Lecture

Leading Cadet Training

Principles of Flight

6Stalling

and Gliding

The StallIn normal flight a wing meets the oncoming air

at a small angle of attack, The more the pilot increases the angle of attack,

the more lift there will be, until an angle of about 15° is reached,

when the airflow becomes turbulent, lift is lost –

so the aircraft STALLS.

α15o

The critical angle of attack(the stalling angle)

varies from one type of wing to another,as does the stalling speed.

The Stall

α15o

The airflow turbulence is calledBoundary Layer Separation

The Stall

TRTOWARDS LOWER PRESSURE -

FASTER

TOWARDS HIGHER PRESSUREPLUS VISCOUS ADHESION –

“SLOWER”

AIRFLOW

TRANSITION POINT FROM LAMINAR TO TURBULENT BOUNDARY LAYER

Boundary Layer Separationat a

Low Angle of Attack

The airflow turbulence is calledBoundary Layer Separation

The Stall

TRTOWARDS LOWER PRESSURE – FASTER

TOWARDS HIGHER PRESSUREPLUS VISCOUS ADHESION –

“MUCH SLOWER”

AIRFLOW

SEPARATION POINT

Boundary Layer Separationat a

High Angle of Attack

The airflow turbulence is calledBoundary Layer Separation

The Stall

TR

TOWARDS LOWER PRESSURE –

FASTERAIRFLOW

COMPLETE SEPARATION

Boundary Layer Separationat a

Stall !!

The main factors which affect the stalling speed are:

Speed

Weight

‘G’ Force

Thrust

Flaps

Ice & Damage

The Stall

The speed at which a clean aircraft (flaps up),at a stated weight,

with the throttle closed,flying straight and level,

can no longer maintain height.

Details of individual aircraft stalling speedsare found in the Pilot’s Notes/Aircrew Manual etc.

Stalling Speed

The Stall

The Effect of Speed

Remember the Lift Formula?

Lift = CL ½ρ V2 S

If we slow the speed down (reduce V) we must keep Lift the same (for Straight & Level Flight)

by increasing CL.

The limit therefore becomes CLMAX,

so the equivalent speed is VMIN (Stalling Speed)

The formula for the Stalling Speed is therefore -

Stalling Speed

The Stall

Lift = CLMAX ½ρ V2MIN S

CL = Coefficient of Lift(the ratio between lift and dynamic pressure).ρ = Density V = True Airspeed S = Surface Area

The Effect of Speed

S

S

CL MAX ½ρCL MAX ½ρ=

=

Lift HEAVY WT

Lift BASIC WT

Stalling Speed

The Stall

The Effect of Weight

=V2

BASIC STALL

V2 HEAVY STALL

=

V2HEAVY STALL

XLift HEAVY WT

Lift BASIC WT

Lift BASIC WT

Lift HEAVY WT

V2BASIC STALL =

and so

CL = Coefficient of Lift(the ratio between lift and dynamic pressure).ρ = Density V = True Airspeed S = Surface Area

CANCELLATION

V2 BASIC STALL

V2 HEAVY STALL

V2 BASIC STALL

V2 HEAVY STALL

Lift HEAVY WT

Lift BASIC WT

= X

THEREFORE

THEREFORE

Stalling Speed

The Stall

The Effect of Weight

V2HEAVY STALL X

Lift HEAVY WT

Lift BASIC WT

V2BASIC STALL =

V HEAVY STALL = V BASIC STALL X

Lift HEAVY WT

Lift BASIC WT

V HEAVY STALL = V BASIC STALL X

Weight HEAVY

Weight BASIC

2 2

CONVERSION

CANCELLATION

CL = Coefficient of Lift(the ratio between lift and dynamic pressure).ρ = Density V = True Airspeed S = Surface Area

Stalling Speed

The Stall

The Effect of Weight

V HEAVY STALL = V BASIC STALL X

Weight HEAVY

Weight BASIC

CL = Coefficient of Lift(the ratio between lift and dynamic pressure).ρ = Density V = True Airspeed S = Surface Area

Load (200 ton)

Empty (50ton) Basic Stall Speed (90kts) X

e.g. V BASIC STALL (90kts) X 4 ton

V HEAVY STALL = 90 X ( = 90 x 2 )

= 180kts

4 (= 2)

Stalling Speed The Effect of ‘G’

V STALL MAN’VRE = V BASIC STALL X ‘g’

SAME FOR PULLING “g”

e.g. V BASIC STALL (90kts) X

V STALL MAN’VRE = 90 X 4 ( = 90 x 2 )

= 180kts

The Stall

V HEAVY STALL XV BASIC STALL = Weight HEAVY

Weight BASIC

4g loop

(= 2)

CL = Coefficient of Lift(the ratio between lift and dynamic pressure).ρ = Density V = True Airspeed S = Surface Area

Stalling Speed The Effect of ‘G’

The Stall

= 180kts

V STALL MAN’VRE = 90 X 4 ( = 90 x 2 )(= 2)V STALL MAN’VRE = 90 X 9 ( = 90 x 3 )(= 3)

When you pull ‘g’, the stalling speed increases, e.g. if you pull 4g the stalling speed doubles !!

If you pull 9g the stalling speed triples !!!

= 270kts

CL = Coefficient of Lift(the ratio between lift and dynamic pressure).ρ = Density V = True Airspeed S = Surface Area

Stalling Speed

The Stall

The Effect of Thrust

Flight PathWeight

Lift

Thrust

Lift TR

Aircraft in level flight have a high nose attitude at the stall,

particularly swept wing aircraft.

Stalling Speed

The Stall

The Effect of Thrust

Flight Path

Thrust

If the engine is at high powerthere are two thrust components:

One acts along the flight path (countering drag).

and the other is vertical (opposing weight).

Therefore less lift is required from wings, so:

SLOWER STALLING SPEED (V) AT CLMAX

TRLift

Weight

Stalling Speed

The Stall

The Effect of Flaps

Relative Airflow

Basic ‘Clean’ Situation

Chord Line

α

Flap Lowered

Effective Increase in Angle of Attack

α

Maintaining the Same Lift

To obtain the same CL, the Attitude is Lowered,

and the Angle of Attack is reduced.

Stalling Speed

The Stall

Other Factors

Ice:Alters the ‘Shape’ of the wing, this will reduce Lift.

Damage:Can also reduce Lift ie after a ‘Birdstrike’.

Natural Stall Warning

NORMAL FLIGHT

Turbulent AirMissingthe tailplane

The Stall SpeedNose Attitude

ControlsLight Buffet

Heavy BuffetNose DropWing Drop

Turbulent Airjust touchingthe tailplane

Natural Stall Warning

STALL WARNINGLIGHT BUFFET

The Stall SpeedNose Attitude

ControlsLight Buffet

Heavy BuffetNose DropWing Drop

Turbulent AirCoveringthe tailplane STALL WARNING

HEAVY BUFFET

Natural Stall Warning

Aircraft Descending

The Stall SpeedNose Attitude

ControlsLight Buffet

Heavy BuffetNose DropWing Drop

Synthetic Stall Warning

The Stall

Firefly/Tutor:Warning HornWarning Light (Firefly only)

Tucano:Warning HornAoA GaugeStick ShakerIndexer

Synthetic Stall Warning

The Stall

Stall Warning Vane

Vane held down by airflow

Micro-switch not made

No stall warning given

Vane lifted up by airflow

Micro-switch is made

Stall warning given

Synthetic Stall Warning

The Stall

The Stall

STANDARD STALL RECOVERY

Move stick Centrally forward until buffet stops.

Open throttle at the same time.

Only then level the wings.

Raise nose at a safe speed and climb.

Gliding

Balance of ForcesIn straight and level flight, at constant speed,

two pairs of forces act on the aircraft.

Thrust opposes Drag and Lift opposes Weight. To maintain a steady airspeed if thrust is removed,pitch the nose down and use weight to descend.

The aircraft is now GLIDING.

WEIGHT

LIFT

DRAG THRUST

Lift

Speed

Lift and Speed reduce.

The Rate of Descent also reduces!

If the Nose is raised,

What happens to Lift and Speed?

Balance of Forces

Speed

Lift

Balance of Forces

If the Nose is lowered,

What happens to Lift and Speed?

Lift and Speed increase.

The Rate of Descent also increases!

Three forces act on a Glider –Due to Gravity a glider descends in a controlled way.

Drag acts along the flightpath,and as the glider descends air flow produces Lift.

Lift

Weight

DragPath of Glider

Balance of Forces

The Lift reduces the rate of descent,and to increase airspeed the nose must be lowered.

So in order to maintain steady flightthe glider must be constantly descending.

Lift

Weight

DragPath of Glider

Balance of Forces

Remember:

If you fly too slowly

Lift will be lost and the glider will Stall.

If you fly too fast

the Rate of Descent will be High.

Lift and Drag

α

CL CD

α0°0°

Lift Drag

Lift and Drag Lift / Drag Ratio

25

20

15

10

5

0

-5

0 5 16

-5 0 5 10 15 20 25o o o o o o

ooo

UsualAngles of flight

Mo

st e

ffic

ien

tA

ng

le o

f at

tack

CL

CDLess LiftMore Drag

Lift and Drag Flight Speed

VIMD

ZERO LIFT DRAG

LIFT DEPENDENT DRAG

DRAG

IAS

We know the best Angle to fly, but what is the best Speed to fly?

Minimum Drag Speed

VIMD

This depends upon the gliding angle and the wind.

The flatter the gliding anglethe further the glider will travel.

A glider with a steep angle does not travel far.

A glider with a shallow angle travels much further.

How Far will a Glider go ?

A Viking Glider’s angle is about 1 in 35.Therefore, from a height of 3,280 ft (1 kilometre),in still air, it will travel about 35 kilometres.

Downwind

Upwind

Equally,

a glider travelling downwind,

will cover a greater distance over the ground

than a glider travelling into the wind.

How Far will a Glider go ?

Most gliders do not have Flaps in their wings.Instead they are fitted with airbrakes.

Airbrakes are panels which lie in the wings,

and can extend to 90°from the upper and/or lower surfaces of the wings

Air Brakes

A glider with Airbrakes IN.

A glider with Airbrakes OUT

produces more drag and must therefore lower the nose to maintain airspeed

= Steeper Descent + Shorter Ground Distance

Air Brakes

Check of UnderstandingAt the stall of any particular wing

which of these factors is not variable?

The airspeed across the wing

The amount of weight supported by the wing

The amount of lift produced by the wing

The angle of attack of the wing

Check of UnderstandingWhich of the following statements is true?

The airspeed at which an aircraft stallsdoes vary

The stall is the same for all aircraft

The airspeed at which an aircraft stallsdoes not vary

An aircraft can stall at any angle of attack

Check of UnderstandingWhich of the following will increase

the stalling speed of an aircraft?

Lowering the flaps

Putting it into a turn

Increasing the power

Reducing the weight

What happens to Stalling Speed :

If Aircraft Weight Increases ?Stalling Speed Increases.If we Lower Flaps ?Stalling Speed Decreases.If we are “Pulling G” ?Stalling Speed Increases.If damaged by a Birdstrike ?Stalling Speed probably Increases.If Using Engine Thrust ?Stalling Speed Decreases.

Check of Understanding

Check of UnderstandingWhat are the three forces acting on a glider

during normal flight?

Force, Thrust and Lift

Drag, Weight and Thrust

Drag, Thrust and Lift

Drag, Weight and Lift

Check of UnderstandingHow does a glider pilot increase airspeed?

Pull the stick back to lower the nose

Push the stick forward to raise the nose

Push the stick forward to lower the nose

Pull the stick back to raise the nose

Check of UnderstandingA Viking glider descends from 1640 ft (0.5 km).

How far over the ground does it travel (in still air)?

8.75 kms

70 kms

17.5 kms

35 kms

Check of UnderstandingWhen flying into a Headwind,

the distance covered over the ground will:

Be the square of the height

Decrease

Increase

Remain the same

Check of UnderstandingDuring flight if the nose of a glider is lowered,

What happens to Lift and Speed?

Both lift and speed increase

Both lift and speed decrease

Lift decreases, speed increases

Lift increases, speed decreases

Check of UnderstandingIn order to maintain steady flight

What must a glider be constantly doing?

Travelling upwind

Descending

Ascending

Spiralling

Principles of Flight

End of Presentation

Leading Cadet Training