Some further non-aerodynamic considerations in wing design

We have mentioned structural problems and how they influence the final design of a wing. There are also a number of other considerations which we will discuss briefly here in order to remind ourselves that the aerodynamicist cannot have things all his own way in the design process.

As well as providing lift the wing usually has other important functions. One of these functions in most aircraft is to act as the main fuel tank. Using the wings for this has a number of advantages. Firstly it uses up an otherwise

unattractively shaped storage volume for a useful purpose. Secondly the fuel weight can be spread over the span of the wing, rather than concentrating it all in the fuselage. Thus we can get away with a lighter wing structure because of the reduced bending moments along the wing.

In many aircraft, particularly transport aircraft, it is very convenient to store all the fuel in the wings and this immediately leads to the requirement that the wing must have a certain minimum volume quite apart from the structural problems we have already mentioned. This may well mean that some comprom­ise had to be made in the aerodynamic performance of the wing. This sort of problem gives some idea of the complexity of the design process. Because the aerodynamic performance is reduced, more fuel will be required, and so the designer must go round the loop of choosing wing capacity and performance until a satisfactory solution is obtained.

Before we leave the subject let us look at a couple of less obvious design choices which must be made. The first of these concerns the question of where we put the main undercarriage legs. With a nose wheel undercarriage these must clearly be behind the aircraft centre of gravity, or the aircraft will topple onto its tail while at rest on the ground. To get a reasonable wheel separation and to keep the fuselage clear it is generally preferable to mount the undercarriage in the wings. However, with a swept wing, the centre of gravity may lie near the trailing edge where the wing is too thin to house the retracted gear, and too weak locally to support the weight of the aircraft. One solution which is commonly employed is to use a cranked trailing edge (Fig. 9.19). This, fortunately, fits in quite well with some of the other requirements which have

already been seen to apply at the centre section. Furthermore, extending the wing chord in this region enables a thick physical section to be used, which is needed for the structure and to house the undercarriage; alternatively the thickness-to-chord ratio can be reduced to give an aerodynamically thinner wing. This again can be helpful in keeping the local Mach number down at the centre section where the local flow speed has been raised by the presence of the fuselage. Another important advantage is that the use of a straight trailing edge close to the fuselage makes it much easier to fit trailing-edge flaps close to the wing fuselage junction.

Another unexpected factor may enter into the design of the cranked inboard portion of the wing. There will clearly have to be a break in the trailing-edge flap to accommodate an underwing pylon-mounted engine. It is therefore con­venient to mount the engine at the junction between the swept and unswept trailing-edge regions (Fig. 9.19). The distance of the engines from the centreline has important implications from the point of view of aircraft controllability in the event of engine failure, particularly at take-off when full thrust is being employed. The further outboard the engine is mounted the larger the fin and rudder assembly needed to provide adequate control in these circumstances. This is one more factor which must be carefully considered, and so we see that we cannot just consider the wing itself in trying to achieve our optimum design for changes in the wing design can have important repercussions elsewhere on the aircraft.

Another factor which may limit the way in which we can achieve our desired wing geometry is the manufacturing process itself. If a conventional wing con­struction of light alloy is to be used, the complexity of the three-dimensional surface which can be achieved is limited, and it may not be possible to build in economically all the variations of twist and camber that we would like if given an entirely free hand. This is another potential advantage presented by more modern composite materials – they offer the possibility, not only of building in tailored stiffness characteristics, but the facility to make more complicated shapes than is possible with more conventional constructional materials (see Fig. 14.6).