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NEXT generation civil Tilt Rotor Interactional aerodynamic tail oPtimisation

Periodic Reporting for period 2 - NEXTTRIP (NEXT generation civil Tilt Rotor Interactional aerodynamic tail oPtimisation)

Reporting period: 2019-06-01 to 2020-08-31

The aerodynamic configuration definition of the NGCTR needs to be confirmed by a large scale powered wind tunnel experiment, with the aim to verify and confirm the key choices of the configuration and to provide guidelines and proposals for potential additional improvement to be implemented.

The partners will work together to modify the NICETRIP model, a 1/5th scale full-span powered wind tunnel model resulting from the EU 6th framework program, perform the wind tunnel test in DNW-LLF and define an optimal empennage configuration based on the wind tunnel test results and given boundaries. NLR will coordinate the NEXTTRIP project and will perform wind tunnel model design and manufacture, test preparations, advanced flow visualization and data analysis. DNW will support test preparation and perform the test. DLR will support test preparation and perform data-acquisition and piloting. Multi-objective aerodynamic empennage optimization will be performed by Hit09 after wind tunnel data analysis. Fokker will, together with NLR, perform a verification of compliance with feasibility and industrial constraints during the empennage optimization process.

By providing a clear understanding of the efficiency (in terms of aircraft static stability) of different empennage configurations exposed to the rotor flow NEXTTRIP provides a vital contribution for the validation of an innovative tilt rotor concept. An optimized light weight and low drag empennage and consequently reduced emission and fuel burn will directly contribute to an efficient configuration capable to meet the ACARE Horizon 2020 goals.
Compared to the classical helicopter configurations, NGCTR will provide more speed, longer range, more productivity to fill the gap between conventional helicopters and other fixed-wing platforms. It will contribute to open a new segment of air transport and mobility, in line with increasing demand for flexible and smarter mobility solutions. Indeed, door to door travel will be significantly improved.

To realize above, the main objectives of NEXTTRIP were defined and can be broken down into three sub-objectives:
O1 Modify the existing NICETRIP model to accommodate two new empennage geometries.
O2 Complete a powered (and non-powered) wind-tunnel campaign for a relevant test matrix which captures the flow phenomena in relevant flight conditions (forces, moments, pressure).
O3 Based on the processed wind tunnel data, define and execute a strategy to propose enhancements for the geometries, supported by CFD analysis.
Explanation of the work carried per WP

Work Package 1: Design and manufacturing of the modified NICETRIP powered components.
A key aspect of the project was to identify the optimal empennage configuration which needs to be effective in stabilizing and controlling the NGCTR both in forward flight and in transition between hover and airplane mode. Two tails were designed and manufactured, to fit into the existing NICETRIP model. In order to preserve geometric similarity between the model and the full-scale prototype, with particular focus on the rotor stream tube/tail interaction, a new set of composite rotor blades was manufactured.

Work package 2: Wind tunnel test preparations of the modified NICETRIP powered model.
All model components were manufactured according plan. All functional checks of instrumentation, model controls, load calibration checks and the GVT were conducted and the model was ready to enter the tunnel end of May 2019.

Work package 3: Wind tunnel test with the modified full span NICETRIP powered model.
The subsonic wind tunnel test was performed in DNW-LLF, the Netherlands. The test matrix primarily covered the low speed range of the flight envelope, from helicopter mode through conversion to aircraft mode. Parametric variation of model component settings were made, such as: “fuselage” incidence and side-slip, outer wing and nacelle tilt angles, control surface settings and rotor operating points. The test program concentrated on aerodynamic interactions and tail effectiveness of both T-tail and V-tail.
The actual test had to be split in two separate entries due to unforeseen circumstances: the 1st from 3-19/6/2019, the 2nd from 28/11-10/12/2019, adding up to over 15 days of tunnel occupation.

Work package 4: Wind tunnel data analysis.
Due to the unforeseen circumstances the data delivery was done in two steps. The project partners used the period between the two test entries to already concentrate on checking and correcting the available data from the first entry. In fact the majority of procedures for final data production were investigated and finalized even before the second entry.
Due to the necessity as input for other projects at Leonardo Helicopters, processing of tail data was already in full swing after the first entry. In order to prevent duplication of processing it was decided
* to focus on further investigations of the tail balance. Tail balance results during NEXTTRIP were suspicious and reliable measurements within HIGHTRIP are of major importance,
* to report the efforts put in FLIGHTLAB modelling of rotor wake and impingement on the tails that were initiated already at the start of the project.
* to perform a limited study of stability behavior.

Work package 5: Empennage optimization to enhance the stability behavior of NGCTR.
In a 4-step approach a multi-criteria multi-constrained Aerodynamic Optimization of 3D finlets was carried out, which resulted in an optimal shape increasing longitudinal and latero-directional stability of the V-tail, while at the same time minimizing drag and lowering rolling moment at the tail root.
In parallel, a parametric study on V-tail/fuselage connection was carried out, analyzing both a fillet and a fairing configuration.
Finally, a DoE-based parametric study was carried out on the V-tail planform, providing dependencies of the aerodynamic coefficients from root offset, dihedral angle and aspect ratio.

Work package 6: Management, dissemination and exploitation.
As part of the project management, a Project Management Plan was issues as were a Dissemination Action Plan and an Exploitation Plan. In due course of the project dissemination and exploitation actions were initiated.
High expectations were on strengthening European competitiveness by enhancing wind tunnel capabilities as on-line wall correction methodology and maturing the Particle Image Velocimetry (PIV) using Helium Filled Soap Bubble visualization technique.

Since the technique was demonstrated before that test and the development of the PIV seeding rake went much faster than originally planned, it was decided to scale up to a bigger seeding rake with dimensions, fit for industrial testing in a large facility. A 3mx3m rake was operational at the end of the second entry of the test campaign and demonstrated the matureness of the technique: providing an excellent flow visualization technique in large industrial wind tunnels up to 60 m/s.
By contributing to the optimization of the tail, which will be implemented on the Technology Demonstrator, the aerodynamics will be improved and effect the effectiveness of the vehicle.

With high maturity of technology integration aimed at completion of this rotorcraft program including flight demonstrations, the possibility to match ACARE goals (sustainable mobility) with actual products to be subsequently developed will be substantiated.
Flow visualization with Helium Filled Soap Bubble particles
NEXTTRIP model with V-tail in helicopter mode in the DNW-LLF
Pre-test of the Helium Filled Soap Bubble (HFSB) seeding rake
Preparation phase - Ground Vibration Test