Aeroelasticity
Study of interactions between inertial, elastic, and aerodynamic forces.
Aeroelasticity is a field of physics and engineering that examines how inertial, elastic, and aerodynamic forces interact when a flexible body moves through a fluid. It splits into two main areas: static aeroelasticity, which looks at steady or unchanging responses, and dynamic aeroelasticity, which focuses on vibrational or time-varying behavior. Because aircraft must be light yet withstand heavy aerodynamic loads, they are especially susceptible to aeroelastic effects. Engineers design planes to avoid three key problems: divergence, where aerodynamic forces twist a wing further, increasing those forces in a loop; control reversal, where moving a control surface creates an opposite aerodynamic moment that weakens or reverses its intended effect; and flutter, an uncontrolled vibration that can destroy the aircraft. These issues are prevented by adjusting a structure’s mass, stiffness, or aerodynamics, verified through calculations, ground vibration tests, and flight flutter trials. Control surface flutter is typically stopped by carefully placing mass balances. When aeroelasticity is combined with thermodynamics, it becomes aerothermoelasticity; combined with control theory, it is called aeroservoelasticity.
Historically, the second failure of Samuel Langley’s prototype plane on the Potomac River was blamed on aeroelastic effects—specifically, torsional divergence. George Bryan published an early scientific work on the subject, *Theory of the Stability of a Rigid Aeroplane*, in 1906. During the First World War, torsional divergence caused repeated problems, which were solved largely through trial-and-error and ad hoc wing stiffening. The first recorded case of aircraft flutter occurred in 1916 on a Handley Page O/400 bomber, which experienced a violent tail oscillation that severely distorted the rear fuselage and caused the elevators to move unevenly. Though the plane landed safely, F. W. Lanchester was consulted during the investigation. He recommended rigidly connecting the left and right elevators with a stiff shaft, a requirement that later became standard. The National Physical Laboratory (NPL) then asked Leonard Bairstow and Arthur Fage to study the phenomenon theoretically. In 1926, Hans Reissner published a theory of wing divergence, sparking further theoretical research.
- Fields
- static aeroelasticity, dynamic aeroelasticity
- Key problems
- divergence, control reversal, flutter
- Prevention methods
- adjusting mass, stiffness, or aerodynamics; calculations, ground vibration tests, flight flutter trials
- Related syntheses
- aerothermoelasticity (with thermodynamics), aeroservoelasticity (with control theory)
- First documented flutter case
- Handley Page O/400 bomber, 1916
- Term coined by
- Harold Roxbee Cox and Alfred Pugsley at the Royal Aircraft Establishment (RAE), Farnborough, early 1930s
Lore & Background
The second failure of Samuel Langley's prototype plane on the Potomac was attributed to aeroelastic effects, specifically torsional divergence. An early scientific work was George Bryan's Theory of the Stability of a Rigid Aeroplane published in 1906. Problems with torsional divergence plagued aircraft in the First World War and were solved largely by trial-and-error and ad hoc stiffening of the wing. The first recorded and documented case of flutter occurred to a Handley Page O/400 bomber in 1916, when it suffered a violent tail oscillation; F. W. Lanchester recommended that left and right elevators be rigidly connected by a stiff shaft, which became a design requirement. The National Physical Laboratory (NPL) investigated the phenomenon theoretically, carried out by Leonard Bairstow and Arthur Fage.
In 1926, Hans Reissner published a theory of wing divergence, leading to much further theoretical research. The term aeroelasticity was coined by Harold Roxbee Cox and Alfred Pugsley at the Royal Aircraft Establishment (RAE), Farnborough in the early 1930s. At Caltech, Theodore von Kármán started a course 'Elasticity applied to Aeronautics,' which he passed to Ernest Edwin Sechler, who developed aeroelasticity in that course and in textbooks. In 1947, Arthur Roderick Collar defined aeroelasticity as 'the study of the mutual interaction that takes place within the triangle of the inertial, elastic, and aerodynamic forces acting on structural members exposed to an airstream, and the influence of this study on design.'
Static aeroelasticity includes divergence, where elastic twist of a wing becomes theoretically infinite, and control reversal, where control surfaces reverse their usual functionality. Dynamic aeroelasticity includes flutter, a dynamic instability caused by positive feedback between body deflection and fluid force; it can be hard or soft flutter. Propeller whirl flutter is a special case involving rotating propeller and nacelle stiffness. Transonic aeroelasticity is dominated by moving shock waves, and a phenomenon called 'transonic dip' can cause flutter speed to approach flight speed. Buffeting is a high-frequency instability caused by airflow separation or shock wave oscillations.
Reader's Guide
Aeroelasticity is significant because it directly impacts aircraft safety and design. The article notes that aircraft are designed to avoid divergence, control reversal, and flutter, which can lead to destruction. These problems can be prevented by adjusting mass, stiffness, or aerodynamics, verified through calculations, ground vibration tests, and flight flutter trials. Flutter of control surfaces is usually eliminated by careful placement of mass balances. The article cites historical failures, such as the second failure of Langley's prototype plane and the destruction of the original Tacoma Narrows Bridge, as consequences of aeroelastic effects. The synthesis of aeroelasticity with control theory (aeroservoelasticity) has demonstrated that automatic control systems can help prevent or limit flutter-related vibration. The legacy of aeroelasticity includes its formal definition by Collar in 1947 and its continued relevance in designing lightweight aircraft structures that endure large aerodynamic loads. The article also notes that even changing mass distribution or stiffness of one component can induce flutter in an apparently unrelated component, underscoring the complexity of the field.
Did You Know?
- The first documented case of flutter occurred on a Handley Page O/400 bomber in 1916, causing violent tail oscillation.
- The original Tacoma Narrows Bridge was destroyed as a result of aeroelastic fluttering.
- Propeller whirl flutter caused failures on two Lockheed L-188 Electra aircraft in 1959 and 1960.
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