Lift Augmentation Flashcards
Explain the basic purpose of lift augmentation devices. And what ones are mainly used
To provide the advantages of high lift at low speed without incurring the disadvantage of generating high drag at high speeds.
Flaps,slats and slots.
Explain the basic principles of trailing edge flaps.
The flaps are lowered to change the shape of the aerofoil section over that part of the wing to which they are attached.
When lowered, increases in camber results in greater pressure differential being generate above and below the wing.
Explain the effects of lowering trailing edge flap on; CL, CD, L/D ratio, CP movement, angle-of-attack and nose attitude.
CL - increases. Meaning more lift is produced with flaps down but reduces stall speed.
CD - increases.
L/D ratio - the increase in Cd is larger then Cl. So the ratio is reduced.
Reduced AoA - Cl max is at a lower angle as the flap setting is increased. V the aircraft will fly level and stall at lower nose altitude when extended.
Taking an average of chord line, the wing will stall at a similar effective Angle as the original geometric angle.
(AoA = geometric AoA where original chord line is reference)
Rearward movement of CoP - TR is increase and CoP is moved rearwards when flaps are lowered.
Distinguish between the effects of lowering leading-edge flap on angle-of-attack and movement of the CP with those of trailing-edge flap.
The stalling angle of attack is increased
CoP moves forward.
Given a description or diagram identify the main types of leading-edge flap.
Drooping flap - the whole of the leading edge surface moves forward and down.
Krueger type - lower leading-edge surface hinges downward from the nose.
Explain the basic principles of slats and slots.
Slat - small aerodynamic surface placed ahead of a main aerofoil, to form a slot through which air can flow.
Low AoA, the effective AoA is negative and is closed.
As AoA increases it brings an increase in upwash over the leading edge and the effective AoA of the slat reaches the point where it produces ‘positive’ lift, it moves up and forward on its track opening the slot.
automatic slats open when required AoA is reached.
Explain the effect of operating leading-edge slats on CL, stalling angle.
Separation is delayed and the pressure envelope over the upper surface is flattened out.
Stall angle will then occur at higher AoA and is delayed.
Cl max is increased.
Show understanding of the theory of spoilers and give examples of their use.
Hinged surfaces which extend and is turn the airflow over the upper part of the wing decreasing lift and increasing drag.
Examples
Gliders use mechanically operated to reduce airspeed and steepen descent path without increasing airspeed.
Large jets use hydraulically operated to dump the lift and increase the weight on the wheels. Improving brake effectiveness and increasing drag.
What are the factors affecting Cl and what can we change with them if we can?
AoA - increase it to critical angle
Condition of the wing surface - keep clean but can change airbourne
Shape of the wing section and planform - where we can have the most impact on increasing CL
How can the wing planform change. (6) (shape, leading edge radius, thickness/chord ratio, sweep, AR, camber)
Shape - cannot change
Leading edge radius - yes with Leading Edge flaps
Thickness/chord ratio - yes with flaps
Sweep - variable geometry
Aspect ratio - yes with flaps
Camber - with flaps,
