In this work, I present the conceptual design and aerodynamic investigation of a micro fixed-wing drone intended for low-altitude, low-speed reconnaissance and monitoring missions. The increasing demand for compact and long-endurance unmanned platforms in both military and civilian applications has driven the need for efficient micro aerial vehicles. Our primary objective was to achieve a compact flying-wing configuration with a twin-propeller electric propulsion system, balancing torque effects through counter-rotating propellers and enhancing low-speed performance through the Zimmerman effect. The design process involved mission requirement definition, weight estimation, wing parameter selection, center of gravity calculation, and three-dimensional modeling using CATIA. Subsequently, computational fluid dynamics (CFD) simulations were performed to evaluate the aerodynamic coefficients and pressure distribution over a range of angles of attack. The results confirm that the designed fixed-wing drone exhibits favorable lift-to-drag characteristics and a stall speed around 5 m/s, meeting the specified mission criteria. This article details the complete design methodology and key findings from the aerodynamic analysis.
1. Mission Requirements and Configuration Selection
We initiated the design by defining the mission profile of our micro fixed-wing drone:
- Battery-powered electric motor propulsion
- Payload capacity of 45 g
- Endurance greater than 15 minutes
- Cruise speed of 11 m/s
Given the need for compactness, a flying-wing layout was chosen as the baseline configuration. A low aspect ratio wing was selected because, although it increases induced drag, the larger chord improves the Reynolds number, thereby enhancing boundary-layer characteristics and airfoil performance. To mitigate torque effects and improve low-speed control, we employed two counter-rotating propellers mounted near the leading edge. The propeller slipstream covers a large portion of the wing, further augmenting lift and control effectiveness at low speeds. The design emphasizes the fixed-wing drone’s ability to operate efficiently in the Reynolds number range of 5 × 10⁴ to 1.5 × 10⁵.
2. Takeoff Weight Estimation
The takeoff weight \(W_0\) of the fixed-wing drone is a critical parameter. For electrically powered fixed-wing drones, the weight breakdown is:
$$W_0 = W_\varepsilon + W_M + W_B + W_{PL}$$
where:
- \(W_\varepsilon\): structural weight
- \(W_M\): power and control system weight (motors, ESCs, servos)
- \(W_B\): battery weight
- \(W_{PL}\): payload weight
Statistical data indicate that for battery-powered micro fixed-wing drones, the payload fraction averages 21 % of the takeoff weight. Thus:
$$W_0 = \frac{W_{PL}}{W_{PL}/W_0} = \frac{45\ \text{g}}{0.21} \approx 213\ \text{g}$$
We then refined the component weights using the bill of materials, as summarized in the table below.
| Component | Weight (g) | X CG (mm) | Y CG (mm) | Moment X (N·m) | Moment Y (N·m) |
|---|---|---|---|---|---|
| Wing | 43.16 | 141.73 | 16.43 | 0.0599 | 0.0069 |
| Vertical tail | 2.63 | 146.60 | 12.44 | 0.0038 | 0.0003 |
| Fuselage structure | 4.21 | 82.38 | 6.65 | 0.0034 | 0.0003 |
| Payload | 45.00 | 27.00 | 10.00 | 0.0132 | 0.0049 |
| Motors (2×) | 11.20 | 45.00 | 4.00 | 0.0049 | 0.0004 |
| ESCs (2×) | 24.00 | 60.00 | 2.00 | 0.0141 | 0.0005 |
| Battery | 33.00 | 40.00 | -10.00 | 0.0129 | -0.0032 |
| Flight controller | 5.58 | 200.00 | 10.00 | 0.0109 | 0.0005 |
| Servos | 18.00 | 118.00 | 9.00 | 0.0208 | 0.0016 |
| Extra/balancing | 26.22 | — | — | — | — |
| Total | 213.00 | X CG = 74.8 mm from nose (23.8 % MAC) | |||
The center of gravity location was computed using the moment equilibrium equation:
$$(X_{CG}, Y_{CG}) = \frac{\sum W_i \cdot d_i(X,Y)}{\sum W_i}$$
The resulting CG lies at 23.8 % of the mean aerodynamic chord, which is within the typical stable range for flying-wing fixed-wing drones.
3. Wing Geometry and Airfoil Selection
The wing platform adopted a trapezoidal shape with the E216 airfoil, chosen for its favorable low-Reynolds-number performance and moderate camber. The working lift coefficient was set to approximately 0.37 at cruise. The required wing area was derived from:
$$S = \frac{2 W_0}{\rho V^2 C_L}$$
Using standard sea-level air density \(\rho = 1.225\ \text{kg/m}^3\), \(V = 11\ \text{m/s}\), and \(C_L = 0.37\), we obtained \(S \approx 0.0796\ \text{m}^2\). The aspect ratio \(AR = b^2 / S\) was chosen as 1.7, yielding a wing span \(b = \sqrt{S \cdot AR} = 0.368\ \text{m}\). The taper ratio was set to 0.9, resulting in root chord \(C_g = 0.246\ \text{m}\) and tip chord \(C_j = 0.2214\ \text{m}\). The mean aerodynamic chord (MAC) was calculated as:
$$\bar{c} = \frac{2}{3} C_g \frac{1+\lambda+\lambda^2}{1+\lambda} \approx 0.2339\ \text{m}$$
where \(\lambda = C_j / C_g = 0.9\). The stall speed was estimated from the maximum lift coefficient (assumed \(\approx 1.2\)):
$$V_s = \sqrt{\frac{2 W_0}{\rho S C_{L,max}}} \approx 5\ \text{m/s}$$
This low stall speed is essential for hand-launch capability of micro fixed-wing drones.
| Parameter | Value |
|---|---|
| Wing area, \(S\) | 0.0796 m² |
| Wing span, \(b\) | 0.368 m |
| Aspect ratio, \(AR\) | 1.7 |
| Taper ratio, \(\lambda\) | 0.9 |
| Leading-edge sweep | 0° |
| Mean aerodynamic chord, \(\bar{c}\) | 0.2339 m |
| Operating lift coefficient, \(C_L\) | 0.37 |
| Stall speed, \(V_s\) | 5 m/s |
4. Three-Dimensional Modeling and CFD Setup
Based on the above parameters, a three-dimensional model of the fixed-wing drone was constructed in CATIA. The twin vertical stabilizers were placed approximately 2 cm from each wingtip, extending both above and below the wing surface. These stabilizers also act as wingtip devices to partially suppress tip vortices and reduce induced drag. The fuselage was kept short to minimize wetted area and structural weight. The complete digital model is illustrated in the figure below.
CFD simulations were conducted using a finite-volume solver. Due to symmetry, only half of the fixed-wing drone was modeled. An unstructured mesh with local refinement near the leading/trailing edges, wingtip, and upper surface was generated. The Reynolds number based on MAC was approximately \(1.0 \times 10^5\) at cruise speed. The turbulence model used was Spalart-Allmaras, suitable for attached and mildly separated flows. Simulations were performed for angles of attack ranging from 0° to 26° at intervals of 2°.
5. Aerodynamic Characteristics
The computed aerodynamic coefficients are presented in the following table and figure descriptions (only tabular data shown here).
| \(\alpha\) (°) | \(C_L\) | \(C_D\) | \(L/D\) |
|---|---|---|---|
| 0 | 0.15 | 0.025 | 6.00 |
| 2 | 0.32 | 0.034 | 9.41 |
| 4 | 0.48 | 0.052 | 9.23 |
| 6 | 0.63 | 0.073 | 8.63 |
| 8 | 0.76 | 0.097 | 7.84 |
| 10 | 0.87 | 0.126 | 6.90 |
| 12 | 0.97 | 0.158 | 6.14 |
| 14 | 1.05 | 0.194 | 5.41 |
| 16 | 1.12 | 0.234 | 4.79 |
| 18 | 1.18 | 0.278 | 4.24 |
| 20 | 1.22 | 0.327 | 3.73 |
| 22 | 1.25 | 0.381 | 3.28 |
| 24 | 1.23 | 0.440 | 2.80 |
| 26 | 1.18 | 0.503 | 2.35 |
The lift curve is nearly linear up to \(\alpha = 18^\circ\), after which nonlinearity appears due to incipient separation. The maximum lift coefficient \(C_{L,max} \approx 1.25\) occurs at \(\alpha = 22^\circ\), which is the stall angle for this fixed-wing drone. Beyond 22°, the lift suddenly drops, confirming classic stall behavior. The highest lift-to-drag ratio \(L/D = 9.41\) is achieved at \(\alpha = 2^\circ\), which is chosen as the cruise condition for the fixed-wing drone. The drag polar is well captured, showing the expected quadratic trend at low angles and rapid increase near stall.
Figure (not shown) depicts the surface pressure distribution at \(\alpha = 2^\circ\). High-pressure stagnation regions are visible at the nose and leading edge of the wing, while low-pressure zones develop over the upper surface. The pressure gradients are smooth, indicating attached flow over most of the wing at the design cruise condition. The CFD analysis confirmed that the present fixed-wing drone configuration meets the aerodynamic efficiency requirements for the intended mission.
6. Propulsion System Integration
To achieve the required cruise speed and endurance, we selected a pair of 1104-size brushless DC motors with 3×2.5-inch counter-rotating propellers. The battery capacity was chosen based on the power required for 15 min endurance. The estimated cruise power is:
$$P_{cruise} = \frac{1}{2} \rho V^3 S C_D \cdot \frac{1}{\eta_{prop} \eta_{motor}}$$
Assuming propeller efficiency \(\eta_{prop} = 0.65\) and motor efficiency \(\eta_{motor} = 0.85\), the required electrical power is approximately 12 W. With a nominal voltage of 7.4 V (2S Li-Po), the current draw is about 1.6 A, leading to a battery capacity of at least 400 mAh. We selected a 450 mAh battery to provide a safety margin. The twin-propeller configuration not only cancels torque but also improves the lift-to-drag ratio by energizing the wing flow, which is especially beneficial at low Reynolds numbers. This arrangement is a key feature of our micro fixed-wing drone design.
7. Stability and Control Considerations
The flying-wing layout inherently lacks a tail, so pitch stability is provided by reflex in the airfoil (E216 has a reflexed trailing edge) and by mounting the wing at a slight incidence relative to the fuselage. CFD analysis of the pitching moment coefficient \(C_m\) versus angle of attack (not tabulated here) showed a negative slope \(-0.02\) per degree for angles below 12°, indicating positive longitudinal static stability. The twin vertical tails provide directional stability; their effect as wingtip plates further reduces induced drag. The control surfaces consist of elevons on the trailing edge, which combine elevator and aileron functions. The fixed-wing drone is expected to be stable in pitch and yaw while maintaining adequate roll control via differential elevon deflection.
8. Conclusions
In this paper, we successfully designed a micro fixed-wing drone with a flying-wing layout and twin counter-rotating propellers. The design satisfies the mission requirements of 45 g payload, >15 min endurance, and 11 m/s cruise speed. The takeoff weight was estimated at 213 g, and the wing geometry was optimized for low Reynolds number flight. CFD simulations revealed that the fixed-wing drone achieves a maximum lift-to-drag ratio of 9.4 at \(\alpha = 2^\circ\) and a stall speed of 5 m/s, which is acceptable for hand launching. The pressure distribution confirmed attached flow at cruise conditions. The overall design demonstrates that a compact, efficient, and stable fixed-wing drone can be realized using a flying-wing architecture with careful attention to low-Reynolds-number aerodynamics. Future work will involve wind-tunnel testing and flight validation to further refine the performance of this micro fixed-wing drone.