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MODELING
Fighting in a diplomat. Tips for a modeler
Directory / Radio control equipment Modern models of air combat, both championship class and "junior", are largely similar in design and, accordingly, in concept. They fully meet the requirements of the "fighters" and differ only in manufacturing technology. However, despite the well-developed scheme, in the "junior" subclass, sometimes unusual solutions appear, the purpose of which, as a rule, are secondary problems. So in the case brought to the attention of the "fighters": the main task of our experiments was the creation of an ultra-compact model of low mass, designed specifically for engines of limited power. It was assumed that such models would be able to compete in mid-level competitions with "fighters" equipped with much more powerful ("professional") engines with comparable maneuverability and speed, but with a reduced average level of cord tension due to low weight. It seems that the experience of working on such non-traditional models and the results and conclusions obtained at certain stages of work can enrich the theoretical and practical knowledge base of modellers. In addition, familiarity with the design and technological findings and errors obtained in the creation of ultra-compact "fighters" will also help in the design of models of other classes and types. First of all - about the tasks that were set when designing non-traditional models. As mentioned above, first of all, it was necessary to significantly reduce the mass and area of the wing, which made it possible to achieve high speed even taking into account the limited power of the engine. At the same time, it was important to preserve such properties of "fighters" as their reliability and ease of starting engines, as well as the reliability of their behavior in any atmospheric conditions at any point in the flight hemisphere. The latter requirements are especially important in terms of operation by schoolchildren who do not have sufficient experience in piloting cord models. Good piloting on takeoff of a "fighter" with a limited wing span is achievable only with maximum compensation of the jet moment from the rotation of the propeller, otherwise, at low forward speed, the model vigorously raises the outer half-wing and goes into a circle with a loss of cord tension. On the model offered to readers, this problem is solved by deepening the engine into the wing. In this case, the propeller approaches the leading edge of the wing and the flow swirled by the propeller is immediately straightened by the wing plane. In this way, most of the reactive torque is compensated. In favor of improving the tension of the cord both on takeoff and in aerobatic mode, there is a difference in the spans of the half-wings, as well as the removal of the elevator, which on the models of the "flying wing" scheme simultaneously functions as a flap to the outside of the engine axis. When the rudder is deflected, there are two side effects that are useful on these compact models: the lift on the outer half-wing is reduced (the “fighter” tries to roll onto the outer half-wing, trying to get out of the circle). At the same time, the aerodynamic drag of the same half-wing also increases. As a result, the model can exit the circle, but in a perpendicular plane. However, when performing smooth figures, both half-wings work equally effectively, due to the equality of their areas.
The choice of the direction of the axis of rotation of the elevator must be recognized as unsuccessful. When working in both directions in conditions of blowing, an aerodynamic moment of force appears on it, directed in a circle. However, calculations have shown that the magnitude of this force is negligible compared to other factors; so the wing was chosen for purely technological reasons (with a different frame design, it would be more profitable to put the rudder perpendicular to the direction of flight or even with a wing in the opposite direction). Preliminary drawings showed that with a quite acceptable value of the specific load on the bearing area, such a compact model is obtained for the MARZ-2,5 engine (or another of a similar type) that it can be easily placed without disassembly in a diplomat-type suitcase. Subsequently, this greatly simplified travel on flights. The construction of the first version of the "fighter" is not difficult for modellers of any level. Therefore, dwelling on the technology of its manufacture does not make much sense. I will only note: to complicate the conditions of the experiment, the motor was boosted to the level of an average quality engine of the KMD type (when operating at high speeds with a light propeller) and at the same time greatly lightened. Centering was set within the generally accepted boundaries; the deflection angles of the small-sized elevator are increased due to its small arm and... confidence: rich experience in piloting extreme devices will in any case allow you to cope with this technique. The very first flights of the unusual "fighter" gave amazing results. With a standard line length of about 16 m, the takeoff of such a small and light model was perfect, regardless of the direction and strength of the throw. Further, the "fighter" quickly picked up speed, and ... something incomprehensible began to happen in horizontal flight. It seemed that someone was systematically pulling either the upper or the lower cord: the model was constantly "dancing", and its flight had to be corrected by a significant deflection of the rudders. On the figures, her behavior stabilized a little, but after returning to level flight, the effect arose again. The thought immediately appeared: the instability is connected with an excessively rear centering. Therefore, to increase the mass of the bow, a single-blade propeller with a counterweight was mounted and at the same time the elevator was replaced. With the same area, it became three times lighter, and the gap between the rudder and the trailing edge of the wing doubled. A single-bladed propeller, among other things, has almost half the moment of inertia, which promised a decrease and the possible influence of the gyroscopic moment. As a result of improvements, the centering has moved forward by almost 10%. Nevertheless, the result of the improvements turned out to be zero: the model flew exactly the same as at the beginning. On takeoff and acceleration - perfect, after picking up speed - you can't imagine worse. I must admit, a puzzle for a person who is well acquainted with aerodynamics is still a puzzle. For some time, the "fight" was postponed, since it was necessary first of all to understand the reasons for what was happening. And at this stage, this was the biggest problem. "Enlightenment" came much later ... It turned out that the whole point was not at all in aerodynamics, but in the control system. The secret was the non-parallelism of the cables suitable for the control rocker. Translated into normal conditions, a complete analogy of a rocking chair with a "reverse sweep" was created. And this one has one hidden feature, which is useful for all cordovans to know, since this effect is manifested on all models without exception, especially heavy and high-speed ones. If you carefully consider the kinematics of this type of rocking chair, it becomes clear that when it deviates from the neutral in any direction, it redistributes the action of the forces from the tension of the cord threads. The result is a different tension of the threads themselves, and the result is their uneven elongation. Since even with slight tensions at standard diameters and lengths of cords (and even more so of twisted cables), the absolute value of the total stretching is calculated in centimeters, with the "reverse sweep" of the rocking chair, the effect of throwing the rudder in the direction deflected by the pilot occurs. Moreover, it manifests itself even with small deviations from neutral. Therefore, it becomes almost impossible to keep the model in level flight. And most importantly - all this is completely independent of the degree of stability of the aircraft itself! It is useful to know that a rocking chair with a "forward sweep", which, in his most successful period of life, was actively used and promoted by the famous American aerobatic pilot Denis Edemsin (he claimed, citing kinematic diagrams, that such a system dramatically increases controllability and improves its character), in fact, it has back effect. The redistribution of the shoulders on it is such that, on the contrary, when deviating from the neutral, forces arise that, due to the difference in the tension of the cord threads, return the rocker to the neutral position. A careful analysis of the graphs and diagrams cited by Edemsin proved, if not erroneous, then at least incorrect conclusions. On a special experimental model, built to test the influence of the "sweeps" of the rocking chair, all variants of the questionable part were sequentially mounted. Test flights fully confirmed the theoretical calculations: "reverse sweep" led to absolute instability of control and flight of the model with any, even excessively forward centering, and "forward sweep" had the effect of pronounced "blunting" at critical centering, not to mention the traditional position of the center of gravity . General conclusion: in all cases, it makes sense to install straight rocking chairs with the holes for the cords and for the central axis on the same line. All measures to improve stability or controllability should be carried out solely due to aerodynamics or balancing of the model itself, but not due to the rocking chair (more precisely, not due to its "sweep"). Attempts to "blunt" an unstable machine by introducing a "forward sweep" of the rocking chair are also doomed to failure: in fact, sluggish control only reduces the effective gear ratio, leaving the model itself unstable in flight and very sensitive to wind gusts. Once again I will clarify: "reverse sweep" not only, as it were, increases the gear ratio of the rocking chair, but also significantly, to an unacceptable degree, changes the nature of the transfer of forces.
When the reasons for the failure with the first compact "fighter" became clear, a second "diplomatic" model was created, but already designed for the MK-17 engine. During the time it took to analyze the kinematics of the control system, new ideas appeared, which were embodied in a new design created specifically for competitions. In addition to increased speed and good maneuverability, the second version of the "fighter" also had to provide a very high take-off reliability without the desire to go into a circle and further increase the likelihood of capturing and cutting off the tape of the opponent's model. The latter was achieved by a sharp "skew" of the wing, as a result of which there was a redistribution of bearing areas between the left and right half-wings (relative to the axis passing through the propeller shaft), which was beneficial for stretching the cord. And the cut of the tape was now carried out not only when it hit the rotating propeller, but also in the case of capture by the beveled leading edge of the left half-wing. The tape, leaning over the edge, independently moved to the center of the "fighter" and there it was chopped with a screw or torn, hitting a drain pipe or engine mount. It should be noted that the proposed solution complies with the rules prohibiting having special devices for cutting off the tape: in our case, there are none, and a break due to hitting the motor mount is quite probable even in conventional equipment with a certain manner of operating the pilot with cords after the tape is bent over the leading edge. We only increased the probability of such a cut-separation, bringing the attacking width of the gripping zone to almost 300 mm (together with the diameter of the screw). In the latest version, the “fighting” has become even simpler and, like the first one, fits into the “diplomat”, however, with the engine removed. Flight tests gave good results in all modes and under any atmospheric conditions. Of course, with reliable operation of the "heart" of the model - the engine. Author: V.Tikhomirov
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