Flying on Mars getting tougher as Ingenuity helicopter gears up for 14th hop

NASA's Ingenuity helicopter, which is being prepared for its 14th Red Planet flight is finding it more difficult due to the thinned Martian air.
Mission team members stated that the upcoming sortie is much simpler than some of NASA's more adventurous scouting flights. It's not hard to understand why. The 4-pound (1.8-kg) chopper will be testing higher rotor spin speeds to determine if it can fly in rapidly changing atmospheric conditions on Mars.

Ingenuity is required to take off, climb to 16 feet (5 m) and then make a sideways maneuver to land. Although the flight was initially scheduled for Sept. 17, it was changed to accommodate the mission team's readiness. As more developments are reported, updates will be posted on Perserverance's official twitter feed.

Related: NASA's Ingenuity helicopter explores Mars' fascinating Raised Ridges

If the test flight occurs, it will likely include a rotor speed around 2,700 revolutions per hour (RPM). This assumes that a ground test of 2,800 RPM spins as planned. Ingenuity flew at 2,537 RPM in its previous Mars experience. Engineers hope that the drone will fly in spite of a thinner atmosphere because it has a higher rotation rate.

"It's actually becoming more difficult [to fly] everyday: I'm referring to the atmospheric density which was already extremely low but is now dropping further because of seasonal variations on Mars," Hvard Grip (Ingenuity chief pilot), from NASA's Jet Propulsion Laboratory, Southern California wrote in the update.

Grip explained that Ingenuity’s flight campaign was only designed to last a few months after Perseverance landed in Mars' Jezero Crater on February. Ingenuity exceeded all expectations and is still flying, testing whether rotorcraft can act as scouts to rovers or human missions.

However, Ingenuity was not designed to change seasonal conditions. The atmospheric density of Jezero Crater was approximately 1.2% to 1.5% higher than the Earth's. Now, the density is close to 1% in the afternoon hours that are most suitable for flight. This is because currents from the ground create less instability for low-flying drones.

Grip explained that although the [atmospheric] differences may not seem significant, they have a significant impact upon Ingenuity's ability to fly. As the Mars atmosphere becomes thinner, Ingenuity's thrust margin (or the amount of thrust that the drone generates above its requirements to hover) has been decreasing. Ingenuity may stall mid-air if the atmospheric density drops too much.

Grip stated, "Fortunately, there is a solution to this problem but it requires spinning the rotors faster than we have been doing so far." They will need to spin faster than any of our test helicopters or Ingenuity. This is something we do not take lightly. Therefore, our next Mars operations will be focused on testing higher rotor speeds to prepare for future flights.

Ingenuity will be investigating potential problems. One possibility is that the rotor blades could hit the atmosphere at 0.8 Mach (or 80% of the speed of sound) due to the higher RPM and wind. Due to Mars' lower atmospheric density, the speed of sound is only three-quarters as fast as on Earth.

Grip stated that if the blade tips reach the speed of sound, they would experience an enormous increase in aerodynamic drag. This would make it prohibitive to fly. "For Ingenuitys' rotor, we don't expect to encounter this phenomenon until higher Mach numbers but it has never been tested on Earth."

Grip stated that engineers will be looking out for resonances that could cause the helicopter's vibrations to occur at certain frequencies. This could at worst "cause damage or lead to a decrease in the flight control system's ability to detect these frequencies." The electrical system will also need to be able to handle higher loads and more power.

Grip stated that while it all amounts to a major challenge, if we approach the issue slowly and methodically, the system will be fully tested at higher speeds to allow Ingenuity to continue flying in the months ahead. Keep checking back for more updates.