In the publication, “Mars Helicopter Technology Demonstrator”, Balaram (2018) described the features of the rotor system of the Ingenuity.
The Ingenuity is equipped with a 1.21 m diameter coaxial counter-rotating rotor to minimize its size and constraints of its host aircraft. The rotor itself is designed with a rigid rotor to reduce flapping; that is the out-of-plane deflection the blades do when a force is applied. The rigid rotor design is due to the thin Martian atmosphere which causes poor aerodynamic damping. Which can cause the aircraft to oscillate during its flight phase. To address the poor damping and at the same time allow some degree of flapping. The blades are unusually stiff, with a rotating flap frequency of about 80–90 Hz. The blades are able to flap at the outboard portion of the blades via elastic deformation. The Martian atmosphere also requires the blades to spin at a maximum rotation of up to 2800 rpm. The bearings and swashplates are enclosed with seals and soft boots to prevent dust build up.
The Ingenuity's rotor system has several important design decisions to address the thin Martian atmosphere that ultimately lead to its successful flight on Mars.
The Ingenuity is designed with a rigid rotor to address the thin Martian atmospheres effect of reducing aerodynamic damping(Balaram, 2018). Aerodynamic damping is the natural tendency for the air in the atmosphere to oppose an aircraft's movement. As a result an aircraft flying on Mars after performing a movement, can have have trouble restoring itself to stable flight (Grip, 2019). Hence to reduce as much excess motion as possible within the rotor, a rigid rotor is used.
To compliment the rigid rotor design, The blades have a rotating flap frequency of about 80–90 Hz.
By using HeliCAT to model the rotor system and CAMRAD II to calculate the modes and shapes of a rotating blade. It was observed that most of the blade deflection occurring outboard of 50% radius (Balaram, 2018).
The rigid rotor however is a direct contrast to Earthen helicopters that often have hinged rotor assemblies. This hinges allow the blades to deflect out of its plane when a force is applied rather than absorb it. Making it particularly important to prevent large longitudinal and lateral moments along the blades from destabilizing the helicopter (Bramwell, 2020).
Addressing the thinner Martian atmosphere again, thinner atmosphere will result in smaller forces acting on the blades thus resulting in less moment, diminishing the need for hinges. With no hinges the blades have less freedom of movement, disallowing them to oscillate freely. The rigid rotor thus is able to provide stable flight on Mars.
References
Balaram, Canham, Duncan, Golombek, Grip, Johnson, Maki, Quon, Stern, and Zhu. (2018). Mars Helicopter Technology Demonstrator https://rotorcraft.arc.nasa.gov/Publications/files/Balaram_AIAA2018_0023.pdf
Bramwell, Done, Balmford (2000). 1 - Basic Mechanics of Rotor Systems and Helicopter Flight.
https://www.sciencedirect.com/science/article/pii/B978075065075550004X
Bramwell, Done, Balmford. (2000). 5 - Flight dynamics and Control.
https://www.sciencedirect.com/science/article/pii/B9780750650755500087
Escobar, Chopra and Datta. (2021). High-Fidelity Aeromechanical Analysis of Coaxial Mars Helicopter.
https://arc.aiaa.org.singaporetech.remotexs.co/doi/10.2514/1.C035895
Grip, Johnson, Malpica, Scharf, Mandić, Young, Allan, Mettler, Martin and Lam. (2019). Modeling and Identification of Hover Flight Dynamics for NASA’s Mars Helicopter
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