1. Wing Embedded Engines For Large Blended Wing Body Aircraft
A Computational Investigation
Michael Farrow
MEng Aerospace Engineering
Hi there.
For those of you who don’t know me, I’m Mike Farrow, and as you can see, I have been investigating embedding of jet engines in blended wing body aircraft. [CLICK]
22/05/2009

2. Introduction Why? Method Results Obtained Problems Encountered
Embedded Engines
Blended Wing Bodies
Method
CAD
Meshing
Solving
Results Obtained
Problems Encountered
Conclusions
Questions
So let me tell you what I’m going to talk about today.
First I’m going to tell you why I chose this project, and a bit about what it’s about.
Then I’ll talk about the method I used to obtain my results, and obviously then, a bit about the results themselves.
Next, I’ll take you through a series of the difficulties I have had in this project, and any affects these problems will have on the results.
Then I will talk about my conclusions drawn from the results,
and after that, you’ll get a chance to ask some questions. [CLICK]
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3. The First Jet Airliner The de Havilland DH-106 Comet 1
First Flew in 1949
Four Fully Embedded dH Ghost 50 Turbojets
Believe it or not, this project starts with the first operational jet airliner, the De Havilland Comet, and ends with an aircraft so new it hasn’t even been built yet.
First flown in 1949, comet was probably far ahead of it’s time in all aspects of it’s design, except perhaps metalligy, as you well know. [CLICK]
One of it’s most distinctive features as you can see is the four fully embedded De Havilland Ghost 50 turbojets. [CLICK]
[1]
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4. Embedded Engines [2] [3] [4] [5] [6] [7]
And there are many other notable examples of large aircraft which utilise wing embedded engines, which is of course, far more common on fast military jets, than passenger aircraft. [CLICK]
Northrop YB-49 Flying Wing
Avro Vulcan
Northrop Grumman B.2 Spirit
Handley Page Victor
Hawker Siddley/BAE Systems Nimrod MR4A
Vickers Valiant
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5. The Embedded Argument For Reduction in Weight
Reduction in Viscous Drag
Potential Reduction in Pressure Drag
Potential Noise Reduction
Against
Optimisation of Inlet Efficiency is Difficult
Engine Failures/Fires are More Dangerous
Maintenance and Upgrade Hampered by Structure
So why do these aircraft use embedded engines?
Reduction in Weight due to removal of engine pylon.
Assuming you have a reduction in wetted surface area, there will therefore be a reduction in viscous, or skin friction drag.
There is also the potential to reduce pressure drag, which is a function of the overall frontal profile. However, this assumes that there is no significant loss of lift due to the presence of the engines.
In addition to this there is a potential noise reduction, because of the deeply embedded engines, which is useful for tightly controlled civilian aircraft
So they potentially offer a useful reduction in weight, drag and noise. So why did we stop using them? [CLICK]
Firstly, having the engine inside the wing makes it difficult to optimise the efficiency of the inlet, and any wetted surface in the captured streamtube upstream of the fan inlet will add losses to inlet.
Secondly, any type of engine malfunction, fire, even blade off failure is much more difficult to deal with. Firewalls have to be installed, and more care has to be taken with fan shrouds to control failures. This adds to the weight of the dry engine.
And lastly, maintenance is more difficult around the engine, due to lack of space. And if you want to upgrade your engine in the future, it is much more difficult, as the engines are integral with the aircraft.
All these are good reasons, but not sufficient to overcome the drag and weight benefits.
The real reason in much more simple.
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6. The Embedded Argument [8]
Today’s high efficiency high bypass engines are simply too big for a standard airliner!
But what if the ‘standard’ aircraft configuration was about to change? [CLICK]
[8]
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7. The Blended Wing Body Smoothly Sweeps Wings into Fuselage
Complete Lifting Body
Large Cargo Volume to Wingspan Ratio
[9]
[10]
Many experts believe that a change in the standard aircraft is about to occur, and that the next step in passenger aviation is the blended wing body.
The blended wing body does away with the traditional tubular fuselage, and uses an enlarged wing swept directly into the fuselage. Half way between a traditional aircraft, and a flying wing. [CLICK]
This means that the entire aircraft body is a lifting surface, and none of the wetted structure of the aircraft is wasted. [CLICK]
This also means the frontal thickness is distributed much more evenly over the span, making the aircraft seem bigger for a given span.
So now, the engines and wings are once more of comparable sizes. So I wanted to know, would it be possible to embed engines in a bwb, while maintaining lift and passenger volume? [CLICK]
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8. Construction of CAD Geometry
Sampled from Public Domain Images
3 Test Cases
The first step was to design a test aircraft in the computer aided design environment.
This was done by sampling public domain images, [CLICK] and constructing a wireframe from image dimensions and standard aerofoil sections. [CLICK]
This then formed the basis for the clean aircraft configuration. [CLICK]
Building on this, it was simple to add in three pylon mounted engines as per the Nasa design. [CLICK]
The embedded configuration required a bit more work, as designs for this do not exist, and three engines are incompatible with the profile. Using the handley paige victor as a benchmark, I designed a simplified system of inlet and exhaust for a four engined embedded case. [CLICK]
All three of these configurations are then halved (as the problem is symmetric), and placed within a control volume, which forms the problem boundaries. [CLICK]
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9. Drag Estimation Required for Engine Sizing Skin Friction Drag
Estimated Using Thin Plate Aerodynamics
Pressure Drag
Function of the Projected Cross Sectional Area
Induced Drag from the Lift Coefficient & Span Efficiency
The next step was to estimate the drag on the aircraft, which is required to size the engine. [CLICK]
Skin friction drag, is a function of wetted surface area, which can be loosely approximated using flat plate aerodynamics, where the wing is broken up into lamina and turbulent boundary layer regions, and an area weighted average of the skin friction drag coefficient used. [CLICK]
In the same way, the pressure drag coefficient can be estimated using the frontal profile area. [CLICK]
Induced drag was estimated as a simple function of the lift coefficient and span efficiency factor.
WITH THE DRAG VALUE IN HAND, WE ALSO HAVE THRUST REQUIREMENTS FOR THE ENGINE, AND AN ENGINE CAN BE SIZED ACCORDINGLY. [CLICK]
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10. Construction of Mesh Unstructured Tetrahedral Mesh using ICEM CFD
Prism Layer Grown Outwards from Surfaces
With the CAD correctly dimensioned, the cases can be meshed. Using ansys ICEM CFD, cad models were imported, and an unstructured tetrahedral mesh grown out from the aircraft. This mesh scheme gives a good balance of accuracy to construction time, and is capable of handling the complex swept surfaces of this type of problem.
A better quality, structured mesh would have been too labour intensive for the timescales of this project, but to insure best possible accuracy in the near wall flow, [CLICK] a prismatic layer was grown outwards from the aircraft surfaces into the flow. This insures good quality mesh in the boundary layer regions of the problem.
All that remains is to solve the problem! [CLICK]
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11. Results - Clean Contours of Static Pressure (Pascals)
Ok, as for results, let us look at the aerodynamic behaviour for each case in some detail.
The first thing you notice on the clean aircraft configuration is the formation of a medium strength normal shockwave over the entire span of the aircraft. This is not abnormal in blended wing body aircraft, but when you think of it in terms of the podded engine configuration, we see that there are going to be problems. [CLICK]
The second thing we notice is that the normal shock is interfering with other shock patterns. We have a lambda shaped weak shock forming on the wing, and a slightly stronger shock being generated by the winglet. This interference in shocks will result in an increase in drag. [CLICK]
Contours of Static Pressure (Pascals)
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12. Results - Clean Contours of Static Pressure (Pascals)
Looking further at the winglet, we see a number of radical shock patterns. There is also evidence of large trailing vortices downstream, which leads us to the assumption that the experimental winglet is not functioning correctly.
Despite this however, there was a strong correlation between the estimated drag force, and the computational one. [CLICK]
Contours of Static Pressure (Pascals)
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13. Results – Podded Contours of Static Pressure (Pascals)
Looking in more detail at the podded case, as mentioned earlier, obviously the same clean aircraft shockwave still exists. [CLICK] Considering the engines, we find that the shockwave upstream of the engine inlet dramatically affects the conditions at the engine intake, which is optimised for free stream flow. This means that the engine would be operating off design in the cruise condition, which is unacceptable.
Due to this shockwave, the engine would have to be reoptimised for the behind shock conditions. [CLICK]
Contours of Static Pressure (Pascals)
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14. Results - Embedded Contours of Static Pressure (Pascals)
For the embedded case, we notice straight away that there is a significant difference to the normal span shockwave of the clean case. [CLICK]
The presence of the exhaust manifolds accelerates the propagation of the existing shockwave, bringing it further upstream. [CLICK]
Overlaying the clean aircraft shock shows this in more detail.
This early shockwave will in turn cause premature boundary layer separation, thus increasing the drag on the aircraft. [CLICK]
Contours of Static Pressure (Pascals)
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15. Results - Embedded
In addition to this, we also have a problem with the engine inlet. The flush inlet manifold causes an expansion wave at the inboard edge as the flow turns to enter the inlet. Therefore the inlet flow is lower pressure and velocity than the expected, and a significant mass flow loss occurs. Effectively starving the engines.
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16. Problems Encountered Insufficient Mesh Quality
Underexpanded Jet Exhaust – Unphysical Results
Further problems were encountered with mesh quality. As you can see here, the shockwave and wake are both being attenuated by the expanding grid. [CLICK]
This has the effect of reducing the magnitude of the shockwaves, and thus reducing their effect. Providing therefore an optimistic value of drag. [CLICK]
In addition to this, the under expanded jet exhaust flow causes a high downstream pressure, which acts upon the rear facing surfaces of the aircraft in both cases. The resulting force causes an unreal additional pressure thrust, rendering pressure drag values inaccurate.
However, even with these inconsistencies, sufficient data is available to identify some reasonable trends in behaviour.
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17. Conclusions Embedded Configuration Optimal - External Aerodynamics
Minimum Lift Loss
Less Viscous Drag than Podded
Less Pressure Drag than Podded AND Clean
However:
Optimisation Required for Both Configurations
Serious Structural Questions Remain
Design & Investigation of Duct Flow Required
With the information at hand, the embedded configuration is clearly better in terms of external aerodynamics. It offers much better viscous and pressure drag performance, with the smallest sacrifice in lift.
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18. Any Questions?
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19. References http://bose.utmb.edu/tdpower/Comet.jpg
NASA Facts – July 1997 – The Blended Wing Body
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20. Additional Slides
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21. Pressure Profile
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22. Duct Flow
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23. Solution Spalart-Allmarus Scheme
Initial Incompressible Solution feeds Compressible
Boundary Conditions - Cruise Conditions for BWB Aircraft
Pressure Far Field
12,000m ISA
Mach 0.85
Engine Inlet & Exhaust Conditions from Engine Model
Spalart Allmarus scheme which is specifically designed for aerospace applications, and is relatively stable.
The problems did not converge directly from a constant initial guess, and therefore each case was run as an incompressible problem first, to give a better initial guess for the compressible problem.
Far field boundary conditions for the control volume are available directly from the aircraft cruise information,
And inlet and exhaust conditions are obtained from the engine model described earlier.
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24. Comparison - Lift
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25. Comparison – Viscous Drag
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26. Comparison – Pressure Drag
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27. Comparison – Fuel Consumption
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28. Comparison – Installation Mass
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29. Engine Modelling GE90-115B Turbofan Thermodynamic Mapping by Stage
Perfect Scaling Assumed
The estimated drag force exerted on the aircraft gives us the thrust requirement for the engine, and we can now go about sizing an engine for the job.
As the most powerful, and one of the most efficient engines available today, the GE90-115B is a suitable benchmark.
Using known engine data in conjunction with known environmental values, the engine can be thermodynamically mapped for temperature and pressure throughout it’s stages.
Once the engine had been mapped, and a specific thrust calculated, mass flow rates can be determined, and then used in conjunction with geometric relationships to determine engine dimensions.
And with the physical size of the engines fixed, it was then possible to return to the CAD to adjust the inlet and exhaust sizes, to ensure that the computational problem correctly reflected the physical one.
This does however make one key assumption, that the engine can be perfectly scaled. This is a rather loose assumption, as the majority of engine components do not scale perfectly, but it serves to give a useful approximation.
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30. Engine Placement
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31. Mesh
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32. Mesh – Prism Layer
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33. Span Loading
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34. Engine Options
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35. Jet Flow
Contours of Mach Number
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