Compressibility
CW2 Dillon
E Co. 1/212th AVN REGT
References
• FM 1-203
• TM 1-1520-237-10
• Some NASA “Rocket Scientist” website
• If the air speed on the top of the wing is
faster than on the bottom, there will be
some speed where the air on top reaches
the speed of sound. This is the critical
mach.
Critical mach is a aeronautics term that refers to
the speed at which some of the airflow on a wing
becomes supersonic.
When this occurs the distribution of forces on the
wing changes suddenly and dramatically, typically
leading to a strong nose-down force on the aircraft.
This effect led to a number of accidents in the
1930s and 1940s, when aircraft in a dive would hit
critical mach and continue to push over into a
steeper and steeper dive.
• When this happens shock waves form on
the upper wing at the point where the flow
becomes supersonic, typically behind the
midline of the chord.
• Shock waves generate lift of their own, so
the lift of the wing suddenly moves
rearward, twisting it down. This effect is
known as mach tuck.
…Not to be confused with Friar Tuck
Subsonic airflow is incompressible
(It acts the same as hydraulic fluid)
Transonic and Supersonic flows become
compressible.
• Shock wave formation
•
•
•
•
Drag increases & Lift decreases
Vibrations Increase
Center of Pressure shifts aft, nose dives
Boundary layer begins to shed
•
•
•
•
Shock Wave increases on top
Shock wave forms on bottom
Shed more boundary layer molecules
CP shifts further aft, nose dives steeper
•
•
•
•
Shock Wave engulfs top and bottom of blade
Supersonic flow begins
More boundary layer sheds
CP moves further aft, Nose dives steeper
•
•
•
•
Shock Wave moves behind the blade
Boundary layer completely missing
Bow Wave forms in front of blade
Catastrophic sequence of events imminent
Conditions Conducive to Compressibility
- ____ Airspeed
Conditions Conducive to Compressibility
- High Airspeed
- ____ Rotor RPM
Conditions Conducive to Compressibility
- High Airspeed
- High Rotor RPM
- ____ GWT
Conditions Conducive to Compressibility
- High Airspeed
- High Rotor RPM
- High GWT
- ____ DA
Conditions Conducive to Compressibility
- High Airspeed
- High Rotor RPM
- High GWT
- High DA
- ____ Temperature
Conditions Conducive to Compressibility
- High Airspeed
- High Rotor RPM
- High GWT
- High DA
- Low Temperature
- ________ Air
Conditions Conducive to Compressibility
- High Airspeed
- High Rotor RPM
- High GWT
- High DA
- Low Temperature
- Turbulent Air
• Corrective action
– ________ Airspeed
• Corrective action
– Decrease Airspeed
– ________ pitch angle (________ collective)
• Corrective action
– Decrease Airspeed
– Decrease pitch angle (Decrease collective)
– ________ G loading
• Corrective action
– Decrease Airspeed
– Decrease pitch angle (Decrease collective)
– Decrease G loading
– ________ RPM
• Corrective action
– Decrease Airspeed
– Decrease pitch angle (Decrease collective)
– Decrease G loading
– Decrease RPM
How do NASA Rocket Scientists figure
when compressibility will be
encountered?
How do Army Aviators figure when
compressibility will be encountered?
For those of you in the back that
couldn’t see the chart clearly,
At: -20ºC
8000’ PA
22,000 GW
You may encounter compressibility at
or above 143 KIAS.
It is possible to encounter compressibility,
however, it is unlikely with:
Proper Performance Planning
QUESTIONS?