The stalling of an aircraft has always been a key feature in flight history. However, what makes an aircraft stall? What methods are being used to determine when a stall may occur? And what is being done to prevent a stall?
This report will discuss aspects of the stall. These include:
• Defining what a stall is
• Boundary layer
• Aerodynamic and control characteristics
• Methods of sensing a stall
o Control surface buffet
o Stall vane
o Stall strips
o Suction activated horn
o Angle of attack sensor and stall warning computer
o Washout
o Vortex generators
• Stall warning and protection
o Stick shaker
o Stick pusher
o Flight deck indicators
DEFINITION OF A STALL
A stall is where an aircraft is climbing at an angle where it can produce no more lift, therefore will result in a stall and the aircraft will fall from the sky. This happens when an angle of 15⁰ has been reached and the airflow across the aerofoil has reached its maximum, this is called the critical angle of attack. The normally smooth airflow on the upper surface of the wing has now become turbulent and chaotic which reduces the lifting characteristics very quickly. As this lift quickly decreases, drag increases significantly. If the airspeed is too low and will not support the aircraft then the stalling speed takes place. This will vary with the shape of the wing and the position of the flaps. Stalls are more likely to happen when performing a maneuver such as a steep bank. This happens because the aircraft will exceed its critical angle of attack (Figure 2). The leading edge of the wing is crucial and needs to be cleared of any contamination, such as ice. Having ice on the leading edge can greatly reduce the lifting characteristics and the critical angle of attack will be reduced, making it easier to stall. A number of stalls can occur during flight, these are:
• Secondary stall – If the first stall has not been fully recovered, a secondary stall may occur or may cause the aircraft to spin. Secondary stall will result if the exit of the stall is too fast before there is enough flying speed to pull out of the stall. To exit the stall, the elevator back pressure should be released, this will then allow the aircraft to naturally recover from the stall and the aircraft will return to steady level flight.
• Cross-Control stall – If a steeply banked turn is overshot from the centerline of the base to the final it tend to result in a stall. This usually happens if the pilot rushes to exit from a stall, usually by using rudder control to turn the aircraft faster. However, this requires the pilot to use the aileron to hold this banking angle which results in the nose dropping and requires the pilot to apply back pressure on the control column.
BOUNDARY LAYER
The boundary layer was first discovered by Ludwig Prandtl, a German engineer who studied aerodynamics. His theory was that there is a very thin layer of air flowing over an aerofoil. The layer directly on the surface, the molecules are motionless. The airflow outside of this area of stagnant air moves faster. Airflow at the top of the boundary layer moves at the same velocity as the airflow outside the boundary layer, this is known as the free stream velocity. The speed within the boundary layer depends on the shape of the wing, angle of attack and viscosity,
As the airflow hits the aerofoil, the first point of contact is the stagnation point. It then flows round the cambered aerofoil to the laminar flow. This section is where the airflow is uniform and organized and creates a suction point which creates lift depending on the camber of the aerofoil and the angle of attack. As the flow continues it reaches a section called the transition point where the laminar flow becomes turbulent flow which is chaotic. The next stage is the separation point, this is where the turbulent flow leaves the surface of the aerofoil and becomes the wake (Figure 3).
As the aerofoil changes in the angle of attack, it can affect the boundary layers characteristics. As the aircraft gets closer to the critical angle of attack, which is 15⁰, the aircraft begins to experience a change in pressure along the upper surface of the wing. The pressure changes from the ambient pressure at the leading edge of the aerofoil to a lower pressure on the surface or the aerofoil and returning to ambient pressure at the trailing edge. Flow separation should occur when low to high pressure (adverse pressure gradient). However, if the pressure gradient becomes too large, then the pressure will overcome the airflow forces. This results in the flow leaving the aerofoil. If the aircraft keeps exceeding the angle of attack, the pressure gradient will increase and will cause the aircraft to lose its loft characteristics all together (Figure 4).
AERODYNAMIC AND CONTROL CHARACTERISTICS
As an aircraft stalls, it will not necessarily stall along the whole wing but certain sections of the wing. Wing tip stall is feature where the tips of the wing stall first, however this causes a significant issue of no aileron control. Having no aileron control means that if an aircraft does stall, there will be no way to maneuver the aircraft out of the stall. This type of stall generally happens on swept wing aircraft. Along with the reduction in lateral control, the centre of pressure would significantly more rearwards causing a nose up pitching moment which is not desired if a stall does occur. If a root stalls first on an aircraft, the pilot will be able to sense the turbulent airflow hitting the horizontal stabilizer b ut it does not have the same effect for tip stalls.
As a tip stall occurs, it results in the centre of pressure moving forward and the aerodynamic centre of the aircraft. If the aerodynamic centre is in front of the centre of gravity then a nose up pitching moment will take place without and horizontal stabilizer control (Figure 5)
CONTROL SURFACE BUFFET
One method of a stall warning is the control surface buffet. As the aircraft nearly reaches the critical angle of attack, the flow of air on the upper surface of the aerofoil will become turbulent and lose its smooth flow. It loses its flow around the aerofoil at the separation point as seen in the boundary layer. As a result, if this turbulent flow goes across the horizontal stabilizer, buffet occurs. This buffet is a stall warning.
STALL VANE
A stall vane is a little tab that sits on the leading edge of the aerofoil which is within the boundary layer. It sits at the stagnation point of the boundary layer, this is an area of low pressure. As the angle of attack changes, so does the stagnation point. If the angle of attack increases, stagnation point moves rearward and decreasing angle of attack moves stagnation point forward,
Therefore, this device is used to inform the pilot what the angle of attack is in relation to the stagnation point and if the aircraft is near stalling (Figure 6)
The spring loaded vane points into the airflow on the lower surface of the aerofoil, where the stagnation point is located. The airflow at the stagnation point pushed the vane, which pushes the spring in the direction of the stagnation point.
As a stall occurs, the speed will decrease and the stagnation point will move rearwards which will trigger the vane to move forward. The forward movement triggers a switch that sends a warning signal to the cockpit so the pilot can make the appropriate maneuver.
STALL STRIP
Stall strip uses the principle of the boundary layer to aid in preventing a stall. As the aerofoil reaches a high angle of attack, the stagnation point will be under the aerofoil. Therefore the air will flow around the upper surface. Without this strip, the oncoming airflow will stay attached to the aerofoil. As the stall strip has such a sharp edge, the airflow can no longer stay attached as easily and begins to separate from the aerofoil before the aerofoil reaches the critical angle of attack. Due to this, an early stall happens directly behind the stall strip before the full surface of the wing stalls.
These stall strips are places close to the root of the wing and are a small device. The reasoning behind placing the strips at the root is to have root stall. Root stall is where it is more desirable to have a stall as this will be the first area to stall and having use of ailerons in this situation is desirable to pull the aircraft out of the stall (Figure 7).
Having these add another benefit as the root stalls first, buffet warning will also happen sooner. The pilot will get these warning much quicker.
SUCTION ACTIVATED HORN
The suction activated horn is another method of detecting when a stall is about to occur. As with most of the stall warning systems, this horn is located on the leading edge of the wing. Pressure difference over the wing will trigger the horn to activate. When the aircraft approaches a stall and the stagnation point moves under the leading edge which causes a pressure reduction on the upper surface. This lower pressure passes over the horn port which senses the negative pressure and triggers the horn activation.. As the angle of attack increases, the horn will get louder as it gets closer to the critical angle of attack. It will supply a warning horn to the cockpit.
ANGLE OF ATTACK SENSORS AND STALL WARNING COMPUTER
The angle of attack sensor or probe, works be sensing the direction of airflow. It is a small device that sits outside of the aircraft on the fuselage. The sensor is continually driven to detect the pressure between the upper and lower surface. Angle of attack probe is a mini aerofoil which will act as the same as a wing but on a very smaller scale. It senses the direction of airflow and the angular position of the probe is connected to an electric output which is where the warning computer comes in (Figure 8).
WASHOUT
Washout is a decrease in the angle of incidence on the wing. This means that the root of the wing and the tip are not on the same straight level, the wing will look twisted. The purpose of this is to make sure the root of the wing stalls before the tip. Having the root stall first is desirable as the aircraft will have the use of the ailerons to control the aircraft and move the aircraft out of the stall before it worsens. It also provides extra control from spinning. In addition to reducing stall, washout also provides a reduction of wingtip vortices which reduces drag.
Washout is a method of stalling. Stalling at the wing tips of the aircraft is dangerous as it could spin the aircraft which will result in crashing. It is highly dangerous at low altitudes. Due to this, a design was made to stall the aircraft at the inboard section of the wing so that during a stall, we can use the ailerons located at the tips so maneuver the aircraft out of the stall. This is called a flat stall, this means the stall is at the root and it is easier to control. Generally, this type of stall will occur on light aircraft.
There are two types of preventative methods of wingtip stall, geometric (Figure 9a) and aerodynamic twist (Figure 9b) .
Geometric stall. The outboard section of the wing will have a lower angle of incidence than the inboard section, similar to twisting the wing slightly. It is reduced between 2-3⁰. At the outside side of the wing, it is rotated downward which provides a gradual decrease in the angle of attack.
Aerodynamic Twist is different to Geometric stall as the outboard section of the wing is higher than the inboard section. It has the same angle of incidence along the span of the wing. As the outboard section as a higher angle of attack, this results in the root of the wing always stalling first. One clear benefit is that when the wind is flowing past the inboard section, turbulent flow will hit against the fuselage, giving an early warning to the pilot.
VORTEX GENERATOR
Vortex Generators are a little component that is placed on the wings which works with the boundary layer. It brings more kinetic energy into the boundary layer to improve the performance of the aircraft when it is in a high angle of attack or flying at low speeds (Figure 10).
Within the boundary layer, as discussed before, at the transition point where the laminar flow changes to turbulent airflow. Despite this, there is a very thin layer close to the surface which contains no turbulence due to dampening effects. If this very thin layer slows down, it will cause separation early and therefore stalling the aircraft. To avoid this happening, the intensity needs to be raised, so we re-energise the boundary layer. Vortex Generators are the solution behind this, it does so by creating a wake which places kinetic energy into the boundary layer (Figure 11).
Having these vortex generators means the aircraft has a higher critical angle of attack at a lower speed. Location of the vortex generators is crucial as they need to be at the transition point. However, finding this location is challenging as it can change if the flow conditions or angle of attack differ. If vortex generators are too close to the leading edge, it can cause a large amount of drag. If it is too far back, it affects critical angle of attack. The perfect location can be calculated or tested by computer simulation or wind tunnel testing.
STICK SHAKER
This is a device which is designed to shake the control stick to alert the pilot of an oncoming stall. It is used as a stall protection system which contains a sensor outside of the aircraft, on the wing. The sensor detects the angle of attack and relays the information to an avionics computer. Here, the data is processed and will alert the pilot if a stall is imminent and results in the stick shaking vigorously along with a sound warning (Figure 12).
The main components of this device is an electric motor that is connected to an unbalanced flywheel. When the shaker is activated, it provides a forceful and loud shaking movement. The shaking movement on the stick is the same frequency and amplitude as the airflow separation as the aircraft is approaching a stall. Using this device in the aircraft is designed as a back up to the main alert tone in the cabin.
STICK PUSHER
A stick pusher is used in aircraft that have poor stall handling characteristics and so that the device can prevent an aerodynamic stall. To prevent this happening, there will be a mechanical or hydraulic device that is in the aircraft with the purpose of pushing the control stick forward when the aircraft reaches a pre-determined angle of attack. When the angle of attack has reduced, the stick will release the pressure significantly (Figure 13).
There is a very large safety requirement when it comes to stall handling. Military and the civilian industry have very demanding requirements.
If some aircraft cannot meet these requirements, it must come up with another way of meeting them. Designers came up with the idea of a device that acts automatically by reducing the angle of attack when approaching the critical angle of attack.
The requirements include:
• Angle of attack
• Wing flap setting
• Load factor
Stick pushers have one crucial flaw, there is a chance that the stick pusher can activate randomly without the need to do so. If this is used in an aircraft, all the crew must know and be alert that this system may act upon itself. The pilot can also decide whether to keep the device on or off.
An import note to notice is that a stick pusher is a is a stall avoidance device but a stick shaker is a warning device.
AURAL AND VISUAL INDICATIONS IN THE FLIGHT DECK
Usually found on light aircraft, there is a small component on the leading edge of the wing. This device is very similar to the suction activated horn discussed earlier. As the angle of attack increases and becomes ever closer to the critical angle of attack, this component, called the reed, sounds an alert in the cockpit to notify the pilot of an impending stall (Figure 14).
The suction activated horn works in a very similar way, also located on the leading edge of the wing. As the low pressure passes over the horn port, the negative pressure triggers the suction activated horn. The sensor gets louder as the angle of attack increases ever closer to the critical angle of attack. An audible warning will be heard within the flight deck for the pilot to take action.
Conclusion
To conclude this report, all methods of stalling have been included. Firstly, the reasoning behind a stall has been explained with relation to the boundary layer and an explanation of what types of stall there is. Secondly, methods of preventing a stall such as:
• Control surface buffet
• Stall vane
• Stall strips
• Suction activated horn
• Angle of attack sensor and stall warning computer
• Washout
• Vortex generators
Lastly, the systems used to for protecting and warning the pilot of an impending stall, These include stick pushers, shaker and visual and audible warnings.
Overall these factors give the reader a greater and clearer understanding of the inner workings of a stall, how these can be prevented and the indicators of when a stall may occur.