Aerodynamic Theory Review 3 ATC Chapter 6. Aim To review stalling and aircraft speeds

Aerodynamic Theory Review 3ATC Chapter 6

Aim

To review stalling and aircraft speeds

Objectives1. Define a Stall2. State manoeuvres where a stall is likely to occur3. State why we refer to a stalling speed4. State factors which affect stalling speed5. State the symptoms of an incipient stall and full stall6. State why we use rudder to check wing drop7. State when stalling is used to maintain structural

integrity

9. State TEM, Airmanship, and Human Factors considerations

8. State the difference between IAS, TAS, CAS & GS

1. Define a StallDefinition:

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.

L

D

4 °AoA

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 °AoA10° AoA

L

D

L

D


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

10° AoA

L

D

L

D

>16° 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

Lift reduces

A large increase in drag occurs

AoA

CL

16°-4°

Stall

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: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 CL is produced at the critical angle

Beyond the critical angle lift reduces rapidly

2. What causes a stall?

AoA

CD

16°-4°

Stall

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:The minimum value of CD

occurs at 0° AoAThe critical angle is around 16° AoA

Beyond the critical angle drag increases rapidly

AoA

CD

16°-4°

Stall

CL

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?

In the stall the CoP moves ___________ and the Tail Down Force from Horizontal Stabiliser _________ causing the nose to....

Pitch down

LIFT

Weight

Tail Down Force

RearwardsReduces


What do you think will happen if one wing reaches the critical AoA before the other?

We will get a wing drop


• Climbing

• Climbing Turns

• Rotation

• Flare• Slow straight and level flight

• Pulling out of a dive

• High Angle of Bank

• Updrafts (Gust factor)

Slow straight and level flight High AoB

Updrafts

Pulling out of a dive

3. Manoeuvres where a stall is likely to occur

Any manoeuvres that require a high AoA may lead to an inadvertent stall. Examples include…

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 . IASAs 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 speedStall Speed

Vs48 KIAS (clean) or Vs0 40 KIAS (full flap)

L

In the C172SP this speed is…

4. Why we refer to a stall speed

48KTS in the clean configuration (Vs)

40KTS with full flap (Vs0)

Stall Speed

There are a number of certification limits under which the stall speed for an aircraft is tested and certified to:

4. Why we refer to a stall speed

MTOW

Most forward CoG

Power Idle

1G

Flaps Retracted

Stall Speed

W = L AoA . IAS

If Weight increases Lift must…

Our critical AoA is fixed

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

The formula we use to calculate our new stall speed is

New stall speed = old stall speed

We know for straight and level flight weight must equal lift

Increase

Increase

5. Factors affecting stall speedWeight

WEIGHT

LIFT

Force

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

L AoA . IAS


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

5. Factors affecting stall speedCentre of Gravity

Ice and frost affect our stall speed in two ways

The first is by increasing the weight of the aircraft which _________our stall speed

The second is by changing the shape of our aerofoil, reducing our co-efficient of lift

L ( CL . AoA ) . IAS



Increases

Increase

5. Factors affecting stall speedIce/Frost




Damage will change the shape of the wing, this will ________ our co-efficient of lift

Reduce

Increase

5. Factors affecting stall speedDamage

L AoA . IAS

If load factor increases lift must…


Therefore our stall speed must…The formula we use to calculate our new stall speed is

New stall speed = old stall speed

Increase

Increase

5. Factors affecting stall speedLoad Factor

If we consider our simplified lift formula we will always stall at the same IAS

L AoA . IASIf however we expand the formula…

L AoA . ( 1/2.ρ.V2 )

We know that as altitude increases, air density… Decreases


Therefore our TAS stall speed for straight and level flight must… Increase

5. Factors affecting stall speedAltitude

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

L AoA . IAS

If our AoA is increased our stall speed must… Reduce

5. Factors affecting stall speedPower

The second is due to the high nose attitude approaching the stall

Reduce

TVertical component of Thrust Flight path

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

Because this component of thrust is acting in the same direction as lift it is reducing the requirement for the wings to produce lift

L AoA . IASOur AoA is fixed

Therefore our stall speed must…

5. Factors affecting stall speedPower


When we extend flap we increase the camber of our wing


Therefore our stall speed must…

AoA

CL

16°-4°

StallThis has the effect of increasing coefficient of lift over all AoA 10˚ Flap

Reduce

Stall

5. Factors affecting stall speedFlap

High AoA

6. Symptoms of a StallIncipient stall (onset)

High or increasing nose attitude

Low and decreasing IAS

Reduced Control Effectiveness

Low External Noise

Stall warning Possible

Pre-stall Buffeting

Aft Control Column

Nose pitch down

6. Symptoms of a StallAt the stall

Buffet

Low and Fluctuating IAS

High Rate of Descent

Possible 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

7. Wing DropWing Drop

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

8. Stalling and structural integrityStructural Integrity

Va = 105 KIAS at MTOW

Vb = 105 KIAS at MTOW (not published)

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

8. Stalling and structural integrityStructural Integrity: V-N Diagram

Definition:

8. IAS, CAS, TAS & GSIndicated Airspeed (IAS)

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

Definition:

8. IAS, TAS, CAS & GSTrue Airspeed (TAS)

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

V2 = TAS

L = CL . ( 1/2.ρ.V2 ) . SC

Definition:

8. IAS, CAS, TAS & GSCalibrated Airspeed (CAS)

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

Definition:

8. IAS, CAS, TAS & GSGround Speed (GS)

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 = DistanceGround Speed

Questions?

Documents

Aerodynamic Theory Review 3 ATC Chapter 6. Aim To review stalling and aircraft speeds