Calculation of rotorcraft inflow coefficients using blade flapping measurements

Induced velocity gives rise to an important component of the angle of attack experienced by a blade element and, hence, the rotor loads. Finite-state induced-velocity models can be found in rotorcraft codes that require fast computation but cannot replicate real flowfield features. However, they do encapsulate in a simple and intuitive way the impact of real wakes on rotorcraft dynamics and hence are popular in codes that are used to study vehicle flight mechanics such as stability, control, and handling qualities. Explicit verification of these models is not possible, but implicit verification is possible if blade flapping is known; and this paper presents results from flight tests of a light gyroplane fitted with a two-bladed teetering rotor instrumented to measure the rotor flapping behavior. An appropriate model structure is developed to allow calculation of induced-velocity components from steady-flight flapping data recorded across the aircraft’s level-flight speed range. Very good agreement is obtained between flight and theory in respect to the uniform and longitudinal components, correlating well with previous studies. However, the lateral component is very poorly correlated, and this is attributed to strong nontrapezoidal behavior in the real wake. Notwithstanding this, it is concluded that the simple finite-state, induced-velocity model is a valid tool for gyroplane flight mechanics studies and rotorcraft autorotation in general