Aeromechanics

Focus

Rotor blades are the principal components that are affected by the inter-dependence of airloads and structural dynamic response. The aerodynamics are driven primarily by blade flap and torsion response (which are themselves coupled both inertially and structurally). Depending on the rotor configuration, lag response may also play a role in the rotor flowfield. Blade motions are in turn driven by centrifugal loading and aerodynamic forcing. The nature of the problem is such that accurate resolution of the aerodynamic environment requires a framework for determining the structural response to a similar fidelity level. The key focus of aeromechanics is to obtain mutually consistent solutions for the aerodynamics, flight dynamics and elastic blade flap-lag-torsion dynamics.

Two types of analysis are commonly used in the field - steady flight analysis (trim) and transient flight analysis (maneuvers). For transient flight, all three aspects (flight dynamics, rotor dynamics and aerodynamics) must be solved together in a consistent manner. For steady flight (trimmed flight), flight mechanics can be eliminated from the picture, and rotor aeromechanics - structural dynamics and aerodynamics - is the key unknown.

Owing to the nature of the rotorcraft trim problem, iterative methods are used to alternately update each component until convergence is achieved. If implemented consistently, these comprehensive analysis tools can provide a strong foundation for synthesizing both predictive design tools and "post-dictive" airloads analysis for validation.

One tool to rule them all

During grad school, I had the opportunity to work with people from both rotorcraft aeromechanics and flight dynamics. After graduating, based on knowledge gained from both sections, I implemented a generalized analysis that could be used to study both aspects of the rotorcraft problem simultaneously. This is ongoing work, and includes multi-rotor modeling (including quad-copters), wind turbine analysis, coupling with a flexible airframe and transmission, composite blade analysis, multiple hub types (including tilt-rotor, bearingless and teetering) using multiple physics-based models and updated software frameworks.

Some single-rotor and multi-rotor validation cases are shown below.

Main Rotor Power Required for UH-60A: Test data vs. Simulation

On-axis pitch rate response On-axis heave acc. response

On-axis yaw rate response On-axis roll rate response

UH-60A on-axis response to pilot stick inputs at hover: Test data vs. Simulation

The previous test data sets are obtained from various dissertations. While additional wind tunnel validation cases are available, I am not sure if I am allowed to disseminate the data. Until I get a definitive yes, it's assumed to be a no.

In a recent paper, we coupled this model to a CFD analysis and successfully obtained a delta-converged coupled solution of the rotor aerodynamics, trim controls and fuselage orientations. Some plots are given below for the sectional loads and comparison to CAMRAD II.

Main Rotor Shaft Power Required for X2 Technology Demonstrator: Model Validation

Variation of deformed-frame airloads at 75% span over CFD-CSD iterations at 55 knots

Contour plots of deformed-frame normal force and pitching moment: CFD vs. CSD