High Speed Flight

Question 1

Mmo

Answer 1

Max operating Mach number

Question 2

Vmo

Answer 2

Max operating IAS

Question 3

What happens to Mach number if temperature changes?

Answer 3

Using formula TAS/LSS.
It doesn’t change as TAS would increase with temperature causing less density and so would LSS. Therefore Mach is constant.

Question 4

What happens to compressibility at high Mach numbers?

Answer 4

Becomes noticeable.

Question 5

Pressure waves

Answer 5

As the aircraft flies it impacts air molecules.
The impact into an air molecule starts a chain affect where the next one hits the next one etc.
This creates slightly high pressure region in front of the aircraft, moving away.
These waves can spread into all directions and expands into a bubble at the LSS surrounding the aircraft.
As we fly around, the aircraft is constantly creating more pressure waves.
When the aircraft gets faster it can keep up with pressure waves being formed.
In front of the aircraft, the pressure waves starts to compress.
Eventually the aircraft gets faster it can enough to keep up with the pressure waves and begin to form a shockwave.

Question 6

Speed regimes

Answer 6

Low subsonic - all airflow <0.1, no compressibility
Transonic - some air <1.0, >1.0, occurs at Mcrit, before Mfs.
Supersonic - all airflow around aircraft is >1.0, occurs at Mfs 1.3.

Question 7

Mcrit

Answer 7

Critical Mach number.
The Mfs at which m=1.0 somewhere at the aircraft for the first time.

Question 8

Mfs

Answer 8

Free Mach number, displayed in aircraft.
Occurs before the aerofoil

Question 9

Ml

Answer 9

Local Mach number.
Measured a particular point.

Question 10

How do you get Mcrit?

Answer 10

As Mfs increases, all the Ml’s increase, eventually one them will reach 1.0.
Normally occurs at max thickness.

Question 11

Do we want Mcrit to be fast or slow?

Answer 11

Fast.
Flying above Mcrit has undesirable effects, so need to stick to Mcrit mostly. So if Mcrit is faster we fly faster.

Question 12

Things that make Mcrit faster?

Answer 12

Decreased camber.
Reduced mass.
More aft CG.
Increased sweepback.
Uses a super critical wing.

As you move faster, pressure waves get closer.

Question 13

Shockwaves

Answer 13

Very thin, dense barrier of air.
As air passes through the shockwave:
Q decrease
Pstat increases

The air compresses through the shockwave:
Temp increases
Ptof decreases
ρ increases

Question 14

Where does shockwave first appear?

Answer 14

On the wing upper surface at the front of max thickness.

Question 15

Shockwaves - less than Mcrit

Answer 15

No shockwave, all airflow is subsonic.
CP is forward.

Question 16

Shockwaves - >Mcrit, Mfs 0.75

Answer 16

Shockwave just forward of max thickness.
Small supersonic religion just forward of it.
Airflow speeds drops as it goes through shockwave.

Sudden change of lift ( and Pstat) at the shockwave.
Increase strength of APG.

CP still forward.

Question 17

Shockwave - Mfs 0.8

Answer 17

Shockwave is more aft, more supersonic airflow.
Lower surface shockwave appears too.

CP gets aft as Mach number increases.
Stronger increase of APG.

Question 18

Shockwaves - Mfs 0.85

Answer 18

Shockwave moves aft, upper and lower surface. More supersonic airflow.

CP moves aft, separation occurs behind shockwave.
If the pressure rise generated by a shock wave is sufficiently strong it can then generate a so-called separated flow

Question 19

Shockwaves - Mfs 0.9

Answer 19

Both shockwaves move aft, similar location respectively to their surface now. Even more supersonic airflow.
Only subsonic at leading edge and near trailing edge.
Stalling region gets smaller.

Question 20

Shockwaves - Mfs 1.0

Answer 20

Shockwaves reach trailing edge. Lots of supersonic flow.
CP at 50% MAC.
No longer stalled, rectangular lift distribution.

Question 21

Shockwaves - Mfs 1.1

Answer 21

Shockwave at trailing edge and ahead of leading edge, still a small amount of subsonic flow at leasing edge.

Question 22

Shockwaves - Mfs 1.3

Answer 22

All subsonic airflow shockwaves at leading edge and trailing edge.

Question 23

Shockwave movement

Answer 23

On a regular cambered Aerofoil, lower surface shockwave moves quicker than upper due to it starting further forward.
But not as aft as upper surface.

On a flatter surface, shockwave changes position quicker

Question 24

Cl vs Mn

Answer 24

Constant Cl up to Mcrit, then increase until it reaches shock induced separation, Cl falls then slightly increases to Mfs 1.3.

Between Mcrit and Mfs1.3 is transonic range?
Cl varies in this area.

Question 25

CD vs Mn

Answer 25

Mdd: The Mn at CD starts to rapidly rise with increasing Mn. Exceeding this = increase drag, instability, Mach tuck

Up to Mdd it’s constant, then rapidly rises to Mfs 1.0, then drag decreases slightly to reach Mfs 1.3.

Drag varies as Mn increases in the transonic range

Question 26

CD vs Mn. swept wing

Answer 26

Constant CD up to Mdd, rapidly increases then decrease slightly.
Highest has highest peak CD.

Question 27

CD vs Mn. Straight wing

Answer 27

Has the lowest Mcrit and Mdd, CD increases to lowest value of the wings and decreases slightly.

Lower MN before increased drag.

Question 28

CD vs Mn. Supercritical wing.

Answer 28

Mcrit at swept wings AC/DC, highest Mdd, rapidly increase almost same CD as swept then decreases not as much as swept.

Travels at faster MN before getting increased drag.

Question 29

CL/CD vs MN

Answer 29

For subsonic higher ratio.
Dog supersonic lower ratio

Question 30

Wave drag

Answer 30

The extra due to shockwaves.

Question 31

Mach tuck (tuck under)

Answer 31

A sudden and possibly uncontrollable pitch down movement that occurs at high Mach numbers.

Question 32

What causes Mach tuck?

Answer 32

Rearward movement of CP.
Loss of downwash due to shock induced separation.

When airflow separates, the negative AOA on the stabiliser is lost causing a loss of downforce which causes a pitch down moment.

Question 33

Mach trimmer

Answer 33

Automatically repositions the THS based on MN to prevent Mach tuck.
Active at high MN - Lessing edge of THS moves down.

Failed Mach trimmer:
Slow down,
Limit the aircraft MN

Stick force stability is lost when aircraft enters Mach tuck.
Because Mach tuck is at high MN which means CG moves aft which decreases stability. Also a steep dive is not stable.

Question 34

Sweepback

Answer 34

Only airflow moving perpendicular to the leading edge produced lift, shockwaves etc.

So sweepback as less lift and shockwaves.

100% Mcrit / cos30 sweep angle = 115%.
This means a swept wing travels at 115% of rectangular wings Mcrit to have same lift and shockwaves.

Swept wing has faster Mcrit because it has a longer effect chord

Question 35

Supercritical wing

Answer 35

Flat upper surface.
Negatively cambered lower surface.
Relatively thick.
Large LE radius.

Gives a faster Mcrit
Gives a faster Mdd
Bigger gap between Mcrit and Mdd.

Unpredictable movement of separation point.
Poor low speed characteristics
Not a practical shape
Large nose down pitching moment majority of lift in rear 30%
Delays onset of wave drag

Question 36

Vortex generator

Answer 36

Renege raise the airflow behind the shockwave, delaying shock induced separation. Placed in front of control surface.

Question 37

Mach number

Answer 37

A measure of compressibility of the air

Question 38

Compressibility

Answer 38

Increases APG, reduces Cl.
Increase stalling speed, decrease critical AOA

Question 39

What happens if low shockwave is behind upper shockwave

Answer 39

Shock induced boundary layer separation.
Or more forward lift distribution

Question 40

What happens to speed stability as Mach number increases

Answer 40

Decreases stability