Effects of Compressibility

Question 1

What is it

Answer 1

● For higher speeds, some of the energy of the aircraft goes into compressing the air.
○ This, of course, is going to change the density of the air. It will increase!
○ With this increase in density, there is also a rise in both the temperature and pressure of the air

● For the pilot, compressibility will be noticed in two major respects:
○ 1) Compressibility will affect the aerodynamics and therefore the handling and flight characteristics of the aircraft.
○ 2) Compressibility will have an effect on determining the true airspeed of the aircraft

Question 2

Compressibility and Aerodynamics

Answer 2

● Compressibility in relation to the handling and aerodynamics is best understood by thinking about the shock wave that forms over the top of the wings as an airplane reaches and passes through Mcrit

Force Divergence Mach Number
● If an aircraft is allowed to accelerate past Mcrit, to speeds about 5% greater than this value, then it reaches what is known as its Force Divergence Mach Number.
○ At this speed, the high speed airflow aft of the shockwave will separate from the wing

● Some of these aerodynamic events are as follows:
○ Uncommanded rolling
○ Aileron or control buzz
○ Buffeting
○ Tuck under

Question 3

Uncommanded Roll

Answer 3

● If the aircraft was trimmed utilizing the aileron trim in slower speed flight, higher speeds may cause a roll due to the sudden increase in air density, or,
● Due to the separated airflow above Mcrit, the trim tab may have limited airflow–and therefore limited effect

Aileron or Control Buzz
● Since the ailerons will likely be in the separated and turbulent airflow aft of the shockwave, they may begin to “buzz.”
● They will also lose their effectiveness, as they are effectively sitting within a stalled airflow

Buffeting
● A continued increase in speed may start a high speed buffet of the aircraft.
● This is due to the turbulent airflow aft of the shockwave.
● Most high speed airplanes have a T-Tail to help offset this effect.
○ Conventional tails will sit in the turbulent airflow, causing greater degree of buffeting

Tuck Under
● Tuck under is the pitching down of the aircraft as it continues to increase its speed above Mcrit.
● This happens for two reasons:
○ Loss of downwash over the tail, resulting a tail up motion–which of course, means nose down!
○ A change in the center of pressure towards the trailing edge of the wing as the shock wave moves backwards

● Tuck under will be very pronounced should an airplane exceed its Mmo (Maximum Operating Mach speed).
● Should it develop, it can feed off of itself, and get progressively worse

Question 4

Compressibility and Airspeed

Answer 4

● Because of the rise in temperature as the air compresses, an airplane will have significant discrepancies in airspeeds

Static Air Temperature:
● The physical temperature of the air which the aircraft is flying through.
● It is also known as the outside air temperature (OAT)

Total Air Temperature:
● The temperature that would be measured by a probe if all of the kinetic energy of the air resulting from the aircraft’s motion was absorbed.
● Because this is impossible, it can never be measured

Ram Rise:
● The difference between Total Air Temperature & Static Air Temperature is called ram rise (RR) and is caused by compressibility and friction of the air at high velocities.
● Faster aircraft push through the air so rapidly that the air can’t get out of the way in time.
● As a result the air is compressed in front of the aircraft and is heated by compression.
● Due to compression the outside air temperature probe or indicator feels a higher temperature than exists in the surrounding air

● The rush of air over the OAT indicator becomes heated by friction, causing further heating and a higher reading.
● This false reading by the OAT indicator or gauge is called temperature rise or ram rise.
● Must be considered when determining TAS