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). enter image description here

  • $\begingroup$ The second picture. Of course, you will measure both induced drag (due to lift) and parasitic drag (due to wingtip vortices). The only way I know of to reduce parasitic drag is to give the wing a high aspect ratio (which is what gliders do). $\endgroup$ – Mike Dunlavey Oct 16 '16 at 20:37
  • $\begingroup$ @MikeDunlavey: With all respect, the bigger aspect ratio will reduce induced drag (and lower the forces overall, so there is relatively more noise). The best one can do is to assume a constant amount of friction drag (measured when no lift is created) and to subtract this amount from all measurements. $\endgroup$ – Peter Kämpf Oct 17 '16 at 7:38

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.

  • $\begingroup$ Thank you very much for the quick response but I do believe I vaguely described the changing variables. The Airflow will stay the same but the angle of attack and the winglet design will change. My project is about winglets and how different vertical length(winglet span) will affect the induced drag created by the wingtip vortices. I do understand that I might be interpreting this wrong, you do have a lot more experience but I feel that it is my fault that I didn't get the answer I really wanted. $\endgroup$ – Olek410 Oct 18 '16 at 4:36
  • $\begingroup$ Part 2: I don't see how viscous drag will matter when the lifting wing shape will stay the same(I know I did not specify this). I guess it could effect the small winglet surface but it should stay overall constant with different winglets will be added to the same lifting wings. I know that viscous drag will affect the winglet surface but the thickness and overall profile shape will stay the same for the winglets with just different lengths. While I could be wrong you have studied most of these topics a lot more than me. $\endgroup$ – Olek410 Oct 18 '16 at 4:42
  • $\begingroup$ @Olek410: Your question nowhere mentioned winglets, so of course my answer did not, too. With winglet span you will change the surface and thus the viscous drag. Not much, but enough that it should be measured for each configuration separately. Now I need to ask what your question actually is - until now I thought you were not sure in which direction to measure drag. Could you please me more clear? $\endgroup$ – Peter Kämpf Oct 18 '16 at 13:37

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