Performance and Loads of a Model Coaxial Rotor Part I: Wind Tunnel Measurements

The results of wind tunnel tests of a model scale, closely spaced, rigid, counter-rotating coaxial rotor system are presented. The system, tested in the Glenn L. Martin wind tunnel, features-two bladed upper and lower rotors, 2.03m in diameter, with uniform section, untwisted rotor blades. Measurements include upper and lower rotor steady and vibratory hub loads, control angles and loads. Blade tip clearance was measured using a custom optical sensor. The rotor system was tested at advance ratios between 0.21 and 0.53, at collective pitches ranging from 2 to 10 degrees achieving blade loadings in excess of 0.10. Sweeps of lift offset up to 20% were performed at each operating condition, with a limited test matrix for variation in rotor speed and inter-rotor phase angle variation. Rotor effective lift-to-drag ratio was found to increase with increasing advance ratio and lift offset resulting in a 30% peak efficiency gain. The lower coaxial rotor was found to operate at higher lift-to-drag ratio than the upper rotor due to aerodynamic interference. Lift offset results in a decrease in blade tip clearance with a corresponding rise in rotor side force. Vibratory loads increase with advance ratio with the largest loads in the two and four-per-revolution harmonics. The coaxial rotor system experiences lower vibratory in-plane forces and torque than the isolated rotors due to cancellation between upper and lower rotor load harmonics. Lift offset, in conjunction with inter-rotor phase angle is shown to modify two-per-revolution in-plane vibratory forces and moments transmitted to the fixed frame, increasing some force components while decreasing others.