Transcript stall - Winnipeg Ground School

Theory of Flight
6.07 Stalls
References: FTGU pages 18, 35-38
6.07 Stalls
• Laminar Flow
• Definition of a Stall
• Centre of Pressure
• Critical Angle of Attack
• Stalls
• Factors affecting stalls
Laminar Flow
• Boundary Layer – The thin layer of
airflow over the wing
• Laminar Layer – Smooth portion of the
boundary layer nearest the leading edge
of the wing
• Transition/Separation Point – Point on
wing where the boundary layer becomes
turbulent
• Turbulent layer – Turbulent portion of
the boundary layer at the trailing edge of
the wing
Stall
What is a stall?
Stall
• When a wing or aerofoil has air flow separation increasing
the drag and reducing the lift
• No longer capable of producing enough lift to counteract
the weight of the aircraft
• As a result, can no longer maintain level flight
Centre of Pressure
• Point on a wing where total aerodynamic pressure acts
Centre of Pressure
• Centre of pressure moves forward as the angle of attack
increases to the point of a stall
• After a stall the centre of pressure moves rapidly back
• If the CoP moves forward of the CG it causes an aeroplane
to become unstable, nose of the aeroplane does not drop
at the stall
Critical Angle of Attack
• The AoA above which
airflow will separate and
become turbulent
• The wing stall will occur
at any speed
Critical Angle of Attack
• Most aerofoil or wing
designs have a stall angle
of 15° to 20°
Stall
• Centre of pressure and separation
point move forward to point of stall
and lift production is increased
• Angle of attack is increased beyond
critical angle of attack
• Wing stops producing lift and stalls
• Centre of pressure moves rapidly
backward
Stall
Symptoms of a Stall
Buffeting
Factors affecting a Stall
• Position of the Centre of Gravity, more forward CG the
higher the Vs
• Weight, increase in wt = increase in Vs
• Turbulence, changes the load factor as well as sudden
changes in AoA (greater then the critical AoA)
• Turns, increases the load factor which increases the Vs
• Snow, Frost, Ice cause early airfoil separation causing an
increase in Vs
Factors affecting a Stall
Centre of Gravity (CG)
• CG forward
– Loading on the horizontal tail
surfaces increases
– Overall weight of aircraft increases
– Vs increases
• CG aft
– Decreased longitudinal stability
– Violent stall characteristics
– Poor or NO stall recovery (very
dangerous!)
– Vs decreases
Factors affecting a Stall
Weight
• The more weight on an aircraft means that it must fly at a
higher AoA (for a given speed)
• Therefore the critical AoA will be reached at a higher
airspeed (instead of stalling at 40 kt stalls at 50 kt)
Attitude to fly straight and level
2500 lbs, at 90 kt (closer to the
critical A of A)
1000 lbs, at 90 kt
Start of a trip
End of a trip
Factors affecting a Stall
Turbulence
• Upward vertical currents cause the aeroplane’s AoA to
increase
• Could result in the aeroplane stalling of the critical AoA is
reached, more likely at reduced speeds (approach)
Factors affecting a Stall
Turns
• As angle of bank increases the load factor also increases
• Therefore, an increased angle of attack is required to
maintain level flight in a turn
• Subsequently, the stall speed in a turn increases, just like
adding more weight to the aircraft
Turns and Stall Speeds
Factors affecting a Stall
Snow, Frost, and Ice
• Accumulation of snow, frost, and ice reduce a wing’s
ability to produce lift
• Increase in Vs
Factors affecting a Stall
Increase Vs
Forward CG
Increased weight
Turbulence
Greater angle of bank
Decrease Vs
Aft CG
Decreased weight
Confirmation Check
Confirmation
1.
Draw the movement of the C of P leading up to the stall.
2.
What are some factors that increase the stall speed?
Confirmation
3. When can an aircraft stall?
4. What are the symptoms of a stall?
Stall Recovery