Tech Block 2

Question 1

1. The primary purpose of sweep back is to..

Answer 1

o Increase the value of the critical mach number

Question 2

2. How does sweep back work?

Answer 2

o 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 3

3. Advantages of a swept wing?

Answer 3

o Mcrit increased
o Higher economical cruise speed (delays onset of compressibility effects)
o Increased lateral stability (roll)
o ‘Softens’ the onset of the force divergence number

Question 4

4. Disadvantages of sweepback?

Answer 4

o Lower CL max (flatter curve giving higher stall speed)
o Extensive use of high lift devices
o High drag at high AOA (CL max obtained at high AoA, overcome by slots & flaps)
o Use of vertex generators, wing fences to reduce wingtip ‘pooling’
o Prone to wing tip stalling (tips are aft part of the wing and separation point is closer to the leading edge.)
o Taper adds to tip stalling but reduces induced drag.
o T tail makes stalling worse as it provides little or no buffet warning (use stick shaker)
o Sweepback increases drag at low speeds as it gives more spanwise flow than high speeds (due to time).

Question 5

5. A swept wing aircraft pitches up or down at a stall?

Answer 5

o 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 6

6. Wash out is…

Answer 6

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

Question 7

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

Answer 7

o Wing fence
o Saw tooth leading edge (generate a vortex to re-energise the flow)
o Vortex generators (re-energise the flow)

Question 8

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

Answer 8

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

Question 9

9. During a turn what happens to the CP of a swept wing aircraft?

Answer 9

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.

Downgoing wing has a relative airflow more from below, upgoing wing has a relative airflow more from above.

Question 10

10. Dihedral is…

Answer 10

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

Question 11

11. Anhedral is…

Answer 11

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

Question 12

12. Area Rule

Answer 12

o 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. The area of cross section should increase gradually to a maximum, then decrease gradually. Giving more streamline shape. Max cross-section area should be approx half-way.

Question 13

13. Formula for Aspect Ratio is…

Answer 13

o AR = Span (width) / Chord (length)

Question 14

14. High Aspect ratio (subsonic speeds) gives…

Answer 14

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

Question 15

15. What is Mach tuck?

Answer 15

o 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 16

16. What happens if Mach tuck is not corrected?

Answer 16

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

Question 17

17. Oscillatory instability is…

Answer 17

o A combined yawing and rolling movement

Question 18

18. What is dutch roll and what causes it?

Answer 18

o It is oscillatory instability when the rolling motion is predominant and,
o It is 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.
o A yaw damper is a gyro system sensitive to changes in yaw which feeds a signal into the rudder which then applies rudder to oppose the yaw. With this device, a Dutch roll will not develop because the yaw which triggers it all off is not allowed to develop. It applies the rudder in the correct direction and in the correct amount, thus preventing the slip starting or building up and stopping all rolling tendency Apart from the swept wing, the basic cause of Dutch rolling tendency is lack of effective fin and rudder area.

Question 19

19. What is a stabilising device to prevent dutch roll?

Answer 19

o Yaw Damper

Question 20

20. What is snaking?

Answer 20

o Oscillatory instability when the yawing motion is predominant

Question 21

21. Oscillatory instability is worse when…

Answer 21

At high altitudes
On sweepback aircraft at low IAS

High altitude = High angle of attack = High dihedral effect = High chance of dutch roll.

Approach at slow speeds = High angle of attack = High dihedral effect = High chance of dutch roll.

Question 22

22. The aircrafts aerodynamic ceiling is…

Answer 22

o The point at which the high airspeed mach buffet and low speed stall buffet merge, called coffin corner.

Question 23

23. The manoeuvre envelope must be…

Answer 23

o At an altitude and airspeed sufficient to avoid stalling and slow enough to avoid structural damage

Question 24

24. Gliding angle depends on…

Answer 24

o Ratio of lift to drag ratio – greater weight does not effect gliding angle or range, does effect speed.

Question 25

25. Rate of vertical descent depends on…

Answer 25

o Angle of glide and airspeed

Question 26

26. Aircraft in constant descent at 300kts, how does weight affect glide angle.

Answer 26

o The heavier the aircraft, the earlier descent must commence. So the more shallow the glide angle.
o Conversely, the lighter the aircraft, the steeper the descent glide angle, for the same speed as the heavier aircraft.

Question 27

27. Define rolling, pitching and yawing movement. About what axis, called what stability?

Answer 27

o Rolling is about longitudinal axis and is lateral stability
o Pitching is about lateral axis and is longitudinal stability
o Yawing is about normal axis and is directional stability

Question 28

28. What is the resultant of using ailerons for turning / rolling, and how is it corrected?

Answer 28

Adverse aileron yaw which is caused by the lowering of one aileron (down going aileron, up-going wing)
Lowered aileron causes additional drag which produces a yawing moment in the opposite direction. By increasing the angle of attack too much causing more drag and by aileron pushing into airflow.

Corrected by use of spoilers and differential or frise ailerons. Or slot-cum-aileron control (Kermode p302)

Question 29

29. What is the difference between differential and frise ailerons?

Answer 29

o Differential ailerons: Up going aileron moves through large rangle to increase drag
o Frise ailerons: Up going aileron projects below as well to cause excessive drag

Question 30

30. Aileron size is limited by…

Answer 30

o Flap size

Question 31

31. The effect of wing twist from operating ailerons at height speed is…

Answer 31

o It will minimise the effect of the ailerons / reverse the effect all together. Turning at high speeds can increase lifting forces on the upgoing wing so much that it twists the wing and reduces the effective AoA. Use inboard ailerons or spoilers at high speeds.

Question 32

32. How do ailerons prevent wing twist at high speeds?

Answer 32

o They are locked out to prevent wing twisting, inboard ailerons are employed to provide roll as well as spoilers.

Question 33

33. How do spoilers produce roll?

Answer 33

o The spoilers are raised on the down going wing reducing the lift on that wing

Question 34

34. What are the function of spoilers?

Answer 34

o Lift dumping in flight – increase the rate of descent
o Speed brakes in flight – to quickly decrease speed in flight
o Ground spoilers to destroy lift – to achieve shorter landing distances
o Assist lateral (roll) control - allows smaller aileron size, avoids aileron reversal
o Direct lift control

Question 35

35. What is direct lift control?

Answer 35

o Use of aerodynamic surfaces (spoilers) to provide control of rate of descent without need to change body angle on approach

Question 36

36. What are differential spoilers?

Answer 36

o They provide roll control when flight spoilers are in use and aileron input is given

Question 37

38. What is the reason for using a variable incidence tail plane?

Answer 37

o To counteract large trim changes through use of fuel and large speed changes allowing the elevator to remain fully effective under all conditions of longitudinal trim
o Less drag at high speed
o Provides control when a shockwave forms on the tail plane

Question 38

39. Power assisted controls provide what?

Answer 38

o Pilot force which is assisted by power units which in turn provides “Feel”
o Provide all the force necessary to operate a control surface, control column feel is provided artificially

Question 39

40. What is Q feel?

Answer 39

o An artificial method of providing the pilot with control column loading using either springs or hydraulics which provide variable loading proportional to airspeed

Question 40

41. Extending flaps increases what?

Answer 40

Wing camber

Wing area and CL

Question 41

42. Slotted flaps allow…

Answer 41

o The airflow to be re-energised avoiding separation

Question 42

43. Large aircraft are usually fitted with what type of flap?

Answer 42

o Slotted fowler (moves down and rearward)

Question 43

44. When does aileron droop occur?

Answer 43

o When flaps are extended (usually the inboard ailerons)

Question 44

45. A Krueger flap does what?

Answer 44

o Extends forward of the leading edge providing an increase camber

Question 45

46. Leading edges slats extend…

Answer 45

o Outwards providing a slot to delay boundary layer separation

Question 46

47. An aft / forward C of G has what effect on the stall speed?

Answer 46

o Aft decreases the stall speed and forward increases the stall speed

Question 47

48. What are the purposes of vortex generators?

Answer 47

o To re-energise the boundary layer & delay separation or air from wing

Question 48

49. What is hydroplaning and what does it depend on?

Answer 48

o	When a tyre is lifted clear of the runway due to a build up of fluid pressure beneath the tyre
o	Tyre pressure
o	Runway surface
o	Tread depth
o	Tyre speed
o	Water depth
o	Runway grooving

Question 49

50. What is the formula for the Hydroplaning Speed?

Rotating and Non-rotating. (T6 speed?)

Answer 49

o Rotating = 9.0 x square root of tyre pressure
o Non-Rotating = 7.7 x square root of tyre pressure
o T6 aquaplane 133 kts

Question 50

37. What is the limitation of spoilers?

Answer 50

o At high speed they can blow back