Aerodynamics Flashcards
What is a Mach Number?
The ratio of TRUE AIRSPEED of the aircraft to the speed of sound in the same atmospheric conditions.
What is VMO
Maximum operating speed expressed in terms of a decimal of Mach speed.
What is the critical Mach number?
The Mach speed at which some portion of the airflow first equals Mach 1.0. This is also the speed at which the shockwave first appears on the airplane.
What is Mach buffet?
As a transonic airplane reaches supersonic speeds, airflow separates over the wings and leads to a buffet.
What is Mach tuck?
An aerodynamic phenomenon that occurs when an aircraft approaches transonic speeds, near Mach 1. As the aircraft reaches this speed, shock waves form on the wings and cause the center of pressure (the point where aerodynamic forces are centered) to move rearward. This rearward shift creates a nose-down pitching moment, causing the aircraft to pitch downward uncontrollably.
To counteract Mach tuck, modern aircraft are equipped with design features like mach trim systems and speed brakes, and pilots are trained to manage transonic flight carefully, particularly with attention to trim and speed.
What is coffin corner?
A flight condition at high altitudes where an aircraft is operating near both its stall speed and its critical Mach number (the speed where shock waves begin to form, leading to potential Mach buffet). At this altitude, the margin between the two speeds is very narrow.
In coffin corner, an aircraft is at risk of either:
- Stalling if its speed decreases, due to the thin air reducing lift,
- Or experiencing Mach buffet if its speed increases, due to the formation of shock waves on the wings.
This delicate balance between high-speed buffet and low-speed stall makes this flight regime particularly dangerous. Aircraft in coffin corner require careful speed management to avoid these risks, as any change in airspeed or altitude could lead to loss of control.
What is the critical Mach number?
The critical Mach number (often denoted as Mcrit) is the lowest Mach number at which some portion of the airflow over an aircraft’s wing or other surfaces reaches the speed of sound, even though the aircraft itself is flying below Mach 1. At this point, shock waves can start to form on the aircraft, particularly on the wings, resulting in an increase in drag (called wave drag), and potentially causing buffeting and a reduction in control effectiveness.
In practical terms, the critical Mach number marks the onset of transonic flight. Once an aircraft exceeds its critical Mach number, significant aerodynamic effects, such as Mach buffet and compressibility issues, become more pronounced. Designers of high-speed aircraft aim to increase the critical Mach number through aerodynamic improvements like swept wings or supercritical airfoils to delay the onset of these effects.
What is “Coffin Corner”
At a jet’s operating ceiling, Maximum Mach Number (MMO) is often close to its stall speed. Get too slow and you’ll stall; get too fast and you’ll your critical Mach number.
