This article explains various technical aspects of the boundary layer ingestion (BLI) concept. Using BLI, airliner designs featuring close-coupled, rear-mounted turbofans are being considered, with a fuselage sculpted to sweep a large part of the fuselage boundary layer into engine inlets for reduced fuel consumption. With an engine array fuselage-centered, rather than splayed out on wings, reduced rudder control is needed in the event of a single engine outage. This reduces the size of a BLI tail assembly, saving weight and reducing drag. A near-future goal of the BLI studies is to determine if modern engine front-mounted fans can be designed to operate efficiently and stably under BLI inlet conditions. The D8 design is aimed at the huge single-aisle, narrow-body market, now dominated by the Boeing 737 and Airbus 320 families. Airframe and engine designers strive to achieve 'clean' inlet flow conditions for jet engines.

## Article

In their frontal engine location, fans and compressors work best with a uniform flow, free of significant total pressure losses. The famous S-shaped inlet duct for the middle engine of Boeing's tri-jet 727 required a lot of engineering attention to minimize inlet distortion for happy engine operation.

However, airline fuel costs have become such a major driving factor, the need for clean inlet flow conditions is being re-evaluated. Using a concept called “boundary layer ingestion” (BLI), airliner designs featuring close-coupled, rear-mounted turbofans are being considered, with a fuselage sculpted to sweep a large part of the fuselage boundary layer into engine inlets for reduced fuel consumption. (Recall that Ludwig Prandtl's boundary layer consists of viscously retarded fluid flow near and in contact with a solid surface, that is a source of aircraft frictional drag.)

With an engine array fuselage-centered, rather than splayed out on wings, reduced rudder control is needed in the event of a single engine outage. This reduces the size of a BLI tail assembly, saving weight and reducing drag.

Just how could the ingestion of the slackened and distorted flow of a boundary layer reduce engine fuel consumption? Given a flight speed of u0 , the average air velocity entering a BLI engine would be u1 < u0 where the magnitude of u1 would depend on the extent of the ingested boundary layer. Following a simplified model by Plas [1], consider an idealized one-dimensional flow entering a BLI jet engine at u1, and leaving the engine nozzle at u2. The thrust force, T, created by the engine is:
$T = m˙u2−u1=m˙ Δ u$
(1)

given by the momentum equation in the axial direction for the engine as the control volume. The mass flow of air through the engine is $m˙$ and we have neglected the small effect of fuel flow.

The mechanical power, P, produced by the engine flow is equal to the rate of change of its kinetic energy, given by:
$P=m˙2u22−u12=Tu1+u22=Tu1+Δu2$
(2)

If we assume the BLI engine will have a the same mass flow, $m˙$, and a thrust force, T, as the conventional engine, Δu in (2) will be a constant. A decrease in the average velocity u1 entering the engine will thus result in a decrease in power P — and consequently the promise of reduced engine fuel consumption.

In a paper on wake ingestion, Smith [2] points out it has been long known in the field of marine propulsion, that ingestion of craft wakes (surface ships, torpedoes or submarines) in a propeller can reduce the propulsive power needed. He points out that Albert Betz [3] (a student of Prandtl) explains that with wake ingestion the power expended can actually be less than the product of the forward speed and craft drag.

Recently, an Aviation Week and Space Technology article [4] highlights research going on at Massachusetts Institute of Technology (MIT), NASA, Aurora Flight Sciences, Pratt & Whitney and United Technologies Research Center to evaluate BLI.

Late last year, tests were carried out at the NASA 14’ x 22’ wind tunnel at Langley Research Center, VA. by the BLI team (the first three of the above organizations). Their D8 configuration test model shown in Fig. 1, was designed by Prof. Mark Drela. It departs from the traditional cylindrical tube and wing shape, to provide more fuselage lift with a roughly elliptical cross section. (Current literature frequently refers to it as a “double bubble” shape. As an early bubble experimentalist, I would comment its cross section is similar to a single non-spherical large bubble rising in a liquid, flattened on the bottom and with more curvature on the top.)

The D8 design is aimed at the huge single-aisle, narrow body market, now dominated by the Boeing 737 and Airbus 320 families. Both Boeing and Airbus are projecting [5] this as a two trillion dollar (US) market for the next 20 years, accounting for more than 23,000 airplane deliveries. (Because of its greater fuselage width, the D8 design with twin aisles would modify the market designation.)

The project's principal investigator is MIT's Prof. Edward Greitzer who is also a past chair of the IGTI Board of Directors. Two goals of the test are to validate the BLI benefit and characterize the flow into aft-mounted propulsors. Both BLI and conventionally podded propulsors were tested, using electrically driven fans. The BLI aerodynamic benefit is determined by comparing the mechanical power produced by the fans in each configuration at cruise conditions, defined as zero net force on the model as determined by the wind tunnel force balance. Preliminary data shows promising power savings of 5 to 8 percent for the aerodynamic effects of the D8 integrated configuration [4], with an estimated 18 percent gain including aircraft systems benefits [6].

A near-future goal of the BLI studies is to determine if modern engine front-mounted fans can be designed to operate efficiently and stably under BLI inlet conditions. Stay tuned - I’m sure there will be more to come on the promise of BLI.

## Reference

Reference
1.
Plas, Angelique, 2006, “Performance of a Boundary Layer Ingesting Propulsion System”, Masters Thesis, MIT, Dept. of Aeronautics and Astronautics, pp. 22-23
2.
Smith
,
Leroy-H
, Jr.
,
1993
,
Wake ingestion Propulsion Benefit
,
AIAA Journal of Propulsion and Power
, Vol.
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Jan.-Feb. pp.
74
82
3.
Betz
,
Albert
,
1966
,
Introduction to the Theory of Flow Machines
,
Pergamon Press
, pp.
215
220
4.
Norris
,
Guy
and
Warwick
,
Graham
,
2013
,
Flush with Potential
,
Aviation Week & Space Technology
, Sept. 30, pp.
40
42
5.
Langston
,
Lee S.
,
2012
,
The Coming Single-Aisle, Narrow-Body Aircraft Bonanza
Global Gas Turbine News
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53
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6.
Greitzer
,
Edward M.
, 2014, private communication, January 26.