Numerical and Experimental Analyses of the Ducted Fan for the Small VTOL UAV Propulsion

Trans. Japan Soc. Aero. Space Sci. Vol. 56, No. 6, pp. 328–336, 2013

By Leesang CHO,1Þ Seawook LEE2Þ and Jinsoo CHO2Þ

1ÞDepartment of Mechanical Systems Engineering, Hansung University, Seoul, Republic of Korea 2ÞSchool of Mechanical Engineering, Hanyang University, Seoul, Republic of Korea

(Received February 9th, 2012)

Ducted fans have higher thrust performance, higher propulsion efficiency and lower noise characteristics due to their duct system compared with commercial isolated propellers. The purpose of this study is to present design procedures in order to improve the aerodynamic performance of the ducted fan for the small VTOL UAV propulsion. In addition, the duct effect of ducted fans is analyzed to satisfy design requirements and improve performance. Aerodynamic design of the rotor and the stator blades of the ducted fan involves a series of steps: meanline analysis, through-flow analysis and aerodynamic analysis, based on consideration of the design requirements. The thrust performance of the ducted fan is somewhat higher compared with that of the rotor only, but wind tunnel test results of the ducted fan do not satisfy the design requirements. The thrust performance of the ducted fan is significantly different in the CFD results and wind tunnel results due to the inconsistency of the intake and the duct shape. Therefore, the thrust performance of the ducted fan would be somewhat improved by the optimization of the intake and the duct shape.

Key Words: Ducted Fan, Free Vortex Design, Through-Flow Analysis, CFD Analysis, Wind Tunnel Test

Nomenclature rotor: rotor efficiency stator: stator efficiency [A]: finite difference coefficient matrix : degree of reaction b: mean stream surface thickness parameter : air density DR: rotor diameter : stream function F: flux vector !~: angular velocity Fz: force vector in z direction G: flux vector 1. Introduction H: flux vector I: relative enthalpy, or rothalpy Autonomous flying vehicles, which are commonly known J: source term as unmanned aerial vehicles (UAVs), are currently used for N: rotational speed military missions and civil applications such as reconnais- p: pressure sance, surveillance and topographical surveys.1) UAVs take R: ducted fan radius advantage of the lower operating cost, the lower risk and r: blade radius the higher practical use compared with manned aircraft in r~: position vector of rotating reference frame warfare and danger zones.2) rtip: blade tip radius Ducted-fan UAVs are typical example of vertical take- T: temperature, thrust off/landing (VTOL) UAVs, which do not need a runway U: solution vector, blade rotational speed and can constantly observe surveillance areas by hovering, Utip: blade tip speed and there is more interest in them than existing fixed-wing V: absolute velocity UAVs.3) Vr: radial component of absolute velocity Ducted-fan UAVs can also operate in hover mode and can Vz: axial component of absolute velocity fly at high speed by pitching over towards a horizontal atti- 4) V: tangential component of absolute velocity tude. Therefore, ducted-fan UAVs have attracted attention Wr: radial component of relative velocity as the future combat system due to their more compact size Wz: axial component of relative velocity and higher static thrust/power ratio than any other VTOL 5) W: tangential component of relative velocity UAV. z: axial component In general, a conceptual description of a ducted-fan UAV : absolute flow angle is shown in Fig. 1. The rotor is the propeller at the center of : relative flow angle the UAV, which is supported by the fuselage. The fuselage fan: fan efficiency is connected to the duct by struts, and the duct encases the remaining internal components. The stator blades, which Ó 2013 The Japan Society for Aeronautical and Space Sciences Nov. 2013 L. CHO et al.: Numerical and Experimental Analyses of the Ducted Fan for the Small VTOL UAV Propulsion 329

Fig. 2. Mission proﬁles of the ducted-fan VTOL UAV.

Most studies on ducted-fan UAVs have used CFD analy- Fig. 1. Schematic diagram of the ducted-fan VTOL UAV.3) sis and wind tunnel tests to investigate performance rather than design optimization to improve aerodynamic performance. Furthermore, the stability of ducted-fan UAVs has straighten the swirling flow induced by the rotor, are located mainly been investigated from the point of view of flight downstream of the rotor inside the duct. Finally, the moving control. flaps (or control vanes) are downstream of the stators, creat- The stability and the flight control of ducted-fan UAVs ing control forces and moments from the high-speed exit are the most important factors. flow to stabilize and steer the UAV.6) As shown in Fig. 2, ducted-fan UAVs demand high The General Dynamics ducted-fan UAVs were developed endurance due to the same thrust performance despite the and flown starting in 1960 with the PEEK aircraft.7) In 1999, small size and light weight. Therefore, the improvement the Micro Craft iStar vehicle was manufactured (two 9-inch of the thrust performance and the propulsion efficiency for diameter flight test vehicles manufactured under DARPA ducted-fan UAVs is required based on a systematic study funding).8) of the aerodynamic design and the aerodynamic analysis Ducted fans have higher thrust performance, higher pro- of the ducted fan. pulsion efficiency and lower noise characteristics due to In this study, systematic design procedures are presented their duct system compared with commercial isolated pro- in order to improve the aerodynamic performance of the pellers.9) Empirical data show that the total thrust produced ducted fan for the small VTOL UAV propulsion. The duct by the ducted rotor system in hover mode is usually 20 to effect of the ducted fan is analyzed to satisfy design require- 50% greater than that of an identical unducted rotor operat- ments and improve performance as well as verify the results ing at the same power.10) presented by McCormick.10) In recent years, aircraft researchers and designers have shown great interest in the use of ducted fans for the propul- 2. Ducted Fan Design sion of UAVs and special air vehicles which can achieve vertical take-off and landing. To improve the aerodynamic performance of the ducted Guerrero et al.3) implemented a multidisciplinary design fan for the small VTOL UAV propulsion, systematic optimization framework to enable conceptual design of design procedures are required for the rotor and the stator ducted-fan UAVs. blades. Design procedures of the rotor and stator blades Ko et al.11) presented the design tool, which enables mul- for the ducted fan are shown in Fig. 3. tidisciplinary design optimization (MDO) and trade studies Aerodynamic design processes for the rotor and stator in the conceptual and preliminary phases of design. blades of the ducted fan follow a series of steps, which are Computational fluid dynamics (CFD) analyses and wind meanline analysis, through-flow analysis and aerodynamic tunnel tests have been performed on the ducted fan by analysis, based on consideration of the design requirements. several researchers. When the performance characteristics of the designed Akturk et al.12) investigated viscous and turbulent flow ducted fan do not satisfy the design requirements at each fields around and inside the ducted fan for hover and for- design step, the design process is restarted again at the ward flight conditions with cross-wind using a commercial previous design step. CFD tool (FLUENT) and the PIV system in the wind tunnel. In this present paper, the rotor and stator blades of the Fleming et al.13) measured aerodynamics characteristics ducted fan are designed using meanline analysis and the of the baseline vehicle and traditional control vanes in simplified meridional flow method16,17) according to the cross-wind using the balance system in the wind tunnel. design requirements. Through-flow analysis uses the matrix Avanzini et al.14) analyzed the flowfield around the air- method18) for the hub-to-shroud flow fields of the meridional 19) frame of the ducted fan vehicle and investigated the per- plane (Wu’s S2 surface ). Aerodynamic analysis of the formance and the stability characteristics. Furthermore, ducted fan is performed using a commercial CFD tool De Divitiis15) calculated aerodynamic coefficients of two (FLUENT) for the steady-state flow.20) Thrust performance ducted fan models to investigate performance and stability of the ducted fan is measured using the balance system in the in significant flight conditions. open type subsonic wind tunnel. 330 Trans. Japan Soc. Aero. Space Sci. Vol. 56, No. 6

Rotor inlet relative flow angle, β β1 Rotor outlet relative flow angle, 2 Stator inlet absolute flow angle, α α2 Stator outlet absolute flow angle, 3 90

Flow angles (deg.) Flow 0 0.2 0.4 0.6 0.8 1

r/rtip

Fig. 4. Flow angle distributions of the rotor and the stator blades for the design requirements.

θ 90 Camber angle, R Stagger angle, ξ } Rotor R Camber angle, θ 75 ξ S Stator Stagger angle, S } 60

15 Blade shape angles (deg.) 0 0.2 0.4 0.6 0.8 1

r/rtip Fig. 3. Design procedures of the ducted fan. Fig. 5. Blade shape angle distributions for the rotor and the stator blades of the ducted fan. 2.1. Free vortex design method The flow in the rotor blade passage of the ducted fan has no radial velocity component, which is commonly known as fan ¼ rotor þð1 Þstator ð3Þ the radial equilibrium axisymmetric flow. 2.2. Fan design results The rotor and the stator blades of the ducted fan are Performance characteristics of the ducted fan are designed using a free vortex design condition with the radial estimated for the variation of design parameters based on equilibrium flow assumption. the design requirements. The simplified radial equilibrium equation is shown as Figure 4 shows flow angle distributions for the rotor and Eq. (1). the stator blade of the ducted fan with the design require- 2 ments. In general, the static pressure rise of the rotor blade 1 dp V ¼ ð1Þ is higher compared with stator blades. Therefore, the magni- dr r tude of the relative velocity and the relative flow angle at the where is the air density, p is the static pressure, r is the rotor inlet is higher than that at the stator inlet. blade radius and V is the tangential component of the abso- Figure 5 shows the blade shape angle distributions for lute velocity. the rotor and the stator blades. The static pressure rise of Free vortex flow means that the product of the blade the rotor is introduced by the difference of the relative radius and the tangential component of absolute velocities velocity between the rotor inlet and the rotor outlet and is a constant and the axial velocity (Vz) is constant along the static pressure rise of the stator blades are affected by the radial direction of rotor blades as shown in Eq. (2). the difference of the absolute velocity. As shown in this figure, stagger angle distributions of the rotor blade are rV ¼ const.; V ¼ const. ð2Þ z higher compared with the stator blades. Approximately, The performance characteristics of the ducted fan are the twist angle of the rotor blade from hub to tip is 33 considered effects of the number of blades, the hub to tip degrees and the stator blade is 31 degrees. ratio, the taper ratio, the solidity and velocity triangles for Figure 6 shows the incidence and deviation angle distri- rotor and stator blade rows. butions for the rotor and the stator blades. Incidence angles The efficiency of the rotor and the stator is calculated of the rotor and the stator blades are designed to be 0 degrees with loss models, which are profile losses,21,22) annulus wall from hub to tip. Deviation angle distributions of rotor blades losses,23) secondary losses24) and tip clearance losses.25) are highly decreased from mean radius to blade tip. This The total efficiency (fan) of the ducted fan is estimated by result means the static pressure rise is excessively increased considering the degree of reaction (), the rotor efficiency at the blade lower surface (pressure side) and that most of (rotor) and the stator efficiency (stator) as follows. the static pressure rise of the ducted fan is produced in the region of the mean radius to blade tip of the rotor. Nov. 2013 L. CHO et al.: Numerical and Experimental Analyses of the Ducted Fan for the Small VTOL UAV Propulsion 331

YZ 60 Incidence angle, iR δ Rotor X Deviation angle, R } Incidence angle, i δS Stator 40 Deviation angle, S }

20 0.1

0 0.075 0.05 r θ 0.05 0 -20 Incidence & deviation angle (deg.) Incidence & deviation 0.2 0.4 0.6 0.8 1 0.025 -0.05 r/r 0 0.02 0.04 0.06 0.08 tip z Fig. 6. Angle distributions for the rotor and the stator blades. Fig. 8. Blade geometries of the rotor and the stator blade.

of the three-dimensional flow in turbomachinery. This method is a solution for the quasi-three-dimensional flow on the meridional or mid-channel stream surface between two blades (Wu’s S2 surface). For the prediction of the flow field on an S2 stream surface, the governing equation can be expressed in terms of derivatives and velocity components on the quasi-orthogonal mesh as in Eq. (4) shown in detail by Katsanis and McNally.18) The matrix method combines the equation of motion and the continuity equation by the use of the stream function ( ), to obtain the following principal equation for Fig. 7. Designed shape of the ducted fan. an arbitrary set of coordinate axis (r and z) in the meridional plane. Table 1. Specifications for the rotor and the stator of the ducted fan. @2 @2 @ @ þ ¼ qr; z; ; ð4Þ Rotor Stator @r2 @z2 @r @z Fan diameter 250 mm 250 mm @ @ ÂÃ@ @ ÂÃ Tip diameter 248 mm 250 mm q ¼ ln ðrbÞ þ ln ðrbÞ @r @r @z @z Hub diameter 49.6 mm 49.6 mm rb @I @s W @ Airfoil shape NACA65 series þ T ðrVÞFz ð5Þ Wr @z @z r @z Camber angle 10 deg @ @ Blade thickness 30% (hub)–10% (tip) ¼ rbWr; ¼rbWz ð6Þ Stagger angle at mid span 69.5 deg 11.2 deg @z @r Blade angle at mid span 20.5 deg 78.8 deg The numerical results described in this paper are obtained Solidity at mid span 0.174 0.4 by assuming a radial S2 surface (Fz 0) for which the flow 1 Number of blades 3 5 angle ( ¼ tan ðW=WzÞ) is known. Rotational speed 7,000 rpm — 3.2. Orthogonal mesh and finite difference method The matrix method involves covering the region of interest with a fixed irregular grid, and writing a finite difference Figure 7 shows the designed shape of the ducted fan. The approximation to the principal equation at every interior number of blades for the rotor is three and the stator is five. grid point. This result is in one algebraic equation for every The stator blades show opposite twist distributions com- interior grid point in terms of the stream function at that and pared with rotor blades. neighboring points. These equations can be expressed in The specifications for the rotor and the stator blade of the matrix form26) as Eq. (7). ducted fan are shown in Table 1. The diameter of the rotor is ½A½ ¼½Qð7Þ 248 mm, and the hub to tip ratio is 0.2. The diameter of duct and stator blades is 250 mm, and the hub to tip ratio of stator Here ½A is the coefficient matrix derived from replacing blades is 0.198. the differential operator r2( ) and ½ is the vector of the quantities qðr; zÞ. 3. Matrix Through-Flow Analysis Figure 8 shows blade shapes of the rotor and the stator blade for the quasi-three-dimensional analysis of the de- 3.1. Governing equations signed ducted fan. The matrix through-flow method, which is adapted from Finite difference mesh grids of the computational domain Wu’s general theory,19) has been widely used in analysis for the meridional plane are shown in Fig. 9. Number of 332 Trans. Japan Soc. Aero. Space Sci. Vol. 56, No. 6

3 W/U 0.125 Streamline 1 tip W /U (r/rtip=0.2) m tip 2 Wθ/Utip 0.1 1

r 0.075 0

-1 0.05 -2 0.025 Nondimensional relative velocity Nondimensional relative -3 0 0.02 0.04 0.06 0.08 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Nondimensional axial distance (z/r ) z tip (a) Hub region (streamline 1) Fig. 9. Finite difference mesh grids for the rotor and the stator blades of 3 W/Utip the ducted fan. Streamline 11 W /U (r/r =0.72) m tip tip W /U 2 θ tip orthogonal meshes for the rotor and the stator blade are 21 21 and computational regions are 61 21. 1 3.3. Matrix through-flow analysis results 0 Meridional and relative velocity distributions for the esti- -1 mation of three-dimensional flow fields in the designed ducted fan are shown in Fig. 10. -2

Figure 10(a) shows the variation of the relative velocity velocity Nondimensional relative -3 components at the hub region of the ducted fan. The magni- -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Nondimensional axial distance (z/rtip) tude of the relative velocity is higher at the rotor inlet than (b) Mean radius (streamline 11) that of the stator blades. This result is due to the rotation of 3 W/U the rotor for the static pressure rise, which introduces the Streamline 21 tip W /U (r/rtip=1.01) m tip thrust production. 2 Wθ/Utip Figure 10(b) shows the relative velocity component dis- 1 tributions at the mean radius. The diﬀerence of the relative velocity between the leading edge (z=rtip ¼ 0:075) and the 0 trailing edge (z=rtip ¼ 0:122) of the rotor blade at the mean -1 radius is higher compared with the hub region. Figure 10(c) shows the variation of the relative velocity -2

components at the shroud (casing) region of the ducted velocity Nondimensional relative -3 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 fan. The difference of the relative velocity at the shroud Nondimensional axial distance (z/rtip) region is dramatically higher than any position of the radius (c) Shroud region (streamline 21) of the rotor blade. This result means the static pressure rise, which introduces the thrust production of the ducted fan, is Fig. 10. Meridional and relative velocity distributions in the designed increased in line with increments in blade radius from hub ducted fan. to tip. Figure 11 shows the nondimensional velocity contours the free vortex design method and the matrix through-flow for the meridional stream surface in the designed ducted fan. analysis method, are fairly well matched. As shown in Fig. 11(a), axial velocity distributions are almost uniform from the fan inlet to the fan outlet. 4. CFD Analysis Figure 11(b) shows the contour of the radial velocity for the through-flow fields in the ducted fan. The magnitude 4.1. CFD simulation schemes of the radial velocity is very small in the ducted fan. This In general, the rotating reference frame (RRF) is used for result is similar to the flow patterns of the axial flow type the steady-state numerical analysis of rotating bodies such turbomachines. as centrifugal compressor impellers and axial compressor As shown in Fig. 11(c), the magnitude of the tangential rotors. velocity is relatively higher and there is a complex pattern In this study, CFD simulation of the ducted fan with con- at the tip regions of the rotor different from the free vortex stant rotational motion is carried out using a single rotating design condition. reference frame (SRF) without additional computing time Figure 12 shows three-dimensional velocity profiles in and application of the user defined function (UDF) of the ducted fan, which are predicted by the free vortex design unsteady state analysis schemes. method and the matrix through-flow analysis method. As Figure 13 shows the rotating coordinate system for the a whole, three-dimensional velocity profiles at each cross- CFD analysis of the ducted fan, where r~ is the position sectional plane of the ducted fan, which are predicted by vector and !~ is the angular velocity. Nov. 2013 L. CHO et al.: Numerical and Experimental Analyses of the Ducted Fan for the Small VTOL UAV Propulsion 333 tip

1 7 DF-UAV(N =3 × 5) V /U 1 z tip . Vz/Utip B 0 N=7,000 rpm Vr/Utip Design 0.9 0.2 } Vθ/U 0.19 Rotor upstream tip Vz/Utip 0.8 0.18 0.3 Vr/Utip 0.17 Matrix method

tip Vθ/U } tip 0.7 0.16

1 /U . 0 θ 0 . 0.15 0.2 1

0.6 6

0.14 ,V 0.5 0.13 tip 0.1

0 /U

0.12 r . 1 7 0.11 0.4 ,V

7 tip 1 0.1 . 0

6 0

0.3 .1 /U

0 z 0 . 6 16 1 . V

Nondimensional radial distance, r/r 0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 Nondimensional axial distance (z/r ) tip r/rtip (a) Axial velocity (a) Rotor upstream (z / r tip = 0.0) tip 1 0 0 V /U

0 r tip 0 0 0 0.1 × V /U 0.9 0 DF-UAV(NB =3 5) z tip 0 0.08 V /U Design

0 N=7,000 rpm r tip

0 } 0.8 0 0.06 Rotor downstream Vθ/Utip 0.04 V /U 0.7 z tip

0 0.3 0.02 Vr/Utip Matrix method 0 0 tip Vθ/U } 0.6 0 tip

-0.02 /U

θ 0.2 -0.04 0.5 0 0 -0.06 ,V tip 0.4 -0.08 0.1 /U r

0 -0.1

0 0 ,V 0.3 0 tip 0

0 /U Nondimensional radial distance, r/r 0.2 z

0 0.1 0.2 0.3 0.4 0.5 0.6 V -0.1 Nondimensional axial distance (z/rtip) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 r/r (b) Radial velocity tip

(b) Rotor downstream (z / r tip = 0.24)

t 8 . .

2 4 0

. . - 0

1 2

- 4 1

. 0 0 . V /U 0 θ 0

- 0 tip

8 1

0 0.9 .

0 0 0.8 . 0 4 × . V /U 4 DF-UAV(N =3 5) 0 z tip -

2 0 0 B

0 0.8 . 1 . 0.6

- . 2

. 0 2 08 . 4 - V /U 0 0 Design 0 . N=7,000 rpm r tip 6 2

- 0 . 0 . .

0 2 0.4 4

. . 0 } 2 V /U 6 - θ 0.7 .2 Stator downstream tip 0 0.2 Vz/Utip 0 0 0.6 0.3 Vr/Utip -0.2 Matrix method

. tip Vθ/U }

0 tip 0.5 -0.4 /U

-0.6 θ 0.2

0.4 -0.8 ,V -1 tip 0.1 /U

0.3 r ,V Nondimensional radial distance, r/r

0.2 tip 0 0.1 0.2 0.3 0.4 0.5 0.6 0 /U Nondimensional axial distance (z/r ) z

tip V -0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 (c) Tangential velocity r/rtip Fig. 11. Velocity contours for through-ﬂow ﬁelds in the ducted fan. (c) Stator downstream (z / r ti p = 0.6)

Fig. 12. Three-dimensional velocity profiles at each cross-sectional 4.2. Governing equations and turbulence model planes of the ducted fan. To predict three-dimensional viscous turbulent flows and thrust characteristics of the ducted fan, governing equations (continuity, momentum and energy equations) are expressed k–! shear stress transport (SST) model.28) This turbulence as in Eq. (8) shown in detail by Anderson.27) An equation of model combines the standard k–" turbulence model,29) the state is applied because the working fluid is air. k–! turbulence model30) and the Johnson-King model.31) Equation (8) can represent the entire system of governing The k–! turbulence model is applied in the sublayer of equations in conservation form where U, F, G, H and J the wall boundary layer, and the standard k–" turbulence are interpreted as column vectors26) for viscous flows. The model is used in the wake region of the boundary layer. In column vector U is called the solution vector. The column general, the two-equations k–! SST model is used to cor- vectors F, G and H are called the flux terms (or flux vectors), rectly predict the onset and amount of separation in adverse and J represents a source term that is zero if body forces and pressure gradient flows. volumetric heating are negligible. 4.3. Computational domain and boundary conditions @U @F @G @H Figure 14 shows the computational domain and the þ þ þ ¼ J ð8Þ grid generation for the hovering condition of the ducted @t @x @y @z fan model. CFD analyses of the ducted fan are performed using the As shown in Fig. 14(a), the shape of the computational pressure-based SIMPLE method with the two-equations domain uses the cylinder type for the rotating blade of the 334 Trans. Japan Soc. Aero. Space Sci. Vol. 56, No. 6

(a) Rotor only

Fig. 13. Coordinate systems of stationary and rotating reference frames.19) (b) Ducted fan

Fig. 15. Velocity contours for the hovering condition of the rotor only and the ducted-fan model.

(a) Computational domain (a) Rotor only

(b) Ducted fan (b) Grid generation Fig. 16. Pathlines for the hovering condition of the rotor only and the Fig. 14. Computational domain and grid generation for the hovering con- ducted-fan model. dition of the ducted fan model. ducted fan. The length of the computational domain is 12 times the rotor diameter (DR) and the radius is five times the fan radius (R) by considering the influence of far field boundary. In Fig. 14(b), computational meshes are composed of the inner cylinder part with unstructured tetra meshes and the outer cylinder part with structured prism meshes. The number of meshes are determined as 2:0 106 to consider the viscous sublayer in typical velocity profiles for the turbulent boundary layer due to the law of the wall32) when the dimen- sionless distance yþ is approximately 17. Boundary conditions for the ducted fan are applied as the Fig. 17. Flow fields of the ducted fan at each cross-sectional plane. pressure inlet and the outlet condition (the atmosphere pressure condition) for the inflow and outflow of the computa- by the stator blades. At the stator downstream, the axial ve- tional domain. The free slip condition is used at the outer locity is somewhat decreased due to the static pressure rise, wall of the computational domain. which introduces the thrust improvement of the ducted fan. 4.4. CFD analysis results Figure 16 shows pathlines for the hovering condition of Figure 15 shows velocity contours for the hovering con- the rotor only and the ducted-fan model. For the single rotor, dition of the rotor only and the ducted fan model. For the the inlet flow of the rotor has an effect 4.8 times of the fan single rotor, the wake flow of the rotor shows the swirl radius on the front far field. For the ducted fan, the inlet flow velocity due to the rotation of the rotor. For the ducted of the ducted fan has an effect 7 times of the fan radius on fan, the swirl velocity of the rotor wake flow is removed the front far field. Therefore, the suction area for the inlet Nov. 2013 L. CHO et al.: Numerical and Experimental Analyses of the Ducted Fan for the Small VTOL UAV Propulsion 335

10 600 Thrust Power } Rotor only 8 Thrust } Ducted fan Power 400 6

4 Thrust (N) 200

2 (W) Input shaft power

0 0 1000, 2000, 3000,,, 4000 5000 6000, 7000, Rotational speed (rpm)

Fig. 19. Thrust and input shaft power characteristics of the single rotor and the ducted fan for the variation of the rotational speed. (a) Schematic diagram (b) Front view

12 CFD Fig. 18. Experimental apparatus of the tested ducted fan. Exp. }Rotor only 10 CFD Exp. }Ducted fan 8 flow of the ducted fan is 2.13 times higher than that of the single rotor. This result means that the thrust performance 6 of the ducted fan is increased approximately 25% compared Thrust (N) 4 with the single rotor as shown in Fig. 20. 2 Figure 17 shows flow fields of the ducted fan at each 0 cross-sectional plane. The wake flow of the ducted fan is 1000,,,,, 2000 3000 4000 5000 6000, 7000, spread up to 16 times of the fan radius on the rear far field. Rotational speed (rpm)

Fig. 20. Comparison of the CFD results and the wind tunnel test results 5. Wind Tunnel Tests for the thrust characteristics of the single rotor and the ducted fan.

5.1. Experimental apparatus and methods The experimental apparatus, as shown in Fig. 18, is setup The thrust to input shaft power ratio of the ducted fan is with the balance system in the subsonic wind tunnel for the increased in line with increments in the rotational speed investigation of the thrust characteristics for the hovering except the range of the low rotational speed. As a result, condition of the designed ducted fan. the tested ducted fan does not satisfy design requirements Total length of the designed ducted fan is 110 mm and the because the duct shape is not considered as the aerodynamic diameter of the fan casing is 250 mm. The specifications for body due to production difficulties in this test. Therefore, the the rotor and the stator blade of the ducted fan are shown in intake and the duct shape of the ducted fan should be modi- Table 1 in detail. fied as the aerodynamic body to satisfy design requirements. The thrust characteristics of the ducted fan are measured Figure 20 shows the thrust characteristics for the hover- for the variation of the rotational speed using the balance ing condition of the single rotor and the ducted fan for the system of the subsonic wind tunnel. The fan thrust is mea- variation of the rotational speed. For the single rotor, the sured by changing the rotational speed of the rotor in CFD results and wind tunnel results are fairly well matched. 1,000-rpm intervals from 1,000 to 7,000 rpm (seven cases). However, the thrust performance of the ducted fan is signif- Input shaft power (L) of the electric motor, which rotates icantly different for the CFD results and wind tunnel results. the rotor of the ducted fan, is calculated by the motor input This result is due to the inconsistency of the intake and the voltage (E), the current (I) and the motor efficiency (m)as duct shape. Therefore, the thrust performance of the ducted shown in Eq. (9). fan would be improved by the optimization of the intake and duct shape. L ¼ E I m ð9Þ The efficiency of the electric motor is obtained from the 6. Conclusions results of calibration tests by the electric motor manufactur- ing company. In this study, numerical and experimental analyses of 5.2. Wind tunnel test results the ducted fan for the small VTOL UAV propulsion were Figure 19 shows the thrust and input shaft power charac- carried out on the hovering flight condition. Matrix teristics of the single rotor and the ducted fan for the varia- through-flow analysis results for the three-dimensional flow tion of the rotational speed. As shown in this figure, the of the ducted fan were investigated by the meanline analysis ducted fan, which consists of the rotor and the stator blades, results according to design requirements. In addition, CFD is somewhat (approx. 5%) higher compared with the rotor analysis results for the thrust performance of the ducted only. fan were compared with the wind tunnel test results. 336 Trans. Japan Soc. Aero. Space Sci. Vol. 56, No. 6

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Therefore, the intake and duct shape of the ducted fan 15) De Divitiis, N.: Performance and Stability Analysis of a Shrouded-Fan Unmanned Aerial Vehicle, J. Aircraft, 43 (2006), pp. 681–691. should be modified in order to satisfy design requirements. 16) Horlock, J. H.: Axial Flow Compressors, Robert E. Publishing Com, (4) For the single rotor, CFD results and wind tunnel Florida, 1958, pp. 96–114. results were fairly well matched. However, the thrust per- 17) Turner, R. C.: Notes on Ducted Fan Design, Aeronautical Research formance of the ducted fan was significantly different for Council C. P. No. 895, 1966, pp. 4–19. 18) Katsanis, T. and McNally, W. D.: Revised FORTRAN Program for the CFD results and wind tunnel results due to the inconsis- Calculating Velocities and Streamlines on the Hub-Shroud Mid-Chan- tency of the intake and the duct shape. 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