Recent developments of the dynamics models for the comprehensive analysis CAMRAD II are described, specifically advanced models of the geometry and material for the beam component, and a force balance method for calculating section loads. Calculations are compared with measurements for beams undergoing large deflection. Bearingless rotor stability and bending loads calculations are compared with the results from a full-scale wind tunnel test. With a reasonable number of beam elements representing the rotor blade, any large deflection effects are captured by the rigid body motion (which is always exact), and a second-order model of the beam element elastic motion is adequate. The deflection method gives unacceptable results for the structural loads in practical cases, and even with uniform blade properties. The force balance method described here gives good results for blade load, without requiring a large number of nodes.

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Rotorcraft Aerodynamics Models for a Comprehensive Analysis

Modern helicopter rotor aerodynamics

Despite the 80-year history of classical wing theory, considerable research has recently been directed toward planform and wake effects on induced drag. Nonlinear interactions between the trailing wake and the wing offer the possibility of reducing drag. The nonlinear effect of compressibility on induced drag characteristics may also influence wing design. This thesis deals with the prediction of these nonlinear aspects of induced drag and ways to exploit them. A potential benefit of only a few percent of the drag represents a large fuel savings for the world's commercial transport fleet. Computational methods must be applied carefully to obtain accurate induced drag predictions. Trefftz-plane drag integration is far more reliable than surface pressure integration, but is very sensitive to the accuracy of the force-free wake model. The practical use of Trefftz plane drag integration was extended to transonic flow with the Tranair full-potential code. The induced drag characteristics of a typical transport wing were studied with Tranair, a full-potential method, and A502, a high-order linear panel method to investigate changes in lift distribution and span efficiency due to compressibility. Modeling the force-free wake is a nonlinear problem, even when the flow governing equation is linear. A novel method was developed for computing the force-free wake shape. This hybrid wake-relaxation scheme couples the well-behaved nature of the discrete vortex wake with viscous-core modeling and the high-accuracy velocity prediction of the high-order panel method. The hybrid scheme produced converged wake shapes that allowed accurate Trefftz-plane integration. An unusual split-tip wing concept was studied for exploiting nonlinear wake interaction to reduced induced drag. This design exhibits significant nonlinear interactions between the wing and wake that produced a 12% reduction in induced drag compared to an equivalent elliptical wing at a lift coefficient of 0.7. The performance of the split-tip wing was also investigated by wing tunnel experiments. Induced drag was determined from force measurements by subtracting the estimated viscous drag, and from an analytical drag-decomposition method using a wake survey. The experimental results confirm the computational prediction.

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A Computational and Experimental Study of Nonlinear Aspects of Induced Drag

. This article summarizes the ability of CHARM+AGDISP to predict the drift and deposition of sprays released from rotary wing unmanned aerial vehicles (UAVs). This predictive capability results from merging algorithms for spray transport, as found in AGDISP (AGricultural DISPersal), with CHARM (Comprehensive Hierarchical Aeromechanics Rotorcraft Model). The resulting software tracks the release of spray droplets from nozzles on the UAV to deposition on the ground. To date, both AGDISP and CHARM, a code that provides a complete representation of the time-varying, unsteady flow field surrounding a helicopter during transient maneuvering flight near the ground, have been extensively validated. The CHARM+AGDISP software is applied to two UAVs to explore the flow field regimes that present challenges for effective UAV operations. The simulations undertaken indicate flight conditions that yield acceptable deposition levels and minimize drift; inversely, conditions are also identified that result in off-target drift that may be problematic.

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Prediction of Aerial Spray Release from UAVs

A method of predicting the minimum induced drag conditions in conventional or innovative lifting systems is presented. The method is based on lifting-line theories and the small perturbation acceleration potential. Assuming linearity and rigid wake aligned with the freestream, the optimal conditions are formulated using the Euler‐Lagrange integral equation subject to the conditions of fixed total lifting force and wing span. The Lagrange multiplier method is applied, and equations for the design optimum are obtained and solved directly. Particular attention is paid to the Hadamard finite-part integrals involved in the solution process. Munk’s drag theorems are also applied in order to verify the quality of the solutions. In this paper, where the theoretical/computational foundation is laid for the induced drag minimization of general lifting-line configurations, the case of the biplane under optimal conditions is extensively analyzed. It is demonstrated that, under optimal conditions, the two wings (which have the same wing span) have the same circulation distribution. Cases of finite, infinite, and infinitesimal distances between the wings are analyzed as well. It is shown that the optimal distribution is, in general, not elliptical. The proposed theoretical approach is general, and it offers a valuable tool for direct prediction of minimum induced drag in both planar and nonplanar lifting systems. Results obtained by the proposed approach shed light on some of the mathematical issues involved, and they can be used for verifying results obtained by numerical math-programming-based optimization.

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Induced Drag Minimization: A Variational Approach Using the Acceleration Potential

A new approach to the prediction of helicopter rotor performance using a free wake analysis was developed. This new method uses a relaxation process that does not suffer from the convergence problems associated with previous time marching simulations. This wake relaxation procedure was coupled to a vortex-lattice, lifting surface loads analysis to produce a novel, self contained performance prediction code: EHPIC (Evaluation of Helicopter Performance using Influence Coefficients). The major technical features of the EHPIC code are described and a substantial amount of background information on the capabilities and proper operation of the code is supplied. Sample problems were undertaken to demonstrate the robustness and flexibility of the basic approach. Also, a performance correlation study was carried out to establish the breadth of applicability of the code, with very favorable results.

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Cho Ong Ching

Papers

Free wake analysis of hover performance using a new influence coefficient method