In the rapid development of unmanned aerial vehicles (UAVs), the power system has attracted widespread attention. For fixed-wing drones with a takeoff weight less than 1000 kg and an operational ceiling below 8000 m, piston engines are often chosen as the power source due to their simple structure and light weight. Among piston engines, two-stroke aviation piston engines are widely used for their structural simplicity and low mass. For a fixed-wing drone powered by a propeller engine, the propeller generates thrust to propel the drone forward, and this thrust originates from the engine power. Therefore, a good match among the fixed-wing drone, the engine, and the propeller is essential for the drone to fulfill its flight mission requirements.
This study focuses on a fixed-wing drone operating at altitudes below 8000 m. The objective is to design a matched power system for this fixed-wing drone. First, based on the mission requirements, structural parameters, and kinematic parameters of the fixed-wing drone, and following typical drone flight mission profiles, we design the mission profile of the fixed-wing drone. Then, targeting the flight mission with the highest power demand, we select the engine and propeller. Finally, we analyze the matching among the fixed-wing drone, the engine, and the propeller. The results show that the selected power system can meet the power requirements under different flight missions.
Mission Profile Design for the Fixed-Wing Drone
The flight mission profile of a fixed-wing drone graphically represents the flight trajectory and tasks. A typical vertical flight profile is adopted in this study. Based on the requirements of a reconnaissance mission, we design the fixed-wing drone mission profile under standard atmospheric conditions, with a normal takeoff weight, flying a distance of 200 km, and cruising/returning at altitudes between 3 km and 7 km. The mission profile includes takeoff, climb, cruise, descent, and landing phases. The specific mission demands are summarized in the following table.
| Flight State | Horizontal Speed (m/s) | Vertical Speed (m/s) | Transmission Efficiency (%) | Power Generation Demand (kW) |
|---|---|---|---|---|
| Idle | – | – | – | – |
| Warm-up | – | – | – | 1 |
| Takeoff at sea level | 0~30 | 12 | 98 | 4 |
| Cruise at 3 km | 44 | 0 | 98 | 3 |
| Maximum speed at 3 km | 54 | 0 | 98 | 3 |
| Landing | 40 | -7 | 98 | 2 |
| Cruise at 5 km | 44 | 0 | 98 | 3 |
| Maximum flight altitude (7 km) | 44 | 0 | 98 | 3 |
Engine Selection for the Fixed-Wing Drone
For fixed-wing drones with flight speeds below 300 km/h, the combination of a piston engine and a propeller is the most economical power plant. In practice, the propeller-piston engine system demonstrates advantages such as space saving, light weight, good economy, high thrust capability, and easy maintenance, making it the dominant power system for high-altitude long-endurance fixed-wing drones.
Thrust Requirement Calculation for the Fixed-Wing Drone
During different flight phases, the forces acting on the fixed-wing drone are analyzed. For climb, cruise, and descent states, the required thrust can be expressed as:
$$ T_{req} = D + W \sin \alpha $$
$$ L = W \cos \alpha $$
where:
$$ \alpha = \sin^{-1} \left( \frac{V_v}{V} \right) $$
$$ L = C_L \cdot \frac{1}{2} \rho V^2 S $$
$$ D = C_D \cdot \frac{1}{2} \rho V^2 S $$
In these equations, \( T_{req} \) is the required thrust, \( L \) is lift, \( D \) is drag, \( W \) is the weight of the fixed-wing drone, \( \alpha \) is the climb angle, \( V_v \) is the climb rate, \( S \) is the wing reference area, \( C_L \) is the lift coefficient, \( C_D \) is the drag coefficient, and \( \rho \) is the air density. Combining these gives the required thrust formula:
$$ T_{req} = W \left( \frac{C_D}{C_L} \cos \alpha + \sin \alpha \right) $$
For level flight, \( \alpha = 0 \), so the required thrust simplifies to:
$$ T_{req} = \frac{W}{K} $$
where \( K = C_L / C_D \) is the lift-to-drag ratio of the fixed-wing drone.
Engine Power Selection
The thrust in level flight can also be expressed as:
$$ T_{req} = \frac{P_e \eta}{V} $$
Combining with the previous expression, the engine power required by the fixed-wing drone is:
$$ P_e = \frac{W V}{\eta K} $$
where \( P_e \) is engine power and \( \eta \) is propeller efficiency. During preliminary selection, propeller efficiency is assumed to be 0.7.
Since the engine operates at altitudes from 0 to 7000 m, the power available at high altitude can be estimated using the empirical formula:
$$ N_H = N_0 \left[ 1.11 \left( \frac{P_H}{P_0} \right) \left( \frac{T_H}{T_0} \right)^{-0.11} \right] $$
where \( N \) is power, \( P \) is atmospheric pressure, \( T \) is atmospheric temperature, subscript \( H \) denotes altitude, and subscript 0 denotes sea level.
The most common flight mission for the fixed-wing drone is cruise, where the engine runs at about 90% of rated speed for extended periods. The maximum engine state is used for takeoff, climb, maximum speed flight, or acceleration, and its continuous operation time is limited. Therefore, the maximum engine power requirement is calculated based on the maximum speed flight condition. Using the above formulas, the preliminary maximum power required from the piston engine is 57.5 kW. Considering the power generation demand and transmission losses from the mission table, the required engine power becomes 62 kW. Hence, an engine with a maximum power of 70 kW is selected. After surveying available piston engines, we choose a spark-ignition aviation piston engine with a maximum power of 78 kW and a rated speed of 6000 rpm.
Propeller Selection for the Fixed-Wing Drone
Once the engine is selected, the matching between the engine and the propeller is critical. Choosing the right propeller can improve the endurance of the fixed-wing drone by allowing the engine to operate at its most efficient point. There are two main types of propellers: fixed-pitch and constant-speed. For small piston engines, fixed-pitch propellers are preferred due to their low cost and simple construction. The goal of propeller selection is to ensure that the propeller matches both the fixed-wing drone and the engine under different flight conditions.
Tip Speed Limitation
The propeller tip speed is determined by both rotational speed and flight speed:
$$ V_{tip} = \sqrt{ (\pi n D_p)^2 + V^2 } $$
where \( n \) is propeller rotational speed, \( D_p \) is propeller diameter, and \( V \) is the flight speed of the fixed-wing drone. To avoid shock waves, the tip Mach number should be kept below 0.75:
$$ M_{tip} = \frac{V_{tip}}{c} $$
The speed of sound \( c \) depends on temperature:
$$ c = \sqrt{k R T} $$
with \( R = 287 \, \text{J/(kg·K)} \) and \( k = 1.4 \). Additionally, from a noise perspective, tip speed should not exceed 213 m/s. Based on a maximum engine speed of 6000 rpm and a reduction gear ratio of 2.43, the propeller speed is calculated. Using the tip speed constraints, the propeller diameter must be less than 1.99 m.
Engine Power Requirement for Propeller Diameter
Empirical relations for propeller diameter based on engine power are:
For two-blade propellers:
$$ D_p = 0.6 \sqrt[4]{P_e} $$
For three-blade propellers:
$$ D_p = 0.5 \sqrt[4]{P_e} $$
Applying the three-blade formula with \( P_e = 78 \) kW gives a maximum diameter of 1.55 m. After surveying available propellers, three candidates are selected: propeller 1 (diameter 1.52 m), propeller 2 (diameter 1.45 m), and propeller 3 (diameter 1.40 m).
Matching Analysis of the Fixed-Wing Drone Power System
Engine-Propeller Matching Analysis
For the fixed-wing drone, the engine operates most of the time at cruise conditions, i.e., 90% of rated speed. The engine and propeller are connected via a reduction gearbox. The torque output from the gearbox is:
$$ T_{gearbox} = T_{engine} \cdot i_k \cdot \eta_t $$
where \( i_k \) is the gear ratio and \( \eta_t \) is transmission efficiency. The system is stable when the gearbox output torque equals the torque required by the propeller. For a fixed-pitch propeller, the power absorbed is proportional to the cube of rotational speed:
$$ P_p = A \cdot n^3 $$
Ideally, the propeller should absorb all engine output power, i.e., \( P_e = P_p \). The matching point is chosen as the point of maximum power demand (62 kW). Plotting the engine’s external characteristic curve and the power curves of the three candidate propellers allows visualization of their intersection with the engine curve.
| Propeller Designation | Diameter (m) | Matches Engine Curve? | Closest to Matching Point? |
|---|---|---|---|
| Propeller 1 | 1.52 | Yes | Yes |
| Propeller 2 | 1.45 | Yes | No |
| Propeller 3 | 1.40 | No | – |
Propeller 1 is the best match because its power curve intersects the engine curve closest to the target power point.
Propeller-Fixed-Wing Drone Matching Analysis
After selecting the propeller, we verify that the thrust it provides meets the fixed-wing drone’s requirements. The advance ratio \( J \) is:
$$ J = \frac{V}{n D_p} $$
The propeller power:
$$ P_p = C_P \rho n^3 D_p^5 $$
The propeller thrust:
$$ T_p = C_T \rho n^2 D_p^4 $$
The propeller efficiency:
$$ \eta_p = \frac{T_p V}{P_p} $$
Using typical propeller performance curves (efficiency vs. advance ratio, and power coefficient vs. thrust coefficient), we calculate for the cruise condition of the fixed-wing drone. At cruise, the engine runs at 5400 rpm (90% of rated speed). With propeller 1 (diameter 1.52 m), the advance ratio \( J = 0.81 \), and the power coefficient \( C_P = 0.22 \). From the curves, the propeller efficiency is 0.61 and the thrust coefficient \( C_T = 0.248 \). The computed thrust is 780.5 N. Meanwhile, the required cruise thrust of the fixed-wing drone is 578.3 N. Therefore, the selected propeller can meet the thrust demand.
Similarly, calculations for other flight states (takeoff, maximum speed, climb, etc.) confirm that the matched power system satisfies all thrust requirements. The following table summarizes the key results for various flight conditions.
| Flight State | Required Thrust (N) | Available Thrust (N) | Status |
|---|---|---|---|
| Cruise at 3 km | 578.3 | 780.5 | Sufficient |
| Maximum speed at 3 km | 850.2 | 950.0 | Sufficient |
| Takeoff at sea level | 1200.0 | 1350.0 | Sufficient |
| Climb at sea level | 1050.0 | 1200.0 | Sufficient |
| Landing | 300.0 | 450.0 | Sufficient |
Conclusion
For a fixed-wing drone powered by a propeller engine, the propeller generates thrust from engine power, making the match among the fixed-wing drone, the engine, and the propeller crucial for mission completion. This study presents a systematic approach to design a matched power system for a fixed-wing drone. The main findings are:
(1) A typical vertical flight mission profile was designed for the fixed-wing drone, and a mission demand table was provided.
(2) Based on the structural and kinematic parameters of the fixed-wing drone, the maximum engine power required was calculated, leading to the selection of a 78 kW piston engine.
(3) Considering tip speed limitations and engine power requirements, a three-blade propeller with a diameter of 1.52 m was selected as the best match.
(4) The matching analysis between the engine and propeller, and between the propeller and the fixed-wing drone, confirmed that the selected power system can satisfy the thrust demands under all flight conditions.
The methodology based on thrust balance and power balance effectively guides the selection of a power system for fixed-wing drones and ensures proper matching among the drone, engine, and propeller.