Years later, while driving home from his office, Bushnell was mulling over the growing demand for ways to get aircraft in tight spaces off the ground. The accepted methods, rotary wing and direct thrust, weren’t enough. Then it hit him. If combined with “circulation control,” a method of generating lift by using jets of air to improve the aerodynamic efficiency of wings the forgotten design could provide another option. “The channel wing couldn’t do it, circulation control couldn’t do it, but maybe they’d be able to do it together,” Bushnell says.
Bushnell directed some money from his discretionary funds, reserved for high-risk, high-payoff projects, and “got back into it” with a program that lasted from 1999 to 2004. The grant money funded Englar’s laboratory work at Georgia Tech. Since then, Bushnell and Englar have co-patented their marriage of circulation control and the channel wing, and Englar continues his work under the auspices of Georgia Tech.
Circulation control is based on the Coanda Effect, named for Romanian aviation researcher Henri Coanda, who in 1910 found that hot gas exiting a jet followed the contour of plates he had installed to deflect the exhaust. Coanda had inadvertently discovered the tendency of a pressurized gas to adhere to an adjacent curved surface. That tendency can be used to increase the lift created by an airplane wing if the exhaust is deflected downward by the wing’s trailing edge.
Circulation control technology works by blowing compressed air—rather than Coanda’s exhaust—over curved trailing or leading edges to achieve very high lift, where and when needed. Researchers believe that circulation control can one day make moving surfaces on aircraft obsolete. By replacing flaps and other mechanical lift maximizers with pneumatic air hoses, engineers can make airplanes lighter, quieter, and easier to maintain.
To find the ideal way to combine circulation control with Custer’s design, Englar used the modern methods of computational fluid dynamics, including data from wind tunnel tests of sensor-studded models. One early goal was to prove that a channel wing with enhanced circulation control could turn a generic twin-engine transport into a super-STOL
The wind tunnel model has an electronic motor that drives either two or three propellers. These can be positioned at various locations to test which placement generates the most lift. In a typical test from 2002, for example, various levels of prop thrust and blowing pressure were tested while the model was kept at a constant angle of attack. In other tests, the angle was changed while the other conditions remained constant.
The research confirmed the potential aerodynamic payoffs of the design in ways that Custer simply could not have. Says Bushnell, “You couldn’t have computed it back then.”
Custer understood that the airflow to generate lift could come either from the airplane’s forward motion or from the engine. But the former auto mechanic and salesman didn’t know—and given the technology of the time, couldn’t have known, engineers now say—that his channel wing caused the air flowing over it to separate and become turbulent. At low speeds and smaller angles of attack, the flow of air detaches from the surface it is traveling across, leading to a loss of the pressure difference that causes lift. Custer could not determine when this would happen, or how to design around it. Also, he didn’t have the digital design tools that could have shown him how to place the external struts of his aircraft without interfering with its aerodynamics.
Englar’s task is to find a way to simultaneously use the channel wing’s ability to generate a lot of lift while weeding out the problems associated with the design. To land, Custer’s airplanes had to be flown at high angles of attack, a dangerous attitude because the pilot can’t see over the nose of an airplane. Also, at a high angle of attack, the failure of one of the two engines could lead to dangerous rolls or stalls, with no way to compensate.