may end with a search for the model or its debris in the surrounding landscape.

However, the biggest problems with the starting height tend to be in meteorological conditions called pure thermals. With it, the wind at the ground is almost zero, only occasionally we feel gusts of different intensity, direction and temperature. Here, the most important thing is to estimate the right moment of the start. It is most advantageous to start with it at a time when the bubble of warm air has just detached from the ground. A symptom of this is a warm breeze for the previous brief lull. The subsequent onset of a cold wind gust, coupled with a reversal of its direction, signals that a thermal bubble has just risen. If we manage to continue this climb, we usually have a long and pleasant flight ahead. Piloting is easy, as even at the top there is only a weak wind, and the model usually does not even have to move too far from the starting point.

Now let's assume that we have just completed a successful launch up to a height of 150 to 180 m. It was already possible to tell in the tow if the model is in thermals. In that case, it rises faster than usual. The rope operator feels a strong pull in the cable, the switch-off - almost overhead - is more difficult and the cable chute descends unusually slowly to the ground. When using a rubber band, everything is similar, but turning it off is even more problematic. Towards the end of the lift, the glider tries to climb further due to the thermals, while the still somewhat stretched rubber - and also its considerable weight - pulls it down. Therefore, the towing hook must not be too closed and must be placed in the right place: approximately 15 mm in front of the center of gravity. However, ending the tow can be facilitated by the opening hook, controlled by the radio, or by firing the model, i.e. accelerating it, pulling it (which causes the ring to slip off the hook) and comparing the model - all with the height rudder. By performing this maneuver vigorously, up to 10 m of extra height can be gained, but it puts considerable stress on the structure, especially the main wing beam.

If the shutdown in thermals has failed, we can try to find it, for example, in the following proven ways: With the model trimmed to minimum descent, we fly straight for a while to places where, given the configuration of the terrain and the direction of the wind, it is possible to assume its occurrence. The model adjusted in this way has a small reserve of stability and reacts sensitively to each gust of air flow by swaying longitudinally. However, it is not always possible to reliably distinguish whether it is a flight into a thermal bubble or a mere gust of wind. The certainty of the estimate can only be obtained through practice.
When searching for thermals, you can also use the rules known to real glider pilots: Consistently oppose everything the model wants to do!" This is especially true in the case when we are only interfering with an updraft. For example, if the right end of the wing passes the edge of the thermal chimney, its left end may still be in still or even descending air. The result is a sharp bank and turn to the left, which is called being thrown from the climb. If it was not just an isolated impact or a gust of wind, it is necessary to immediately counter with the rudder and ailerons so that the model enters the climb. However, it must be added that the opposite is often true, especially for novice pilots: "The less you touch the controls, the better it flies." This second rule can be understood in the sense that with each deflection of the rudders, the additional resistance of the aircraft increases, which it worsens its aerodynamic finesse with all the negative consequences. That's why it's sometimes better to let the model, abv, enter the standing area alone. rather than countering very roughly ("oversteering") with the necessity of subsequent opposite deviations.

Bird probes are perhaps the most reliable way to visualize thermals. Especially larger birds of prey, storks and seagulls like to seek out thermal chimneys, in which they unerringly concentrate and glide for long minutes without bending their wings. The instructions for use are simple: Go after them immediately and try to copy their flight. If they are too high and far away, they are at least reliable evidence for us that they wear it today. If we are not flying over a landscape so devastated that you hardly meet any living creatures in it, you can really see birds flying every once in a while during more pronounced thermals.

Img 3: The technique of sailing in a wind-driven thermal chimney

What should we do if we have found a rising current and want to use it optimally? The basis of gliding in a thermal chimney is steady circling in its central area, where the rise is greatest. Ideally, however, you can circle only when steering with the rudder, elevator and ailerons, otherwise the resistance of the aircraft increases with various slips and slips. In addition, the chimney is a very turbulent environment and usually drifts in the direction of the blowing wind, so the centering of the model in it must be constantly corrected (Img. 3). If the maximum pitch is to be achieved, there is an optimal turning radius with an optimal angle of inclination for a given chimney and model. From the theoretical analysis (literature 1), findings emerge, some of which may seem paradoxical, although they are in complete agreement with practical experience:

- The narrower the chimney and the steeper the rise in it, the smaller the radius of gyration and the greater the tilt must be given to the model.
- Even in a chimney with a large diameter, it is necessary to circle in small circles and with a relatively large inclination (guide values: chimney diameter 120 m, circle radius 10 m, inclination 30°).
- If a light and a heavier model are circling in the same chimney, the lighter one must copy smaller circles with a smaller inclination, the heavier one, on the contrary, larger circles with a greater inclination. The maximum climb rate of the lighter model will be greater.
- Even when circling in the chimney, the model must be adjusted to minimum descent, and therefore flies just above the stall speed. As the pitch increases, the stall speed increases, which must be kept in mind when piloting.

It is clear from the stated principles that the optimal use of updrafts is not a simple matter, and even many experienced pilots have reservations in this regard during top sailing.

However, everything is easier when flying in stronger thermals and under favorable winds. Under such conditions, a sufficiently strong updraft can be found in just a few tens of meters and in a short time a height of 200 to 300 m can be reached in it, even without particularly careful piloting. However, we don't come across ideal conditions every day.

So we have gained considerable height, the model continues to climb and we are thinking about the longest possible flight. The first obstacle that stands in our way at this stage is the ever-decreasing visibility and thus the increasing difficulty of piloting the model, which becomes more or less free: It is very difficult to determine whether it is rising or falling, if it is properly trimmed longitudinally, what is its lateral tilt, etc. The height at which these complications begin varies roughly between 200 and 500 m and depends on the size, color and stability of the model, on the purity of the air and the pilot's vision. So we have to - even if we don't like it - leave the rising current. The most effective way is a sharp downward spiral. However, this is a hard test of the strength and rigidity of the entire model, and only he can use it without chills. who is sure of the stated properties of his creation. Studying the literature will help prevent excessive optimism in this direction (3). Height can be spent more sparingly, for example, in this way. that we accelerate the model by trimming the elevator and fly elsewhere. But rather far away, because at these heights the rising fields tend to be extensive. If that doesn't help, press down slightly for a few seconds with the elevator control lever. when the model gains speed. Soon after releasing the lever, it tries to slow down again and float to its original height, but we will prevent it from doing so by further timely suppression, etc. By repeating this procedure, we will eventually get it to the desired flight level. However, it requires patience and not to panic if it doesn't work right away.

In my experience, this, as well as any further reduction in model altitude, are critical moments in terms of achieving long flight duration. A small decrease in height will not help much, and too much does not guarantee that we will be able to follow the same upward current again, or find another one. That is why the vast majority of flights that look promising at the beginning end after 10 to 15 minutes, but there are many dozen significantly shorter ones of thirty or more minutes. The degree of risk that we are willing to take when flying at high altitudes also plays a significant role here. And that, in the form of a crash or loss of the model (absorbed by a cloud, it has already happened!), is after all greater than in the case of just recreational flying. For this reason, it will also be more reasonable to use simple, purpose-built models for long flight attempts rather than laborious and expensive mock-ups.

Last spring, with the Mimóza model (MO 1990/9) after starting from a 150-meter rubber slingshot, in pure thermals and with a minimal desire to take any risks, I achieved a time of 42:23 min:s. To the idea of a high-end glider, suitable for breaking records, the Mimóza has far, and not only because of its relatively small dimensions. And so, given the number of good models and the pilot in our country, we can perhaps hope that some of them will get carried away by the mentioned rough time and try to overcome it. A report on this, as well as any other improvement on the record of the previous one, could perhaps be published in the section Supporters of silent flight. It would therefore be a kind of variation of the correspondence competition, either in the form of an affidavit by the pilot, who would also be his own timekeeper (let's hope no one is interested in a made-up record!), or a confirmation of the achieved performance by a brief entry of the pilot and another person in the function of timekeeper , sent to the trainer for the RC Glider categories.