I would like the lecture videos to be of either MIT/ Stanford/Harvard/ UCLA. The lectures should be dealing with derivations of equations of motion using Euler- lagrange methods.
I've used the first edition of Seddon's book to teach some basic helicopter aerodynamics. I recommend it thoroughly for this purpose. But there is no explicit use of the Euler - Lagrange equation in the book (first or second edition).
I do not know any derivation of helicopter motion equations by Euler Lagrange methods, but I think that vehicle equations of motion derived with enough generality are valid for helicopters. The problem is in modeling the dynamics of the rotor (or rotors) and coupling them with that of the vehicle. Please let me recommend you my book "Basic Equations of Flight Dynamics" for a derivation with this degree of generality. I hope this is not offensive for you.
Vitalii Pertsevyi: I consider that you souhld, at least, inform correctly about the content of your links. One can think that you have not even open the files. First link is for first edition and second link is second edition of Seddon's book. The error in the name of the file is spreading on the net precisely because people are sharing links only for the title.