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Support to aerodynamic analysis and design of propellers of a compound helicopter

Periodic Reporting for period 1 - PROPTER (Support to aerodynamic analysis and design of propellers of a compound helicopter)

Reporting period: 2015-12-01 to 2017-01-31

PROPTER addresses the analysis and design of propellers operating in the complex flow field around a compound helicopter where a strong interaction with airframe and lifting rotor occurs. The investigation of the interactional flow forms an essential part of the project. The specific challenge to be dealt with in PROPTER is due to the compound configuration consisting of a multitude of aerodynamic surfaces: the main rotor, the two propellers, the low-aspect ratio wing and the fuselage, to deliver the forces and moments necessary for cruise, hover, auto-rotation and manoeuvres.

The compound configuration relieves the rotor in high-speed forward flight conditions, by transferring the role of producing thrust to two propellers installed on the tip of a low-aspect ratio lift-producing wing. The two propellers, on the starboard-side and port-side of the helicopter, do not necessarily have a same geometry. During cruise the flow field around the propeller is not symmetrical due to the downwash originating from the rotor. During hover the propellers must counter the torque generated by the rotor. It is natural to expect that an optimum design of the propellers should result in two different propeller geometries, each to be installed on the starboard and port side.

The compound helicopter concept aims at a step change in the efficiency and speed capability of the next generation rotorcraft. The optimum propeller will bring direct environmental impacts in terms of reduced fuel burn and CO2/NOx emissions. The project will produce numerical figures for input into engine cycle analyses in order to precisely quantify the environmental impacts. The significance of the project for society will manifest itself in a higher mobility, as a compound helicopter equipped with properly designed propellers (for high flight speeds) will effectively introduce a new dimension in the intermodal passenger transport promoting the ACARE goal of door-to-door travel within 4 hours.

The overall objectives of PROPTER are three-fold:
1) To design and determine the flow field characteristics of a wing-mounted propeller that interacts with an overhead rotor and a nearby airframe.
2) To advance high-fidelity CFD analysis and design of an installed propeller as a key enabling technology to TRL-6.
3) To tailor the knowledge gained into the multi-disciplinary industrial environment
During the reporting period CFD steady-state flow simulations and unsteady time-accurate flow simulation for the cruise, hover and autorotation flight conditions have been carried out. The computational results have given important insight into the performance and flow characteristics in the isolated and installed configuration.

A modular approach of the setup and domain CFD modelling have been opted to give versatility in simulations for changing pitch-angle and flow condition. For consistency in comparing the isolated and installed propeller performance, the volume grids around the propeller blades have been made identical in both isolated and installed situations. Properties of the resulting computational domain can be summarized as folllows: about 9000 blocks distributed over three domains (propeller, flap and airframe) containing a total of 150 million grid cells. The temporal resolution of the time-accurate simulation takes a time step equivalent to 1 degree rotation of the propeller.

The simulations of the complete installed configuration have been performed using the RANS solver of ENFLOW. For all operating conditions the strategy to achieve converged periodic solutions has been determined. Converged periodic solutions in cruise and autorotation were achieved without much difficulty. Finding a converged periodic solution in the hover condition was problematic. A special technique to achieve this solution with the least computational cost had to be developed.

The simulation results obtained so far have formed a foundation for a complete sound understanding of the aerodynamics of the compound helicopter. More understanding will still be gained during the coming periods of the project with optimized propellers and more variations in the flight condition.
To the best knowledge of the consortium and the topic manager, the large-scale simulation that has so far been performed within PROPTER is unique in the world. The versatility of the simulation setup encompasses a wide range of tasks allowing an efficient execution of the flow simulation, e.g. the flow simulations of the complete compound helicopter configuration trimmed towards a target thrust by varying the blade pitch angle. The results so far obtained are being used to a unique formulation of multi-point propeller design, which should allow a simultaneous optimization of the blades in multiple azimuthal positions. The knowledge gained so far is promising in terms of the achievement of societal impacts expressed in the ACARE goals.
logo of propter (compound helicopter architecture still confidential)