Coefficient lift and Angle of Attack chart
Factors affecting Stall Speed
Most aircraft accidents occur during the take-off or landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. In this section of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight.
What factors influences the stall speed of an aircraft? Which can a pilot alter or change to his advantage so that the stall speed remains low and the flight is carried out safely. This and more will be the subject here.
Before we continue lets look at the definition of the stall: It is a condition in flight where the angle of attack of the wing exceeds the critical angle and where the airflow begins to separate. The actual speed and angle of attack where that happens depends on a number of factors as you will see.
When the wing stalls the airflow on the upper surface breaks away from trailing edge moving forward, and the amount of lift will be reduced to below what is needed to keep the wing flying (total lift will then be less than actual aircraft weight). If one wing stalls before the other: the aircraft tends to roll in that direction (wing drop) and it might possibly enter into a spin. To remedy this: at the wing drop, apply opposite rudder and move the stick/yoke forward to reduce the angle of attack, after that the addition of engine power speeds up a full recovery.
A change in weight will not change the angle of attack with which the wing will stall (CLmax is fixed for a given wing configuration), but it changes the speed where the stall will occur. We know that for any level flight (not climbing) the amount of lift must be equal to the weight of the aircraft, thus if all up weight is lower then the amount of lift required is less too. To calculate the new stall speed: Vs new = Vs old weight x √(new weight / old weight).
For normal, straight and level flight the load factor is exactly one (1). As lift opposes and is equal to weight, load factor (G) is G = L / W. But, for example, if you would place the aircraft in a level 60° banked turn the story changes. To calculate the load factor for a certain bank angle use this formula: G = 1 / cos (bank angle). The accelerated or increased stall speed becomes then Vsacc=Vs x √G.
To the right above we took some common bank angles, used the formula's and we showed the increase in stall speed and load factor for a typical aircraft. As you can see: from 45° bank angle and higher the stall speed and load factor go up rapidly.
As can be seen in the table above, when the aircraft banks 30° the stall speed increases with 7% due to the fact that aircraft weight in a level turn increases by 15% caused by the increased load factor. Although load factor increases, the stalling angle of attack will remain the same.
Image shows clearly that in a 45° banked turn the amount of generated lift is larger than the vertical component of lift which keeps the aircraft from descending in a level turn.
Climbing / descending turns
In a climbing turn the higher wing (with the greatest rate of turn) has the highest angle of attack and will stall before the inboard wing. As a result the aircraft will therefore roll level. When the aircraft descends the lower wing has the highest angle of attack, at the stall the wing will drop increasing the bank angle even more.
Given the lift formula: L = 1/2 ρ V2 x S x CL, the amount of lift generated by a given wing depends on AOA (CL) and airspeed, altitude is set by 1/2 ρ. So when the aircraft climbs the factor '1/2 ρ' decreases and as CL remains the same, true airspeed must increase to obtain the same indicated airspeed (IAS). And as stall speed is directly related to AOA it also remains the same, but the TAS where the stall occurs increases with altitude because of the lower air density (1/2 ρ).
These are comb like protrusions on the top of the wing. The effect they have is that the airflow is energized during high angles of attack and therefore sticks better to the wing surface, so that the separation of the airflow is delayed even more and stall speed is lower with a higher AOA.
Power & climb
During slow or climbing flight the power thrust line is pointed upward and if power is applied the upward vector of thrust offsets the weight. As a result less lift is needed and as CL is constant -> IAS is lower. Conclusion: the stall speed is lower with engine power applied.
Added to this is the fact that slipstream from the propeller over the inner part of the wing and tail section improves effectiveness and delays the separation of the airflow near the wing root. At the stall the airflow separates nearer to the wing tips than the root. The effect will be a more pronounced wing drop but with less effective ailerons.
Flaps or Slats
Extending flaps or slats increases the wing camber (curvature), the CL increases and the geometric AOA reduces. The lift formula tells us that if CL increases IAS must decrease if lift is to remain constant. So therefore, as a result, stall speed and AOA both are reduced.
Lowering the flaps also increases drag, especially when beyond 25°. This helps stabilizing your airspeed during approach and landing. On some aircraft equipped with full wing ailerons (flaperons) extending flaps will introduce a lot of adverse yaw, and coordinating with more rudder will be necessary.
Wings covered with rime, ice, hoar frost and remains of bugs and other dirt will cause an early separation of the boundary layer and the stall speed will increase, sometimes even by 4%. Remember that the same applies to the propeller, but then a decrease in thrust will be the result.