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

Periodic Reporting for period 3 - PROPTER (Support to aerodynamic analysis and design of propellers of a compound helicopter)

Reporting period: 2018-08-01 to 2019-03-31

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 first 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. 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.

During the second reporting period, the design optimization was performed consisting of automatic gradient-based optimizations, and afterwards manual parametric studies to further maximize the improvement. Relative to the baseline propeller provided by AH, improvement for the starboard propeller has been achieved at all design points. Investigation to determine the most proper procedure to achieve a converged periodic solution has been performed. A certain proper combination of spatial and temporal resolution sequencing was found to be necessary to obtain a good periodic convergence.The effects of variation of angle of attack and sideslip have been assessed. The isolated all-blade propeller and the complete compound helicopter configurations have been considered for the simulations. For efficiency, NLR has provided an email-based eCFD framework to allow TUD to conduct the large-scale CFD simulations remotely from their premise. Such a framework has accelerated the simulation processes and proved to be effective for an efficient collaboration between NLR and TUD in planning, trouble-shooting and executing the necessary simulations. In the scope of dissemination, a joint TUD, NLR and AH paper led by TUD has been written and presented at the AHS International 74th Annual Forum & Technology Display, Phoenix, Arizona, USA, May 14–17, 2018.

During the third (final) period, CFD setup including grid generation was prepared to assess the installed performance of the optimized propeller. Two additional configurations required for the bookkeeping analysis of the interactional flow: the compound helicopter configuration (i) without the propellers and (ii) without the fuselage. Large-scale simulations for the compound helicopter configuration were performed, followed by the post-processing analysis and flow visualization in the scope of: (i) assessment of the installed performance of the optimized propeller, (ii) bookkeeping analysis of the interactional flow, and (iii) performance data base concerning characteristics at boundaries of the operating envelope in support of control law design.

In the scope of dissemination, PROPTER has contributed to the article and interview by the journalist Mr. P. Siller (assigned by CSJU), and to the poster and podium presentation of the Clean Sky 2 Events held on 9-10 April 2019. A webinar to present eCFD used during the project was planned and invitation was sent to 19 propeller manufacturers by NLR, for which there was no response. eCFD would allow the manufacturers to perform customized CFD simulations remotely through an email interface. An investigation has been conducted to know the reasons behind this, mainly because time unavailability and limited capacity of the small companies. As a result of this, in the place of a live demonstration of the planned webinar, NLR has prepared extended presentation slides ready to be sent to the interested parties individually after the project closure.

Preparation for two journal articles to be sent to Aerospace Science & Technology after project closure has been started:
• A joint journal article led by TUD will focus on the breakdown of aerodynamic Interactions for the lateral rotors on a compound helicopter.
• A journal article led by NLR wil focus on the propeller design optimization method and results.
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. Also, although the assessment of the impact of the propeller design optimization is still on-going, the results obtained so far have delivered significant installed performance gains in terms of power. This is promising in terms of the achievement of societal impacts expressed in the ACARE goals.