High speed aero

Question 1

Basics

Answer 1

● At standard temperature, the speed of sound at sea level is about 660 knots or 761.2 MPH.
● At higher altitudes or in the areas of extremely cold weather even at sea level, the speed of sound decreases.

Question 2

Mach Number

Answer 2

● These categories are:
○ Subsonic Flight – Mach 0 - about Mach 0.75
○ Transonic Flight – Mach 0.75 – Mach 1.2
○ Supersonic Flight – Above Mach 1.2 (at this point, the airflow over all of the aircraft’s surfaces will be above the speed of sound
● It is important to note that aircraft are grouped not by the speed of the aircraft itself, but by the speed of the airflow over the surface of the aircraft

Compressibility
● At low speeds, air molecules move on either side of objects moving through them.
● At high speeds they start to pile up increasing pressure and temperature.

Question 3

Transonic Speeds

Answer 3

● As the aircraft moves into transonic speeds, the air accelerating over parts of the wing becomes supersonic.
● Parts of the aircraft start moving through the air faster than the air molecules can move around it.
● This results in a massive increase in air density and a shock wave forms at the boundary between the disturbed and undisturbed air

Shock-wave
● As air passes through a shock-wave:
○ Pressure increases
○ Temperature increases
○ Velocity decreases
● Supersonic flow is one of compressibility (pressure changes at high speed are large and significant)

High-Speed Buffet
● As the aircraft accelerates, the shock wave moves aft and separates the airflow behind it.
● This disturbed air starts to affect the horizontal stabilizer and can be felt in the controls as a High-Speed Buffet similar to a stall.
● High-speed buffets can happen at different Mach numbers depending on altitude and aircraft loading

Question 4

Coffin Corner

Answer 4

● Remember that our stall speed increases with altitude.
● At high altitudes and high weight, stall speed starts to approach Mmo max mach operating speed.
● The small area between stall speed and Mach limit is known as Coffin Corner. At high enough altitude, stall speed and mmo will meet

Buffet margin
● Optimum altitude is reduced by an increase in OAT.
● If an increase in temperature is encountered while in level flight, buffet margins will be reduced and a descent may be necessary

Question 5

IAS/Mach No./TAS in Climb

Answer 5

● During the initial part of the climb, the crew will fly the aircraft referenced to a fixed IAS.
● This causes TAS and Mach number to increase as altitude is gained. (Refer to table)
● When “Changeover Level” is reached, the flight crew will thereafter climb the aircraft referenced to a fixed Mach number until Top of Climb is reached

● Climbing at a fixed Mach number will cause TAS and IAS values to decrease.
● The TAS decreases because the aircraft is climbing into ever colder air.
● The Speed of Sound (SoS) varies according to air temperature, such that it is slower in cold air and faster in warm air.
● When climbing at a fixed Mach number we are are flying at a certain percentage of the speed of sound.
● Because the SoS is decreasing with altitude gained, so TAS will decrease also