Open AccessArticle

Experimental Aerodynamics of a Small Fixed-Wing Unmanned Aerial Vehicle Coated with Bio-Inspired Microfibers Under Static and Dynamic Stall

Flow Control and Aerodynamics Lab, Department of Mechanical Engineering, Texas Tech University, Lubbock, TX 79409, USA

Flow Raider LLC, 3911 4th St., Lubbock, TX 79409, USA

Author to whom correspondence should be addressed.

^†

These authors contributed equally to this work.

Aerospace 2024, 11(11), 947; https://doi.org/10.3390/aerospace11110947

Submission received: 16 October 2024 / Revised: 9 November 2024 / Accepted: 11 November 2024 / Published: 17 November 2024

(This article belongs to the Special Issue Aerodynamics, Flight Dynamics and Control of Advanced Air Mobility Vehicles)

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Figure 1
(A) Concept of a shark skin denticle, (B) close perspective of bio-inspired microfibers, scale bar ≈ 100 µm, (C) surface coating from top, and (D) flow mechanism within the fibers and outside. Adapted from [<a href="#B26-aerospace-11-00947" class="html-bibr">26</a>]. "> Figure 2
Planform drawing of UAV model with microfiber coverage (dimensions in mm). "> Figure 3
(a) Microfiber schematic (dimensions in µm); (b) wing covered with microfiber coating (zoomed-in picture adapted from Doosttalab et al. [<a href="#B26-aerospace-11-00947" class="html-bibr">26</a>]). "> Figure 4
Wind tunnel model setup of the ‘high-speed, long-range’ (HSLR) variant of the Switchblade UAV. "> Figure 5
Lift coefficients, CL as a function of angle of attack, α. "> Figure 6
Drag polars; lift coefficients, CL as a function of drag coefficients, CD. "> Figure 7
Lift-to-drag ratio, L/D as a function of angle of attack, α. "> Figure 8
High angle of attack, α lift-to-drag ratio, L/D enhancement. "> Figure 9
Time-averaged velocity over a curved APG section representative of an airfoil in turbulent flow with a freestream velocity of 30 m/s. "> Figure 10
Elevon deflection performance: pitching moment coefficient, CM as a function of elevon deflection angle, δe for the baseline and micropillar cases. "> Figure 11
Dynamic pitch coefficients for different surface cases and wing coverage: (a) CA; (b) CN; (c) CM. The black arrow indicates the direction of the pitch up maneuver. "> Figure 12
Dynamic derivatives in pitch: <math display="inline"><semantics> <mrow> <msub> <mi>C</mi> <mrow> <msub> <mi>A</mi> <mi>q</mi> </msub> </mrow> </msub> </mrow> </semantics></math>; <math display="inline"><semantics> <mrow> <msub> <mi>C</mi> <mrow> <msub> <mi>M</mi> <mi>q</mi> </msub> </mrow> </msub> </mrow> </semantics></math>; <math display="inline"><semantics> <mrow> <msub> <mi>C</mi> <mrow> <msub> <mi>M</mi> <mi>q</mi> </msub> </mrow> </msub> </mrow> </semantics></math> as a function of mean angle of attack. ">

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Abstract

A passive flow control technique in the form of microfiber coatings with a diverging pillar cross-section area was applied to the wing suction surface of a small tailless unmanned aerial vehicle (UAV). The coatings are inspired from ‘gecko feet’ surfaces, and their impact on steady and unsteady aerodynamics is assessed through wind tunnel testing. Angles of attack from −2° to 17° were used for static experiments, and for some cases, the elevon control surface was deflected to study its effectiveness. In forced oscillation, various combinations of mean angle of attack, frequency and amplitude were explored. The aerodynamic coefficients were calculated from load cell measurements for experimental variables such as microfiber size, the region of the wing coated with microfibers, Reynolds number and angle of attack. Microfibers with a 140 µm pillar height reduce drag by a maximum of 24.7% in a high-lift condition and cruise regime, while 70 µm microfibers work best in the stall flow regime, reducing the drag by 24.2% for the same high-lift condition. Elevon deflection experiments showed that pitch moment authority is significantly improved near stall when microfibers cover the control surface and upstream, with an increase in CM magnitude of up to 22.4%. Dynamic experiments showed that microfibers marginally increase dynamic damping in pitch, improving load factor production in response to control surface actuation at low angles of attack, but reducing it at higher angles. In general, the microfiber pillars are within the laminar boundary layer, and they create a periodic slip condition on the top surface of the pillars, which increases the near-wall momentum over the wing surface. This mechanism is particularly effective in mitigating flow separation at high angles of attack, reducing pressure drag and restoring pitching moment authority provided by control surfaces.

Keywords:

unmanned aerial vehicle aerodynamics; bio-inspired surfaces; passive flow control

1. Introduction

The popularization of unmanned aerial vehicles (UAVs) and their success at performing a myriad of roles have resulted in exponential development and commercialization. It is estimated that the drone market will expand at a compound annual growth rate of 57.5% through 2028, with an expected revenue of USD 501.4 billion, according to [1]. This indicates an enormous growth potential in the field, encouraging improvement opportunities in UAV technology.

Various methods of passive flow control are regularly employed in airplanes to improve aerodynamics by diminishing separation or to correct undesirable flow mechanics. Different techniques such as vortex generators, wing fences, notches and strakes are commonly designed for commercial and military aircraft [2,3]. In a turbulent regime, the boundary layer has greater resistance to detachment, resulting in better performance characteristics in regard to flow separation [4]. In contrast, small UAVs usually operate in the laminar or transitional regimes, where laminar separation becomes an issue, increasing pressure drag significantly. In these situations, passive flow control techniques such as vortex generators, bumps, trips and roughness elements can cause the flow to transition earlier, reducing pressure drag due to flow detachment [5,6,7,8,9].

Immense technological advances in manufacturing and the miniaturization of electronics have made it possible for scientists and engineers to be inspired by nature for the creation of the next generation of vehicles. Increasing focus has lately been pointed to bio-inspired features, mostly elements found on the skin surface of fish, aquatic mammals, special feathers in birds [10] or flight mechanisms observed in insects and bats [11,12,13]. Protuberances found on humpback whale flippers, named tubercles, have been widely studied and found to increase performance and delay stall [14,15,16]. Fish scales’ effect on wall-bounded flow behavior has been analyzed experimentally and numerically, indicating that subtle geometric features on the skin can enhance hydrodynamic performance of aquatic vehicles by generating vortices that reduce drag [17,18,19,20]. Similarly, small-scale textures of shark skin known as denticles (Figure 1A) have been studied and found to minimize pressure drag by reducing the degree of flow separation [21,22,23,24,25]. Similar textures inspired from gecko feet surfaces or ‘setae’ (Figure 1B,C) were applied to a small fixed-wing UAVs in this study, and their effect was assessed through wind tunnel experiments. In previous two-dimensional studies, boundary layer re-energization through pressure modulations was identified as a consequence of their use, causing a reduction in flow separation. This reflected less flow re-circulation in the separation area (Figure 1D) [26,27]. In those instances, the microfibers accomplished that without increasing turbulent kinetic energy, which would lead to higher viscous losses—an underlying issue with flow control methods that transition the boundary layer. Furthermore, these microfibers are molded onto adhesive films that can be conveniently applied to any smooth surface instead of being 3D printed, which represents a considerable advantage over other methods explored in the literature. That limits the shortcomings linked to additive manufacturing, such as a long time and high cost to produce small-scale structures.

The aircraft model used in this study is based on a design named Switchblade, an unmanned aerial vehicle developed under a family of reconfigurable aircraft with a modular framework [28,29,30,31]. The reconfigurability feature enables multiple UAV variants with the same center body structure but different mission profiles including ‘low-speed, high-endurance’ (LSHE) and ‘high-speed, long-range’ (HSLR) to be set up for flight. In this work, microfibers were applied to the HSLR variant to evaluate how its aerodynamic performance may be enhanced further.

2. Test Setup

2.1. Switchblade UAV Model

The Switchblade UAV concept is a flying-wing, blended-wing-body aircraft that allows changing wing modules according to mission performance requirements. In this study, a half-body model of the HSLR variant was tested in a wind tunnel. The aircraft has a moderate aspect ratio and wing planform area, with a moderately swept wing to assist with longitudinal stability and control. The cruise speed corresponds to an airspeed of approximately 34.3 m/s near sea-level conditions. The model is illustrated in Figure 2, and its geometric variables are in Table 1. The numbered regions represent parts of the wing suction surface covered with bio-inspired coatings. Region 1 is upstream of the elevon (region 2), and region 3 is the wing surface closest to the center body. The aircraft was partially covered due to the challenge of sticking the microfiber adhesive to tight corners, such as the nacelle and the leading edge. The microfiber films were guaranteed to be placed upstream of the maximum thickness line.

The aircraft structure was built almost entirely via 3D printing, employing a short carbon-fiber-infused plastic and sanding the surface to a smooth finish, to make sure that the baseline surface was free of imperfections that could affect the results. Due to its reconfigurable feature, the model has the wing subassembly attached to the center section with tubular spars, allowing for different geometries to be used in the experiments—although that was not explored in this study. The wind tunnel model was manufactured with the same methods as a flying prototype, which offers the additional possibility of evaluating its structural airworthiness in operating conditions for further research. The nacelle tip shown in Figure 4 is removable to enable the installation of a motor and propeller in dynamic propulsion tests, something that was accomplished in [28].

2.2. Bio-Inspired Microfiber Films

The micro-structures used in this study consist of diverging cylindrical pillars, such as described in Figure 3a. They are built with an approximate glass shape, with a base and a diverging cylindrical pillar that increases in diameter towards the tip, where the flat face interacts with the external flow. The substrate to which the fibers are attached is an adhesive film, firmly adhered to the aircraft surface. Small fibers have a height of approximately 40 µm, medium fibers 70 µm, and tall fibers 140 µm. These dimensions are according to the manufacturer’s available options for adhesive films. The fibers are tightly packed on the substrate, forming an array with 120 µm spacing between each pillar. The base and tip diameter of each pillar are 40 µm and 75 µm, respectively. This geometry and spacing allow two main flow mechanisms to develop: an internal high-speed and low-pressure flow under the canopy, and a low-speed, high-pressure flow at the interface layer, as shown in Figure 1D. An in-depth study of the flow physics involved with the microfibers was provided by [26,27]. These flow mechanisms are the basis for the pressure modulation effect that is transferred to the external flow. The experimental model covered with microfiber coating is shown in Figure 3. These coatings were produced and supplied by Setex Technologies (Pittsburg, PA, USA), and they were tested to find the effect of microfiber pillar height, which refers to the size of the cylindrical pillars on the surface shown in Figure 1. We experimented with two types of microfiber coatings, with approximately 70 µm and 140 µm pillar heights. These sizes were selected considering the work performed in [26,27], where medium fibers were applied. The taller fibers were also selected under the assumption that they would have a stronger effect on the performance. Small fibers were not used due to previous experience showing that their effect was negligible under the experimental conditions.

2.3. Wind Tunnel

The experiments were conducted in the National Wind Institute (NWI) closed-loop, subsonic wind tunnel at the Reese Technology Center. The test section is 1.2 m tall and 1.8 m wide, which is large enough to accommodate the aircraft half-model at its true scale, as shown in Figure 4. The half-model is attached to a circular splitter plate and connected to an ATI 9150 Net Gamma 6-DoF load sensor with a resolution of 0.025 N for the lift and drag forces and 0.00125 N-m for the pitching moment. The force sensor readings were recorded with a National Instruments DAQ model 6353, using a data acquisition rate of 1000 Hz for a minimum of 30 s in each test run. The pitch attitude was set automatically with a high-torque stepper motor with a positioning resolution of 0.1°. The angle was measured throughout each experiment with a digital inclinometer, with a 0.1° resolution. To enable control surface deflection, the model was also equipped with a servo motor installed in the wing module, as in the flying prototype.

2.4. Static Angles of Attack Program

The surface coatings were used in distinct parts of the wing to analyze correlations between the coverage amount and its effect on aerodynamic performance. The experiments were split in three main cases: (i) top of wing (all regions covered); (ii) outboard, covering elevon and upstream (regions 1 and 2 only); (iii) smooth wing (baseline case). In every scenario, two different wind tunnel airspeeds were tested: 30 m/s and 17.3 m/s, representing the UAV cruising velocity and its estimated stall velocity, respectively. The corresponding Reynolds numbers, Re based on wing mean aerodynamic chord (mac) are Re_mac ≈ 3.7 × 10⁵ and 2.1 × 10⁵ in the laboratory air conditions. This flow regime is representative of the actual flight conditions the Switchblade UAV was designed for, and the flows are named in the test cases as “cruise Re” and “stall Re”, respectively.

In cases where the control surface was not deflected, aerodynamic forces were measured for a range of angles of attack between −2° and 17°, in increments of 1°. The model position was changed with a pitch link and the model locked in position, turning the wind tunnel off between test runs to avoid the hysteresis effect or biasing in the force sensor. The elevon performance tests were accomplished by changing the deflection angle of the control surface from −18.7° to 24.7°, at different UAV model angles of attack: 2°, 7° and 12°. According to the standard sign convention, a negative deflection angle is upwards and causes a reduction in the sectional lift produced, or a nose-up pitch response [32]. The static cases explored are shown in Table 2.

2.5. Dynamic Stall Program

To evaluate the effect of microfiber coatings on aircraft performance in the dynamic regime and under the effects of dynamic stall, forced oscillation tests were executed at multiple combinations of mean angle of attack, airspeed, reduced frequency and pitch amplitude. The reduced frequency, k for the dynamic pitch experiments was calculated according to the following expression,

k = \frac{ω \bar{c}}{U_{\infty}}

(1)

where ω is the pitch frequency,

\bar{c}

is the mean aerodynamic chord of the wing and U_∞ is the airspeed corresponding to cruise or stall. For the dynamic tests, three surface conditions were evaluated: no fibers (smooth), 70 µm fibers covering the whole wing (regions 1, 2 and 3) and 70 µm fibers covering the outboard section of the wing (regions 1 and 2). Table 3 summarizes the types of forced-oscillation experiments.

During each forced oscillation run, the data were recorded through a minimum of 30 cycles, such that a one-cycle average could be calculated for the axial and normal force coefficients (C_A and C_N), and the pitching moment coefficient (C_M). These coefficients were then used to calculate the dynamic derivatives with respect to pitch rate

C_{A_{q}}

C_{N_{q}}

and

C_{M_{q}}

, with the single-point method described in [33], using Equations (2)–(4).

C_{A_{q}} = \frac{C_{A_{(q m a x)}} - C_{A_{(q m i n)}}}{\frac{c_{r e f}}{2 V} (q_{m a x} - q_{m i n})}

(2)

C_{N_{q}} = \frac{C_{N_{(q m a x)}} - C_{N_{(q m i n)}}}{\frac{c_{r e f}}{2 V} (q_{m a x} - q_{m i n})}

(3)

C_{M_{q}} = \frac{C_{M_{(q m a x)}} - C_{M_{(q m i n)}}}{\frac{c_{r e f}}{2 V} (q_{m a x} - q_{m i n})}

(4)

where q_max and q_min are the maximum and minimum pitch rates in the cycle, c_{re f} is the mean aerodynamic chord and V the airspeed. Computing the dynamic derivatives effectively condenses the forced-oscillation data, which simplifies their further analysis.

3. Results and Discussion

3.1. Static Angles of Attack

The lift, drag and pitch moment loads were normalized with the dynamic pressure and wing planform area to obtain aerodynamic coefficients. The lift coefficients with respect to the angle of attack, drag polar and lift-to-drag ratio L/D are shown in Figure 5, Figure 6 and Figure 7, respectively. The linear sections of the C_L vs. α graphs do not show an appreciable difference or improvement in the lift produced using microfibers. However, there is a marked difference after α = 10° when the flow begins to separate for the baseline case near the trailing-edge region of the wing. Alternatively, the flow over the wing surface with microfibers is able to maintain higher suction pressure and therefore an appreciable increase in the lift coefficient until slightly after stall. For the UAV surface covered with 140 µm microfibers, C_L was 3.61% higher in the cruise regime and α = 14°. For the outboard coverage microfiber case, C_L was 3.89% higher in equal conditions. The 70 µm microfiber case with entire wing coverage led to a 3.28% rise in C_L when comparing to the smooth case at α = 15°.

In the stall Reynolds regime, the increments in C_L with microfiber coatings were smaller for either microfiber sizes. It is theorized that they produce a cyclical yet weak positive momentum injection mechanism to the outer flow, which increases the mean flow speed marginally. Moreover, the distance by which the flow passes through the surface coating prior to flow separation also affects the ability to reduce detachment. More space traveled allows the momentum transfer to accumulate and become effective. In this sense, passive microfibers are a subtle and distributed flow control device, in contrast with active synthetic jets, which are strong but concentrated flow control devices that should be placed close to the point of flow separation to become effective.

The bio-inspired surface coatings have a more pronounced and desirable result in drag reduction, as observed in the drag polars (Figure 6). In the cruising regime condition, the case with all wing regions covered attained a maximum drag reduction of 24.7% for C_L = 0.90 and α = 12°. For the wing with partial coverage, drag was lowered by 23.3% with respect to the smooth case for the same C_L. We found that 70 µm tall microfibers lowered drag by 19.9%. Considering the same C_L of 0.9 for the stall Reynolds regime, the drag was reduced by 14.8% on the wing with all regions covered with 140 µm microfibers, by 14.9% on the wing with only the outboard section covered and by 24.2% on the wing that was entirely covered with 70 µm microfibers.

Using microfibers minimally affects the maximum aerodynamic efficiency at cruise regime Reynolds, with the largest gain in the lift-to-drag ratio being 1.8%, in the 140 µm microfiber case, covering the outboard wing (Figure 7). For the stall Re airspeed, the maximum L/D was marginally reduced by the use of microfibers. In this regime, 140 µm microfibers covering the outboard area of the wing showed a 6.9% reduction. When looking at high α, every coated case indicated an improvement in L/D compared to smooth cases (Figure 8). With the cruise Reynolds number, the highest performance was found with 140 µm microfibers covering the entire wing surface (regions 1, 2, 3), demonstrated by a 15.3% improvement in L/D at α = 10°. In the stall regime, 70 µm microfibers on the wing were the best case, showing a 14.9% higher L/D for the same α.

A summary of the trends seen in the results is as follows: (i) regarding surface coverage, a small distinction was identified between the “top of wing” (regions 1, 2 and 3) and the outboard cases (regions 1 and 2). This shows that for the UAV design, bio-inspired coatings are more useful to mitigate flow separation over the region that is close to the tip of the wing at high α. (ii) Flow regime has an impact on the way microfibers perform; at higher Reynolds, all experiments provided similar L/D benefits at α = 10°, with 70 µm fibers experiencing a sharp decline in their effect at α = 11°. In comparison, at lower Reynolds numbers, the experiments showed a reduced improvement of L/D at α = 11°, followed by a further decrease at higher angles. Upon observation of the results for all microfiber cases at α = 10°, it is theorized that microfiber height strongly impacts aerodynamic performance improvements at the onset of flow separation, given that the shorter fibers provide less benefit at a higher Re. With a more turbulent flow, the 70 µm coating does not create a strong enough effect to reenergize the boundary layer as with the 140 µm coating due to weaker secondary vortices produced and thus lower turbulent kinetic energy. Conversely, in the less turbulent flow, the highest performance enhancement is provided by the smaller fibers due to their ability to increase momentum in the outer flow without introducing excessive turbulent kinetic energy.

Considering that the microfibers provide the best performance at higher angles of attack, it is suggested that they could be used in applications where separation is a pre-dominant effect, such as in bio-inspired unmanned aerial vehicles. In these novel concepts, flapping wings or a combination of flapping and rotary wings have been explored, in which flow separation has a considerable effect during flight [34,35]. For each stroke of the wings, a strong separation effect is present, which could be minimized with special microfiber textures to improve their performance and capabilities.

In order to gain more understanding of how microfibers modulate the flow in the boundary layer over an airfoil, two-dimensional particle image velocimetry (PIV) flow measurements were taken on the surface of a curved adverse pressure gradient (APG) section in turbulent flow with a LaVision PIV system. The APG section was coated throughout with microfibers containing h = 140 µm and subjected to a freestream velocity of 30 m/s. Two cameras with 10 cm by 10 cm measurement frames were centered about the approximate location of flow separation. Two thousand instantaneous velocity (U, V) images were acquired and processed into time-averaged flow quantities. The mean velocity contours are presented in Figure 9.

The turbulent boundary layer (TBL) is estimated to separate at an absolute position, x_s ≈ 10 mm where the TBL thickness, δ_i ≈ 18 mm. At the inlet of the measurement domain, the flow is attached and δ_i ≈ 3 mm. Hence, the microfiber pillar height to boundary layer thickness, h/δ_i ≈ 0.046. The tips of the microfibers are within the buffer layer in the region 5 < y⁺ < 30 in terms of viscous wall units. Further upstream, where the APG strength progressively decreases until the pressure gradient becomes zero, the boundary layer thickness was measured as approximately 1.3 mm. At this point, h/δ ≈ 0.108 where the microfibers modulate the flow within the log layer, where 30 < y⁺ < 0.15δ [36].

We theorize that the microfibers are most effective when the pillar height scales with this layer given by the law of the wall,

u^{+} = \frac{1}{k} \log (y^{+}) + B

(5)

The von Kármán constant, κ and intercept, B were found to be κ = 0.384 and B = 4.17 according to Österlund et al. [37]. The log layer is characterized by not only laminar shear but also turbulent shear produced by naturally occurring velocity fluctuations. The micropillars mainly serve to intensify wall-normal velocity fluctuations, creating alternate low-speed, high-pressure and high-speed, low-pressure regions as the flow travels throughout the micropillars. The net effect is a positive momentum transfer into the outer turbulent layer (y⁺ > 0.15δ), which is used to overcome the external adverse pressure gradient and delay flow separation.

The elevon deflection results displayed in Figure 10 show that variations in pitch by applying bio-inspired coatings to the wing depend on the angle of attack of the airplane, and in some scenarios, the microfibers have a detrimental effect. At α = 2°, covering the entire suction surface of the wing lowered C_M by 11.1% for a control surface deflection angle, δ_e = −15°. At a moderate angle of attack of α = 7°, the effect of microfibers was considered negligible for pitching performance. Finally, for α = 12°, microfibers were very effective for control surface performance, increasing C_M magnitude by a maximum of 15.6% for δ_e = −15° with the suction surface fully coated with microfibers.

Alternatively, in the outboard coated wing, the increment in C_M was 22.4% at the same elevon deflection angle. This behavior displays some nonlinear spanwise interaction on the flow between different areas of microfibers, since covering the surface of the wing with additional microfiber becomes detrimental or less efficient. A similar phenomenon has been found on helicopter blades with synthetic jets under a transition regime [38]. The most efficient use of synthetic jets in terms of improving the figure of merit of the rotor occurred in the outboard region. The activation of additional synthetic jets in the blade middle and root regions showed a smaller return in rotor performance when considering the additional energy expense of the jets. The clear similarity between the finite UAV wing in this work and the mentioned helicopter rotor is that the flow is considerably three-dimensional (3D) with spanwise flow on the wing and blade tip region. These results show that microfibers can be utilized as an effective passive flow control technique for finite wings with 3D flow and at higher angles of attack where it becomes more critical to avoid stall and thus a loss in flight control.

3.2. Dynamic Stall

Each dynamic angle of attack run from forced oscillations in pitch was generated for a one-cycle average with reduced frequency, k = 0.039, as shown in Figure 11. Given the high volume of data, it would be inconvenient to display the plots for all cases in a single figure; therefore, the coefficients across all cases were used to calculate the dynamic derivatives, shown in Figure 12. As expected from the static test results, the strongest effect by the fibers is observed at the highest angle of attack, α = 10°. A hysteresis is defined in this study by the absolute value difference between the force or moment coefficient produced during dynamic pitch relative to static pitch at a given angle of attack. At α = 10°, in the dynamic axial force coefficient (C_A), the existence of microfibers seemed to marginally reduce the hysteresis effect. For the normal force coefficient, the hysteresis on the top side of the loop was moderately increased in the case with microfibers covering the entire wing, as seen in C_N (Figure 12b) and C_M (Figure 12c). This indicates a minor effect between the 70 µm microfibers and the dynamic damping of the aircraft.

The combined pitch derivatives are shown in Figure 12a–c. The derivatives lie in two groupings: lower speed (17 m/s) and higher speed (30 m/s), which shows a small dependence on airspeed in the results.

C_{A_{q}}

is higher at 17 m/s (Figure 12a), whereas

C_{N_{q}}

is higher at 30 m/s (Figure 12b). No particular trend is seen on

C_{M_{q}}

in respect to airspeed (Figure 12c). The effect of mean angle of attack on the dynamic behavior is small, as all cases show similar dispersion with an increasing angle of attack. The normal force derivative

C_{N_{q}}

decreases modestly with an increasing angle of attack, as seen in Figure 12b, although not enough to suggest intensification of stability nonlinearities characteristic of high-α flight.

Considering that the dynamic derivatives on their own do not provide a good physical sense of the aircraft’s performance, a simple exercise was performed to evaluate a steady pull-up maneuver. Equation (6), found in [32], was used to estimate the load factor variation δn with respect to a unit elevon deflection δη.

\frac{δ n}{δ η_{steady state}} = - \frac{m_{η} z_{w} U_{e}}{m_{q} z_{w} - m_{w} U_{e}}

(6)

In this equation, m_η, m_q and m_w are the derivatives of pitching moment with respect to elevon angle, pitch rate and vertical speed, respectively. z_w is the derivative of vertical force with respect to vertical speed, and U_e is the equilibrium longitudinal speed. All the derivatives are in a concise form and defined in the body coordinate system. Although

C_{A_{q}}

C_{N_{q}}

and

C_{M_{q}}

are not in the equation, they are implicit in the definitions of m_η, m_q and m_w through the aircraft’s equation of state. Equation (6) was solved for a steady pull-up maneuver at the cruise speed (30 m/s) and pitch angles of 0, 5 and 10°. These flight conditions were chosen to match the data obtained from the forced oscillation tests since the derivatives are calculated about specific mean angles of attack. Additionally, Figure 12 shows that the maximum difference between the baseline (smooth) and microfiber cases happens at the highest angle of attack. Table 4 shows the results for Equation (6).

The results show that the marginal increase in dynamic damping seen in Figure 11, with the use of microfibers, has an effect on the aircraft’s maneuverability. Covering the top of the wing or just the outboard region with 70 µm fibers increased the aircraft’s response to a unit elevon input at the lowest angle of attack. From −0.14 g/deg of elevon deflection, the load factor went up to −0.21 g/deg at α = 0°. To better understand the significance of these performance metrics, consider that the Lockheed F-104A Starfighter, an interceptor aircraft, generates a response of −0.94 g/deg during a comparable maneuver [32]. For the tailless UAV, although less maneuverable due to the short moment arm between the control surface and center of gravity, an improvement of 0.07 g/deg is considerable for a small change in surface treatment. For a 5° or 10° angle of attack, the microfibers had a lower impact on load factor response, with a decrease being observed at 10°. The pressure modulation created by the microfibers re-energizes the external flow under adverse pressure gradients, reducing flow separation, though it causes the aircraft to exhibit a slower response in symmetrical maneuvers at high angles of attack.

4. Conclusions

Special coatings with textures inspired by nature were used on the surface of a small-scale unmanned aerial vehicle, and the impact on several performance metrics of aerodynamics was analyzed using wind tunnel experiments. The relationship with flow regime Re, pitch angle α, region of distribution and microfiber pillar size was measured for the lift production and drag reduction, together with aerodynamic efficiency L/D. For cruising conditions, with 140 µm microfibers, the best minimization in drag was identified, 24.7%, when the entire wing was covered. The significance of drag reduction at high angles of attack for a flying UAV is higher climb rates and/or energy savings that increase the range of the aircraft. In the stall flow regime, with 70 µm microfibers, performance is maintained with a drag reduction of up to 24.2%. Performance enhancement was more pronounced at a higher pitching attitude, where flow detachment effects are predominant in generating drag, since microfibers delay flow separation. In this manner, microfibers are functionally similar to other forms of passive flow control devices, yet are not detrimental to cruising. At moderate angles, for example, α = 5°, the microfibers affected aerodynamic efficiency marginally; in cruise conditions, L/D saw an increase of 1.8%, and in stall conditions, a reduction of 6.9%. It is believed that a positive increase in L/D can be achieved by more careful microfiber design including tuning of the microfiber pillar height so that it remains within the log-layer of the boundary layer for the tested stall and cruise Reynolds number. The control surface with microfiber experiments yielded increased pitching moment authority for the HSLR variant at α = 12°, enhancing C_M by 22.4% with an elevon deflection of −15°. Microfibers for the first time were tested under dynamic stall conditions on a pitching wing. Under a mild stall at α = 10°, microfibers indicated a marginal reduction in the hysteresis of the dynamic axial force coefficient, while an increase in hysteresis for the normal force coefficient was shown. This indicates a decrease and an increase in the primary force components that make up drag and lift, respectively, which serve to increase the dynamic lift-to-drag ratio with microfibers compared to the baseline case. The small increase in dynamic damping also yielded unexpected consequences of the slower pitching moment response of the UAV model to an elevon input. The overall results demonstrate the potential of employing bio-inspired microfibers with a diverging pillar cross-section area, as presented in this study, to the design of small-scale, fixed-wing UAVs. They can be particularly beneficial for flight at higher angles of attack, or so called high-α flight, which can be related to an unmanned aerial vehicle with improved maneuverability, akin to unmanned combat aerial vehicles.

Author Contributions

Conceptualization, D.S. and V.M.; methodology, A.D. and D.S.; software, A.D.; validation, D.S. and G.D.F.; formal analysis, G.D.F. and D.S.; investigation, G.D.F.; resources, A.D.; data curation, G.D.F.; writing—original draft preparation, D.S., G.D.F. and V.M.; writing—review and editing, V.M.; visualization, D.S.; supervision, V.M.; project administration, V.M. and D.S.; funding acquisition, V.M. The authors have contributed equally to all sections and tasks. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Department of Mechanical Engineering, Texas Tech University, as part of the startup package of Victor Maldonado.

Data Availability Statement

Data will be provided upon request.

Acknowledgments

The authors would like to acknowledge David Myers for his indispensable experience and help in manufacturing the experimental setup used in this work. Without him, most of the experiments would not have been possible.

Conflicts of Interest

Author Ali Doosttalab was employed by the company Flow Raider LLC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:

UAV	Unmanned Aerial Vehicle
UAS	Unmanned Aerial System
HSLR	High-Speed, Long-Range
LSHE	Low-Speed, High-Endurance
AoA	Angle of Attack

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Figure 1. (A) Concept of a shark skin denticle, (B) close perspective of bio-inspired microfibers, scale bar ≈ 100 µm, (C) surface coating from top, and (D) flow mechanism within the fibers and outside. Adapted from [26].

Figure 2. Planform drawing of UAV model with microfiber coverage (dimensions in mm).

Figure 3. (a) Microfiber schematic (dimensions in µm); (b) wing covered with microfiber coating (zoomed-in picture adapted from Doosttalab et al. [26]).

Figure 4. Wind tunnel model setup of the ‘high-speed, long-range’ (HSLR) variant of the Switchblade UAV.

Figure 5. Lift coefficients, C_L as a function of angle of attack, α.

Figure 6. Drag polars; lift coefficients, C_L as a function of drag coefficients, C_D.

Figure 7. Lift-to-drag ratio, L/D as a function of angle of attack, α.

Figure 8. High angle of attack, α lift-to-drag ratio, L/D enhancement.

Figure 9. Time-averaged velocity over a curved APG section representative of an airfoil in turbulent flow with a freestream velocity of 30 m/s.

Figure 10. Elevon deflection performance: pitching moment coefficient, C_M as a function of elevon deflection angle, δ_e for the baseline and micropillar cases.

Figure 11. Dynamic pitch coefficients for different surface cases and wing coverage: (a) C_A; (b) C_N; (c) C_M. The black arrow indicates the direction of the pitch up maneuver.

Figure 12. Dynamic derivatives in pitch:

C_{A_{q}}

;

C_{M_{q}}

;

C_{M_{q}}

as a function of mean angle of attack.

Figure 12. Dynamic derivatives in pitch:

C_{A_{q}}

;

C_{M_{q}}

;

C_{M_{q}}

as a function of mean angle of attack.

Table 1. Wing geometric parameters.

Wing Area [m²]	Aspect Ratio	Span [m]	LE Sweep	Taper Ratio
0.324	6.3	1.43	24°	0.35

Table 2. Static angle of attack experimental variables.

Surface	Region	AoA (α) [deg]	Airspeed, U_∞/Re	Elevon Deflection, δ_e
no fibers	N/A	−2° to 17°	cruise	N/A
no fibers	N/A	−2° to 17°	stall	N/A
140 µm fibers	1, 2, 3	−2° to 17°	cruise	N/A
140 µm fibers	1, 2, 3	−2° to 17°	stall	N/A
140 µm fibers	1, 2	−2° to 17°	cruise	N/A
140 µm fibers	1, 2	−2° to 17°	stall	N/A
70 µm fibers	1, 2, 3	−2° to 17°	cruise	N/A
70 µm fibers	1, 2, 3	−2° to 17°	stall	N/A
no fibers	N/A	2°	cruise	−18.7° to 24.7°
no fibers	N/A	7°	cruise	−18.7° to 24.7°
no fibers	N/A	12°	cruise	−18.7° to 24.7°
140 µm fibers	1, 2, 3	2°	cruise	−18.7° to 24.7°
140 µm fibers	1, 2, 3	7°	cruise	−18.7° to 24.7°
140 µm fibers	1, 2, 3	12°	cruise	−18.7° to 24.7°
140 µm fibers	1, 2	2°	cruise	−18.7° to 24.7°
140 µm fibers	1, 2	7°	cruise	−18.7° to 24.7°
140 µm fibers	1, 2	12°	cruise	−18.7° to 24.7°

Table 3. Dynamic stall experimental variables: k is the reduced frequency and A is the pitch amplitude.

Surface	Region	Mean AoA (α) [deg]	Airspeed, U_∞/Re	k	A [deg]
no fibers	N/A	0.0, 5.0, 10.0	cruise, stall	0.022, 0.039	5.0
no fibers	N/A	0.0, 5.0, 10.0	cruise, stall	0.011, 0.020	5.0
no fibers	N/A	0.0, 5.0, 10.0	cruise, stall	0.022, 0.039	10.0
no fibers	N/A	0.0, 5.0, 10.0	cruise, stall	0.011, 0.020	10.0
70 µm fibers	1, 2, 3	0.0, 5.0, 10.0	cruise, stall	0.022, 0.039	5.0
70 µm fibers	1, 2, 3	0.0, 5.0, 10.0	cruise, stall	0.011, 0.020	5.0
70 µm fibers	1, 2, 3	0.0, 5.0, 10.0	cruise, stall	0.022, 0.039	10.0
70 µm fibers	1, 2, 3	0.0, 5.0, 10.0	cruise, stall	0.011, 0.020	10.0
70 µm fibers	1, 2	0.0, 5.0, 10.0	cruise, stall	0.022, 0.039	5.0
70 µm fibers	1, 2	0.0, 5.0, 10.0	cruise, stall	0.011, 0.020	5.0
70 µm fibers	1, 2	0.0, 5.0, 10.0	cruise, stall	0.022, 0.039	10.0
70 µm fibers	1, 2	0.0, 5.0, 10.0	cruise, stall	0.011, 0.020	10.0

Table 4. Load factor increment per unit elevon input [g’s/deg].

Case	Mean AoA	Load Factor Increment, ∆n
Baseline	0.0	−0.14
70 µm, top of wing	0.0	−0.17
70 µm, outboard	0.0	−0.21
Baseline	5.0	−0.10
70 µm, top of wing	5.0	−0.08
70 µm, outboard	5.0	−0.10
Baseline	10.0	−0.04
70 µm, top of wing	10.0	−0.03
70 µm, outboard	10.0	−0.02

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Santos, D.; Fernandes, G.D.; Doosttalab, A.; Maldonado, V. Experimental Aerodynamics of a Small Fixed-Wing Unmanned Aerial Vehicle Coated with Bio-Inspired Microfibers Under Static and Dynamic Stall. Aerospace 2024, 11, 947. https://doi.org/10.3390/aerospace11110947

AMA Style

Santos D, Fernandes GD, Doosttalab A, Maldonado V. Experimental Aerodynamics of a Small Fixed-Wing Unmanned Aerial Vehicle Coated with Bio-Inspired Microfibers Under Static and Dynamic Stall. Aerospace. 2024; 11(11):947. https://doi.org/10.3390/aerospace11110947

Chicago/Turabian Style

Santos, Dioser, Guilherme D. Fernandes, Ali Doosttalab, and Victor Maldonado. 2024. "Experimental Aerodynamics of a Small Fixed-Wing Unmanned Aerial Vehicle Coated with Bio-Inspired Microfibers Under Static and Dynamic Stall" Aerospace 11, no. 11: 947. https://doi.org/10.3390/aerospace11110947

APA Style

Santos, D., Fernandes, G. D., Doosttalab, A., & Maldonado, V. (2024). Experimental Aerodynamics of a Small Fixed-Wing Unmanned Aerial Vehicle Coated with Bio-Inspired Microfibers Under Static and Dynamic Stall. Aerospace, 11(11), 947. https://doi.org/10.3390/aerospace11110947

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