Stability and control through induced drag reduction

The energy of wake vortex is created by heavier-than air flight, not by anything we design or put on an airplane, and it is 100% lost to us unless we can make use of it for something. In the 1970s Burt Rutan did so marvelously with the first-ever application of Whitcomb’s non-planar winglet technology. Being vertical stabilizers located behind the CG, Rutan winglets create yaw stability while reducing the energy of wake vortex.

Let’s take it a step farther. The swirl of wake vortex acts in a 360 degree circle. We can put airfoils in it as we like, and depending on where they are located and how they are oriented, we can redirect the molecules we had previously whipped off into a frenzied spin and use them to create stabilizing forces… while leaving them slightly less energized than they were when we slammed into them. The ‘cost’ of doing this at the wing tip is therefore mostly ‘paid for’; whereas on most aircraft we pay double: first we throw air at the ground, paying an induced drag penalty, then we catch some of it (right in the middle of the downwash, where we tried to move it the fastest) and pay another induced drag penalty at the tail while we reduce the lift we just made. Even when we minimize the downforce of a tail, going faster brings it right back into focus because our most efficient airfoils have a strong negative pitching moment.

After decades of study, I have discovered that the ideal place for a tail is connected to a wingtip by a tall, shared vertical winglet. In this location a deliberately strong negative lift from the tail can achieve its stabilizing action by reducing the strength of wake vortex, while constructively interacting with the other flight surfaces from the proper safe distance. Its downward force reduces wing bending moment, and in a swept wing design, counters wing twist.

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