Active Gurney Flap

Project Information

AGF

Grant agreement ID: 298182

Closed project

Start date 1 May 2012

End date 30 April 2015

Funded under

• Specific Programme "Cooperation": Joint Technology Initiatives

Total cost € 299 577,00

EU contribution € 202 422,50

Coordinated by

Active Space Technologies, Actividades Aeroespaciais S.A.
Portugal

Final Report Summary - AGF (Active Gurney Flap)

Executive Summary:
Rotor blades are primary helicopter systems, providing lift, but at the same time undesirable external noise. Huge efforts have been dedicated to improving their performance via advanced aerodynamic surfaces and aeromechanic configurations, maximizing overall efficiency (hence reduced fuel consumption), reduced external noise and reduced aircraft vibration. The next level of improvement is sought from blade adaptive or morphing techniques and/or ‘active’ technologies, the subject of this work. Enhancements such as Active Gurney flaps are therefore being investigated.
To further this research, Active Space Technologies (Portugal) developed a new concept of a model rotor ‘Active Gurney Flap’. Collaboration occurred with AgustaWestland, and other pan European partners, in the framework of the CleanSky Green Rotorcraft Consortium. The project consisted of the design and manufacturing of a Gurney flap mechanism (with actuators), to be fitted within a set of four model scale helicopter blades, and the associated electronic controller. The completed blades/control system was tested at the Politecnico Di Milano wind tunnel using AgustaWestland’s advanced rotor rig. The precision Active Gurney Flap system was compliant with a set of challenging requirements e.g. massive blade accelerations, demanding actuation frequency (160 Hz), demanding Gurney flap deployment schedule, tight spatial constraints and minimal weight.
The final system was based on piezoelectric actuators and with a miniaturized mechanical design (machined aluminium). It proved capable of resisting the challenging rotating blade centripetal forces and flapping accelerations whilst providing full Gurney flap advancement and retraction at the high operating frequencies. The mechanism, blade and system allowed important wind tunnel data to be gathered which AgustaWestland is already using for full scale blade developments.

Project Context and Objectives:
The challenging requirements related to the harsh operational environment of the model rotor system, were refined into functional and performance objectives. These guided a compact electromechanical solution compatible with a scaled model rotor blade (chord 95 mm, span 1,100 mm, thin aerofoil section) and tested at Politecnico Di Milano's wind tunnel.
The Active Gurney Flap deploys on a once per blade revolution basis and near perpendicular to the lower blade surface in response to external commands. Its maximum flap extension out of the blade was 1.4 mm at frequencies up to 160 Hz. The lightweight mechanical structure, fitting into a ‘pocket’ in the blade, was designed to withstand the flapping and centripetal accelerations without compromising the flap’s deployed height by more than 0.1 mm.
The project started with a comprehensive analysis of the current state of the art (SOTA) focusing on miniaturized mechanisms used in high frequency applications and capable of sustaining extreme accelerations during operation. The SOTA defined the enormous difficulty of miniaturized mechanisms to deal with significant accelerations found in such model rotor blades, and associated friction from the rotating joints. For example, this study demonstrated that for the necessary rotating elements, small bearings were not capable of supporting the expected forces whilst bushes would introduce high friction forces with associated accelerated wear. Electrical rotating motors were therefore not suitable.
The development was split into two phases. Firstly, prototypes were designed and tested in order to validate potential technologies and concepts and refined to an initial solution. From this, a final ‘production’ Active Gurney Flap system was developed, then fabricated before being installed in seven model scale rotor blades, the latter being designed and built by other consortium members.

> PHASE 1: Preliminary prototype
The main goal of the preliminary prototype phase was the study and testing of potential technologies and components to assess their suitability to operate on the conditions of the Active Gurney Flap system.
Based on the outputs of the SOTA analysis, several mechanical and actuating concepts were considered. A thorough assessment of the advantages and disadvantages of candidate systems was performed, resulting on the pre-selection of a mechanical design composed of an interface and a support structure, and two differing actuating technologies, namely piezoelectric and electromagnetic actuation.
The pre-selected technologies and concepts were designed and structurally analysed using numerical simulations (Catia and Patran/Nastran). This helped ensure that the preliminary models would satisfy the requirements, and helped define the needs for the actuators, both in terms of forces required to move the flap and of forces that have to be internally sustained. The resulting preliminary model was prototyped and assembled in a customized mechanical set-up that was allowed for several testing configurations.
The preliminary prototypes were initially tested on a fixed bench (not a rotating blade). The Gurney flap however could be operated to its full deployment range, obtaining 150 Hz with the piezoelectric configuration but only 120 Hz with the electromagnetic configuration. The latter was limited by its electromagnet dimensions and its sensitivity to changes in pre-tension springs. As a result, the piezoelectric solution was selected.
The next step was to test the piezoelectric actuated Gurney flap prototype system in a blade representative dynamic environment. For this a dedicated spin rig was manufactured, integrating the prototype systems in a mock-up blade that was spun up to 1,500 rpm using a 4 kW motor. The results were promising, demonstrating that the actuators could cope with the high aerodynamic forces and centrifugal loads of the rotating test set up.
The preliminary prototype testing produced other valuable results in terms of other technologies qualified for the final Active Gurney Flap model, such as:
- The flex pivots, used instead of bearings to facilitate rotational motion on a rotating shaft demonstrated very good performance and reliability;
- The mechanical transmission designed to translate the linear movement of the piezo into the rotation of the shaft proved simple and effective;
- The Hall Effect sensor demonstrated a good accuracy on the positional measurement of the deployed Gurney flap.

> PHASE 2: Final prototype
The next stage was to configure an Active Gurney Flap unit that could fit inside the tiny volume of a scale model rotor blade whilst retaining the functionalities and performances demonstrated so far. The final prototype had to fit within maximum blade internal height of 9 mm, whilst maintaining flap excursion and operating frequency. Additionally, the final deliverable had to include sensing and driving control systems to enable a full blade testing campaign at Politecnico Di Milano's wind tunnel.
Two major changes were needed to achieve this. The transmission link between the piezo actuator and the main rotating shaft needed development as did the installation of the Hall Effect sensor. These new challenges required several prototyping and verification stages. For the former simple rotating joints (axle and flange) were used. The new placement of the Hall Effect sensor was straightforward, because the sensor component and the neodymium magnet are very small and lightweight whilst adequate space existed.
The mechanical support and interface structures were also redesigned to fit inside the blade envelope, however there concept was not significantly altered. They were modelled and simulated in Catia and Nastran/Patran in order to optimize their behaviour in terms of minimizing deflections induced by the flapping and centripetal accelerations of the blade.
The distribution of actuator electrical energy into the blade and sensor signal recovery out of the blade, was achieved with a 3 layer polyimide based Flex PCB, which proved a thin and convenient solution to route the wires. The customized shielded design ensures good characteristics in terms of EMC and EMI, and the end points to connect the devices were designed to facilitate the assembly and minimize the need for unshielded wires.
The control system, required to provide electrical commands to the actuators now embedded within the blade and which were remotely located away from the blade was based in a high performance Compact RIO embedded solution from National Instruments. This consisted of a fast processor and an FPGA to perform the most challenging real time calculations. In addition to the graphical interface with the operator, the control software (implemented in LabVIEW), receives the actuating reference profiles from external analogue channels and applies the processed signals directly to the piezoelectric power amplifier.
To allow for a more accurate control of the piezo actuators, the control software implements a mathematical algorithm to correct the effect of the intrinsic hysteresis of the actuators, and performs a closed loop PID control, so as to achieve better tracking of the actuation reference signal.
Finally, seven final Active Gurney Flap prototypes structures/units were produced in aluminium (Al 7075-T6 and Al 6082-T6), tested by Active Space Technologies, and then integrated by Airborne (programme partner) and Active Space Technologies in seven carbon fibre blades. Active Space Technologies performed the final deployment calibration of all blades after which they were readied for testing on the AgustaWestland Model rotor rig at Politecnico Di Milano. During testing support, several on-the-fly corrections and adjustments were performed within the embedded software to overcome the unforeseen problem with blade flexibility negatively affecting the accuracy of the deployment measurements.

Project Results:
The final phase of the testing campaign consisted of the wind tunnel tests performed at Politecnico Di Milano, where the Active Gurney Flap prototypes assembled inside the carbon fibre blades were installed on a four bladed rotor hub. This rotor rig was able to rotate the blades up to 1,600 rpm, with independent blade pitch control whist inside the wind tunnel.
The wind tunnel tests verified the performance improvements caused by the active Gurney flap, on a rotorcraft and provided testing data to validate numerical models. Moreover, these tests allowed preliminary assessment of the mechanical and electrical reliability of the Gurney flap when subject to the harsh operational environment seen by Helicopter rotor blades. This included high temperatures, vibration, bending stress, and centripetal forces.
The wind tunnel tests were considered an overall success, although the range of achievable Gurney flap deployment was limited by practical constraints. The most relevant was the inability to operate the Active Gurney Flap in a closed loop configuration during tests, which would have provided valuable information about the precise level of flap deployment, important to calibrating numerical models and to allow for a comprehensive study on the aerodynamics improvements. Nonetheless, significant high quality data was generated.

During the design, build and testing of Active Gurney Flap system many additional foreground lessons were learnt, such as:
1. The system concept (interface structure and support structure) proved an excellent solution for integrating a moving device within a rotor blade. The interface structure provides stiffness for the instrumented segment of the blade, while the second structure provides support to the moving device (Gurney flap);
2. Bearings and bushes are not well suited for applications subjected to high environmental accelerations, because the first are not designed to sustain such accelerations, and the latter introduces high friction forces that largely reduces the efficiency of the system;
3. Rotating joints requiring small angular displacements can be efficiently handled by flex pivots. They do not introduce any friction forces and the required bending force is not significant;
4. Piezoelectric actuators provide a good ratio between force and dimensions/weight, which are parameters very important to applications in which the available space is very limited. Nevertheless, the need for high driving voltages could be a drawback in some sensitive applications;
5. Measuring displacements with Hall Effect sensors are a very common and accurate solution for many applications, but care should be taken to avoid wrong measurements due to magnet misalignments and unexpected movements over non controlled axis. In applications such the Active Gurney Flap, this measurement method can only be used if calibrated in relation to the rotational velocity of the blade, otherwise, the direct measures do not allow for proper closed loop control;
6. Polyimide flex PCBs were demonstrated to be a robust and reliable solution to route the signals and power along the instrumented blades whilst being of minimal weight. Conventional wiring looms would have been too large/heavy. As these PCBs can be manufactured with high dimensional accuracy, they are a good solution for holding sensors at correct operational positions. The ability to manufacture multilayer PCBs also proved a good choice for combating electromagnetic interference.

Potential Impact:
# POTENTIAL IMPACT
The results of the project advance the development and understanding of Gurney flaps mechanisms for the rotorcraft industry. The benefits aimed for include helicopter noise reduction, reduced fuel burn and reduced associated environmental footprint.
In fact, the goals of the Clean Sky JTI, as set by ACARE - Advisory Council for Aeronautics Research in Europe, are to demonstrate and to validate "the technology breakthroughs that are necessary to make major steps towards the environmental goals sets the European Technology Platform for Aeronautics & Air Transport and to be reached in 2020":
- 50% reduction of CO2 emissions through drastic reduction of fuel consumption;
- 80% reduction of NOx (nitrogen oxide) emissions;
- 50% reduction of external noise;
- A green product life cycle: design, manufacturing, maintenance and disposal / recycling.

Within the framework of Clean Sky, the results of the project have the potential to fast-track the above mentioned objectives since it advances on the understanding of active Gurney flaps, which enable helicopters to operate with a lower rotation operation by reducing tip speed of its main rotor whilst preserving current flight performance capabilities.

# DISSEMINATION ACTIVITIES
The dissemination activities of the project included presentation of the results in fairs, press releases which resulted in several online and paper newspaper articles, and a Masters dissertation.

# EXPLOITATION of RESULTS
The Active Gurney Flap Model rotor programme is one of four elements that make up the AGF programme in total. The others are a full scale blade section testing at the Universtity of Twente, dynamic blade testing at CIRA (Italy) and finally a full flight demonstration on an AgustaWestland aircraft. Each of these four ‘elements’ provides data as ‘stepping stones’ that builds towards the flight test activity. The purpose of the model rotor test was to provide initial data, in a controlled environment and scientific manner and of the performance changes that could be achieved by the introduction of an Active Gurney Flap to a rotor system. At the same time, it allowed for the exploration of the impact of the system into flight regimes that would be a risk for a real helicopter. Furthermore, it also allowed for an exploration of different deployment strategies of the Gurney flap as the blade rotated around the azimuth. Finally, it allowed for an exploration of the deficits a Gurney flap can also introduce if incorrectly used.

The AGF model rotor programme involved a wide ranging consortium of European partners in a complex contractual arrangement. Despite the programme and technical challenges a significant volume of important data was obtained that AgustaWestland is now using to advance and advise their Flight blade activity.

List of Websites:
Project coordinator
- Abel Mendes ([e-mail])