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IN Part I wc saw how structural flexibility could introduce aerodynamic forces which might eventually lead to instability, or to the complete nullification of a desired aerodynamic effect. The phenomenon of flutter presents another problem in stability, but in this case an oscillatory instability is threatened. It must be realized at the outset that flutter is no mere resonance phenomenon such as the bad vibrations a motor‐car may exhibit at a particular engine speed. Flutter is a vibration in which energy is extracted from the airstrcam to help build up the amplitude, and a catastrophic failure can easily occur within a second of the start of the flutter.

Computational efficiency is always the major concern in aircraft design. The purpose of this research is to investigate an efficient jig-shape optimization design method. A new jig-shape optimization method is presented in the current study and its application on the high aspect ratio wing is discussed.

First, the effects of bending and torsion on aerodynamic distribution were discussed. The effect of bending deformation was equivalent to the change of attack angle through a new equivalent method. The equivalent attack angle showed a linear dependence on the quadratic function of bending. Then, a new jig-shape optimization method taking integrated structural deformation into account was proposed. The method was realized by four substeps: object decomposition, optimization design, inversion and evaluation.

After the new jig-shape optimization design, both aerodynamic distribution and structural configuration have satisfactory results. Meanwhile, the method takes both bending and torsion deformation into account.

The new jig-shape optimization method can be well used for the high aspect ratio wing.

The new method is an innovation based on the traditional single parameter design method. It is suitable for engineering application.

Ground effect is one of the important factors in the enhancement of wing aerodynamic performance. This study aims to investigate the aerodynamic forces and performance of a flapping wing with the bending deflection angel under the ground effect.

In this study, the wing and flapping mechanism were designed and manufactured based on the seagull flight and then assembled. It is worth noting that this mechanism is capable of wing bending in the upstroke flight as big birds. Finally, the model was examined at bending deflection angles of 0° and 107° and different distances from the surface, flapping frequencies and velocities in forward flight in a wind tunnel.

The results revealed that the aerodynamic performance of flapping wings in forward flight improved due to the ground effect. The effect of the bending deflection mechanism on lift generation was escalated when the flapping wing was close to the surface, where the maximum power loading occurred.

Flapping wings have many different applications, such as maintenance, traffic control, pollution monitoring, meteorology and high-risk operations. Unlike fixed-wing micro aerial vehicles, flapping wings are capable of operating in very-low Reynolds-number flow regimes. On the other hand, ground effect poses positive impacts on the provision of aerodynamic forces in the take-off process.

Bending deflection in the flapping motion and ground effect are two influential factors in the enhancement of the aerodynamic performance of flapping wings. The combined effects of these two factors have not been studied yet, which is addressed in this study.

MUCH reference is made in the aeronautical field to the flutter problem and the subject is receiving the attention of many persons engaged in research, testing, and design. Many aeronautical engineers are well acquainted with some aspect of the problem, and although only a few are concerned with its several phases it is safe to say that all aeronautical men regard it with some degree of interest. It is fitting, therefore, that although it has been adequately treated by many authors from other points of view, a statement be here made summarizing the flutter problem as one of the aeroplane designer. In order that the exact nature of this problem be appreciated it is first necessary that a few of the fundamentals be reviewed.

WE concluded Part II of this series with the remark that a different outlook is needed for problems of control surface flutter than for those of wing flutter. There are two reasons for this. Wing flutter must be investigated carefully early on in the design of an aircraft so as to provide a safe aircraft without a severe weight penalty, whereas the weight penalty of avoiding control surface flutter is usually small, although not negligible, and modifications can often be made at short notice, so it is important to make a full investigation as late as possible before flight when all the data are available in a reliable form. The second reason is that with wing flutter, as with aileron reversal and divergence, it is usual to think of safety margins in terms of forward speed or possibly wing torsional stiffness; with control surface flutter, on the other hand, quite different types of safety factor become the rule.

The separation of energy conversion and propulsor is a promising aspect of hybrid-electric propulsion systems, allowing for increased installation efficiencies and setting the basis for distributed propulsion concepts. University of Stuttgart’s Institute of Aircraft Design has a long experience with electrically powered aircraft, starting with Icaré 2, a solar-powered glider flying, since 1996. Icaré 2 recently has been converted to a three-engine motor glider with two battery-powered wing-tip propellers, in addition to the solar-powered main electric motor. This adds propulsion redundancy and will allow analyzing yaw control concepts with differential thrust and the propeller-vortex interaction at the wing-tip. To ensure airworthiness for this design modification, new ground vibration tests (GVTs) and flutter calculations are required. The purpose of this paper is to lay out the atypical approach to test execution due to peculiarities of the Icaré 2 design such as an asymmetrical aileron control system, the long wing span with low frequencies of the first mode and elevated wing tips bending under gravity and thus affecting the accuracy of the wing torsion frequency measurements.

A flutter analysis based on GVT results is performed for the aircraft in basic configuration and with wing tip propulsors in pusher or tractor configuration. Apart from the measured resonant modes, the aircraft rigid body modes and the control surface mechanism modes are taken into consideration. The flutter calculations are made by a high-speed, low-cost software named JG2 based on the strip theory in aerodynamics and the V-g method of flutter problem solution.

With the chosen atypical approach to GVT the impact of the suspension on the test results was shown to be minimal. Flutter analysis has proven that the critical flutter speed of Icaré 2 is sufficiently high in all configurations.

The atypical approach to GVT and subsequent flutter analysis have shown that the effects of wing-tip propulsors on aeroelasticity of the high aspect ratio configuration do not negatively affect flutter characteristics. This analysis can serve as a basis for an application for a permit to fly.

The presented methodology is valuable for the flutter assessment of aircraft configurations with atypical aeroelastic characteristics.

Biomimetic study existing natural biological elements to produce engineering products with similar performance and abilities. The purpose of this paper is to highlight biomimetic studies to produce a new type of airplanes: adding remiges, bending ability and flapping mechanisms.

The used methodology was to thoroughly investigate the literature, to define the proper endurance and fatigue parameters, to perform a series of numerical studies and report improvement percentages relevant to defined parameters.

By adding remiges and the bending mechanism, the authors managed to reach – numerically – the preset desired structure goal. Efficiency increased using remiges with less drag force. In addition, with the help of the bending wing technique, the drag force was improved. The flapping mechanism showed high vibration rates. Last but not least, applying multiple winglets gave a better optimization of the endurance parameter.

Research is conducted at a university without any research facilities. No laboratories exist, and acquiring research papers is mostly difficult and costly.

The research study is original in the sense of its numerical investigation. Proposing biomimetic was at the heart of the airplane invention and cannot be stated as an original contribution. Rather the field has been recently abandoned, and performing this major literature review can be considered as original in a sense it summarizes recent to somewhat old advancement.

The purpose of this paper is to consider divergence of composite plate wings as well as slender wings with thin-walled cross-section of small-size airplanes. The main attention is paid to establishing of closed-form mathematical solutions for models of wings with coupling effects. Simplified solutions for calculating the divergence speed of wings with different geometry are established.

The wings are modeled as anisotropic plate elements and thin-walled beams with closed cross-section. Two-dimensional plate-like models are applied to analysis and design problems for wings of large aspect ratio.

At first, the equations of elastic deformation for anisotropic slender, plate-like wing with the large aspect ratio are studied. The principal consideration is delivered to the coupled torsion-bending effects. The influence of anisotropic tailoring on the critical divergence speed of the wing is examined in closed form. At second, the method is extended to study the behavior of the large aspect ratio, anisotropic wing with box-like wings. The static equations of the wing with box-like profile are derived using the theory of anisotropic thin-walled beams with closed cross-section. The solutions for forward-swept wing with box-like profiles are given in analytical formulas. The formulas for critical divergence speed demonstrate the dependency upon cross-sectional shape characteristics and anisotropic properties of the wing.

The following simplifications are used: the simplified aerodynamic theory for the wings of large aspect ratio was applied; the static aeroelastic instability is considered (divergence); according to standard component methodology, only the component of wing was modeled, but not the whole aircraft; the simplified theories (plate-lime model for flat section or thin-walled beam of closed-section) were applied; and a single parameter that defines the rotation of a stack of single layers over the face of the wing.

The simple, closed-form formulas for an estimation of critical static divergence are derived. The formulas are intended for use in designing of sport aircraft, gliders and small unmanned aircraft (drones). No complex analysis of airflow and advanced structural and aerodynamic models is necessary. The expression for chord length over the span of the wing allows for accounting a board class of wing shapes.

The derived theory facilitates the use of composite materials for popular small-size aircraft, and particularly, for drones and gliders.

The closed-form solutions for thin-walled beams in steady gas flow are delivered in closed form. The explicit formulas for slender wings with variable chord and stiffness along the wing span are derived.

IN Parts I, II and III of this series we have discussed the physical nature of divergence, control reversal and various forms of flutter, and have seen how these phenomena can be predicted by theory. The flutter problem is so complicated, however, that the aircraft designer needs the assistance of certain guiding principles; otherwise he may find when the aircraft is ready to fly that the flutter calculations which are just completed show that drastic modifications to the aircraft are necessary. These principles form the basis of this concluding part of the series and have two main objects: first to avoid large changes in design on flutter grounds and secondly to obtain a high efficiency from the flutter calculations.

IN our report of the tenth annual meeting of the Institute of the Aeronautical Sciences we shall not follow precisely the order in which the sessions occurred nor at all times classify the papers in exactly the manner of the meeting. Unfortunately, certain of the papers presented will not be found in our review owing to lack of preprints, but this in no way reflects on the value or timeliness of the papers omitted in the review.