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The study of the damage of aeronautical materials is important because it may change the microscopic surface structure profiles. The modification of geometrical surface properties can cause small instabilities and then a displacement of the boundary layer. One of the irregularities we can often find is surface roughness. Due to an increase of roughness and other effects, there may be extra momentum losses in the boundary layer and a modification in the parasite drag. In this paper we present a speckle method for measuring the surface roughness on an actual unmanned aircraft wing. The results show an inhomogeneous roughness distribution on the wing, as expected according to the anisotropic influence of the winds over the entire wing geometry. A calculation of the uncertainty of the technique is given.

Numerous examples are known in which the surface roughness plays an important role in the operation and performance of all types of technological systems. In aeronautical engineering the use of specialty paints (coating), whose aim is to minimize the friction of the object relative to the fluid in which it moves, and the ice build-up and pollution on the wings of an aircraft are some examples in which the control of the roughness is important. In all these and many other cases, the increase in roughness can lead to significant negative effects. For these reasons, among others, the roughness and its effects have been investigated in-depth for a long time.

One specific case which we are interested is to study of the erosion (damage) on the wings of an actual aircraft, due to the mechanical influence of the wind and other factors that contribute to the increase of roughness. Taking into account the geometrical characteristics of a wing (length and cross-section), flight conditions and erosive effect of the wind, damage will be distinct on different parts of the airfoil. As a consequence, a non-homogeneous roughness distribution must appear.

An airfoil-shaped body which moves through a fluid produces an aerodynamic force [

In fluid mechanics the layer of fluid in the immediate vicinity of a bounding surface is known as the boundary layer. In this area the factor that creates the majority of drag experienced by the boundary body is the fluid viscosity. One problem when increasing wing surface roughness is the change in the boundary layer flow of the airfoil, leading to a premature transition of this layer, which results in early turbulent separation. Any change in its contour will cause a detachment of the boundary layer at lower angles of attack. As a result a greater overall drag value, that is to say, the force opposing the advance of the plane, will be obtained. Therefore, because an aircraft having rough wings must fly with higher angles of attack to maintain lift (the boundary layer separation occurs before), drag force will be greater than in the case of the smooth airfoil. Consequently, fuel consumption will increase, as the airplane it will have to counter increased drag. Notwithstanding, this negative influence is not only limited to the wings of an aircraft.

Direct and indirect effects on the different mechanical elements have been extensively studied to date, so a similar effect may be found when studying wind turbines and other machines. Thus, the roughness modification of the rotor blade airfoils because of contamination also leads to power loss and to an increase of the boundary layer thickness [

In this article the measurement of the different degrees of surface roughness on a wing of an unmanned aircraft (MILANO unmanned aircraft-UAV) is presented. Traditionally the most used method has been the profilometer technique. However, it has been revealed that this procedure is not useful to apply to the case of an entire airfoil wing considering that this system is based on the registration of the surface topography of a small sample by means of a type stylus. In order to avoid this drawback, an optical method, which is based on the speckle phenomenon, has been employed. There are distinct speckle procedures for measuring roughness [

In the field of the speckle techniques, the two most important set-ups used are the following: (1) the angular speckle correlation method (ASC) and (2) the spectral speckle correlation technique (SSC) [

Assuming a plane surface (macroscopically) and a Gaussian distribution for its surface heights, and neglecting multiple scattering and shadowing effects, it can be demonstrated that, for a focal length given the correlation of two speckle patterns in the Fourier plane corresponding to two incidence angles of the laser beam is [_{h}_{i}_{o}

Substituting both results in

In this paper we chose the condition _{i}_{o}_{h}

In this section the systematic uncertainty of the measurement by the ASC technique is computed. With this aim the basic theory of uncertainties [_{h}_{j}

For the case analyzed _{s}^{−3} rad. On the other hand, assuming _{s}_{s}^{−2} rad,

It may be observed that the second term contributes more to the uncertainty than the rest. It corresponds to the angular difference between incident beams. To reduce this uncertainty and improve the measurement, a higher precision when measuring the angle is necessary. However, with this set-up and experimental values the precision of the technique is 0.1 μm.

The above calculation is only valid if the angles of incidence and observation are the same. However, if one of them is different, then the

This expression may be simplified to give:

Let us suppose that the observation angle (45°) is maintained, but the incidence angle changes. For this specific case the equality becomes:

By comparing this result with _{s}_{h}_{s}_{h}_{s}_{i}_{s}_{o}_{s}

However, even though this inequality is difficult to resolve, as the first term of the second member always adds, this relation is always fulfilled if:
_{i}

In order to investigate the effect of the wind on the surface of an actual aircraft wing, measurement of the roughness on different parts of the wing were carried out. To perform the experiments a device as shown in

Owing to the different angle of incidence, the two speckle patterns are displaced [see

All experiments were made for _{i}_{o}_{h}

The initial roughness of the wing was 2 μm, and then these results show that after many hours of flight all parts deteriorate, but differently, and on places near the leading edge, both above and below the aircraft wing, the wear is less than on the flap. The interpretation of all these numerical values can be understood by means of fluid mechanics.

In effect, near the leading edge of the wing up to the shoulder, a favorable pressure gradient exists and the airflow is laminar [

In this paper a study of the roughness on an actual unmanned aircraft wing is presented. By using the angular speckle correlation technique (ASC), the degree of damage at different small areas of the airfoil is measured. The study reveals a non-homogeneous resultant mechanical effect of the wind, and other factors, on the surface microstructure. As expected, because of the apparition of turbulences over the flap and near to the intersection between the wing and the fuselage, the erosion is greater over these areas than on the leading edge, where the airflow is almost laminar. Besides, a study of the uncertainty of the technique is made, and a recommendation for the best experimental set-up is given.

The authors wish to express their gratitude to INTA for its material support and especially to the director of LINES Belenguer for his helpful discussions.

The authors declare no conflict of interest.

Layout. (_{0}, lateral displacement of the CCD camera.

Experimental set-up employed. The picture corresponds to the measurement at a point located on the extreme of the wing flap (the length of the wing is 1.30 m, approximately).

Experimental device. In this case the photograph shows a point located on the middle of the wing near the leading edge.

Experimental plot of the correlation C versus displacement (in μm) of the CCD camera. Notice that the maximum is reached away of the first exposure where the camera was not yet displaced.

Speckle patterns corresponding to the maximum correlation. (^{2}).

This picture shows the roughness at each point measured on the airfoil. Point 3 is a special case because it does not have coating.

Experimental roughness obtained for the selected points of the aircraft wing.

_{q}(μm) | ||
---|---|---|

1 | Extrados | 9.2 |

2 | Intrados | 10.1 |

3 | Intrados | 12.4 |

4 | Extrados | 8.2 |

5 | Intrados | 6.9 |

6 | Intrados | 6.3 |

7 | Intrados | 6.7 |

8 | Extrados | 6.2 |

9 | Extrados | 6.2 |