High Speed Flight 69 - 101

High Speed Flight 69 - 101

1. Three Speed Regions

   • Subsonic
   • Transonic
   • Supersonic

2. Subsonic Region

   • All speeds around the aircraft are below the speed of sound
     • Air is incompressible at low speed

3. Transonic Region

   • Some speeds around the aircraft are below speed of sound, other are above

4. Supersonic Region

   • All speeds around aircraft are higher than the speed of sound
   • Small changes in pressure and almost no change in density
   • Air becomes compressible
   • Shock waves produced
     • Large pressure and density changes

5. Low Speed

   • Sound waves move ahead of the object
   • Airflow immediately ahead of object is influenced by forward moving pressure field
     • Acts as 'pressure warning' to the leading edge and there is a change of flow direction ahead of leading edge

6. Above the Speed of Sound

   • Airflow ahead of object is not influenced by pressure field
     • Sound waves can't move ahead of the object
   • Approaching speed of sound: compression wave forms
   • Changes in velocity, pressure and density take place suddenly and sharply
   • Airflow ahead of object receives no pressure warning because the air particles are suddenly forced out of the way by the shock wave

7. Mach

   • Subsonic: <0.8 Mach
   • Transonic: 0.8 - 1.2 Mach
   • Supersonic: 1.2 - 5.0 Mach

8. Critical Mach Number

   • Highest Mach number we can have without supersonic airflow
   • Boundary between subsonic and transonic flight
   • When exceeded, an area of supersonic airflow is created
     • Local Mach number of the wing will be higher than aircraft speed
       • Wing profile accelerates airflow over the upper surface
   • Normal shock wave takes place at Mach 1.2
     • Leads to large increase in static pressure behind wave
     • Strong shockwave causes boundary layer to have too little energy to withstand increase in static pressure
       • Flow separation
         • Reduced lift
   • As flight Mach number reaches 1, areas of supersonic flow increase
     • Shock waves move closer to trailing edge
   • Bow Wave
     • Forms at leading edge when flight exceeds speed of sound
     • Has a detached normal shockwave region with an area of subsonic flow behind wave and oblique shock wave regions
       • SUpersonic flow behind the wave
     • If speed increased to a higher supersonic value
       • Detached normal shock portion of the bow wave moves closer to trailing edge

9. Wave Drag

   • Portion of total drag due to shock waves
   • Two ways to reduce
     • Vortex Generators
       • Produce a vortex which energises the boundary layer
         • Reduces flow separation
       • Produces oblique shock wave inside supersonic airflow
         • Reduced airspeed behind oblique shockwave produces smaller normal shockwave
           • Reduces wave drag
       • Increase parasite drag slightly
     • 'Area Rule'

10. Swept Wing Effect

    • Wings designed to carry weight and house fuel tanks
    • If profile is 1.5m thick and chord length 10m, thickness is 0.15 or 15%
      • Profile thickness = actual thickness/chord length
    • Most modern jets have sweep angle of 30 degrees
      • Reduces thickness more and increases critical Mach number
    • Variable Sweep Angle
      • Allows improvement at low speed where thicker chord to length ratio makes for greater lift for take-off and landing
        • And high speed flight where fully swept position has greater critical speed