9th AIAA Aviation Technology, Integration, and Operations Conference (ATIO) <br>and<br>Air
21 - 23 September 2009, Hilton Head, South Carolina
AIAA 2009-7098
Potential of the Cross-Flow Fan for Powered-Lift Regional
Aircraft Applications
Corin Gologan*, Sebastian Mores†, Hans-Jörg Steiner*, Arne Seitz*
Bauhaus Luftfahrt, 80807 Munich, Germany
Previous investigations have indicated that the integration of cross-flow fans at the
trailing edge of lifting surfaces has a high potential to increase the maximum lift coefficient,
concurrently reducing drag through thrust generation and wake filling. Several studies have
investigated these effects for thick airfoils. However, these airfoils are not applicable to
commercial aircraft cruising at high subsonic Mach numbers. The application of cross-flow
fans within propulsive wing configurations for commercial aircraft concepts is constrained
by airfoil geometry. Besides that, two different operational modes including high-lift at low
speed and high efficiency at cruise conditions have to be considered for the development of
conceptual design solutions. Now, the major challenges associated with the described crossflow fan application are the understanding of the aerodynamic behavior and the
constructive integration into the wing’s trailing edge. Presented in this paper is an analysis
of the potentials of cross-flow fan based propulsive-wing configurations for the application
to commercial aircraft featuring extreme short take off and landing (ESTOL) capabilities.
The presented results involve kinematic considerations as well as aerodynamic simulations.
Based hereon, a potential propulsive-wing concept using cross-flow fans is introduced.
Nomenclature
AR
Cp
Cd, CD
Cl, CL
D
e
N
Pi
Ps
p
p0
pt1
pt2
Q
q
S
Sref
U
α
φ
ω
ηt
*
†
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
Wing aspect ratio
Pressure coefficient (p-p0)/q
Drag coefficient (2-dimensional, 3-dimensional)
Lift coefficient (2-dimensional, 3-dimensional)
Fan diameter
Oswald efficiency factor
Fan rotational speed
Ideal power
Shaft power
Static pressure
Ambient static pressure
Total pressure at cross-flow fan inlet
Total pressure at cross-flow fan exit
Volumetric flow rate
Free stream dynamic pressure
Area of wing section
Wing reference area
Free stream velocity
Angle of attack
Fan flow coefficient U/ωD
Fan angular velocity
Total efficiency Q(pt2-pt1)/PS
Research Associate, Projects Engineering, Lyonel-Feininger-Str. 28, AIAA member.
Graduate Student, now with Euro Engineering AG.
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Copyright © 2009 by Bauhaus Luftfahrt. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
I. Introduction
P
OWERED-LIFT systems (PLS) have been investigated for over 60 years. Detailed investigations based on wind
tunnel experiments and CFD simulations have been performed for Upper Surface Blowing (USB), Externally
Blown Flaps (EBF), Circulation Control and other system solutions. Several prototypes have been built and flown
and even serial production has been achieved for EBF with the Boeing C-17 and for USB with the Antonov AN-74.
Different projects in the US as well as in Europe are currently
investigating powered-lift regional aircraft for ESTOL application as a
potential solution to airport congestion1,2,3. These research activities
focus on the above mentioned powered-lift systems. The cross-flow fan
might constitute an additional potential solution for the realization of
efficient powered-lift.
The cross-flow fan is a drum-like rotor with forward curved blades,
embedded into a housing (Figure 1) and has been patented by Mortier in
18934. The main activities in cross-flow fan research ever since are
documented by Dang and Bushnell5. Accordingly, the unique advantage
of the cross-flow fan with regard to aircraft propulsion and flow control
is that there is no limitation in the ratio of length to diameter. Thus, with
4
a cross-flow fan embedded into a wing natural distributed propulsion Figure 1. Cross-Flow Fan of Mortier
and/or flow control is possible. Due to wake-filling the theoretical
maximum for the propulsive efficiency is 2.0. A disadvantage of the cross-flow fan is the relatively low compression
efficiency compared to state of the art axial turbo compressors.
Experimental studies of a housing-embedded cross-flow fan have been performed by the Vought Systems
Division (VSD) of LTV Aerospace Corporation. The Naval Postgraduate School (NPS) performed extensive
experiments on a very similar model for which efficiencies of almost 80%6 were determined. Yu et al.7 used the
same model to validate their computational fluid dynamics (CFD)
simulations which were performed using the software ANSYS. It was
found that the performance of the cross-flow fan was well predicted by
the computational simulation. Kummer and Dang validated FLUENT
CFD simulations using experimental results of the VSD and the NPS and
found excellent agreement8. Subsequently they simulated a propulsive
airfoil of 34% relative thickness with a cross-flow fan integrated near the
trailing edge (Figure 2). Thereby the cross-flow fan creates a suction
effect at the fan inlet, draws the flow into the duct and blows the flow out
at the trailing edge of the airfoil creating a jet flap effect. The maximum
achieved lift coefficient was 6.4 caused by supercirculation and the jet
flap effect, while the cross-flow fan operated at a total efficiency of 60%
Figure 2. Cross-flow fan integrated
and the drag coefficient was negative. Attached flow at angles of attack into wing trailing-edge at high angle
of up to 40deg was demonstrated.
of attack8
9
A similar model was investigated by Dygert and Dang . They performed wind-tunnel tests and used the
experimental data for CFD validation. The results showed very good agreement of the simulations and the measured
pressure data. The maximum documented lift coefficient CLmax was 7 for a fan flow coefficient of 0.16. A flying
aircraft model based on these investigations was built by Propulsive Wing LLC (www.propulsivewing.com), using a
thick airfoil with an integrated cross-flow fan for thrust generation and circulation control. Further simulations of a
modified thick Griffith/Goldschmied airfoil with an integrated cross-flow fan were performed by Casparie and
Dang10 showing lift coefficients higher than 10 and significant thrust generation.
Hancock11 performed experimental investigations of a cross-flow fan embedded into a housing and achieved a
maximum efficiency of 82%. He proposed the application of the cross-flow fan for short take off and landing
(STOL) aircraft with transonic airfoils. His ideas included cross-flow fan integration into the wing as well as
integration into the wing section (Figure 3). However, he did not perform aerodynamic investigations of the
proposed propulsive airfoils.
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Figure 3: Cross-flow fan in wing and airfoil proposed by Hancock 11
Based hereon, the goal of this study is to find conceptual design solutions for the cross-flow fan integrated into a
thin airfoil and to assess the aerodynamic potential for the application to powered-lift regional aircraft.
In the following sections essential parts of the present study are discussed in detail. The performed study
includes conceptual design solutions for the cross-flow fan integration, accordant aerodynamic analyses as well as
preliminary performance estimations on aircraft system level. First, a large number of different concept solutions for
the cross-flow fan integration were generated and qualitatively assessed. For the three most promising integration
concepts the kinematic mechanisms were verified in more detail. For the purpose of aerodynamic analyses the wind
tunnel setup of Syracuse University was modeled and simulated in order to validate the applied CFD code.
Subsequently, a supercritical airfoil was selected for further aerodynamic investigation of a promising conceptual
solution. For this model, a 2-D simulation was performed for 15° angle of attack and a fan rotational speed of
12,000rpm representing a potential take-off condition. Based on this reference point an initial estimation of the
implications on a 3-D propulsive-wing is discussed.
II. Concept Solutions
The integration of a cross-flow fan at the trailing edge of a transport aircraft wing section represents a
challenging task, since the wing’s aerodynamic behavior as well as its structural concept is strongly affected. The
requirement for ESTOL transport aircraft to produce high-lift at very low speeds and to be highly efficient at cruise
condition superimposes the necessity of an adequate kinematic mechanism for wing geometric adaptation. These
requirements yield two operational modes to be realized for the wing structure:
 Low-speed mode operating the cross-flow fan as a high-lift device
 High-speed (cruise) optimized mode with closed airfoil contour
During high-speed operation the cross-flow fan may be used for boundary layer control and drag reduction by
wake-filling, which, however, was not primary subject to the here conducted investigations and is one of the open
questions concluded by Dang and Bushnell5.
In order to find feasible concepts for the desired cross-flow fan integration, 30 potential concept solutions were
developed and evaluated. Therefor, a qualitative assessment of multidisciplinary aspects involving aerodynamics,
system complexity, actuation, structural impact, system safety and geometric integration was performed. The three
most promising conceptual solutions were then investigated in more detail including feasibility studies of the
respective kinematic mechanisms using CATIA V5.
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High-Speed Operation
Low-Speed Operation
Concept 1
Concept 2
Concept 3
Figure 4: Kinematic concept solutions
Figure 4 shows the resultant favorable concepts for the integration of cross-flow fans at the trailing edge of a
transonic airfoil. Integration concept 1 is characterized by two flaps providing intake and nozzle characteristics
during low-speed operation. Concept 1 was considered advantageous due to low complexity of its kinematic
mechanism enabling simple actuation. However, airfoil circulation was assessed suboptimal due to expected flow
separation behind the upper flap during low-speed operation. In integration concept 2 the upper shell of the wing
structure is elevated during low-speed operation in order to abate flow separation behind the cross-flow fan air
intake. The outlet duct is designed so that the nozzle is located close to the wing’s trailing edge forming a split flap
shape. The aerodynamic characteristics of concept 2 are therefore evaluated beneficial, while the impact on wing
structure is considered severe. Concept 3 is mainly characterized by the kinematic extension of the wing’s trailing
edge for low-speed operation, thereby opening the cross-flow fan’s air intake and outlet duct. Its kinematic
mechanism is considered robust while the extension of the trailing edge yields an enhancement of effective wing
area. However the cross-flow fan outlet location is regarded unfavorable compared to concept 2.
The basic kinematic functionality of the discussed conceptual solutions was verified using CATIA’s kinematic
toolbox. Finally, concept 2 was chosen as a basis for the subsequent CFD analysis.
III. Aerodynamic Simulation
As a consecutive step during concept evaluation aerodynamic studies were performed in order to investigate
airfoil circulation characteristics due to the proposed cross-flow fan integration. For the aerodynamic simulations,
the ANSYS toolbox (ICEM and CFX) was chosen. This software toolbox was validated by simulating a wind-tunnel
model of a cross-flow fan propulsive wing section built at the Syracuse University. For a first 2-D CFD simulation,
concept 2 was selected as it seemed intuitively to be the most promising concept from an aerodynamic point of
view. However, an aerodynamic comparison of the three concepts has yet to be performed.
A. Validation of CFD Toolbox
For the validation of the toolbox, the wind tunnel model investigated by Dygert and Dang9 was modeled based
on the published data. Figure 5 shows the generated mesh with a total number of mesh elements of 65,000 which led
to approximately 3 weeks of computation time using a 3.4 GHz Pentium 4 processor, 2GB RAM workstation. The
k-ε model was used for the modeling of the turbulences.
The inlet boundary condition was set to 3.92 m/s which equals the wind-tunnel free stream velocity, the outlet
condition was set to ambient pressure. The cross-flow fan region was modeled as a rotating domain with a sliding
interface to the static domain. Instationary simulations were performed using the results of stationary simulations as
a starting condition.
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Figure 5: Mesh of validation model
Cl
Figure 6 shows the lift curve slope taken from the wind-tunnel data of the reference study. The lift coefficient is
a function of the fan flow coefficient φ. The results of the simulated test-points for different angles of attack and fan
flow coefficients showed a good agreement in terms of lift coefficient with the wind-tunnel (WT) results (see Figure
6) taking into account that the simulated model does not exactly match the wind tunnel model geometry. A
comparison of the drag and pitching moment coefficients was not possible as wind-tunnel data for these parameters
was not available.
Validation
8
7
6
5
4
3
2
1
0
WTWT-4000rpm
φ=0.16
WT-3000rpm
WT-
φ=0.22
WT-2000rpm
WTφ=0.33
WT-1000rpm
0
5
10
15 20
α[ ]
25
30
35
WTφ=0.65
Figure 6. CFD results compared to wind-tunnel data
B. CFD Simulation
The basic airfoil selected for the investigated propulsive airfoil was the Lockheed C-141 supercritical airfoil. An
analysis of different supercritical airfoils available in the database of the University of Illinois at UrbanaChampaign12 showed that this airfoil provides the highest internal space for the integration of the cross-flow fan near
the trailing edge. The selected chord length is 2.24 m and the relative airfoil thickness 15%.
The conducted CFD analysis is based on the previously described kinematic solution of concept 2. However, for
the powered-lift operational mode, the airfoil contour shape was adapted to improve aerodynamic performance.
Therefor, the upper surface contour behind the cross-flow fan’s position was modified to reduce undesirable wake
flow effects (see Figure 7).
The cross-flow fan modeled has 18 blades and is located at 80% of the chord. The outer diameter D of the fan is
0.13m. The lower part of the outlet is slightly deflected downwards to create a jet flap effect and represents a
potential take-off configuration.
Figure 7: CAD model of the investigated concept
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The unstructured mesh of concept 2 was modeled similar to the mesh of the validation model and had around
90,000 cell elements (see appendix Figure 13). The inlet boundary condition was set to a free stream velocity of
U=35 m/s (Mach 0.1). The outlet condition was set to ambient pressure. The Reynolds number was 6.7•106
calculated based on the airfoil chord length and 1.5•105 calculated based on the blade chord. The time step was
2.77e-5s corresponding to 180 positions of the cross-flow fan calculated for one revolution. y+ values were less than
5 while the other pre-processor settings were kept similar to the settings of the validation model.
A simulation at 15° angle of attack and a fan rotational speed of 12,000rpm (corresponding to a flow coefficient
of φ=0.21) was performed for a first estimation of the potential of the system and to get an understanding of the
effects occurring.
Figure 8 shows the flow field for concept 2 at this operational point compared to the basic airfoil without crossflow fan. The pressure coefficient and Mach number for the propulsive airfoil concept are shown in Figure 9. All
results refer to a time-averaged solution.
Figure 8. Flow field for basic airfoil and concept 2 at α=15°
For the same angle of attack it can be seen from Figure 8, that the wing circulation is significantly higher for the
cross-flow fan featured airfoil. The cross-flow fan creates a suction effect at the fan inlet (see Figure 9 left) which
draws the flow into the duct. Thus, for the propulsive airfoil concept the flow is attached while for the basic airfoil
this angle of attack represents the beginning of flow separation.
The fan suction causes a decrease in static pressure on the upper side of the airfoil (see Figure 9 left) which
strongly increases aerodynamic lift. Inside the fan the flow is compressed which results in an increase of the velocity
at the exit plane of the fan (see Figure 9 right). The accelerated flow exits the duct at the trailing edge of the wing
creating a jet-flap effect. Mach numbers of above 0.7 occur inside the duct, while the maximum Mach number for
this operational case is 1.0.
For this operational condition the indication is given that the effects which were demonstrated for the propulsive
airfoil concepts with thick airfoils by Dygert and Dang9 as well as Kummer and Dang8 also apply to a cross-flow fan
imbedded into a supercritical airfoil. However, the flow characteristic at higher angles of attack and other fan flow
coefficients are open issues that have to be analyzed and optimized in future simulations to determine the maximum
lift coefficients and the angles of attack where flow separation occurs.
Figure 9. Pressure coefficient Cp (left) and Mach number (right) for propulsive airfoil α=15° and φ=0.21
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Figure 10 shows the flow-field of the cross-flow fan intake and outlet. The color code represents the velocity
with a maximum speed of 200 m/s. Flow separation can be observed within the inlet duct. Here, improvement could
be achieved with an increase of the inlet lip radius that has been proven for a generic fan-in-wing model in a recent
study13. A backflow appears in the outlet of the cross-flow fan duct, which indicates that additional improvement
could be achieved with an optimization of the flap and deflector geometry.
Figure 10. Streamlines for the cross-flow fan flow
Table 1 shows the lift and drag coefficients for the basic airfoil and the propulsive airfoil. The total lift
coefficient for the propulsive airfoil is 5.29 (calculated with the average density of 1.184 kg/m³), which is 3.5 times
the lift coefficient of the basic airfoil which amounts to 1.5 at 15° angle of attack. The resulting total drag coefficient
for the propulsive airfoil has a negative value of -1.31, which means that thrust is produced. The wind tunnel model
investigated by Dygert and Dang9 had a lift coefficient of 4.35 at the same angle of attack and a comparable fan flow
coefficient (0.22). However, an overall comparison of the aerodynamic performance with the wind-tunnel is not
possible as there is no information available neither about the drag produced nor about the power required available.
Cl
Cd
Basic airfoil
1.5
0.01
Propulsive airfoil
5.29
-1.31
Table 1. Lift and drag coefficients for the basic airfoil and propulsive airfoil at α=15° and φ=0.21
The shaft power Ps required for the cross-flow fan is 223 kW per meter span for this operational case. The shaft
power required seems to be high, but one has to consider that a considerable amount of thrust is produced by the
system. The ideal power is 85.5 kW/m resulting in a total efficiency ηt =38%. This value is low compared to the
total efficiencies published by Kummer and Dang8 (between 55% and 60%) for their propulsive airfoil. The low
efficiency shows the high optimization potential for the cross-flow fan and the accordant entire propulsive airfoil
system.
kW/m
Ps
223
kW/m
Pi
85.5
kg/s
Mass flow per meter
10
pt2/ pt1
1.1
ηt
38%
Table 2. Results for cross-flow fan parameters
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IV. Potential Regional Aircraft Application
A wing with an area of 50m², an aspect ratio of 10, and a taper ratio of 0.5 was considered representative for a 50
seat regional aircraft. Based on the 2-D simulation results for the investigated operation point, 3-D aerodynamic
coefficients and power requirements are estimated. The wing area is divided into three sections according to Figure
11. The inner part that intersects with the fuselage (SFuselage) has a total area of 6.4 m² and represents the wingfuselage intersection. The middle part represents the section with the integrated cross-flow fan (SCFF=34²) and the
outer part the aileron-section with no cross-flow fan installed (SAileron=9.6m²) giving the total reference area of 50 m².
Figure 11: Wing section definition for calculation of 3-D polar
The 3-D lift-coefficient CL is then calculated as the area-average of the lift coefficients of the three sections:
CL
ClCFF S CFF
Cl Aileron S Aileron
(1)
S ref
For the cross-flow fan section the lift coefficient of the 2-D CFD simulations (Table 1) and for the outer part the
coefficient for the basic airfoil (1.5) is used. It is assumed that the fuselage section does not produce lift. The 2-D
drag coefficient presented in chapter III represents the cross-flow fan airfoil section only. For the calculation of the
3-D drag coefficient CD, again, the area-average of the drag coefficients of the three sections is calculated.
Additionally, fuselage drag and lift induced drag are added, as they are not included in the 2-D data. Thereby it is
assumed, that the lift induced drag characteristic while the fan is operating is the same compared to a conventional
wing. This trend has been proven for externally blown systems by the help of wind-tunnel data14:
CD
C dCFF S CFF
C d Aileron S Aileron
S ref
C D 0Fuselage
1
C L2
AR e
(2)
The Oswald efficiency factor e is assumed to be 0.75. To account for the fuselage, a drag coefficient CD0Fuelage of
0.03 was added. The power required is scaled linearly with the span of the cross-flow fan section taking the value
from Table 2 as a reference. Table 3 shows the resulting values for these assumptions. The lift coefficient is high
(3.8) while the thrust produced by the cross-flow fan is higher than the drag of the aircraft, which results in a total
drag coefficient of -0.26 and a climb angle of 3.9°.
CL
3.80
CD
-0.26
°
Climb angle
3.9
kW
Installed power
3,211
Table 3. Performance data for aircraft system, α=15° at N=12,000rpm (φ=0.21)
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Based on these coefficients the reference aircraft could lift 14,366 kg at 35 m/s which corresponds to a wing
loading of 287 kg/m² or 59lb/ft² which is slightly lower than typical values for turbo-prop aircraft. At this condition
the excess-thrust produced is 9.4 kN while the power required is 3,211 kW. Given the fact that at this operational
point the flow is completely attached and higher angles of attack may be realizable, the cross-flow fan integrated
into the wing has the potential to serve as a powered-lift device that produces high-lift as well as a significant thrust.
However, these findings are based on a very simple method for the determination of the three dimensional
performance for one operational point only. Three dimensional effects that are not determined here may occur for
swept and tapered wings. These effects will have to be determined with 3-D CFD simulations.
Finally, Figure 12 shows an artist view of a regional aircraft with a cross-flow fan powered-lift system. Within
this idea the cross-flow fans are powered by the over wing mounted engines. However, depending on the high-speed
performance of the cross-flow fan at low angle of attack, it could be used as distributed embedded wing propulsion
system without additional propulsion systems.
Figure 12: Artist view of regional aircraft with cross-flow fan powered lift system
V. Conclusion
Presented in this paper is an analysis of the potentials of cross-flow fan based propulsive wing configurations for
the application to commercial aircraft featuring extreme short take off and landing capabilities. Therefor, kinematic
mechanisms for three cross-flow fan propulsive airfoil concept solutions were evaluated in a preliminary assesment
enabling a low-speed operation as well as high-speed operation mode. The most promising of these concept
solutions from the aerodynamic point of view was selected for CFD simulations. For the purpose of aerodynamic
analyses the wind tunnel setup of Syracuse University was modeled and simulated in order to validate the applied
CFD code showing good agreement with the experimental data in terms of lift coefficient for different angles of
attack and fan rotational speeds. Subsequently, a thin supercritical airfoil was selected for aerodynamic
investigations of the preferred concept solution. For this model, a 2-D simulation was performed at a free stream
velocity of 35 m/s for 15° angle of attack and a fan rotational speed of 12,000rpm representing a potential take-off
condition. The supercirculation and jet-flap effect described by reference studies using thick airfoils could be
demonstrated for the investigated system based on a thin airfoil. The lift coefficient was found to be greatly
enhanced (5.29) compared to the basic airfoil (1.5) and thrust was produced (drag coefficient of -1.31). Based on
these 2-D results a first estimation of 3-D lift and drag coefficients was performed with simple methods. The
calculated lift coefficient for an aircraft with a reference area of 50m² was 3.8 while the total drag coefficient was 0.26 resulting in a climb angle of 3.9° and a wing loading of 287kg/m² at 35m/s.
The gained results are promising, considering that the concept developed was not yet optimized. Potential for
improvement lies in the design of the cross-flow fan blades, ducts, as well as the airfoil shape. These improvements
can enhance the aerodynamic characteristics and increase the total efficiency resulting in a lower power
requirement.
In further research, additional simulations for different airspeeds as well as rotational speeds, flap angles and
angles of attack have to be performed for the 2-D airfoil to find maximum lift and drag coefficients achievable as
well as the points where the flow is separating. CFD simulations for a wing section have to be performed to
determine the effects caused by wing sweep and taper. The solutions for the kinematic mechanisms will have to be
extended to account for the three dimensional integration aspects and the resulting actuation forces.
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Appendix
Figure 13: Mesh of concept 2
Acknowledgments
The authors would like to thank Prof. Max Platzer for introducing us to this interesting topic. Nicolas Thouault is
gratefully acknowledged for his support with the CFD simulations. Dr. Andreas Sizmann and Dr. Jost Seifert are
acknowledged for fruitful discussion and valuable advice. Additional thanks goes to Michael Hembera and Nikolaus
Spyra for excellent cooperation and Matthias Bühner for research assistance.
References
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4
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9
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10
Casparie, E.D. and Dang, T.Q., "High lift/low drag thick subsonic Goldschmied/ Griffith airfoil with integrated cross-flow
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11
Hancock, J.P., "Test of a high efficiency transverse fan," AIAA/ SAE/ASME 16th joint propulsion conference, Hartford,
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12
Selig, M. UIUC Airfoil Coordinates Database.
13
Thouault, N., Breitsamter, C. and Adams, N.A., "Aerodynamic Characteristics of a Lift-Fan under Large Inflow Distortion
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14
Johnson, W.G.J., "Aerodynamic and Performance Characteristics of Externally Blown Flap Configurations," Conference on
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