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081 03 01 00 The stall
081 03 01 01 Flow separation at increasing α
(02) X Describe the thickness of a typical laminar and turbulent boundary layer.
(03) Describe the properties, advantages and disadvantages of the laminar boundary layer.
Less drag due to the smaller speed gradient – there is less heat and therefore energy loss.
(04) Describe the properties, advantages and disadvantages of the turbulent layer.
Turbulent layer has more energy, helps resist separation. Has a steeper velocity gradient.
A deeper boundary layer which causes more drag.
(05) Define the ‘transition point’.
The point where laminar flow turns into turbulent flow.
(06) Explain why the laminar boundary layer separates easier than the turbulent layer does.
Because the turbulent layer has more energy.
(07) Describe why the airflow over the aft part of a wing slows down as the α increases.
Because peak flow rises, i.e a larger adverse pressure gradient – it rises aft of the suction peak which opposes the relative flow.
(08) Define the ‘separation point’ and describe its location as a function of α.
Where the lower layers of the boundary layer no longer have enough kinetic energy to remain attached. It moves forward with increase of alpha.
(09) X Define αCRIT.
(10) Describe in straight and level flight the influence of increasing the α on:
1 - the forward stagnation point;
2 - the pressure distribution;
3 - the CP location (straight and swept-back wing);
4 - CL;
5 - CD and D (drag);
6 - the pitching moment (straight and swept-back wing).
1 – The stagnation point moves downwards and to the underside of the wing.
2 – Increases the total reaction with a lower pressure towards the leading edge which…
3 – …moves the centre of pressure forwards, but at the stall it moves aft for straight wing. For swept wing, it moves forwards then continues to move forward.
4 – The CL increases with increasing alpha.
5 – Drag increases.
6 – Pitching moment, straight wing will nose down if it is at the stall we are talking about. A swept wing may nose up at the stall due to further CP movement forward.
(11) Explain what causes the possible natural buffet on the controls and on the aeroplane in a pre-stall condition.
(12) Describe the effectiveness of the flight controls in a pre-stall condition.
(13) Describe and explain the normal post-stall behaviour of a straight-wing aeroplane.
(14) Describe the effect and dangers of using the controls close to the stall.
081 03 01 02 The stall speed
(01) Explain VS0, VS1, VSR, and VS1G.VS0 – Stall speed in landing config.
VS1 – Stall speed in a specific condition.
VSR – Because swept wings don’t have clearly defined stall onset. VSR is a margin above the stall for TO and Landing.
VS1G – 1g stall speed in level flight.
(02) Solve VS1G from the lift formula given varying CL.
VS1g = sqrt (2Mg /rho S CLmax)
(03) Describe and explain the influence of the following parameters on stall speed:
— CG;
— thrust component;
— slipstream;
— wing loading;
— mass;
— wing contamination;
— angle of sweep;
— altitude (for compressibility effects, see 081 02 03 02).
CG – Forward increases stall speed – because wings must create extra list.
Thrust component – Reduces at high alpha because some of the thrust replaces lift.
Slipstream – increases flow and lift so reduces stall speed.
Wing loading – Increases adverse pressure gradient helping separation – stall speed increases.
Mass – High mass = more lift needed = higher stall speed – steeper adverse pressure gradient.
Wing contamination – wing can’t produce the same amount of lift – stall speed increases.
Sweep angle – flow is not perpendicular to the wing so stall speed increases.
(04) X Define the ‘load factor n’.
(05) Explain why the load factor increases in a turn.
(06) Explain why the load factor increases in a pull-up and decreases in a push-over manoeuvre.
(07) Describe and explain the influence of the ‘load factor n’ on stall speed.
(08) X Explain the expression ‘accelerated stall’.
Remark: Sometimes, accelerated stall is also erroneously referred to as high-speed stall. This latter expression will not be used for Subject 081.
(09) Calculate the change of stall speed as a function of the load factor.Vs = Vs1g x sort of the load factor.
(10) Calculate the increase of stall speed in a horizontal coordinated turn as a function of bank angle.
Load factor = 1 / cos theta
Vs as above formula.
(11) Calculate the change of stall speed as a function of the gross mass.
Vs new = Vs old x sqrt of weight new / weight old.
081 03 01 03 The initial stall in spanwise direction
(01) Explain the initial stall sequence on the following planforms:— elliptical;
— rectangular;
— moderate and high taper;
— sweepback or delta.
Elliptical will probably stall at the same time across the whole wing.
Rectangular will probably stall at the roots first because of the higher effective angle of attack. (More downwash at the tips)
Moderately tapered will stall first in the middle of the wing then progressive outwards.
Delta or sweptback will stall at the tips first.
(02) Explain the purpose of washout.
(03) Explain the effect of aileron deflection.
(04) Explain the influence of fences, vortilons, saw teeth, vortex generators, and strakes on engine nacelles.
Fences – a physical barrier to span wise flow.
Saw teeth – Produces a blocking vortex.
Vortilons – little spikes things which produce a vortex to reenergise the boundary layer – delaying separation.
Strakes reduce upwash and create strong vortexes at high alpha reenergising the wing..
081 03 01 04 Stall warning
(01) X Explain why stall warning is necessary.(02) X Explain when aerodynamic and artificial stall warnings are used.
Shut up.
(03) Explain why CS-23 and CS-25 require a margin to stall speed for take-off and landing speeds.
(04) X Describe:
— buffet;
— stall strip;
— flapper switch (leading-edge stall-warning vane);
— angle-of-attack vane;
— angle-of-attack probe;
— stick shaker.
1 – An aerodynamic warning of an impending stall.
2 – Creates aerodynamic stall warning early to alert the pilot.
3 – Reduced pressure sucks the flapper up to active a stall warner.
4 – Measure the angle relative to the airflow.
5 – Uses the difference in dynamic pressure to measure alpha.
6 – Linked to the above sensor to physically alert the pilot.
(05) Describe the recovery after:
— stall warning;
— stall;
— stick-pusher actuation.
Reduced alpha and accelerate to a safe speed.
Stall – reduce alpha and recover, don’t use ailerons until well out of stall range.
Pusher – add power, trim, no A/P.
081 03 01 05 Special phenomena of stall
(01) X Describe the basic stall requirements for commercial air transport (CAT) aeroplanes.Vsr may not be less than Vs1g.
Minimum speed below 35 must be Vsr1 x 1.13
Vref landing speed, not below 1.23Vso
(02) Explain the difference between power-off and power-on stalls and recovery.
(03) Describe stall and recovery in a climbing and descending turn.
(04) Describe the effect on stall and recovery characteristics of:
— wing sweep (backward sweep);
— T-tailed aeroplane.
Swept – May nose up, difficult to recognise the stall, reduce alpha, init.. T-tail may be out of the turbulent flow so pre-stall buffet may not exist.
(05) Describe super stall or deep stall.
(06) Describe the philosophy behind the stick-pusher system.
(07) Describe the factors that can lead to the absence of stall warning and explain the associated risks.
(08) Describe the indications and explain the consequences of premature stabiliser stall due to ice contamination (negative tail stall).
(09) Describe when to expect in-flight icing.
(10) Explain how the effect is changed when retracting/extending lift-augmentation devices.
Retracting flaps may provoke a stall on an iced wing.
(11) Describe how to recover from a stall after a configuration change caused by in-flight icing.
Reduced alpha or reinstate previous condition.
(12) Explain the effect of a contaminated wing on the stall speed and αCRIT.
(13) Explain airframe contamination and the aerodynamic effects when parked and during ground operations in winter conditions.
(14) Explain de-icing/anti-icing holdover time and the likely hazards after it has expired.
Reicing
(15) Describe the aerodynamic effects of heavy tropical rain on stall speed and drag, and the appropriate mitigation in such conditions.
Heavy rain can affect the boundary layer and increase skin friction. Mitigation is probably to avoid such areas.
081 03 01 06 The spin
(01) Explain how to avoid spins.(02) List the factors that cause a spin to develop.
(03) Describe an ‘incipient’, ‘developing’ and ‘developed’ spin, recognition and recovery.
Incipient – Aircraft has stalled and it has not rotated through one rotation
Developing – Not sure.
Developed – Not control input are required to continue autorotation. Steady rolling, yawing and pitching often with a low nose and high rate of descent.
(04) Describe the differences in spin attitude with forward and aft CG.
081 03 02 00 Bufet onset boundary
081 03 02 01 Definition and relationship with Mach buffet
(01) Explain shock-induced separation, shock stall, and describe its relationship with Mach buffet.
As the wing passes MCrit the boundary layer separates at the bottom of the shockwave, the turbulent flow this creates is the mach buffet.
*Removed. (02) X Define ‘shock stall’.
Remark: For theoretical knowledge examination purposes, the following description is used for shock stall: Shock stall occurs when the lift coefficient, as a function of Mach number, reaches its maximum value (for a given α).
081 03 02 02 Buffet onset
(01) Explain the concept of buffet margin, and describe the influence of the following parameters on the concept of buffet margin:— α;
— Mach number;
— pressure altitude;
— mass;
— load factor;
— angle of bank;
— CG location.
It is the range of speeds between low speed stall and high speed buffet.
1-
2-
3-
4-
5-
6-
7-
(02) Explain how the buffet onset boundary chart can be used to determine:
— manoeuvrability;
— buffet margin.
Look at the chart, init…. Load factor affects so choose the maximum load factor that you will use.
(03) Describe the effect of exceeding the speed on buffet onset.
Flow separation and general nastiness.
(04) Explain ‘aerodynamic ceiling’ and ‘coffin corner’.
(05) Explain the concept of the ‘1.3g’ buffet margin altitude.
(06) Find (using an example graph):
— buffet free range;
— aerodynamic ceiling at a given mass;
— load factor and bank angle at which buffet occurs at a given mass, Mach number, and pressure altitude.
(07) Explain why descent increases the buffet free range.
081 03 03 00 Situations in which buffet or stall could occur
081 03 03 01 Explain why buffet or stall occurs
(01) Explain why buffet or stall could occur in the following pilot- induced situations, and the methods to mitigate them:
— inappropriate take-off configuration, detailing the consequences of errors associated with leading-edge devices;
— steep turns;
— go-around using take-off/go-around (TOGA) setting (underslung engines).
- Not enough lift….leading edge increase alpha crit so if you don’t use them you are a bell.
- Higher load factor causing accelerated stall.
- Pitch up when close to alpha crit
(02) Explain why buffet or stall could occur in the following environmental conditions at low altitude, and how to mitigate them:
— thunderstorms;
— wind shear and microburst;
— turbulence;
— wake turbulence;
— icing conditions.
Ts – Down draughts corrected beyond stall angle
WS – Instant change of flight conditions.
Turbulence – Vertical speed changing RAF and EAF
Wake – Roll taking down going wing beyond alpha crit.
Ice – Changes lifting ability of the wing.
(03) Explain why buffet or stall could occur in the following environmental conditions at high altitude, and how to mitigate them:
— thunderstorms in the intertropical convergence zone (ITCZ);
— jet streams;
— clear-air turbulence.
Turbulence, ice, high wind speeds etc.
(04) Explain why buffet or stall could occur in the following situations, and how to mitigate them:
— inappropriate autopilot climb mode;
— loss of, or unreliable, airspeed indication.
Vertical speed mode set.
081 03 04 00 Recognition of stalled condition
081 03 04 01 Recognition and explanation of stalled condition
(01) Explain why a stalled condition can occur at any airspeed, or attitude or altitude.(02) Explain that a stall may be recognised by continuous stall- warning activation accompanied by at least one of the following:
— buffet, that can be heavy;
— lack of pitch authority;
— uncommanded pitch down and uncommanded roll;
— inability to arrest the descent rate.
(03) Explain that ‘stall warning’ means a natural or synthetic indication provided when approaching the stall that may include one or more of the following indications:
— aerodynamic buffeting;
— reduced roll stability and aileron effectiveness;
— visual or aural clues and warnings;
— reduced elevator (pitch) authority;
— inability to maintain altitude or arrest a rate of descent;
— stick-shaker activation.
Yep.