NASA X-59’s Latest Testing Milestone: Simulating Flight from the Ground

NASA's X-59 has recently completed a significant and pivotal round of tests. These tests are an essential part of the development process, aimed at assessing the aircraft's performance and capabilities in a controlled environment. During this phase, engineers and researchers simulated various flight conditions, meticulously evaluating how the jet would behave while cruising at high altitudes over the expansive California desert, without leaving the ground.

“The idea behind these tests is to command the aircraft’s subsystems and flight computer to function as if it is flying,” said Yohan Lin, the X-59’s lead avionics engineer at NASA’s Armstrong Flight Research Centre in Edwards, California. “We thought we might find a few things during the tests that would prompt us to go back and tweak them to work better, especially with some of the software, and that’s what we wound up experiencing. So, these tests were very helpful”.

The objective of ground-based simulation testing was to ensure that the hardware and software enabling the X-59 to operate safely are functioning correctly in conjunction and are capable of managing unforeseen issues. Each new aircraft integrates various systems, and pinpointing the necessary minor adjustments to optimise performance is a crucial component of a systematic approach to flight.

Completing the tests marks another milestone off the checklist of things to do before the X-59 makes its first flight this year, continuing NASA’s QueSST mission to help enable commercial supersonic air travel over land.

During the testing, engineers from NASA and contractor Lockheed Martin turned on most of the X-59’s systems, leaving the engine off. For example, if the pilot moved the control stick a certain way, the flight computer moved the aircraft’s rudder or other control surfaces, just as it would in flight.

At the same time, the aircraft was electronically connected to a ground computer that sends simulated signals – which the X-59 interpreted as real – such as changes in altitude, speed, temperature, or the health of various systems. Sitting in the cockpit, the pilot “flew” the aircraft to see how it would respond.

“These were simple manoeuvres, nothing too crazy,” Lin said. “We would then inject failures into the aircraft to see how it would respond. Would the system compensate for the failure? Was the pilot able to recover?”

In contrast to standard astronaut training simulations, where flight crews are unaware of potential scenarios, the X-59 pilots were largely informed about the conditions the aircraft would face during each test. They also contributed to the planning of these tests to concentrate more effectively on the aircraft systems' responses.

In the field of aircraft development, this process is referred to as "iron bird" testing. This term originates from the use of a basic metal framework where representations of the aircraft’s subsystems are installed, connected, and evaluated. Constructing such a test-bed is a standard procedure in development programs where multiple aircraft will be produced. However, given that the X-59 is a unique aircraft, officials determined that utilising the aircraft itself would be more cost-effective and efficient.

Consequently, engineers referred to this series of exercises as “aluminium bird” testing, given that the X-59 is primarily constructed from this metal. Unlike the traditional “iron bird” tests, which involve evaluating copies of an aircraft’s systems on an unremarkable frame, the “aluminium bird” approach utilised the actual aircraft and its systems. This methodology provided greater confidence in the design due to the direct and accurate test results obtained.

“It’s a perfect example of the old tried and true adage in aviation that says ‘Test what you fly. Fly what you test,” Lin said.

Following the completion of aluminium bird testing, the next significant step for the X-59 on its journey to its inaugural flight involves conducting taxi tests on the airport taxiways adjacent to Lockheed Martin’s Skunk Works facility in Palmdale, California, where the aircraft was constructed. The first flight will occur after these taxi tests.

Already in the X-59’s logbook since the fully assembled and painted aircraft made its public début in January 2024:

• A Flight Readiness Review was conducted in which a board of independent experts from across NASA completed a study of the X-59 project team’s approach to safety for the public and staff during ground and flight testing.

• A trio of important structural tests and critical inspections that included “shaking” the airplane to make sure there were no unexpected problems from the vibrations.

• Firing up the GE Aerospace jet engine for the first time after installation into the X-59, including a series of tests of the engine running with full afterburner.

• Checking the wiring that ties together the X-59’s flight computer, electronic systems, and other hardware to be sure there were no concerns about electromagnetic interference.

• Testing the aircraft’s ability to maintain a certain speed while flying, essentially a check of the X-59’s version of cruise control.

Watch this video about the X-59 aluminium bird testing. It only takes a minute. Well, 59 seconds to be precise.