#pilotstuff

Air New Zealand

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INTRODUCTION

The days of the a380 and the humped 747 are gone and the 787 dreamliner is at the forefront of this revolution making it my favourite commercial airliner.

MATERIAL

About 50 percent of it is made of carbon fiber composite making it the first commercial jet in the world to be made up primarily of this material.

Composites are composed of 2 or more materials. Carbon reinforced plastics are composed of extremely strong carbon fibres bound together by plastic.

Carbon fibre is very strong and very light. It is 5 times stronger than steel and twice as stiff. Since they are very thin strands, thy can’t create solid structures, therefore, are bounded together by the plastic resin otherwise they would just form a strong but flexible fabric. Which is useful as it can be made into any shape required thus helping in forming the smooth curves of a jet.

Fun Fact- Boeing had to make customised ovens for the resin to heat in after it had been laid on the mould.

The 787 can also accommodate large windows on its fuselage as it is made up of composites. A hole this large on an aluminium jet would result in build up of stress around the window boundaries due to deviation of stress contours around the holes. Over a period of time, this would lead to damage on the body of the jet which will shorten the lifespan of the jet by a lot.

Aluminium based jets also pose another problem that has been solved by the use of carbon fibre composite. Use of joints and fasteners to rivet the pieces of aluminium together created small bumps and imperfections on the surface of the jet that created a lot of drag. Since the fuselage is now essentially a monolithic structure made of carbon composite, this drag is eliminated.

WINGS Wing Spar is the main structural component of a wing and its main job is to resist the upward bending force. In the 787 dreamliner, it is made up of carbon fibre composites and are structured by aluminium plates. This structure is hollow and acts as a space to store fuel in the jet. Carbon fibre composites have another quality that make them better for making wings. Carbon fibre composites can deform about 1.9% before entering the plastic region whereas aluminium can deform less than 1%.

(stress-strain graph depicting elastic and plastic regions. Source- https://www.smlease.com/entries/mechanical-design-basics/stress-strain-curve-diagram/)

Therefore, the wings can be super flexible. During the flight, the wingtips of a 787 dreamliner can move upward by about 3m.

The wings of a 787 are not the same as that of other aluminium jets. Due to its high flexibility, engineers designed the wings with a high aspect ratio. Aspect ratio is the ratio of the wing span to the mean chord of the wing. Gliders have high aspect ratio and the delta wings of a fighter jet have low aspect ratio. The 777 had an aspect ratio of 9 but the 787 has the aspect ratio of 11!

Thus even though composites are stiffer than aluminium, the wings of 787 bend more due to higher aspect ratio.

Higher aspect ratio means a larger wingspan as the vortex drags at the tip of wing, by spreading the area of wing over a longer wing, we minimize the pressure that drives vortices thus, the energy loss due to vortex drag is reduced. Another difference wings of the 787 dreamliner is the airfoil itself. The 787 uses a supercritical airfoil.

Tested in the early 1970s by NASA at the Dryden Flight Research Centre is now universally recognized by the aviation industry as a wing design that increases flying efficiency and helps lower fuel costs. Conventional wings are rounded on top and flat on the bottom. The SCW is flatter on the top, rounded on the bottom, and the upper trailing edge is accented with a downward curve to restore lift lost by flattening the upper surface.

 THE PROBLEM WITH COMPOSITES

One of the things that stands out the most in the material composition of the 787 is the extensive use of titanium over aluminium since titanium is an expensive metal.

Aluminium on its own doesn’t corrode but when kept near carbon fibre, it oxidises rapidly. This is due to a phenomenon known as galvanic corrosion. When 2 materials with dissimilar electric potentials are kept in contact with each other with an electrolyte such as salt water, exchange of ions takes place.  

Fun Fact- To reduce the cost of manufacturing Titanium, Boeing partnered with Norsk Titanium which 3-D print titanium parts thus eliminating the wastage of metal.

There is another issue with composites, even though carbon fibre is a good conductor of electricity, the plastic resin is an insulator and thus doesn’t allow electricity to conduct through it in case of lightning strikes. Thus, Boeing had to add strips of copper all around the fuselage to help it conduct lightning.

BIBLIOGRAPHY:

https://www.nasa.gov/pdf/89232main_TF-2004-13-DFRC.pdf

https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-044-DFRC.html

https://www.smlease.com/entries/mechanical-design-basics/stress-strain-curve-diagram/

https://www.innovativecomposite.com/what-is-carbon-fiber/#:~:text=Carbon%20Fiber%20is%20a%20polymer,steel%20and%20twice%20as%20stiff.

https://www.sciencedirect.com/topics/engineering/boeing-787-dreamliner

https://www.aerospace-technology.com/projects/dreamliner/

https://www.boeing.com/commercial/787/by-design/#/advanced-composite-use