In static bending tests, I use normal and tangential forces to get tip defle ction without converting them to lift an drag.
Is it incorrect?
You've to define what normal/tangential forces directions are. The airload (lift/drag) distribution along the blade is required to obtain the final tip deflection (which may not remain in either plainly blade normal or chordwise direction, but will be in a plane which could be normal to blade elastic axis) and this is valid for unsteady as well as static blades. For a uniformly distributed lift load along z-direction on a static blade, for example, the tip deflection will be along z-direction. There are a lot of example of this in literature (see link below).
http://www.engineersedge.com/beam_calc_menu.shtml
The simple answer to your question: It should be correct if the resulting force vector and axis (center of action) is equal. The coordinate system used has no influence on the result since it is anyhow an artificially defined breakup of one force into linear independent components.
The complicated answer: Simply taking the integral lift and drag and applying it at a discrete position at the blade will not achieve the same deformation as for the real load under surface distributed aerodynamic forces. But you may validate a structural model of the blade by this that you can feed into a CFD-CSM coupled simulation.
It is not an easy task to model the aerodynamic forces acting on an aerodynamic body by distributed discrete forces. My suggestion would be to at least apply multiple forces in spanwise direction to model the spanwise variation of loads (best obtained by partial integration of pressure and friction forces). You can estimate the error by evaluating the acting forces and the resulting bending energy (which will not be able to be matched exactly at the same time).
If you know normal and tangential forces (to the rotor disc? To the section?) you can directly use them (be careful to express forces in the same reference of the structural equations, there are only geometric relations). But often you don't know normal and tangential forces and instead you can evaluate drag and lift with some kind of model. It is possibile to pass from lift and drag to normal and tangential, but it requires not only geometric relations but also the knowledge of inflow (induced by wind and wake) over the disc. Here you can find some useful informations:
Book Principles of Helicopter Aerodynamics
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