Assessment of Fountain Flow Effects on Two Non-Overlap Tandem Rotors Performance

Very few issues are yet to be reported about the fountain flow beneath the tandem rotors configuration body in ground effect. In this paper, a series of tests have been performed using multipurpose test stand to study the fountain flow pattern of a sub-scale model of a generic tandem rotor helicopter in ground effect. Pressure and velocity measurements were performed by the pressure ports embedded longitudinally along a generic tandem rotor helicopter fuselage. The presence of a second rotor in tandem rotors configuration causes a fountain flow formation in the longitudinal center region below the helicopter and a semi-quiescent flow was there. The positive effects of this flow were detected such as increasing the sub-body pressure lifting force, the pressure distribution balance, and reinforcing the lift by the formation of the positive pressure gradient on sidewalls of the airframe. Tuft test observations confirmed the location of the fountain flow formation concluded from pressure and velocity measurements. The mentioned fountain flow aerodynamic effects must be taken into account in the tandem rotor helicopter flight controls trims, stability, and control augmentation system and lift force calculations.