Aluminium Wings v Composite and Future Wing Materials

The comparison of the properties of the materials used in aluminium and composite wings and the advantages and disadvantages of which they both possess and make them suitable for use within the manufacture of wings. A discussion of future materials which have been developed and are suitable for the use in wings will also take place. Collected information came from appropriate websites and books.

Aluminium is the widest used material for the manufacture of aircraft wings to date since the first time it was used in the 1920’s. Now the use of composites is becoming greater utilized in the manufacturing of aircraft wings at present instead of traditional aluminium wings. This is mainly to do with the weight saving properties that composites can posse. Weight saving properties is just one of the advantages of composite materials, another can be stiffness, but there are also disadvantages to using composites compared to aluminium, such as if they get damaged they need replacing immediately unlike aluminium which is very tolerant to damage. Aluminium production and repair is also much easier than that of composites. Aluminium and composites both have their own advantages and disadvantages and their properties have to be taken into account before any material changes are made. Future developments will hopefully provide a material that which will provide sufficient advantages and minimal disadvantages compared over composites or aluminium.

This report will look at the Boeing 737, which features aluminium wings, and the Boeing 787 dreamliner, which incorporates composite wings, and refer to them for the comparison of the different properties and structures of the two wing types. It will look at each type of material found in a traditional aluminium aircraft wing structure at present and will go into depth about the use of composites in wings instead of aluminium at present and in the future. The types of composites used, as well as investigating whether the structure of the wing had to be altered to compensate for the different properties of the composites will be discussed. The strength and weight properties of each different type of aluminium and composites used in an aircraft wing will also be examined. Types of corrosion which occur on an aluminium wing, including the inspection and repair of it will also be included, as well as the inspection and repair of composite materials and the types of damage which can occur in composites, such as delamination. The cost of production and repair of composites compared to aluminium and aluminium alloys, as well as the weight saved resulting in lower running costs for the company will be examined. The collected information will then be compared and advantages and disadvantages of each type of wing will be produced. It will also look at future aircraft wing materials, such as the use of incorporating aluminium with composites, and if they will change the way aircraft looks at present. The different properties of the new materials will also be examined and compare to the properties of both aluminium and composite wings. An overall conclusion of all the main findings and collected information will also be given. Recommendations will also be given at this point.

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Methodology

After deciding what the topic of the report was going to be about, the research undertaken would need to be relevant. The first part of the study was to find information about Aluminium wings and the materials and structures which made them. This part incorporated finding and recording relevant information from certain websites off the internet. Another source used was finding appropriate books which gave suitable information about the subject in hand. Finding information on composite materials and structures was carried out by the same method. Locating appropriate information about future aircraft wing materials was carried out only with the use of the internet.

Findings

Aluminium wings

Types

Aluminium (Al) has been used in aircraft since the 1920’s due to it being lightweight while also being relatively strong. It is used over steel as aluminium is three times the density less then that off steel, this means that for the same density the aluminium would be three times thicker, resulting in it being much stronger. Aluminium is also has good corrosion resistance, which is an advantage as an aircraft is subject to all weather conditions. Nowadays aluminium is joined with other elements to change the properties of the metal, improving specific areas of it, creating an aluminium alloy. At the present time, Aluminium alloys make up a vast total of a commercial aircrafts unloaded weight.

Alloys

Adding different elements to aluminium improve different properties, for example adding zinc to aluminium will improve the strength of the material. The added zinc allows the aluminium to be heat treated, where the metal is heated and cooled which in turn changes the structure of the metal along with its properties. More than one element can be added at the same time resulting in different properties being produced from having the same main alloying element. Even tho some of the properties of the aluminium will improve, the alloying elements need to be correctly chosen as other properties within the metal will be sacrificed. Certain aluminium alloys are used in the manufacturing of aircraft wings, the types of aluminium alloys, along with where it is used, the elements which are used to create the alloy and the improved properties are listed in the table below.

Al Alloy

Area Used

Elements (%)

Properties

7178

Spars, Beams, upper wing skin

Zinc, magnesium, copper

High compressive strength to weight ratio

7055

Lower wing skin

Zinc, magnesium, copper

Improved stress corrosion and fatigue resistance

7075

Wing ribs

zinc

Improved stress corrosion cracking resistance, high mechanical properties

2024

Slats, flaps

copper

Good fatigue performance, fracture toughness, slow propagation rate

The Boeing 777 also uses the aluminium alloy 7055 due to it having a greater compressive strength than other alloys that had been tried before. Due to this, it was able to be used in the manufacture of parts of the wing, in the stringers and the upper wing skin.

Corrosion

Even though Aluminium has good corrosion resistance, it is still susceptible to corrosion. Aluminium is somewhat protected from corrosion as an aluminium oxide film forms on the surface. This is due to the aluminium being protected from additional oxidation by the existing aluminium oxide film. Minimal corrosion, such as light surface or small pitting corrosion, does not normally cause a problem to the metal. Heavier corrosion occurring in metals used on aircraft is not wanted as it can lead to a weakening in the structural rigidity of the metal. If this is not rectified it can lead to a structural failure within part of the aircraft. Corrosion can occur in many different forms, which include pitting, intergranular, and galvanic corrosion.

Pitting

This is one of the main types of corrosion which occurs on an aircrafts wing. This type of corrosion is a localised type and starts on the surface of a metal, whether it is on the skin panels of the aircraft or within the aircraft itself. It works its way through the surface protection of the metal, and then penetrates its way further into the metal creating a hole within the metal itself. Due to metals have different mechanical and chemical properties, when pitting corrosion occurs, the pits created will be different from one metal to another, as shown in on the right. This hole decreases the strength of the metal due to the grain damage caused by the pitting corrosion.

Detection

One way of detecting certain corrosion is by using x-rays or gamma rays to take a picture of the piece of metal suspected of having corrosion. Once the picture is developed, it is clear to see where the corrosion, such as pitting, is taking place in the metal, as it produced a darker spot on the film. This is due to less of the radiation being absorbed where the corrosion is taking place. If pitting corrosion is taking place, the image can be used also to establish the depth of the pit within the metal.

Another way of determining whether pitting corrosion has occurred on a piece of metal is by the use of Eddy currents. This type of non-destructive testing uses magnetic fields, where the metal object being tested is placed. The magnetic field is produced by putting an alternating current through a coil. An alternation in the back EMF (Electromotive force) occurs when the eddy current gets disturbed by a pit in the metal. This alternation is amplified so it can be seen as an image or heard as a sound by the operator.

Prevention

There are lots of ways to try and prevent corrosion from occurring. One method is to uses surface treatments which protect the surface of the metal, therefore reducing the chance of corrosion and painting the metal surface can also prevent corrosion as no air or moisture can tough the metal. The use of cathodic protection can also prevent corrosion.

Composites

The use of composites within aircrafts is a relatively new concept. They were first introduced in the 1980s in secondary aircraft components, such as wing leading and trailing edges, and then as more composites were produced they made their way into larger structures in the 1990s. The Boeing 787 dreamliner tries to make the fullest use out of composite materials that is possible. Around 50% of the full aircraft, including several parts of the wings are manufactured using composites. The rest is manufactured using other materials, such as aluminium, which incorporate properties which at the present cannot be bettered by composites. At the moment composites are used mainly on non structural parts of the wings, and are used on parts such as the wings skins and the flaps.

The great attraction for airline industries to use composites within the manufacture of their aircraft is because composites can be strong, and at the same time be lightweight. This means that heavier metals can be replaced with lighter weight composites which have the same strength. This causes the overall weight of the aircraft to decrease, resulting in a more fuel efficient aircraft as less fuel is needed to be burned to move the aircraft. This is an advantage to an airline company as it would result in lower running costs for that aircraft. Costs in manufacturing were also managed to be reduced as during assembly, a smaller quantity of fasteners were needed and there were also a smaller amount of parts required to construct the component.

Composites do have disadvantages compared to metals for use within aircraft. One of these is that damage to composites can be difficult to see. Another is due to the fact that composites do not conduct electricity which may cause a problem if the aircraft is struck by lightning. These have also been concerns regarding the safety of the use of composites if there was a crash.

Make up

Composites are made up by joining together two or more materials which creates a material with improved properties compared to that of both original materials. Composites are made up of a matrix, which is a resin which joins together with a reinforcing material, which is a fibre. There are different types of reinforcing fibre and matrix which individually have different properties and need to be carefully chosen to make sure that they will be suitable for their purpose within the aircraft if chosen. The most commonly used reinforcing fibre used in aircraft is Kevlar. This is due to it having the greatest impact resistance and tensile strength compared to all other reinforcing materials while still being reasonably light.

Types

Carbon fibre reinforced plastic is the composite used within the manufacture of the Boeing 787 aircraft wing. This composite is used as it has lightweight qualities while also being very strong, and can have the equivalent strength to steel. It is manufactured using carbon fibre as the reinforcing fibre and the matrix is usually epoxy.

Damage

One of the main disadvantages with the use of composites is the difficulty to tell if damage has occurred within it, this can be known as barely visible damage. This is due to the way in which the composite structure is manufactured and that the majority of the damage will occur behind the surface. The surface of the composite may only seem to have a small bit of damage, such as a bit of scratched paintwork, while behind it the inside of the structure has been badly damaged.

Delamination can happen due to moisture being able to go through the surface of the composite. If this moisture freezes, which can occur at high altitudes, it will start to force the layers of the composites apart. This could continue to occur if undetected causing serious damage to the composite structure. Fibre damage, where the fibres within the reinforcing material break, and matrix damage, where the matrix splits, may also occur if there is damage to the composite.

Inspection

There are several ways of testing for damage to composites. The simplest one of these is tap testing. This is where the surface of the composite is tapped using either a light hammer or a coin. An area of which is undamaged will make a ringing sound where as a duller note will be heard if the area is damaged. A more accurate version of this method can be had with the use of an electronic tap tester.

Other methods of detecting damage are with the use of ultrasonic or x-ray machines. All these forms of testing are known as non destructive testing. This is due to no damage is needed to be made to the component getting tested by any of these methods.

Repair

Unlike Aluminium which can withstand damage and still be useable, composites when damaged have to be either repaired or replaced immediately. Repairing a composite panel is considerably more difficult than repairing an aluminium panel. This means that the repair will take a longer time in comparison, and will mean that the aircraft will be out of service longer. The cost of the materials to replace the damaged part is also more expensive, and may not be available at the airport where the damage is detected. Special training for working with composites may also be needed, resulting in even greater costs for the airline operator.

Lightning Strikes

The use of metal wing skins meant that if there was a lightning strike on the aircraft, it would be dispersed over the whole body of the aircraft and would dissipate at the end of the wings, through static dischargers, due to its conductive nature.

The problem with the use of some composites as a wing skin is that they are considerable less conductive compared to a metal wing skin. Therefore, this could lead to damage occurring to the composite panel as the intensity of the lightning strike would be concentrated on the spot it hit as there would be no way for the energy to disperse due to the non conductive nature of the composite. The main danger of this is that the energy of the lightning bolt may be able to penetrate through the surface of the skin enough to produce a spark inside the wings where the fuel tanks are. This spark could cause the fuel vapour within the tanks to ignite, causing an explosion within the wing.

Boeing have created several ways to prevent this scenario from occurring within their 787 dreamliner. The main method is having a thin metal mesh on the outside of the composite. This causes the composite skin panel to act in the same way as the metal one, and disperse the energy of the lightning strike over the whole surface of the aircraft. They also make sure that each fastener holding the composite skin panel to the wing structure is tightly fitting, preventing sparking from occurring between the spaces. Edge sealant will also be used to make sure there are no gaps present, and can be of either a glass fibre or goop. The use of a nitrogen generating system will be used to add nitrogen into the fuel tank, which will mix with the fuel vapours creating a safer non-flammable mixture should a spark occur.

Future Materials

New materials are continually being created by the aviation industry to try and lighten their aircrafts, and thus making them more appealing to airline operators. There has been increased competition to try and make composite materials which can be used throughout an aircraft. Other manufacturers are looking for slightly different ways to improve on materials that are available at present, with the use of shape memory alloys.

Composite Spar

Al/Composite

The continued development of composites has lead to the creation of a material which incorporates both aluminium and composite. This material would be ideal for the use in aircraft wings due to several properties in which it possesses. The main one being that it is virtually fully resistant to metal fatigue. Metal fatigue comes about due to the cyclic loading of material. This will lead to a failure of the metal after a crack starts within the component then increases in size. This is relevant in aircraft wings as they experience cyclic loading as the lift generated by them changes during flight, such as take off and during patches of turbulence.

Compared to the manufacturing costs of composites, the manufacturing costs of this material are significantly lower. As well as this, repairs to damaged sections are more straightforward compared to composites, which reduce the cost.

The strength properties in which this material holds are greater than the composites which are used in aircraft wings at the present time. The most noticeable being the Boeing 787 which incorporates carbon fibre reinforced plastic. Due to this increased strength, the thickness of the material needed can be reduced and this can lead to a weight saving of around twenty percent, which is equivalent of between 600 to 800kg. This reduction in weight will cause a reduction in fuel use, along with the reduction in maintenance cost will reduced the overall running costs imposed on the airline operator.

Morphing wing

Shape memory alloys have existed for a reasonable long time, but it is only recently in which it has found a purpose within the airline industry. The use of shape memory alloys within the manufacture of aircraft wings is being looked at to improve the efficiency of the wing. This would happen as the flight crew would be able to change the shape of parts of the wing during different flight operations. There has been research into the development of a fully morphing wing and also that of a morphing winglet. Both of these ideas would lead to several advantages, but there are also disadvantages of the use of shape memory alloys.

The main advantage of this material is that it can remember its shape after being deformed. When the material has been deformed, if the material is then heated to a certain temperature it will return to original shape. These materials also incorporate the property of Pseudo elasticity, which is super elasticity. This is when if the alloy is subjected to load it will stretch and change form. The load imposed on the material will then be absorbed, and it will return to its original form and shape. Shape memory alloys, such as Nickel Titanium, can be polished to give very smooth finishes resulting in a reduction in drag as the air flows over it.

There are disadvantages which hold up the development of shape memory alloys, which include the difficulty and the cost of manufacture. The main problem with the use of this material in aircraft wings is that it does not have very good fatigue properties, which one needs.

A shape memory alloy is manufactured to the shape in which it will take when heat is applied to it. As the reactivity of titanium is high, the use of a vacuum during manufacture is common. Hot working is one of the methods used to create these types of alloy and is where the material is heated up to temperatures of 900oc and then shaped. Cold working is another method that can also be used, but comes with the disadvantage that the material need constant heat treatment due to work hardening occurring.

The use of this material in winglets would allow the winglets to change shape depending on the flight conditions such as the relative airspeed of the aircraft. This would allow them to have the most efficient angle between them and the wing. A reduction in wing vortices would then be able to occur over each flight operation. The drag experience on the aircraft at each point would be minimised, in turn reducing the fuel consumption of the aircraft as less thrust is required to move the aircraft would decrease. The idea of the winglets flattening out during takeoff and landing is also being examined as the wing would produce more lift at the slower speeds. This means there would be a reduction of noise generated from the engines as less thrust would be required.

Constructing a wing out of smart alloy materials has been look at as it could lead to several advantageous properties, such as weight saving and reduction in drag. This means that the wing could change shape during flight operations to make them more efficient. The wing surface would be continuous as there would be no gaps in between flap and the surface would be smoother as there would be fewer rivets needed. This would result in a reduction of drag generated from the surface of the wing. A reduction in weight could be seen from the removal of the hydraulic system needed to move the control surfaces of the wing at present. There has also been investigation into using the shape memory alloy for use in just the leading and trailing edges as a replacement for the traditional metal flaps. The overall result of using shape memory alloys to replace traditional wings would be better fuel consumption as there would be a reduction of drag and weight.

Conclusion

I recommend that there should be a continued development of composites within the airline sector. This will lead to the manufacture of composites which are strong enough to be used on the main structural parts of the wings, and which could also be used on other components of the aircraft. The more widely use of composites would also lead to a reduction of weight of the aircraft, making them more fuel efficient and more environmentally friendly. This would also be an advantage for the airline company as there would be a reduction in the amount of fuel needed resulting in reduced running costs.