A good summary of the aerodynamic characteristics of our aircraft is the drag polar. It is the locus of lift and drag coefficients as angle of attack is evolving. So, this is a CD as a function of CL curve, which is a parabola just turned by 90 degrees. Because, of course, it is more natural to draw the lifts, CL, on the vertical axis and the drag, CD, on the horizontal one. As you can see, as fortunately, drag coefficients are far smaller than lift coefficients. The drag axis is magnified by a factor of 100. This gives us a precise and easy-to-read drag polar. You can also notice that this is not, of course, a perfect parabola. As for high angle of attack, CL is limited to CLmax by stall, while drag keeps on increasing due to flow separation. And on the lower part of the curve, we observe the minimum drag coefficient, CD0, obtained here at zero lift. Although, on a well-optimized airplane, this minimum can occur for a non-null lift coefficient. It is very important to note that the shape of the whole aircraft drag polar is significantly different from the one of a single airfoil in a 2D flow. This is mainly due to the induced lift, of course, a typical 3D effect, as well as the extra drag brought by the non-lifting surfaces, fuselage, antennas, landing gear, etc. So, drag polar allows us to directly visualize lift and drag, as the corresponding lift and drag coefficients CL and CD are just multiplied by the same scaling factor, which is the dynamic pressure and the reference area to obtain the actual force applying on the airplane. We can see how lift and drag evolve when angle of attack is changed. At low angle of attack, for example, we have a significant drag and a very small lift. The blue line shows us, directly, to which extent is the aerodynamic resultant is leaning backward. And, in fact, the slope of this blue line is just CL/CD. The lift to drag ratio, that we call finesse in French. So, to find the maximum lift to drag ratio, we just have to find the angle of attack for which the blue line has the highest slope. And, of course, this occurs when it is tangent to the drag polar. In our case, the best lift/drag ratio is obtained for an angle of attack of six degrees. Let us see now what happens if we increase the parasitic drag of our airplane CD0. This may result from adding antennas or other appendices on the fuselage or collecting bugs on the leading gauge or extending the landing gear or whatsoever. In that case, I just add an increased delta CD to my former parasitic drag, and it results in the translation of the whole drag polar to the right. This results, of course, in a reduction of the maximum lift to drag ratio, as the slope of the tangent is reduced. But also in a change of the corresponding angle of attack. So, the maximum lift/drag ratio is quite sensitive to parasitic drag. And when it is increased, the corresponding angle of attack is also increased, and goes closer to the stall angle of attack. This demonstrates, one more time, that those values are completely disconnected from those of the 2D airfoil drag polar. [SOUND]
