INTRODUCTION
The aim of this report is to research the operation and requirements for stall protection systems. This should develop the author/readers knowledge of stall as well as the methods used in an aircraft to sense that it might be in danger of stalling and the systems used to warn and protect the aircraft from this occurring. Stall is very dangerous and is something that pilots need to ensure that they try and avoid, in order to prevent the pilot from taking the aircraft into a stall there is many warning systems in place in which shall be analysed throughout this report.
STALLING OF AN AIRCRAFT INCLUDING STALL ANGLE AND STALL SPEED
Stall
Stall is potentially really dangerous especially if the pilot is unable to regain control of the aircraft. Stall tends to occur when the angle of attack is exceeded past the point in which the aerofoil can no longer produce lift which results in a loss of lifting capabilities of the wing. The critical angle of attack or the stall angle is the point in which the aircraft will reach its maximum lift coefficient before it stalls. The stall angle of an aerofoil can be tested using a wind tunnel in which is a rather exciting experiment as you can see how the aerofoil reacts to the fluid and how steady the flow is until the critical angle of approximately 15 degrees is reached.
There are many types of stall some being more dangerous than others these range from departure and arrival stall, accelerated stall, secondary stall as well as Cross-control stall. Essentially stall isn’t just a loss of power to the engines although this can be the case. Stall is when there is a sudden loss of lift coefficient and this can occur due to different reasons as well as any air speed or at any altitude. As a result of this an aircraft has many systems in place in order to prevent and warn the pilot of a potential stall occurring in which shall be explained later in this report. It is very important that pilots in training practice putting the aircraft into stall conditions in which they feel safe in, in order to familiarise themselves with the stalling characteristics without actually putting the aircraft into dangerous conditions. This is important as it allows a pilot to recognise when an aircraft is close to a stall and how to react.
Figure 1: Lift coefficient against angle of attack
Figure 1 shows lift coefficient in relation to angle of attack which is very useful to aid the understanding of how a cambered aerofoil generates lift. As the angle of attack reaches 0 degrees there is already a positive lift coefficient which is a property of most cambered aerofoil where as a symmetrical aerofoil wont produce lift at an angle of attack at 0 degrees. There is a steady increase in the lift coefficient as the angle of attack increases until about 12 degrees at this point the lift coefficient still increases but not as steady and at the 15 degrees the aerofoil reaches its lift coefficient maximum and also the stall angle causing a loss in lift coefficient.
Boundary layer
The boundary layer was discovered and named by Ludwig Prandtl. The boundary layer itself is essentially just a thin layer of air that sticks the surface in which the effects of viscosity and friction are found. There are two types of flow in a boundary layer the first being laminar flow which is calm, smooth and tends to flow in layers. The second type of flow is found after the transition point, called turbulent flow which is chaotic and has no noticeable pattern. When air flows over a surface it tends to try and stick to the surface its flowing over but only a thin layer appears to be affected by this as shown in Figure 2 its only the airflow closest to the surface that is slowed down. Essentially there isn’t an edge to the boundary layer it just fades but the part in which relates to the edge of a boundary layer is when the flow speed reaches approximately 99% of its free stream value.
Figure 2: boundary layer
At the leading edge of the boundary layer the thickness of the boundary layer is said to be zero but as you go further along the wing you can see that the boundary layer thickness increases as shown in Figure 2. As the boundary layer thickens the amount of skin friction drag created also increases and the laminar flow turns into turbulent. This is where higher velocities next to the surface can also be found.
At high angles of attack, the airflow separates and this is known as the separation point. The separation point is the part in which the airflow leaves the surface and a wake is formed at the trailing edge of the wing. This occurs due to the air flowing from low pressure to high pressure also known as the adverse pressure gradient which is responsible for flow separation.
Aerodynamic and control characteristics
Pilots have the ability to alter the control characteristics of an aircraft through the use of the flight controls. The most important control surface in relation to stall would be the ailerons which provide a rolling motion around the longitudinal axis giving the aircraft lateral control. The reason being that if an aileron is moved after a sudden loss of lift coefficient then this can aggravate the stall and could cause a complete stall at the wing tip. In order for a pilot to recover from a stall it is important that flight is coordinated with the rudder. The pilot should be looking to lower the nose of the aircraft which is a tendency most aircraft have due to the centre of gravity being located in front of centre of pressure. The add thrust to restore the airflow over the wing as well as reducing the angle of attack. This should then allow the pilot to have use of his ailerons allowing them to safely correct for a wing drop.
When stall occurs, it is doesn’t mean that the full wing has stalled it can stall at the wing tip first or at wing root but it is desirable that the wing root stalls first as this would still allow use of the ailerons. Swept wing aircraft are susceptible to wing tip stall which is undesirable as there is also little warning given for this type of wing design. Due to the lower camber at the wing tip and the shorter cord length the wing tip is more likely to stall first as oppose to the wing root. Tip stall results in the centre of pressure moving forward of the centre of gravity which causes the aircraft to pitch up making it very difficult for the pilot to overcome.
METHODS USED FOR SENSING AN IMPENDING STALL, ANGLE OF ATTACK
Control surface buffet
Buffet is a type of vibration and in this case it is used to provide a warning that stall is approaching. This warning is only provided by an aerodynamic buffet. The airflow on the upper surface of the cambered aerofoil starts its transition phase from laminar to turbulent as it loses contact with the wing surface and if this turbulent airflow affects the horizontal stabiliser then a buffet occurs giving the pilot warning of the incoming stall.
Stall vane
The stall warning vane is an angle of attack sensor. Their only tends to be one stall vane and that is located on the outboard, leading edge of the left wing. The stall vane operates through a metal strip that is connected to an electrical sensor and if the wing comes close to the critical stall angle and the forward stagnation point gets below the stall vane it is then pushed up and will transmit a signal to the cockpit operating the horn. By sounding the horn this gives the pilots audible warning that they are reaching the stall conditions. The sensor also has an anti ice feature preventing it from being affected by ice conditions.
Figure 3: Stall vane (warning) and Stall strip
Stall Strips
Stall strips are also a pretty important part of the wings design and allow the aircraft to have more control if it does fall
into stall conditions. The stall strips actually cause the aircraft to stall at lower angles of attack which isn’t exactly desirable but it makes up for it by allowing the aircraft to stall at the wing roots first rather than the wing tips in which results in the pilots having a better chance of recovering from the stall than they would if it stalled at wing tips.
Suction activated horn
This type of sensor is essentially just a small hole that can also be found on the leading edge of the wing. It is activated by a pressure difference being created over the wing causing suction to the small hole drawing air. The suction activates the horn and it will increase in volume as the aircraft gets closer to the critical angle of attack
Figure 4: Suction horn stall sensor
Angle of attack sensor and stall warning computer
The angle of attack sensor tends to be located on the fuselage and is used to measure the angle of attack between the wing and the airflow. This information is then processed through to the stall warning computer in the cockpit and delivers the information to the pilots giving them an angle of attack display as well as a stall warning when needed.
Washout
Washout is a design concept of the wing which creates an angle of attack difference between the wing root and wing tip. This causes the wing tip to meet the airflow at a lower angle of attack than the root of the wing causing it to stall first. Not all aircrafts need this design as some planes can fly without it but it can be very effective because if the root stalls first lateral control is still available to the pilot.
Vortex generators
Vortex generators are effectively just small fins that can be located on the upper surface of the leading edge of an air foil. Vortex generators are placed vertical to the upper wing surface, and are positioned so that they meet the laminar flow coming over the wing with a slight angle attack. This generates vortices which regenerate the boundary layer and delay stall.
Figure 5: Vortex generators
SYSTEMS USED FOR STALL WARNING AND PROTECTION
Stick shaker
The stick shaker is a mechanical device which shakes to warn the pilot of the beginning of stall which should give him enough time to react and to move the aircraft in time preventing the stall from occurring.
Stick pusher or nudger
A stick pusher is can be referred to as a stick nudger. This type of system can also be connected with stick shaker for aircraft that are affected by deep stall with these systems being designed to prevent the pilot from entering a stall. The stick pusher collects information from the angle of attack, flap setting, airspeed and load factor and if it is believed that the aircraft is in a stall the stick pusher activates and pushes the control column forward which will bring the elevators down and should bring the nose of the aircraft down as well. This will stay activated until it reaches an agreed angle of attack which is set by the computers.
Aural and visual indications in the flight deck
In order to aid the pilot by flying the aircraft safely, the flight decks are fitted with aural and visual indications for many different errors that occur. If an aircraft approaches stall conditions and has been installed with a lift detector stall warning horn, then a loud noise blares in the cockpit regardless of the aircrafts speed or type of manoeuvre the aircraft is trying to perform providing it is in a high angle of attack. This warns the pilots that if nothing is done then the aircraft is going to stall. Some aircraft might be fitted with a visual warning where a light shall turn on which also makes the pilot aware of this.
The reed horn stall warning system is an improvement of the lift detector stall warning horn and is found on Cessna aircrafts but works in similar fashion but can function without electric wiring or power. The horn operates as negative air pressure develops at the leading edge of the wing allowing the horn to sound before stall occurs. This system is very useful as it doesn’t weigh much and its very easy to install.
CONCLUSION
Throughout this report we learned about stall and how dangerous it can be if the conditions are not spotted by the pilot. We also learnt about Ludwig Prandtl and his discovery of the boundary layer with relation to how the effects of this can cause the aircraft to stall. The methods of sensing an impending stall proved how important each individual sensor can be and essentially that the pilots shouldn’t be able to miss that the aircraft is heading into a stall as there is also systems in the cockpit that allows them to hear, feel and visualise that a stall is occurring.
The results of this investigation therefore lead to the conclusion that it is important that all aircrafts have stall protection systems and pilots are aware of stall conditions. Airline industries are always looking to improve their aircrafts with as many protection systems in place as possible but without incurring too much weight. This will give the pilots a better chance of flying safely and also gives passengers a bit of comfort knowing these systems are in place.
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