Wind Tunnel Experimental Study of Lift Enhancement and Drag Reduction on a Swept Wing Based on a Co-Flow Jet Technique

Abstract

Co-flow jet (CFJ) is an active flow control technique that significantly enhances aerodynamic performance metrics such as the maximum lift and maximum lift-to-drag ratio of airfoils or wings. Currently, investigations into lift enhancement and drag reduction on three-dimensional CFJ swept wings are limited. To address this issue, we used a low-speed, high-lift NPU-LS 0515 airfoil as a baseline and designed a wind tunnel experimental model of a CFJ swept wing, with the CFJ driven by internally mounted ducted fans and guided by injection ducts. We investigated the effects of jet direction, jet momentum coefficient, and injection slot size on lift enhancement and drag reduction performance of the CFJ swept wing through wind tunnel experiments. Experimental results showed that chord-wise vortices generated by the interaction of the deflected jet flow with the main flow through shear stress effectively enhanced mixing effect and energy transfer, improving the lift coefficient of the CFJ swept wing. Compared to the baseline configuration, the CFJ swept wing achieved over a 20% increase in maximum lift coefficient and more than a 50% reduction in drag coefficient at high angles of attack. Consequently, the lift-to-drag ratio of the swept wing improved substantially.

1. Introduction

Flow control has long been a key research focus in aircraft design, as it provides an effective means to significantly enhance the aerodynamic performance of aerial vehicles. Passive flow control methods such as winglets [1,2], Gurney flaps [3,4], and vortex generators [5] have been extensively implemented in transport aircraft, commercial airliners, and other fields. Compared to passive methods, active flow control methods can more effectively modify the flow field, considerably enhancing the lift-to-drag ratio and stall angle of attack (AoA) of the aircraft while expanding the flight envelope. This demonstrates its broader application prospects.

The co-flow jet (CFJ) is a zero-net-mass-flux active flow control method [6] that involves placing injection slots near the leading edge and suction slots near the trailing edge on the upper surface of an airfoil while integrating air pumps or fans within the airfoil to draw air through the suction slot and eject it through the injection slot. Numerous studies on CFJ airfoils [7,8,9,10,11,12,13,14,15,16] have demonstrated that the CFJ method suppresses flow separation at high AoAs, significantly increases stall AoA, considerably enhances maximum lift, and reduces drag. Furthermore, it provides lift enhancement and drag reduction effects under attached flow conditions at low AoAs and can generate thrust at specific AoAs.

CFJ has also been applied to three-dimensional (3D) wings, yielding promising results. For example, Lefebvre et al. [17] applied CFJ to straight wings and studied its effects on lift enhancement and drag reduction during cruise and takeoff/landing conditions. Furthermore, they examined how the location of injection/suction slots, airfoil thickness, and aspect ratio influence CFJ performance. Their findings suggested that CFJ significantly improved aerodynamic efficiency during cruise and enhanced maximum lift coefficient during takeoff and landing. Yang et al. [18,19] applied CFJ to low-speed electric aircraft wings and achieved a maximum lift coefficient and stall AoA of 6.9 and 50°, respectively, under high-altitude conditions. Based on these results, they proposed a wing design concept that allows overall wing rotation to adapt to various flight conditions and compared the aerodynamic efficiency of CFJ wings at different aspect ratios. They observed that, as the lift coefficient increased, the induced drag impact on CFJ wings considerably reduced compared with conventional wings. Xu et al. [20,21] studied the application of CFJ on aircraft control surfaces and investigated the effects of two different CFJ configurations on improving control surface effectiveness. Wang et al. [22,23,24] examined the effects of Mach and Reynolds numbers on lift enhancement and the drag reduction performance of CFJ wings and assessed their energy consumption during cruise. Boling et al. [25,26] compared the wingtip vortex flow states of high-aspect-ratio straight wings before and after CFJ implementation. Additionally, they explored lift enhancement and drag reduction of 3D CFJ transonic wings and found that CFJ increases the lift-to-drag ratio by 10%, whereas the wing sweep weakens this effect. Lei et al. [27] studied the low-speed aerodynamic performance of a supersonic CFJ delta wing equipped with flaps. They observed that positioning the CFJ on deflected flaps improved takeoff and landing performance compared to placement on the main wing. Lei et al. [28] further investigated how the CFJ layout affects aerodynamic characteristics on finite-span straight wings. They showed that CFJ has a limited effect within 20% of the span near the wingtip. Arranging CFJ in multiple discontinuous regions minimized performance losses in lift enhancement and drag reduction, especially when the spacing between regions was small. Wang et al. [29] applied CFJ to a 3D flying wing configuration and confirmed that CFJ increased the lift coefficient by 5.1% and reduced the drag coefficient by 15.3% for the wing. Zhu et al. [30] applied CFJ to a straight wing with simple lift enhancement devices and studied its aerodynamic performance during low-speed takeoff and landing, as well as the effects of jet momentum coefficient and sweep angle on CFJ lift enhancement and drag reduction.

Existing CFJ-related studies have focused on two-dimensional airfoils and 3D straight wings, leaving research on swept wings limited. The key purpose of this study is to verify the lift enhancement and drag reduction effect of CFJ on a swept wing. Additionally, it investigates how wing sweep and spanwise flow influence CFJ flow control. We used the NPU-LS 0515 airfoil as the baseline and designed a CFJ swept wing wind tunnel experimental model powered by internally mounted ducted fans and guided by injection ducts. Wind tunnel experiments were conducted to investigate the effect of jet direction, jet momentum coefficient, and injection slot size on the aerodynamic performance of the CFJ swept wing. We analyzed how jet direction affects the mixing effect and energy transfer between the main and jet flows under different typical duct configurations. The mechanisms responsible for lift enhancement and drag reduction were analyzed and clarified. The experimental results provide a reference for future numerical and experimental studies of CFJ wings and provide a foundation for the engineering application of CFJ technology on wings.

2. Wind Tunnel Experimental Design of a CFJ Swept Wing

2.1. Design of the CFJ Swept Wing Wind Tunnel Model

The CFJ swept wing wind tunnel experimental model was designed to implement CFJ flow control using internally mounted ducted fans and guiding ducts. The NPU-LS 0515 airfoil, a low-speed, high-lift airfoil with a thickness of 15%, was selected as the baseline, providing sufficient internal space for the CFJ components. The profile for the NPU-LS 0515 airfoil is shown in Figure 1.

A new CFJ airfoil, based on the NPU-LS 0515 airfoil, was developed to accommodate internal ducted fans and guiding ducts within the wind tunnel model. The design follows geometric parameter guidelines by Wang et al. [31], specifying an optimal suction slot at approximately 70% chord and placing the injection slot as far aft as feasible. The resulting CFJ airfoil, designated NPU-CFJ 0515, served as the basis for constructing the CFJ swept wing. The key geometric parameters and the schematic of the CFJ airfoil are presented in

Table 1. Key CFJ geometric parameters of the NPU-CFJ 0515 airfoil.

Geometric Parameters	Detail Value
Injection slot location	8% chord-wise
Injection slot angle γ	75°
Suction slot location	70% chord-wise
$\mathrm{Suction}\mathrm{slot}\mathrm{size}{d}_{suc}$	2.5% chord-wise
Suction slot angle θ	45°
Suction surface translation	0.3% chord-wise

and Figure 2, respectively.

The wing has a sweep angle of 20°, a chord length of 700 mm, and a span of 1300 mm. Figure 3 presents the model setup in the wind tunnel and a schematic of its structure. The model is installed vertically in the wind tunnel and divided into upper, middle, and lower sections. The upper and lower sections adopt the baseline airfoil profile to establish swept flow field conditions for the middle test section and are connected to the model base via internal structural components. The middle section is mounted directly to an internal wing spar, which is connected to a balance mounted on the model base through brackets, forming an integrated load-bearing structure. Importantly, the middle section and its brackets are isolated from the upper and lower sections to avoid interference during force measurements.

Furthermore, as shown in Figure 4, four sets of ducted fans and duct components are mounted spanwise on the wing spar within the middle section, serving as the jet source for CFJ. Two distinct upper surface shells are prepared for the middle wing section: one designed for the baseline wing configuration and the other for the CFJ wing. When the CFJ upper surface is installed, its front end is connected to the duct outlet via elastic metal strips and incorporates an adjustment mechanism that allows the injection slot size to be varied. Figure 5 illustrates this adjustment mechanism and provides a comparison of the two typical injection slot sizes.

Xu et al. [20] reported that jet direction is severely deflected when internal ducts and the leading edge of the wing are sweep back. This deflection complicates mixing effect between the main and jet flows, affecting lift enhancement and drag reduction. Therefore, we designed a novel duct configuration to minimize jet flow deflection. Figure 6 compares the two duct geometries (Ducts A and B), while Figure 7 presents the streamline diagrams of airflow through each duct. Duct B eliminates sweep-back at the turning section, effectively preventing jet direction deflection. The CFJ swept wings using Ducts A and B are designated as Configurations A and B, respectively.

2.2. Experimental Equipment

The wind tunnel experiment was conducted in the NF-3 low-speed wind tunnel facility at Northwestern Polytechnical University. This tunnel is a low-speed, open-circuit facility with a test section measuring 1.6 m in height, 3 m in width, and 8 m in length, and it provides a stable wind velocity range of 10–130 m/s with a turbulence intensity of 0.045%.

To measure the aerodynamic characteristics of the CFJ swept wing model, a six-component strain-gauge balance was mounted on the model base and connected to the mid-span wing spar through the model bracket for force measurements. To improve force measurement accuracy, the contact surfaces between the balance, model base, and bracket were polished to achieve the required flatness and surface roughness. The force data were acquired using the VXI data acquisition system of the wind tunnel and processed through the post-processing system.

Four ducted fans (FMS 2627-KV4500 model, manufactured by Ruiqi Power Technology Co., Ltd., Shenzhen, China) were mounted on the wing spar of the middle test section. Each pair of ducted fans was powered by an MW S-600-15 power supply (manufactured by MINGWEI Electronic Holdings Co., Ltd., Shenzhen, China) at 15 V. The motors were throttle-controlled using electronic regulators (50 A, HOBBYWING, manufactured by Hobbywing Technology Co., Ltd., Shenzhen, China) and speed controllers. They were connected to a watt meter (130 A, G.T. POWER RC, manufactured by G.T.Hobbies Co., Ltd., Shenzhen, China) for power monitoring and regulation. The motors were wired internally using silicone copper wires that passed through the wind tunnel wall. They were then connected to the external power supply, electronic regulator, and watt meter at the operating console outside the wind tunnel. Figure 8 and Figure 9 show photographs of the relevant experimental equipment.

To evaluate the reliability of the measurement approach used in this study, a repeatability test was conducted on the baseline swept wing under a typical condition of $V_{\infty }$ = 15 m/s and an AoA of 11°. An uncertainty analysis was then conducted based on the repeated measurements. The relative uncertainties were determined to be 0.6% for the force balance, 1.14% for the lift coefficient, and 4.58% for the drag coefficient.

2.3. Basic Parameters of CFJ

The strength of CFJ flow control is evaluated using the jet momentum coefficient $C_{\mu }$, defined as follows:

$$C_{\mu }={\displaystyle \frac{\dot{m}{V}_j}{0.5{\rho }_{\infty }{V_{\infty }}^2{S}_{ref}}}$$

(1)

where $\dot{m}$ and $V_j$ represent the mass flow rate and jet velocity at the injection slot, respectively; ${\rho }_{\infty }$ and $V_{\infty }$ represent the freestream density and velocity, respectively; and $S_{ref}$ represents the reference area of the wing.

2.4. Wind Tunnel Testing Procedure

The wind tunnel testing procedure for the CFJ swept wing model is outlined as follows:

• Install the wind tunnel test model and calibrate the wind tunnel testing apparatus.
• Start the wind tunnel and stabilize the wind velocity to the target value.
• For CFJ swept wing configurations, start the internally mounted ducted fans and stabilize the fan power at the pre-set value.
• Use the VXI data acquisition system of the wind tunnel to collect and record the forces measured by the balance.
• Adjust the AoA of the wing model using the AoA control system.
• When the flow is stable, record the force data again.
• Repeat steps 5 and 6 until balance measurements at all AoAs are completed.
• Shut down the internally mounted ducted fans, reset the AoA to the initial angle, and stop the wind tunnel operation.
• Reconfigure the wing model or adjust the wind tunnel freestream velocity, then repeat steps 2–8 until all test cases are completed.
• Use post-processing software to analyze the experimental data and obtain aerodynamic coefficients of the wing.

Wind tunnel tests were performed on the baseline and CFJ swept wing configurations at freestream velocities of 10 and 15 m/s. For the CFJ swept wing, injection slot sizes of 0.5% and 0.7% chord-wise were tested, and the jet momentum coefficient was varied from 0.05 to 0.15.

2.5. Hot-Wire Measurement Test of Jet Outlet Mean Velocity

Before the wind tunnel experiment, measurements were conducted to determine the relationship between the ducted fan power and the mean jet velocity of the CFJ swept wing model for various duct geometries and injection slot sizes. The ducted fan power and the corresponding mean jet velocity were measured using a watt meter and a hot-wire anemometer, respectively.

A CTA-04A hot-wire anemometer measured the mean velocity at the jet outlet for different fan power levels after model installation. Figure 10 presents the experimental setup used for jet velocity measurement. The measurement device had a velocity range of 0.1–150 m/s and a frequency response of 50 kHz. Prior to conducting formal measurements, a careful calibration of the probe was performed. The probe was positioned at the center of the injection slot during the measurement. After being filtered and amplified through the data acquisition and processing system, the output voltage was then converted to mean jet velocity using a calibrated velocity–voltage relationship.

2.6. Flow Visualization Experiment

To further investigate the influence of the two duct configurations on the jet flow direction of the CFJ swept wing, a flow visualization experiment was conducted using fluorescent tufts. The experiment targeted the upper-surface region between the injection and suction slots to analyze tuft motion patterns.

3. Experimental Results and Analysis

3.1. Measurement and Calibration of Jet Outlet Mean Velocity

To determine the relationship between the ducted fan power and the mean jet velocity of the CFJ swept wing model under different combinations of ducts and injection slot sizes, measurements were conducted before the wind tunnel experiment. The results are presented in Figure 11. The results indicate that the relationship was minimally affected by the injection slot size and duct shape. A spline fitting was then applied to the measured data. Additional measurements of mean jet velocity were taken at ducted fan power levels of 30, 70, 110, and 150 W to validate the spline fitting results.

Table 2. Comparison between measured and fitted jet velocities at different ducted fan power levels.

Power of Ducted Fan (W)	Cubic Spline Fitting (m/s)	Hot-Wire Velocity Measurement (m/s)
30	20.48	20.76
70	29.30	28.97
110	34.71	34.89
150	38.56	38.20

compares the measured mean jet velocity with the spline fitting results, demonstrating good agreement.

Using the measured relationships,

Table 3. Correspondence between jet momentum coefficient, jet velocity, and ducted fan power at the injection slot (V_∞ = 10 m/s).

Jet Momentum Coefficient	Dinj = 0.5% Chord-Wise		dinj = 0.7% Chord-Wise
	Required Jet Velocity (m/s)	Ducted Fan Power (W)	Required Jet Velocity (m/s)	Ducted Fan Power (W)
0.07	26.46	53.9	22.36	36.5
0.09	30.00	74.5	25.35	48.7
0.11	33.17	97.0	28.03	61.2
0.13	36.06	122.7	30.47	73.7
0.15	38.73	152.2	32.73	86.2

and

Table 4. Correspondence between jet momentum coefficient, jet velocity, and ducted fan power at the injection slot (V_∞ = 15 m/s).

Jet Momentum Coefficient	dinj = 0.5% Chord-Wise		dinj = 0.7% Chord-Wise
	Required Jet Velocity (m/s)	Ducted Fan Power (W)	Required Jet Velocity (m/s)	Ducted Fan Power (W)
0.05	33.54	100.1	28.35	64.2
0.06	36.74	129.6	31.05	81.6

list the jet velocities and ducted fan power levels corresponding to various jet momentum coefficients at different wind velocities.

3.2. Analysis of the Effects of Different Ducts on the Aerodynamic Characteristics of the CFJ Swept Wing

A systematic comparison of aerodynamic forces was conducted among the baseline swept wing and two CFJ configurations (A and B) to elucidate the influence of jet direction on the aerodynamic performance of the CFJ swept wing. Tests were conducted at freestream velocities of 10 and 15 m/s, with the injection slot size fixed at 0.5% chord-wise, and comparative analyses were performed on the lift coefficient, drag coefficient, and lift-to-drag ratio.

3.2.1. Analysis of the Effects on Lift

Figure 12 shows a comparison of lift coefficients between the baseline swept wing, Configuration A, and Configuration B.

Table 5. Comparison of lift coefficient enhancement at typical AoAs for the two CFJ swept wing configurations (V_∞ = 10 m/s and C_µ = 0.11).

Configuration	${\mathit{C}}_{\mathit{l}}$		$\mathsf{\Delta }{\mathit{C}}_{\mathit{l}}$
	AoA = 5°	AoA = 15°	AoA = 5°	AoA = 15°
Baseline swept wing	0.865	1.662	-	-
CFJ swept wing (Configuration A)	1.185	2.056	37.0%	23.7%
CFJ swept wing (Configuration B)	1.093	1.972	26.4%	18.7%

and

Table 6. Comparison of lift coefficient enhancement at typical AoAs for the two CFJ swept wing configurations (V_∞ = 15 m/s and C_µ = 0.06).

Configuration	${\mathit{C}}_{\mathit{l}}$		$\mathsf{\Delta }{\mathit{C}}_{\mathit{l}}$
	AoA = 5°	AoA = 15°	AoA = 5°	AoA = 15°
Baseline swept wing	0.763	1.511	-	-
CFJ swept wing (Configuration A)	0.905	1.722	18.5%	14.0%
CFJ swept wing (Configuration B)	0.884	1.685	15.9%	11.5%

summarize the lift coefficients at representative AoAs for Configurations A and B, compared with the baseline wing.

At a freestream velocity of 10 m/s, the slope of the lift curve for the CFJ swept wing was slightly higher than that of the baseline wing. The absolute increase in lift coefficient was larger at high angles of attack, while the percentage increase was greater at low angles. Additionally, Configuration A exhibited higher lift coefficients than Configuration B. At AoAs of 5° and 15°, the lift enhancement for Configuration A exceeded that of Configuration B by 10.6% and 5.0%, respectively. However, Configuration B had a slightly higher stall AoA than Configuration A. The maximum lift coefficients of Configurations A and B increased by 21.1% and 23.3%, respectively, compared to the baseline wing. At a freestream velocity of 15 m/s, the lift coefficient trends of both configurations remained similar to those observed at 10 m/s. Configuration A continued to outperform Configuration B in terms of lift coefficient. The maximum lift coefficients of Configurations A and B exceeded that of the baseline wing by 14.7% and 11.8%, respectively. Overall, Configuration A provided slightly better lift enhancement than Configuration B.

3.2.2. Analysis of the Effects on Drag

Figure 13 shows comparisons of the drag coefficients and lift-to-drag ratios for the baseline and the two CFJ swept wing configurations. At a freestream velocity of 10 m/s and C_µ = 0.11, the CFJ swept wing exhibited limited drag reduction at negative AoAs. However, in the positive AoA range, particularly between 15° and 27°, the drag was significantly lower than that of the baseline wing. A comparison of drag coefficients between Configurations A and B shows that Configuration B achieved better drag reduction than Configuration A.

Table 7. Comparison of the effects of two duct configurations on drag reduction at a typical AoA for CFJ swept wings (V_∞ = 10 m/s, α = 15°, and C_µ = 0.11).

Configuration	${\mathit{C}}_{\mathit{d}}$	$\mathsf{\Delta }{\mathit{C}}_{\mathit{d}}$	$\frac{\mathit{L}}{\mathit{D}}$	$\mathsf{\Delta }\frac{\mathit{L}}{\mathit{D}}$
Baseline swept wing	0.0815	-	20.4	-
CFJ swept wing (Configuration A)	0.0383	−53.0%	53.9	163.3%
CFJ swept wing (Configuration B)	0.0270	−66.9%	73.0	258.2%

lists the drag coefficients and lift-to-drag ratios of both configurations at AoA = 15° and a freestream velocity of 10 m/s, compared to the baseline wing. The CFJ wing could reduce drag by more than 50% at relatively high AoAs. This significant drag reduction substantially improved the lift-to-drag ratio of the wing. At a freestream velocity of 15 m/s and C_µ = 0.06, the drag reduction trend was similar to that at 10 m/s. Configuration B continued to outperform Configuration A in both drag reduction and lift-to-drag ratio improvement.

3.2.3. Flow Visualization Results and Analysis of the CFJ Swept Wing

Figure 14 compares the upper-surface flow fields of the two CFJ swept wing configurations. In Configuration A, the jet near the injection slot was strongly deflected, forming a significant angle relative to the main flow direction. Conversely, the jet near the injection slot in Configuration B remained nearly aligned with the main flow. This phenomenon indicates that the jet flow direction at the injection slot strongly depends on the geometry of the turning section of the guiding duct.

Numerical simulations were performed on simplified wind tunnel models at 10 m/s and representative angles of attack. This aimed to investigate jet-main flow interactions and clarify how CFJ enhances lift and delays flow separation. Figure 15 compares the velocity contours in the CFJ region of the wing. Results indicate that the jet significantly accelerates the main flow near the leading edge of the CFJ swept wing compared to the baseline. This generates stronger suction effects on the upper surface, increasing lift. Moreover, the high-energy main flow improves its ability to overcome the adverse pressure gradient downstream on the upper surface. Meanwhile, the suction slot removes the separated boundary layer, delaying flow separation and significantly reducing drag at high angles of attack.

Chord-wise vortices may form at the interface of the flows owing to shear stress and viscosity when there is an angular deviation between the jet and main flow. Figure 16 shows the vorticity contours along the flow direction on the upper surface of both CFJ swept wing configurations. These vortices extend downstream along the chord-wise flow over the upper surface and facilitate energy transfer from the jet to the main flow. This effect enhances lift, similar to that produced by vortex generators and leading-edge strakes on modern fighter aircraft. However, jet deflection can reduce drag reduction. The generated chord-wise vortices increase surface turbulence and friction. The deflection reduces the momentum component of the jet in the freestream direction, thereby weakening the ability of the CFJ to counteract drag. These effects were confirmed by the wind tunnel experimental results.

Overall, the CFJ technology maintained substantial aerodynamic benefits for the swept wing configuration, demonstrating: (1) sustained lift enhancement with a maximum lift coefficient increase exceeding 20% under tested conditions, (2) remarkably high-angle-of-attack performance with drag reduction exceeding 50%, and (3) significant improvement in the lift-to-drag ratio. Furthermore, the two duct configurations affected the jet direction and effectiveness of CFJ flow control. Duct A induced jet deflection at the injection slot, enhancing flow mixing effect and energy transfer to the main flow. This increased vortex-induced lift on the wing surface, even though it resulted in higher drag. Conversely, Duct B effectively suppressed jet deflection. Although lift enhancement was slightly reduced, the improved drag reduction substantially increased the lift-to-drag ratio of the wing. However, the complex geometry of Duct B imposes higher manufacturing demands and requires additional internal wing space, which may pose challenges for practical implementations. These challenges can be mitigated through optimized duct design.

3.3. Analysis of the Effects of Jet Momentum Coefficients on the Aerodynamic Characteristics of the CFJ Swept Wing

To investigate the effect of varying jet momentum coefficients on the aerodynamic characteristics of the CFJ swept wing, experiments were conducted at a freestream velocity of 10 m/s with an injection slot size of 0.5% chord-wise. The jet momentum coefficients were controlled by adjusting the power of the ducted fans in Configurations A and B.

Figure 17 shows the aerodynamic force trends for the two CFJ swept wing configurations under different jet momentum coefficients. For both configurations, the lift coefficient at a given AoA increased with a higher jet momentum coefficient, with the maximum lift coefficient also showing a steady rise. However, the rate of lift increase in response to changes in the jet momentum coefficient differed between the two configurations. Figure 18 presents the trends in lift coefficient increment versus jet momentum coefficient for both configurations at 10 m/s and an AoA of 11°. Configuration A exhibited a rapid increase in lift coefficient increment with increasing C_µ, whereas Configuration B showed a considerably lower growth rate. As C_µ increased further, the rate of lift increment continued to rise. As previously discussed, the jet deflection at the injection slot in Configuration A generates chord-wise vortices through its interaction with the main flow, enhancing the mixing effect and energy transfer between the flows. Consequently, this increases the lift coefficient and makes the lift increment more sensitive to C_µ. Although Configuration A had a lower stall AoA than Configuration B, its maximum lift coefficient increment increased rapidly from 11.5% to 27.4% as C_µ increased from 0.07 to 0.15. By comparison, Configuration B experienced a smaller maximum lift coefficient increment, increasing from 19.1% to 24.7% over the same C_µ range.

The drag coefficient of both CFJ swept wing configurations decreased with increasing jet momentum coefficient C_µ. Figure 19 shows the relationship between drag reduction and C_µ at a typical AoA. Configuration B achieved greater drag reduction than Configuration A because Duct B effectively limits jet deflection. This increases the freestream momentum component of the jet, enhancing its drag-counteracting effect. Although Configuration A reduced drag less effectively, the performance gap between the two configurations decreased as C_µ increased.

3.4. Analysis of the Influence of Different Injection Slot Sizes on the Aerodynamic Characteristics of the CFJ Swept Wing

To study the effect of injection slot size on the aerodynamic characteristics of the CFJ swept wing, experiments were performed on Configuration B at a freestream velocity of 10 m/s, using jet momentum coefficients of 0.07 and 0.09. The injection slot sizes were adjusted to 0.5% and 0.7% chord-wise using the duct outlet mechanism. Figure 20 presents the results, while

Table 8. Comparison of increments in maximum lift coefficient for the CFJ swept wing for different injection slot sizes.

Injection Size	Cµ = 0.07		Cµ = 0.09
	${\mathit{C}}_{\mathit{l}\mathit{m}\mathit{a}\mathit{x}}$	$\mathsf{\Delta }{\mathit{C}}_{\mathit{l}\mathit{m}\mathit{a}\mathit{x}}$	${\mathit{C}}_{\mathit{l}\mathit{m}\mathit{a}\mathit{x}}$	$\mathsf{\Delta }{\mathit{C}}_{\mathit{l}\mathit{m}\mathit{a}\mathit{x}}$
0.5% chord-wise	2.262	19.2%	2.287	20.5%
0.7% chord-wise	2.033	7.1%	2.134	11.4%

compares the maximum lift coefficient increments for the different injection slot sizes.

Experimental results showed that reducing the injection slot size increases the lift coefficient of the CFJ swept wing. At C_µ = 0.07 and 0.09, a 0.5% chord-wise injection slot increased the maximum lift coefficient by 19.2% and 20.5%, respectively. Conversely, a larger 0.7% chord-wise slot increased the lift by only 7.1% and 11.4%. Meanwhile, a smaller injection slot reduced the drag coefficient. At a constant jet momentum coefficient, the smaller injection slot increased jet velocity. Consequently, stronger negative pressure between the injection and suction slots enhanced the upper-surface suction, improving lift coefficient. Additionally, a larger velocity gradient between the main flow and jet flow enhanced the accelerating effect of the jet on the main flow. This improved the ability of the main flow to overcome the adverse pressure gradient. These effects collectively increased the stall AoA of the CFJ swept wing with a 0.5% chord-wise injection slot.

4. Conclusions

The CFJ swept wing wind tunnel experimental model was designed with internally mounted ducted fans and guiding ducts. Two CFJ swept wing configurations, differing in duct geometry, were tested in a wind tunnel to compare their jet directions and aerodynamic performance. The associated flow mechanisms were also analyzed. Additionally, the effects of jet momentum coefficient and injection slot size on lift enhancement and drag reduction were systematically investigated.

The results indicate that guiding duct geometry strongly influences the jet flow direction at the injection slot. Jet deviation from the main flow induces chord-wise vortices, enhancing mixing effects and energy transfer between the jet and main flow, which increases lift but also moderately increases drag. Furthermore, relatively high energy transfer efficiency contributes to more energy being acquired by the main flow as the jet momentum coefficient increases. Consequently, the increment of lift coefficient becomes more sensitive to variations in jet momentum coefficient.

Conversely, when the jet direction is nearly aligned with the main flow, the CFJ swept wing improves lift and drag performance across a wider AoA range, enhancing overall aerodynamic efficiency. This alignment yields at least a 20% increase in the maximum lift coefficient and approximately a 50% drag reduction at high AoA compared to the baseline wing.

A smaller injection slot accelerates the main flow more effectively, resulting in increased lift and reduced drag. Moreover, this configuration improves the ability of the main flow to overcome the adverse pressure gradient at high AoAs, exerting a favorable impact on stall-delay performance.