While geometric washout seems to be relatively easy to understand (you twist the wing at the tip and it stalls later because of it's lower AoA - simple concept for the most part), aerodynamic twist seems to be more complex. Here is a statement by myself that was made in the "Fast Forward to the 1930s" thread started by ccg, and I have read statements by others that seem to say more or less the same thing:
I was suggesting that a higher Clmax would equate to a later stall at the tip. On reviewing some information in TOWS (Theory of Wing Sections, Abbott and Von Doenhoff), I see that this is not true. For example, with a root section NACA 4415, it's stall occurs at an AoA of 12°. If the tip section is the NACA 4418, it's stall will occur at an AoA of 14°, both according to the NACA lift curve charts at a Reynolds number of 3 million. Barring additional variables, this suggests that the 4415 root section will stall 2° before the 4418 tip section. Both sections have similarly gentle looking stall breaks, though the 4418 has a slightly more gentle one. The important thing here, with respect to the statement that I made above in ccg's thread, is that the NACA 4418 tip section actually has a slightly LOWER Clmax. "Stick and Rudder" constantly states that what is imprtant to how the airplane flies is the ANGLE OF ATTACK and not any other parameter. Regardless of the lift coefficient, the airfoil will stall at its stalling AoA. For washout, this section's stalling AoA needs to be higher than that of the root sections stalling AoA.My belief at present is that there needs to either be geometric washout or that the outer panel section needs to have a higher Clmax,...
If the tip section is changed for the much thinner NACA 2412 section, the tip stall angle is now approximately 15°, but the stall break is much more abrupt due, I am guessing, to the thinner airfoils sharper leading edge radius. The symmetrical NACA 0012 also stalls at an AoA of 15° but it too has a very abrupt stall break.
Billksi has stated many times that thinner wing tip airfoils are not a good idea, and I'm just now beginning to grasp the reason why. There is way more to this, though, including the influence of the spanwise lift distribution. Orion has also talked about this many times.
I can't explain everything there is to know (or much of anything, actually) about aerodynamic washout, but its effect on handling and safety is obviously very important to anyone who will design and fly an aircraft.
It would be nice if there were a sticky with a good concise explanation of aerodynamic washout, considering the importance of this subject to safety.