Aerodynamics Flashcards
Stagnation Point
Point at which air cannot decide whether to pass under or over the aerofoil
Constant Total Pressure
Constant Total Pressure = Static pressure + Dynamic Pressure
As static pressure increases, dynamic pressure decreases and vice versa
Affect of Temp on Air Pressure
As temp increases, total air pressure and density decrease
As temp decreases, total air pressure and density increase
Lift Formula
L = Cl x r/2 x V^2 x S Cl = AoA + camber r = air density v = velocity S = surface area
Ways of Increasing Lift
Increase camber, velocity and AoA
Centre of Pressure
The point at which lift acts from, which moves along the chord line as the AoA varies
If AoA increases, the centre of pressure moves forwards
Parasite Drag
Caused by movement of the aircraft through the air
Types of Parasite Drag
Form: Shape
Interference: Where surfaces meet
Skin Friction: air passing over a surface
Cooling: engine cooling
Induced Drag
Caused by the creation of lift
Wingtip Vortices
Results from span wise flow
Span Wise Flow
Results in airflow above the wing moving towards the wing root and below the wing moving towards the wing tip
Wake Turbulence
Due to wingtip vortices
Can be destroyed by wind
Ways to Reduce Induced Drag
- Wing tapering: smaller wingtip chord line and therefore smaller wingtip vortices
- Aspect ratio: a measure of an aircrafts wing span to its wing chord, increased aspect ratio wings produce increased lift for decreased drag
- Washout: the ‘angle of incidence’ at the wing root is greater than at the wing tip
- Wingtip design: design features such as wingtip fuel tanks, winglets and modified wingtips can help to reduce leakage of airflow around the wingtip and reduce induced drag
- Wing fences: reduce span wise flow
Lift/Drag Ratio
Most efficient L/D ratio
Min amount of drag created for an x amount of lift
Airspeed
Indicated airspeed (IAS) Calibrated airspeed (CAS): corrected for aircraft equipment errors True airspeed (TAS): corrected for temp and pressure, aircrafts true speed Ground speed (GS): TAS with wind allowance, aircraft speed relative to the ground
Stability
The tendency of an aircraft to return to its original state, following a disturbance, without any pilot input
Dynamic Stability
Unstable and will get further away from the original path
Longitudinal Stability
After a disturbance in pitch it returns to the original AoA, without pilot input
Directional Stability
After a disturbance in yaw the aircraft returns to the original heading, without pilot input
Improved Longitudinal and Directional Stability
Increase surface area or move the centre of gravity forward
Lateral Stability
After a disturbance in roll the aircraft returns to its original state, without pilot input
Improved Lateral Stability
Dihedral
Sweepback
High wing/low centre of gravity
Stronger Directional Stability Than Lateral Stability
After a roll creates a spiral dive
Spiral instability
Stronger Lateral Stability
Will cause a dutch roll
Ground Effect
A cushioning effect caused by the air between the wing and the ground
At a height less than approx a wingspan above the surface
Ground surface restricts airflow by:
- reducing up and downwash
- restricting the formation of wingtip vortices (if not formed fully there is little to no induced drag)
As the aircraft takes off it increases in induced drag, decreases in airspeed and increases in lift
Ancillary Controls
Flaps
Trim
Engine
Flaps - Leading Edge Devices
Helps to reenergise the smooth airflow over the leading edge of the wing
Provide additional lift at higher AoAs and allows the aircraft to fly slower and at higher AoAs prior to stalling
Eg. Slats, kruger flaps and leading edge flaps
Flaps - Trailing Edge Flaps
Are mounted on the trailing edge and move together
Increases the camber of the wing, increases the Cl for any AoA and increases drag, reducing airspeed and decreasing stalling angle and speed, and causes a nose down pitching moment
Simple/Plain/Camber Flap
- The rear section of the wing hinges down
Slotted Flap
- A slot is formed between the wing and the flap
- Re-energises airflow
Split Flap
- Forms part of the lower surface of the wing at the trailing edge
- Undesirable feature: creates a sudden decrease in lift
Fowler Flap
- Extends from the wing, increases area and camber of wings, moves rearwards and downwards
When using flap the centre of pressure moves back
Trim
Relieves pilot of control loads
Creates a small aerodynamic force which holds the elevator in place
Forces in a Climb
L < W
T = D + Rcw
T > D
Types of Climb
Normal Climb: Increased visibility, better engine cooling, increased airspeed
Max Rate of Climb (Vy): Best rate, best L/D ratio
Max Angle of Climb (Vx): Avoid Obstacles and controlled airspace
Factors Decreasing Climb Performance
Reduced power Increased weight Inaccurate airspeed being maintained Increased air temp Decreased air density and pressure
The Cruise Descent
Helps to ensure engine warmth and passenger comfort
The Glide Descent
Max range and endurance for engine failure
L < W
D + L = W
D = FCW
Performance will vary due to: flaps, weight, inaccurate speed maintenance and wind
Forces in a Turn
Centripetal force pulls into a turn
Load factor = how hard the wing is working
G-factor = 1 in straight and level flight
Range
Best range - Vimd (indicated speed for min drag)
When aiming for max range, we are achieving:
- max distance per litre of fuel
- max fuel burn over a given distance
To max, we need to min the fuel burn per mile
- fuel burn/distance = fuel burn per hour/distance per jour = fuel flow/groundspeed
Fuel flow is directly related to power and groundspeed to TAS, so:
- Fuel flow/GS is directly related to Power/TAS
Also power = thrust/drag, so thrust = power/TAS
Min fuel burn means min thrust
Thrust = drag in level flight, max range is achieved by flying at the speed for min drag (best L/D ratio)
Endurance
With max endurance, we achieve:
- Max time aloft for a given amount of fuel
- A given time in flight for the min amount of fuel
Aiming for min fuel flow
Fuel flow is directly related to power, therefore the best speed for max endurance is the speed for min power
Best Range and Endurance
Must account for aerodynamics and engines
Normally aspirated engines
- Best range: at full throttle height (power setting gives min drag speed)
- Best endurance: flying as low as possible (when TAS is almost = to IAS, air more dense and therefore less power to maintain IAS
Supercharged Engine
- Best range = high altitude
- Best endurance = flying low
Lack of Stability on the Lower Speed Range
Between stall speed and min power speed, it is difficult to maintain constant speed
If a gust decreases speed, total drag increases, reducing the speed further, therefore increasing power
If a gust increases speed, opposite applies and decreased power is required
Must be quick on the throttle to maintain airspeed
Effects of Changes in Weight On AoA and IAS in level flight
- As weight decreases, less lift is required, therefore IAS decreases by reducing power
- Or maintain power, decrease AoA and therefore, increase IAS
Effect of Altitude On IAS and AoA
- Will be the same (same amount of lift required)
- Same IAS will give increased TAS with Increased altitude
Effect of Wind On Endurance and Range
On endurance:
- Not affected as time airborne
On range and glide range:
- To achieve max range: fly faster than best range speed to minimise the effect of headwind or fly slower than best range speed to max the effect of a tailwind
Power
Min power required to keep airborne means: min fuel flow which gives max endurance
Thrust
Needs to = drag to maintain airspeed
The difference in airspeed before and after the propellor disk, therefore thrust decreases as IAS increases
Max thrust when we have min drag gives max range (min fuel per nm)
Speed that gives max ‘excess’ thrust is max angle of climb
Effects of Changes in Weight On Range and Endurance in Level Flight
- As weight decreases and less lift is required we can either decrease IAS by reducing power = greater endurance or decrease AoA while maintaining IAS, drag decreases and therefore range increases
Effects of Changes in Weight On Glide Range and Endurance
- Maintain AoA (optimum), therefore decreased weight = decreased lift and IAS to maintain best glide speed (constant), therefore glide range is not affected but with a lower speed rate of descent will decrease, increasing endurance
Effects of Changes in Weight on Turn Performance
No effect
Effect of Altitude On Range and Endurance
- Little effect on range as no affect on drag and thrust
- Best endurance = maintain a certain IAS, therefore increase TAS required at increased height. As power required depends on TAS, endurance decreases with increased height
- Fly at the altitude with most favourable wind
Effect of Altitude On Turn Performance
- Turn at certain angle of bank and IAS, TAS will increase with height, therefore the radius of the turn increases and it is a lower rate of turn
