Takeaways from “So you think you know lift?”

April 2023

Our discussion in the February issue about aerodynamic lift with Paul Bevilaqua and Haitham Taha sparked some ideas from Associate Fellow Aaron Altman of Dayton, Ohio. Here are excerpts from his letter:

“I have struggled to explain how lift is generated for over 30 years,” Altman writes. “That journey has provided some additional thoughts and comments related directly to the discussion in the article.”

Regarding viscosity, Altman points to a 1959 Ph.D. dissertation by Paul Palmer Craig, “Observations of perfect potential flow and critical velocities in superfluid helium II.” The paper discusses an experiment in superfluid helium, which Altman explains “very closely approximates the behavior of an inviscid fluid,” meaning one with negligible viscosity. “The dissertation centered on the measurement of lift force on a flat plate twirled at angle of attack in this near inviscid fluid. The result for the lift force measured in the near-absence of viscosity? Negligible measurable lift force!”

But, he adds, “The characterization of the Wright Brothers as not working from an operable theory deserves gentle refute. It is precisely because they were successful in determining the error in the best operable theory of the time (taught to them through the work of [Octave] Chanute and [Otto] Lilienthal), that they were successful in getting the vehicle to fly for those 59 seconds. It’s worth noting that they flew for considerably longer within a few years of that first flight, using the same operable theory and their experimentally determined correction to the error in the Smeaton coefficient that had endured for close to 150 years. It was through their multiple early failures that they determined something must have been wrong with the accepted theory of the time. A highly significant contribution to the theory of the time!”

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