1. Aerodynamic Theory Review 3
ATC Chapter 6

2. Aim
To review stalling and aircraft speeds

3. Objectives 1. Define a Stall
2. State manoeuvres where a stall is likely to occur
3. State why we refer to a stalling speed
4. State factors which affect stalling speed
5. State the symptoms of an incipient stall and full stall
6. State why we use rudder to check wing drop
7. State when stalling is used to maintain structural integrity
8. State the difference between IAS, TAS, CAS & GS
9. State TEM, Airmanship, and Human Factors considerations

4. 1. Define a Stall Definition:
A stall is defined as the point where an increase in AoA coincides with a rapid decrease in the ability to create lift.
The angle at which this occurs is known as the stalling or critical angle.

5. 2. What causes a stall? Airflow Over The Wing L D 4 °AoA
At low angles of attack there is relatively little disturbance to the airflow as the aerofoil travels through it
CoP is typically around 1/3 chord length L
4 °AoA D

6. 2. What causes a stall? Airflow Over The Wing L L D D 4 °AoA 10° AoA
As AoA increases the airflow must increasingly deviate from its path and accelerate to follow the contour of the wing
The air toward the aft of the aerofoil begins to separate L
As AoA increases CoP moves forward L
4 °AoA
10° AoA D D

7. 2. What causes a stall? Airflow Over The Wing L L D D 10° AoA
Beyond an AoA of around 16 ° the change in direction and speed is too great, the airflow can no longer conform to the shape of the aerofoil and becomes turbulent
CoP moves rapidly rewards L
Lift reduces
A large increase in drag occurs
>16° AoA
10° AoA L D D

8. 2. What causes a stall? The co-efficient of lift graph
This graph shows the variation of CL with AoA
Important things to note are:
Stall
Because our aerofoil is cambered it will produce lift at 0° AoA CL
The zero lift angle for a general purpose aerofoil is around -4° AoA
The critical angle is around 16° AoA
The maximum CL is produced at the critical angle
Beyond the critical angle lift reduces rapidly
-4°
16°
AoA

9. 2. What causes a stall?

10. 2. What causes a stall? The co-efficient of drag graph
This graph shows the variation of CD with AoA
Important things to note are:
Stall
The minimum value of CD occurs at 0° AoA CD
The critical angle is around 16° AoA
Beyond the critical angle drag increases rapidly
-4°
16°
AoA

11. 2. What causes a stall? The aircraft will sink
When we combine the CL and CD graphs we can see the large reduction in lift and large increase in drag when we exceed the critical angle.
What effect do you think this will have on our aircraft?
Stall CL
The aircraft will sink CD
-4°
16°
AoA

12. 2. What causes a stall?
In the stall the CoP moves ___________ and the Tail Down Force from Horizontal Stabiliser _________ causing the nose to....
Rearwards
Reduces
Pitch down
LIFT
Weight
Tail Down
Force

13. 2. What causes a stall?
What do you think will happen if one wing reaches the critical AoA before the other?
We will get a wing drop

14. 3. Manoeuvres where a stall is likely to occur
Any manoeuvres that require a high AoA may lead to an inadvertent stall. Examples include…
Slow straight and level flight
Updrafts (Gust factor)
Flare
High Angle of Bank
Climbing
Rotation
Pulling out of a dive
Climbing Turns
Slow straight and level flight
High AoB
Pulling out of a dive
Updrafts

15. L  AoA . IAS 4. Why we refer to a stall speed Stall Speed
Does the C172SP have an angle of attack indicator?
We know from our straight and level flight; as IAS reduces we must increase AoA to maintain lift
L  AoA . IAS
As our speed reduces we will eventually reach our critical AoA and the aircraft will stall. The IAS at which this occurs is known as the stall speed

16. 4. Why we refer to a stall speed
In the C172SP this speed is…
48KTS in the clean configuration (Vs)
40KTS with full flap (Vs0) L Vs
48 KIAS (clean) or Vs0 40 KIAS (full flap)

17. 4. Why we refer to a stall speed
There are a number of certification limits under which the stall speed for an aircraft is tested and certified to:
MTOW
Most forward CoG
Power Idle 1G
Flaps Retracted

18. 5. Factors affecting stall speed
Weight
We know for straight and level flight weight must equal lift
If Weight increases Lift must…
Increase
Our critical AoA is fixed
Therefore to maintain straight and level flight our stall speed must…
Increase
W = L  AoA . IAS
The formula we use to calculate our new stall speed is
New stall speed = old stall speed × new weight old weight

19. 5. Factors affecting stall speed
Centre of Gravity
We know that in a correctly loaded aircraft the tail plane will create a small force…
Downwards
This force is opposing lift, increasing the requirement of the wing to produce lift. As CoG moves forwards the tail down force increases, further increasing the requirement for lift
Our critical AoA is fixed
LIFT
Therefore to maintain straight and level flight our stall speed must…
Increase
L  AoA . IAS
Force
WEIGHT

20. 5. Factors affecting stall speed
Ice/Frost
Ice and frost affect our stall speed in two ways
The first is by increasing the weight of the aircraft which _________our stall speed
Increases
The second is by changing the shape of our aerofoil, reducing our co-efficient of lift
Our critical AoA is fixed
Therefore to maintain straight and level flight our stall speed must…
Increase
L  ( CL . AoA ) . IAS

21. 5. Factors affecting stall speed
Damage
Damage will change the shape of the wing, this will ________ our co-efficient of lift
Reduce
Our critical AoA is fixed
Therefore to maintain straight and level flight our stall speed must…
Increase
L  ( CL . AoA ) . IAS

22. 5. Factors affecting stall speed
Load Factor
If load factor increases lift must…
Increase
Our critical AoA is fixed
Therefore our stall speed must…
Increase
The formula we use to calculate our new stall speed is
L  AoA . IAS
New stall speed = old stall speed × Load Factor

23. 5. Factors affecting stall speed
Altitude
If we consider our simplified lift formula we will always stall at the same IAS
If however we expand the formula…
L  AoA . IAS
We know that as altitude increases, air density…
Decreases
Our critical AoA is fixed
Therefore our TAS stall speed for straight and level flight must…
Increase
L  AoA . ( 1/2.ρ.V2 )

24. 5. Factors affecting stall speed
Power
Power affects our stall speed in two ways
The first is due to slipstream. The air being accelerated by the propeller is traveling faster and meeting the inboard sections of the wing at a slightly lower AoA. This delays separation of the airflow over this area of the wing, allowing us to fly at a higher AoA
If our AoA is increased our stall speed must…
Reduce
L  AoA . IAS

25. 5. Factors affecting stall speed
Power
The second is due to the high nose attitude approaching the stall
We can see from the diagram below at high nose attitudes the thrust line is inclined upwards relative to the flight path of the aircraft
This creates a vertical component of thrust
Vertical component of Thrust T
Flight path
Because this component of thrust is acting in the same direction as lift it is reducing the requirement for the wings to produce lift
Our AoA is fixed
L  AoA . IAS
Therefore our stall speed must…
Reduce

26. 5. Factors affecting stall speed
Flap
When we extend flap we increase the camber of our wing
Stall
This has the effect of increasing coefficient of lift over all AoA
Stall CL
10˚ Flap
Our critical AoA is fixed
Therefore our stall speed must…
Reduce
L  ( CL . AoA ) . IAS
-4°
16°
AoA

27. 6. Symptoms of a Stall Incipient stall (onset) High AoA
High or increasing nose attitude
Low and decreasing IAS
Reduced Control Effectiveness
Low External Noise
Stall warning Possible
Pre-stall Buffeting
Aft Control Column

28. 6. Symptoms of a Stall At the stall Nose pitch down Buffet
Low and Fluctuating IAS
High Rate of Descent
Possible wing drop

29. 7. Wing Drop Wing Drop Wing drop occurs when...
one wing stalls before the other.
The wing has stalled because...
it has exceeded the critical AoA.
Using aileron to ‘pick up’ with dropped wing will...
increase AoA and further stall the wing
Therefore we can use Secondary Effect of Rudder : ROLL

30. 8. Stalling and structural integrity
Manoeuvring speed (Va); ensures below that speed any full or abrupt control deflection in one axis will stall the aircraft before it is overstressed.
Va = 105 KIAS at MTOW
Turbulence Penetration Speed (Vb); ensures below that speed, certified turbulence of 66 ft/sec will stall the aircraft before it is overstressed.
Vb = 105 KIAS at MTOW (not published)

31. 8. Stalling and structural integrity
Structural Integrity: V-N Diagram

32. 8. IAS, CAS, TAS & GS L = CL . ( 1/2.ρ.V2 ) . S ( 1/2.ρ.V2 ) = IAS
Indicated Airspeed (IAS)
Definition:
The indicated airspeed is a measure of dynamic pressure
The Airspeed Indicator displays the dynamic pressure in a measurement of knots (NM/hr)
The IAS also is dependent on density, pressure and temperature
As IAS is measured with respect to dynamic pressure, IAS is a function of the lift equation
L = CL . ( 1/2.ρ.V2 ) . S
( 1/2.ρ.V2 ) = IAS

33. 8. IAS, TAS, CAS & GS True Airspeed (TAS) L = CL . ( 1/2.ρ.V2 ) . S
Definition:
The TAS is the speed of the aircraft relative to the air mass in which it is flying
With an increase in altitude we know that we have a reduction in:
Pressure
Temperature
Density
Therefore a reduction in the atmospheric conditions require a higher TAS to maintain the aerodynamic forces i.e Lift
L = CL . ( 1/2.ρ.V2 ) . S
L = CL . ( 1/2.ρ.V2 ) . S C
V2 = TAS

34. 8. IAS, CAS, TAS & GS Calibrated Airspeed (CAS) Definition:
The CAS is the IAS corrected for errors in the pitot & static sensors in addition to the ASI itself
CAS may be thought of as the actual IAS
The flight manual will state both IAS & CAS for limiting speeds

35. 8. IAS, CAS, TAS & GS Ground Speed (GS) Definition:
Ground speed is the speed of an aircraft relative to the ground
Ground speed = TAS + Headwind / Tailwind
Ground speed is used for angle of climb performance and navigation
Estimated Time Interval = Distance
Ground Speed

36. Questions?