Planform Effects During High Speed Flight Flashcards
What is the effect on Cl of a swept wing compared to a straight wing?
- Swept wings have overall lower Cl for same AoA as straight wings
- Swept wings can achieve a higher angle of attack tho.
To achieve the same amount of lift for swept wing as straight wing, what needs to occur on the swept wing?
- Higher AoA
What is the effect on drag of a swept wing at large AoA? What causes this?
- 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
For subsonic flow, do swept wings tend to stall at the wing tips? Why is this?
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.
For subsonic flow, what causes the pitch up after the tip stall on a swept wing?
- 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.
What are swept wings effects of Mcrit and Mcdr?
- 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.
What are swept wings effects on Mcdr?
By sweeping the wings, Mcdr for fuselage and wings can be at different speeds, reducing the effects of them so they don’t combine.
What are the effects of sweep on the shockwave pattern across the wing?
- 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.
What is the difference between delta wing and swept wing?
Delta wind as a swept leading edge but generally a straight trailing edge (as the simplest form).
What are the three primary design aspects of delta wings?
1) Sweep of the trailing edge
2) Sweep of the line of maximum thickness
3) Sweep of the trailing edge
For optimum performance of delta wing, what should leading edge sweep be kept in?
The Mach Cone
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?
a) Increased Mcrit and Mcdr
b) Reduced wave drag
c) Decreased T/C ratio
d) Increased drag at high AoA
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
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.
REVISION: What are the 3 main characteristics of high speed aerofoils?
1) Low T/C ratio
2) Sharp leading edge
3) Reduced camber
REVISION: What are the 3 main disadvantages of high speed aerofoils?
1) Lower ClMax (higher stalling speed and reduced maneuverability)
2) Abrupt leading edge stall
3) Shallow Cl vs AoA (higher nose attitude for landing)
What does variable geometry refer to?
- 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.
What are some variable geometry features that may be included?
1) Trailing edge flaps
2) Leading edge flaps
3) Drooping wingtips
4) Variable sweep
What is the purpose of variable sweep wings?
- 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.
What are the four main disadvantages of variable swept wings?
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
Are canards used for stability and control considerations or high speed considerations?
Stability and control predominantly
What is a canard configuration?
One which has a foreplan located forward of the wing
What are the two types of canard design?
1) Foreplane acts as a pure trimmer
2) The foreplane acting as a surface that has the same functions as a conventional tailplane
What is the primary advantage of canards?
The up force on the foreplan contributes to lift
What are the canards effect on AoA for a given speed? What effect does this have on drag and stalling speed?
- Because of lift contributed by canards, reduced AoA is needed to produced lift.
- This reduces drag and lowers the stalling speed
Can both stable and unstable configurations be achieved using a canard?
Yes they can.
What can the canard be an advantage with WRT stalling?
If canard is rigged to stall before main wing, it will provide a pitch down moment at the stall, giving a stable configuration
What occurs if the canard is rigged to stall after the main wing?
- 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
Are canards effected by main wing shockwaves?
Nah not really
What can canard affect WRT airflow over main wing?
Vortices over canard can either help or hinder airflow over the main wing depending on its design.
What type of designs are canards typically used for?
Those with the wing closer to the rear of the aircraft such as delta wings.
What is the area rule? Why is it important?
- The distribution of the aircraft’s cross sectional area to closely represent that of a high speed body.
- Important to reduced wave drag
In simple terms, what is the general cross section of an aircraft that has area rule considerations?
Cross sectional area increases gradually to a maximum and then decrease gradually
What are the four artificial feel systems used for powered controls?
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
What are slab surfaces used for?
To reduce the effects of shockwaves and turbulent flow over the control surfaces to allow the aircraft to still be controlled.
What can spoilers be used for WRT roll?
Can roll the aircraft by extending innto the airflow above the wing, causing a loss of lift on the required side.
What are the 3 advantages of spoilers for roll?
1) Wing trailing edge is left clear for full span flaps
2) Reduced control forces due to lower hinge moments
3) Favorable yaw
What are 4 disadvantages of spoilers?
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
When are inboard ailerons used?
At high speeds when outboard ailerons would cause too much deflection and twisting.
What are 2 advantages of inboard ailerons?
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
What is disadvantage of inboard ailerons?
Outboard ailerons are more suitable at low speed due to their longer moment arm
What can be used to augment and improve flight control characteristics electronically?
Advanced flight control Fly by wire computers.
What are undesirable features of flap ailerons at high speeds?
1) Potential to cause twist on long, thin wings
2) Adverse yaw leading to directional instability.
