BASICS OF AN AERODYNAMIC STALL

This is written from a pilot’s point of view with the aim of giving a theoretical aerodynamic background to the actions that one takes in the cockpit. These actions would become instinctive if one understands the basic theory. We are not designers or engineers, but we need to fly the aircraft safely from A to B, under all conditions. Under normal conditions we would never be operating close to stall. The highest AsOA that we encounter in flight are during the take-off and landing phases.  However, when operating under conditions that are not normal we are still required to fly that aircraft safely and thus we need to understand how our aircraft behaves at all times.

• ·         In flight we have four forces that we should be aware of at all times. To maintain steady straight and level flight the Lift has to balance weight and the thrust has to balance the drag, and the sum of the moments has to be zero. This also implies that the Power available is equal to the power required. If the Power available is more than the Power required then the aircraft would have a ROC, and vice versa. Power required = Drag X TAS. Power available is Thrust X TAS.
• ·         A stall is a condition of flight in which the aircraft has exceeded the stalling angle of attack or the critical angle leading to a sharp decrease in co-efficient of lift and a sharp increase in the co-efficient of drag. (Please refer to the AOA vs Cl/ Cd graphs).
• ·         The decrease in Cl leads to loss of lift as L = Cl ½ pv2S. In straight and level flight the lift is now no longer able to balance the weight.
• ·         The increase in co-efficient of drag increases the drag leading to a power deficit, as the power available at the same throttle settings is now less than the power required. On a training aircraft with rectangular wings, propeller slipstream and high thrust line, the wing stalls and the nose pitches down, helping in recovery of the aircraft from the stalled condition.
• ·         This situation changes in aircraft that fly at high altitude, high speed, have sweep back wings and a thrust line below the CG (due to low slung engines). Sweep back wings tend to stall at the tips first causing the nose to pitch up; any addition of thrust from the low slung engines would aggravate the pitch up situation and lead to a stall.
• ·         In a power deficit situation, the aircraft will develop a descent. When the aircraft commences descent, the direction of the relative airflow changes, as the flight path of the aircraft has changed.  The relative airflow comes from further below than earlier leading to an increase in AOA – reducing Cl further and increasing Cd further. We are now entering into a deep stall zone which is more pronounced on sweep back and delta wings. The positioning of the tail also has an effect.  Any other phenomenon like icing could also lead to reduction of Cl and increase in Cd.
• ·         The only way out is to unstall the aircraft by reducing the AOA by aligning the aircraft with the relative airflow coming from below OR in pilot’s parlance by pushing forward on the control column. The throttle is advanced to increase the thrust so that the power available increases to match the power required.
• ·         Once the aircraft unstalls, the aircraft can be eased out. This procedure has to be followed on all aeroplanes – the basic theoretical considerations do not change very much and you can understand the reason why ; the manufacturers give the finer points which would be peculiar to that aircraft type.
• ·         The basic fact is that whether one flies the C-152 or the A-380, the aircraft needs to be supported by the air in which it flies.
• ·         The airflow going around the aerofoil is what generates the lift and the aerofoil (and in turn the wing too) stalls when the airflow ‘separates’ from the aerofoil at a forward position on the chord, leading to sharp loss of lift. (As the AOA increases, the separation point keeps moving forward.)
• ·         Every aerofoil has different aerodynamic characteristics, as can be seen in the books. When icing takes place the aerofoil shape changes, and so do its aerodynamic characteristics – Cl, Cd and separation of airflow too. It would happen every time the aerofoil shape changes – it could be because of dirt, pigeon shit on the wing or any other phenomenon.
• ·         Standard operating Procedures are laid down for all normal operations that a company undertakes, and are a pilot’s life line and should be followed meticulously to go from A to B safely under all conditions – normal or not normal.