Shock Stall (Supersonic Aerodynamics)
Overview
A shock stall in supersonic aerodynamics is a stall induced by shock waves that form when airflow over an aircraft's wings is disturbed at or above the aircraft's drag divergence Mach number, leading to a decrease in lift and increased drag.
Supersonic Flight and Shock Waves
When an aircraft flies faster than the speed of sound (supersonic), it creates shock waves, which are regions of compressed air.
Transonic Regime and Drag Increase
As an aircraft approaches and passes through the transonic regime (near the speed of sound), shock waves can form over the wings, leading to a sharp increase in drag, known as "wave drag".
Shock Stall Mechanism
A shock stall occurs when the airflow over the wings becomes unstable due to these shock waves, causing the boundary layer (the thin layer of air near the wing surface) to separate. This separation leads to a sudden loss of lift and a significant increase in drag, which can be dangerous.
History
Early Problems
Early supersonic aircraft experienced unexpected and dangerous behavior, including loss of control, due to shock stalls.
Ralph Virden's Fatal Accident
A notable example is the 1941 crash of a test pilot, Ralph Virden, who lost control of his aircraft due to a shock stall, highlighting the need for understanding and mitigating this phenomenon.
Kelly Johnson and the Investigation of Compressibility
Engineers like Kelly Johnson began investigating the effects of compressibility on aircraft, which is crucial for understanding supersonic flight and shock stall.
Hugh Dryden and Theodore von Kármán
The term "transonic" was coined by NACA director Hugh Dryden and Theodore von Kármán of the California Institute of Technology to describe the region around the speed of sound.
Mitigation Strategies
- Supercritical Airfoils: These airfoils are designed to delay the onset of shock waves and boundary layer separation.
- Swept-Back Wings: Swept-back wings help to delay the onset of supersonic flow over the wings, thus postponing the formation of shock waves.
- Wing Fences: Wing fences can prevent the entire wing from stalling at once.
- Stick Shaker and Stick Pusher Systems: These systems provide warnings and assist in recovery from stalls, especially in aircraft using supercritical wings.
