It sounds similar to heart burn.
Boundary Layer Ingestion (BLI) is an idea studied by NASA researchers to reduce emissions by burning less fuel in jet engine and this reduce operating cost of aircraft.
In simple words, the engines of airplanes are located near the rear in BLI. The air flowing over the body of aircraft mix with air being taken by the engine and is accelerated from the back.
According to Jim Heidmann, manager of the Advanced Air Transport Technology Project at NASA’s Glenn Research Center in Cleveland, this concept is not new completely. What is studied now is new technologies that can enable to bring out more benefits from BLI.
What Exactly is BLI? What are its potential environmental and economic benefits?
A quick look at the basics. Four major forces act on a flying airplane – lift, trust, drag and weight. Thrust is the forward force that moves an aircraft ahead, drag is the force acting opposite to thrust trying to slow the airplane down. Lift and weight act opposite to each other, one keeps the aircraft in the sky and the other wants to bring the airplane to the ground.
BLI technique works to reduce the drag which an aircraft experiences in the sky.
When an aircraft moves through the air, a layer of slower moving air starts to form along the skin of the wings and fuselage. This layer is called the boundary layer, and causes additional drag.
The thickness of the boundary layer is zero at the front of the airplane. This layer becomes thicker as this air moves back over the surface of the aircraft wings and fuselage. This layer is almost a foot or more deep by the time it reaches the read of the aircraft.
The jet engines are hung below the wings in a conventional tube and wing airplane, this is the end of story for the boundary layer. The drag-inducing, slower airflow continues till the back end of the airplane and mix with the peaceful air there.
The aerodynamics of an aircraft change when its engines are place in the path of the boundary layer, meaning at the extreme back end of the airplane.
The placement of the engines is different from what is seen in some business jets and airliners today, and is directly behind or atop the main fuselage.
The slower boundary layer air enters the engines placed at these location. It is ingested and leaves the engines at accelerated speed.
It does not matter if this airflow route around the engine core, through the fans and out the back or gets compressed, mixed fuel, burned and forms part of the hot jet exhaust.
This ingested boundary layer of air does not have any effect on the efficiency of the engines.
What Changes Happen?
There is a reduction in the amount of total drag caused by slower moving air around the airplane’s body . A lot of this slow moving air is taken up engines and thrown back at faster speed.
The drag is still there, loss is still there, but it will reduce significantly. And this is the main benefit.
When the drag is reduced, engines will have to create less thrust to move the aircraft ahead. This will ultimately result in less fuel burn, reduced emissions and less fuel expenditure.
All this sounds pretty cool on paper. In practical terms, lot of engineering challenges need to be overcome still.
Engines for Boundary Layer Ingestion
First take a look at the conventional wing and tube design. The engines hang from the bottom of the wings. The air that moves inside these engines is nice, clean and uniform. The airflow slows a little as it is sliced in by the first set of fan blades.
The engine blades experience constant environmental conditions with every revolution – the same air speed and pressure.
When these engines are placed at the back end of the fuselage, the fan blades encounter additional stresses with every revolution. Reason, the airflow coming at this place is distorted.
This is the main challenge faced by NASA research team. They want to design and build an engine whose blades can withstand the additional force. They are testing a BLI engine configuration at Glenn in the 8′ by 6′ wind tunnel.
NASA is performing these experiments as it does not know how to design fan blades that can work in uneven airflow. This distorted airflow hits the blades almost like a hammer in every revolution.
Initial results indicate, it is possible to achieve a proper design. Still lot needs to be done to find the right solution something of a less aerodynamically efficient, heavier engine design. The end design should not nullify the fuel burn efficiency created by reduced drag through BLI.
Numerous airplane concepts are being studied by NASA’s aeronautical researchers and industry partners. Their objective is to utilize BLI to reduce fuel burn.