The physics of producing lift with a wing creates an aircraft wake which has a large vortex (tornado) generated at each wingtip. The strength of this vortex increases as aircraft weight increases. The flow from a wingtip of a crop duster picks up the seeds or chemicals and sends them far from the area directly below the aircraft. The strong spinning flow in the vortex can also cause a problem for other aircraft under some conditions. NASA and the FAA had done extensive research beginning in the 1970s and continuing today to understand and predict flow behind transport aircraft to determine safe separation distances (spacing of aircraft). We believed that this research could help the "Aerial Applications Research" program, perhaps, if the physics of the flow behind crop dusters could be understood, predicted, and tamed.
Because we see computer images on TV of airplanes the size of counties flying over a map of the U.S., we come to think of the airspace as crowded. In reality, the airspace is not crowded. Instead, the runways and the arrival queue of airplanes lined up to land are crowded. The airplanes are spaced in the queue that keeps each safely following an airplane away from the spinning wake vortex of the previous airplane.
Before this picture was taken, the NASA researchers involved pulled together all of the known theory and experimental information about the vortex, with the idea of developing a computer program to predict this flow pattern. (This was in the very early days of computing -- think magnetic tape.) Along the way we met with a collection of interesting folks including the holder of a patent for "Distributor Wing Aerial Applications Aircraft." Here is the original patent.
The idea in this patent was to blow air and seed or fertilizer through channels inside the wing and use the vortex to spread the materials on a wider swath. This intriguing concept reached a limited level of fruition in Russia in the form of the Polish-built PZL (Milek) M-15, turbofan-powered agricultural aircraft.
Our research led to the work with a startup computational methods company in Princeton, NJ, Continuum Dynamics, Inc., to develop a comprehensive CFD code for predicting where spray and seeds would fly behind the wing of a crop duster. That computer work led to the tests at NASA's Wallops Flight Research Center, where we took this picture, to prove to ourselves that the computer method worked. It did.
In the photo, underneath the flight path, plastic pipes are visible on which we rolled out sticky tape to collect glass beads of varying sizes that were released from movable canisters located under the wing. The results of the testing proved that the computer was an excellent tool for the task of designing spray systems on aircraft.
That task resulted in the most famous vortex flow picture of all time (perhaps a little over-stated), shown here. This photo shows up virtually anywhere the subject of wake turbulence, wake vortex, and aircraft wakes is discussed. We never imagined the value of the photo back then.
And now, as my fellow Minnesotan, Paul Harvey, used to say, you know "the rest of the story."
Bruce J. Holmes, retired from his NASA career in public sector entrepreneurialism, is now practicing the art in the private sector as CEO, NextGen AeroSciences.
Image: NASA Langley Research Center.