Briefing: Slow Flight and Stalling

Introduction

Aim:

  • You will recognise the symptoms of the approaching stall, and know what avoiding action to take
  • become familiar with the characteristics of the full stall, and learn how to recover with the minimum loss of height
  • learn to avoid inadvertent stalling.
  • The flying exercises will show what it feels like to stall.

What do we know?
  • What is a stall?
  • At what speed can a stall occur?
  • Do all gliders stall?
  • When is a stall dangerous?
Theory

Terminology and References
Terms

  • Horizontal Fuselage Datum
  • Attitude
  • Chord
  • Chord Line
    • Note how the Chord Line can be altered by the control surfaces
  • Flight Path
  • Relative Airflow
  • Angle of Attack (AoA): angle between the relative airflow and the chord line.
Lift, and what affects it

The Wing must generate sufficient Lift to carry the Weight of the glider.

The amount of lift produced by a wing is affected by:

  • Aerofoil’s Camber (overall shape)
  • Wing Area
  • Airflow Speed
  • Angle of Attack
Factors affecting Airflow Speed

  • Things we control:
    • Change of Attitude
    • Drag (use of Airbrakes, Flaps)
  • Other things, beyond our control:
    • Horizontal gusts
    • Wind sheer
    • Wind gradient
The Angle of Attack

For each Speed, there is a specific Angle of Attack that generates the required Lift:

  • At high speed, a low AoA generates sufficient lift
  • In slow flight a high AoA is necessary

At slower speeds, the required lift can be generated by increasing the AoA – but there is a limit: The Critical AOA.

The Critical Angle of Attack

  • The AoA at which the maximum amount of lift is produced at a given airspeed
  • Beyond which:
    • Lift reduces (sometimes sharply)
    • Drag continues to increase.
  • It is
    • Aerofoil specific
    • Typically c. 15 degrees
Flight Path and Pitch Attitude

Flight Path: the direction in which the glider is travelling

Pitch Attitude: the direction in which it is pointing, relative to the horizontal

A glider will stall in any attitude, and at any speed, if the AoA reaches the Critical Angle.

How a Stall occurs

As the AoA increases, turbulence initially forms at the wing root trailing edge, then progressively extends outwards and forwards.

When at the stall:

  • The upper airflow breaks away becoming turbulent:
    • Lift is lost
    • Drag increases
    • Weight is not supported
Stalling Speed
  • Stalling Speed is the speed at which the wings will stall.
  • A ‘higher stalling speed’ means the glider must fly faster to avoid stalling.
The Stalling Speed depends on
  • Airspeed
  • Wing loading
    • Weight
    • Vertical accelerations:
      • Changes in direction: Turning, pulling out of a dive
      • Cable tension in a winch launch.
  • Contamination
    • Bugs, Rain, Dirt
  • Use of Airbrakes (reduces the effective wing area, and reduces the lift)
One May day, it just didn’t feel the same – after one hour’s flying! The entire leading edge was splattered.
Further Points

Flying close to the stall, or partially stalled
  • Inefficient
  • Dangerous:
    • close to the ground
    • in thermals
Most gliders will stall in a progressive and smooth manner:
  • Different aerofoils used near the tip
  • Washout
  • Drag and Sink increase markedly
The ailerons may continue to work
  • Increasingly sluggish and ineffective
Aileron input close to the stall may result in very rapid roll in the opposite direction
  • Down-going aileron (intended up-going wing) stalled
Secondary effect of rudder is more marked close to the stall
  • A lot of roll with little yaw
Elevator may fail to raise the nose

Or the nose may drop regardless of stick position:

  • the most important symptom of a stall.
Airflow noise changes
  • Quieter,
  • or Louder
  • or ‘different’
  • Yaw may alter it too.
Buffet
  • Separated airflow may create buffet (on tailplane)
ASI Flicker

Turbulent airflow across the static ports.

Slow flight: 43kts vs 38kts
Symptoms
  • Attitude: nose higher than ‘normal’
  • Airspeed is slow or reducing
  • Airflow noise is changing
  • ASI is flickering
  • Airframe buffeting
  • The effectiveness of the controls is changing
  • Unusual control positions for the phase of flight: the stick is further back than ‘normal’ / lots of out of turn aileron
  • High rate of descent (for the attitude)
  • Elevator ineffective
Recovery

Standard Stall Recovery:

  • Ease the Stick straight forward to pitch the glider below the normal gliding attitude
  • Allow speed to build to regain flying speed
  • Roll wings level, or return to the required attitude for the phase of flight
  • Height loss: Up to 300 feet
Prevention

In slow flight, subtle signs:

  • Pitch Attitude
  • Airspeed
  • Controls Response
  • Controls Position
  • Airflow Sounds different

Recovery:

  • Ease the back pressure on the stick.
  • Ask yourself: Why did it happen? Are you paying attention / flying fast enough for the conditions?
  • Height loss: Less than 30 feet.
Unaccelerated, or 1G, Stalls

Stalls which occur without increasing the Wing Loading.

  • May occur at speeds above the ‘1G Stall Speed’
Examples of Unaccelerated Stalls:

  • Mushing Stall
    • Stall without a nose drop
    • Requires more ‘stick forward’ movement to recover than a stall with nose drop.
Airspeed: 31kts Sink: 4.95kts
  • Stall with a nose drop
  • Stall with a wing drop
    • recover after first regaining speed
    • level wings with coordinated aileron and rudder
    • ease out of the dive
  • Stall with Airbrakes (or Spoilers) open
    • Stalling speed is higher
    • Stall may be very sudden, and steep
    • Recovery: includes closing the Airbrakes or Spoilers

Video: unaccelerated stalls

Accelerated, or High G, Stalls

  • The glider will stall at any speed above the ‘normal’ 1G stall speed if:
    • the Angle of Attack is high enough
    • you pull enough G (Wing Load)
  • They occur quickly
  • Have fewer warnings
Examples of Accelerated Stalls:

Stall in a Turn
  • Controls likely to be in unusual positions
  • Recovery: as per Stall with Wing Drop
  • Prevention: ease the back pressure on the stick
Stall in Climbing Attitude
Stalled, dropping a wing, nose high
  • As if climbing into a thermal turn at speed, or after experiencing a winch launch failure during the climb
  • Nose drop may be the first symptom
  • Stall will be severe
  • Recovery still requires the stick to go forward
  • Prevention:
    • monitor ASI;
    • take prompt recovery action after a winch launch failure.
Stall in a Steep Turn
ASI: 55kts – Sink: 4kts
6 secs later: ASI: 52kts – Sink: 6.5kts
  • Symptoms, such as buffet, appear at higher speed than a 1G stall.
  • Recovery:
    • ease the back pressure on the stick.
    • in very steep turns, reduce bank before re-establishing the glider in the turn
  • Prevention:
    • monitor ASI, allowing for a higher stall speed
    • ease back pressure on stick
  • Steep turns require higher speed.
‘High Speed’ Stall
  • The glider will stall at speeds above the unaccelerated stall speed if the G load is increased – e.g. when recovering from a stall or a spin too quickly, even if
    • the wings are level,
    • the glider is in an attitude at or below the normal gliding attitude.
  • Stall may be more dramatic than the 1G stall
  • Recovery: Standard Stall Recovery, but be careful to ease back on the stick during the recovery.
  • Prevention: recover from the previous event carefully.

Prevention and Recovery is the same as Unaccelerated Stalls.

Sensations in a Stall

Reduced G as the nose drops.

  • Is reduced G a symptom of a stall?
Recap

How much height might you lose in a stall?

How much height would be lost if a stall is prevented?

What are the symptoms of a stall – before the stall? When stalled?

How many stall symptoms are required before taking prevention or recovery action?

Can you stall when flying above the glider’s stated stall speed?

What is the ‘Standard Stall Recovery’?

TEM

  • Loss of height. Prevention, Lookout-Attitude-Instruments
  • Loss of control. Recovery: SSR
  • Collision. Lookout, HASSLL checks (Height (set minimum to start and finish), Airframe (Max Manoeuvring Speed), Security, Straps, Location, Lookout)

Self-briefing lesson on 1G Stalls