Aerodynamic Design Optimisation of a Helicopter Fuselage including a Rotating Rotor Head

Executive Summary:

The reduction of emissions is clearly one of - if not - the most challenging task of our society and the aeronautical industry today. Within CleanSky environmental issues in the rotorcraft domain are addressed by the Green Rotorcraft Integrated Technology Demonstrator (GRC ITD). Even though, fixed wing aircraft generally outperform rotorcraft in fuel efficiency, range, speed and noise, rotorcraft are still of high importance.

There are several reasons for that. First, rotorcraft provide unique Vertical Take-Off and Landing (VTOL) capabilities. Thus, they can operate in areas with limited infrastructure or in airspace where other aircraft cannot. Second, they can excel in scenarios where economy of time is crucial, e.g. search and rescue (SAR) missions. Finally, rotorcraft are deployed when they outperform other applicable forms of transport in dynamic productivity, i.e. payload multiplied by range and speed divided by costs. Examples are the crew change on oil platforms or the transport of service personnel to offshore wind farms. To be able to provide these services at reduced environmental impact, measures are taken by the GRC to reduce emissions and increase fuel efficiency of rotorcraft.

In many cases, the missions described above are performed by light weight utility helicopters. Therefore, it is desirable raising the efficiency of this helicopter class. The power requirements of light weight utility helicopters in fast level flight results to 55 % from parasite drag, to 40 % from the main rotor and to 5 % from the tail rotor. Thus, aiming on parasite drag constitutes a promising approach for obtaining a more efficient utility helicopter design.

Parasite drag is generally reduced by optimizing an aircraft’s shape. Unfortunately, practical considerations dominate the design process of utility helicopters. In consequence it is not always possible choosing the optimal shape. Therefore, solutions for the aerodynamic design have to be developed accounting for these operational constraints.

The main objective of the GRC project ADHeRo (Aerodynamic Design Optimization of a Helicopter Fuselage including a Rotating Rotor Head) is reducing the parasite drag of light weight utility helicopters in fast level flight, without increasing the fuselage down force. The latter fact is important to avoid additional power requirements for the main and tail rotor, which would deteriorate the gain in efficiency obtained through parasite drag reduction.

Analyzing the drag decomposition of this helicopter class reveals that the parasite drag of the fuselage, the landing gear and the rotor head causes some 70 % of the total parasite drag. Thus, significant reduction of emissions and fuel consumption can be achieved by optimizing the aerodynamic design of these components. For this reason, the focus within ADHeRo is set on the aerodynamic optimization of the fuselage, the landing gear and the rotor head. Whenever feasible, aerodynamic design optimizations are achieved by shape optimization. As already mentioned, this is not always possible. In these cases, efficiency gains are achieved through the application of passive flow control devices, i.e. strakes and vortex generators.

The aerodynamic analyses are primarily performed through wind tunnel (W/T) experiments. Besides the identification of feasible drag reduction methods, the experiments allow determining the gain in efficiency through the selected design modifications. In summary, this led to a sounded database not available before. The database provides a large set of global and local data including aerodynamic forces, surface pressure distributions and velocity field data. For the data collection a new W/T model was designed and manufactured. Since detailed drag analysis is the main scope of ADHeRo, the model design reflects to that. One important constraint for precise drag evaluation through W/T experiments is the elimination of aerodynamic interaction of the model support with the flow field of the model. Hence, a specific tail sting mount is used. Furthermore, the model rotor head reproduces the full kinematic complexity of the original design. This includes the rotation of the rotor head and the collective and cyclic pitch motion of the rotor blades. In addition to the experimental investigations, numerical simulations were performed for selected cases to support the analysis of experimental results.

During the course of the project several design modifications to a baseline model were considered. Through the first phase of the project the status quo was determined for the baseline configuration representing current production type light weight utility helicopter. Thus providing the reference data for all subsequent design modifications. In the second and third phase of the ADHeRo project the effect on drag of modifications to the original design were studied. The modification include new faired landing gears (18 % drag reduction), a smoothed cabin bottom (2.7 % drag reduction), passive flow control devices at the aft-body (1.4 % drag reduction) and a new mast fairing (1.5 % drag reduction). Thus the investigated configurations exceeded the expected drag benefits with almost 24 % on aggregate. In consequence, the power requirements for light weight utility helicopters could be reduced by 10 % or more through ADHeRo. This could result in a reduction in fuel consumption of similar magnitude. Through the wind tunnel tests performed on the selected modifications, already technology readiness level four is achieved (TRL4, i.e. component and/or breadboard validation in laboratory environment). Considering the planned flight tests on the technology demonstrator even technology readiness level 6 will be reached (TRL 6, i.e. system/subsystem model or prototype demonstration in a relevant environment). Hence, the prospective benefits could enter market within a short period after project completion. Thus, ADHeRo helps reducing the environmental impact of services provided by light weight utility helicopters and their operational costs in the near future. Overall, this could further improve the societal implication of helicopter services as a beneficiary means both for society and industry.