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High Speed Flight 101 - 155

Transonic Profile
1. Flatter upper surface
  1. Airflow decelerates due to flat upper surface which gives much smaller shockwave
    1. No flow separation behind shockwave so that this area can also be used to generate lift
2. More curved leading edge
  1. Airflow immediately accelerates to supersonic because of rounded leading edge
3. Thinner trailing edge
  1. Aft lower surface has negative camber
    1. Local velocity reduced, increasing static pressure
      1. Increases lift in this region
4. Also called 'rear loaded wing'
5. Advantages
  1. Allows for thinner and lighter material during construction
  2. Allows for greater wingspan without huge weight increase
    1. Reduces drag
  3. Greater wing chord gives greater fuel capacity
  4. Greater lift means wing can be smaller than conventional wing and higher Mach numbers mean sweep-back angle can be reduced
    1. Reduction in sweep-back angle and rounded leading edge improve low speed characteristics and allows simpler lift devices
6. Disadvantages
  1. Drag is greater on transonic wing than conventional until just above critical Mach number
Profile Comparison
1. Cruise Mach number for trans sonic profile is 15% higher than conventional
2. At cruise Mach number, thickness to chord ratio for transonic profile is 42% higher than conventional
Control Surfaces in Transonic Regions
1. Shock wave appears on wing root first because this is the thickest part
2. Aircraft reaction is the same as a stall due to high angle of attack
  1. Flow separation causes shock stall or high speed stall
3. During shock stall, centre of lift moves toward wing tip and rear of the aircraft
4. Aircraft has nose down reaction after passing critical Mach number
  1. Known as the 'tuck under effect' or 'Mach tuck'
5. Horizontal stabilzer used to correct tuck under effect
  1. Must increase downward acting force to compensate
6. System works automatically and is known as the Mach trim system
Normal Shock Wave
1. When supersonic airflow passes through shock wave
  1. Density increases
  2. Pressure increases
  3. Velocity decreases
  4. temperature increases
2. Shock wave wastes energy
3. Two Types of Waves
  1. Shockwaves
    1. Normal
      1. Detached from leading edge
      2. Right angle to the air-stream
      3. Formed in front of the object
      4. No change in airflow direction
      5. Airflow slowed to subsonic
      6. Static pressure, density and temperature increased, and useful energy or total pressure reduced
    2. Oblique
      1. Consumes less energy than a normal shockwave
      2. Touches leading edge
      3. Change in airflow direction
      4. Static pressure, density and temperature increase but not as much as normal shockwave
      5. Useful energy or total pressure reduced, not as much as normal shockwave
  2. Expansion Waves
    1. Formed where supersonic air-stream turns away from preceding flow direction
    2. Unlike shockwave, flow around a corner doesn't cause sharp or sudden changes in airflow
    3. When supersonic air-stream passes through expansion wave, direction follows the surface as long as there is no flow separation
    4. Velocity increases
    5. Static pressure, density and temperature decrease, no change in useful energy or total pressure
Flat Plate Profiles
1. At positive angle of attack, upper surface airflow passes through an expansion wave at leading edge and oblique shockwave at the trailing edge
  1. Uniform suction pressure on the upper side
2. Airflow at the under surface passes through an oblique shockwave at the leading edge and an expansion wave at the trailing edge
  1. Uniform positive pressure on the lower side
3. Net lift is produced by distribution of pressure on a surface
4. Profile lift is the force from perpendicular to the free air-stream
5. Inclination of net lift from profile lift produces drag
Supersonic Profiles
1. Two types
  1. Double Wedge
    1. Increase in pressure on forward half of the chord
      1. Decrease in pressure on the aft half of the chord
    2. No net lift
    3. Pressure distribution produces an inclined net lift and inclination of net lift from the profile lift produces drag
  2. Circular Arc
    1. Airflow passes oblique shockwave at the leading edge
    2. Airflow undergoes gradual and continuous expansion until it passes another oblique shockwave at the trailing edge
2. If flow over profile is supersonic, centre of lift is located at 50% chord position
3. If flow is subsonic, centre of lift is 25% chord position
4. Stability increases during supersonic flight because distance between C of G and C of L is greater
Supersonic Engine Inlets
1. Air entering compressor must be slowed to subsonic
2. Air must be slowed with least waste of energy
3. Normal Shock Diffuser Inlet
  1. Least complicated engine inlet
  2. Employs single shock wave at the inlet
    1. Slows air to subsonic
  3. Suitable for low supersonic speeds because normal shockwave is strong
    1. Causes great reduction in total pressure
4. Single Oblique Shock Inlet
  1. Employs external oblique shockwave to slow airflow before normal shock occurs
5. Multiple Oblique Shock Inlet
  1. Employs a series of weak oblique shockwaves to gradually slow airflow before the normal shock
  2. Normal shock doesn't have to be very strong
  3. Combination of weak shockwaves leads to least waste of energy
6. Variable Supersonic Inlets
  1. Equipped with actuator operated panels
  2. At speeds below Mach 1, engine inlet fully open
  3. Flight speeds above Mach 1, actuators change position of panels and inlet employs normal shockwave
  4. High Mach numbers, actuators operate panels so the employ 3 oblique shockwaves and a normal shock
Aerodynamic Heating
1. Temperature increases are caused by friction between surface of object and high velocity free air-stream
2. When air flows over an aerodynamic surface, theres a reduction in velocity and increase in temperature
3. Greatest reduction in velocity and increase in temperature occurs at various stagnation points on the aircraft
4. Aluminium alloy loses 80% of its strength if temperature increases to 250 degrees
  1. Parts of Concorde and some military aircraft are made from titanium alloy

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High Speed Flight 101 - 155

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	Created by Kenzie Evans almost 7 years ago