To many pilots, density altitude remains a nebulous notion because it is often taught in a vacuum, textbook treatments teeming with technical terminology. NASA’s magnificent achievement can provide a more down-to-earth illustration of the relationship between an atmosphere’s density and an aircraft’s ability to fly in it.
As summer approaches in our Northern Hemisphere, flying activity will soar, which means more flying in hot weather. Heated air has lower density than cool air, and when air density decreases, it hinders the (nonturbocharged) engine of an aircraft from producing power. Reduced air density also lessens wing lift and propeller performance plummets. A pilot intending to fly a Cessna 182 from a short runway at a high elevation might run the numbers for expected takeoff-and-climb performance under possible density-altitude scenarios and conclude that reducing the load or departing in the cooler (denser) air of dawn or dusk is necessary.
Although that’s not rocket science, many accident reports describe high-density-altitude takeoff attempts in which the aircraft never quite gets off the ground or gets no higher than ground effect before “mushing” back to the surface or stalling.
On Mars, the challenge wasn’t heat, but low atmospheric pressure, which exerts a proportional effect on density. Although Mars’ gravity is only about one-third Earth’s, flying on Mars is still a heavy lift because Mars has an extremely thin atmosphere, its surface pressure less than 1 percent of sea-level pressure on Earth. “This means there are relatively few air molecules with which Ingenuity’s two 4-foot-wide (1.2-meter-wide) rotor blades can interact to achieve flight,” NASA said.
Like the Skylane pilot figuring out the best time to take off from a mountain airstrip, NASA’s solution was to fly the autonomous, solar-powered Ingenuity at 12:33 Local Mean Solar Time (Mars time) when the four-pound aircraft—that’s 0.001287 the maximum ramp weight of an S-model Skylane—“would have optimal energy and flight conditions.”
Ingenuity, which weighs about 1.5 pounds in Mars gravity, maneuvered up to 10 feet above the surface of Mars for 39 seconds—a modest height, but worlds above what some earth pilots achieve when they rely only on moonshot optimism to fly in high-density-altitude conditions.