11.1.2 High Speed Flight Flashcards
What is the speed of sound?
The speed at which pressure waves travel through a medium.
What is the speed of sound at sea level (15 degrees c)?
340m/s or 760mph
What is the speed of sound at 36,000 feet?
295m/s or 660mph
What happens when an aircraft approaches the speed of sound?
It experiences compressibility effects which become shockwaves, sudden increase in drag and lift buffeting.
What phase a typical engines optimised for?
Subsonic
What design adaptations does supersonic flight require?
Advanced engine and inlet designs as well as specialised propulsion systems.
How is Mach number worked out?
Velocity of object divided by speed of sound
What are the three ranges of aircraft speed?
Subsonic
Transonic
Supersonic
What happens as an aircraft approaches the speed of sound?
A compression wave is formed at the leading edge.
What is subsonic mach number?
Less than 0.8
What is transonic mach number?
0.8 to 1.2
What is supersonic mach number?
1.2 to 5.0
What is the behaviour of the air in the subsonic region?
Incompressible.
What happens to the air in transonic flight as it hits the upper surface of the wing?
It reaches mach 1, and forms a shockwave at 90 degrees to the airflow.
What is the name of the shockwave formed in transonic flight?
Normal
What challenges can transonic flight lead too?
Stability challenges due to the shock wave which can lead to flow separation.
What other effect can a normal shockwave have on the aircraft in transonic flight?
CL moves rearward causing nose down pitch.
What is the speed region above mach 5 called?
Hypersonic
In supersonic flight where does the shockwave move too?
The trailing edge of the aircraft.
What is the result of the air being compressible in the supersonic region?
Reduction in the coefficient of lift and increased coefficient of drag.
The coefficient of drag is higher in supersonic flight than which region?
Subsonic, but les than the transonic region.
If the mach number is 0.6 what percentage of the speed of sound is this?
60%
What is the critical mach number of an aircraft?
The lowest mach number where airflow over the aircraft at some point reaches the speed of sound.
What is the incipient shock wave?
Sharp increase in density at the point of maximum thickness where the airflow over the wing reaches mach 1.
What happens to the air as it passes through the shockwave?
Decelerates, kinetic energy is dissipated as heat and temp rise, pressure increases.
As speed of airflow increases what happens to the supersonic flow?
It grows larger and forms another shockwave.
What happens to the boundary layer behind the shockwave?
It thickens, becomes turbulent and loses kinetic energy.
How is an oblique shock wave formed?
Bow wave that attaches to the leading edge and slopes backwards.
What are expansion waves?
Not a shockwave.
How is an expansion wave formed?
When supersonic airflow changes direction around a curve.
What two typical supersonic profiles are there?
Double wedge and circular arc.
On a double wedge profile at 0 degrees AoA what shockwaves are experienced?
Oblique @ LE
Expansion @ Mid
Oblique @ TE
Where is the centre of lift on a circular arc profile when supersonic?
50% chord
What is wave drag?
Portion of total drag which is due to the shock waves.
What two ways are used to reduce wave drag?
Using vortex generators
Applying the area rule
What are vortex generators designed to do?
Create vortex’s that reenergise the boundary layer.
What benefit does a vortex generator have in supersonic airflow?
Creates an oblique shockwave, this reduces the airspeed behind the shockwave producing a smaller normal wave than without the vortex gen.
What is a disadvantage of vortex generators?
They increase parasite drag slightly.
What is area rule sometimes referred too as?
Transonic area rule.
What is area rule?
A design rule used to reduce an aircrafts drag at transonic speeds.
What is the Sears-Haack body shape?
The profile with the lowest theoretical wave drag in supersonic flow
What does the area rule state?
Two aircraft with the same longitudinal cross-sectional area distribution have the same wave drag.
What is aerodynamic heating also known as?
Friction heating.
What is aerodynamic heating?
Heating of the airframe caused by the friction of the air.
When is the only time that friction heating is significant?
Supersonic flight.
Where does the greatest reduction in velocity and increase in temp occur?
Stagnation point.
How much strength is lost in aluminium when the temp increases too 250 degrees c?
80%
What is the max axial velocity for GTE?
Mach 0.4
Why can GTE only handle a certain range of axial velocity?
It can change the angle of attack of the compressor blades.
For supersonic aircraft what must the air entering the compressor be?
Slowed to subsonic.
What is the least complicated inlet design?
Normal shock diffuser inlet.
How does a normal shock diffuser inlet work?
Single normal shockwave at the inlet to slow the air to subsonic velocity.
Why is a normal shock diffuser inlet not suitable for high supersonic speeds?
Because the shockwave is really strong and causes a great reduction in the total pressure recovered.
What happens to ‘normal airflow” in a converging duct?
The velocity increases and the static pressure decreases.
What does supersonic air do in a converging duct?
The air chokes at the narrowest point and actually starts to back up, decelerate and compress.
Behaviours of subsonic flow to supersonic flow in a convergent or divergent duct are what?
Opposite. Subsonic air is the classic rules of venturi.
How does a single oblique shock inlet work?
It forms an external oblique shock to slow the supersonic flow before the normal shock.
How does a multiple oblique inlet work?
Uses a series of weak obliques to slow the air before the normal shock
What is the benefit of a multiple oblique inlet?
It leads to less wasting of energy and the highest pressure recovery.
What is the point of a variable supersonic inlet?
Allows the optimum shape of the inlet for varying angles of attack and different speeds.
What are the difficulties of transonic flight?
Shockwave formation and an associated flow separation.
How can the adverse effects of transonic flight be reduced?
If the shockwave induced separation and the intensity of the shock waves can be delayed and reduced.`
What is one way of reducing the profile thickness and delay the critical mach number?
Sweep back of the wing.
What is the sweep angle on most modern aircraft?
30 degrees
What 3 major benefits do sweepback wings offer?
Reduce frontal area of the aircraft
Reduce the profile drag
Improve lateral stability
What is the disadvantage of swept wings?
Increased span wise flow which can cause stagnation in the boundary layer.
