Our friends aerodynamicists, have given us a quite simple expression for drag coefficient, CD = CD0 the constant term called parasitic drag plus Ki CL square a term dependent on the lift and called induced drag. A simplistic explanation to this dependence on lift, comes from the fact that to create lift. The wing pushes down on the air. This creates a downwash behind the wing. Of course, we cannot have a discontinuity between the downwash and still air of the wing. And as you can see on this beautiful picture from NASA, a vortex appears at the wingtip. This large motion of the mass of air around the airplane, requires a significant transfer of kinetic energy from the airplane to the air, which is obtained through an increase of the drag. The corresponding theory called the lifting line theory, has been developed by Ludwig Prandtl. He has shown that the induced drag coefficient Ki should be equal to 1/ Pi lambda with lambda z aspect ratio. This is true for an elliptic lift distribution along the wing. Later, if the additional e0 efficient, called the Oswald efficiency factor, was proposed to account for non elliptic lift distribution. Practically speaking, those coefficients may vary from the theory. as CD0 will capture all the constant rate contributions, and Ki all the contributions dependent on lift and angle of attack, even if not directly induced by lift. The constant parasitic drag coefficient, CD0. accounts for friction drag, a direct effect of air viscosity and pressure drag, due in particular to boundary layer separation. Obviously ,this drag is formed not only on the wing, but all over the wet surface of the entire airplane. This "wet" surface is the one that would be covered with paint, if I dip the whole airplane in a paint bucket. This includes wings, fuselage, empennage, landing gear antennas, etc. For this reason, wet area must be kept to a minimum, to reduce parasitic drag. The last contribution to CD0 is wave drag, due to sonic shock. As wave drag is due to sonic shock, it does not exist below the critical Mach number. It appears in the transonic domain, when a pocket of supersonic flow forms on the upper surface of the wing. However, it remains relatively small, up to Mdd, the drag-divergence Mach number, where it starts to increase dramatically with Mach number. The precise definition of Mdd may vary, depending on the threshold value retained for the gradients CD over delta M. A great improvement of airliner wings in the last decades, has been to increase the separation between Mc and Mdd and push the last closer to Mach one. While keeping a thick wing, which is lighter to design and hosts more fuel, on supersonic airplanes fitted with thin wings. The way of drag increase is less dramatic and reduces above Mach 1. Reasonable values are recovered above typically Mach 1.3. But it is never interesting to fly close to Mach 1. [SOUND]
