081.02 HIGH-SPEED AERODYNAMICS

Question 1

The regime of flight up to M_crit is called the (…) range. In this range (all local flow < M1 / all local flow > M1 / some local flow > M1 and some < M1).

Answer 1

The regime of flight up to M_crit is called the (subsonic) range. (all local flow < M1)

Question 2

The regime of flight from about Mach 1.3 to about Mach 5 is called the (…) range. (all local flow < M1 / all local flow > M1 / some local flow > M1 and some < M1)

Answer 2

The regime of flight from about Mach 1.3 to about Mach 5 is called the (supersonic) range. (all local flow > M1)

Question 3

The regime of flight from M_crit up to approximately Mach 1.3 is called the (…) range. (all local flow < M1 / all local flow > M1 / some local flow > M1 and some < M1)

Answer 3

The regime of flight from M_crit up to approximately M 1.3 is called the (transonic) range. (some local flow > M1 and some < M1)

Question 4

Compressibility means that density can change along a streamline. It has to be considered at speeds above M(…).

Answer 4

Compressibility has to be considered at speeds above M(0.4).

Question 5

The least energy loss through a normal shockwave occurs when the local Mach number is (just below/exactly/just above/well above) Mach 1.

Answer 5

The least energy loss through a normal shockwave occurs when the local Mach number is (just above) Mach 1.

- below and exactly at M1 no shockwaves present and well above M1 the energy loss is large -

Question 6

Explain the effects on the following in a normal shock wave:

• velocity and Mach number behind wave
• static pressure
• density
• temperature
• total pressure

Answer 6

Explain the effects on the following in a normal shock wave:

• velocity and Mach number behind wave → decrease to subsonic
• static pressure → increase
• density → increase due to compression
• temperature → increase (static pressure increase)
• total pressure → decrease (= airflow energy)

Question 7

Increasing mass at constant Mach number will (increase/decrease) shock wave intensity.

Answer 7

Increasing mass at constant Mach number will (increase) shock wave intensity.

- more mass at constant M means flying a larger AoA which intensifies the shock wave due to higher local speeds -

Question 8

If a shock wave appears on the upper side of a wing, the centre of pressure will (not move/move towards leading edge/move towards trailing edge).

Answer 8

If a shock wave appears on the upper side of a wing, the centre of pressure will (move towards trailing edge).

- remember CP at supersonic speed is at 50% chord -

Question 9

A normal shock wave (only occurs on the upper wing/can occur at different points on the aeroplane).

Answer 9

A normal shock wave (can occur at different points on the aeroplane), in transonic flight.

Question 10

The front of a shock wave moves across the Earth’s surface at (equivalent/true/ground) speed.

Answer 10

The front of a shock wave moves across the Earth’s surface at (ground) speed.

Question 11

Drag due to conversion of kinetic energy into heat inside the shock wave and seperation of the boundary layer caused by the normal shock waves is called (…) drag.

Answer 11

Drag due to conversion of kinetic energy into heat inside the shock wave and seperation of the boundary layer caused by the normal shock waves is called (wave) drag.

Question 12

Define ‘M_crit‘.

Answer 12

Flight speed below Mach 1 at which a part of the airflow anywhere on the aeroplane first reaches a local speed of Mach 1 (local sonic flow).

Question 13

Describe the C_D/Mach number graph in the transonic region.

Answer 13

First an increase, then a decrease..

Question 14

Describe the C_L/Mach number graph in the transonic region.

Answer 14

First an increase, then a decrease and afterwards an increase again.

Question 15

In the transonic range C_Lmax will (increase/decrease) and V_s1g will (increase/decrease).

Answer 15

In the transonic range C_Lmax will (decrease) and V_s1g will (increase).

Question 16

On a typical transonic airfoil, the transonic rearward shift of the CP occurs at about Mach (0.75/0.89/0.91) to Mach (0.89/0.98/1.4)​.

Answer 16

On a typical transonic airfoil, the transonic rearward shift of the CP occurs at about Mach (0.89) to Mach (0.98).

Question 17

Positive stick force stability means that when an airplane accelerates, the stick must move (forward/backward) in order to keep the plane level. The effect of exceeding M_crit on stick force stability on an aeroplane with swept-back wings without stability augmentation is an (increase/decrease) in stability due to (…).

Answer 17

Positive stick force stability means that when an airplane accelerates, the stick must move (forward) in order to keep the plane level. The effect of exceeding M_crit on stick force stability on an aeroplane with swept-back wings without stability augmentation is an (decrease) in stability due to (rearward movement of CP creating a nose down moment).

- so instead of pusing while acceleration you have to pull, hence a decrease in stability -

Question 18

an increase in stall speed (IAS) with increasing altitude is due to (an increase in TAS/compressibility effects/the larger AoA necessary to compensate for lower air density/the excedance of M_crit).

Answer 18

an increase in stall speed (IAS) with increasing altitude is due to (compressibility effects).

Question 19

Aircraft fitted with swept-back wings will reach M_crit (earlier/later), have (an earlier/delayed) onset of the transonic drag rise and a (lower/higher) C_D in supersonig flight than aircraft with straight wings.

Answer 19

Aircraft fitted with swept-back wings will reach M_crit (later), have (/delayed) onset of the transonic drag rise and a (higher) C_D in supersonig flight than aircraft with straight wings.

- higher C_D because swept wing aircraft need a higher AoA to maintain level flight because it has a lower C_L than a straight wing -

Question 20

Vortex generators cause a(n) (increase/decrease) in (M_crit/wave drag).

Answer 20

Vortex generators cause a (decrease) in (wave drag).

Question 21

Compared to a straight wing of the same airfoil section a wing with a 30º sweep-back should theoretically have an Mcrit (sine 30/cosine 30/1.154/1.141) times the Mcrit of the straight wing. However, in practice the swept wing will gain (only half/twice) that increase.

Answer 21

Compared to a straight wing of the same airfoil section a wing with a 30º sweep-back should theoretically have an M_crit (1.154) times the Mcrit of the straight wing. However, in practice the swept wing will gain (only half) that increase.

• → Cos (30°) = 0.866 → The red line is 0.866 times the green line → the green line is 1.154 times the red line. The distance is 1.154 as long, so M_crit is increase 1.154 times.*
  source: https://imgur.com/sJAmqvQ

Question 22

The purpose of a supercritical wing is to eliminate shock wave formation when flying at high speeds, even (slightly/greatly) above M_crit. They are characterized by their flattened (upper/lower) surface, (sharp pointed nose/large nose radius) and both negative as positive camber.

Answer 22

The purpose of a supercritical wing is to eliminate shock wave formation when flying at high speeds, even (slightly) above M_crit. They are characterized by their flattened (upper) surface, (large nose radius) and both negative as positive camber.

Question 23

Lowest M_crit values are produced by (thick/thin) aerofoils and (small/large) angles of attack.

Answer 23

Lowest M_crit values are produced by (thick) aerofoils and (large) angles of attack.

Question 24

Sweep-back (increases/decreases) C_Lmax, (increases/reduces) efficiency of high-lift devices and stalls first at the (root/wing tips) which therefor creates a (pitch-up/pitch-down) stall behavior.

Answer 24

Sweep-back (decreases) C_Lmax, (reduces) efficiency of high-lift devices and stalls first at the (wing tips) which therefor creates a (pitch-up) stall behavior.

Question 25

Sweep of a wing or rotorblade delays appearance of shock waves due to a(n) (increase/decrease) of the velocity of the air (parallel/perpendicular) to the leading edge.

Answer 25

Sweep of a wing or rotorblade delays appearance of shock waves due to a (decrease) of the velocity of the air (perpendicular) to the leading edge.