Interview 2


Interviewee profile: Research Center, involved in WP1
Tags used: Hybrid Laminar Flow Control (HLFC) - Permit to fly - Computational Fluid Dynamics (CFD).


Head of Transport Aircraft Department, German Aerospace Centre (DLR)

Q1: The consortium has been working on specification of sensors of the HLFC Verification System. Could you please describe us the strategy followed for selection of the sensors? Which parameters have been taken into account?

A1: The HLFC Verification System (HVS) will be an autarkic measurement system required to record all sensor data and transmit selected sensor data to an in-flight data monitoring system installed in the cabin during FT which allows the assessment of the functionality of the HLFC-system. We will measure chamber pressures and temperatures, pressure and temperature in the pressure duct as well as the humidity in each chamber and in the pressure duct. The identification of appropriate sensors depends on the requirements with respect to environmental conditions, expected data accuracy, geometrical size of the sensors as they have to fit into the HLFC-structure, as well FT qualification issues, etc. The selection process naturally starts by completion of the sensor requirements and specifications. From here we screen available sensors and defined related system architectures. As not much space is available inside the HLFC-suction system, sensor size and robustness by maintaining high accuracy and responds times equivalent to specifications had been the main challenges. Especially the humidity sensors gave us some headache but promising sensor candidates have been identified. However, we will have a HVS which will record and transmit data during flight test as specified but can be extended in future for HLFC-system control, but this is not subject of AFLoNext. The HVS allows us to compare the measured status of each suction chamber and the pressure duct with the predicted status at the current operating point of the system. Icing, chamber blockage, leakage etc. can be identified during FT which allows almost instantaneous corrections, for example of the flight path in case of indicated risk of chamber icing. Besides these scientific benefits the HVS will save time and budget in FT.

Q2: Several partners have been involved in the FT coordination in order to prepare the ground for obtaining the “Permit to fly”. How is the consortium setting up the appropriate roads towards the Permit? Could you present the stakes of this planning and the necessary coordination amongst the workpackages?

A2: The FT will be performed on the DLR A320 ATRA with DLR as operator of the aircraft. As the Flight Tests covers experimental modifications across three work packages involving a lot of different partners contributing to flight test instrumentation and other hardware as the HLFC-system, Nose landing gear door, etc. the procedure for FT qualification had to be identified and consolidated among the partners and the responsible design organization of the aircraft operator. The size and constitution of the FT-team is challenging especially considering that not all partners do have an airworthiness design organization which requires a precise definition of the root to “Permit to fly”, but we are on good track.

Q3: A CFD grid generation has been successfully performed. What was the objective of performing this grid generation? What results did it produce? How will the results be useful for the project activities?

A3: Now, the aerodynamic design of the HFLC system will lead to a target suction velocity distribution sufficient to achieve the desired downstream shift of the transition at a minimum required suction power. To achieve this goal the pressure distribution and boundary layer development on the vertical tail plane (VTP) and its dependence on HTP-setting angles within the HLFC operation envelope has to be known precisely. The flow field at the VTP is influenced by several parameters such as the aircraft configuration, the HTP trim angle, angle of attack, side slip angle, engine settings, etc. Hence, we perform numerical simulations of the complete flight test aircraft within the complete HLFC-envelope at various flow conditions and parameter settings of the aircraft. The results serve further as input for the structural design activities of the HLFC-system. Structural displacements are passed back into the aerodynamic analysis resulting in new aerodynamic loading of the VTP which will be brought to convergence. The CFD-data are further used to design the anti-contamination-device installed at the first leading edge segment of the VTP. Boundary layer stability analysis including optimized suction velocity distributions will be analysed for transition predictions a priori to the flight test. So we will have pre-calculated test data for flight test to directly compare test data with predictions. This will not only answer the question of the level of functionality of the HLFC-system but will give us clear validation of our design methodology, applied processed and tools. As it is clear that the real flight test points might differ from the pre-calculated test data points (w.r.t. side slip angle, rudder deflection angle, Reynolds number etc.) the grid generation is done in a way, that grid deformation techniques can be applied to set HTP trim angle and the VTP rudder deflection angle. With this methods all generated grids are of the same “family” which allows the setup of ROM. The ROMs enable us to identify the CFD-solution even an intermediate CFD-point between the computed ones to match flight test data as close as possible. This technique allows us to rapidly post-process flight test data close to real time with respect to predicted and detected transition locations. This is an advantage for flight test as corrections to the flight test points can be applied during the flight test almost instantly which saves costs and time.