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Answer: We will surely look into your suggestions. thanks
Answer: There is no difference in derivation in the two books. You should interpret them correctly. One can refer to any of the two books, my personal favorite is Perkins among these two.
Answer: You should not play with weight. L = W is valid in level flight at any altitude. What you have to do is, keep weight constant (unless you want to account for consumed fuel weight in going from sea-level to 11km), and take the correct density of air at that altitude and then find the velocity that would be required to maintain the level flight condition. Change should be found in velocity and thus in thrust required and not in the weight.
Answer: The aerodynamic center of an airfoil is the point about which there is no change in pitching moment coefficient with respect to the angle of attack. That is mathematically saying, dCm/dalpha=0. The resultant aerodynamic load (or the lift you may want to call it) due to pressure distribution over the bottom and the top surface of the airfoil acts at a point (center of pressure, about which moment is zero) on the airfoil which lies behind the aerodynamic center of a positively cambered airfoil resulting in a negative pitching moment about the aerodynamic center. As the negatively cambered airfoil is nothing but an inverted positively cambered airfoil, exactly opposite happens here.
