Axial Descent of Variable-Pitch Multirotor Configurations: An Experimental Investigation under Free-Flight Settings for Mars Deployment Applications

For future helicopter-only Mars missions, NASA-JPL has proposed a novel entry, descent, and landing technique, in which the rotorcraft is deployed from the aeroshell in mid-air before landing. However, this approach is likely to subject the rotorcraft to unfavorable vortex ring state aerodynamics during deployment. To address this, the performance of a variable-pitch multirotor in axial descent was experimental investigated using untethered, free-flight wind tunnel tests. This approach allow to examine the rotor performance as a function of descent rate without restricting vehicle dynamics with a rigid mounting. Results indicated significant mean thrust losses of up to 20% compared to hover conditions, as well as heavily amplified rotor thrust fluctuations and vehicle attitude oscillations with increasing descent rate. Experimental findings were compared to analogous computational efforts utilizing the tool RotCFD, showing considerable discrepancies in the critical descent rates at which maximum thrust losses occur.