New BSc thesis - Numerical Modelling of the Influence of Quadcopter Arm Length

The Bachelor’s Thesis of Pol Mut Gosp, student of UPV’s Aerospace Engineering degree, and developed in the framework of ENOLA, investigates the influence of quadcopter arm length on the aerodynamic flow field and overall vehicle performance. Two of the ENOLA UAV (affectionately called Octavio by the team) configurations, with arm lengths of 30 and 35 cm, were analysed using CFD simulations and a numerical performance model under hover, vertical flight and forward flight conditions.

Figure 1. Quadcopter configurations analysed, with arm lengths of 30 cm and 35 cm.

The two configurations were defined while keeping the remaining vehicle components unchanged. The 30 cm configuration was assigned a total mass of 6.00 kg. The additional mass resulting from extending each of the four arms by 5 cm was estimated from their geometry and material density, obtaining an increase of 0.226 kg. Consequently, the total mass of the 35 cm configuration was estimated as 6.23 kg. The aerodynamic behaviour of both configurations was then evaluated using CFD simulations, considering the complete vehicle geometry and the interaction between the rotor wakes, arms, central body and landing gear. In addition, a numerical performance model was used to assess how the change in arm length affects the power requirements and flight performance of the quadcopter.

The results show that increasing the arm length mainly modifies the spatial distribution of the induced flow. The 35 cm configuration produces a greater separation between the rotor wakes, reducing their proximity to the central fuselage, while the overall wake direction remains primarily governed by the flight condition. This effect is particularly visible in forward flight.

Figure 2. Comparison of the velocity magnitude fields for the 30 cm and 35 cm arm configurations under hover, vertical-flight and forward-flight conditions.

From a performance perspective, the longer-arm configuration requires greater thrust and mechanical power because of its higher mass. In forward flight, it also produces greater aerodynamic drag, mainly due to the increased contribution of the arms. However, the larger distance between the rotors and the centre of gravity reduces the difference in rotational speed required between the front and rear rotors to balance the pitching moment.

Figure 3. Comparison of the main aerodynamic performance parameters of both configurations in forward flight.

Overall, the results reveal a trade-off between wake separation, aerodynamic performance and vehicle control. Shorter arms reduce mass, power requirements and forward-flight drag, whereas longer arms provide greater wake separation and reduce the differential rotor speed required for longitudinal trim. The study therefore highlights the importance of considering arm length as part of the overall aerodynamic design of multirotor UAVs.