Final answer:
Drag rise or drag divergence is an aerodynamic phenomenon where drag increases sharply as an aircraft nears the speed of sound, leading to Mach tuck, a condition that causes the aircraft's nose to pitch down. Mmo is the Maximum Operating Mach Number, which is set to prevent these conditions. It's favorable that Mach tuck occurs below Mmo to allow for safe flight operations.
Step-by-step explanation:
Drag Rise and Drag Divergence
Drag rise, also known as drag divergence, is a phenomenon observed in aerodynamics where the drag on an aircraft dramatically increases as its speed approaches the speed of sound. As an aircraft's speed increases towards the speed of sound (Mach 1), the air pressure waves can no longer move out of the way quickly enough, causing these waves to compress and form shock waves, leading to a sharp rise in drag.
Mach Tuck
Mach tuck is an aerodynamic effect that occurs when an aircraft approaches the speed of sound and the center of pressure on the wing moves rearward. This causes the nose of the aircraft to pitch down, which can be dangerous as it may result in a loss of control if not corrected. Mach tuck is related to the critical Mach number, where this behavior starts becoming significant.
Maximum Operating Mach Number (Mmo)
Mmo, or Maximum Operating Mach Number, is the highest speed an aircraft is certified to fly under normal operating conditions. It is set below the speed where undesirable effects, such as Mach tuck or control issues due to compressibility effects, become significant. Operating below Mmo is favorable to ensure safety and to prevent any potential aerodynamic issues like Mach tuck.
It is certainly favorable that Mach tuck occurs below Mmo because this allows pilots the margin to react and recover before the aircraft reaches a speed where the effect becomes uncontrollable.