Aerodynamic Theory Review 3

ATC Chapter 6

Aim

To review stalling and aircraft speeds

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

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.

2. What causes a stall?

Airflow Over The Wing 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 4 ° AoA

2. What causes a stall?

Airflow Over The Wing 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 As AoA increases CoP moves forward

L L

4 ° ° AoA

2. What causes a stall?

Airflow Over The Wing 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 Lift reduces A large increase in drag occurs >16 ° AoA 10 ° AoA

L D

2. What causes a stall?

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

Stall

-4 ° 16 ° AoA

2. What causes a stall?

2. What causes a stall?

The co-efficient of drag graph This graph shows the variation of C D with AoA Important things to note are: The minimum value of C D occurs at 0 ° AoA The critical angle is around 16 ° AoA Beyond the critical angle drag increases rapidly C D

Stall

-4 ° 16 ° AoA

2. What causes a stall?

When we combine the C L and C D 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

C L

The aircraft will sink

C D -4 ° 16 ° AoA

2. What causes a stall?

from Horizontal Stabiliser _________ causing the nose to ....

Pitch down

LIFT Weight Tail Down Force

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

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 High Angle of Bank Pulling out of a dive • • • Updrafts (Gust factor) Climbing Climbing Turns • • Flare Rotation Slow straight and level flight Pulling out of a dive Updrafts High AoB

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

4. Why we refer to a stall speed

Stall Speed In the C172SP this speed is… 48KTS in the clean configuration (V

s

)

40KTS with full flap (V

s0 ) L V s 48 KIAS (clean) or V s0 40 KIAS (full flap)

4. Why we refer to a stall speed

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

5. Factors affecting stall speed

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

AoA

. IAS New stall speed = old stall speed × new weight old weight

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 Therefore to maintain straight and level flight our stall speed must… Increase

LIFT

L 

AoA

. IAS

WEIGHT Force

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 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 

( C

L

. AoA )

. IAS

5. Factors affecting stall speed

Damage Damage will change the shape of efficient of lift Our critical AoA is fixed Therefore to maintain straight and level flight our stall speed must… Increase L 

( C

L

. AoA )

. IAS

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

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… We know that as altitude increases, air density… Decreases Our critical AoA is fixed L 

AoA

. IAS Therefore our TAS stall speed for straight and level flight must… Increase L 

AoA

. ( 1 / 2 .ρ.V

2 )

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

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 Flight path

T

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 Therefore our stall speed must… Reduce L 

AoA

. IAS

5. Factors affecting stall speed

Flap When we extend flap we increase the camber of our wing This has the effect of increasing coefficient of lift over all AoA Our critical AoA is fixed C L Therefore our stall speed must… Reduce 10˚ Flap L 

( C

L

. AoA )

. IAS -4 ° 16 ° AoA

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

6.

Symptoms of a Stall

At the stall Nose pitch down Buffet Low and Fluctuating IAS High Rate of Descent Possible wing drop

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

8. Stalling and structural integrity Structural Integrity

Manoeuvring speed ( V a

); ensures below that speed any full or abrupt control deflection in one axis will stall the aircraft before it is overstressed.

V a = 105 KIAS at MTOW Turbulence Penetration Speed ( V b

); ensures below that speed, certified turbulence of 66 ft/sec will stall the aircraft before it is overstressed.

V b = 105 KIAS at MTOW (not published)

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

8. IAS,

CAS, TAS & GS

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 =

C

L . ( 1 / 2 .ρ.V

2 ) .

S ( 1 / 2 .ρ.V

2 ) = IAS

8. IAS,

TAS, CAS & GS

True Airspeed (TAS) 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 = C L . ( 1 / 2 .ρ.V

2 ) .

S L = C C L . ( 1 / 2 .ρ.V

2 ) .

S V 2 = TAS

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

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

Questions?