Study of the wings

Computations are run in steady flow over the clean wing or equipped with one of the tips designed. The pressure distribution at various spanwise stations is collected in order to have a better understanding of the physical phenomenon observed. For the interest of this study, more stations were examined near the tip region than on the inner wing. Only few data is selected to be presented in this report as most of the phenomena observed reappear in all computations. I chose the one computed at M=0.8 to be a reference as they exhibit the most complex flow features.

The pressure distributions presented always refer to the clean wing, called wing in the caption, the wing with the raked tip, called raked tip in the caption, and the wing with the winglet, called winglet in the caption. The critical pressure coefficient (Cp*) over the wing is also displayed to get an idea of the flow Mach number over the section and to estimate the strength of the shocks that might appear. The critical pressure coefficient is not constant over a section because of the difference in sweep angles between the leading and the trailing edge.

4.2.1.1. Root section

A very strange pressure distribution is observed at the root section (figure 26). The first thing to remark about this plot is that this distribution is very different from what was seen during the design procedure. There is no near sonic rooftop, the loading at the leading edge equals almost zero and a shock wave appears near the trailing edge. The acceleration of the flow that should appear near the leading edge is underestimated and the flow does not reach the critical pressure before 55% of the chord.

Chapter 4: Comparison of aerodynamic performance of a raked wing tip and a winglet  graph displays this pressure distribution for the clear wing, the wing with the winglet and the wing with the raked tip.

At first, I thought this phenomenon was due to the symmetry boundary condition which could be inadequate even if this was not encountered on the ONERA M6 wing. Some computations involving the full span wing have shown that this was not the case. As a result, I think this phenomenon could be attributed to the fact that I cut the wing designed at the crank and the flow over the inner part of the wing might affect the pressure distribution over this section.

The wing loading should not be much affected as the lift that is not produced by the acceleration of the flow near the leading edge should be nearly recovered by the very low pressure appearing before. The main issue related to this inaccuracy is the presence of a shock wave which might affect dramatically the drag produced. However, as we can see on figure 26, the pressure distribution at this station is the same for the three

Chapter 4: Comparison of aerodynamic performance of a raked wing tip and a winglet 

wings and the increase in drag resulting from this problem should not affect the performance of a wing in comparison with another.

4.2.1.2. Outer sections

The problems encountered at the root section damp out very rapidly when moving outwards. It completely disappears at 25% of the semi span and a rooftop is observed outboard of this station. The pressure distribution over the three wings is nearly the same until 87% of the span (y=13m). There, tip effects start to be observed and the tip devices highly influence the nearfield flow.

Figure F6 shows a comparison of the pressure distribution over the three wings at y=14m, one meter inboard from the wing tip. We can clearly see the influence of the tip vortex on the wing. The aerofoil effective incidence is reduced due to the downwash and the resulting lift produced at this station is reduced. The pressure distribution over the wing with a tip device is less affected by this phenomenon and a higher difference of pressure between lower and upper surfaces can be noticed near the trailing edge.

A peak starts to form near the trailing edge of the wing equipped with a winglet, but this can be better seen at y=14.8m (figure 27). This low pressure peak appears due to interactions of the flow over the wing and over the winglet. The flow is accelerated above both of these lifting devices resulting in very high velocities obtained at the   junction. At this station, the lift produced by the wing is highly affected by the tip

vortex leading to a decrease of the loading. The raked tip still shows a near sonic rooftop, the tip of the wing mounted with this aerodynamic device being still 1.9 meters far from this station.

At a freestream Mach number M=0.5 (figure F7), we can see that the interferences on the wing equipped with a winglet are less important. There is no shock wave because of 

Chapter 4: Comparison of aerodynamic performance of a raked wing tip and a winglet 

the low speed. However, the same physical phenomena can be observed over the three wings.

Figure 27: Pressure distribution over the section y=14.8m (the tip of the wing being at y=15m) at M=0.8 with zero incidence. The graph displays this pressure distribution for the wing clean, the wing with the winglet and the wing with the raked tip.