Stalling And Spinning Flashcards
Explain the stalled condition of an aerofoil
- Angle of attack exceeds its critical (or stalling) angle, Airflow breaks away (separates) from the upper surface. Great reduction of lift results.
Explain basic stall speed and relate it to the lift formula.
The IAS at which an aircraft reaches its stall angle in straight and level flight while in the ‘clean’ power off configuration.
Since lift = Cl x IAS x S
To remain in level flight e.g with throttle closed, pilot must match reduction in airspeed with increases in Cl(AoA) and soon an IAS must be reached where the associated AoA reaches the Cl max value and stall.
Describe typical symptoms and other indications of the approach to the stall. (7)
- reducing control effectiveness
- buffet
- sink
- stall warning devices
- high nose attitude
- lower noise level
- low airspeed.
Describe the changes in the airflow over the wing, movement of the CoP, and aircraft attitude as the point of stall is reached.
At stall, separation point moves forward rapidly and streamline flow over the most of the upper wing separates and large turbulent wake forms.
CoP will have moved forward by the time the critical angle is reached and as flow over the upper surface breaks down at the stall, CoP moves rapidly rearward.
The rearward movement results in a nose-down pitch, which reduces AoA and unstall the wings.
Explain the standard recovery from the stalled condition.
Stick power rudder
Describe how the following factors affect stalling speed:
(a) aircraft weight;
Level flight lift must equal and opposite to weight.
Therefore heavier aircraft require more lift and therefore since Cl Max would be a fixed quantity only IAS can be changed so therefore the stall speed will have to increase with an increase in weight and opposite with decrease in weight.
What is the stall speed formula (new)?
New stall speed = basic stall speed x √(new weight)/(old weight)
Describe how the following factors affect stalling speed:
(B) load factor;
The measure of how much extra lift is being produced by the wings in a turn is called the load factor.
The effect of load factor on the stalling speed is the same as for increase/decrease in weight. If load factor is increased AoA of the wing must be increased to produce the extra lift required.
Describe how the following factors affect stalling speed:
(C) power
Increased power decreases the stalling speed. This is because with an increase in power the AoA required to maintain level flight at any given IAS is reduced .
Describe how the following factors affect stalling speed:
(D) altitude
As altitude is increased, density decreases, although stall IAS remains constant, as altitude is increased, the stall speed in terms of TAS is increased.
Describe how the following factors affect stalling speed:
Use of flaps and slats
Flaps
Since L = Cl x IAS x S
Flaps increase camber of aircraft and therefore Cl max so for Cl max to increase IAS must decrease. So stall speed is reduced and they also reduce the geometric stall angle of attack. (Lower nose attitude in level flight)
Slats
Slats rejuvenate the boundary layer and delay separation of the airflow so Cl max is increased and therefore stall speed is decreased and theres also a higher AoA.
Describe how the following factors affect stalling speed: contamination of the wing surfaces
When contaminated, early separation of the boundary layer is encouraged.
results in a reduction of the Cl max being experienced at a lower AoA which means the aircraft will stall at a higher speed than normal.
Describe the conditions which encourage a wing-drop at the stall. (6)
Surface condition:
presence of contamination which can result in flow separation from one wing ahead of the other.
Unbalanced flight approaching the stall:
when aircraft with dihedral slip/skids develops a higher effective AoA on one wing and small amounts of either are induced as stall is approached.
Use of ailerons near stall:
using them can result in the outer section of the wing with the down aileron reaching its stall angle ahead of the up aileron.
Approaching the stall during climbing or descending turns:
AoA of each wing are different in climbing and descending turns, one wing can reach stall angle before the other.
Use of flap approaching the stall:
Flap extended some aircraft may display an increased tendency to drop a wing at the stall.
Use of power:
In conjunction with flaps theres a tendency to encourage stalling of the outer sections first.
Explain the design measures taken to reduce the tendency for wing-drop. (3)
Wing root stall is desired an aircraft with rectangular wing planform shape have this characteristic.
Adding washout which enables the inner section of the wing to reach stall angle first.
Flow strips (stall strips) to inboard leading edges which encourage early flow separation at higher AoA
Explain the caution against using aileron near or at the stalling angle.
If aileron is used to try to prevent wingdrop stall near the stalling angle.
The AoA on the down-going wing will be further increased and will take it further beyond the stalling angle adding to the unwanted roll and subsequent yaw.
