Zusammenfassung der Ressource
High Speed Flight 101 - 155
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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