POF 7 - Lift Augmentation and Degradation

Types of Trailing-edge flaps

Plain flaps - Simple construction, Mainly used on low speed aircraft, good lift but lots of drag.

Split flaps - Forms part of the lower surface of the wing trailing edge.  Same lift increase of plain flap at low alpha but greater at high alpha. Increased drag compared to plain

Slotted flaps - Purpose of the slot is to guide higher pressure air from the lower surface over the flap and re-energise the boundary layer. Slotted flap gives more CLmax than a plain or split flap but with less drag. More complex

Fowler flaps - increase Surface area, increase camber. May be slotted. Gives the greatest increase in lift while having the least drag due to the slots.

Coefficient of lift and TE flap type

Pitching Moments and Flap movement

• Flap movement will cause a change in the overall pitching moment of the aeroplane 

• For trailing edge flaps, it will cause a pitch down moment.

• CP movement on the main wing - Pitch down moment

• Downwash -  pitch up moment 

• Depends of type of wing, and position of tail plane

What is Adverse pressure gradient

• Low static pressure in the throat causes higher PSTAT to reverse flow up the wing

What is Alpha Critical?

 AoA which is the last point before adverse pressure gradient causes a lift to not maintain level light

TE flaps reduce Alpha critical

TE flaps increase CL max for slow flight

Flap Problems and Risk management

Flap Overspeed and Overstress

Risk mitigation

• IAS sensor

• Overspeed protection 

Flap problems - Flap Asymmetry 

Effects

• Force unbalance

• Roll control problems

• Yaw control issue - blocks airflow over the wing with body

• Possible loss of control

Risk mitigation

Excess torque sensor

Flap asymmetry detection

Leading edge Lift Augmentation

There is two methods of increase the lift of the wing

1. Increasing leading-edge camber

• Lead-edge flap

2. Re-energising the boundary layer

• leading -edge slot or slat

LE - SLOTS/SLATS

• Slots 

  • A gap from the lower surface to the upper surface of the leading-edge.

  • They allow air to pass through

  • Re-energised the boundary layer which delays the flow separation to a higher angle of attack.

• Automatic slots

  • Not controlled by the pilot, open by changes of pressure.

  • Mainly used on small aircraft

• Disadvantage of slots 

  • Required a high AOA to work, limited visibility for landing

LE Flaps

• Krueger flaps

  • Lower surface of the leading-edge, which rotates abouts its forward edge

  • Not as effective as variable camber flaps

• Variable Camber Flap

Leading Edge flaps change the chord of the basic aerofoil and increase the camber because the profile changes. The same as trailing-edge flaps

High Lift Devices

Effects of Flaps on L:D ratio

Increase in lift, but increase in drag is greater therefore L:D ratio decreases

•  Decreased glide range

• Increased fuel burn

• Reduced range

• Reduced endurance (time in the air)

• All flaps reduce L/D ratio

• All flaps slow VMD

Overview of Vortex Generators

• Energies the boundary layer by introducing high energy free stream flow into layers of the boundary layer

• Increases Alpha critical

• Delays flow separation

Saw tooth/dog tooth - acts barreis to reduce spanwise flow, reduces vortex

Nacelle strake- Used to bring air over the wing which is blocked by the engine.

Advantages - More aerodynamic efficiency, better control performance, less aircraft noise (fuel vents on a320)

Disadvantages - Create extra drag but they rescue the flow separation, less profile drag will be created as well

Flap operation

First 50% of deflection, mainly increases CL

• Used for take-off

Second 50% is mostly CD increase ( used to slow aircraft, allow for steeper angles of descent)

• Used for landing

Speedbrakes

Affecting drag and lift are called spoilers as they spoil part of the lift force

Speed brake - 50% deflection

Lift dump - 100% deflection -weight on wheels. 

Stall speed will increase due to induced flow separation  by extending the spoilers.

Aerodynamic Degradation

Contaminants on the airframe will modify the profile of the zerofoil and increase the roughness of the surface.

Locations of possible Ice build-up

• Aerodynamic surfaces - leading-edge of the wing, horizontal and vertical stabiliser

• Components - windshield, wipers, landing gear

• Instrument probes - pitot, static, TAT, AoA

• Powerplant - fan blades

Possible contaminants 

• Liquid water - rain, mud

• Solid water - snow, ice, frost

• Biological - insects, bird strike

• Mechanical - ageing, dents, scratches, dirt

• Lift (CLmax) - 

• Drag - increase

• Stall speed Increase - Decrease in CLmax

• alphaCrit - decrease

• Stability - Less table

• Controllability - Less