It sounds like a cause of heart burn.
Instead, Boundary Layer Ingestion – or BLI – is a promising idea NASA researchers are studying to reduce fuel burn in jet engines, thus reducing emissions and the cost of operating the aircraft.
At its simplest: With BLI, an airplane’s engines are located near the rear of the aircraft so that air flowing over the aircraft body becomes part of the mix of air going into the engine and is then accelerated out the back
“The idea isn’t completely new,” said Jim Heidmann, manager of the Advanced Air Transport Technology Project at NASA’s Glenn Research Center in Cleveland. “What we’re testing now are new technologies that can help us derive benefits from BLI.”
So, more specifically, what exactly is BLI and how does it lead to potential economic and environmental benefits?
A quick review: When an airplane is flying, it has four major forces acting on it – thrust, drag, weight and lift. Thrust makes an airplane go forward, while drag tries to slow it down. Lift offsets the weight to keep an airplane in the sky.
BLI deals specifically with the drag part of the equation by, ultimately, trying to reduce the total drag an airplane experiences in the sky.
It all starts with the fact that as an airplane flies through the air, a layer of slower moving air begins to build up along the skin of the fuselage and wings, which is fittingly called the boundary layer. This slower moving air causes additional drag.
At the front of the airplane the thickness of the boundary layer is zero, but as the air flows back over the surface of the airplane’s fuselage and wings, the layer grows thicker. By the time it gets to the rear of the airplane it can be a foot or more deep.
In a conventional tube and wing airplane, where the jet engines are hung beneath the wings, that’s the end of the boundary layer story. The slower, drag-inducing airflow just continues off the rear of the airplane to mix with the undisturbed air there.