High Speed Flight 69 - 101

Three Speed Regions
1. Subsonic
2. Transonic
3. Supersonic
Subsonic Region
1. All speeds around the aircraft are below the speed of sound
  1. Air is incompressible at low speed
Transonic Region
1. Some speeds around the aircraft are below speed of sound, other are above
Supersonic Region
1. All speeds around aircraft are higher than the speed of sound
2. Small changes in pressure and almost no change in density
3. Air becomes compressible
4. Shock waves produced
  1. Large pressure and density changes
Low Speed
1. Sound waves move ahead of the object
2. Airflow immediately ahead of object is influenced by forward moving pressure field
  1. Acts as 'pressure warning' to the leading edge and there is a change of flow direction ahead of leading edge
Above the Speed of Sound
1. Airflow ahead of object is not influenced by pressure field
  1. Sound waves can't move ahead of the object
2. Approaching speed of sound: compression wave forms
3. Changes in velocity, pressure and density take place suddenly and sharply
4. Airflow ahead of object receives no pressure warning because the air particles are suddenly forced out of the way by the shock wave
Mach
1. Subsonic: <0.8 Mach
2. Transonic: 0.8 - 1.2 Mach
3. Supersonic: 1.2 - 5.0 Mach
Critical Mach Number
1. Highest Mach number we can have without supersonic airflow
2. Boundary between subsonic and transonic flight
3. When exceeded, an area of supersonic airflow is created
  1. Local Mach number of the wing will be higher than aircraft speed
    1. Wing profile accelerates airflow over the upper surface
4. Normal shock wave takes place at Mach 1.2
  1. Leads to large increase in static pressure behind wave
  2. Strong shockwave causes boundary layer to have too little energy to withstand increase in static pressure
    1. Flow separation
      1. Reduced lift
5. As flight Mach number reaches 1, areas of supersonic flow increase
  1. Shock waves move closer to trailing edge
6. Bow Wave
  1. Forms at leading edge when flight exceeds speed of sound
  2. Has a detached normal shockwave region with an area of subsonic flow behind wave and oblique shock wave regions
    1. SUpersonic flow behind the wave
  3. If speed increased to a higher supersonic value
    1. Detached normal shock portion of the bow wave moves closer to trailing edge
Wave Drag
1. Portion of total drag due to shock waves
2. Two ways to reduce
  1. Vortex Generators
    1. Produce a vortex which energises the boundary layer
      1. Reduces flow separation
    2. Produces oblique shock wave inside supersonic airflow
      1. Reduced airspeed behind oblique shockwave produces smaller normal shockwave
        Reduces wave drag
    3. Increase parasite drag slightly
  2. 'Area Rule'
Swept Wing Effect
1. Wings designed to carry weight and house fuel tanks
2. If profile is 1.5m thick and chord length 10m, thickness is 0.15 or 15%
  1. Profile thickness = actual thickness/chord length
3. Most modern jets have sweep angle of 30 degrees
  1. Reduces thickness more and increases critical Mach number
4. Variable Sweep Angle
  1. Allows improvement at low speed where thicker chord to length ratio makes for greater lift for take-off and landing
    1. And high speed flight where fully swept position has greater critical speed

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High Speed Flight 69 - 101

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	Created by Kenzie Evans about 7 years ago