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Developing eco-friendly aircraft cargo doors

07 June 2021

Redesigning cargo doors will help to make the aircraft of the future lightweight and more environmentally-friendly, contributing to the Clean Sky’s own objectives as well as the ACARE Flightpath 2050 targets, in alignment with the European Green Deal. 

Four door demonstrators (called TD1-TD4 respectively) are being finalised, ready for a Clean Sky 2 virtual demonstration day on 8 June. Registration is now open – you can sign up here

For the first two doors, TD1 and TD2, the MISSP project demonstrated stretch forming, creep forming and laser beam welding, while OASIS project focused on friction stir welding, spot welding. laser beam welding. AddMan worked on additive manufacturing with titanium, CORDIAL developed new fasteners intended for fully automated one-way assembly, and finally RODEO investigated orbital drilling of carbon fibre reinforced stackups of inner skins assembled on top of aluminium parts.

For the TD1 door, friction stir welding (FSW) was used to join frames and stringers to the skin panel, which itself was joined from two separate sheets. The advantage of friction stir welding is that two parts can be joined together without pre-drilled holes and without adding material to the structure, making the process both less time consuming and more weight efficient. The idea to weld separate sheets together to a bigger sheet allows for larger parts to be assembled from smaller subassemblies.

For the TD2 door, laser beam welding (LBW) was used to join frames and stringers to a skin in an aluminium structure. The advantage of laser beam welding, compared to a riveted structure, is reduced weight. It also avoids holes in the structure. Laser beam welding of aluminium-magnesium alloys (5000-series aluminium) is already well known, but a new alloy, AA5028-H116 containing scandium, was investigated here. The new alloy is weldable, and lacks the decrease in strength that’s been noticed in other weldable aluminium alloys in the past.

Welding methods for structural aerospace aluminium alloys have developed greatly over the last few years. Automation of the welding process has led to precision improvements for both laser beam welding and friction stir welding. 

The same TD2 door structure was also used as a demonstrator for new fasteners (Ergotech and Optiblind) which are starting to appear on the market. The blind fasteners show similar properties to traditionally 2-piece titanium fasteners such as Hi-Lok and Lockbolts, while offering the advantage of being suitable for automated one-way assembly. The fasteners require no machining afterwards and leave a flush surface, which is not the case with the most common blind fasteners on the market today. 

Optiblind fasteners being installed on the TD2 demonstration door
Optiblind fasteners being installed on the TD2 demonstration door

The door was sent to the south of France for a demonstration of orbital drilling as part of the RODEO project. The orbital drilling equipment was integrated into a small robotic end-effector combined with a pressure foot to apply gentle pressure on the structure during drilling. This allowed for the drilling of high quality holes whilst applying very small forces to the structure, hence eliminating contamination of trapped chips and burrs in the interface of the joined airframe parts.

Completed TD3 Structural Door demonstrator
Completed TD3 Structural Door demonstrator

The TD3 door is a somewhat non-conventional demonstrator and only consists of two parts. The skeleton itself is a relatively simple structure machined from a single preformed aluminium billet, the lift fitting has been designed and 3D printed by Fraunhofer using a high strength aluminium alloy (Scallmalloy) and the only fasteners are those which attach the lift fitting to the door. Both the door and lift fitting design are the result of a topology optimisation design process and the manufacturing processes make use of recent advances in high speed machining and 3D printing.

The striking three colours of the door structure show off the three major phases of the powder coating solution. Firstly, the ‘naked’ aluminium (after anodising), then in the central part of the door the aluminium has been painted with primer and the grey area has subsequently been powder coated. This provides a corrosion protection level suitable for the inside of a typical aluminium aircraft structure.

In some ways this demonstrator is so simple but in other ways it is at the cutting edge of what is possible!  

Details about the manufacture of the skeleton and the lift fitting of the T3 Door Demonstrator can be read here. Now the door has been painted and the lift fitting has been assembled, marking the closure of a very successful demonstrator where all the major goals have been delivered.

The TD4 Door will be one of the fully functional metallic cargo door demonstrators. Two full size ‘skeletons’ have been successfully bonded, each skeleton consisting of a skin, two doubler sheets, seven frames, one upper and one lower beam. Each skeleton has been cured in a single shot in an autoclave. 

This bonded solution has the potential for substantial weight and cost savings compared to a traditional riveted structure. A bonded structure has improved load transfer capability compared to a riveted/bolted structure; instead of concentrated load transfer points (rivets) the loads are spread over a much larger area providing the potential for weight savings and fatigue strength.

In addition to the environmental benefits of aircraft weight savings there are also environmental benefits during the manufacture process. No chemical etching of the skin is required and a new chromate free and water based adhesive primer provides significant health and safety improvements compared to the more traditional adhesive primers. 

The tooling and production technology has been changed radically compared to traditional adhesive bonding techniques. The key here is in the complex tooling which allows a relatively complex assembly to be bonded in a single autoclave cycle.

Before and After: Adhesive bonded and powder coated (top half) door structure

 

Before and After: Adhesive bonded and powder coated (top half) door structure
Before and After: Adhesive bonded and powder coated (top half) door structure in jig with system installed

 

Door structure being positioned in the tooling prior to autoclave curing
Door structure being positioned in the tooling prior to autoclave curing

Now the next collaboration phase of the project is underway. The second 3D printed lift fitting from Fraunhofer (again in Scalmalloy) has been assembled and Saab Avionic Systems has finalised the installation of the electromechanical systems into the TD4 door. The door has now been sent to the ZAL Centre for Applied Aviation Research in Hamburg for installation and testing in an A321 demonstration fuselage in collaboration with Airbus.

Anders Rydbom, Chief Engineer for Saab Aerostructures R&T programs, summarises: ‘With the completion of the 4 full-scale cargo door technology demonstrators Saab, together with partners, have managed to increase the maturity level of a number of very promising manufacturing technologies for metallic structures. The optimum choice for a future metallic aircraft door (cargo or passenger door) will depend on the expected manufacturing rate and the required time-to-market, since different methods have different characteristics regarding investments needed, industrialisation leadtime, automation possibilities and recurring manufacturing cost.’