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Any vortex formed in the airstream produces drag. A vortex generator is, by definition, a disruption of the airflow. I do not doubt, however, that in the case of the door deletion kit that the increase in drag produced using small, controlled vortices is less than that produced by the large, uncontrolled ones which would originate at the sharp wheel well edges and total drag could in fact be less than that produced by the original fuselage form as it wraps around the rear of the aircraft. I do not wish to appear petulantly argumentative but trying to measure such a small change in total drag with an original equipment airspeed indicator would be like trying to measure the diameter of human hair with a yardstick. And on two different airplanes, no less. Such a test has not even begun to control for all of the variables involved. Full scale wind tunnel and super computer might predict an approximate delta drag.
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