I am a 9th grader doing a science fair project at my local high school. I am doing a science experiment on different plane wings and how the angle of attack affects drag. I need to create a rig to measure the drag created by lift(induced drag). I don't know if the spring scale(the tool to measure the drag) should be connected to the front of the model plane should and stay level with the plane as the angle of attack increases or should the spring scale stay level with the horizon as the plane itself only changes its angle of attack to correctly measure induced drag. Picture attached below(sorry for pathetic attempt to recreate question).
Lift and drag are defined relative to the direction of flow ahead of the airplane. Lift is orthogonal and drag parallel. Therefore, the lower picture with the parallel spring shows the better arrangement, because it will just measure drag, nothing else. Note that it will measure the sum of induced and viscous drag, so you still need some corrections to isolate the induced drag once you have measured the force.
In a first-order approximation, viscous drag is constant over angle of attack, so you measure the minimum force while varying the angle of attack and subtract that force from all other measurements.