PRINCIPLES OF FLIGHT 28 WWW.GOFLY.ONLINE Y ONLI NE FLI GHT SCHOOL The factors which will result in stall speed changes The speed at which a wing stalls is influenced by many factors including weight, wing loading, the use of flaps, power or atmospheric conditions such as altitude or density altitude. See below for more detail on the factors that affect the stall speed. Flaps Flaps are a lift generating device that changes the shape of the wing or camber. An aircraft in level flight requires a certain angle of attack to create the amount of lift required to balance the weight. If we deploy flaps, we create more lift, which means we can now maintain a slower airspeed at the same angle of attack. Flaps reduce the airspeed at which the aircraft will stall. Power When power is on during a stall, the propeller slipstream adds more airflow over the surface of the wing and therefore delays the separation of the airflow from the wing at the onset of the stall. As the aircraft approaches the stall with power on, the higher nose attitude means some of the thrust force acts vertically. This vertical component of thrust has the effect of allowing the aircraft to fly at a lower airspeed for the same corresponding angle of attack. Power reduces the airspeed at which the aircraft will stall. Altitude One of the factors of a stall is the loss of dynamic pressure over the wing. As an aircraft’s altitude increases, the air density and dynamic pressure over the wing decreases. This results in less lift being available and therefore at a higher altitude the aircraft will stall at a higher airspeed. Altitude increases the airspeed at which the aircraft will stall. Weight An increase in weight requires an increase in lift to maintain level flight. A higher weight means a higher angle of attack is required for the same airspeed at a lower weight. Effectively this means that an increase in weight will increase the aircraft’s stall speed. Weight increases the airspeed at which the aircraft will stall. G Force/Load factor When an aircraft is in a turn or pulling out of a dive it experiences centripetal force, the opposite, resultant force is centrifugal force. The resultant force or G loading has the same effect as increasing the weight of the aircraft. At sixty degrees angle of bank the aircraft will experience 2G, and the pilot will feel twice their normal weight. The stall speed increases by the square root of the load factor. This means that at a 2G loading, the stall speed will increase by a factor of 1.4 The formula to work out the new stall speed is: Vs(new)=Vs(old) x square root of load factor. For instance, if the original stall speed is 60 kts and the G loading is 2, the formula would be 60 times the square root of 2, giving a new stall speed of 84 kts. G Force/Load Factor increases the airspeed at which the aircraft will stall. Wing shear, gusts and thermals Wind shear, wind gusts and thermal activity can change the angle of attack and add some sudden G loading to the aircraft, therefore depending on the gust or thermal it could potentially decrease or increase the stall speed suddenly.
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