Stalling Flashcards
Reducing IAS does what do CL
CL need to increase to maintain level flight
Straight Wing Stall
Distinct Symptoms (nose drop/buffering)
- Lift reduces
- Total drag much greater from large increase in form drag
- CP moves aft
You stall what you gonna do
REDUCE AOA
DO NOT USE AILERON OR RUDDER UNTIL YOU UNSTALL
Factors Contributing To Wing Drop
Manufacturing differences between each wing
Small Lateral imbalances in lift prod
Loss of lift at tip rather than root (moment)
Ellipetical Wing Stall Category
Same vortex spanwise = same downwash spanwise
All area of wing working equally as hard
! Sudden stall - controls become ineffective quickly
! Large roll rates
Stalling With Tapered Wings
Mid section vortex least so least downwash here (greatest EAOa at mid section)
Stall starts in mid section
Not as bad as Ellipetical wing
Stall With Swept Wing
Less vortices at tip - EAOA greatest at tip
Tip will stall first
CP moves forward and inwards
AC pitches further up (Deep stall)
Increased Downwash on tailplane adding to the deep stall conditions
Deep Stall
Wake of wing impinges tail surfaces making them ineffective
Worse on high T tail planes
A/C with sweepback wings
Anti Stall Devices
Vortilon - Generate vortex to help spanwise flow
Wing Fence - Physical barrier to spanwise flow
Reduce camber/washout towards tip to reduce local CL and prevent tip stall
Cross Section - Thicker more progressive inboard stall
Nacelle Stakes - Located on engine tight vortex to reenergise
Stall Strips - Used on tip to sharpen LE at root
Wing Loading on stall speed
Greater Wing Loading = Higher Stall Speed
Lower Wing Loading = Lower Stall Speed
Weight/area
Flaps/Slats on Stall Speed
CLMAX increased + VS speed decreases
LE - Increase stalling Angle
TE - Reduce Stalling Angle
Slates/Slots - Increase delaying flow separation to higher AOA
Measures to delay flow separation
Vortex Generators
Engine nacelle strakes (lift vanes)
Sharper vs Larger radius leading edge
Sharper LE the tighter the radius of turn more energy consumed
Large radius from LE less energy loss by flow
Stagnation point moves backward onto underside of aerofoil as alpha increases
Critical Angle Of Attack
Highest achievable AOA before the stall
No the actual stall
Maximum lift produced at the critical angle
AC will always stall at the same critical angle of attack
Aspect Ratio on stalling angle
High aspect ratio produces same lift with less span wise flow/less downwash = shallower effective airflow
High aspect ratio the effective angle of attack = low wing angle of attack
Low aspect ratio produces more span wise flow and more downwash for same lift = small effective angle of attack and therefore stalling angle of attack is much larger
Rectangular Wing on stalling
Stalls at the roots first then the tips
Shallower effective airflow at wing root produces a greater effective angle of attack and therefore stalls first
Spread from root to the tip as alpha increases
Why does a sweep back wing pitch up at the stall
Wings CP moves forward and inwards
Increased downwash on horizontal stabiliser (inner wing has to produce more lift increasing downwash airflow onto tail)
CS25 Stall Criteria
Nose down pitch that cannot be readily arrested
Buffeting of magnitude to deter further speed reduction
Pitch control reaches aft stop no more pitching
Deterrent Buffet
Point at which buffet becomes a strong and effective deterrent
Vertical accelerations of 0.5g+
Engine nacelle (Lift vanes)
Reduces upwash ahead of wing delaying onset of seperation locally in area of wing influenced by nacelle
Create large vortex over upper surface which reenergises boundary layer locally delaying separation to high alpha
Cross section on stalling
Sharper thinner leading edge stalls more sudden (high-speed wing)
High-thickness chord ratio with greater leading edge has a more gradual stall
VS1
Minimum steady flight speed in obtained config
VS0
Stall speed in landing config
EASA regs for MEP aircraft in class B
VR = 1.1vs1
T/O Screen = 1.2vs1
L/D(vref) = 1.3vs1
EASA regs for stall speeds in class A
VR = 1.1vs1
VSRI (T/O Screen) = 1.13VSR1
VSRD (Landing) = 1.23VSRD
10%
13%
23%
EASA C25 Reg for stall warning speed
VSW must be 5kt or 5% greater
What happens towards the stall
AOA is increasing
Stagnation point moving rearward
Kinetic energy airflow over the wing is reducing
Seperation point moving forward
Adverse pressure gradient increasing
What causes a swept wing to pitch up at the stall
Roots are having to generate more lift are tips have stalled
More downwash on tailplane
Increasing AOA on horizontal tailplane
Increased downforce
Causing nose to pitch up
Swept wing “pitch up” phenomenon
Cp moves forward with AOA
Beyond critical AOA CP continues to move forward due to swept wings stalling at tips
Further nose up pitch
Centre of pressure of straight wing at stall
CP moves forward with AOA
Beyond critical AOA the CP moves aft
Nose down pitch moment
Swept Wing CP movement at stall
Swept wing moves forward with AOA
Beyond critical AOA keeps moving forward
Further nose up pitch
Common method to reduce tip stalling
Modify cross section
Washout and reduced camber at tips to reduce local CL
Landing Gear effect on stall speed
Extension increases stall speed
Creates a drag force which forms an arm with CG causing a nose down pitching moment
Increased downforce produced by tailplane the counter so more lift needed
Speed needs to be increased
Where are stall speed determined
With the least favourable cg position.
The most forward cg limit
This leads to downwards force which needs to be compensated for
Higher stall speed at aft cg
How are minimum controls speed determined (conditions)
Cg on aft limit will provide small arm and least ability to oppose yawing moments on engine failure
Flapper Switch
Mounted on LE - on/off device that activates audio warning when AOA exceeds set value
Works by:
AOA increasing stagnation point moves down and aft once stagnation point is behind flapper switch airflow pushes flapper switch into its up position
AOA Vane
Pivots around its own axis to align with airflow
Angle is detected relative to longitudinal axis electrically
Processed by computer to give a calculated wing AOA and provide continuous display of aoa and trigger warning on approach to stall
Fixed Prob Stall warner
Two ports which measure pressure
When aoa increases airflow into each port differs
Different pressure sensed at each port sent to computer to determine AOA
Works even if pitot tube freezes
Rotating Probe
Probe with two ports that rotates to null out pressure difference at both ports. Orientation is then measured by computer to calculate wing AOA.
Stick Shakers Role
At computed AOA stick shaker forcefully vibrate control yoke to stimulate light buffet in ac with no natural buffet
Operates until pilot reduces AOA
Low Mounted Engine hazards at the stall
Below ac CG therefore full power at the stall could pitch up
CS23 Stall Measured by
Engines at idle/throttle closed
CG at max stall speed - forward limit and max mass
Vr
Speed at which pilot begin to apply control input on T/O
1.1 x vs1
VToss (50ft)
Speed you must be at 50ft after take off
1.2 x VS1
VREF
Landing speed
1.3 x VS0
Minimum speed at 35ft after take off on CS25
1.113 x VSR1
Minimum landing speed C25
Referred to as Vref
1.23 x vs0
CS23 Single engine Vr speed
For must not be less than VS1
CS23/25 rules for stall warning speeds
Must be activated 5kt or 5% above relevant stall speed and operate until above the limit
Airbus 3 AOA protection systems
Alpha protection
Alpha Floor
Alpha Maximum
What do you do if you experience buffeting after lowering the flaps
Return flaps to previous setting
Vne
Never exceed speed used on smaller ac - CS23
Vmo
Maximum operating speed/maximum operating Mach number - used on cs25
Vmc
Minimum control speed - calibrated airspeed of ac below which directional or lateral control of ac can no longer be maintained after failure of one engine.
Maintain straight flight with bank angle of no more than 5 degrees with max thrust on engine
Vmca established conditions
Max take off power
Aeroplane trimmed for takeoff
Unfavourable CG position
Max seal level take off weight
Critical take off config (gears ups)
Ground effect negligible
Washout
To stall the root first
3 AOA protections in normal Law of a320
Alpha protection
Alpha floor
Alpha maximum
Power increase from idle in stall will do what to clmax and the stall speed
CLMAX unchanged
Stall speed will decrease
Angle of sweep on stall speed
Swept wing less efficient at producing lift than straight wing therefore as cl decreases speed must increase.
Decrease in clmax from swept wing increases the stall speed
Vortilons
Fixed aerodynamic devices on ac winds to improve handling at low speed - energises the boundary layer over the wing turbulent flow
Usually fitted under the wings leading edge
Help improve aileron performance at high AOA also
What does weight do to critical alpha
No effect on critical alpha only the design of the wing will change the critical alpha
