Delft University of Technology Aerodynamic installation effects of lateral rotors on a novel compound helicopter configuration Stokkermans, Tom; Soemarwoto, Bambang; Voskuijl, Mark; Fukari, Raphaël; Veldhuis, Leo; Eglin, Paul Publication date 2018 Document Version Accepted author manuscript Published in Annual Forum Proceedings - AHS International Citation (APA) Stokkermans, T., Soemarwoto, B., Voskuijl, M., Fukari, R., Veldhuis, L., & Eglin, P. (2018). Aerodynamic installation effects of lateral rotors on a novel compound helicopter configuration. Annual Forum Proceedings - AHS International, 2018-May. Important note To cite this publication, please use the final published version (if applicable). Please check the document version above. 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Aerodynamic Installation Effects of Lateral Rotors on a Novel Compound Helicopter Configuration Tom Stokkermans Mark Voskuijl Leo Veldhuis PhD Candidate Assistant Professor Full Professor Delft University of Technology Delft University of Technology Delft University of Technology Delft, the Netherlands Delft, the Netherlands Delft, the Netherlands Bambang Soemarwoto Raphael¨ Fukari Paul Eglin Senior Scientist Engineer Aeromechanics senior expert Netherlands Aerospace Centre NLR Airbus Helicopters Airbus Helicopters Amsterdam, the Netherlands Marignane, France Marignane, France ABSTRACT Installation effects of the lateral rotors for a compound helicopter were investigated by means of unsteady CFD sim- ulations. The helicopter featured a box-wing design for additional lift in cruise and wingtip-mounted lateral rotors in pusher configuration for additional thrust in cruise and counter-torque in hover. It was found that propeller per- formance installation effects for a compound helicopter are highly beneficial in cruise, while some penalties exist in extreme and specific cases in hover and autorotation. In cruise the main interaction was between the wing and lateral rotors, resulting in a propulsive efficiency increase up to 10:6% due to wingtip vortex energy recovery. In hover the main rotor slipstream resulted in a near perpendicular inflow to the lateral rotors, with a disturbance from the wings due to the deflection of the main rotor slipstream. For higher than nominal lateral rotor thrust settings, this resulted in a performance penalty. In autorotation a small power installation penalty was present due to inflow disturbances. NOTATION Dpt Total pressure difference w.r.t. freestream d f Flap deflection CD Drag coefficient hp Propulsive efficiency hp = TV¥=P CL Lift coefficient j Blade phase angle P 3 5 CP Power coefficient CP = =r¥n Dp Cp Pressure coefficient Subscripts T 2 4 CT Thrust coefficient CT = =r¥n Dp a Axial c Chord inst Installed Dp Propeller diameter iso Isolated E Richardson extrapolation uncertainty mr Main rotor hhover Hover altitude p Propeller L CFD domain length t Tangential n Rotational speed ¥ Freestream P Shaft power q Dynamic pressure INTRODUCTION R Radius r Radial coordinate A compound helicopter is a helicopter where the function of T Thrust the main rotor of providing lift and thrust is supported by V Velocity different means. Following Yeo and Johnson (Ref.1) com- p2T 2 Vd Downwash farfield velocity Vd = mr=r¥pRmr pounding can be split in lift compounding by the addition of y+ Dimensionless wall distance wings to the fuselage and thrust compounding by the addition of propulsive devices. Combined lift and thrust compound- Greek ing is often called full compounding. The goal is to expand a Angle of attack the flight envelope, especially improve the high speed capabil- ity of the helicopter while maintaining a helicopters efficient Presented at the AHS International 74th Annual Forum & hover advantage over fixed wing aircraft. Technology Display, Phoenix, Arizona, USA, May 14–17, 2018. Copyright c 2018 by AHS International, Inc. All rights reserved. As part of the Clean Sky 2 (CS2) research programme, the 1 right upper wing right nacelle right propeller in Figure2 (b). Bending moments of the propeller blades due to this skewed inflow may be significant (Ref.3). Further- fuselage more, a propeller close to the main rotor may affect the rotor right spinner flapping amplitude and bending moments due to its pressure V field, as is known from the extensive experimental investiga- ∞ tion of Bain and Landgrebe (Ref.7). Not only the propellers right lower wing but also the wings experience a large variation of angle of at- left upper wing tack in the flight envelope as shown by Lynn (Ref.8). The wings may cause an additional disturbance to the inflow of the propellers. Because of these unknowns, the impact of this left lower wing complex interactional flow on the propeller performance and left nacelle unsteady aerodynamic loading is investigated. This will pro- left propeller left spinner vide the necessary insight for mitigation of possible problems by propeller blade shape optimisation. Fig. 1: Sketch of the Airbus RACER without tailplanes. Airbus RACER (Rapid And Cost-Effective Rotorcraft) is be- ing developed. This full compound helicopter design is opti- mized for a cruise speed of 220 kts (Ref.2). Figure1 shows a sketch of the helicopter with its tailplanes removed. Addi- tional lift in the cruise condition is provided by a box-wing design, while wingtip-mounted lateral rotors in pusher con- figuration provide additional thrust in the cruise condition (a) Interaction in the cruise condition. and counter-torque in the hover condition. The box-wing de- sign reduces the overall surface affected by the downwash Tmr from the main rotor in hover while providing the required lift in cruise. As part of the CS2 PROPTER project (Sup- port to aerodynamic analysis and design of propellers of a T compound helicopter), this paper investigates the effects of p installation on the lateral rotor performance, focusing on the aerodynamic interaction with the box-wings and main rotor slipstream. For this research an extensive amount of unsteady Reynolds-averaged Navier-Stokes (RANS) CFD simulations have been performed. Comparison with simulation results of (b) Interaction in the hover condition. the isolated propellers provides the necessary insight into the propeller installation effects. For improved reading of the pa- Fig. 2: Main rotor and propeller aerodynamic interaction. per the lateral rotors will be referred to as propellers. Furthermore, at low speed conditions the right propeller needs In the cruise condition the aerodynamic interaction between to provide reverse thrust for counter-torque. Reverse propeller the main rotor and propellers is limited since their slipstreams thrust flow phenomena are not well described in literature, are separated, as sketched in Figure2 (a). Orchard and New- apart from the experimental investigation by Roosenboom and man (Ref.3) underline that propellers are particularly advan- Schroder¨ (Ref.9). This paper provides some insight in reverse tageous as propulsion device for compound helicopters con- thrust performance effects for compound helicopters. sidering their high efficiency at the moderate cruising speed. An additional efficiency advantage for these propellers can be expected because of the installation on the wingtip in pusher COMPUTATIONAL SETUP configuration, potentially resulting in wingtip vortex energy A perspective view of the simulated body is sketched in Figure recovery by the propellers and wing induced drag reduction 1. The helicopter fuselage with box-wings was simulated with (Refs.4–6). The impact of the novel box-wings on these ef- two fully resolved six-bladed tip-mounted propellers turning ficiency benefits is investigated in this paper, including the inboard-up, all represented by no-slip walls. All four wing effect of trailing edge flap deflection on all four wing halves. halves were equipped with plain flaps over the majority of the These flaps allow a change in the ratio of wing lift to main span. In the remainder of the paper this body without the pro- rotor lift in forward flight. peller blades is referred to as the airframe. The model was Although propellers may be beneficial in the cruise condition simplified by removal of the tailplanes and the time-averaged where the main rotor slipstream generally passes over them, effect of the main rotor was introduced by an actuator disk the lack of shielding of the propellers may pose a problem at implementation that introduces radially and circumferentially low airspeed and particularly in the hover condition where the varying momentum and energy jump conditions based on pro- main rotor slipstream impinges on the propellers as sketched vided blade loading distributions. The domain for the cruise 2 and autorotation condition and the enlarged domain for the PERIODICITY AND GRID CONVERGENCE hover condition are sketched in Figure3. For the cruise and autorotation condition the dimension in the freestream direc- Solutions of the aerodynamic loading were expected to be in tion L was chosen larger than the other dimensions