Stalls - Power on and Power Off

Stalls - Power On and Power Off

DEFINITION

As the wing angle of attack (AOA) increases to or beyond the critical AOA (approximately 16-20°), smooth airflow over the wing is disrupted, resulting in great increase in drag and loss of lift: a stall

SAFETY FACTORS

A primary objective of stall training is to enhance safety by helping assure inadvertent stall avoidance or prompt stall recovery
Stall avoidance is promoted by understanding
Flight situations where unintentional stalls may occur
The relationship of various factors to stall speed (Vs)
Recognition of the first indications of a stall
Recovery technique
Practice stall entry:
above 2,000 feet AGL to recover above 1,500 feet AGL
lights on
CLEAR area with 90° turns, left and right
mix rich, prop high RPM
check carb temp

TOLERANCES

Private Pilot PTS, VII, A & B:

Explain relevant aerodynamic factors, flight situations, recovery procedures, hazards of stalling uncoordinated
Select entry altitude allowing recovery above 1,500 feet AGL
Power-off stall
Approach or landing configuration with throttle reduced or idle
Straight glide or gliding turn with 30o, +10o bank
Maintain attitude that will induce a full stall
Promptly recover by decreasing AOA, leveling wings, and adjusting power as necessary to regain normal attitude
Retract flaps and gear and establish SLF or climb
Avoid secondary stall, excessive airspeed or altitude loss, spins, and flight below 1,500 feet AGL
Power-on stall
Takeoff or normal climb configuration
Establish takeoff or climb airspeed before power increase
Straight or bank 20o, +10o and pitch to stall
Recover as above

Commercial Pilot PTS, V, A. Imminent Stalls

Explain aerodynamic factors related to stalls in various configurations and flight situations; effects of various factors on VS; recovery procedure
Allow recovery above 1,500 feet AGL
Stabilize airplane appropriately during entry
Maintain heading +10o straight or bank +10o turning
Pitch for imminent stall while maintaining coordinated flight
Recover promptly at first indication of stall
Recover with minimum altitude loss consistent with safety during power-on recoveries; recover to the glide airspeed, +10 kts, during power-off recoveries
Avoid full stalls, excessive pitch changes, spirals, spins, or flight below 1,500 feet AGL

OBJECTIVES

To familiarize the pilot with the conditions that produce stalls
To develop knowledge and skill in recognizing imminent and full stalls
To develop the habit of taking prompt preventive or corrective action
Power-on stall: to understand what could happen if the airplane were climbing at an excessively nose-high attitude immediately after takeoff or during a climbing turn
Power-off stall: to understand what could happen if controls are improperly used during a turn from the base leg to final approach or on final approach

PROCEDURES

Discuss definition, safety factors, tolerances, objectives, and other elements of stalls
Aerodynamics of stalls
Lift of a wing is given by L = CL(ρv²A)/2 where

CL= coefficient of lift
ρ = air density
v = velocity, airspeed
A = wing area

CLincreases with AOA to CLmax, where wing stalls
Just before wing stalls, nose pitches down due to reduction in tail down force
Most airplanes' wings stall progressively outward from the roots
Rectangular wings have this "wash out" characteristic Wing "twist" (tips have smaller angles of incidence and attack than roots) helps assure wash out
Allows aileron effectiveness at relatively high AOA
In a stall, ailerons loose effectiveness; rudder should be used to maintain direction or level wings

Relationship of various factors to stall speed (Vs)
Landing gear: 09T: no demonstrated effect on power-off Vs
Flaps increase CL, decrease Vs(09T: 1-4 kts)
Increasing weight increases Vs
Lift must be increased by increasing CLby increasing AOA
As CG moves forward, Vsincreases
Greater tail down force and thus greater total lift are required
Aft CG decreases Vs, but aft of safe CG range, stall may be unrecoverable
VS is proportional to the square root of the load factor
Increasing bank increases Vs
Turbulence can increase load factor and Vs
Even small amounts of snow, frost, or ice increase Vs
Flight situations where unintentional stalls may occur
Power-on
Takeoffs and departure climbs, esp. short fields with obstacles
Go-arounds
En-route best-angle climbs
Power-off
Approach and landing
Turning base to final (may be crossed-control)
After engine failure
During glides; attempting to "stretch" a glide
Recognition of the first indications of a stall
Vision: see relatively nose-high attitude and decreasing airspeed
Hearing: quieter air flow, sounds of vibration, stall warning horn
Best: kinesthesia: sense of change in direction or speed of motion; feeling of decrease in speed, settling or "mushing"
Feeling of decreased effectiveness of controls
POWER-ON STALLS
Performance of power-on stalls in climbing flight (straight or turning)

May be practiced in

Takeoff configuration (gear down, flaps 20° for short field simulation)
Clean (gear and flaps up)

Entry technique and minimum entry altitude
Enter above 2,000 feet AGL to recover above 1,500 feet AGL
Lights on, CLEAR area with 90° turns, left and right
During second clearing turn
Mix rich, prop in high RPM, carb temp check
Throttle back slow to VR= 55 KIAS
+ Gear down and flaps 20°
At 55 KIAS:
Throttle to 25" (or 30"), carb heat OFF
Pitch up slow (heading straight or 207deg; coordinated bank)
Maintain pitch up until airspeed slows to Vs
Coordination of flight controls
Right rudder to counteract torque and P-factor
Maintain coordination even if controls are crossed (right turn)
Stall will break straight away from pilot if coordinated
Release right rudder as stall breaks
In uncoordinated stalls, the airplane usually yaws to the side with excess rudder (rolls or falls away from the ball)
Recovery technique and minimum recovery altitude
Recover above 1,500 feet AGL
Technique:
Promptly release elevator back pressure
Lower nose to horizon, check power to 30" MP
Regain SLF coordinated
Safe altitude, accelerate to 90 kts, retract flaps and gear

POWER-OFF STALLS
May be practiced in SLF, descending straight and turning flight
To simulate landing approach stalls
gear down
flaps down
power reduced to idle
Entry technique and minimum entry altitude
Enter above 2,000 feet AGL to recover above 1,500 feet AGL
Lights on, CLEAR area with 90° turns, left and right
During second clearing turn
Gear down <140 kts
Throttle back approximately 19", prop in high RPM, mix rich
Flaps down as airspeed slows
Carb temp check + carb heat on
Throttle back to idle
For level stall, hold nose up to maintain altitude until Vs
Coordination of flight controls - as above
Recovery technique and minimum recovery altitude
Recover above 1,500 feet AGL
Technique
Promptly release elevator back pressure
Throttle to 30 in. MP, carb heat OFF, flaps up to 20°
Level wings coordinated
Then either
Gear up, flaps up, resume SLF or
"Go-around": 70 KIAS climb, gear up, flaps up slow, 80 KIAS
Demonstrate power-off stalls and recoveries with and without power
Coach student practice of power-off stalls
Demonstrate power-on stall
Coach student practice of power-on stalls
Critique student performance

COMMON ERRORS

Failure to establish the specified landing gear and flap configuration prior to entry
Improper pitch, heading, and bank control during straight ahead stalls
Use outside and instrument references
Right rudder in nose-high power-on condition; release at break
Improper pitch and bank control during turning stalls
Rough or uncoordinated control technique
Failure to recognize the first indications of a stall
Failure to achieve a stall
Improper torque correction
Poor stall recognition and delayed recovery
Excessive altitude loss or excessive airspeed during recovery
Secondary stall during recovery