Ch 28 - Introduction To High Speed Flight

Question 1

Speed of Sound

Answer 1

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)

Question 2

Mach Effects and Mach Number

Answer 2

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)

Question 3

Local and Free Stream Mach Number

Answer 3

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

Question 4

Speed Ranges: Sub-Sonic

Answer 4

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

Question 5

Speed Ranges: Transonic

Answer 5

Where airlines try to operate

ML < 1.0

ML = 1.0

ML > 1.0

Boundary between transonic and super sonic is at MDET

Question 6

Speed Ranges: Supersonic

Answer 6

All ML > 1.0

Question 7

Speed Ranges: Hypersonic

Answer 7

All ML > 5.0

Question 8

Wave Characteristics

Answer 8

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.

Question 9

Normal Shock Wave

Answer 9

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

Question 10

Oblique Shock Wave

Answer 10

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

Question 11

Bow Wave

Answer 11

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)

Question 12

Mach Waves (Cones)

Answer 12

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

Question 13

Mach Angle

Answer 13

As AC speed increases, the Mach cone angle reduces

Sin(Mew - U) = 1 / Mach Number (e.g 1.2)

Question 14

Area of Influence

Answer 14

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

Question 15

Expansion Waves

Answer 15

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

Question 16

CAS, TAS and Mach Number

Answer 16

Mach number = TAS / a

Cold to warm air = a increases therefore TAS must increase

Mach Constant therefore CAS remains the same

TAS does what Temperature does (temp up, TAS up)

Question 17

The Climb on TAS, CAS and Mach Number

Answer 17

C hicken T ika M asala

Constant TAS as you climb = Decreasing TAS, Increasing MACH

Constant CAS = Increasing TAS, Increasing Mach

Above tropopause, TAS and Mach are the same, CAS is the only one that changes

Inversion = CMT

Question 18

Climb Schedule Summary

Answer 18

At constant CAS:

CAS, AoA, CL all remain constant

The pitch angle and angle of climb reduce

At Constant Mach Number:

CAS decreases

AoA and Cl increase

Pitch angle and climb angles decrease

Question 19

Descent

Answer 19

At constant Mach:

CAS increases
Decreasing AoA and CL
Decreasing Pitch angle 
FPA increases 
(Pitch angle can be negative)

At Constant CAS:

Mach number decreases
AoA and CL remains Constant
Pitch and Descent angles remain constant

Question 20

Mcrit

Answer 20

The free stream Mach number of which the local flow first reaches Mach 1

Flow is sonic

No shock waves are present