Application of Asymmetric Airfoils to Main Rotor Blades of Small Helicopters
Asymmetric airfoils for helicopter rotor blades enhance aerodynamic efficiency, particularly at low Reynolds numbers typical for small helicopters. Unlike symmetrical profiles, they provide higher lift at zero angle of attack while minimizing pitching moments. This is particularly relevant for UAVs and light helicopters with small rotors.[7]
Aerodynamic Basics of Asymmetric Airfoils
Asymmetric (cambered) airfoils differ from symmetrical ones by having camber, generating lift at zero angle of attack. For main rotor blades, key requirements include low pitching moment about aerodynamic center (C_m ≈ 0), high max lift coefficients, and delayed stall. Symmetrical profiles like NACA 0015 are common in small helicopters for stability, but asymmetric, including reflexed (NACA 230-series), better handle lift dissymmetry in forward flight.[4][7] At low Re (10^4-10^5) typical for small rotors, cambered flat plates (4-6% camber) outperform conventional airfoils, providing up to 7% higher thrust and 5% better figure of merit (FM). This is due to early turbulence transition at sharp leading edges, avoiding laminar separation bubbles.[1][2]
Advantages in Small Helicopters
In small helicopters and UAVs, low Reynolds numbers cause early stall on symmetrical airfoils. Asymmetric profiles like cambered plates improve FM by 54% and reduce power via optimized taper and twist.[6] Airfoil combinations along span (inboard/outboard) boost FM in hover and L/D forward.[1] Supercritical airfoils (NASA SC(2)-0714) increase thrust 5-10% at high speeds.
Flow Phenomena at Low Re
At Re < 10^5, laminar separation bubbles (LSB) degrade conventional airfoil performance. Sharp leading edges of asymmetric plates trigger KH instabilities, leading to turbulent reattachment and vortex shedding, boosting L/D 17-41%.[2] In Mars-like conditions (akin to small rotors), cambered plates outperform MH airfoils. Transition depends on freestream turbulence and rotor vibrations.[1]
Optimization and Applications
Multi-objective optimization (GA + UMARC2) for Hart-II blades shows gains from sectional asymmetric airfoils.[1] In RC helicopters, cambered optimized blades enhance efficiency. Russian sources note modified NACA 230-13M for Mi-2, near-symmetrical but cambered for efficiency.[9]
References
1. Bousman, W.G. Airfoil Design and Rotorcraft Performance. NASA Ames, 2002 [1]. 2. Koning, W.J.F. et al. Low Reynolds Number Airfoil Evaluation for the Mars Helicopter Rotor. NASA TP, 2018 [2]. 3. Safdar, M.M. et al. Multi-Objective Optimization of Helicopter Rotor Blade. AIAA SciTech 2025 [1]. 4. Forward Flight Performance Analysis of Supercritical Airfoil. Tech Science Press, 2022 . 5. Herniczek, M. et al. Rotor blade optimization and flight testing of a small UAV rotorcraft. Carleton Univ., 2019 [6]. 6. Лопасти несущего винта Ми-2. ooobskspetsavia.ru, 2015 [9].