Tandem Rotor Inflow Modeling and its Effect on Vehicle Dynamics

Abstract

Inflow modeling is necessary for accurate predictions of performance, aeromechanics, handling qualities analyses and flight simulation of single and/or multi-rotor configurations. There are complete inflow theories, i.e., finite state dynamic wake theory, for single rotor configurations which were shown to correlate well with the test data. However, inflow models of multi-rotor configurations such as CH-47 Chinook helicopter are still heavily dependent on empirical corrections. The physical effects behind adding correction factors are reasoned to wake interference between front and rear rotors. However, effect of the interference on steady-state conditions and mechanism of how it affects the transient response are left unanswered. This paper aims to answer physical reasoning behind the correction factors. For this goal, an industry standard, empirically correlated inflow model, viz., Boeing Helicopter Simulation CH-47 Chinook Inflow Modeling Method (BHSimIMM), is compared with Pressure Potential Superposition Inflow Model (PPSIM). In addition, a high fidelity Viscous Vortex Particle Method (VVPM) is included in the study for gaining further insight into rotor-to-rotor inflow interference. In this study, it is shown that body pitch rate response is greatly affected by uniform inflow interference in hover and forward flight. The body roll rate response characteristic can be improved by having fore-to-aft inflow coupling and uniform to fore-to-aft inflow interference in hover. It is also shown that interference on the front rotor quickly diminishes as speed increases while rear rotor is always under the influence of front rotor.