Aerodynamic Lift
No One Can Explain Why Planes Stay in the Air
In December 2003, the New York Times published an article commemorating the Wright brothers' centennial, asking a fundamental question: What keeps planes airborne? When interviewed, John D. Anderson Jr., curator of aerodynamics at the National Air and Space Museum, provided a surprising answer: "There is no simple one-liner answer to this." Over 15 years later, multiple competing theories still exist, each defended with considerable passion by experts.
The challenge exists across two levels of explanation. Technical mathematical analysis successfully predicts aircraft behavior through complex equations and computational models. However, providing an intuitive, physical explanation remains contentious. This gap between mathematical accuracy and conceptual clarity generates ongoing debate among aerodynamicists.
Two Competing Theories
Bernoulli's Explanation
Daniel Bernoulli's 1738 principle states that fluid pressure decreases as velocity increases. Applied to wings, this theory suggests air moves faster across the curved upper surface, creating lower pressure and generating lift.
However, Bernoulli's principle has significant limitations. It fails to explain why air accelerates over curved surfaces. The "equal transit time" theory — suggesting air parcels must reunite at the trailing edge — contradicts empirical observation. Additionally, aircraft with symmetrical or flat airfoils successfully fly inverted, contradicting predictions based solely on Bernoulli's principle.
A common classroom demonstration proves problematic: blowing across a curved paper sheet makes it rise, yet blowing across a flat vertical sheet produces no movement, despite obvious velocity differences. This reveals that pressure differences alone don't fully explain the phenomenon.
Newton's Third Law Approach
The alternative explanation relies on Newton's principle of action and reaction: wings push air downward, generating equal upward force. This approach applies universally to all wing shapes and flight orientations, addressing limitations inherent in Bernoulli's explanation.
However, Newton's third law alone cannot explain the lower pressure region above the wing that persists regardless of wing shape. This pressure difference remains an inescapable component requiring separate explanation.
Historical Context
Neither Bernoulli nor Newton attempted explaining aviation — they preceded mechanical flight by centuries. When the Wright brothers' achievement necessitated understanding aerodynamic principles, scientists initially applied simplified models assuming incompressible, frictionless fluids.
Notably, Albert Einstein contributed to early aerodynamic theory. Publishing "Elementary Theory of Water Waves and of Flight" in 1916, Einstein proposed explanations assuming ideal fluids. He subsequently designed the "cat's-back wing," resembling a stretched cat's back. A test pilot reported the resulting aircraft "waddled like a pregnant duck." Einstein himself later called this "youthful folly."
Contemporary Understanding
Modern computational fluid dynamics employs the Navier-Stokes equations, accounting for air's actual viscosity. These mathematical solutions accurately predict pressure distributions and airflow patterns, yet remain fundamentally equations rather than intuitive explanations.
Aerodynamicist Doug McLean proposed a more comprehensive approach. His explanation identifies four interdependent components:
- Downward airflow turning
- Increased airflow speed
- Lower pressure regions
- Higher pressure regions
Crucially, McLean emphasizes these elements exist in reciprocal cause-and-effect relationships: they support each other and none would exist without the others. This mutual reinforcement, driven by Newton's second law, creates self-sustaining pressure dynamics when wings move through air.
Remaining Questions
Mark Drela, Massachusetts Institute of Technology professor, explains that air parcels follow wing curvature because a vacuum would be created if they separated. This partial vacuum pulls parcels downward, maintaining airfoil contact and creating the reduced pressure region. As a side effect, this suction accelerates approaching air upstream.
However, complete disagreement persists. Cambridge aerodynamicist Holger Babinsky disputes this explanation, noting flow sometimes separates from surfaces despite vacuum theory predictions. He acknowledges: "There is no quick and easy explanation."
Despite a century of aviation and sophisticated mathematical frameworks, scientists cannot provide a universally accepted, intuitive explanation of aerodynamic lift. Multiple valid perspectives exist, each explaining certain aspects while leaving others unclear. The phenomenon remains understood mathematically and operationally, yet resists simple physical explanation — a remarkable gap in scientific understanding given that evolution solved this problem millions of years ago through bird flight.