Example Of Research Paper On Stalls

In fluid dynamics, a stall is defined as a sudden reduction or fall in the lift generated by an aerofoil when the critical angle of attack is attained or exceeded. The critical angle is normally about 15 degrees but may vary depending on the foil, fluid and Reynolds number. Formally defined, a stall can be said to be a condition in aerodynamics and aviation where the angle of attack increases way past a particular point such that the aircraft begins to experience a decrease in the lift. The angle at which this situation occurs is what is called the critical angle of attack.
The critical angle is often dependent on the planform of the wing, its profile and aspect ratio among other factors. This critical angle of attack can then be said to be the angle of attack on the lift coefficient versus the angle of attack curve at which maximum lift coefficient occurs. However, typically, the critical angle of attack lies in the range of 8 to 20 degrees relative to the incoming wind for a number of subsonic foils.
During a stall, a fixed wing aircraft might experience a change in attitude. This is because attempts to increase the angle of attack at higher speeds can cause the aircraft to climb or alternatively cause a high-speed stall. Any yaw of the aircraft as it enters a stall or during a stall can cause the aircraft to go into autorotation, also known as the spin. Depending on the type of aircraft and its design, a stall can pose extremely adverse properties of balance as well as control.

The graph below shows the relationship between the angle of attack and the lift.

Factors influencing a stall
Weight: A change in weight cannot change the AoA with which an aircraft wing will stall but can change the speed where the stall will take place. For any level flight, the amount of the lift experienced must be equal to the weight of the aircraft. Therefore in the event that the all up weight is low, the amount of lift needed will be less as well.

Load factor: For a normal level and straight flight, the load factor is exactly 1.

Descending/Climbing turns: In a climbing turn, the wing with the greatest rate of turn has the greatest angle of attack and will stall before the inboard wing. The aircraft will, therefore, roll level, as a result. In the event, that the aircraft descends, the lower wing has, the higher AoA hence at the stall the wing will drop, resulting in an increase in the bank.
Altitude: The amount of lift generated by a wing always depends on the angle of attack as well as the airspeed.
Vortex generators: Vortex generators are the comb-like protrusions on the top side of the wing. Their effect is that the airflow is energized during high AoA and thus sticks better to the surface of the wing so that the separation of the airflow is further delayed, and stall speed is lower with a higher angle of attack.

Power and climb: the stall speed is much lower when the engine power is applied.

Wing contamination: aircraft wings covered with ice, rime, remains of bugs, hoar frost and other forms of dirt will result in an early separation of the boundary layer, and consequently the stall speed will increase. The same applies to the propeller only that the result will be a decrease in thrust.

Symptoms of an approaching stall

One common symptom of an approaching stall is sloppy and slow controls. As the aircraft speed reduces approaching the stall, there is less air moving over the wing. This results in less air being deflected by the control surfaces at that lower speed. Some buffeting might also be experienced from the turbulent flow on the top side of the wing as the stall is reached.

Characteristics of stalling

Stalling characteristics vary from one type of aircraft to another. A benign stall is the one whereby the nose of the aircraft drops gently while the wings remain level throughout. Slightly more demanding is a stall whereby one wing stalls moments before the other causing the wing to drop rapidly with the possibility of resulting in a spin. A dangerous stall is where the nose of the aircraft rises thereby pushing the wing deeper into the stalled state hence leading to a hardly recoverable deep stall. This can frequently occur in T-tailed aircrafts where the turbulent air flow from the stalled wing can cover the control surfaces at the tail of the aircraft.

Stall speed

Stalls depend majorly on angle of attack and not airspeed. However, the slower an airplane moves, the more angle of attack it needs to produce a lift equivalent to the airplane's weight. With the speed decreasing further, it will reach a point where the angle of attack will be equivalent to the critical angle of attack. This speed is what is referred to as the stall speed. Any aircraft flying below its stall speed will not stop descending while one flying at its stall speed will not climb. Any attempts to do so by increasing the angle of attack without increasing the airspeed will result in a stall. The real stall speed will vary depending on the aircraft’s configuration, weight, vertical and lateral acceleration and altitude.

Accelerated and turning flight stall

The normal stall speed always refers to a straight as well as a level flight where the load factor is equivalent to 1g. If the airplane is pulling up or turning from a dive, then additional lift is needed to provide the lateral or vertical acceleration thus leading to a higher stall speed. A stall which occurs under such conditions is what is referred to as an accelerated stall. In order to achieve an extra lift the lift coefficient and consequently the angle of attack will need to be significantly higher than it would have been in the event of a straight and level flight at the same speed. For this reason, provided that the stall occurs at the same critical angle of attack, by increasing the load factor such a critical angle as well as the stall will be reached with the airspeed remaining steadily above the normal stall speed.

Dynamic stall

This is a nonlinear, non-steady aerodynamic effect which takes place when airfoils quickly change the angle of attack. This quick change can cause a strong vortex to be shed from the leading edge of the aerofoil and travel backward above the wing.

Deep stall

Also referred to as the super stall is an extremely dangerous type of stall which affects certain aircraft types, especially those with a T-tail configuration. In such designs, it is quite difficult to recover from a stall.

Tip stall

The airflow along the chord mainly contributes to the lift. This implies that the wing then begins to develop less lift at the tip as opposed to the root. In severe cases, this situation can lead to the tip of the wing entering a stall way earlier before the whole wing. In such a case the average lift of the wing as a whole moves forward, the inboard sections continue to generate the lift are in front of the CoG generally while the tips remain behind the center of gravity since they are no longer contributing. This produces a strong nose-up pitch in the airplane that can lead to increased wing stalling, further forward movement of the lift and much more.

References

Phillips, W. F. (2004). Mechanics of flight. New Jersey: Wiley.
Stowell, R. (2007). The Light Airplane Pilot's Guide to Stall/Spin Awareness. California: Rich Stowell Consulting.
Swatton, P. J. (2011). The principles of flight for pilots. West Sussex: Wiley.
Torenbeek, E. (1996). Synthesis of subsonic airplane design. Dordrecht: Kluwer.