Lift And Drag Flashcards
(59 cards)
1
Q
The lift formula
A
Lift = Cl 1/2pV^2S
Cl = Coefficient of lift
1/2p = 1/2 Air density
V^2 = velocity squared
S = Wing plan surface area
2
Q
Coefficient of lift (Cl)
A
- Dimensionless number that represents how effectively an aerofoil generates lift in response to the conditions it encounters.
3
Q
What does Cl depend on?
A
- Wing shape
- Angle of attack
4
Q
What is 1/2pV^2?
A
- The TAS^2 influenced by the air density
i.e. equivalent to dynamic pressure, or the indicated airspeed.
- Shows that lift is proportional to the square of the TAS.
If you increase the TAS x2, the lift will = x4
5
Q
Wing plan area (S)
A
The surface of the wing from wingtip to wingtip and leading to trailing edge.
6
Q
When considering a constant wing shape, area and altitude, how can you rewrite the lift formula?
A
AoA x IAS
7
Q
Coefficient of lift curve
A
- Represents the coefficient of lift of a general-purpose aerofoil.
- Increases linearly with an increase of AoA until approaching the critical AoA
- After the wing is stalled, the Cl reduces rapidly
8
Q
Camber
A
- Increase in camber = increase in Cl
- High camber - aerofoil has an increase in Cl over the entire range of AoA
- Symmetrical aerofoil - has lower coefficient of lift
9
Q
Surface roughness
A
- Smooth surfaces = better airflow
- Rough surfaces - leads to earlier flow separation as the AoA increases
10
Q
Cause of downwash?
A
- Airflow meeting at angle to each other at the trailing edge
- Create vortices and thus spirals
11
Q
Coefficient of drag
A
Ability of an aerofoil to produce drag
- Want low min, because you will have less drag to overcome in flight.
- Higher cruise speed attainable
12
Q
Good lift/drag ratio
A
- Most lift for least drag
- Greater range
13
Q
Small and stable movement of CoP
A
- Only have to strengthen the place where the CoP is going to act, reduces costs
- Remains read of aircraft CoG
14
Q
Sufficient depth of wings
A
- More space for fuel tanks and landing gear
-Greater depth of the spar allows less wight for a given strength
15
Q
Where is the pass of least resistance for air on an aerofoil?
A
From upper to lower surfaces via the wing tips.
16
Q
Where does air flow from?
A
High to low pressure
17
Q
Where is the area of low pressure on an aerofoil?
A
On top of the wings
18
Q
Where is the area of high pressure on an aerofoil?
A
Underneath the wings
19
Q
What bends the chordwise flow of air travelling from the leading to the trailing edge of the wing?
A
Spanwise flow
20
Q
What are created when the air flowing from the lower and upper surfaces meet at the trailing edge?
A
Rotating vortices.
21
Q
What is induced downwash?
A
When air curling around the wingtip also moves backward as the aircraft is advancing, resulting in curling airflow pushing down on the air behind the trailing edge of the wing.
22
Q
RAF
A
Relative airflow:
The airflow at the front of the aerofoil and that hasn’t been disturbed by the wing.
23
Q
ERAF
A
Effective relative airflow:
The entire air flow around the aerofoil, taking into account how the wing disturbs it. AKA the average airflow
24
Q
Due to the downwash, which airflow is steeper?
A
ERAF
- By having a steeper ERAF, the EAoA is steeper than the geometric AoA
25
Aspect Ratio (AR)
The ratio between the span and the chord of the wing of an aircraft.
AR = Span/Chord
Or
AR = Span^2/Wing surface area
26
Example of high aspect ratio wing
Glider
27
Example of low aspect ratio wing
Alpha 160A
28
High aspect ratio wings
- Air has a longer distance to travel from high -pressure area under the wing to the low pressure area above it.
- Lower pressure gradient
- Vortices formed at wingtips are weaker
29
Low aspect ratio wings
- Air travels a shorter distance from the high-pressure area to low pressure area.
- Steeper pressure gradient - therefore faster moving air along the span of the wing
- Stronger vortices, more pronounced downwash effects
30
Compared to a high aspect ratio wing, when will a low aspect ratio wing stall?
At a steeper geometric AoA.
31
Drag
Aerodynamic force that opposes an aircraft’s motion through the air
32
Components of total drag
1. Zero lift drag - proportional to V^2
2. Lift dependent drag (induced drag) - penalty paid for the production of lift. Inversely proportional to V^2 (1/V^2)
33
Factors affecting skin friction drag
- Speed - high speed leads to more shear stress
- Surface condition - smoother = less drag, roughness = turbulent, more drag
- Size, shape, and surface area
- AoA - increased AoA = transition point moved forward, increasing length of turbulent boundary layer, increasing drag.
34
Factors affecting form drag
- Surface conditions - ice and damage increases drag
- Size - form drag is proportional to the size of surface.
- AoA - as AoA increases, air conforms less. Separation point moves forward and form drag will increase.
- Speed - form drag increases with airspeed
35
Reducing form drag
- Streamlining
36
Reducing interference drag
- Streamlining areas where different parts of the aircraft meet
- Filleting (curved junctions) at intersections of wings, tails, and fuselage
37
The boundary layer
- The area of air right next to a surface where the effects of viscosity are significant
- Laminar (smooth and orderly) or turbulent
- 2mm for laminar
- 20mm for turbulent
- Laminar results in less skin friction drag than turbulent, but more susceptible to separation
38
Induced drag
- Created whenever lift id produced.
- (1/IAS^2)
Drag is maximum at slow speed and reduced with an increase in IAS.
- Cause by the downwash created by the trailing edge and wingtip vortices
39
Factors affecting induced drag
- Aspect ratio - increasing AR decreases induced drag
- Wing planform shape
- AoA - induced drag increases with an increases in AoA
- Airspeed - at higher speed, induced drag will be lower
- Weight - More weight requires a high AoA for a given airspeed, thus increasing induced drag
40
Reducing induced drag
- You must reduce amount of downwash created by trailing edge and wingtip vortices, therefore reduce the size and strength of vortices.
1. Straighten the spanwise flow
2. Reduce the pressure gradient between the upper and lower surfaces
41
Drag formula
D = Cd 1/2pV^2S
Where:
- Cd = coefficient of drag
- 1/2 = constant
- p = air density
- V^2 = TAS^2
- S = frontal area
42
Total drag
Combination of lift dependent drag and parasite drag
43
Coefficient of drag
- How much drag an aerofoil generates
- Minimum around 0 degrees AoA, increases steadily to 16 degrees
44
Lift to drag ratio
- Reflects the balance between two opposing forces in flight
- A goof lift to drag ratio means the aircraft can produce enough lift to stay in the air with as little drag as possible.
45
Methods to reduce induced drag
1. Aspect ratio - have a high aspect ratio
2. Washout - reduce the angle of incidence towards the wingtip, results in a smaller AoA, reduces the pressure differential at wingtips.
3. Wing taper - chord taper of camber taper. Increases aspect ratio and reduces pressure differential respectively.
4. Wing fences - strengthen the chord wise flow, reduce spanwise flow
5. Pressure gradient reduction:
- winglet
- sharklet
- drooping wingtip
- hoerner tip
- curved wing
46
Basic purpose of lift augmentation devices
- The heavier the aircraft, the more lift they need.
- Lift augmentation devices alter the aircraft’s wing shape and planform area
47
Trailing and leading-edge flaps
Changes the shape of the camber, thus increasin the coeffficient of lift.
48
Increased coefficient of lift
- Lowering the flaps
- Lowers the stall speed
- Extended flaps, aircraft can manoeuvre more safely at lower speed
49
Inreased coefficient of drag
- Lowering the flaps
- Requires more thrust to maintain a given airspeed
- Steeper descent angles
- Improved visibility
- Safer obstacle clearance
- Lower speeds
- Shortens landing distance
50
Decreased lift/drag ratio
- Lowered flaps, increase in drag is proportionally greater that lift
- Different degrees of flap create more or less drag
51
Rearward movement of the CoP
- Lowering flaps alters pressure distribution.
- Increases pressure differential near the rear where the flaps are
- Increases total reaction
- Shift CoP inwards (towards the wing root)
- Nose-down pitching tendency when flaps extended
52
Reduced angle of attack
- When flaps are lower, camber is increased
- This increases lift
- The required AoA is lower at any given speed as well as the stalling angle.
53
Simple/plain flap
- Trailing section of wing lowers into the relative airflow - increasing camber.
- Flap stalls, resulting in a smaller lift increase
- Increases Cl by 50%
54
Split flap
- Only the lower surface of the aerofoil moves
- Upper surface stays constant - prevents the early airflow separation
- Produce more drag
- increases Cl by 60%
55
Slotted flap
- Simple flat with a slot that opens ahead of the flap when lowered
- Air flows through the slot
- Accelerates over the flap nose and the upper surface
- Re-energises the boundary layer - delaying separation and allowing for a higher max lift
- Increases Cl by 65%
56
Fowler flap
- Similar to slotted flap
- Deflects downward and backwards
- Shifts the CoP further back
- Largest lift increase with the smallest drag increase
- Increase Cl by 90%
57
Leading edge flaps
- Typically used along with trailing edge flaps
- Main role is to increase the wing's camber
- Increased stalling AoA
- Forward movement of the CoP
58
Slats and slots
- Slat = lift augmentation device ahead of the main aerofoil.
Creates a slot of air to flow through.
mainly used on older aircraft - create too much drag
- As the wings AoA increases, causing more upwash, slat's angle becomes positive.
- It slides forward from the leading edge and creates a slot.
- Reducing the AoA does the opposite
59
Spoiler
- Hinged surfaces, that when extended, disrupts the air flow and increases drag
- Glider pilots use mechanically operated spoilers
- Jets use hydraulically operated spoilss
