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Flight-tests with multi-functional coatings

Final Report Summary - LH-LHT-RFT (Flight-tests with multi-functional coatings)

Executive Summary:
Fuel Efficiency is a core topic in several research and development projects in the aviation industry. Environmental aspects as well as economic flight operations are the key driver for these projects.
One solution to reduce the drag of an aircraft and thereby increase the fuel efficiency is to focus on an optimisation of the aircraft’s surface by applying multi-functional coatings such as “riblets” (also called “shark-skin coatings”) as shown in Figure 1. These coatings essentially shall support natural laminarity of the air flow, besides providing the “normal” functions of an aircraft coating such as protection of the sheet metal or composite.

Former research projects have shown that by using riblet surfaces fuel consumption of an aircraft can be reduced by 1 % or more, which offers attractive saving potentials for aircraft operators.
Recently developed paint systems allow for embossing riblet structures directly into the paint itself. No additional riblet layers on foils are needed, therefore leading to a significant reduction of weight for the coating.

For a wide acceptance in commercial aviation, fine structured multi-functional coatings need to have a high durability comparable to those of a conventional coating system. If a fast degradation of the functionality occurs, frequent repairs or repaints are necessary, which significantly reduces or eliminates the cost saving potential of the shark-skin coating.
The project “LH-LHT-RFT— Flight-tests with multi-functional coatings” has performed durability tests of riblet coatings under realistic environmental conditions. Intense inspections of riblet specimens applied on two commercially operated aircraft have provided basic data to judge the durability behaviour of multi-functional coatings. The positive results have led to the launch of further projects aiming at an industrial scale application of riblets on commercial aircraft.
Project Context and Objectives:
The specific target of this project was to test the functionality and durability of newly developed coating systems during commercial flight operation. Previous flight tests were performed on test aircraft or aircraft with limited operational variations, e.g. on Airbus’ Beluga Aircraft, operating mainly within Europe. These tests revealed good basic results, for a wide role-out of riblets into commercial aviation, however, the testing environment had to be adapted to the actual conditions in this field.

For the specified flight tests, two Airbus A340-300 aircraft of Lufthansa’s fleet were selected. These aircrafts type operates to near eastern as well as to mid and far eastern countries, were they are exposed to UV radiation, high temperatures on ground (+50°C to +65°C or more) and very low temperatures at high altitudes (-50°C to -60°C) as well as to atmospheric pollution (dust and sand). Aircraft speeds (air speed) at lower altitudes (below 10,000 feet) during take-off and landing are typically between 250 and 500 km/h and at cruise altitudes (above 30,000 feet) between 800 to 880 km/h.
The long term functionality, respectively efficiency, of multifunctional surface structures depends on its capability of resistance against these environmental attacks. For the future research and development of multi-functional structures and adequate surface coating materials (paint-systems), it’s a prerequisite to study and understand the wear and tear of these structures under real operating conditions. Lab test have limited correlation to natural tests, especially to flight test. As there are no detailed load structures available which are close to reality with respect to this complex superposition of environmental effects, simulations cannot replace real flight tests.

In the project, multi-functional structure test areas were applied to those zones of the aircraft structure which are exposed to maximum environmental load. For this purpose, 10 patches (size 10 cm x 10 cm) were applied on 2 Lufthansa A340-300 aircraft.

After having prepared the test aircraft accordingly - including the technical release of the aircraft for commercial flight operations - the core objective of the project was the collection of data to facilitate a detailed analysis of riblet degradation behaviour in the laboratories of the project partners. The flight tests lasted 24 month. During the test duration, frequent inspections were performed. The inspection was done by a replica impression, by photos or by roughness measurements and was recorded during the test. Furthermore, a documentation of the number of flight hours and flight cycles was ensured.

The partners in the project have taken complementary roles: Airbus and IFAM developed the coating itself as well as the related application and the training for the de-moulding processes. Lufthansa Technik together with Lufthansa Airline were providing the test-aircraft, the facilities and manpower to ensure proper application and de-moulding on commercial aircraft operation.

Project Results:
Work Package 1 - Certification of Application / Job Cards

WP1.1 Selection of aircraft

In order to obtain a certain variety of testing environment related to commercial flight operations, two Lufthansa long range aircraft were chosen covering a wide range of different flight patterns.

The aircraft selected for riblet test flights was an Airbus A340-300.
The registrations (tail signs) were chosen related to the planned fligt pattern, i.e. to ensure a certain share of extreme weather conditions, such as monsoon, sand or heat:
Airbus A340-300 D-AIGI
Airbus A340-300 D-AIGH

A special crew information paper regarding riblet test flights was prepared by the project team and deposited on board the aircraft in order to provide some detailed information to the crew. On some occasions, the paper was used to inform interested passengers on the Fuel Efficiency efforts of Lufthansa.

WP1.2 Location of Patches on the Aircraft

To evaluate the durability of riblet coating, different areas of the aircraft were selected for the application to ensure a range of exposure scenarios. Figure 3 shows these areas on an A340-300 aircraft.

WP1.3 Material

The micro-structured coating and the anti-erosion coating were provided by Fraunhofer IFAM and two Airbus suppliers (Akzo Nobel and Mankiewicz):
• IFAM – Fraunhofer: LJ2dirtfree and LJ1Beluga
• Akzo Nobel: UV Clear
• Mankiewicz: CYCON-UVClear

The coatings for the micro-structured patches (riblets) are UV hardening coatings, which allow for an embossing of the intended riblets. Figure 4 demonstrates the priciples of this process.

WP1.4 Documentation for Application and Inspection on Lufthansa aircraft

To allow the application of the intended test patches on flying aircraft, the airworthiness of such application has to be proven and documented. For this purpose, the following two Documents (“Engineering Order”) were prepared by Lufthansa Technik Engineering Department in Frankfurt:

WP1.4.1 EO Document 331819 – Modification

This document contains the following process description regarding application and removal of the patches:

a) Application of micro-structured coating

During suitable downtime of the aircraft in a hangar, the area which had to be coated was masked to protect the surrounding surface. The top coat had to be grinded and cleaned to activate the surface prior to riblet application.
The two components of the coating material, base and hardener, were mixed to a homogeneous liquid and then coated on an embossing foil with the negative texture by help of a coating knife. This coated mould was then pressed onto the previously masked spot and the coating applied by help of a roller.

The coating was then cured in two steps: first the mould texture was frozen by UV
radiation and the complete curing was driven by room temperature. An UV lamp cured the coating in a short period (approx. 50 seconds), and the mould was removed afterwards as well as the masking tape. The job description clearly ordered the mandatory use of a sufficient protection against UV radiation.

The texture of the patch was stable after the UV curing, and the second part of the curing was done by room temperature.

A rectangular cut in the left edge of the patch was then made to fix a reference point for orientation during microscopic texture check in a later stage.

In brief, the application process contained following work steps:
• Masking of coating area
• Cleaning with isopropyl alcohol
• Activation of surface by sanding
• Cleaning with isopropyl alcohol
• Mixing of pre-conditioned (about 20°C) base and hardener
• Application of liquid coating material on silicone mould
• Adjustment of mould on spot
• UV curing
• Removal of mould
• Removal of masking tape
• Implementation of rectangular cut in upper left edge after 12 hours curing time at room temperature

b) Application of anti-erosion coating

As a reference for the erosion behaviour, an anti-erosion patch was applied at the wing tip as well as tip of the horizontal stabilizer. The anti-erosion coating was a sprayable material and was applied in a smooth condition with standard curing process at room temperature.

c) Removal of the coating

During suitable downtime of the aircraft in a hangar, the micro-structured coating samples and the anti-erosion coating samples installed according to the Lufthansa Technik EO were removed per standard procedure; all remaining residuum was clean and removed. The debris of the patches were disposed according environmental and health & safety requirements.

WP1.4.2 EO Document 331818 – Multiple Inspection:

In this document, the process for inspection of the riblet patches is outlined, taking into account the different material types – micro-structured and anti-erosion coating.

The first inspection was planned one day after the initial application of the micro-structured coating (as per EO 331819). Thereafter, the inspections were repeated approx. every 2 months. In order to safe ground time of the respective aircraft, standard layover (A- or C-Checks) events were used for inspection, even if they were a little bit before or after 2 month.

a) Inspection of riblet coating

The check of riblet coating patches consisted of a texture de-moulding and a visual

Texture de-moulding:
A two component poly-addition silicon paste, usually used by dentists, with a high
De-moulding precision is used for this step. The ambient temperature was at min. 14°C to ensure a proper curing of the casting compound. According to the procedure agreed with Airbus, no cleaning of the patches was performed prior to de-moulding.
After curing, the impress mass was collected in specific boxes and sent to Airbus for further evaluation.
In summary, the de-moulding process is divided in following steps:
• Masking
• Cleaning of patch with isopropyl alcohol
• Distribution of silicone compound in patch by cardboard
• Patch removal after 12 minutes curing time
• Removal of masking tape

Visual Inspection:
In addition to de-moulding, the patches were visually checked for any defects like chipping, soiling, delamination or scratches; such defects were recorded by means of photos and a remark in the inspection report.

b) Inspection of anti-erosion coating

The visual appearance as well as gloss were determined in frame of the check. The inspection results were recorded in the applicable reporting sheets.

Visual appearance:
The patches were also visually checked for any defects like chipping, soiling, delamination or scratches; the defects were recorded by means of photos and a remark in the inspection report.

c) Further Measurements

Prior gloss measurement the patches were cleaned with isopropyl alcohol (IPA) or an approved external cleaning agent like e.g. Webetec 54-67. The gloss retention was determined at 60° angle in 3 spots on each patch according to ISO2813. Each spot
consisted of several measurement points in spanwise direction, started in mid of the patch and then continued with a distance of 2 cm.

d) Reporting

The Reporting Sheet with data from visual inspection was submitted to Airbus as well as the boxes with the riblet impress.

WP1.4.3 Job Cards

Based on the EO Documents as developed in WP1, Job Cards were produced for the application / removal process and the inspection process, respectively. Those Job Cards were the binding guideline for Lufthansa Technik employees performing the process.

Work Package 2 - Training of Technical Staff / Application of Test Patches on Aircraft

In an earlier stage of the project, special test panels were prepared at Lufthansa Technik to test the application process of riblets (Figure 5). The basic process for test patch application has been performed before by Airbus on Beluga aircraft. Based on the respective experience, the process as outlined in the Lufthansa Technik EO was designed.

Before starting the application of the patches on the aircraft, staff training and testing of the multifunctional coating application and de-moulding was performed. Training included the following steps:
• Introduction to project “flight tests”
• Process description in line with EO
• Safety requirements
• Description of material used / properties
• Trial application process

The paint material was shipped by the supplier in accordance with the selection made by Airbus. Consumables were provided by LHT. As IFAM / Airbus is the knowledge holder regarding the application process for the coating and the corresponding de-moulding, Airbus experts did the training of Lufthansa Technik Painter Team in Frankfurt in the 2-3 weeks before the application of the first patch on the dedicated aircraft.

The final coating application to the dedicated aircraft areas and the reference de-moulding was performed by LHT staff assisted by Fraunhofer IFAM and Airbus Material and Process Surface Technology. Figure 6 demonstrates some basic steps of the riblet patch preparation.

Table 2 gives an overview on the different patch locations as well as the material used on both Airbus A340-300 aircraft.
Upon request from Airbus, the respective manufacturer and product names for the selected patches will not be published and are therefore replaced by letters / numbers.

Work Package 3 - De-Moulding

During the 24 month flight test phase, inspections including de-mouldings and roughness measurements of hard coat patches were arranged in line with the process outlined in the EO Documents. Figure 7 demonstrates the basic steps of the de-moulding procedure.

The de-moulding was performed at Lufthansa Technik Base Frankfurt by Lufthansa Technik staff, formerly trained by Airbus. All inspections during the monitoring period as well as the de-application at the end were performed by Lufthansa Technik staff. Beside the de-moulding putty for the multifunctional coatings, all consumable material was provided by Lufthansa Technik. The de-moulding specimen were sent to Airbus for further investigation, accompanied by the replenished Reporting Sheets. An example of a reporting sheet is shown in Figure 8.

Table 3 summarizes the dates of the de-mouldings as well as the Flight Hours and Cycles recorded for both test aircraft. On aircraft D-AIGH, two more de-mouldings than initially scheduled were performed. On aircraft D-AIGI, one more de-moulding was performed.

Mid 2013, the de-moulding silicon paste was slightly modified. In order to ensure a suitable comparison of the quality of the samples, two de-mouldings with both pastes were performed in March 2013 on both Aircraft.

According to Airbus, the accuracy of the de-mouldings was excellent and allowed for detailed investigations regarding the degradation behaviour of riblet coatings.
The further analysis of the patches at Airbus pursued the following path:

• Optical Measurements
• White Light Inferometry
• Production of enlarged model and drag analysis from Oil Channel Tests

The results of the respective tests are intellectual property of Airbus and can currently not be disclosed.

Tables 4 and 5 display the results of the visual inspection performed on the patches prior to de-moulding. In addition, gloss measurements on the erosion patches are recorded.

The summary and comparison of the visual inspections is shown in Table 6.

On both aircraft, only one patch on the wing upper area showed significant erosion: Patch #2 “Mb1”. A second wing patch, Patch #2 “Ma1” developed traces of erosion on aircraft D-AIGH. In general, the patches on D-AIGH seemed to be affected a bit more by erosion. A stronger wear of the paint was also detected on the erosion patches on wing and horizontal stabilizer.

In order to enable a relation between the degradation of the riblet structure and the environmental conditions, flight data was recorded during the tests. Table 7 displays the respective routes per aircraft.

It is shown, that the routes of both test aircraft were similar, with a strong focus on Asian and North American routes.
The flight crews were asked to record abnormal weather or environmental conditions. However, none of these circumstances were observed during the tests. Based on these data, the slightly stronger degradation of the patches on aircraft D-AIGH cannot be related to different flight routes. Different weather conditions or treatment of the aircraft by e.g. washing or de-icing could be an explanation. Respective data, however, was not recorded to allow a deduction of the root cause.

As a conclusion, two of the four test materials show already traces of changed visual appearance that might also lead to a degraded functional behaviour of the riblet structure. Further investigations of this behaviour after surface alteration were perfomed by Airbus and are currently not available for publication.

Work Package 4 – Removal of Test Patches

After 24 month and the last inspection, the test patches on aircraft D-AIGH were removed by Lufthansa Technik in line with the procedures in the EO Document and the respective job cards. After removal, the patch was discarded.
On Aircraft D-AIGI, the patch is still on the aircraft to allow for further investigations also after the closing of this project.

Potential Impact:
The project covered the flight tests with multifunctional coating patches applied to an aircraft in commercial operation. The strategic aim was to identify whether the microstructure is in proper shape or possible erosion had affected the patches and therewith the efficiency of the microstructure coating has been influenced with regard to airflow properties and fuel consumption.
So far, the results have shown a good stability of riblet coatings under realistic environmental conditions. In addition, the tests have significantly contributed to research on friction and fuel consumption in internal Airbus projects.

Such coatings have never been applied to any aircraft in commercial operation. This test program has delivered the first results of the effects of such coatings on flights to destinations in Asia, Middle East and North America with different air quality and weather conditions than possibly found in Europe.

For Airbus and Lufthansa, the flight tests have given enough evidence that riblet structure embossed into paint show sufficient durability to go for the next step towards increased fuel efficiency: the large scale application of riblets on a commercial aircraft. In order to achieve this goal, Lufthansa Technik and Airbus have founded a consortium and applied for German public funding (“Luftfahrtforschungsprogramm LuFo V”). The funding was granted as of December 2013, and the new project has started on January 1st 2014. The goal is the development of an automatic riblet application device allowing for a fast, economic and yet high quality coating process.

This progress demonstrates how valuable the basic studies done with riblet patches were for strengthening the position of the European aviation industry in the field of innovative technologies.

Throughout the project period, Lufthansa Technik has frequently published its activities on enhancing fuel efficiency of the Lufthansa fleet by applying riblet coatings. The positive feedback on these articles was intense. Furthermore, the Fuel Efficiency Programme of Lufthansa has obtained the “Airline Strategy Reward - category Technology” by the Airline Business Magazine in July 2013.

List of Websites:
Dr. Georg Fanta
Director Aircraft Painting Services
Lufthansa Technik AG
Aircraft Painting Services HAM WD6
Weg beim Jaeger 193
22335 Hamburg, Germany
Phone: +49 40 5070 1182
Fax: +49 40 5070 98 1182
Mobile: +49 151 589 15718