Structural Design and Aeromechanical Analysis of Unconventional Blades for Future Mars Rotorcraft

The structural design of rotor blades with ultra-thin, unconventional airfoils is conducted in support of the NASA Rotor Optimization for the Advancement of Mars eXploration (ROAMX) project. The outer mold line was provided by NASA, and the internal structural design was developed at the University of Maryland using a CAD-based three-dimensional (3D) aeromechanical analysis. The main objectives of this paper are to document the unique aeroelastic behavior encountered due to the low Reynolds number (down to 15K) and high subsonic Mach number (up to 0.95). Four different blade designs are considered, with the pitch axis varied from quarter-chord to midchord to determine the effect of center of gravity (C.G.) offset on natural frequencies, blade deformations, root loads, and 3D stresses. Torsional stability is emphasized for each of the designs - especially important due to the low Lock number on Mars. The designs are first studied in vacuum, and significant reductions in root loads and 3D stresses are achieved by moving the pitch axis closer to midchord to reduce the C.G. offset. Next, the design with the pitch axis at 40% chord is selected for a lifting-line aeromechanical analysis. The blade control load, airloads, deformations, and 3D stresses are studied for steady hover. Dynamic control load and dynamic 3D stresses are studied for unsteady hover. Interesting elastic twist is observed due to the trapeze effect and propeller moment, in turn affecting the spanwise distribution of aerodynamic loads. The dynamic control load is found to increase significantly due to inertial coupling from the C.G. offset. The dynamic stresses also increase but still have factors of safety greater than two for both tensile and compressive stress.