Mach buffet is an aerodynamic phenomenon characterized by a distinct vibration or shaking of an aircraft, primarily occurring during high-speed flight when approaching or exceeding the speed of sound (Mach 1). It arises specifically when airflow separates on the upper surface of a wing behind a shock wave, leading to turbulent air and a noticeable buffeting sensation.
Understanding the Phenomenon
At slower speeds, air flows smoothly over an aircraft's wings, generating lift. As an aircraft accelerates to higher speeds, especially into the transonic range (roughly Mach 0.75 to Mach 1.2), the airflow over certain parts of the wing, particularly the upper curved surface, can accelerate to supersonic speeds even when the aircraft itself is still subsonic.
This local supersonic flow eventually must slow down to subsonic speeds again as it moves towards the trailing edge of the wing. This rapid deceleration creates a shock wave – a sudden, drastic change in air pressure, density, and temperature.
The crucial aspect of Mach buffet is that the air directly behind this shock wave loses a significant amount of its kinetic energy. This energy loss can cause the airflow to separate from the wing's surface, creating a turbulent, chaotic wake instead of smooth flow. This turbulent wake impinges on the tail and other parts of the aircraft, resulting in the characteristic shaking or "buffeting."
Factors Influencing Mach Buffet
The strength of the Mach buffet is directly related to the characteristics of the shock wave and the extent of airflow separation.
- Shock Wave Strength: All other things being equal, shock wave strength increases as the local airflow speed ahead of the shock wave increases. A stronger shock wave leads to greater energy loss in the airflow behind it, making airflow separation more severe and the buffet more pronounced.
- Aircraft Speed: As an aircraft accelerates, the shock waves become stronger and move rearward on the wing, intensifying the buffet.
- Altitude: At higher altitudes, the air is thinner, meaning a higher true airspeed is required to reach a given Mach number. This affects the onset and severity of Mach buffet.
- Aircraft Design: Wing sweep, airfoil shape, and wing loading all influence how and where shock waves form, and thus, the aircraft's susceptibility to Mach buffet. Modern aircraft use designs like swept wings and supercritical airfoils to delay or minimize the onset of strong shock waves and mitigate buffet.
Types of Buffet
While Mach buffet specifically refers to the high-speed phenomenon, it's often discussed in contrast to or in conjunction with other types of buffet:
| Type of Buffet | Primary Cause | Typical Speed Regime | Characteristics |
|---|---|---|---|
| Mach Buffet | Airflow separation behind a shock wave | High-speed (transonic) | Vibrations due to shock wave formation and airflow breakdown |
| Pre-Stall Buffet | Airflow separation from the wing's leading edge | Low-speed (approaching stall) | Vibrations signaling an impending aerodynamic stall |
| Buffet Margin | The speed range between Mach buffet onset and stall buffet onset | Both | Critical for safe flight envelope management |
Operational Implications and Mitigation
Mach buffet serves as a natural warning sign to pilots that they are approaching the aircraft's operational limits. It indicates that the aircraft is nearing its critical Mach number, where aerodynamic efficiency decreases significantly, and structural loads can become excessive.
- Pilot Action: Upon experiencing Mach buffet, pilots typically respond by slightly reducing airspeed or adjusting altitude to move out of the critical buffet region.
- Flight Envelope: Aircraft operate within a defined flight envelope, which includes limits to avoid severe Mach buffet. Exceeding these limits can lead to dangerous situations, including loss of control or structural damage.
- "Coffin Corner": In high-altitude, high-speed flight, there's a point known as the "coffin corner." This is where the indicated airspeed for Mach buffet onset (high-speed limit) and the indicated airspeed for stall (low-speed limit) converge. The safe operating window for the aircraft becomes very narrow, demanding precise control.
- Aircraft Design: Designers incorporate features like wing sweep to increase the critical Mach number and allow for higher cruising speeds before buffet onset. Modern commercial airliners are designed to cruise comfortably at Mach numbers just below their buffet threshold.
Mach buffet is a crucial consideration in the design and operation of high-speed aircraft, representing a key aerodynamic boundary that pilots must respect for safe and efficient flight.
