The AkaModell team of the University of Stuttgart won the Air Cargo Challenge, held early August at Oto Airbase in Portugal. This was the second win in the competition for the team.
The Air Cargo Challenge is an aeronautical engineering competition that is held every two years and is primarily directed to aeronautical and aerospace engineering students.
The main objective is to design and build an electric powered and radio controlled aircraft that can fly with the highest possible payload according with the rules established in the competition regulations, which vary in each edition. The team’s score is not only given by the performance demonstrated in the flight competition part, but also by the technical quality of the project, through the evaluation of the design report and drawings. From 2003 onwards, the competition grew to an international level and the winning team got the possibility of organizing the next edition.
The wings’ airfoil was specifically developed for the Air Cargo Challenge, two airfoils have been used successfully since AkaModell participated for the first time in 2007: the S1223 designed by Michael Selig (plus more or less promising derivatives) and, in 2009, AkaModell's two element airfoil ACC09-SK33. Multiple element airfoils were first mentioned by Lachmann and Handley Page in 1921 as a means to improve maximum lift compared to conventional airfoils. Nowadays, multiple element airfoils are used commonly in commercial and military aviation to reduce take-off and landing runway length. However, at Reynolds numbers relevant for model airplanes, little research has been conducted up to now. The development of the two element airfoil was the topic of the diploma thesis of an ACC 2009 team member. Preliminary design was performed using the inverse design routines of MSES. Flow around airfoils in the relevant range of Reynolds numbers (125,000 to 250,000) is governed by laminar separation bubbles, which can only be modeled by approximation. Separation bubbles on main element and flap constitute a highly non-linear system leading to exponentially growing modeling errors. Thus fine tuning of slot width, flap angle and overlap had to be done by means of wind tunnel measurements. Since polars for both airfoils have been measured at the model wind tunnel of the Institute for Aerodynamics and Gas Dynamics (IAG), the resulting data set is more reliable than calculated polars. The ACC09-SK33 provides nearly 50% more lift than the S1223 at the same free stream conditions, whereas the drag coefficients of both airfoils are of the same order of magnitude.
The bending load of an airplane wing is nearly carried solely by the wing spar. For calculation purposes it is sufficient to take only the wing spar into consideration. The wing’s root bending moment resulting from the lift can be calculated by integrating the lift distribution along the wing span. For man-carrying airplanes an elliptical lift distribution is often assumed to simplify calculations. Since the present lift distribution is far away from being elliptical, we assumed a constant lift coefficient cl (y) for the load calculation, which adds an extra safety margin together with the actual local wing chord c (y). Two load cases are considered. For the first case, a load factor of n = 4.5 at a maximum take-off weight of 17 kg is assumed in flight to account for emergency maneuvers and gust loads. The second case is the structural validation test at the competition, when the plane has to withstand the load from being supported at the wing tips being fully loaded. A concentrated load of 200 N acting at the center of the wing is assumed for this load case, including a safety margin.
The bending loads are distributed over a sophisticated spar construction in combination with a D-box structure, which is responsible for taking most of the torsional loads applied to the wing. The wing spar consists of the upper and lower spar cap and a shear web. The spar caps are supposed to sustain only to tension and compression loads, while the shear web is supposed to withstand the shear loads. Ruben Buehler, head of engineering at AkaModell Stuttgart, says: ‘The wing´s torsional stiffness had to be as high as possible. Otherwise a torsional wing deformation could occur during flight and decrease the outer wing sections angle of attack and thereby its lift. Additionally, wing flutter could become an issue. The aim was to build a wing as stiff as possible at a very low weight, which was only possible due to the extensive use of TeXtreme® Spread Tow high modulus carbon fabric 60 gsm in much of the parts,. Rohacell is used as sandwich core material for the wing skins as well as for the shear web. The additional flaps are permanently aligned to the main wing, while the outer wings have moveable ailerons for lateral control. The winglets have been built with a hot-wired EPS foam core, vacuum-bagged with glass fabric layers without the aid of additional molds. For joining the five wing sections, carbon fiber joiners with a Rohacell core were laminated in milled molds.
The fuselage accommodates the RC equipment and the propulsion system. One challenge was to design the fuselage in two parts, since it is not possible to fit the long fuselage into the transportation box in one piece. The fuselage has been divided right after the wing. To put the two pieces together a hexagonal, tapered connection was integrated into the fuselage. Hence, a self-retaining, non-twistable connection is guaranteed. The tail boom and the engine support is constructed as a mainly tubular part, employing high modulus UD plies for bending and TeXtreme® Spread Tow carbon fabric 60 gsm oriented in ± 45° for torsional stiffness. Therefore, the bladder molding technique was used to achieve high fiber volume fractions.
The horizontal and vertical tail was built conventionally with ribs and spars made of balsa wood, which was, despite the large surface area, still the most lightweight solution. Specific areas have been additionally reinforced with pultruded semi-finished carbon profiles. The covering was made with an iron-on polyester film.
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