The main-planes of aircraft are the primary lifting surfaces, hence structurally they have to be very strong and durable in order to take enormous aerodynamic loadings during flight, and whilst at the same time be able to carry fuel within the structure. There are various different types of wing structures but the two main types used in subsonic aircraft are the mass boom and box beam type, with each having its own benefits and drawbacks.
1.2 Aims
The aim of this report is to analyse the construction and performance of the above mentioned types. To discuss those factors that affect the choice of materials used in different sections of the wings and to explain the reasons behind their choice. This report is also looking to identify what a designer can do to reduce the maximum bending moment on a wing and to analyse why a fatter wing is lighter than a thinner wing designed for the same loading and flight conditions.
(Materials assignment sheet)
2 Different types of wing construction
The following section explains the construction of mass boom and box beam type of wing structures, also highlighting their associated benefits and drawbacks.
Mass Boom
In the mass boom type of wing structure the flanges (booms) of one or two spars take the span wise bending load, whilst the skin caters for shear loads it can also aid with torsional load if used in conjunction with spar webs. Normally slow aircraft with thick wings that are lightly loaded use mass boom structures. Figure 1 shows a typical single spar mass boom type of wing structure. (Torenbeek, 1982)
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Figure 1, Single spar mass boom structure, Synthesis of Subsonic Airplane Design, Torenbeek, 1982, p260
The main advantages of this arrangement are that apart from ease of manufacture of these tapered booms, high stress levels are achievable without damage occurring if ribs are spaced closely to prevent the booms buckling. The added benefit of the ribs being closely spaced is that the requirement for stringers is removed as the skin is only subjected to shear forces, hence this simplifies the manufacture process for them. Due to the requirement of only two main frames it's very easy to attach the wing to the fuselage with this configuration. (Torenbeek, 1982)
The main disadvantages of this type of structure is that a failure of a spar (boom) would be disastrous, hence as there is no fail safe characteristics for this type it has been phased out of future transport aircraft designs. There are large deflections under bending loads if high stresses are placed on the spar booms, which require high number of ribs to stabilise. Also, the skin is not used efficiently as it does not contribute to absorbing the bending moment; and would ultimately buckle if no stringers were to be used. (Torenbeek, 1982)
Box Beam
In the box beam structure the main difference is that the skin panels are stressed to take shear loading along with the end bending load. This added benefit makes it better then the mass boom type as it's classified as a fail-safe design; this is so because the skin can be divided into multiple load paths in conjunction with span wise splice members to allow it to be flexed. By using joint straps this type of wing construction can allow for in-service crack to emerge without causing a catastrophe, resulting in a near better fail-safe design. Figure 2 shows a typical box beam structure and where span wise splices are employed. (Torenbeek, 1982)
The main benefit of box beam structure is that it is damage tolerant, and a more efficient design compared to the mass boom type. This can be clearly observed when considering the skin thickness required to gain a sufficient amount of torsional rigidity for wings that are designed for high speed, or thin high aspect ratio wings. For lightly loaded wings the upper skin stress levels are kept low to avoid it buckling and the difference in weight is also small when compared with the mass boom type. (Torenbeek, 1982)
But the drawbacks to this structure are that it is costly and complex, as the large number of different components needed for its construction. Also there are many joints which add weight and in the future could be stress points which might set off cracks. In addition to these issues, it is also difficult to seal integral tanks when they are employed within this structure. (Howe, 2004)
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Figure 2, Typical box beam structure, Synthesis of Subsonic Airplane Design, Torenbeek, 1982, p260
2 Use of different materials
The main factor behind the development of aerospace materials is to reduce weight. Generally materials that have high stiffness, high strength and light weight are ideal for aircraft construction. The reason behind why Aluminium and Titanium have been the most used materials in aircraft construction is because of their stiffness to weight and strength to weight ratios. Nevertheless the advancement of fibre reinforced composites is likely to change this position, as composites allow for weight savings of 30-40% over their metal counterparts whilst also being stronger. Hence these are likely to take over in the future of aircraft structural design as can be seen by the new Boeing '787 dreamliner' which employs high levels of composites in its structure.
(Characteristics of aircraft structures & materials, no date)
Cost and structural performance are the primary two factors that affect the choice of aircraft materials. The costs include the initial material, manufacturing and the maintenance costs. The following list shows the main material properties that are important to the structural performance:
Density (weight)
Strength (ultimate & Yield strength)
Stiffness (Young's modulus)
Durability (fatigue)
Damage tolerance (fracture toughness & crack expansion)
Corrosion
(Characteristics of aircraft structures & materials, no date)
Steel alloys have the highest density in comparison to the other metallic materials and are used where high strength is required on highly loaded fittings such as the wing root. Steel alloys have poor corrosion resistance and must be plated to protect against corrosion.
Aluminium alloys have been the most used material in aircraft structures for a long time, this is because they have good mechanical properties along with being light weight. The 2024-T3, T42 aluminium alloy provide excellent fracture toughness, slow crack growth rate as well as a good fatigue life, hence it is used on the lower wing skins which usually face fatigue stress due to application of cyclic tensile stresses. The upper wing skins are normally subjected to compressive stresses and so aluminium alloy 7075-T6 is used. Recently we have seen the development of aluminium lithium alloys that give better performance as they are 10 percent stiffer, 10 percent lighter and have better fatigue life over conventional aluminium alloys.
Titanium alloys such as Ti-6a1-4v are though heavier then aluminium, their yield stress is almost double and they have better corrosion resistance properties then both aluminium and steel.
Fibre reinforced composites are stiff, strong and light hence are ideal for aircraft structures. They are normally used as unidirectional laminate with different fibre orientation to provide for 'multidirectional load capability'. They have excellent fatigue life, damage tolerance and are corrosion resistance making them ideal material for a strong wing structure. (Characteristics of aircraft structures & materials, no date)
3 Bending moment reduction
The wings of aircraft produce lift as result of different pressures at the top and bottom surfaces. This action creates a shear force and a bending moment, both which are greatest at the wing root. Hence the structure at this point has to be very strong to resist the loads and moments, whilst at the same time be sufficiently stiff to limit the wing bending. The wing at this point is usually very thick to provide maximum stiffness for the minimum increase in weight. A clear advantage of having wing mounted engines is that they are located next to the area which produces lift. This reduces the fuselage weight and hence the shear and bending moments are also reduced at the wing root. By having the fuel stored in the correct position of the wing also results in smaller bending moments. As fuel near the wing tips reduces the bending moment, the tanks are normally emptied from the wing root towards the wing tip. (Aircraft Structures Summary, No date)
It can be said that the greater the aspect ratio of a wing the greater the bending moment at the root. This is because the thickness of high aspect ratio wing is less than the thickness of low aspect ratio, meaning the structure is heavier as the spar depth is smaller and hence the end loads are higher on the booms. Fuel tanks and engines situated outboard of a wing can help to decrease the bending moment and enable lighter structures to be designed. As high aspect ratio wing carries the lift further out from the root this results in large bending moments, along with larger loads on the booms and hence greater weights as when compared to a wing with low aspect ratio. The increased weight that is caused by the high aspect ratio is due to the need to meet the increased bending moments. (Stinton, 1998)
4 Effects of Wing thickness
Figure 3, Wing spar, Anatomy of the Aeroplane, Stinton, 1998, p274
From figure 3 we can see that the shear force is represented by Wx and this is reacted by end loads in the top and bottom booms. For example if the bending moment is 100,000 lbs per inch and the depth of the spar (Z) is 20 inches then the tension in the top boom would be 100,000/20 = 5,000lbs per inch; and the compression in the bottom boom would also be 5,000 lbs per inch. So therefore if the same bending moment was to be met by a spar of half the depth then the end loads of tension and compression would both increase to 10,000 lbs per inch respectively. This is an approximation but nevertheless validates the point that the deeper the spar the smaller the end loading on the booms and hence the required structure is lighter. The required amount of shear and bearing strength still determine the overall amount of material used in the cross-section. But from this short example we can clearly see that a thicker wing is lighter than a thinner wing designed for same loading and flight conditions as it has smaller end loadings on the booms.
(Stinton, 1998)
