Ch 28 - Introduction To High Speed Flight Flashcards
Speed of Sound
Movement through the air impacts air molecules which go on to impact other molecules
This impact is transmitting through the air as expanding waves
- Areas of higher and lower density (due to compressibility which is now a factor) - Known as Pressure waves
Warmer = Higher speed of sound Colder = Lower speed of sound (temp decreases with alt so LSS (a) usually decreases with alt)
LSS (also ‘a’)= 39((square root)K)
M1 is roughly 10 miles a minute (0.7mach = 7 miles per minute)
Mach Effects and Mach Number
In front, the molecules are getting compressed, if you continue to get faster eventually you will over come them.
This effect changes the; lift, drag, stability and pressure properties of the object moving through the flow and is caused by the relationship between TAS and LSS
Mach Number = TAS/a (LSS)
Local and Free Stream Mach Number
Local Mach number is the speed of the air at some point on the aerofoil. The local Mach numbers around an aircraft are either faster, equal to or slower than the free stream Mach number.
The free stream Mach number is the speed at which the free stream air is moving
Speed Ranges: Sub-Sonic
Broken down into low and high and will always be less than M1.0
Low is less than 260kt or M0.4
The boundary between Sub-sonic and Transonic is at Mcrit ~M0.75 (but depends on sweep and thickness of aerofoil) and means that somewhere around the aerofoil there is sonic flow
Speed Ranges: Transonic
Where airlines try to operate
ML < 1.0
ML = 1.0
ML > 1.0
Boundary between transonic and super sonic is at MDET
Speed Ranges: Supersonic
All ML > 1.0
Speed Ranges: Hypersonic
All ML > 5.0
Wave Characteristics
A shockwave is about 0.0025mm thick across which the airflow’s pressure, temp and density change rapidly. A shockwave remains in the same position relative to the aircraft for the same speed of flow and aerofoil design.
Normal Shock Wave
A normal shockwave is a surface of extreme pressure, temperature and density change with no change in flow direction.
In front of the normal shockwave; flow is always supersonic, behind = subsonic (flow velocity and ML greatly decrease - roughly the same decrease after, as increase before)
Total pressure energy loss is converted to an increase in temp - the loss of energy is known as wave drag
Static pressure and density increase behind the shockwave
Acts normal to the direction of flow
Only extend a short distance off the aircrafts surface. Strength and length increases with an increase in MFS
Oblique Shock Wave
Same as normal shockwave, just less severe
Flow; Supersonic before, slower supersonic behind = lesser compression and smaller effects on flow properties
Loss of total pressure energy which is converted into temp (wave drag) but less than Normal Shock wave
Increase in static pressure and density behind the shockwave but less severe than normal
Flow direction always changes across an oblique shockwave and is no longer normal to the flow
Considerably longer than normal shock waves and can extend down to the ground
Only occur at fast transient and supersonic speeds
Bow Wave
The shockwave at the leading edge
Forms just above MFS 1.0 in the transonic region
Compromises of a normal shockwave in its central section and develops into an oblique shockwave further out.
Oblique shockwave also forms at the trailing edge (also known as a fishtail)
Mach Waves (Cones)
Is the surface of a very weak oblique shock that forms around the AC above Mach 1.
Produced by the pressure wave disturbances that radiate outwards from the aircraft. Spherical moving out from the aircraft in all directions.
Felt as a sonic boom on the ground - moves over the ground at the TAS of the AC
Size of the cone depends on speed
Mach Angle
As AC speed increases, the Mach cone angle reduces
Sin(Mew - U) = 1 / Mach Number (e.g 1.2)
Area of Influence
Anything outside the Mach cone cannot be influenced by pressure changes initiated by the aircrafts structure and its control surfaces.
This means that when designing a wing to go supersonic the control services must be within the Mach Cone
Expansion Waves
Expansion waves form an expansion region where the flow direction and speed changes but remains attached to the surface.
Different to the other two:
Velocity; Supersonic to faster supersonic
Static Pressure and Density Decreases
Temperature and local speed of sound decreases
Total pressure energy remains the same