Trajectory planning for a multihelicopter transportation system requires optimizing numerous variables while considering obstacle avoidance, formation shape change, multibody dynamics and other feasibility constraints, making it challenging to achieve real-time effectiveness and convergence. To overcome this issue, a real-time trajectory planning method based on differential flatness and a minimum control effort (MINCO) trajectory expression is proposed. This method enables the implicit consideration of dynamic constraints and formation shape change requirements, and it handles obstacle avoidance and feasibility constraints linearly, reducing the complexity of planning. Specifically, the flatness characteristics and related assumptions of the system are given in detail by considering two cases where the load is modeled as a mass point or a rigid body. The obstacle avoidance trajectory planning problem is transformed into an unconstrained optimization problem based on MINCO and its spatial–temporal deformation. This optimization aims to minimize the trajectory energy and total time consumption while considering cost functions related to load obstacle avoidance, load state feasibility, helicopter obstacle avoidance, swarm avoidance between helicopters, cable obstacle avoidance, and cable force feasibility constraints. Finally, simulations are conducted in both cases where the load is modeled as a mass point and a rigid body to verify the feasibility of the proposed method. Overall, the proposed method has the potential to solve the problem of trajectory planning with obstacle avoidance in a multihelicopter transportation system in real time with improved efficiency and effectiveness.

Laminar separation bubbles (LSBs) on low-Reynolds-number airfoils are sustained by intrinsic unsteadiness driven by Kelvin–Helmholtz (K-H) growth in the separated shear layer. Using incompressible 2D URANS with the SA-$\gamma$ transition model for a NACA 0012 airfoil at $Re=5.3\times {10}^4$, we reveal that numerical dissipation behaves as a critical bifurcation parameter. Validated against the recent Jardin (2025) experimental benchmark, the physical state correctly resolves the LSB-induced pressure plateau ($C_p$) and local negative skin friction ($C_f<0$). However, when numerical dissipation exceeds the K-H instability growth rate, the physical limit-cycle oscillation collapses into a spurious fixed-point attractor—a phenomenon defined as numerical quenching. This pseudo-convergence triggers a catastrophic ∼30% deficit in mean lift ($C_l$). Furthermore, at $\alpha =6^{\circ }$, a drag-mechanism inversion is identified: while the physical branch is dominated by LSB-induced pressure (form) drag, the quenched branch exhibits a non-physical drag surge that exceeds the fully turbulent baseline. Phase portraits and power spectral densities ($St\approx 0.2$) provide objective diagnostics, demonstrating that standard residual convergence is a deceptive indicator of physical fidelity in transitional separated aerodynamics.

Rotor dynamic inflow modeling is essential for rotorcraft flight dynamics analysis and control, particularly under the rarefied Martian atmospheric conditions. This study develops a reduced-order state-space dynamic inflow model for Mars coaxial rotors based on the Pitt–Peters formulation, incorporating aerodynamic coupling between the upper and lower rotors. High-fidelity unsteady aerodynamic responses are obtained using the viscous vortex particle method (VVPM), whose steady aerodynamic predictions are evaluated against experiments conducted under simulated low-density conditions. A two-stage identification strategy is adopted to reduce the number of simultaneously estimated parameters. The isolated-rotor inflow parameters are first identified and subsequently retained as fixed diagonal blocks, while the additional inter-rotor coupling parameters are identified from coaxial rotor responses. The identified model exhibits transient inflow characteristics that differ from those predicted by the conventional Pitt–Peters model and reproduces the main dynamic responses obtained from VVPM. Under the investigated Martian hovering conditions, the model yields normalized root mean square errors of 4.12% and 4.86% for the isolated-rotor and coaxial rotor identification cases, respectively. The steady thrust reductions in the upper and lower rotors are approximately 10.3% and 20.7%, respectively, relative to their corresponding isolated-rotor conditions, indicating stronger aerodynamic interference on the lower rotor. These results demonstrate the potential of the proposed identification approach to characterize asymmetric coaxial rotor inflow coupling within a reduced-order framework under the prescribed hovering conditions.