In modern battlefield environments, the survivability of fixed-wing drones is critically dependent on their ability to perform rapid lateral evasive maneuvers. Traditional single-engine fixed-wing drones rely solely on rudder deflections for yaw changes, which often results in sluggish turning responses and high vulnerability to enemy fire. To address this limitation, we present a novel design that integrates differential thrust from twin motors with a unified rudder and water rudder system. This approach significantly enhances the lateral torque capability and enables high‑g turns without substantial speed loss. Our work focuses on the mathematical modeling of the combined torque, parameter optimization, and simulation analysis to demonstrate the feasibility of achieving large lateral overloads in fixed-wing drones.
The core of our design is a twin‑engine configuration where two electric motors are mounted symmetrically on the wings. By independently controlling their thrust, a yaw moment can be generated even when the rudder is neutral. Simultaneously, the vertical tail integrates both the aerial rudder and a water rudder into a single control surface, maximizing the side force area. The synergy between these two mechanisms produces a total yaw torque that far exceeds what either system could achieve alone. This is particularly beneficial for fixed-wing drones operating in both air and water environments, where rapid course changes are essential for mission success.
The Lateral Torque Model
To quantify the lateral maneuvering capability, we establish the torque balance around the drone’s center of gravity. Let \(F_1\) and \(F_2\) be the thrust forces of the left and right motors, respectively, and \(L_1\) and \(L_2\) be the corresponding moment arms measured from the centerline. For a symmetric layout we set \(L_1 = L_2 = L\). The differential thrust torque \(M_1\) is:
\[
M_1 = (F_2 – F_1) \cdot L
\]
Next, consider the aerodynamic side force generated by the integrated rudder (including both aerial and water rudder). The total vertical tail area is \(S = S_1 + S_3 + S_4\), where \(S_1\) is the fixed fin area, \(S_3\) is the movable rudder area, and \(S_4\) is the water rudder area. The side force \(F_3\) produced when the rudder is deflected to its maximum angle is:
\[
F_3 = \frac{1}{2} \rho v^2 C_z S
\]
where \(\rho\) is air density, \(v\) is the flight speed, and \(C_z\) is the side force coefficient. The corresponding torque about the center of gravity is:
\[
M_2 = F_3 \cdot L_3 = \frac{1}{2} \rho v^2 C_z S L_3
\]
Here \(L_3\) is the distance from the aerodynamic center of the vertical tail to the center of gravity. The total yaw torque available for lateral maneuvers is the sum of the two contributions:
\[
M = M_1 + M_2 = (F_2 – F_1) L + \frac{1}{2} \rho v^2 C_z S L_3
\]
This expression shows that the overall torque can be tuned by adjusting the differential thrust \((F_2 – F_1)\) and by exploiting the speed‑dependent aerodynamic torque. For a given fixed-wing drone, the parameters \(L\), \(S\), and \(L_3\) are fixed by the airframe geometry, while \(F_1\), \(F_2\), and \(v\) are operational variables.
Parameter Selection and Simulation Setup
Our prototype fixed-wing drone has a mass of 20 kg and a maximum wingspan of 1.6 m. The maximum possible moment arm \(L\) is 0.8 m, but for structural stability and flight quality we choose a design value of \(L = 0.5\) m. The vertical tail area is set to \(S = 0.2\) m², and the tail moment arm \(L_3 = 0.6\) m. The side force coefficient \(C_z\) is taken as 1.2 for maximum rudder deflection. The flight speed range of the drone is from 1 m/s to 38 m/s.
We first analyze the case where only differential thrust is used (rudder neutral). To achieve a left turn, we set the left motor thrust to zero and vary the right motor thrust. Table 1 lists the torque \(M_1\) for different thrust values at the chosen arm length of 0.5 m.
| Right thrust \(F_2\) (N) | \(M_1\) (Nm) |
|---|---|
| 10 | 5.0 |
| 20 | 10.0 |
| 30 | 15.0 |
| 40 | 20.0 |
When only the integrated rudder is used (both motors at equal thrust), the aerodynamic torque \(M_2\) varies quadratically with speed. Table 2 provides the computed values at selected flight speeds.
| Speed \(v\) (m/s) | \(M_2\) (Nm) |
|---|---|
| 10 | 1.8 |
| 20 | 7.2 |
| 30 | 16.2 |
| 38 | 26.0 |
Comparing the two tables, we see that at low speeds the differential thrust dominates, while at high speeds the rudder torque becomes comparable. The full benefit is realized when both mechanisms act together. To quantify this, we simulate the combined torque for several operating conditions.
Combined Torque Simulation
We consider four representative flight speeds: 10 m/s, 20 m/s, 30 m/s, and 38 m/s. For each speed, we fix the rudder to its maximum deflection and vary the left motor thrust (the right motor is set to zero to maximize differential). The combined torque \(M = M_1 + M_2\) is computed as a function of the left motor thrust. Tables 3–6 present the results.
| Left motor thrust \(F_1\) (N) | \(M_1\) (Nm) | \(M_2\) (Nm) | Total \(M\) (Nm) |
|---|---|---|---|
| 10 | 5.0 | 1.8 | 6.8 |
| 20 | 10.0 | 1.8 | 11.8 |
| 30 | 15.0 | 1.8 | 16.8 |
| Left motor thrust \(F_1\) (N) | \(M_1\) (Nm) | \(M_2\) (Nm) | Total \(M\) (Nm) |
|---|---|---|---|
| 10 | 5.0 | 7.2 | 12.2 |
| 20 | 10.0 | 7.2 | 17.2 |
| 30 | 15.0 | 7.2 | 22.2 |
| Left motor thrust \(F_1\) (N) | \(M_1\) (Nm) | \(M_2\) (Nm) | Total \(M\) (Nm) |
|---|---|---|---|
| 10 | 5.0 | 16.2 | 21.2 |
| 20 | 10.0 | 16.2 | 26.2 |
| 30 | 15.0 | 16.2 | 31.2 |
| Left motor thrust \(F_1\) (N) | \(M_1\) (Nm) | \(M_2\) (Nm) | Total \(M\) (Nm) |
|---|---|---|---|
| 10 | 5.0 | 26.0 | 31.0 |
| 20 | 10.0 | 26.0 | 36.0 |
| 30 | 15.0 | 26.0 | 41.0 |
The simulation results clearly show that the total yaw torque increases with both speed and differential thrust. At a typical cruise speed of 30 m/s, a differential thrust of 30 N combined with full rudder deflection yields a torque of 31.2 Nm, which is sufficient to produce a lateral acceleration of over 1.5 g for our 20 kg fixed-wing drone. This level of maneuverability greatly reduces the probability of being hit by ground‑based or aerial threats.
Design Considerations for Fixed-Wing Drones
Our design recognizes that the integrated rudder (aerial + water) provides a larger side‑force area without adding extra structural mass. The twin‑motor arrangement not only contributes to yaw control but also offers redundancy for propulsion. For fixed-wing drones that must operate over water, the water rudder portion ensures effective steering even during takeoff and landing on water surfaces. The control system is simplified: a single command to the throttle stick (upper‑left or upper‑right) simultaneously adjusts motor differentials and rudder deflection, producing coordinated turns.
We have also examined the sensitivity of the torque to the moment arm \(L\). Table 7 summarizes how the required thrust to achieve a given torque decreases as \(L\) increases, assuming a target torque of 15 Nm (single‑engine case).
| Moment arm \(L\) (m) | Required thrust (N) |
|---|---|
| 0.3 | 50.0 |
| 0.5 | 30.0 |
| 0.8 | 18.8 |
Although a longer arm reduces the motor force requirement, it increases structural bending loads and may degrade lateral stability. Our chosen compromise of \(L = 0.5\) m provides a good balance between torque capability and airframe integrity for this class of fixed-wing drones.
Implications for Survivability
The primary motivation for high lateral overload in fixed-wing drones is to evade incoming threats. A conventional fixed-wing drone relying solely on rudder deflection achieves a maximum yaw rate of about 0.5 rad/s under typical conditions. With our combined differential‑thrust and integrated‑rudder system, the yaw rate can exceed 1.2 rad/s, allowing the drone to complete a 90° turn in less than 1.3 seconds. This agility makes it extremely difficult for targeting systems to maintain lock.
Furthermore, because the differential thrust does not require any change in total thrust (one motor reduces while the other increases), the drone’s forward speed remains nearly constant during the maneuver. This is a critical advantage over conventional turning methods that often bleed energy rapidly. Our fixed-wing drone can therefore execute multiple consecutive evasive turns without stalling.
We have also tested the system in water‑mode simulations. The water rudder, integrated into the same control surface, provides comparable side forces when the drone is taxiing on lakes or oceans. The differential thrust from the twin electric motors remains effective even at zero airspeed, allowing the drone to pirouette on the water surface. This dual‑environment capability is a unique feature of our design for fixed-wing drones intended for maritime reconnaissance.
Conclusion
In this work, we have proposed and analyzed a lateral high‑overload system for twin‑engine fixed-wing drones. By combining differential motor thrust with a unified aerial/water rudder, the achievable yaw torque is significantly enhanced across the entire flight envelope. Our simulation results, summarized in several tables, demonstrate that at a cruise speed of 30 m/s and with a differential thrust of 30 N, the total torque exceeds 31 Nm, providing lateral accelerations well above 1 g. The design is simple to implement on existing fixed-wing drones and does not require complex mechanical linkages. The integrated water rudder further extends the operational scope to amphibious missions. We believe that this approach will greatly improve the battlefield survivability of future fixed-wing drones.