1. Advanced, High Speed AerodynamicS

Question 1

What is Mach number

Answer 1

The ratio of an aircraft true air speed to the local speed of sound

Question 2

What are the main factors affecting the coefficient of lift

Answer 2

Angle of attack, cleanliness of wing surface, wing shape in plan form and cross section, local speed of sound

Question 3

What is the critical Mach number (MCRIT)

Answer 3

The free stream Mach number at which Sonic flow is reached at any point on the aircraft

Question 4

Speed of sound at sea level Isa conditions

Answer 4

661kt (38.94√15+273)

Question 5

How does the speed of sound change with an increase in altitude

Answer 5

Reducing speed of sound with reducing temperature increases Mach number

Question 6

What is detachment Mac number of an aircraft

Answer 6

The mach free stream at which the bow shockwave attaches to the leading edge of the wing

Question 7

What is the only factor that affects the speed of sound through the air

Answer 7

Temperature. LSS proportional to T

Question 8

What is the relationship between local speed of sound and temperature? Proportional or in proportional?

Answer 8

Proportional

Question 9

How is air modified as it flows through a contracting nozzle at supersonic speed

Answer 9

Velocity reduces, temperature, pressure and density all increase

Question 10

How does true airspeed vary in the climb at a constant Mach number in Isa conditions

Answer 10

Constant Mac, reducing true airspeed.

Question 11

What are speed transition and the crossover altitude

Answer 11

Crossover altitude is the altitude at which the process of speed transition occurs, where reference to speed moves from IAS to MaCH number in The climb, and from MACH number to IAS in The descent

Question 12

What are the points A B and C on the Drag curve

Answer 12

MCRIT, Mcdr, M1.0 (max Drag)

Question 13

What is the lambda foot?

Answer 13

The shape of the base of the wings upper shockwave caused by high pressure in the shockwave creeping forward into the subsonic boundary layer.

Question 14

Where does MCDR occur?

Answer 14

At a slightly higher Mach number than MCRIT, when the upper shock Wave forms, increasing coefficient of drag significantly

Question 15

What does the formation of a definite shockwave on the upper surface of the wing indicate?

Answer 15

That Mcrit has been exceeded.

Question 16

Advantages of a supercritical aerofoil section?

Answer 16

Delayed Mcrit due to reduced acceleration on top surface.
Increased Gap between Mcrit and Mcdr due to delayed shock wave formation
Increased high-speed cruise efficiency and still relatively high coefficient of lift Max for slow flight

Question 17

Advantages of a swept wing

Answer 17

Higher MFS Mcrit

Delays shockwave formation, Mcdr

Question 18

Disadvantages of swept wing

Answer 18

High stall angle of attack
High nose attitude due to increased induced drag (downward vortices)
High drag at high angle of attack
Higher chance of wing drop stall (tip stalling)
Forward movement of centre of pressure as tips stall, causing nose up pitch.
Nose up pitch can deepen, causing deep stall on t-tail aircraft

Question 19

What design features result in a higher Mcrit

Answer 19

Sweepback, supercritical wing, low thickness to chord ratio

Question 20

Design characteristics of a supercritical aerofoil section?

Answer 20

Flat top of wing, reflex Camber in rear third of wing, relatively high thickness to chord ratio to provide space for carriage of fuel.

Question 21

Advantages of a low thickness to chord ratio wing

Answer 21

Coefficien of drag is less at stall, Mcrit and Mcdr both delayed (Faster TAS)

Question 22

What is mach tuck

Answer 22

Entering transonic phase of flight, as shockwave form, further and further rearwards, centre pressure moves aft, thus creating a nose pitch down moment.

Question 23

How does the ad bridge centre of pressure of a swept wing aircraft move at the point of stall?

Answer 23

Due to wing tips stalling first, centre of pressure moves forwards, creating a nose up moment, further deepening the stall

Question 24

Advantages of wing sweep

Answer 24

Higher Mac free stream for Mcrit (V1 vector less than IAS, delayed Mcdr- meaning shockwave formation is delayed

Question 25

Disadvantages of wing sweep

Answer 25

Low coefficient of lift Max, low coefficient of lift at any angle of attack, high angles of attack to produce lift, therefore greater drag to produce given coefficient of lift, Dutch roll, greater chance of wing drop stall due to tip stalling, deep stall from reduction of downwash on tail plane

Question 26

How is air affected as it passes through a shockwave?

Answer 26

Above the wing is a converging duct, so velocity of air reduces at supersonic, raising temperature density and pressure

Question 27

Design features that lead to a greater Mcrit

Answer 27

Low thickness to chord ratio wing, supercritical wing, wing sweepback