Hi XPMortem
Regarding those CD vs CL graphs of NACA airfoils:
I thought that (for positive CLs) CD was always increasing with AoA, wether with or drag bucket or not, since CD is basically the sum of CD parasite (constant with AoA) and CD induced (increasing with AoA).
I am not sure now but the graphs I remember comparing conventional and laminar NACA airfoils had its minimum drag coefficient at CL=0.
However, in the 767 graphs you show, both airfoils have the minimum drag at a positive CL.
What is the reason for that effect?
¿Do you also have the L/D graph versus AoA for the 767?
Thank you
As for the energy calculations ¿What is your opinion about the following?
Let's consider a descent from cruising altitude to a given altitude at a constant IAS for two identical airplanes, light and heavy.
Their potential energy is to be dissipated, transferred to the air. The heavier, the more potential energy. For a similar rate of dissipating energy (for a similar drag) the heavier needs more time to dissipate its potential energy. Same speed, longer time, more distance.
At a given airspeed, the heavier has more drag (or so I used to believe) which means a faster rate of spoiling energy. But this effect does not overcome the effect of the greater potential energy of the heavy one.
We should consider kinetic energy too, since at constant IAS there is a decrease in TAS as we descend. This decceleration is identical in both heavy and light. However, the decrease in kinetic energy is greater in the case of the heavy, because of its mass. So, not only the heavier has to transfer more Potential energy but it also has to transfer more Kinetic energy. At a similar rate of loosing energy, or Drag, the heavier will need more time and hence more distance.
Again, the heavy has more drag for the same airspeed, but the mass effect is stronger.