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

Prediction of Aerodynamic Rotor-Airframe Interactions in Forward Flight

Results from the comprehensive analysis CAMRAD II are presented, illustrating recent developments in the aerodynamics and dynamics models, and demonstrating the technology that is needed for an adequate calculation of rotorcraft behavior. Calculations of rotor performance, airloads, structural loads, and stability are presented, including comparisons with experimental data.

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Rotorcraft aeromechanics applications of a comprehensive analysis

Main rotor blades with plain trailing edge flaps are investigated as a potential means of vibration reduction using a comprehensive rotorcraft analysis. The analysis is modified to incorporate an unsteady aerodynamic model including flap effects. Predicted blade and pitch link loads show adequate correlation with experimental data. An initial open loop study shows that, for a typical articulated four bladed rotor, significant reductions in fixed system 4/rev vertical shears and hub moments are possible with open loop flap deflections at 3/rev and 4lrev. The reductions are most closely associated with changes in the response of the blade third flatwise bending mode and torsional deflections. A reduction in blade torsional stiffness reduced the effectiveness of the flap in reducing vibration.

Helicopter vibration reduction with trailing edge flaps

The loading of an airfoil during dynamic stall is examined in terms of the augmented lift and the associated penalties in pitching moment and drag. It is shown that once stall occurs and a leading-edge vortex is shed from the airfoil there is a unique relationship between the augmented lift, the negative pitching moment, and the increase in drag. This relationship, referred to here as the dynamic stall function, shows limited sensitivity to effects such as the airfoil section profile and Mach number, and appears to be independent of such parameters as Reynolds number, reduced frequency, and blade sweep. For single-element airfoils there is little that can be done to improve rotorcraft maneuverability except to provide good static C(l(max)) characteristics and the chord or blade number that is required to provide the necessary rotor thrust. However, multi-element airfoils or airfoils with variable geometry features can provide augmented lift in some cases that exceeds that available from a single-element airfoil. The dynamic stall function is shown to be a useful tool for the evaluation of both measured and calculated dynamic stall characteristics of single element, multi-element, and variable geometry airfoils.

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Airfoil Dynamic Stall and Rotorcraft Maneuverability

Comparisons were made of the performance and blade vibratory loads characteristics for an advanced rotor system as predicted by analysis and as measured in a 1/5 scale model wind tunnel test, a full scale model wind tunnel test and flight test. The accuracy with which the various tools available at the various stages in the design/development process (analysis, model test etc.) could predict final characteristics as measured on the aircraft was determined. The accuracy of the analyses in predicting the effects of systematic tip planform variations investigated in the full scale wind tunnel test was evaluated.

D. Jepson

Papers

Analysis and correlation of the test data from an advanced technology rotor system