Correlations of Rotor/Wake-Airframe Interaction Measurements with Flow Vizualization Data

Narayanan M. Komerath Associate Professor School of Aerospace Engineering Georgia Institute of Technology Atlanta, Georgia 30332 revised version of a paper included in the Proceedings of the 46th Annual Forum of the AHS. ABSTRACT Interaction between the aerodynamics of the rotor and the airframe causes large unsteady pressure fluctuations on rotorcraft airframes. A two-bladed rotor and a hemispherecylinder airframe model are used to study these pressure fluctuations in a wind tunnel, simulating low-speed forward flight conditions. Controlled displacement of the airframe is used to obtain finelyspaced pressure data. These are correlated with azimuth-resolved, quantitative laser sheet visualization of the dynamics of the tip vortices and the inboard vortex sheets from the rotor, along the top of the airframe. Three prominent periodic interactions are observed on the airframe surface. These are the effects of blade motion, the approach and impingement of the tip vortex, and of the vortex sheet. These interactions determine, to a large degree, the overall pressure distribution on the airframe surface, and hence the airframe unsteady airloads. The pressure signatures from the blade passage and the vortex impingement are quite different, and are explained using 2-dimensional models. NOMENCLATURE Cp Pressure coefficient based on freestream dynamic pressure CPmean (Pmean -P∞)/q∞ CPinst (Pinst -P∞)/q∞ CPsteady CP(V∞/Vsteady)2 CPunsteady Punsteady/q∞ CT Rotor thrust coefficient, T/(ρπΩ2R4) h Vertical distance between 2-D vortex core and surface H Vertical spacing between rotor hub center and airframe centerline Pmean Mean static pressure P∞ Freestream static pressure Pinst Instantaneous static pressure Punsteady Unsteady component of static pressure (Pinst Pmean) q∞ Freestream dynamic pressure R Rotor radius T Rotor Thrust U Velocity at surface beneath 2-D vortex Vblade Velocity of blade section Vsteady Far field velocity in a steady reference frame. *: This work was part of the first author's Ph.D. dissertation, performed at the School of Aerospace Engineering, Georgia Institute of Technology. This is a V∞ Tunnel free-stream speed X Streamwise distance from hub center XL Local horizontal coordinate Revised from paper at the AHS Forum, 1990. XN X co-ordinate of airframe nose