General Questions

Question 1

Wave drag is

Answer 1

• The separation of the airflow behind the shock wave

Question 2

A shock stall is the result of

Answer 2

• The boundary layer behind the shock wave becoming turbulent and separating, spilling rearwards and striking the tail plane, creating buffet and rearward CP movement. Rear movement of the CP causes nose pitch down

Question 3

Wing design for high speed flight takes into consideration

Answer 3

• Minimal camber to delay shockwaves
• Maneuverability
• Low thickness chord ratio

Question 4

The primary purpose of sweep back is to

Answer 4

• Increase the value of the critical Mach No.

Question 5

How does sweep back work

Answer 5

• By sweeping the wing the freestream air that travels along the effective chord is less therefore less acceleration is achieved resulting in a lower speed over the wing and a higher achievable aircraft speed before Mcrit is reached.

Question 6

Advantages of a swept wing?

Answer 6

• MCRIT increased.
• Higher economical cruise speed
• Increased lateral (roll) stability

Question 7

Disadvantages of sweepback

Answer 7

• Lower Cl
• Extensive use of high lift devices
• High drag @ high AOA
• Use of vortex generators, wing fences to reduce wingtip ‘pooling’

Question 8

A swept wing aircraft pitches up / down at a stall

Answer 8

• A nose pitch up results from the wing tips stalling first moving the CP inwards and forwards (wash out is used to try and prevent tip stalls)

Question 9

Wash out is

Answer 9

• A decrease in incidence from root to tip – to prevent wing tip stall

Question 10

What devices are used to prevent wing tip stall (spanwise flow)

Answer 10

• Wing fence
• Saw tooth leading edge
• Vortex generators

Question 11

What happens to the center of pressure in a stall in a swept wing aircraft

Answer 11

• Tips will stall first so CP moves inward and forwards & nose tends to pitch up

Question 12

During a turn what happens to the CP of a swept wing Aircraft

Answer 12

• As the wing gets higher in a turn the outer portion become higher than the inner portion which creates its own form of washout resulting in a lower AOA and causing the CP to move inwards and pitch the nose upwards.

Question 13

Dihedral is

Answer 13

• The upward inclination of the wing to the lateral axis to provide lateral (roll) stability

Question 14

Anhedral is

Answer 14

• Negative dihedral – usually with high mounted swept wings to combat dynamic instability (dutch roll)

Question 15

Area Rule

Answer 15

This is a design function to ‘blend’ areas where wings, tail, join the fuselage to minimise the increasing and decreasing cross section, minimizing the amount of drag formed by shockwaves

Question 16

Aspect ratio

Answer 16

• = span (width) ÷ chord (length)

Question 17

High Aspect ratio (subsonic speeds)

Answer 17

• Better lift
• Better lift/drag ratio
• Less induced drag due reduced wing tip vortices

Question 18

Mach tuck

Answer 18

• Is when the aircraft is accelerated through the transonic range causing the CP to move rearwards and increasing the lift generated by the tail plane due to modified airflow from the wing causing a nose pitch down.

Question 19

If Mach tuck is not corrected the result will be

Answer 19

• The nose pitch down causes further speed increase which causes further movement rearwards of the CP which causes further nose pitch down…..etc

Question 20

Oscillatory instability is

Answer 20

• A combined yawing and rolling movement

Question 21

Dutch roll is

Answer 21

• Oscillatory instability when the rolling motion is predominant

Question 22

Dutch roll is a result of

Answer 22

• A yaw to the left or right which makes the outside wing speed up producing more lift resulting in a roll, after which because of the greater exposed area of the faster wing it has a higher drag component therefore causes a yaw in the opposite direction, resulting in a roll in the direction of the yaw

Question 23

A stabilising device to prevent Dutch roll is

Answer 23

• A yaw damper (gyroscopically operated stabilisers)