Stalling Flashcards
What does the stall cause
A large reduction of lift and large increase in drag.
How is a stall caused
Increase in angle of attack. Separation of boundary layer from majority of wings upper surface.
When is an adverse pressure gradient greatest
When the distance is short and pressure difference across upper surface is high.
Key influencing factor is amount of pressure change caused by AOA as distance is approximately constant. FOR LOW SPEED FLIGHT
What happens to surface pressure gradient as AOA increases.
As AOA increases surface pressure gradient increases. On cambered wing separation point moves forwards.
What is the effect of the stall on lift and drag
At point of stall lift reduces substantially, total drag much greater (large increase in form drag)
CP has moved aft.
What happens in Level flight with reducing airspeed and increasing AOA
Lift remains constant, total drag increases due to induced drag, CP moves forward
What is the stalling angle/critical angle
AOA in CL is maximum. Angle beyond this which wing stalls at. An aerofoil section of a given design always stalls at same AOA.
Effective airflow
And stall
Less downwash shallow EAF will be. Less downwash results in larger effective AOA for same AOA.
Effective AOA determines CL and so determines when a wing/part of wing will stall.
Relationship between aspect ratio and stall
Low aspect ratio has relatively small AOA therefore stalling angle is higher. High aspect ratio wings produce lift quicker but stalling angle is less.
Relationship between rectangular wing and stalling
Stalls at root first and progressively moves across wing. Root in front of wing section so turbulent air hitting tail creates buffeting warning of approaching stall. Reduced downwash increases pitch down moment at the stall.
Ailerons remain usable for some time so still have some lateral control in early stages of stall.
Wing drop less pronounced.
Relationship between elliptical wing and stall
As elliptical wing produces lift across all wing span when stall occurs it sudden, ailerons become ineffective quickly and large roll rates if a wing does drop.
Relationship between moderately tapered wing and stall
Stall midspan as downwash least and effective AOA is greatest. Progress out to tip and root. Stall onset rapid, no stall warning, ailerons quickly ineffective and wing drop greater than rectangular wing.
Relationship between swept wing and stall
Least downwash at tips so effective AOA highest. Stall occurs at. Tips first and moves inboard. Swept wings have tendency for wing tip flow separation because boundary layer is thicker and more sluggish at tips.
Effect of downwash on EAF
Determines EAF and therefore the effective AOA and CL.
What is deep stall
Wake/turbulent air of wing becomes impinged on tail section and renders it ineffective. Occurs mainly on T-Tail Planes. Cannot get out of deep stall. Only way to prevent is to not get into deep stall.
