Aviv Rosen

Optimizing Electric Propulsion Systems for Unmanned Aerial Vehicles

The propeller model and a model of the electric system, together with various optimization schemes, are used to design optimal propulsion systems for a mini unmanned aerial vehicle for various goals and under various constraints. Important design trends are presented, discussed, and explained. Although the first part of the investigation is based on typical characteristics of the electric system, the second part includes a sensitivity study of the influence of variations of these characteristics on the optimal system design.

The Influence of Boundary Conditions on the Buckling of Stiffened Cylindrical Shells

Theoretical and experimental studies on the influence of boundary conditions on the buckling of stiffened cylindrical shells and their vibrations are discussed. The effect of prebuckling deformations on the buckling loads and vibrations of stiffened shells is studied and compared with that in the case of unstiffened shells. The in-plane boundary conditions are found to be of particular importance for stiffened cylindrical shells and their effect differs significantly from that in unstiffened shells. The effect of axial restraints, which are found to be of prime importance in stringer-stiffened shells, are also studied.

Active aerodynamic stabilization of a helicopter/sling-load system

A general theoretical model is used in order to investigate the possibility of an active aerodynamic stabilization of a helicopter/sling-load system in the case of a single suspension point. The stabilization system includes two vertical aerodynamic surfaces that are located on the external sling load. The incidence angles of these surfaces are varied according to a set of control laws. This study leads to a system that is relatively easy to realize. The input measurements of the load stabilization system (LSS) include the helicopter and sling load lateral/directional angular rates. It is shown that adding a feedback loop that is based on the load lateral acceleration increases further the stability of the whole system. Studies also show that a single rear surface is very efficient and can be used alone. The proposed LSS is capable of stabilizing the system in a wide range of airspeeds, load weights, suspension methods (geometries), and aerodynamic properties of the external sling load.

Aeroelastic stability and response of horizontal axis wind turbine blades

Coupled flap-lag-torsion equations of motion of an isolated horizontal axis wind turbine (HAWT) blade have been formulated. The analysis neglects blade-tower coupling. The final nonlinear equations have periodic coefficients. A new and convenient method of generating an appropriate time-dependent equilibrium position, required for the stability analysis, has been implemented and found to be computationally efficient. Steady-state response and stability boundaries for an existing (typical) HAWT blade are presented. Such stability boundaries have never been published in the literature. The results show that the isolated blade under study is basically stable. The tower shadow (wake) has a considerable effect on the out-of-plane response but leaves blade stability unchanged. Nonlinear terms can significantly affect linearized stability boundaries; however, they have a negligible effect on response, thus implying that a time-dependent equilibrium position (or steady-state response), based completely on the linear system, is appropriate for the type of HAWT blades under study.

Optimization of Propeller Based Propulsion System

Propeller design is a complex task that involves a variety of disciplines such as: aerodynamics, structural analysis, and acoustics. A new method of designing an optimal propeller which is based on a MDO (Multidisciplinary Design Optimization) approach is presented. By combining various analysis tools with an optimization tool, a powerful and flexible design method is obtained. During the design process three different optimization schemes are used, leading the design to its optimal goal. This new method is applied for the design of a propeller for an Ultralight aircraft. Several optional designs for different design goals are presented. The results of the new method are compared with results of the classical design method, based on Betzs condition, which considers only the aerodynamic performance of the propeller. The importance of addressing the characteristics of the entire air-vehicle, its aerodynamic characteristics and its propulsion system (engine, gear box, etc.), rather than only the isolated propeller, is emphasized.

Vibrations and buckling of axially loaded stiffened cylindrical shells with elastic restraints

Abstract A theory is derived for calculation of the influence of elastic edge restraints on the vibrations and buckling of stiffened cylindrical shells. The stiffeners are considered “smeared” and the edge restraints can be axial, radial, circumferential or rotational. Extensive computations are performed for special kinds of stringer-stiffened shells, and the theoretical predictions are compared with experimental results. A method of definition of equivalent elastically restrained boundary conditions by use of vibration tests is discussed. Application of this technique to tests on 10 shells significantly reduces the scatter in the ratio of experimental to predicted buckling loads.

Numerical model of the nonlinear behavior of curved rods

Abstract A theoretical model for the nonlinear behavior of curved rods is derived. The model is very general and it allows any combination of geometry, structural properties distribution, load distribution, and boundary conditions. A set of nonlinear equilibrium equations and boundary conditions is obtained. The equations are solved using the Galerkin Method. Two kinds of methods to calculate the cross sectional resultant forces and moments along the rod are described. One method includes differentiation of the displacements along the rod while the other method is based on integration of the loads along the rod. In order to show the use of the model an example is presented. The different nonlinear effects are indicated and discussed.

Design of Quiet Propeller for an Electric Mini Unmanned Air Vehicle

Designing a quiet and efficient propeller is a demanding task because these two goals often lead to contradictory design trends. The task becomes even more complicated when additional constraints are introduced. This paper presents a multidisciplinary design optimization process for designing a quiet propeller under various constraints. The acoustic model of the propeller is presented in detail, including its validation. The new process is used to design a quiet propeller for an electric mini unmanned air vehicle. This design is subjected to power, structural, and side constraints. The design variables include blade geometry, blade cone angle, propeller radius, number of blades, and rotational speed. It is shown that the motor characteristics have an important influence on the optimization, and it is therefore important to take them into account during the design process. The influences of the various parameters on the design and propellers characteristics are presented and discussed in detail.

Novel Approach to Axisymmetric Actuator Disk Modeling

Actuator disk models are commonly used for the analysis of rotary wing systems. The blade-element momentum model is probably the most popular one because of its simplicity, efficiency, and good accuracy in many cases. Yet momentum models fail to give satisfactory results in many other cases. The reason is probably the fact that momentum models include a basic assumption that the integral form of the equation of conservation of momentum can be replaced by its differential form. This paper presents a new actuator disk model that does not include the aforementioned assumption. It is assumed that the pressure difference between both sides of each point of the disk is a time average of the pressure difference between both sides of the blade elements that pass through that point. In addition to calculating the axial components of the induced velocity through the disk, the solution procedure also includes calculations of the radial component. The new model includes an iterative solution procedure that converges relatively fast and requires relatively small computing resources and short computing time. The paper describes the new model, presents the solution procedure, and compares the new results with known results from the literature.

Nonlinear analysis of pretwisted rods using 'principal curvature transformation'. II - Numerical results

This, the second part of a two-part paper, makes use of the method developed in the first part to investigate the nonlinear behavior of rods. A first example illustrates the complexity of the nonlinear relative to the linear case. A second example examines such aspects as convergence of the iterative solution, the number of generalized coordinates needed, and four different levels of nonlinearity. A comparison of the results calculated using the theory for the second example with existing experimental results shows good agreement. A third example deals with the resulting moment distribution along the rod. Three methods for calculating the components of this resultant moment are presented, compared, and discussed. The fourth example involves the nonlinear behavior of a pretwisted rod.