Application of Computational Fluid Dynamics/Computational Structural Dynamics Coupling for Analysis of Rotorcraft Airloads and Blade Loads in Maneuvering Flight

Abstract

This paper presents coupled calculations of both the airloads and structural loads for the UH-60A main rotor during the UTTAS pull-up maneuver performed under the NASA/Army UH-60A Airloads Program. These calculations were performed using OVERFLOW-2, a computational fluid dynamics (CFD) solver, coupled to the Rotorcraft Comprehensive Analysis System (RCAS), a rotorcraft comprehensive analysis. For time-varying maneuvers, the two codes were tightly coupled and exchanged airloads and structural deflections at every time step. The coupled solution methodology gives improved airload prediction because of the ability to model three-dimensional transonic effects on the advancing blade, stall events on the retreating blade, as well as the aeroelastic deformations. Correlation with data for both the airloads and structural loads is reasonably good. Control load predictions also show good correlation with data, which is a substantial improvement over conventional analyses. A quasisteady loosely coupled approximate solution was also examined and was found to give good airload and structural load predictions, for this relatively slow maneuver. γ flight path angle, deg θ aircraft pitch attitude, deg μ advance ratio σ rotor solidity φ aircraft roll angle, deg ψ aircraft yaw angle, deg rotor rotational frequency, rpm CFD/CSD MethodologyThis section describes the CFD and CSD modeling including a brief description of the codes and the fluid/structure coupling interface. The RCAS comprehensive rotorcraft analysis code is used for the structural calculations, whereas the OVERFLOW-2 code is used for the flow calculations. These two codes are coupled using file i/o in both loosely coupled manner for steady-state calculations and tightly coupled manner for time-accurate calculations. RCAS UH-60A modelingThe RCAS is a comprehensive multidisciplinary, computer software system for predicting rotorcraft aerodynamics, performance, stability and control, aeroelastic stability, loads, and vibration for a wide range of complex rotorcraft configurations operating in hover, forward flight, and in maneuvering conditions. RCAS was developed by the Aeroflightdynamics Directorate, U.S. Army Aviation and Missile Research, Development, and Engineering Center (RDECOM), to provide 032007-2

Prediction and Analysis of Main Rotor Loads in a Prescribed Pull-Up Maneuver

“…As the load factor begins to increase when the helicopter starts to pull up, there is a significant difference between the prediction and the flight-test measurement (around 6000 lb). This may be due to the contributions from the fuselage and the horizontal tail, and it has been clarified by Bhagwat et al [7]. Figure 17 shows the sectional normal forces at 86:5%R, predicted by the coupled analysis.…”

“…Abhishek et al [9] carried out a simpler lifting-line analysis, also for an isolated rotor, with an attempt to calculate the rotor pitch control angles: it was unsuccessful due to large errors stemming from the unknown horizontal tail lift during the maneuver and an inability to predict the maneuver trajectories in absence of detailed aircraft data. Subsequently, with availability of flight-test control angles, several researchers have predicted loads for this prescribed maneuver: Abhishek et al [10] and Yeo [11] focused on lower-fidelity lifting-line predictions, [7,8] compared the lifting-line model's capabilities with CFD/CSD analysis, [12] employed wake-coupling CFD/CSD approach, and [13] examined the effect of time-accurate coupling using RANS. The lifting-line analyses in general shows less satisfactory correlation with measured flight-test data when compared with CFD analysis.…”

“…A high-fidelity simulation of the prescribed UTTAS pull up was carried out by Bhagwat et al [7,8] using a multibody finite element structural model coupled with a Reynolds-averaged Navier-Stokes (RANS) model. This work demonstrated RANS capability in predicting two rotor dynamic stall cycles for the first time for a maneuver and showed that the oscillatory blade structural loads could be predicted with increased accuracy using an isolated rotor calculation.…”

Rotor wake and flow analysis using a coupled Eulerian–Lagrangian method

“…Additionally, the highly energetic concentrated tip vortex generated and rolled up in the blade tip region constantly remains within the vicinity of the operating blades. The close interaction between the wake and the blade may lead to impulsive airloads and increase the rotor's vibration and noise levels (Bhagwat, Ormiston, Saberi, & Xin, 2012). Given the complexity stated above, rotor-wake dynamics and performance predictions remain the most challenging tasks for rotor aerodynamics modeling.…”

Investigation of Rotor Performance and Loads of a UH-60A Individual Blade Control System

“…However, in-depth correlation studies have not been performed. In recent years, there has been significant progress in aeromechanics prediction capability using coupled computational fluid dynamics (CFD) / rotorcraft computational structural dynamics (CSD) analyses [11,12]. The CFD methods, which use a high fidelity, Navier-Stokes, overset grid methodology with first-principles-based wake capturing, overcame the limitations of the conventional lifting line aerodynamics used in rotorcraft comprehensive codes.…”