In the rapid development of drones, the power system has attracted widespread attention. For fixed-wing drones, the piston engine combined with a propeller is the most economical choice for flight speeds below 300 km/h and altitudes under 8000 m. To ensure that a fixed-wing drone can complete its mission profile, a proper matching among the drone, engine, and propeller is essential. In this work, we present a systematic method for designing and matching the power system of a high-altitude long-endurance fixed-wing drone. Our approach begins with a mission profile design, followed by engine and propeller selection based on the maximum power demand, and finally a detailed matching analysis using thrust balance and power balance principles. The results demonstrate that the selected power system satisfies all flight phase requirements.
Mission Profile Design
We designed a typical vertical flight profile for a fixed-wing drone, starting from takeoff, climbing, cruising, descending, and landing. The drone operates at altitudes between 3 km and 7 km with a range of 200 km. The mission requirements under standard atmospheric conditions are summarized in Table 1.
| Flight Phase | Horizontal Speed (m/s) | Vertical Speed (m/s) | Transmission Efficiency (%) | Power Demand for Generator (kW) |
|---|---|---|---|---|
| Idle | – | – | – | – |
| Warm-up | – | – | – | 1 |
| Takeoff at sea level | 0–30 | 12 | 98 | 4 |
| Cruise at 3 km | 44 | 0 | 98 | 3 |
| Max speed at 3 km | 54 | 0 | 98 | 3 |
| Landing | 40 | −7 | 98 | 2 |
| Cruise at 5 km | 44 | 0 | 98 | 3 |
| Max altitude 7 km | 44 | 0 | 98 | 3 |
Engine Selection for Fixed-Wing Drones
Based on the mission profile, we calculated the thrust required for each flight phase. The forces acting on a fixed-wing drone during climbing, cruising, and descending are given by:
$$ T_{\text{req}} = D + W \sin \alpha $$
$$ L = W \cos \alpha $$
$$ \alpha = \sin^{-1}\left( \frac{V_v}{V} \right) $$
$$ L = C_L \cdot \frac{1}{2} \rho V^2 S $$
$$ D = C_D \cdot \frac{1}{2} \rho V^2 S $$
Combining these, the thrust required is:
$$ T_{\text{req}} = W \left( \frac{C_D}{C_L} \cos \alpha + \sin \alpha \right) $$
For level flight (\(\alpha = 0\)), the required thrust simplifies to:
$$ T_{\text{req}} = \frac{W}{K} $$
where \(K = C_L / C_D\) is the lift-to-drag ratio.
The power that the engine must deliver can be expressed as:
$$ P_e = \frac{T_{\text{req}} V}{\eta} = \frac{W V}{\eta K} $$
We initially assumed a propeller efficiency \(\eta = 0.7\). Considering the maximum speed cruise at 3 km (54 m/s) as the most demanding condition, and accounting for generator power (4 kW) and transmission losses (98% efficiency), we estimated the required engine power to be 62 kW. Therefore, we selected a spark-ignition aviation piston engine with a maximum power of 78 kW at 6000 rpm. The engine altitude performance was corrected using the empirical formula:
$$ N_H = N_0 \left( 1.11 \frac{P_H}{P_0} \sqrt{\frac{T_H}{T_0}} – 0.11 \right) $$
Propeller Selection for Fixed-Wing Drones
We chose a constant-speed three-blade propeller for its balance of efficiency and compactness. The propeller diameter must respect tip-speed limitations (Mach number less than 0.75 and tip speed below 213 m/s). The tip speed is:
$$ V_{\text{tip}} = \sqrt{(\pi n D_p)^2 + V^2} $$
Using the engine maximum speed of 6000 rpm and a reduction gear ratio of 2.43, the propeller speed is about 2470 rpm. At sea level, the local speed of sound is 340 m/s, limiting the diameter to about 1.99 m. The empirical formula for a three-blade propeller gives:
$$ D_p = 0.5 \sqrt[4]{P_e} $$
With \(P_e = 78\) kW, this yields \(D_p \approx 1.55\) m. Based on available commercial propellers, we selected three candidates with diameters of 1.52 m, 1.45 m, and 1.40 m (labeled Propellers #1, #2, and #3).
Matching Analysis of the Power System
Engine-Propeller Matching
The propeller absorbs power according to:
$$ P_p = A \cdot n^3 $$
For ideal matching, the engine power output \(P_e\) should equal \(P_p\). We plotted the engine’s external characteristic curve (power vs. speed) and the power absorption curves of the three candidate propellers (see Figure 4 in the original study). Only Propellers #1 and #2 intersected the engine curve; Propeller #3 did not reach the required speed. Among the two, Propeller #1’s intersection point was closest to the design point (62 kW), indicating the best match.
| Propeller ID | Diameter (m) | Intersection with Engine Curve | Matching Quality |
|---|---|---|---|
| #1 | 1.52 | Yes | Best |
| #2 | 1.45 | Yes | Acceptable |
| #3 | 1.40 | No | Poor |
Propeller-Drone Matching
Using the selected propeller #1, we calculated the thrust available during cruise. The advance ratio \(J\) is:
$$ J = \frac{V}{n D_p} $$
At cruise conditions (V = 44 m/s, n = 2470 rpm = 41.17 rps, D_p = 1.52 m), we obtain \(J = 0.81\). From the propeller performance charts (power coefficient \(C_P = 0.22\), efficiency \(\eta_p = 0.61\), thrust coefficient \(C_T = 0.248\)), the thrust produced is:
$$ T_p = C_T \rho n^2 D_p^4 $$
Assuming standard air density at 3 km (\(\rho = 0.909\) kg/m³), we compute \(T_p = 780.5\) N. The required thrust for cruise at 3 km is 578.3 N, so the propeller provides sufficient margin. Similar checks for takeoff, climb, and maximum speed phases confirmed that the power system meets all requirements.
Conclusion
We have developed a systematic method for matching the power system of fixed-wing drones, integrating mission profile design, engine selection, propeller selection, and comprehensive matching analysis. By applying thrust and power balance, we ensured that the selected piston engine and three-blade propeller can deliver the required thrust and power across all flight phases. The study provides a practical framework for engineers to design power systems for fixed-wing drones, emphasizing the critical interactions among the drone, engine, and propeller. Our approach successfully validated with a 78 kW engine and a 1.52 m propeller, demonstrating that proper matching is essential for mission completion of fixed-wing drones.