What happens when an aircraft overspeeds?

What happens when an aircraft overspeeds?

Aircraft overspeed Airbus A350

An incident took place with a Ryanair flight EL-EBW on Jan 2017. It was Boeing 737-8AS. As the aircraft was descending into a high altitude jet stream, there was an associated increase or rise in the headwind which caused the aircraft to overspeed. In order, to control the aircraft's overspeeding, the pilot switched the autopilot to manual, which eventually led to a nose-up pitch input at the control column. This incident caused few crew members to collapse and undergo severe injury.

Overspeed is that situation or condition of an aircraft in which it's engine is made or forced to operate beyond it's designed limits. However, the consequences that arise when a particular engine is made to run too fast vary depending upon the engine type and the model used. Other factors that determine the type of failure or consequence on the engine on overspeeding are judged mainly by the duration of the overspeeding condition and the speed reached. Few engines can be affected severely by reducing their engine life or cause catastrophic failures even by momentary overspeeding. The engine overspeed is measured in revolutions per minute.


Overspeed conditions arise when an aircraft exceeds the Never Exceed speed or the Vne of the airframe. Although the speed level doesn't bother the engines used in an aircraft, it is preferable for the pilot to stay within the indicated limits.

An aerodynamic phenomenon, commonly known as Mach tuck, which is too involved to get into in a forum is experienced by Jets and speeding aircraft. But generally, it is observed that it's the aerodynamic regime that results to uncontrollability of the aircraft after the published speed has been exceeded, usually with a fraction or percent of margin.

Induced drag, parasite drag, skin friction drag and many other kinds of drag are created on an airframe as the aircraft flies through the atmosphere, few of them resulting to the point of uncontrollability or complete or partial failure when exceeded. These values are usually determined by the manufacturers of the aircraft. Regardless of being an aerodynamic failure or a structural failure, it is not often published in the POH when the speed is exceeded. To avoid these failures it is best expected for the pilot to stay within the published limits.

EPR or N1 and the EGT temperature present on the engine stack are used to monitor the overspeed factor of an engine.The limits for various flight conditions like take-off and cruise is provided in the POH, which includes the temperature and density altitude at standard conditions.

Aircraft overspeed


In most aircraft, designed to avoid overspeeding failures a governor or a regulator is often used to make the overspeeding condition impossible or very less likely to be attained and hence giving no chance for failure due to the same reason. For instance, Some of the aircraft are embedded with constant - speed units which have the capability to change the propeller pitch without any human intervention. This helps the aircraft or the engine to run at the optimal speed.

Aircraft overspeed prevention

Overspeed condition

In propeller driven aircrafts, overspeed conditions are attained when the propeller which is ordinarily connected directly to the engine of the aircraft, has been made or forced to operate at a very fast pace with high-speed airflow during the aircraft performing dive, or in case it moves to a flat pitch in cruising flight due to failure of the governor or any feathering failure from/in the engine.

Coming to jet driven aircraft, overspeed conditions are attained when the maximum operating speed has been exceeded by the axial compressor of the engine. As a result, we notice mechanical failures of the turbine blades or flameouts or even complete destruction of the engine.

Thanks for reading!

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Aircraft Fan Drive Gear System

Fan Drive Gear System

A very commonly boasted off phrase by aircraft engine manufacturers these days is, "The Ultra High By-Pass Ratio". Have you ever wondered what do they mean by this ultra-high bypass ratio? What is the reason behind the ultra-high bypass ratio?

Aircraft Fan Drive Gear System

Before starting the discussion about the ultra-high bypass ratio and the fan drive gear system let us have a brief introduction to the basic composition and working of an aircraft engine. The engine consists of basically five sections
  1. Fan
  2. Compressor ( low pressure and high pressure)
  3. Diffusion and combustion chamber
  4. Turbine ( low pressure and high pressure)
  5. Exhaust Nozzle

working of the aircraft jet engine

The ambient air is sucked into the engine by the fan, the air coming inside from the fan is divided into two parts the bypass air and the secondary air. As it can be seen from the image the air entering the core engine is called the secondary air and the part of the air that does not enter the core engine and goes through the engine is called the bypass air. The air which enters the core engine is initially compressed in the compressor stage of the aircraft engine which increases the pressure and temperature of the air. After compression, the air is sent to the diffusion and combustion chamber where the compressed air is burnt along with air and fuel mixture which further increases the pressure and temperature values to a very high value. After this, the high pressure and temperature air is sent in the turbine section where high pressure and temperature air is expanded which increases the velocity of air to very high values at the expense of its pressure. The expansion process also produces rotary motion of the N1 and N2 shafts which is used to drive the compressor and fan. After expansion, the high-velocity air is passed through the exhaust nozzle which further increases the velocity of the air which is leaving from the aft of the engine. Now the thrust is produced as according to Newton's Third law of motion that for every action there is an equal and opposite reaction. The secondary air coming out from the aft of the engine is the action for which we get a forward push which is the reaction. Thrust is not only produced by the air coming out of the exhaust nozzle but also due to the by pass air. To know more about the construction of an aircraft and thrust production in an aircraft engine you can read here

A very interesting fact which should be remembered is that the major amount of thrust produced by an aircraft engine ( approx 80% )  is by the by pass air and the rest 20 % is produced by the air passing through the core engine. The ratio of the f mass flow rate of the by pass air to the mass flow rate of the secondary air is called the by pass ratio. For ex- 10:1 by pass ratio means that 10 kg of air passes through the by pass duct for every 1 kg of air passing through the core engine.

Airbus A320

As you all have understood about the by pass ratio, let us move our discussion towards ultra high by pass ratio. For a normal turbojet engine such as the V2500 engine the by pass ratio is around 5-6 :1 but in case of PW1100G-JM the by pass ratio increases to a value of 12:1, ye that is ultra high by pass ratio.

Advantages of ultra high by pass ratio

1. Lower fuel consumption ( upto 20% fuel consumption reduction )

2. Lesser noise created by the engine

Simple thrust equation of aircraft propulsion is


Here Pe and Po are the pressures at the exit and inlet of the engine.
        Ve and Vo are the velocity of the air at the exit and inlet of the engine.
        Me and Mo are the mass flow rate at the exit and inlet of the engine.

 We can very clearly see that as the value of Me that is the mass of air leaving the engine increases the value of thrust force F provided by the engine increases. You will be easily be able to relate to the fact that increasing the mass of air coming inside the engine would surely increase the mass of air leaving from the engine hence a very high value of by pass ratio would increase the thrust value.

Now the other method of increasing thrust would have been increasing the velocity of air leaving the engine. The only method of doing that is to increase the mass of air fuel mixture burning inside the combustion chamber which would increase the fuel cost.

So we can say that having an ultra high by pass ratio is better than burning more amount of fuel both environmentally as well as economically.

After knowing about what is ultra high by pass ratio and what does it affects now let us examine the technology behind ultra high by pass ratio.


Fan drive gear system was initially named as ATFI Advanced Technology Fan Integrator then the it again got a new name as Geared turbofan technology and now we know it as the Fan Drive Gear System. Though the name of technology has changed over time but the engineering behind the technology has always remained the same.

Aircraft engine fan

 If moving the larger mass of the air is the solution then we can simply make the fan blades very large and make them spin very slowly or we can make more air to pass through the core engine. But both the situations have some shortcomings in the first case as we have the fan coupled directly to the low pressure compressor hence we can not rotate the fan at lower speeds as that would require higher compressor stages. Now if we move the fan at a higher speed then we would get a disadvantage in form of tip losses at the tip of the fan blade hence lowering the engine's efficiency. Now the next solution was to make more to pass through the core engine, that would create a great hike in the fuel requirement as well as a major increment in the noise produced by the engine due to the friction of air.

To counter these problems a person at Pratt and Whitney brought a new technology after working for around 20 years endlessly. A 3:1 ratio gearbox is used between fan and the low pressure compressor shaft, (assuming that you have read the above mentioned article at least once) now the fan is rotated at slow speeds as required and the LPC is made to rotate at high speed compared to the fan and that too in opposite direction. The high pressure spool connecting the high pressure compressor to the high pressure turbine and the fan rotate in the same direction but the fan rotates at a much lower speed as compared to the fan and the low pressure compressor along with the low-pressure turbine rotate in the opposite direction as compared to both of them hence apart from solving the first problem as we discussed earlier we also get a rotational balancing in the engine.

This ends up discussion about the fan drive gear system apart from creating the ultra high by pass ratio the FDGS have many other advantages such as reduction in compressor and turbine stages and more......

Thanks for reading!

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How Aircraft hydraulic system works?

Aircraft hydraulics system

Aircraft hydraulic system

Today, an aircraft in the modern times uses a hydraulic system to operate many of its critical components. Taking time back, hydraulic systems well still incorporated into the aircraft as a form of hydraulic breaks. With the passage of time, as the technologies evolved, so did the use of the hydraulic system. At present, hydraulics are used in many aspects in the aircraft, which includes, landing gears, brakes as well as flight control surfaces. Regardless of the size and shape of the aircraft, the basic principles and functionality of the hydraulic systems virtually remain the same.

The main reason behind this increased usage of the hydraulic systems are;

- They are very cost effective to install,
- Relatively easier maintenance,
- Can operate at maximum efficiency even at the toughest flight conditions.

 Apart from this, the components used in a hydraulic system are typically lightweight, their installation is not complicated and they process a simple investigation methodology. Adding to their design and incorporation of fluid dynamics, the hydraulics have maximum efficiency with negligible frictional losses.


All though hydraulic systems have proved to be an extremely safe and durable introduction in the aircraft industry, certain redundant systems are embedded into the aircraft to enable safe operation in case of failure of the hydraulic systems.

The basic components of a hydraulic system are:

Pump – The pump is a device that is used to generate power in order to pressurize the system.

Reservoir – The reservoir is the vessel that is used to store the hydraulic fluid which is supplied to the system.

Actuating Cylinder – The actuating cylinder is that part where the major function of the hydraulic system is operated. For instance, these cylinders are installed at the flaps of the aircraft so that the pilot can withdraw and extend them as per his/her requirements.

Pressure Relief Valve – The pressure relief valve is used to protect the hydraulic system in case of excess pressurization.

Heat Exchanger – The heat exchanger is a device used to help maintain the hydraulic fluid at an optimum temperature.

These are basic elements that constitute a hydraulic system. However, in larger aircraft or in aircraft with more scope have additional components.

Aircraft hydraulic system


The hydraulic system of an aircraft works on the principle that, it uses a pressurized liquid to help in movement of certain body components from one position to another position. The hydraulic system has a wide range of pressure capacity from a few hundred pounds per square inch to more than 5000 pounds per square inch. This wide range of the hydraulic system is to cope with different sizes of aircraft and different loads.

Pascal's law is the main science used behind this principle of hydraulics system, which states that when you apply pressure anywhere onto a liquid which is enclosed in a system will exert the same pressure by distributing it in the surrounding medium.

During the flight, the pilot brings the hydraulic system to activation method by switching on the input or flight control device. As the pump starts operating, the actuation begins to move.

The actuator forwards it's motion by directing it to a control surface or any other device that needs to be moved to the desired motion. The pressure is released when the system needs to be moved in the reverse direction.

Aircraft hydraulic system


Hydraulic fluids majorly function in order to convey power between components. However, they are also used for other reasons such as protection of hydraulic machine components. There are many important factors that one needs to keep in his/her mind before selecting an optimum fluid to be used in the hydraulic system, they are:

Supposed to be non compressible ( with high bulk modulus), Less volatility, Moderate heat transfer, Optimum thermal capacity and conductivity, good viscosity, be able to minimize internal leakage, High viscosity index, Special function, Fire resistance, Friction modifications, Radiation resistance, Environmental impact, Low toxicity when new or decomposed, Biodegradability, Functioning life and Material compatibility.

Usually, hydraulic fluids incorporated are based on mineral oil or water. Skydrol is the widely used hydraulic dluid in the aircraft.

Thanks for reading!

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Windowless Cockpit Concept by Airbus

Windowless Cockpit Concept by Airbus

                            WINDOWLESS COCKPITS 

Imagine that you are boarding an aircraft and you find out that there are no cockpit windows, rather an opaque cone of metal sheet covering the entire cockpit window area. You would feel dumbfounded be reluctant to board the aircraft, right? Well, in a few years you may find every Airbus aircraft without cockpit windows. This may look stupid to many of the readers, but Airbus advocates that it may be the future of aircraft design.
In 2014, Airbus registered a patent featuring the idea of “windowless” cockpit which would increase pilots’ field of view.

windowless cockpits
Windowless Cockpit

Looking at the bigger picture, this invention is basically replacing the glazed cockpit window with a large screen displaying a digital image of the outside environment.
The cockpit is the room reserved for pilots comprising of all controls necessary to fly the aircraft. Conventionally, it is located in the front of the fuselage with slanted window panes which allows the pilots to have an unobstructed view of both forward and downward of the aircraft.
For maximum aerodynamic efficient design, the nose of the aircraft should ideally be of lancet shaped (like a knife) but to accommodate the radome and the landing gear, it is modified to give a larger radius of curvature.

Alternative cockpit
Alternative location for a cockpit

This invention solves this problem by shifting the cockpit at another location which allows the engineers to modify the shape of the nose. Since there is no need for the pilot to actually look outside, the entire cockpit can be shifted to another location freeing up space in the front of the fuselage.
This not only gives a better aerodynamic design to the aircraft, but also increases the pilots’ field of view. Airbus also claims that this invention can increase the payload capacity of the aircraft in their patent.

concept of windowless cockpits
Digital screen concept

The digital image would be projected in the form of a hologram or any other advance technology. The image would be generated from the data supplied by on-board video camera located in the front and the rear of the aircraft.
Example : When the aircraft is taxiing on the ramp, the screen would display a 3D image of the airport generated from the data captured from the video camera and the data stored in data banks on ground. If an obstacle is detected in the path of the aircraft (both on ground and in air) it is possible to modify the video image by displaying the obstacle using Augmented Reality with warnings to alert the pilot.
Holographic guidance system
Holographic Guidance System

Using this technology, the flight path to avoid the obstacles or any turbulence or bad weather can be also displayed on the screen. Airbus prefers that the screen used in the cockpit be made of OLED (Organic Light Emitting Diode) which helps in designing various complex shapes on the screen.

Airbus is yet to make the proof of concept of this invention. Such complex technology takes decades to develop and be trustworthy enough to replace the cockpit windows. It may take more than a decade to see an aircraft with no cockpit windows.

How Aircraft engine oil system works?

The Oil System of an Aircraft Engine

What is lubrication 

In a chilly morning of the month of January you would find yourself rubbing your hands to keep yourself warm, Won't you?

Now just imagine if I put some oil in between your hands would you be able to keep yourself warm? You would answer in a shivering voice, "NO". What exactly happened ? Where has the heat absconded?

Initially, when you were rubbing your hands without any oil layer, you were actually generating heat due to friction and that heat was helping you to keep warm. Heat generation by friction in simple words can be attributed to a phenomenon that at a very microscopic level of magnification it can be seen that surfaces of both the hands were having some irregular bumps and grooves on their surfaces. Now imagine bump of one surface gets interlocked in the groove of the other surface, then it would require a greater amount of force to break that interlocking. This interlocking prevents the motion between two hands which is called friction. Now when these bumps and grooves come in contact particles of both the surfaces get bonded to each other. When the bonds between these particles are broken a considerable amount of heat is generated which was helping you to keep yourself warm.

What happened when there was a layer of oil present in your hands,? The fluidity of oil helps the layers of oil to easily slide over each other, due to which it acted as a lubricant and prevented the formation of a bond between the particles of the hands which further reduced the amount of heat generated. Though here we have talked only about the liquid form of lubricant but lubricants can be solid, liquid and even gases. 

With this, we have learned a very important concept of lubrication

Informal engineering words lubrication is defined as the action of applying a substance such as oil or grease to any machine component so as to minimize the friction and allow smooth movement. 

Oil System 

The oil system is very important for any machine may it be a small sewing machine used at home to the huge aircraft engine. You may easily develop a confusion between oil system and fuel system but they are way different from each other. The fuel system is used to supply the fuel inside the combustion chamber to increase the pressure and temperature of the incoming air inside the engine whereas the oil system is used to supply oil in various parts of the engine. The main parts which are to be taken care by the oil system are the bearing compartments and the gearboxes.

The oil system performs five basic functions :

1. Lubrication
2. Cooling
3. Cleaning
4. Vibration reduction

To summarize the first function of oil system that is lubrication, we can say in simple language that the process of reducing friction between two machine components which may be any components
inside the engine say a shaft rotating in the journal bearing as seen in the figure where oil acts as lubricant and prevents the contact between the two surfaces reducing friction between them which further reduces the wear and tear of these components increasing their life. 

The second function of cooling which also has been discussed earlier may be summarized as the function of oil in which it reduces the friction between the mating parts which reduces the heat generation. It also takes away the heat generated despite reduced friction. 

The third and a very important function which the oil system performs in cleaning, while relative movement of machine components against each other they tend to produce some chips and burs. The lubricant carries away these particles with it which helps in the cleaning process. 

Vibration Reduction is also done by the lubricant by absorbing the vibration produced as the components of an aircraft engine move at very high rpm,

You would be able to understand the above functions of the oil system more clearly once you come across the functioning of the oil system.

Firstly we would study the working of the oil system at the specific level and then we would take it as a broader concept.

Working of the oil system

Looking at the image above you can easily relate this to a shaft rotating between the balls of a ball bearing the yellow color signifies the lubricant. The lubricant enters the bearing compartment at a very high pressure and low temperature. After coming into the bearing compartment the high-pressure lubricant pushes away the old lubricant present in the compartment. The outgoing lubricant takes away the heat generated inside the bearing compartment along with any chips or debris that are produced due to the relative motion between the shaft and the balls of the bearing. After that, the outgoing oil is filtered and cooled down and again fed at high pressure to the bearing compartment. 

The complete oil system

Basic Components

1. Oil Tank 

The oil tank usually found in most common engines ranges from a capacity of 20-30 liters. Hot tank system and cold system are the two technologies used to store the oil inside the tank. In hot oil system as the name suggests the oil present inside the oil tank is having a considerably high temperature and vice versa for the cold tank system.

2. Pressure pump and scavenge pump with filter

As discussed earlier the oil sent in the bearing compartment is in a pressurized state and this pressurization is done with the help of pressure pumps. Generally used pumps are the gear type pumps or the vane pumps. The pressure produced ranges about 150 psi. The reason behind pressurization is to send the clean oil at high pressure so that the old and used oil present inside the compartment can be removed easily and completely. It is just like to throw a ball at another ball so that the first ball can displace the second ball from its position. 

After the pressurized oil has entered the compartment the dirty oil has to be removed and cleaned for removing the dirty oil we use similar type pumps as we used for pressurization but this type their function is to scavenge the dirty oil and the cleaning of dirty oil is done with help microparticle filters.

A very interesting fact is that in an oil system we have a single pressure pump and 5+ scavenge pumps. This is because as the dirty and heated oil would deteriorate the moving components and would increase their wear rate hence the used oil needs to be removed as quickly as possible hence more scavenge pumps are used so that scavenging can be done quickly.

3. Deoiler

Deoiler is also a very important component of the oil system. The oil used as a lubricant is aircraft engines is very costly and every single drop of it is precious hence efforts are made to prevent its wastage. For doing this a device called deoiler is used which separates out oil from the air that is used for breathing of the bearing compartments and the gearboxes.

4. Fuel Cooled Oil Cooler (FCOC) and Air Cooled Oil Cooler (ACOC)

When the heated oil is scavenged from the bearing compartment and gearboxes is required to be cooled before reusing it. This cooling of oil is done by either giving its heat to the fuel being used in the engine or the air passing through the engine. FCOC and ACOC are simple counter flow heat exchangers.

The Complete Oil system

Let's sum up the oil system by talking about its flow chart, the cool and clean fuel is taken from the oil tank with help of pressure pumps after which it is then taken to the oil filter where any impurity which may be present in the oil is removed. After this, the clean oil is then sent to the three bearing compartments, angle gearbox, and the main gearbox. A deoiler is mounted on the AGB which separates out any oil is present in the air being breathed out. Then the heated and dirty oil from the bearing compartments and the gearboxes is taken away with the help of scavenge pumps and then it is filtered in the scavenge filter after which the oil is cooled in the FCOC and ACOC and then the cool and clean oil is sent to the oil tank.

Thanks for reading!

What is the Coriolis effect?

How earth's rotation affects aircraft motion?

What is Coriolis effect?

What is the Coriolis effect?

Right from our school days, we have been studying about the fact that earth rotates about its axis from west to east direction i.e. in the counterclockwise direction and completes one rotation in 24 hours. Now according to simple physics, each and every particle on earth would follow the same rotational motion.

What is Coriolis effect?

 Let's understand this in a more simpler way, let's go to the nearby park an imagine the merry go round swing as the earth, come let us sit on the swing and one of our friends would move the swing for us. Wooah!! everything is rotating you are rotating I am rotating the air around us is rotating and even a piece of small stone on the floor of swing is rotating. Now let us make this game a bit more complicated you will move towards the center of rotation of the swing (the swing is stopped for a moment) as shown in the figure and I will stay at my place. The swing starts rotating again, both of us will stretch out our hands at exactly the same time and in the same direction. After five minutes of swing we stop the swing again, do our hands point in the same direction? Are they still aligned with each other?

What is Coriolis effect?

Yes they are, this brings out a very interesting concept that you and I were at different radius of rotation at the swing but still our hands were aligned at every moment which brings out that the distance traveled by you and me was different as the circumference of circle followed by us can be given by 2*π*R (R-radius of rotation). The radius of circle followed by me is larger hence I have to cover a larger distance as compared to you in the same time now to do that my speed of rotation will be greater than yours.

What is Coriolis effect?

Here A and B are are mine and your initial locations receptively, B' is your new location, the dotted circles show the path followed by us while rotation.

Hence it can be said that if a body A situated at a rotating body B moves towards the axis of rotation of body B then its speed of rotation will decrease given that its mass remains constant, i.e. the radius of rotation is directly proportional to the speed of rotation.

Now after learning about this fact lets move towards the main topic of our discussion the Coriolis effect. Let us now apply the principle learned for a merry go round to the earth. Consider you are at equator right now then your speed of rotation will be around 1647 km/h but once you will start moving towards the pole the radius of rotation will keep on decreasing and the speed of rotation will also decrease as discussed earlier. At some point between the equator and pole, your speed of rotation will be much less than that at the equator and at exactly at the pole your speed of rotation will be zero.

Ok, let's imagine one thing a hypothetical paper plane has to travel from Chennai which is near to the equator to New Delhi which is away from the equator in the north direction. If you would throw this paper plane in straight direction would it reach exactly at New Delhi? No, it won't, here come the Coriolis effect into play.

Before moving forward I need to explain a simple physics term called the angular momentum which is equal to Mass * Radius of rotation * Speed of rotation. This quantity always remains conserved in a rotational motion until and unless an external unbalanced torque is applied to the system.

What is Coriolis effect?

Now as the plane leaves from Chennai its angular momentum is having some value A and when it reaches New Delhi, the value of angular momentum becomes B but as we know after traveling to New Delhi the value of angular momentum decreases as both the speed of rotation as well as the radius of rotation are decreasing. But according to the above said principle we need to have A= B, for that the path of motion of particle adjusts itself so that the angular momentum remains conserved i.e. A= B + C. Here C is the angular momentum generated due to the path change.

We can understand this in a more simpler way, Chennai is moving much faster as compared to Delhi hence a body leaving from Chennai won't be able to reach Delhi it would reach a point deviated in the anticlockwise point from Delhi. The solid line in the above image shows the desired that and the dotted line shows the actual path followed.

So to bring out the crux of Coriolis effect we can say that if a body is thrown in north direction from the rotating earth then its path of motion will be redirected or deflected in the right direction and vice versa for the southern direction i.e. body thrown towards south would get deflected towards the left direction. And if we would throw a ball from the north pole towards equator then it would get deflected the other way around.

What is Coriolis effect?

Coriolis Effect on airplanes

As discussed earlier the effect of Coriolis effect on a hypothetical plane, the word hypothetical was used just to signify that the frame of reference for watching the plane was several lakhs kilometers away from the atmosphere of earth. If this frame of reference is taken on earth itself i.e. we are observing a plane flying over our head standing on the surface of earth then Coriolis effect won't play any role as we are also moving with the atmosphere of earth. So it is all about the frame of reference which we are talking about.

Coriolis effect would play an important role in case of Rocket Propulsion as rockets leave the atmosphere of earth.

Though the Coriolis effect does not affect the airplanes directly it has an indirect effect on the flight path by producing various phenomenon related to wind motion such as vortex winds, Jet streams etc.

Thanks for reading!

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Types of flaps


Types of Flaps

Will an aircraft produce the same amount of lift or could the lift produced by it be increased? An aircraft could produce the different magnitude of lift depending upon the angle of attack and wing configuration. The angle of attack is the angle in which the relative wind velocity makes with the chord line. Chord line is the line joining the leading and the trailing edge of the wing.

The concept of high lift devices:

As the birds are able to change the configuration of their feathers, similarly an aircraft could make use of the high lift devices to increase the amount of lift generated. High lift devices include slats, slots, and flaps. Wings have to generate a lot of lift during take-off and landing to reduce the chances of a stall.

The speed below which the aircraft goes into a stall is known as stall speed. So the aircraft must always fly at a speed greater than stall speed. Stall speed would be more if the wings are producing more lift. Therefore high lift devices are used to decrease the stall speed, which would result in the decreased requirement of runway length for take-off and landing.

Different high lift devices:

Trailing edge flaps are extended and moved downwards to increase the curvature of the wings. Leading edge flaps are present on the front of the wings and are moved out to increase the lift. Leading edge slats extend out from the front of the wing. When the high lift devices are extended, then the net area and the curvature of the wing are increased. This causes increased lift but also creates a lot of drag. Both these things are desirable during landing.

High lift devices are used during landing as well as take-off but their configuration is different for different purposes. Higher flap settings are used for landing whereas lower flap settings are used for take-off, because higher drag, which is desirable during landing, is created through higher flap settings.

Understanding Flaps:

Flaps are the secondary control surfaces. These are usually present on the trailing edge of the wing. Chord line gets altered when the flaps are extended. The effective angle of attack increases when the flaps are extended. Lift varies with varying lift coefficient which in turn is dependent upon the angle of attack.

With flaps down larger lift could be generated at the same speed as compared to flaps up configuration. Stall speed is decreased with flaps down configuration; therefore the aircraft would now be able to fly at a slower speed without stalling. The pilot gets improved visibility as the pilot could now fly at a lower nose attitude without stalling.

Different types of flaps:

Plain Flaps are the simplest form of flaps. They are hinged at the rear part of the wing. When extended they help in changing the curvature of the wing. They provide more lift but at the same time chances of flow separation increases. Chances of flow separation is increased as the flow losses energy because now it has to travel a larger distance. Fairey Hamble Baby was the first aircraft to fly with flaps.  Cessna A185F is one of the aircraft which use plain flaps.

Cessna A185F
Cessna A185F
Split Flaps are hinged below the wings. When these are rotated they increase lift but causes more drag as compared to the plain flap design due to disturbance of airflow around the wings. Split Flaps when fully extended could also work as a spoiler. Split flaps are used by many aircraft and Douglas DC-1 was one of the first aircraft to use them. 

Douglas DC-1

Slotted Flaps are similar to the plain flap design but there is a slot or gap which helps in reducing the chance of flow separation. Chances of flow separation are reduced as the higher pressure air from below the wings flows up and energizes the boundary layer. There are many aircraft which are installed with slotted flaps, most of the commercial airliners use slotted flaps.

Slotted flaps

Fowler Flaps are similar to slotted flaps but unlike slotted flaps, they could be extended both back and down. This increases affected wing surface area as well as the effective angle of attack. Fowler Flaps increases effective chord length as well as curvature of the wing. P-47d thunderbolt and many other aircraft use fowler flaps. 

 P-47d thunderbolt
P-42d thunderbolt
Krueger Flaps are installed on the leading edge of the aircraft, unlike other flaps which are present on the trailing edge. The total chord length of the wing is increased when Krueger flaps are deployed. Wings get a new leading edge when the Krueger flaps are deployed. Some of the portions of lower wing is rotated so as to bring it in the front of leading edge of the main wing. A mixture of inboard Krueger flaps and outboard slats are used in Boeing 727.

Boeing 727
Boeing 727

Thanks for reading!

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