Planform Effects During High Speed Flight

Question 1

What is the effect on Cl of a swept wing compared to a straight wing?

Answer 1

• Swept wings have overall lower Cl for same AoA as straight wings
• Swept wings can achieve a higher angle of attack tho.

Question 2

To achieve the same amount of lift for swept wing as straight wing, what needs to occur on the swept wing?

Answer 2

• Higher AoA

Question 3

What is the effect on drag of a swept wing at large AoA? What causes this?

Answer 3

• Large increases in lift-induced drag
• Due to ram’s horn vortices resulting from:
  
  • leading edge separation
  • Flow around wing tips
  • Spanwise flow of boundary layer

Question 4

For subsonic flow, do swept wings tend to stall at the wing tips? Why is this?

Answer 4

Yes
- Due to the boundary layer flowing outwards to the wing tips which when coming into contact with adverse pressure gradient causes the air to slow, forming a thick boundary layer which is prone to separation.

Question 5

For subsonic flow, what causes the pitch up after the tip stall on a swept wing?

Answer 5

• Occurs after the tip stall due to the forward movement of the centre of pressure and increased downwash on the tailplane.
• Cp moves closer to CoG and reduces the lift weight couple, causes nose up pitch moment.

Question 6

What are swept wings effects of Mcrit and Mcdr?

Answer 6

• Mcrit and Mcdr are increased
• Increased spanwise flow means chordwise flow is reduced for the same Mfs, and shocks are delayed
• Further, effective chord is reduced with sweep and thickness to chord ratio is reduced (this reduces drag humps)
• A/C can fly faster without experiencing any transonic flow effects.

Question 7

What are swept wings effects on Mcdr?

Answer 7

By sweeping the wings, Mcdr for fuselage and wings can be at different speeds, reducing the effects of them so they don’t combine.

Question 8

What are the effects of sweep on the shockwave pattern across the wing?

Answer 8

• Each element of the wing creates it’s own shockwave
• Sweep means the outboard sections will be behind the shockwaves caused by the inboard sections.
• Flow across the inboard sections can therefore be actually subsonic, and can take on more subsonic aerofoil profiles.

Question 9

What is the difference between delta wing and swept wing?

Answer 9

Delta wind as a swept leading edge but generally a straight trailing edge (as the simplest form).

Question 10

What are the three primary design aspects of delta wings?

Answer 10

1) Sweep of the trailing edge
2) Sweep of the line of maximum thickness
3) Sweep of the trailing edge

Question 11

For optimum performance of delta wing, what should leading edge sweep be kept in?

Answer 11

The Mach Cone

Question 12

What are the advantages of delta wings (as well as swept wings) WRT a) Mcrit and Mcdr; b) Wave drag; c)T/C ratio d) Drag at high AoA?

Answer 12

a) Increased Mcrit and Mcdr
b) Reduced wave drag
c) Decreased T/C ratio
d) Increased drag at high AoA

Question 13

What are the additional 4 advantages of delta wings over swept wings WRT a) tip stalling tendencies; b) Wing loading; c) Stall angle; d) T/C ratio and stowage space

Answer 13

1) Tip stalling tendencies are reduced because the lower aspect ratio reduces the boundary layer outflow
2) Increased wing area reduces wing loading, permitting high altitude flight and lower stalling speeds
3) Stall angle is increased as leading edge vortex can be controlled to a greater degree
4) Longer chord allows a smaller thickness to chord ratio for a given thickness (or same T/C as swept with greater thickness), meaning more space for stowage and structure.

Question 14

REVISION: What are the 3 main characteristics of high speed aerofoils?

Answer 14

1) Low T/C ratio
2) Sharp leading edge
3) Reduced camber

Question 15

REVISION: What are the 3 main disadvantages of high speed aerofoils?

Answer 15

1) Lower ClMax (higher stalling speed and reduced maneuverability)
2) Abrupt leading edge stall
3) Shallow Cl vs AoA (higher nose attitude for landing)

Question 16

What does variable geometry refer to?

Answer 16

• An aircraft’s ability to change its shape in flight to suit the prevailing conditions.
• Used to improve handling characteristics of supersonic aircraft at other speeds.

Question 17

What are some variable geometry features that may be included?

Answer 17

1) Trailing edge flaps
2) Leading edge flaps
3) Drooping wingtips
4) Variable sweep

Question 18

What is the purpose of variable sweep wings?

Answer 18

• Used to improve the efficiency of the aircraft through a broader speed range
• By sweeping the wings forward and back, can get benefits of straight wings for subsonic flight and swept wings for supersonic.

Question 19

What are the four main disadvantages of variable swept wings?

Answer 19

1) Heavier structure due to internal sweep mechanism
2) More development work require during design
3) Reduces internal volume due to internal mechanisms
4) Fuel transfer may be required to ensure CoG remains within limits

Question 20

Are canards used for stability and control considerations or high speed considerations?

Answer 20

Stability and control predominantly

Question 21

What is a canard configuration?

Answer 21

One which has a foreplan located forward of the wing

Question 22

What are the two types of canard design?

Answer 22

1) Foreplane acts as a pure trimmer

2) The foreplane acting as a surface that has the same functions as a conventional tailplane

Question 23

What is the primary advantage of canards?

Answer 23

The up force on the foreplan contributes to lift

Question 24

What are the canards effect on AoA for a given speed? What effect does this have on drag and stalling speed?

Answer 24

• Because of lift contributed by canards, reduced AoA is needed to produced lift.
• This reduces drag and lowers the stalling speed

Question 25

Can both stable and unstable configurations be achieved using a canard?

Answer 25

Yes they can.

Question 26

What can the canard be an advantage with WRT stalling?

Answer 26

If canard is rigged to stall before main wing, it will provide a pitch down moment at the stall, giving a stable configuration

Question 27

What occurs if the canard is rigged to stall after the main wing?

Answer 27

• A divergent upward pitching moment is created
• This unstable config is advantageous as it increases maneuverability but requires an electronic FCS to ensure that control is not lost

Question 28

Are canards effected by main wing shockwaves?

Answer 28

Nah not really

Question 29

What can canard affect WRT airflow over main wing?

Answer 29

Vortices over canard can either help or hinder airflow over the main wing depending on its design.

Question 30

What type of designs are canards typically used for?

Answer 30

Those with the wing closer to the rear of the aircraft such as delta wings.

Question 31

What is the area rule? Why is it important?

Answer 31

• The distribution of the aircraft’s cross sectional area to closely represent that of a high speed body.
• Important to reduced wave drag

Question 32

In simple terms, what is the general cross section of an aircraft that has area rule considerations?

Answer 32

Cross sectional area increases gradually to a maximum and then decrease gradually

Question 33

What are the four artificial feel systems used for powered controls?

Answer 33

1) Spring feel - A spring provides a resistance force to the controls that is independent of aircraft speed
2) ‘Q’ Feel - control force is proportional to dynamic pressure, taht is V^2
3) G Feel, control feel is altered depending on amount of ‘g’ to prevent overstress

Question 34

What are slab surfaces used for?

Answer 34

To reduce the effects of shockwaves and turbulent flow over the control surfaces to allow the aircraft to still be controlled.

Question 35

What can spoilers be used for WRT roll?

Answer 35

Can roll the aircraft by extending innto the airflow above the wing, causing a loss of lift on the required side.

Question 36

What are the 3 advantages of spoilers for roll?

Answer 36

1) Wing trailing edge is left clear for full span flaps
2) Reduced control forces due to lower hinge moments
3) Favorable yaw

Question 37

What are 4 disadvantages of spoilers?

Answer 37

1) Non-linear relationship between deflection and response
2) Use on swept wing can lead to movement of centre of pressure and pitch up
3) Small deflections may increase lift causing a roll in the wrong direction
4) Use of spoilers at lower speed may aggravate a stall

Question 38

When are inboard ailerons used?

Answer 38

At high speeds when outboard ailerons would cause too much deflection and twisting.

Question 39

What are 2 advantages of inboard ailerons?

Answer 39

1) Fitment near wing root decreases the risk of wing flex and aileron reversal
2) Shorter moment arm offsets increased effectiveness at high speed reducing the change of over controlling the A/C

Question 40

What is disadvantage of inboard ailerons?

Answer 40

Outboard ailerons are more suitable at low speed due to their longer moment arm

Question 41

What can be used to augment and improve flight control characteristics electronically?

Answer 41

Advanced flight control Fly by wire computers.

Question 42

What are undesirable features of flap ailerons at high speeds?

Answer 42

1) Potential to cause twist on long, thin wings

2) Adverse yaw leading to directional instability.