Weight Prediction of the Lifting System for an Unconventional Aircraft Configuration

In this paper a tool, previously developed and validated for the prediction of wing box structural weight in very early design stages, is improved to better match effects on panel sizing of stability constraints. It is applied to an unconventional configuration aircraft based on the Best Wing System concept, introduced by Ludwig Prandtl in 1924, to achieve minimum induced drag: such lifting system is composed of two swept wings (fore and aft) connected by vertical wings at their tips and two fins connecting the rear wing to the fuselage. The system is over-constrained to the fuselage and, thus, the structural design, as well as the static aeroelasticity and flutter characteristics, totally differs from a conventional aircraft. An optimization method finds out a suitable prediction of the structural weight by defining the wing span stiffness behavior compatible with a mix of global and local design constraints, but without any claim about a structural design. The link among stiffness properties and structural weight is made by means of simplified models of the box cross-sections, suitable also to supply responses for the approximated evaluation of local constraints

Analytically Driven Experimental Characterisation of Damping in Viscoelastic Materials

The damping assessment of highly dissipative materials is a challenging task that has been addressed by several researchers; in particular Oberst defined a standard method to address the issue. Experimental tests are often hindered by the poor mechanical properties of most viscoelastic materials; these characteristics make experimental activities using pure viscoelastic specimens prone to nonlinear phenomena. In this paper, a mixed predictive/experimental methodology is developed to determine the frequency behaviour of the complex modulus of such materials. The loss factor of hybrid sandwich specimens, composed of two aluminium layers separated by the damping material, is determined by experimental modal identification. Finite element models and a reversed application of the modal strain energy technique are then used to recover the searched storage modulus and loss factor curves of rubber. In particular, the experimental setup was studied by comparing the solutions adopted with the guidelines given in ASTM-E756-05. An exhaustive validation of the values obtained is then reported

Weight Prediction of the Liftig System for an Unconventional Aircraft Configuration

In this paper a tool, previously developed and validated for the prediction of wing box structural weight in very early design stages, is improved to better match effects on panel sizing of stability constraints. It is applied to an unconventional configuration aircraft based on the Best Wing System concept, introduced by Ludwig Prandtl in 1924, to achieve minimum induced drag: such lifting system is composed of two swept wings (fore and aft) connected by vertical wings at their tips and two fins connecting the rear wing to the fuselage. The system is over-constrained to the fuselage and, thus, the structural design, as well as the static aeroelasticity and flutter characteristics, totally differs from a conventional aircraft. An optimization method finds out a suitable prediction of the structural weight by defining the wing span stiffness behavior compatible with a mix of global and local design constraints, but without any claim about a structural design. The link among stiffness properties and structural weight is made by means of simplified models of the box cross-sections, suitable also to supply responses for the approximated evaluation of local constraints

Efficient Discrete Element Modeling of Particle Dampers

Particle dampers’ dissipative characteristics can be difficult to predict because of their highly non-linear behavior. The application of such devices in deformable vibrating systems can require extensive experimental and numerical analyses; therefore, improving the efficiency when simulating particle dampers would help in this regard. Two techniques often proposed to speed up the simulation, namely the adoption of a simplified frictional moment and the reduction of the contact stiffness, are considered; their effect on the simulation run-time, on the ability of the particle bed to sustain shear deformation, and on the prediction of the dissipation performance is investigated for different numerical case studies. The reduction in contact stiffness is studied in relation to the maximum overlap between particles, as well as the contacts’ duration. These numerical simulations are carried out over a wide range of motion regimes, frequencies, and amplitude levels. Experimental results are considered as well. All the simulations are performed using a GPU-based discrete element simulation tool coupled with the multi-body code MBDyn; the results and execution time are compared with those of other solvers

Multilevel Structural Optimization for Preliminary Wing-Box Weight Estimation

This paper deals with the weight estimation of the wing box of a commercial aircraft by means of a procedure suitable for very large liners and/or unconventional configurations for which statistical data and empirical formulas may not be sufficiently reliable. Attention is focused on the need to account for aeroelastic interaction from a very preliminary stage of the design cycle. The procedure exploits the first of three levels of a multilevel structural optimization system conceived for the preliminary design of the wing primary structure and a simplified evaluation of the cross-sectional properties. The comparison between weight estimates obtained with the present procedure and predictions supplied by available literature shows a satisfactory agreement

The Lifting System of a Prandtlplane, Part 1: Design and Analysis of a Light Alloy Structural Solution

The lifting system of a PrandtlPlane is a box in the front view, made of two swept wings set at different heights and of vertical wings at their tips; one or two fins connect the rear wing to the upper fuselage, and the front wing is clamped to the same; thus, the wing system is over-constrained, and the structural design becomes a challenge. In this paper, starting from a preliminary solution, an optimization procedure is applied to minimize the overall weight with the constraints of maximum allowable stress, stability of compressed structures, minimum aileron effectiveness, aeroelastic effects on load distribution and flutter. The system is approximated as beams with 54 elements. Two models of the wing section are presented: in the first, the wing section is approximated with three parameters (skin thickness, spar web thickness and stringer section total area) for a total of 162 degrees of freedom; in the second one, two other parameters are adjoined in any section (areas of two spar flanges) with a total of 270 d.o.f. According to the first model, the overall weight, including the fins, is about 17.5% of the maximum takeoff weight, while, in the second one, it is reduced to 15.8%

Laser and vision-based measurements of helicopter blade angles

In this paper, three novel non-contact measurement systems for helicopter rotor blades, based on the 2D laser triangulation, the single camera and the stereo camera respectively, are designed and developed. The three measurement systems are applied to reconstruct the spatial position of the blade, and consequently its attitude angles. The measuring qualities of the three systems are assessed by means of experiments, including vibration tests, coupled rotation and vibration tests, and accuracy tests with complex motion configurations. These tests demonstrate that the three solutions can perform continuous operation correctly in a relevant dynamic environment. The results of accuracy tests show that, while all the systems can be successfully applied for the measurement of the angles of a helicopter blade, the stereo camera system provides a better accuracy than the other two systems. In particular, for the stereo camera system, the discrepancies of the three angles are comprised between 0.1 and 0.3 degrees in case of realistic blade motion conditions

ROTOR FOR A HOVER-CAPABLE AIRCRAFT AND METHOD FOR DETECTING THE ATTITUDE OF A BLADE WITH RESPECT TO A HUB OF SUCH A ROTOR

A rotor (3, 3') for a hover-capable aircraft (1) is described that comprises: a drive mast (6); a hub (5) operatively connected to the drive mast (6) and rotatable about a first axis (A); and at least two blades (9) hinged to the hub (5), via a rigid or elastically deformable connection, so as to be able to assume an attitude rotated about and/or translated along at least a second axis (B, C, D) with respect to said hub (5); the aircraft (1) further comprising sensor means (25) configured to detect the attitude of at least one said blade (9) with respect to the hub (5); the sensor means (25) are configured to acquire an optical image associated with the attitude of the blade (9) with respect to the hub (5)