PoF - Stalling Flashcards
What is Stalling?
Occurs when the airflow over the wing separates from the upper surface
When does Stalling occur?
When a/c exceeds its Critical AoA
A/C can stall at any speed so long as Critical AoA is exceeded
Why does the wing stall?
We want the air to flow rearwards over the wing
The air wants to flow in the direction of pressure gradients (high to low)
What is the Adverse Pressure Gradient (APG)
and consequences
Pressure gradient ring to reverse the airflow over the wing
Consequences
~Reduced lift
~Increased drag
~L/D ratio decrease
~CP moved aft of stall (fwd on swept)
~Nose pitch down (up on swept)
~Uncommanded roll
~Struggle to maintain alt
~Sluggish controls
Stall Speeds
Vs - Current stall speed
V s1g - 1g stall speed
Vs1 - Stall speed for given configuration
Vs0 - Stall speed in the landing configuration
Vsr - Stall reference speed used by low A.R a/c
Load factor vs stall
Flying at 1 x V s1g, you can stall at 1g
Flying at 0.5 x V s1g, you can stall at 0.25g
Flying at 2 x V s1g, you can stall at 4g
Flying at 3 x V s1g, you can stall at 9
Vs² ∝ n
A.R vs Stalling
On a high A.R a/c, if you exceed ∝crit, CL drops quickly and you’d have a high rate of decent
On a high A.R a/c, if you exceed ∝crit, CL has a small reduction
~may only notice stall after exceeding ∝crit by a lot, risking deep stall
What is a deep stall?
When the horizontal stabiliser is stalled, the makes it much harder to recover from the stall
Hori stabiliser is designed to stall after wing
T tail more prone to deep stall
Swept wing are more prone to stalling due to pitch up tendencies at the stall
What must a/c have if they are prone to deep stalling?
Deep stall protection system
E.g. stick puller - auto pushes the stick forward before a stall
Ailerons vs stalling
If a pilot attemps to roll right close to the stall, they will encounter a wing drop stall
~ Wing drop stall is where the a/c rolls in opposite direction to what the pilot attempted
Called a low speed reverse aileron roll
~Prevented by not using aileron close to stall speed
Landing gear vs stall speed
Lowering landing gear, increased stall speed
Flaps vs Stall speed
Training edge flaps increase camber
Leading edge devices (reduce stall speed) - add energy to airflow, allowing it to push against there APG for longer
Thrust vs Stall speed
Weight of a/c opposed by thrust (cancelled out)
Which means vertical lift is less
Power on stall speed - slower
Power off stall speed - higher
On prop a/c, it has even more ‘bonus’ lift it consumes air accelerated over the wings as the engines are attached on the wings
Alt vs stalling
As alt increases, there’s a change in Indicated Vs
At higher speeds, the air has less warning that the wing is approaching, so when it flows over the wing it has a ore serve turn around the leading edge - vunerable to AGP due to separation of air
At lower speeds, air gets a longer waring that the wing is approaching and takes a gentle path over the wing
Stall calculations
Vs ∝ √W (mass can be used instead of weight)
Stall speed changes in a turn due to increased load factor
(trig)
n = 1/cosϑ
