Leading Cadet Training Principles of Flight Leading Cadet Training Stalling and Gliding Lecture 6

The Stall α In 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. 15o α

The Stall α The critical angle of attack (the stalling angle) varies from one type of wing to another, as does the stalling speed. 15o α

The Stall The airflow turbulence is called Boundary Layer Separation at a Low Angle of Attack TR TOWARDS HIGHER PRESSURE PLUS VISCOUS ADHESION – “SLOWER” TOWARDS LOWER PRESSURE - FASTER AIRFLOW TRANSITION POINT FROM LAMINAR TO TURBULENT BOUNDARY LAYER

The Stall The airflow turbulence is called Boundary Layer Separation at a High Angle of Attack TOWARDS LOWER PRESSURE – FASTER TR TOWARDS HIGHER PRESSURE PLUS VISCOUS ADHESION – “MUCH SLOWER” AIRFLOW SEPARATION POINT

The Stall The airflow turbulence is called Boundary Layer Separation at a Stall !! TR COMPLETE SEPARATION TOWARDS LOWER PRESSURE – FASTER AIRFLOW

The main factors which affect the stalling speed are: Weight ‘G’ Force Thrust Flaps Ice & Damage

The Stall Stalling Speed The Effect of Speed 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 speeds are found in the Pilot’s Notes/Aircrew Manual etc.

The Stall Stalling Speed The Effect of Speed Lift = CL ½ρ V2 S CL = Coefficient of Lift (the ratio between lift and dynamic pressure). ρ = Density V = True Airspeed S = Surface Area Stalling Speed 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 - Lift = CLMAX ½ρ V2MIN S

The Stall Stalling Speed The Effect of Weight so Lift HEAVY WT CL = Coefficient of Lift (the ratio between lift and dynamic pressure). ρ = Density V = True Airspeed S = Surface Area Stalling Speed The Effect of Weight Lift HEAVY WT CL MAX ½ρ = V2 HEAVY STALL V2 HEAVY STALL S and so = Lift BASIC WT CL MAX ½ρ = V2 BASIC STALL V2 BASIC STALL S Lift HEAVY WT CANCELLATION V2 HEAVY STALL = Lift BASIC WT V2 BASIC STALL THEREFORE Lift HEAVY WT Lift HEAVY WT V2BASIC STALL X = V2HEAVY STALL Lift BASIC WT Lift BASIC WT THEREFORE = X

The Stall Stalling Speed The Effect of Weight Lift HEAVY WT CL = Coefficient of Lift (the ratio between lift and dynamic pressure). ρ = Density V = True Airspeed S = Surface Area Stalling Speed The Effect of Weight Lift HEAVY WT V2HEAVY STALL 2 V2BASIC STALL 2 = X Lift BASIC WT CONVERSION Lift HEAVY WT V HEAVY STALL = V BASIC STALL X Lift BASIC WT CANCELLATION Weight HEAVY V HEAVY STALL = V BASIC STALL X Weight BASIC

Basic Stall Speed (90kts) X The Stall CL = Coefficient of Lift (the ratio between lift and dynamic pressure). ρ = Density V = True Airspeed S = Surface Area Stalling Speed The Effect of Weight Load (200 ton) Basic Stall Speed (90kts) X Empty (50ton) e.g. V BASIC STALL (90kts) X 4 ton V HEAVY STALL = 90 X 4 (= 2) ( = 90 x 2 ) V HEAVY STALL = V BASIC STALL X Weight HEAVY Weight BASIC = 180kts

The Stall Stalling Speed The Effect of ‘G’ = 180kts V HEAVY STALL 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’ V HEAVY STALL X V BASIC STALL = Weight HEAVY Weight BASIC SAME FOR PULLING “g” V STALL MAN’VRE = V BASIC STALL X ‘g’ e.g. V BASIC STALL (90kts) X 4g loop V STALL MAN’VRE = 90 X 4 (= 2) ( = 90 x 2 ) = 180kts

The Stall Stalling Speed The Effect of ‘G’ = 180kts = 270kts 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’ 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 !!! V STALL MAN’VRE = 90 X 4 ( = 90 x 2 ) (= 2) V STALL MAN’VRE = 90 X 9 ( = 90 x 3 ) (= 3) = 180kts = 270kts

The Stall Stalling Speed The Effect of Thrust Lift Lift Thrust Weight TR Thrust Flight Path Weight Aircraft in level flight have a high nose attitude at the stall, particularly swept wing aircraft.

The Stall Stalling Speed The Effect of Thrust Lift Thrust Weight TR Thrust Flight Path Weight If the engine is at high power there 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

The Stall Stalling Speed The Effect of Flaps To obtain the same CL, Flap Lowered Basic ‘Clean’ Situation Maintaining the Same Lift Chord Line α α Relative Airflow To obtain the same CL, the Attitude is Lowered, and the Angle of Attack is reduced. Effective Increase in Angle of Attack

The Stall Stalling Speed Other Factors Ice: Damage: Alters the ‘Shape’ of the wing, this will reduce Lift. Damage: Can also reduce Lift ie after a ‘Birdstrike’.

The Stall Natural Stall Warning Speed Nose Attitude Controls Light Buffet Heavy Buffet Nose Drop Wing Drop Natural Stall Warning Turbulent Air Missing the tailplane NORMAL FLIGHT

The Stall Natural Stall Warning Speed Nose Attitude Controls Light Buffet Heavy Buffet Nose Drop Wing Drop Natural Stall Warning Turbulent Air just touching the tailplane STALL WARNING LIGHT BUFFET

The Stall Natural Stall Warning Speed Nose Attitude Controls Light Buffet Heavy Buffet Nose Drop Wing Drop Natural Stall Warning Turbulent Air Covering the tailplane STALL WARNING HEAVY BUFFET Aircraft Descending

The Stall Synthetic Stall Warning Firefly/Tutor: Tucano: Warning Horn Warning Light (Firefly only) Tucano: AoA Gauge Stick Shaker Indexer

The Stall Synthetic Stall Warning Stall Warning Vane Vane held down by airflow Micro-switch not made No stall warning given Stall Warning Vane

The Stall Synthetic Stall Warning Vane lifted up by airflow Micro-switch is made Stall warning given

STANDARD STALL RECOVERY 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 Forces In 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

Balance of Forces Lift Speed If the Nose is raised, What happens to Lift and Speed? Lift and Speed reduce. The Rate of Descent also reduces!

Balance of Forces Lift Speed If the Nose is lowered, What happens to Lift and Speed? Lift and Speed increase. The Rate of Descent also increases!

Balance of Forces 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. The Lift reduces the rate of descent, and to increase airspeed the nose must be lowered. So in order to maintain steady flight the glider must be constantly descending. Lift Drag Path of Glider Weight

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 Drag Path of Glider Weight

Lift and Drag Lift Drag CL CD α α 0° 0°

Lift and Drag Lift / Drag Ratio CL CD Usual Angles of flight Less Lift Angle of attack Most efficient Less Lift More Drag

Lift and Drag Flight Speed We know the best Angle to fly, but what is the best Speed to fly? Minimum Drag Speed VIMD DRAG VIMD ZERO LIFT DRAG LIFT DEPENDENT DRAG IAS

How Far will a Glider go ? This depends upon the gliding angle and the wind. The flatter the gliding angle the further the glider will travel. A glider with a steep angle does not travel far. A glider with a shallow angle travels much further. 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.

How Far will a Glider go ? Equally, a glider travelling downwind, will cover a greater distance over the ground than a glider travelling into the wind. Upwind Downwind

Air Brakes 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

Check of Understanding At the stall of any particular wing which of these factors is not variable? The amount of weight supported by the wing The angle of attack of the wing The amount of lift produced by the wing The airspeed across the wing

Check of Understanding Which of the following statements is true? The stall is the same for all aircraft An aircraft can stall at any angle of attack The airspeed at which an aircraft stalls does not vary The airspeed at which an aircraft stalls does vary

Check of Understanding Which of the following will increase the stalling speed of an aircraft? Putting it into a turn Reducing the weight Increasing the power Lowering the flaps

Check of Understanding What happens to Stalling Speed : If Aircraft Weight Increases ? Stalling Speed Increases. If we Lower Flaps ? Stalling Speed Decreases. If we are “Pulling G” ? If damaged by a Birdstrike ? Stalling Speed probably Increases. If Using Engine Thrust ?

Check of Understanding What are the three forces acting on a glider during normal flight? Drag, Weight and Thrust Drag, Weight and Lift Drag, Thrust and Lift Force, Thrust and Lift

Check of Understanding How does a glider pilot increase airspeed? Push the stick forward to raise the nose Pull the stick back to raise the nose Push the stick forward to lower the nose Pull the stick back to lower the nose

Check of Understanding A Viking glider descends from 1640 ft (0.5 km). How far over the ground does it travel (in still air)? 70 kms 35 kms 17.5 kms 8.75 kms

Check of Understanding When flying into a Headwind, the distance covered over the ground will: Decrease Remain the same Increase Be the square of the height

Check of Understanding During flight if the nose of a glider is lowered, What happens to Lift and Speed? Both lift and speed decrease Lift increases, speed decreases Lift decreases, speed increases Both lift and speed increase

Check of Understanding In order to maintain steady flight What must a glider be constantly doing? Descending Spiralling Ascending Travelling upwind

Leading Cadet Training Principles of Flight Leading Cadet Training End of Presentation