Why is CL/Cd equal to L/d? I'm making a school project where I intend to design and calculate an rc airplane. Reading some books I found something I can't understand, it says that CL/Cd is equal to L/d, but the only possible way to make that true is to use the same surface on Drag and Lift equations. Isn't drag equation supposed to use frontal surface and Lift equation to use wing surface?
The frontal surface is indeed used for the drag coefficient of cars.
For airplanes, however, almost* all coefficients use the wing area as their reference. This makes sense exactly because it allows to express L/D as c$_L$/c$_D$.
* For the nitpickers: Control surface hinge moment coefficients use the area of their respective control surface, not the wing surface.