Designers rethink jet aircraft in the quest for shorter take-offs
What's wrong with this picture? If you said the engines are upside down, you'd be wrong. The odd engine placement is part of a cruise-efficient, short take-off and landing (CESTOL) aircraft concept from the Georgia Tech Research Institute which also sees mechanical wing-flaps replaced by high-speed blasts of air to generate extra lift. It's hoped that the development of such craft will make more airports available to fixed-wing jet aircraft by enabling take off and landing at steep angles on short runways, as well as reducing engine noise.
The research at the Georgia Tech Research Institute (GTRI) as part of NASA's Hybrid Wing-Body Low-Noise ESTOL Program focuses on developing a CESTOL aircraft similar in size to a Boeing 737 which can carry 100 passengers and travel at 600mph.
"To take off or land on a short runway, an aircraft needs to be able to fly very slowly near the runway," said GTRI principal research engineer Robert J. Englar. "The problem is that flying slowly decreases the lift available for taking off and landing. What's needed is a powered-lift approach that combines low air speed with the increased lift capability required for successful CESTOL operation."
The GTRI team placed turbo-fan engines above the wing of the conceptual CESTOL aircraft, rather than below it. Over-the-wing placement enables very high lift while still providing the necessary engine thrust for take-off and high-speed level flight. As an added bonus it also reduces engine noise.
The powered-lift design relies on a circulation control wing or "blown-wing" which sends high-speed blasts of air over the upper surface of the wings during take-off and landing to generate extra lift.
In most fixed-wing aircraft, Englar explains, mechanical flaps are used at take off and landing to increase the size of the curved wing and augment lift. But the lift generated by conventional wings isn't sufficient for the low flight speeds and steep ascents and descents required by CESTOL aircraft.
The blown wing in the GTRI design uses only one small flap. The key is a narrow slot running along the entire trailing edge of each wing through which compressed air is blown.
The wing flap rotates downward on take-off and landing to form a highly curved aft surface; then air from the slot can be blown over that curved surface to generate high lift – two to four times higher than a conventional mechanical flap.
The GTRI design also uses the interaction between the air coming from the wing slot and the exhaust of the plane's over-the-wing jet engines to achieve even greater lift.
"This strategy allows an aircraft to be flying at a very low speed, while the wing is seeing much higher relative wind speeds on its curved upper surface," Englar said. "We have measured lift coefficients between 8.0 and 10.0 on these pneumatic powered-lift wings at a level flight condition during testing. The normal lift coefficient on a conventional wing at a similar flight condition is less than 1.0." "Our design has to incorporate several trade-offs, yet the entire wing-engine powered-lift system has to perform all of its functions well," said Englar, who leads the aerodynamics portion of GTRI's work.
NASA's Hybrid Wing-Body Low-Noise ESTOL Program is a four-year program led by California Polytechnic State University which includes GTRI.