NASA’s Sustainable Flight Demonstrator full-scale single-aisle research plane designed to explore net-zero aviation emissions and faster, more economical passenger flight near the speed of sound has been named the X-66A by the US Air Force.
Beginning in the 1940s, the US X-plane program was dedicated to building aircraft; it wasn’t to produce prototypes for production, but as pure research vehicles for making aerospace breakthroughs at a fundamental level, resulting in such firsts as the supersonic X-1 and the hypersonic X-15.
For the X-66A, NASA and Boeing are dealing with a basic problem of aerospace engineering, which is that every aircraft is an exercise in compromise. Ideally, what engineers want is to have an airframe with a perfect wing installed on it. Unfortunately, such a wing has to be infinite in span, which isn’t exactly practical.
As a result, engineers have to make compromises. In part, this is because of the role a wing plays. If you look at the shape of bird wings, they are adapted for different functions and have not only different shapes, but aspect ratios, cambers, and tips to deal with the vortexes caused by the incredibly complex flow of air over them. Crows have elliptical wings for speed and maneuverability, eagle wings are broad with slotted tips for slow speeds with lots of lift, hummingbirds have wings designed to flap rapidly for hovering, and albatrosses have long, thin wings for soaring at high speeds.
It’s the albatross wing that comes the closest to that of the X-66A, which is called a transonic truss-braced wing (TTBW). Its purpose is to fly in the transonic region. That is, at speeds between Mach 0.8 and 1.2. This is the range just before the so-called sound barrier when an aircraft begins to be subject to severe stresses called shock waves that increase drag, hamper controls, and cause other problems.
What is odd about the transonic region is that it isn’t that the entire plane ever exceeds the speed of sound. Instead, the air flowing over different parts of the aircraft is doing so at different speeds. As a result, most of the plane is subsonic while other bits are supersonic. The best way to understand this is from the plight of fighter pilots during the Second World War, whose prop-driven planes would often make high-speed dives. The fighter was flying below the speed of sound, but the propeller and parts of the airfoils were going faster than sound relative to the air, sometimes with unpleasant results.
All of this is of more than academic interest. Airlines are keen on building passenger and cargo aircraft that can fly transonic, hence Boeing’s involvement in the X-66A. The problem is overcoming the challenges needed to reach the right compromise when you don’t have a perfect wing.
In the case of the X-66A, what is wanted is a long, thin, and narrow wing set high on the fuselage. One drawback of this is that such a wing will tend to bow and wobble, so a truss is added to stabilize it. This in itself introduces new problems to the already formidable one of creating realistic computer models of the complex airflow across the wing, fuselage, and engines. The goal is to build up a better understanding of the design to build a demonstrator aircraft that may one day lead to a transonic airliner that is 30% more efficient and could lead to net-zero air travel.
“To reach our goal of net zero aviation emissions by 2050, we need transformative aircraft concepts like the ones we’re flying on the X-66A,” said Bob Pearce, associate administrator for NASA’s Aeronautics Research Mission Directorate, who announced the designation at the American Institute of Aeronautics and Astronautics Aviation Forum in San Diego. “With this experimental aircraft, we’re aiming high to demonstrate the kinds of energy-saving, emissions-reducing technologies the aviation industry needs.”