8.4 high speed flight

what % of the speed of sound of do aircraft typically flight

87%

low speed flight is based that air is

incompressible

what is the flow mixture in the transonic region

some supersonic some transonic

what is the formula for local speed of sound

(LSS)=39x (square root of absolute temperature)

sound waves/pressure waves radiate in what directionn

outwards equally in all directions

the speed pressure/sound waves depend on temperature but what else

density of the material

at sea level (15 degrees0 what is the aprox speed of sound in mph

761 mph

what altitudes will speed of sound stay constant

11,000 meters to 27,000 meters

at subsonic speed how far a head can the air “feel” the aircraft coming and what is this called

100 meters in front - called the doppler effect

give a example of the doppler effect

emergency vehicle driving past and the change in the pitch

high speed flight is based that the air is

compressible

what happens to velocity pressure and density in subsonic flow through a duct

in subsonic when flow os converging

pressure degreases

velocity increases

constant density

when flow is diverging

pressure increases

velocity decreases

constant density

what happens to velocity pressure and density in supersonic flow through a duct

in supersonic flow when converging

pressure increases

velocity decreases

density increases

when flow is diverging

pressure decreases

velocity increases

density decreases

what part of the aircraft are effected the most by the transonic region

the wings

as the aircraft is travelling at the speed of sound what happens to the pressure waves

they form a shockwave

shock waves upset the aerodynamic balance of?

the wing

what causes a mach cone

the aircraft travelling faster than its pressure wave

a aircraft traveling exactly at the speed of sound is travelling how fast

mach 1

what its the critical mach number

the minimum speed were air reaching the the maximum thickest part of the wing is travelling supersonic

what is free stream air (MFS)

the mach number of a point unaffected by the presence of a aircraft

what is the local mach number (ML)

when a aerofoil is travelling in subsonic airflow and some air is accelerated and some slowed down to speed to a specified region

what is critical mach number

the lowest MFS which for a given angle of attack will give a rise to mach 1

what type of wing can fly faster in the transonic region

a thin wing (supercritical wing)

how does a supercritical wing overcome the less lift produced by the shallow camber

it has a longer chord

at slow speeds a supercritical wings aircraft needs…

high lift devices

what is the few unwanted effects as the critical mach number is approched

buffeting, shockwaves, drag, movement of the centre of pressure

what is compressibility mach number

the mach number that because of compressibility effects control of the aircraft becomes difficult and beyond loss of control is probable

what shape is a normal shock wave

90 degrees or perpendicular to the flow

what angle is a oblique shock wave

at a angle to the direction of flow

in a normal shock the air flow down stream is

subsonic

what creates a normal shock

a blunt body in a supersonic flow and the pressure wave building up

when normal shock wave builds up and is forced to change the air around the body this is called ?

bow shock

how is a oblique shock formed

when supersonic flow is turned going through the shock wave

how fast is the air downstream of a oblique shock

the air could be subsonic or supersonic

the mach number calculated behind the oblique shock is calculated from

the upstream shock and the angle the flow has turned

what is a expansion fan opposite to

oblique shocks

what happens to temperature pressure and density in a explanation fan

temperature pressure and density falls, velocity increases

how many mach waves in a expansion fan

infinite

the first indication of a change in the nature of the flow

breakaway from the airflow resulting in turbulence

as speed increases what happens to the point of separation

it moves forward

when a shockwave is produced what happens to the pressure distribution

its alters the values of lift, drag and affects control operations

a normal shock wave is a boundary between

between subsonic and supersonic airflow

drag increases how much more when there is a shockwave

10 times

if the centre of pressure is altered what will it affect

the pitching moment

the extra drag caused by a shockwave is known as

shock drag

what two components is shock drag made up from

wave drag and boundary layer drag

the air leaving the shockwave remains

supersonic

in shock wave energy is also dissipated as what

heat

once a bow wave attaches to the wing the airflow over the wing will be

supersonic

in terms of wave drag what causes the boundary layer to separate

rise in pressure

what is stronger normal or oblique shock waves

normal

what causes wave drag

the pressure increase causing the boundary layer to separate and de creasing lift and increasing drag

at what point does wave drag start

Mcrit

if speed increases the shock waves will move

towards the trailing edge

when the speed increases ad the shockwave moves back what happens to the lift and drag

lift increases and drag decreases

for the same AOA the Cl is higher in subsonic or supersonic

subsonic

the loss of lift due to a shock wave is simillar to a

low speed stall

what causes more energy drag a normal shock or oblique

normal

what is the term for energy drag

the nature of changes occurring as flow crosses a shock wave

what is the term for boundary layer separation

flow separation

how to reduce wave drag

make the shock waves weak as possible with a sharp leading edge and thin wing to minimise the pressure gradient

what happens to the CL as a aircraft flys fro subsonic to supersonic

the CL increases and there is a sharp drop when the the shock waves forms approx 30% of the incompressed value

once the shock waves reach the trailing edge the CL is 10% of the basic value and the CP is approx 45% of the chord

when the bow wave is attaching to the leading edge CL is reduced to a value less than 30%

the fall in lift flying from subsonic to supersonic can be masked by

trim changes to smooth out the gradient

what causes aerodynamic heating

friction and the compression process at high speeds

the increase in surface temperature is proportional to

the square of the speed

what effects could happen to the skin because of aerodynamic heating

makes them brittle and causing thermal stress as the structure expands

what can be considered to mitigate aerodynamic heating

titanium alloy and stainless steal

what is area rule used for

to reduce the drag of aircraft flying in the transonic region

what is the “perfect’ aerodynamic shape known as

sears-haack body ( shaped like a cigar and pointy at both end)

what is the area rule defined as

for the minimum drag at connections the variation of the aircrafts total cross sectional area along its length should be ideal to minimise wave drag

in terms of area rule were the fuse joins the wings what would reduce

either/or you just cant have them both

the area rule holds true at higher than the speed of sound but what the bias of the wings

rear bias

what type of intakes do you need to fly through all the ranges

variable geometry intakes

bullet farings

plug intakes

how to raise the Mcrit without structural limitations of a thin wing

sweep back wings

sweep wings reduce the production of shock waves and reduces the severity of ?

shock stall

how is the sweep wing measured

25% of the chord line or the leading edges or in sections

what has a higher CL no sweep or sweep

no sweep

a sweep back of 45 degrees will lower the CLmax by

30%

CL of the swept wing is reduced in the ratio of the ?

cosine of sweep angle

why cant sweep back wing fly at its highest angle of attack CLmax when landing

undesirable characteristics near stall

sweep back raises the critical mach number by

the sweep splits the vector components into 2 the one flowing to the chord is only affected and the span wise only affects skin friction

and mean aerodynamic chord delaying Mcrit more

why is a straight wing have more CL than a sweep back

premature separation from the upper surface at the tips

why do sweep wing need a higher landing speed

less production in lift

how is drag affected on sweep back wings

higher induced drag

how does sweep back wings overcome the high drag at low speeds

high lift devices

what is the ideal supersonic airfoil

diamond shaped in cross section

the faster you go the mach cone becomes

narrower