Ingenuity Mars HelicopterIn July of 2020 NASA launched a mission to search for signs of life on Mars. The key tool for this mission was the Perseverance rover, but mounted in the belly of the rover was a small experimental helicopter named Ingenuity. Ingenuity was deployed from the rover shortly after arrival, and has successfully flown many missions on Mars. It achieved its goal of demonstrating flight on Mars, and it still flies today supporting the rover’s mission. HistoryFor as long as scientists have conetmplated sending a rover to Mars, they’ve also considered sending an aircraft. In 2014 NASA and AeroVironment published a design for a helicopter to play this role. Within 2 years, the team received $15M from the US government, to develop and test the model. In 2018, another $23M was provided and it was announced that the chopper could be included in the 2020 mission to Mars. In the following year the model was tested and shown to perform satisfactory for the Mars mission. Ingenuity was launched from CapeCanaveral on July 30, 2020. It landed on February 18, 2021 at the Octavia Butler landing site, within the Jezero Crater on Mars. It was not released from the rover at the landing site, but carried to a more ideal “airfield.” Various tests were performed, as planned, on Ingenuity in the first 30 days. The first flight occurred on April 19, 2021. In this flight, the helicopter hovered about 10 feet above the ground. In the ensuing weeks, Ingenuity flew incrementally further in each flight. Ingenuity has far surpassed its planned mission, which was simply to demonstrate flight on Mars. As of the time of this writing, Ingenuity still makes periodic flights. It’s reportedly helping the science mission by seeking out routes and attractive targets for Perseverance. Not all scientists are on board with this, however. Many claim the helicopter adds too much complexity/cost to the mission and is a distraction from the ultimate goal—collecting soil and rock samples while searching for signs of life. Purpose / MissionAccording to NASA, Ingenuity is “a technology demonstration to test powered, controlled flight on another world for the first time.” It reportedly completed this mission after the first 3 successful flights. After successful flight demonstration, Ingenuity transitioned to a new mission—to demonstrate flight operations that future aerial craft could utilize. In this role, Ingenuity is supporting the rover science mission, primarily by mapping and scouting the terrain. Flight environmentTwo key variables for flight are air density \(\rho\) and weight/gravity. The Martian atmosphere is only about 1% as dense as a sea level standard atmosphere on Earth. This means only about 1% as much lift and drag will be created in a given flight scenario. The gravitation force on Mars is about ⅓ what we feel on Earth. This means Ingenuity only needs to create about ⅓ as much thrust to hover, compared to performing a hover on Earth. Let’s think about what these requirements mean. How must the rotor operate compared to one on Earth? A section of a rotor blade creates lift proportional to \(\rho v^2 c\), where \(v\) is its airspeed and \(c\) is the blade chord length. Since gravity is ⅓ the value on earth, the amount of lift required will be roughly a third as much. However, with 1% as much \(\rho\), it’s necessary for \(v^2 c\) to be about 33x larger on Mars. Increasing blade chord can help a bit, but it won’t achieve the 33x target. It can also be accomplished by increasing rotor speed about 6x or, more realistically, a combination of the two. Nightly temperatures on Mars may drop as low as -140C. This does not play a role in aerodynamics, but is a complicating factor for many components on the aircraft that must be kept in a fixed temperature range. Specifications
Rotor System
Ingenuity has two 2-bladed rotors. Each rotor spins about the same central mast. Like all coaxial rotors, the two spin in opposite directions to cancel the yaw moment they create. The rotors are not controlled by varying rotor speed, like many drones. Such control would not allow Ingenuity to pitch, roll and fly along non-vertical flight paths. Instead, each rotor has a swashplate that moves vertically and tilts to provide blade feathering and more precise flight. Each swashplate is actuated by three servos, mounted 120deg apart (equally spaced) in the rotor azimuth. This is akin to full-scale, traditional helicopters. As discussed above, the rotors must operate at a relatively high rotational speed (to makeup for the low air density). This high speed exagerrates a phenomenon known as the tennis racket moment—the fact that the centrifugal force due to rotation, acting through the CG of blade cross sections, tends to twist blades flat into the rotor plane. This would require relatively high forces to be applied to the swashplate servos, but is instead countered using an old helicopter trick: Chinese weights. When the rotor spins, these weights twist the blade in the opposite direction as the tennis racket moment, reducing the loads on the servos. Brushless DC motors are mounted above each rotor, and only power the rotor immediately below them. There’s no gearing to to provide power to the other rotor in case of a failure—if/when one of these motors fail, Ingenuity will no longer be able to fly. NavigationRemote human-piloting was not an option for ingenuity. It takes minutes for signals to travel to Earth from Mars, hence pilot reactions to signals received would be way too late. Ingenuity had to perform autonomous flight. Before each flight, scientists upload a flight plan. During the flight, Ingenuity runs its control algorithms using input from the sensors below to execute the flight plan.
The Ingenuity chief pilot described how many maneuvers are performed in an article: What We’re Learning About Ingenuity’s Flight Control and Aerodynamic Performance. MediaNASA provides many pictures and videos taken from Ingenuity and Perseverance. Wikipedia's Ingenuity article also includes a nice gallery. |
