Coefficient lift and Angle of Attack chart
Factors affecting Stall Speed
Most aircraft accidents occur during the take-off and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. On this part 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. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.
What influences the stall speed? What factors can a pilot take care of so that the stall speed is low and the flight is safe. That will be the subject here.
Influencing factors
Wing Stall
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 where the airflow begins to separate. And the actual speed and angle of attack where that happens depends on a number of factors.
When the wing stalls the airflow breaks away from the upper surface and the amount of lift will be reduced to below what is needed to keep the wing flying. If one wing stalls before the other (wing drop) the aircraft tends to roll in that direction and possibly into a spin. At the wing drop, apply opposite rudder and stick forward to reduce the angle of attack, add power for a full recovery.
Weight
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. For any level flight the amount of lift must be equal to the weight of the aircraft, if all up weight is lower then the amount of lift needed is lower too. To calculate the new stall speed: Vs new = Vs old weight x √(new weight / old weight).
Load factorFor a normal straight and level flight load factor is exactly one (1), lift opposes weight, load factor (G) equals G = L / W. But put the aircraft in a level 60° banked turn and the story changes. To calculate the load factor for a certain bank angle: G = 1 / cos (bank angle). The accelerated stall speed is Vsacc=Vs x √G. To the right we have calculated some common bank angles to showing the increase in stall speed and load factor on an aircraft. |
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Turning
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 much more than the vertical component which keeps the aircraft from descending in a level turn.
Altitude
The amount of lift generated by a given wing depends on AOA (CL) and airspeed. Given the lift formula: L = 1/2 ρ V2 x S x CL, 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 remains the same, but the TAS where the stall occurs increases with altitude because of the lower air density (1/2 ρ).
Vortex generators
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 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 and 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. Thus 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 with less effective ailerons.
Flaps or Slats
Extending flaps or slats increases the wing camber (curvature), the CL and the geometric AOA both reduce. The lift formula tells us that if CL increases IAS must decrease if lift is to remain constant. So therefore stall speed and AOA reduce.
Lowering the flaps also increases drag, especially when beyond 25°. This helps stabilizing the airspeed during approach and landing.
Wing contamination
Wings covered with rime, ice, hoar frost and remains of bugs and 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.
