LAMINAR AERODYNAMIC

With the word “laminar aerodynamic” we wean every possible technology that could defer the transition between a laminar flow to a turbulent one in the boundary layer on the upper surface of the wing. As a matter of fact, when the airflow interacts with the upper side of the wing, it generates a boundary layer which is, at the beginning, a laminar flow. This latter is a laminar regime when the airflow occurs with the sliding of infinitesimal layers on each other without any type of fluid mixing, even on a microscopic scale, the flow is governed by viscous forces and is constant over time.

On the other hand, a turbulent regime is a motion of a fluid (the airflow, in our case) in which the viscous forces are not sufficient to counteract the forces of inertia. As a consequence, the motion of the resulting fluid particles occurs chaotically, without following ordered trajectories as in the case of regime laminar. At some point, on the upper wing surface, the airflow passes from laminar to turbulent and that causes performance decreases in term of generated lift. For this reason, the later this transition occurs, the better. The laminar–turbulent transition is an extraordinarily complicated process, which at present is not fully understood and it is not the goal of this thesis to fully describe it. However, we do know which factors influence this transition. The main ones are: the viscosity of the flow, its speed, the shape of the wing, the roughness of the wing surface and the skin fraction between the boundary layer and the wing itself.

As we cannot do anything about the first two of them, it is possible to work on the last three. We will not deal with changing the shape of the wing in order to get an optimal laminar-turbulent transition for it would require a detailed description and it goes beyond the purposes of this document. What we are going to deal with are riblets and machining that allow to reduce the wing surface roughness. For what concerns the first ones, we can say that they are normally small grooves (or protrusions) aligned with the local air flow. Many studies indicated that the effects of this technology on a turbulent boundary layer decreases the local skin fraction in the order of 10%. By decreasing it, the passage from laminar to turbulent happens closer to the tail edge, thus increasing the generated lift. Another effect this technology brings is reducing the drag that would be generated by the turbulent laminar flow. In fact, the closer to the tail edge the transition happens, the less drag will be generated. If the overall drag is reduced, the aircraft would need less fuel to accomplish its mission as it will be less difficult for it to fly through the airflow. On the other hand, surfaces of real objects are usually affected by micro-geometric irregularities, these ones cause the object (the wing in our case) to have roughness on it. The rougher the wing is, the more drag it will produce and, more important, the closer to the wing tip the laminar-turbulent separation will happen.

These last two factors drastically increase the fuel required per mission. Nowadays there are many surface finishing treatments allowing to reduce the roughness of a wing surface. The two technologies have in common that they both decrease drag and move the laminar-turbulent transition point closer to the wing tip. Unfortunately, if surface finishing machining are available today, we cannot say the same for riblets. In fact, these latter will only be available on commercial aircrafts around 2020 (they have a TRL of 7). If we combine these two technologies together, we will obtain some advantages:

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Fuel saving As previously said, decreasing drag means decreasing the fuel needed every mission as an airplane equipped with both of them would face less air resistance during its flights

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Easy design As shown in Figure 2.1, riblets are not complicated to manufacture and for this reason they are quite cheap and easy to produce comparing to other new technologies

However, riblets and surface finishing treatments have their disadvantages too. The main ones are:

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Expensive machining Unfortunately, the operations needed to decrease the roughness of the wing surface are expensive and sometimes it is not worth to spend a huge amount of money for a slight decrease of roughness

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Ultraviolet damage Some tests that have been conducted on riblets showed that they have a small resistance to ultraviolet radiation. Because of these reason, an increased airframe maintenance would be necessary

If we combine these two technologies together and we imagine mounting them on a commercial aircraft; the way they change the cost categories is clearly shown in table 3.8.

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Table 3.8Cost categories for riblets and surface finishing machining [38]

As it can be easily understood, riblets are not too difficult to research, develop and test as they are basically small grooves (or protrusions) aligned with the local air flow (see Figure 2.1). For this reason, “only” 1 million US dollars are enough for this cost category. The annual operating costs are slightly improved due to the fuel saved thanks to the drag reduction and the increase of lift these two technologies can obtain. On the other hand, maintenance costs increase only because of the presence of riblets (there is no need for extra maintenance on finished surfaces as they are treated like normal surfaces) which are additional elements to take care of. For what concerns the production costs we can say that they are slightly increased mainly due to the need to do surface finishing treatments in order to decrease the roughness of the wing surface.

As a matter of fact, these operations are quite expensive nowadays and, on the other hand, this cost category is not much afflicted by rubles for it is very easy to manufacture and produce them. The on-aircraft investment costs are around 1 million dollars as the presence of riblets on the wing itself do change the design of the wing, even if with a little change. Eventually, around the same amount of money is required for the retrofits costs because the installation method of the wing on the aircraft change if riblets are on it. Surface finishing treatments do not affect these last two cost categories at all. Last but not the least, the four cost categories of chapter one applied to to these two technologies can be expressed as follows:

1) RDTE costs Around 1 million dollars. As previously said, riblets are not complicated items to research and test. Of all the other technologies we will deal with in this chapter, they are the cheapest one regarding the RDTE costs. This cost category does not apply to surface finishing machining, as it is possible to apply it on current airplanes

2) Acquisition costs Assuming zero profit on these two technologies (which is not that far from the truth for profits are normally set for massive new technologies

EIR

Annual operating

costs

On-aircraft investment

costs

Retrofits costs

Maintenance costs

Production costs

RIBLETS + SURFACE FINISHING MACHININ

G

1 M −0.7 ^% 1 M 1 M +0.4 % +1.3 ^%

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such as the morphing wing, the geared turbofan, etc…) we can assume a %, due to the expensive processes needed for decreasing the wing surface roughness 3) Operating costs % as shown in table 3.8

4) Disposal costs % as a surface finished wing will be disposed the same as a classic wing and because riblets can be disposed without too many efforts for they are small and they are not made of toxic materials.

Nel documento Effetto delle nuove tecnologie sul costo del velivolo = EFFECTS OF NEW TECHNOLOGIES ON THE AIRPLANE FINAL COST (pagine 87-90)