High Speed Flight 101 - 155

High Speed Flight 101 - 155

1. Transonic Profile

   • Flatter upper surface
     • Airflow decelerates due to flat upper surface which gives much smaller shockwave
       • No flow separation behind shockwave so that this area can also be used to generate lift
   • More curved leading edge
     • Airflow immediately accelerates to supersonic because of rounded leading edge
   • Thinner trailing edge
     • Aft lower surface has negative camber
       • Local velocity reduced, increasing static pressure
         • Increases lift in this region
   • Also called 'rear loaded wing'
   • Advantages
     • Allows for thinner and lighter material during construction
     • Allows for greater wingspan without huge weight increase
       • Reduces drag
     • Greater wing chord gives greater fuel capacity
     • Greater lift means wing can be smaller than conventional wing and higher Mach numbers mean sweep-back angle can be reduced
       • Reduction in sweep-back angle and rounded leading edge improve low speed characteristics and allows simpler lift devices
   • Disadvantages
     • Drag is greater on transonic wing than conventional until just above critical Mach number

2. Profile Comparison

   • Cruise Mach number for trans sonic profile is 15% higher than conventional
   • At cruise Mach number, thickness to chord ratio for transonic profile is 42% higher than conventional

3. Control Surfaces in Transonic Regions

   • Shock wave appears on wing root first because this is the thickest part
   • Aircraft reaction is the same as a stall due to high angle of attack
     • Flow separation causes shock stall or high speed stall
   • During shock stall, centre of lift moves toward wing tip and rear of the aircraft
   • Aircraft has nose down reaction after passing critical Mach number
     • Known as the 'tuck under effect' or 'Mach tuck'
   • Horizontal stabilzer used to correct tuck under effect
     • Must increase downward acting force to compensate
   • System works automatically and is known as the Mach trim system

4. Normal Shock Wave

   • When supersonic airflow passes through shock wave
     • Density increases
     • Pressure increases
     • Velocity decreases
     • temperature increases
   • Shock wave wastes energy
   • Two Types of Waves
     • Shockwaves
       • Normal
         • Detached from leading edge
         • Right angle to the air-stream
         • Formed in front of the object
         • No change in airflow direction
         • Airflow slowed to subsonic
         • Static pressure, density and temperature increased, and useful energy or total pressure reduced
       • Oblique
         • Consumes less energy than a normal shockwave
         • Touches leading edge
         • Change in airflow direction
         • Static pressure, density and temperature increase but not as much as normal shockwave
         • Useful energy or total pressure reduced, not as much as normal shockwave
     • Expansion Waves
       • Formed where supersonic air-stream turns away from preceding flow direction
       • Unlike shockwave, flow around a corner doesn't cause sharp or sudden changes in airflow
       • When supersonic air-stream passes through expansion wave, direction follows the surface as long as there is no flow separation
       • Velocity increases
       • Static pressure, density and temperature decrease, no change in useful energy or total pressure

5. Flat Plate Profiles

   • At positive angle of attack, upper surface airflow passes through an expansion wave at leading edge and oblique shockwave at the trailing edge
     • Uniform suction pressure on the upper side
   • Airflow at the under surface passes through an oblique shockwave at the leading edge and an expansion wave at the trailing edge
     • Uniform positive pressure on the lower side
   • Net lift is produced by distribution of pressure on a surface
   • Profile lift is the force from perpendicular to the free air-stream
   • Inclination of net lift from profile lift produces drag

6. Supersonic Profiles

   • Two types
     • Double Wedge
       • Increase in pressure on forward half of the chord
         • Decrease in pressure on the aft half of the chord
       • No net lift
       • Pressure distribution produces an inclined net lift and inclination of net lift from the profile lift produces drag
     • Circular Arc
       • Airflow passes oblique shockwave at the leading edge
       • Airflow undergoes gradual and continuous expansion until it passes another oblique shockwave at the trailing edge
   • If flow over profile is supersonic, centre of lift is located at 50% chord position
   • If flow is subsonic, centre of lift is 25% chord position
   • Stability increases during supersonic flight because distance between C of G and C of L is greater

7. Supersonic Engine Inlets

   • Air entering compressor must be slowed to subsonic
   • Air must be slowed with least waste of energy
   • Normal Shock Diffuser Inlet
     • Least complicated engine inlet
     • Employs single shock wave at the inlet
       • Slows air to subsonic
     • Suitable for low supersonic speeds because normal shockwave is strong
       • Causes great reduction in total pressure
   • Single Oblique Shock Inlet
     • Employs external oblique shockwave to slow airflow before normal shock occurs
   • Multiple Oblique Shock Inlet
     • Employs a series of weak oblique shockwaves to gradually slow airflow before the normal shock
     • Normal shock doesn't have to be very strong
     • Combination of weak shockwaves leads to least waste of energy
   • Variable Supersonic Inlets
     • Equipped with actuator operated panels
     • At speeds below Mach 1, engine inlet fully open
     • Flight speeds above Mach 1, actuators change position of panels and inlet employs normal shockwave
     • High Mach numbers, actuators operate panels so the employ 3 oblique shockwaves and a normal shock

8. Aerodynamic Heating

   • Temperature increases are caused by friction between surface of object and high velocity free air-stream
   • When air flows over an aerodynamic surface, theres a reduction in velocity and increase in temperature
   • Greatest reduction in velocity and increase in temperature occurs at various stagnation points on the aircraft
   • Aluminium alloy loses 80% of its strength if temperature increases to 250 degrees
     • Parts of Concorde and some military aircraft are made from titanium alloy